United States 

Environmental Protection Agency

Office of Water

4304T

EPA-822-B-08-001

June 2008

Nutrient Criteria 

Technical Guidance Manual

Wetlands

U.s. Environmental Protection Agency

Nutrient Criteria

Technical Guidance Manual

Wetlands

June 2008

Nutrient Criteria–Wetlands i

DISCLAIMER

This manual provides technical guidance to States, Authorized Tribes, Territories and other authorized 

jurisdictions to establish water quality criteria and standards under the Clean Water Act (CWA), in order 

to protect aquatic life from acute and chronic effects of nutrient over-enrichment. Under the CWA, States 

and Authorized Tribes are directed to establish water quality criteria to protect designated uses. States 

and Authorized Tribes may use approaches for establishing water quality criteria that differ from those 

recommended in this guidance. This manual constitutes EPA’s scientific recommendations regarding the 

development of numeric criteria reflecting ambient concentrations of nutrients that protect aquatic life. 

However, it does not substitute for the CWA or EPA’s regulations; nor is it a regulation itself. Thus, it cannot 

impose legally binding requirements on EPA, States, Authorized Tribes, or the regulated community, and 

might not apply to a particular situation or circumstance. Further, States and Authorized Tribes may choose to 

develop different types of criteria for wetlands protection, including narrative criteria. EPA may change this 

guidance in the future.
ii Nutrient Criteria–Wetlands

Disclaimer ........................................................................................................................................i

List of Figures ......................................................................................................................................iv

List of Tables ......................................................................................................................................iv

List of Internet Link References .............................................................................................................v

Contributors .....................................................................................................................................vii

Acknowledgments ........................................................................................................................vii

Executive Summary .........................................................................................................................ix

Chapter 1. Introduction .................................................................................................................1-1

1.1 Introduction .......................................................................................................................................1-1

1.2 Water Quality Standards and Criteria ..........................................................................................1-2

1.3 Nutrient Enrichment Problems .........................................................................................................1-3

1.4 Overview of the Criteria Development Process ...........................................................................1-6

1.5 Roadmap to the Document .........................................................................................................1-6

Chapter 2. Overview of Wetland Science ..................................................................................2-1

2.1 Introduction ......................................................................................................................................2-1

2.2 Components of Wetlands .........................................................................................................2-2

2.3 Wetland Nutrient Components .........................................................................................................2-5

Chapter 3. Classification of Wetlands ..................................................................................3-1

3.1 Introduction ......................................................................................................................................3-1

3.2 Existing Wetland Classification Schemes ...........................................................................3-2

3.3 Sources of Information for Mapping Wetland Classes ..........................................................3-10

3.4 Differences in Nutrient References Condition or Sensitivity to

Nutrients Among Wetland Classes....................................................................................................3-10

3.5 Recommendations ......................................................................................................................3-11

Chapter 4. Sampling Design for Wetland Monitoring ....................................................................4-1

4.1 Introduction ......................................................................................................................................4-1

4.2 Considerations for Sampling Design ..........................................................................................4-2

4.3 Sampling Protocol .......................................................................................................................4-5

4.4 Summary ......................................................................................................................................4-9

Chapter 5. Candidate Variables for Establishing Nutrient Criteria ...........................................5-1

5.1 Overview of Candidate Variables ..........................................................................................5-1

5.2 Supporting Variables ........................................................................................................................5-3

5.3 Casual Variables .......................................................................................................................5-5

5.4 Response Variables .......................................................................................................................5-9

5.5 Summary ....................................................................................................................................5-12

CONTENTS
Nutrient Criteria–Wetlands iii

Chapter 6. Database Development and New Data Collection .........................................................6-1

6.1 Introduction ........................................................................................................................................6-1

6.2 Databases and Database Management ............................................................................................6-1

6.3 Quality of Historical and Collected Data ...........................................................................................6-4

6.4 Collecting New Data .........................................................................................................................6-6

6.5 Quality Assurance / Quality Control ............................................................................................6-7

Chapter 7. Data Analysis ...............................................................................................................7-1

7.1 Introduction .........................................................................................................................................7-1

7.2 Factors Affecting Analysis Approach ............................................................................................7-1

7.3 Distribution-based Approaches ...........................................................................................................7-2

7.4 Response-based Approaches ...........................................................................................................7-3

7.5 Partitioning Effects Among Multiple Stressors ..............................................................................7-5

7.6 Statistical Techniques ..........................................................................................................................7-5

7.7 Linking Nutrient Availability to Primary Producer Response ................................................7-8

Chapter 8. Criteria Development .................................................................................................8-1

8.1 Introduction ........................................................................................................................................8-1

8.2 Methods for Developing Nutrient Criteria .............................................................................8-1

8.3 Evaluation of Proposed Criteria ..........................................................................................................8-6

References ......................................................................................................................................................R-1

Supplementary References .......................................................................................................................R-21

Appendix A. Acronym List and Glossary ...........................................................................................A-1

Acronyms ......................................................................................................................................................A-1

Glossary ......................................................................................................................................................A-2

Appendix B. Case Study: Deriving a Phosphorus Criterion for the Florida Everglades .................B-1

Literature Cited ......................................................................................................................................................L-1

CONTENTS (continued)
iv Nutrient Criteria–Wetlands

List of Figures

Figure 1.1 Flowchart providing the steps of the process to develop wetland nutrient criteria ..................1-8

Figure 2.1 Schematic of nutrient transfer among potential system sources and sinks ..................2-1

Figure 2.2 Relationship between water source and wetland vegetation. Modified from Brinson (1993) ......2-2

Figure 2.3 Schematic showing basic nutrient cycles in soil-water column of a wetland ......................................2-7

Figure 2.4 Range of redox potentials in wetland soils .....................................................................................................2-8

Figure 2.5 Schematic of the nitrogen cycle in wetlands .....................................................................................................2-9

Figure 2.6 Schematic of the phosphorus cycle in wetlands ...................................................................................2-10

Figure 3.1 Map of Omernik aquatic ecoregions .....................................................................................................3-3

Figure 3.2a Map of Bailey ecoregions with coastal and estuarine provinces ......................................................3-3

Figure 3.2b Legend (of Map of Bailey ecoregions with coastal and estuarine provinces) .....................................3-4

Figure 3.3 Examples of first four hierarchical levels of Ecological Units .....................................................................3-5

Figure 3.4a Cowardin hierarchy of habitat types for estuarine systems .....................................................................3-6

Figure 3.4b Cowardin hierarchy of habitat types for Palustrine systems .....................................................................3-7

Figure 3.5 Dominant water sources to wetlands, from Brinson 1993 .....................................................................3-8

Figure 3.6 Dominant hydrodynamic regimes for wetlands based on flow pattern .....................................................3-8

Figure 3.7 Interaction with break in slope with groundwater inputs to slope wetlands (Bottom)

 Palustrine systems, from Cowardin et al. 1979 .....................................................................................3-8

Figure 5.1 Conceptual model of causal pathway between human activities and ecological attributes ......................5-2

Figure 7.1 Biological condition gradient model describing biotic community condition as levels of 

 stressors increase ......................................................................................................................................................7-1

Figure 8.1 Use of undisturbed wetlands as a reference for establishing criteria versus an effects-based 

 approach ....................................................................................................................................................8-3

Figure 8.2 Tiered aquatic life use model used in Maine .....................................................................................................8-4

Figure 8.3 Percent calcareous algal mat cover in relation to distance from the P source showing the loss 

 of the calcareous algal mat in those sites closer to the source .....................................................................8-5

List of Tables

Table 1 Observed consequences of cultural eutrophication in freshwater wetlands ......................................1-5

Table 2 Comparison of landscape and wetland classification schemes ...................................................................3-12

Table 3 Features of the major hydrogeomorphic classes of wetlands that may influence background nutrient 

 concentrations, sensitivity to nutrient loading, nutrient storage forms and assimilative capacity, 

 designated use and choice of endpoints ...................................................................................................3-13

Table 4 Comparison of Stratified Probabilistic, Targeted, and BACI Sampling Designs ....................................4-10
Nutrient Criteria–Wetlands v

List of Internet Links/References/glossary/appenDIX

Internet Links

Executive Summary http://www.epa.gov/waterscience/criteria/nutrient/guidance/index.html

 http://www.epa.gov/owow/wetlands/

 http://www.epa.gov/owow/wetlands/watersheds/cwetlands.html

Chapter 1 http://www.epa.gov/waterscience/criteria/nutrient/guidance/index.html

 http://www.epa.gov/owow/wetlands/initiative

 http://www.epa.gov/waterscience/criteria/nutrient/strategy

 

Chapter 2 http://www.arl.noaa.gov/research/programs/airmon.html

 http://nadp.sws.uiuc.edu/

Chapter 3 http://www.epa.gov/emap/remap/index.html

 http://www.epa.gov/bioindicators/

 http://www.epa.gov/waterscience/standards/nutrient.html

 http://el.erdc.usace.army.mil/wetlands/pdfs/wrpde9.pdf

 http://water.usgs.gov/GIS/metadata/usgswrd/XML/ecoregion.xml

 http://www.nwi.fws.gov

 http://www.wes.army.mil/el/wetlands

http://www.epa.gov/waterscience/criteria/wetlands/7Classification.pdf

 

Chapter 4 http://www.epa.gov/owow/wetlands/bawwg/case/or.html

 http://www.epa.gov/waterscience/criteria/wetlands/

 http://www.epa.gov/waterscience/criteria/wetlands/17LandUse.pdf

 http://www.epa.gov/owow/wetlands/bawwg/case/mtdev.html

http://www.epa.gov/owow/wetlands/bawwg/case/fl1.html

http://www.epa.gov/owow/wetlands/bawwg/case/fl2.html

 http://www.epa.gov/owow/wetlands/bawwg/case/oh1.html

 http://www.epa.gov/owow/wetlands/bawwg/case/mn1.html

 http://www.epa.gov/waterscience/criteria/wetlands/

Chapter 5 http://www.epa.gov/waterscience/criteria/wetlands/

 http://water.usgs.gov/nawqa/sparrow/

 http://www.epa.gov/waterscience/criteria/wetlands/16Indicators

 http://www.epa.gov/waterscience/criteria/wetlands/10Vegetation.pdf

 http://www.epa.gov/waterscience/criteria/wetlands/11Algae.pdf

 http://www.epa.gov/waterscience/criteria/wetlands/9Invertebrate.pdf

 
vi Nutrient Criteria–Wetlands

Chapter 6 http://www.epa.gov/emap/html/data/index.html

 http://www.epa.gov/emap

 http://www.usgs.gov

 http://water.usgs.gov/nrp/webb/about.html

 http://hydrolab.arsusda.gov/arssci.html

 http://www.nps.ars.usda.gov/programs/nrsas.htm

 http://www.fs.fed.us/research/

 http://www.usace.army.mil/public.html

 http://www.usbr.gov

 http://nhd.usgs.gov

Chapter 7 http://el.erdc.usace.army.mil/wrap/wrap.html

 http://www.socialresearchmethods.net/

 http://www.yorku.ca/isr/scs/ 

 http://www.surveysystem.com/sscalc.htm

 http://www.stat.ohio-state.edu/~jch/ssinput.html

 http://www.stat.uiowa.edu

 http://www.epa.gov/waterscience/criteria/wetlands

Chapter 8 http://www.epa.gov/waterscience/biocriteria/modules/wet101-05-alus-monitoring.pdf

 http://www.wbi.lsu.edu/wbi/web-content/index.html

 http://data.lca.gov/Ivan6/app/app_c_ch9.pdf

 http://www.epa.gov/owow

References http://www.dep.state.fl.us/labs/library/index.htm 

 http://www.epa.gov/owow/wetlands/pdf/RapanosGuidance6507.pdf

 http://www.epa.gov/waterscience/criteria/nutrient/policy20070525.pdf

 http://www.wwwalker.net/dmsta

Glossary http://www.epa.gov/waterscience/criteria/nutrient/ecoregions/index.html

 http://www.epa.gov/owow/wetlands/pdf/RapanosGuidance6507.pdf

 http://www.epa.gov/owow/wetlands/

Appendix B http://www.dep.state.fl.us/water/wqssp/everglades/docs/DOTechSupportDOC2004.pdf

http://www.epa.gov/owow/wetlands/bawwg/case/fl2.html

http://www.dep.state.fl.us/water/wqssp/everglades/pctsd.htm

 http://www.www.sfwmd.gov
Nutrient Criteria–Wetlands vii

CONTRIBUTORS

Nancy Andrews (U.S. Environmental Protection Agency)

Mark Clark (University of Florida)

Christopher Craft (University of Indiana)

William Crumpton (Iowa State University)

Ifeyinwa Davis (U.S. Environmental Protection Agency)

Naomi Detenbeck (U.S. Environmental Protection Agency)

Paul McCormick (U.S. Geological Survey)

Amanda Parker (U.S. Environmental Protection Agency)

Kristine Pintado (Louisiana Department of Environmental Quality)

Todd Rasmussen (University of Georgia)

Ramesh Reddy (University of Florida)

R. Jan Stevenson (Michigan State University)

Arnold van der Valk (Iowa State University)

ACKNOWLEDGMENTS

The authors wish to acknowledge the efforts and input of several individuals. These include members of our 

EPA National Nutrient Team: Jim Carleton, Lisa Larimer, Steve Potts, and Robert Cantilli; members of the 

EPA Wetlands Division: Kathy Hurld, Chris Faulkner and Donna Downing; members of the Office of Research 

and Development and their Office of Science Policy: Valerie Chan, Joseph Schubauer-Berigan, Charles Lane, 

John Morrice, Christine Weilhoefer, Tony Olsen, and Walt Nelson; the Office of General Counsel: Leslie 

Darman. We also want to thank Kristine Pintado (DEQ, Louisiana) for her contributions and her careful 

review and comments.

This document was peer reviewed by a panel of expert scientists. The peer review charge focused on 

evaluating the scientific validity of the processes and techniques for developing nutrient criteria described in 

the guidance. The peer review panel comprised Dr. Robert H. Kadlec, Dr. Lawrence Richards Pomeroy, Dr. 

Eliska Rejmankova and Dr. Li Zhang. Edits and suggestions made by the peer review panel were incorporated 

into the final version of the guidance.
viii Nutrient Criteria–Wetlands
Nutrient Criteria–Wetlands ix

Executive Summary

The purpose of this document is to provide scientifically defensible guidance to assist States, Authorized 

Tribes, Territories, and other authorized jurisdictions—hereafter referred to as States—in assessing the 

nutrient status of their wetlands, and to provide technical assistance for developing numeric nutrient criteria 

for wetland systems in an eco-region. The development of nutrient criteria is part of an initiative by the U.S. 

Environmental Protection Agency (USEPA) to address the problem of cultural eutrophication, i.e., nutrient 

pollution caused by human activities (USEPA 1998a). Cultural eutrophication is not new; however, traditional 

efforts at nutrient control have been only moderately successful. Specifically, efforts to control nutrients 

in water bodies that have multiple nutrient sources (point and nonpoint sources) have been less effective 

in providing satisfactory, timely remedies for enrichment-related problems. Development and adoption 

of numeric criteria into water quality standards aids nitrogen and phosphorus pollution control efforts by 

providing clear numeric goals for water quality protection. Furthermore, numeric nutrient criteria provide 

specific water quality goals that will assist researchers in designing improved best management practices. 

In 1998, the USEPA published a report entitled, National Strategy for the Development of Regional Nutrient 

Criteria (USEPA 1998a). This report outlines a framework for development of waterbody-specific technical 

guidance that can be used to assess nutrient status and develop region-specific numeric nutrient criteria. The 

document presented here is the wetland-specific technical guidance for developing numeric nutrient criteria. 

The Nutrient Criteria Technical Guidance Manuals for Rivers and Streams (USEPA, 2000b), Lakes and 

Reservoirs (USEPA, 2000a) and Estuarine and Coastal Marine Waters (USEPA, 2001) have been completed 

and are available at: http://www.epa.gov/waterscience/criteria/nutrient/guidance/index.html.

Section 303(c) of the Clean Water Act directs States to adopt water quality standards for waters that are 

“waters of the United States,” including wetlands that are waters of the United States1. A water quality 

standard consists of three main elements: (1) one or more designated uses of each of the State’s waters, such as 

recreation or propagation of fish; (2) criteria expressed as pollutant concentration levels or narrative statements 

representing a quality of water that supports a designated use; and, (3) an anti-degradation policy to protect 

existing uses and high quality waters.

The information used in developing the technical approaches in this document came from references about 

studies of wetlands in a wide range of conditions, but not wetlands with a high degree of modification (e.g., 

wetlands that are considered “prior converted cropland” or artificial wetlands specifically engineered to protect 

or improve downstream water quality). This guidance is to assist States in developing numeric nutrient criteria 

for wetlands, should they choose to do so. States may choose to develop different types of criteria for wetlands 

protection, including site-specific or narrative criteria for wetlands protection, as long as they are scientifically 

defensible and protective of the designated uses (40 CFR § 131.11). This Guidance Manual includes chapters 

dealing with the following topics:

1

For further information regarding the scope of ‘waters of the U.S.’ in light of the U.S. Supreme Court’s 2006 decision in Rapanos v. 

United States, see “Clean Water Act Jurisdiction Following the U.S. Supreme Court’s Decision in Rapanos v. United States & Carabell v. 

United States,” which was jointly issued by the U.S. Environmental Protection Agency and the Army Corps of Engineers and is available 

at: http://www.epa.gov/owow/wetlands/.
x Nutrient Criteria–Wetlands

Classification of Wetlands

Classification strategies for nutrient criteria development include:

• physiographic regions

• hydrogeomorphic class

• water depth and duration

• vegetation type or zone

Choosing a specific classification scheme will likely depend on practical considerations, such as: whether a 

classification scheme is available in mapped digital form or can be readily derived from existing map layers; 

whether a hydrogeomorphic or other classification scheme has been refined for a particular region and wetland 

type; and, whether classification schemes are already in use for monitoring and assessment of other waterbody 

types in a State or region.

Sampling Design

Three sampling designs for new wetland monitoring programs are described:

• probabilistic sampling

• targeted/tiered approach

• BACI (Before/After, Control/Impact)

These approaches are designed to obtain a significant amount of information for statistical analyses 

with relatively minimal effort. Sampling efforts should be designed to collect information that will 

answer management questions in a way that permits robust statistical analysis. In addition, site selection, 

characterization of reference sites or systems, and identification of appropriate index periods are all of 

particular concern when selecting an appropriate sampling design. Careful selection of sampling design will 

allow the best use of financial resources and result in the collection of high quality data for evaluation of the 

wetland resources of a State. 

Candidate Variables for Establishing Nutrient Criteria

Candidate variables to use in determining nutrient condition of wetlands and to help identify appropriate 

nutrient criteria for wetlands consist of supporting variables, causal variables, and response variables. 

Supporting variables provide information useful in normalizing causal and response variables and categorizing 

wetlands. Causal variables are intended to characterize nutrient availability (or assimilation) in wetlands 

and could include nutrient loading rates and soil nutrient concentrations. Response variables are intended to 

characterize biotic response and could include community structure and composition of macrophytes and 

algae. Recommended variables for wetland nutrient criteria development described in this chapter are: 

1. Causal variables – nutrient loading rates, land use, extractable and total soil nitrogen (N) and 

 phosphorus (P), water column N and P; 

2. Response variables – nutrient content of wetland vegetation (algal and/or higher plants), above 

 ground biomass and stem height, macrophyte, algal, and macroinvertebrate community structure 

 and composition; and,

3. Supporting variables – hydrologic condition balance, conductivity, soil pH, soil bulk density, soil 

 organic matter content. 
Nutrient Criteria–Wetlands xi

Database Development and New Data Collection

A database of relevant water quality information can be an invaluable tool to States as they develop nutrient 

criteria. In some cases, existing data are available and can provide additional information that is specific to 

the region where criteria are to be set. However, little or no data are available for most regions or parameters, 

and creating a database of newly gathered data is strongly recommended. In the case of existing data, the data 

should be geolocated, and their suitability (type and quality and sufficient associated meta data) ascertained.

Data Analysis

Data analysis is critical to nutrient criteria development. Proper analysis and interpretation of data determine 

the scientific defensibility and effectiveness of the criteria. Therefore, it is important to evaluate short and 

long-term goals for wetlands of a given class within the region of concern. The purpose of this chapter is to 

explore methods for analyzing data that can be used to develop nutrient criteria consistent with these goals. 

Techniques discussed in this chapter include: 

• Distribution-based approaches that examine distributions of primary and supporting 

 variables (i.e., the percentile approach); 

• Response-based approaches that develop relationships between measurements of nutrient 

 exposure and ecological responses (i.e., tiered aquatic life uses); 

• Partitioning effects of multiple stressors; 

• Statistical techniques; 

• Multi-metric indices; and,

• Linking nutrient availability to primary producer response.

Criteria Development

Several methods can be used to develop numeric nutrient criteria for wetlands. They include, but are not 

limited to, criteria development methods that are detailed in this document:

• Comparing conditions in known reference systems for each established wetland type and class 

 based on best professional judgment or identifying reference conditions using frequency 

 distributions of empirical data and identifying criteria using percentile selections of data plotted 

 as frequency distributions;

• Refining classification systems using models, and/or examining system biological attributes in

 comparison to known reference conditions to assess the relationships among nutrients, vegetation 

 or algae, soil, and other variables and identifying criteria based on thresholds where those 

 response relationships change; and, 

• Using or modifying published nutrient and vegetation, algal, and soil relationships and values to 

 identify appropriate criteria. 

A weight of evidence approach with multiple attributes that combine one or more of the development 

approaches will generally produce criteria of greater scientific validity. 

The purpose of this document is to provide guidance on developing numeric nutrient criteria in a scientifically 

valid manner, and is not intended to address the multiple, complex issues surrounding implementation of 

water quality criteria and standards. Implementation will be addressed in a different process and additional 

implementation assistance will also be provided through other technical assistance projects provided by 

EPA. For issues specific to constructed wetlands, States should refer to http://www.epa.gov/owow/wetlands/

watersheds/cwetlands.html. 
xii Nutrient Criteria–Wetlands
Nutrient Criteria–Wetlands 1-1

Chapter 1: Introduction

1.1 Introduction/Purpose

The purpose of this document is to provide technical guidance to assist States in assessing the nutrient status 

of their wetlands by considering water, vegetation, and soil conditions, and to provide technical assistance for 

developing regionally-based, scientifically defensible, numeric nutrient criteria for wetlands. In this document, 

the term “wetlands” or “wetland systems” refers to wetlands that are considered as “waters of the United 

States.” However, States may, at their discretion, use this document to develop water quality criteria and 

standards for wetlands that are considered waters of the State.

EPA’s development of recommended nutrient criteria is part of an initiative by the U.S. Environmental 

Protection Agency (USEPA) to address the problem of cultural eutrophication. In 1998, the EPA published 

a report entitled, National Strategy for the Development of Regional Nutrient Criteria (USEPA 1998a). The 

report outlines a framework for EPA’s development of waterbody-specific technical guidance that can be used 

to assess nutrient status and develop region-specific numeric nutrient criteria. This document is the technical 

guidance for developing numeric nutrient criteria for wetlands. Approaches to nutrient criteria development are 

similar for freshwater and tidal wetlands, however, this document has a freshwater emphasis. EPA recognizes 

that wetlands are different from the other types of waters of the U.S. in that they frequently do not have 

standing or flowing water, and the soils and vegetation components are more dominant in these systems than 

in the other waterbody types (lakes, streams, estuaries). Additional, more specific information on sampling 

wetlands is available at: http://www.epa.gov/waterscience/criteria/nutrient/guidance/index.html.

BACKGROUND

Cultural eutrophication (human-caused inputs of excess nutrients in waterbodies) is one of the primary causal 

factors that impair surface waters in the U.S. (USEPA 1998a). Both point and nonpoint sources of nutrients 

contribute to impairment of water quality. Point source discharges of nutrients are relatively constant and are 

controlled by the National Pollutant Discharge Elimination System (NPDES) permitting program. Nonpoint 

source pollutant inputs have increased in recent decades, resulting in degraded water quality in many 

aquatic systems. Nonpoint sources of nutrients are most commonly intermittent and are usually linked to 

runoff, atmospheric deposition, seasonal agricultural activity, and other irregularly occurring events such as 

silvicultural activities. Control of nonpoint source pollutants typically focuses on land management activities 

and regulation of pollutants released to the atmosphere (Kronvang et al. 2005; Howarth et al. 2002; Carpenter 

et al. 1998).

The term eutrophication was coined in reference to lake systems. The use of the term for wetlands can be 

problematic due to the confounding nature of hydrodynamics, light, and the differences in the responses of 

algae and vegetation. Eutrophication in this document refers to human-caused inputs of excess nutrients and 

is not intended to indicate the same scale or responses to eutrophication found in lake systems and codified in 

the trophic state index for lakes (Carlson 1977). This manual is intended to provide guidance for identifying 

deviance from natural conditions with respect to cultural eutrophication in wetland systems. Hydrologic 

alteration and pollutants other than excess nutrients may amplify or reduce the effects of nutrient pollution, 

making specific responses to nutrient pollution difficult to quantify. EPA recognizes these issues, and presents 

recommendations for analyzing wetland systems with respect to nutrient condition for development of nutrient 

criteria in spite of these confounding factors. 

Cultural eutrophication is not new; however, traditional efforts at nutrient control have been only moderately 

successful. Specifically, efforts to control nutrients in waterbodies that have multiple nutrient sources (point 

and nonpoint sources) have been less effective in providing satisfactory, timely remedies for enrichment-
1-2 Nutrient Criteria–Wetlands

related problems (Azzellino et al. 2006; Merseburger et al. 2005; Carpenter et al. 1998). Development and 

adoption of numeric nutrient criteria into water quality standards aid State nutrient pollution control efforts 

by providing clear numeric goals for nutrient concentrations. Furthermore, numeric nutrient criteria provide 

specific water quality goals that will assist researchers in designing improved best management practices. 

1.2 WATER QUALITY STANDARDS AND CRITERIA

States are responsible for setting water quality standards to protect the physical, biological, and chemical 

integrity of their waters. “Water quality standards (WQS) are provisions of State or Federal law which consist 

of a designated use or uses and water quality criteria for such waters to protect such uses.2 Water quality 

standards are to protect public health or welfare, enhance the quality of the water, and serve the purposes of 

the Act (40 CFR 131.2 and 131.3(i))” (USEPA 1994). A water quality standard defines the goals for a wetland 

by: designating its specific uses, setting criteria to protect those uses, and, establishing an antidegradation 

policy to protect existing water quality. 

Designated uses are a State’s concise statements of its goals and expectations for each of the individual surface 

waters under its jurisdiction. With designated uses, States can work with their publics to identify a collective 

goal for their waters that they intend to strive for as they manage water quality. EPA encourages States to 

evaluate the attainability of these goals and expectations to ensure they have designated the appropriate uses. 

Generally, the effectiveness of designated uses in guiding water quality management programs is greater if 

they:

• Identify specific expectations based on as much data as possible to reduce ambiguity;

• Recognize and accommodate inherent natural differences among surface water types; and 

• Acknowledge certain human caused conditions that limit the potential to support uses.

Designated uses may involve a spectrum of expectations depending on the type of wetland and associated 

hydropatterns, where the wetland is situated with respect to natural landscape features and human activity, 

and the historical and anticipated future functions that the wetland provides. Criteria to protect specific uses, 

in turn, should reflect these differing expectations where appropriate. The information used in developing the 

technical approaches in this document was drawn from references about studies of wetlands in a wide range 

of conditions, but not wetlands with a high degree of modification (e.g., wetlands that are considered “prior 

converted cropland” or artificial wetlands specifically engineered to protect or improve downstream water 

quality).

Water quality criteria may be expressed as numeric values or narrative statements. As of this writing, most 

of the Nation’s waterbodies do not have numeric nutrient criteria, but instead rely on narrative criteria that 

describe the desired condition. Narrative criteria are descriptions of conditions necessary for a water to attain 

the designated uses. An example of a narrative criterion for nutrients is shown below: 

2

EPA published guidance on water quality standards for wetlands in 1990 (USEPA 1990c). Examples of different state approaches for 

standards can be found at: http://www.epa.gov/owow/wetlands/initiatives/.

In no case shall nutrient concentrations of a body of water be altered so as to cause an imbalance in natural 

populations of aquatic flora or fauna. 
Nutrient Criteria–Wetlands 1-3

Numeric criteria, on the other hand, are values assigned to measurable components of water quality to protect 

designated uses, such as the concentration of a specific constituent that is present in the water column. An 

example of a numeric criterion for specific waters is shown below:

In addition to narrative and numeric criteria, some States use numeric translator mechanisms—mechanisms 

that translate narrative (qualitative) standards into numeric (quantitative) values for use in evaluating water 

quality data. Translator mechanisms may be useful internally by the State agency for water assessment and 

management and serve as an intermediate step between numeric and narrative criteria. 

Numeric criteria provide distinct interpretations of acceptable and unacceptable conditions, form the 

foundation for measurement of environmental quality, and reduce ambiguity for management and enforcement 

decisions. The lack of numeric nutrient criteria in State water quality standards for most of the Nation’s 

waterbodies makes it difficult to assess the condition of waters of the U.S. with respect to nutrients, and 

thus hampers the water quality manager’s ability to protect designated uses and improve water quality. EPA 

encourages States to adopt numeric nutrient criteria into their water quality standards (USEPA 2007b).

Many States have adopted some form of nutrient criteria for surface waters related to maintaining natural 

conditions and avoiding nutrient enrichment. Most States with nutrient criteria in their water quality standards 

have broad narrative criteria for most waterbodies and may also have site-specific numeric criteria for certain 

waters of the State. Established criteria most commonly pertain to phosphorus (P) concentrations in lakes. 

Nitrogen (N) criteria, where they have been established, are usually protective of human health effects or relate 

to toxic effects of ammonia and nitrates. In general, levels of nitrate (10 ppm [mg/L] for drinking water) and 

ammonia high enough to be problematic for human health or toxic to aquatic life (1.24 mg N/L at pH = 8 and 

25°C) will also cause problems of enhanced algal growth (USEPA 1986). 

Numeric nutrient criteria can provide a variety of benefits and may be used in conjunction with State 

and Federal biological assessments, Nonpoint Source Programs, Watershed Implementation Plans, and 

in development of Total Maximum Daily Loads (TMDLs) to improve resource management and support 

watershed protection activities at local, State, and national levels. Information obtained from compiling 

existing data and conducting new surveys can provide water quality managers and the public a better 

perspective on the condition of State waters. The compiled information can be used to most effectively budget 

personnel and financial resources for the protection and restoration of State waters. In a similar manner, 

data collected in the criteria development and implementation process can be compared before, during, and 

after specific management actions. Analyses of these data can determine the response of the wetland and the 

effectiveness of management endeavors. 

1.3 NUTRIENT ENRICHMENT PROBLEMS

Water quality can be affected when watersheds are modified by alterations in vegetation, sediment transport, 

fertilizer use, industrialization, urbanization, or conversion of native forests and grasslands to agriculture and 

silviculture (Turner and Rabalais 1991; Vitousek et al. 1997; Carpenter et al. 1998). Cultural eutrophication, 

one of the primary factors causing impairment of U.S. surface waters (USEPA 1998a), results from point and 

nonpoint sources of nutrient pollution. Nonpoint source pollutant inputs have increased in recent decades 

(4) Phosphorus Criterion. 

(a) The numeric phosphorus criterion for Class III waters shall be a long-term geometric mean of 10 

ppb, but shall not be lower than the natural conditions of the Class II waters, and shall take into account 

spatial and temporal variability. Achievement of the criterion shall be determined by the methods in this 

subsection. Exceedences of the provisions of the subsection shall not be considered deviations from the 

criterion if they are attributable to the full range of natural spatial and temporal variability, statistical 

variability inherent in sampling and testing procedures, or higher natural background conditions.
1-4 Nutrient Criteria–Wetlands

and have degraded water quality in many aquatic systems (Carpenter et al. 1998). Control of nonpoint source 

pollutants focuses on land management activities and regulation of pollutants released to the atmosphere 

(Carpenter et al. 1998). 

Nutrient enrichment frequently ranks as one of the top causes of water resource impairment. EPA reported to 

Congress that of the waterbodies surveyed and reported impaired 20 percent of rivers and 50 percent of lakes 

were listed with nutrients as the primary cause of impairment (USEPA 2000c). Few States currently include 

wetland monitoring in their routine water quality monitoring programs (only eleven States reported attainment 

of designated uses for wetlands in the National Water Quality Inventory 1998 Report to Congress (USEPA 

1998b) and only three States used monitoring data as a basis for determining attainment of water quality 

standards for wetlands); thus, the extent of nutrient enrichment and impairment of wetland systems is largely 

undocumented. Increased wetland monitoring by States will help define the extent of nutrient enrichment 

problems in wetland systems. 

The best documented case of cultural eutrophication in wetlands is the Everglades ecosystem. The Everglades 

ecosystem is a wetland mosaic that is composed primarily of oligotrophic freshwater marsh. Historically, 

the greater Everglades ecosystem included vast acreage of freshwater marsh, small stands of custard apple 

and some cattail south of Lake Okeechobee, and Big Cypress Swamp, which eventually drains into Florida 

Bay. Lake Okeechobee was diked to reduce flooding. The area directly south of Lake Okeechobee was then 

converted into agricultural lands for cattle grazing and row crop production. The cultivation and use of 

commercial fertilizers in the area now known as the “Everglades Agricultural Area” have resulted in release 

of nutrient-rich waters into the Everglades for more than thirty years. The effects of the nutrient-rich water, 

combined with coastal development and channeling to supply water to communities on the southern Florida 

coast, have significantly increased soil and water column phosphorus levels in naturally oligotrophic areas. In 

particular, nutrient enrichment of the freshwater marsh has resulted in an imbalance in the native vegetation. 

Cattail is now encroaching in areas that were historically primarily sawgrass; calcareous algal mats are 

being replaced by non-calcareous algae, changing the balance of native flora that is needed to support vast 

quantities of wildlife. Nutrient enriched water is also reaching Florida Bay, suffocating the native turtle grass 

as periphyton covers the blades (Davis and Ogden 1994; Everglades Interim Report 1999 2003; Everglades 

Consolidated Report 2003). Current efforts to restore the Everglades are focusing on nutrient reduction and 

better hydrologic management (Everglades Consolidated Report 2003). 

Monitoring to establish trends in nutrient levels and associated changes in biology has been infrequent for 

most wetland types as compared to studies in the Everglades or examination of other surface waters such as 

lakes. Noe et al., (2001) have argued that phosphorus biogeochemistry and the extreme oligotrophy observed 

in the Everglades in the absence of anthropogenic inputs represents a unique case. Effects of cultural 

eutrophication, however, have been documented in a range of different wetland types. Existing studies are 

available to document potential impacts of anthropogenic nutrient additions to a wide variety of wetland types, 

including bogs, fens, Great Lakes coastal emergent marshes, and cypress swamps. The evidence of nutrient 

effects in wetlands ranges from controlled experimental manipulations, to trend or empirical gradient analysis, 

to anecdotal observations. Consequences of cultural eutrophication have been observed at both community 

and ecosystem-level scales (Table 1). Changes in wetland vegetation composition resulting from cultural 

eutrophication of these systems have been demonstrated in bogs (Kadlec and Bevis 1990), fens (Guesewell et 

al. 1998; Bollens and Ramseier 2001, Pauli et al. 2002), meadows (Finlayson et al. 1986), marshes (Bedford et 

al. 1999) and cypress domes (Ewel 1976). Specific effects on higher trophic levels in marshes seem to depend 

on trophic structure (e.g., presence/absence of minnows, benthivores, and/or piscivores, Jude and Pappas 1992; 

Angeler et al. 2003) and timing/frequency of nutrient additions (pulse vs. press; Gabor et al. 1994; Murkin et 

al. 1994; Hann and Goldsborough 1997; Sandilands et al. 2000; Hann et al. 2001; Zrum and Hann 2002). 

The cycling of nitrogen (N) and phosphorus (P) in aquatic systems should be considered when managing 

nutrient enrichment. The hydroperiod of wetland systems significantly affects nutrient transformations, 
Nutrient Criteria–Wetlands 1-5

availability, transport, and loss of gaseous forms to the atmosphere (Mitsch and Gosselink 2000). Nutrients 

can be re-introduced into a wetland from the sediment, or by microbial transformation, potentially resulting in 

a long recovery period even after pollutant sources have been reduced. In open wetland systems, nutrients may 

also be rapidly transported downstream, uncoupling the effects of nutrient inputs from the nutrient source, 

and further complicating nutrient source control (Mitsch and Gosselink 2000; Wetzel 2001). Recognizing 

relationships between nutrient input and wetland response is the first step in mitigating the effects of cultural 

eutrophication. When relationships are established, nutrient criteria can be developed to manage nutrient 

pollution and protect wetlands from eutrophication.

3

Similar impacts in tidal and estuarine wetlands have been documented, but are not included in this table.

Observed Impact References

Loss of submerged aquatic plants that have high light 

compensation points

Phillips et al. 1978

Stephenson et al. 1980

Galatowitsch and van der Valk 1996

Shifts in vascular plant species composition due to shifts in 

competitive advantage

Wentz 1976

Verhoeven et al. 1988

Ehrenfeld and Schneider 1993

Gaudet and Keddy 1995

Koerselman and Verhoeven 1995

Increases in above-ground production Barko 1983

Bayley et al. 1985

Barko and Smart 1986

Vermeer 1986

Decreases in local or regional biodiversity Mudroch and Capobianco 1979

Guntenspergen et al. 1980

Lougheed et al. 2001

Balla and Davis 1995

VanGroenendael et al. 1993

Bedford et al. 1999

Increased competitive advantage of aggressive/invasive species 

(e.g., Typha glauca, T. latifolia and Phalaris arundinacea)

Woo and Zedler 2002

Svengsouk and Mitsch 2001

Green and Galatowitsch 2002

Maurer and Zedler 2002

Loss of nutrient retention capacity (e.g., carbon and nitrogen 

storage, changes in plant litter decomposition)

Nichols 1983

Davis and van der Valk 1983

Rybczyk et al. 1996

Major structural shifts between “clear water” macrophyte 

dominated systems to turbid phytoplankton dominated systems 

or metaphyton-dominated systems with reduced macrophyte 

coverage

McDougal et al. 1997

Angeler et al. 2003

Shifts in macroinvertebrate composition along a cultural 

eutrophication gradient

Chessman et al. 2002

Table 1: Observed consequences of cultural eutrophication in freshwater wetlands.3
1-6 Nutrient Criteria–Wetlands

1.4 OVERVIEW OF THE CRITERIA DEVELOPMENT PROCESS

The National Strategy for the Development of Regional Nutrient Criteria (USEPA 1998a) describes the 

principal elements of numeric nutrient criteria development. This document can be downloaded in PDF format 

at the Web site: http://www.epa.gov/waterscience/criteria/nutrient/strategy. The Strategy recognizes that a 

prescriptive, one-size-fits-all approach is not appropriate due to regional differences that exist and the scientific 

community’s current technical understanding of the relationship between nutrients, algal and macrophyte 

growth, and other factors (e.g., flow, light, substrata). The approach chosen for criteria development therefore 

may be tailored to meet the specific needs of each State. The EPA Strategy envisions a process by which State 

waters are initially monitored, reference conditions are established, individual waterbodies are compared to 

known reference waterbodies, and appropriate management measures are implemented. These measurements 

can be used to document change and monitor the progress of nutrient reduction activities and protection of 

water quality.

The National Nutrient Program represents an effort and approach to criteria development that, in conjunction 

with efforts made by State water quality managers, will ultimately result in a heightened understanding of 

nutrient-response relationships. As the proposed process is put into use to set criteria, program success will 

be gauged over time through evaluation of management and monitoring efforts. A more comprehensive 

knowledge-base pertaining to nutrient, and vegetation and/or algal relationships will be expanded as new 

information is gained and obstacles overcome, justifying potential refinements to the criteria development 

process. 

The overarching goal of developing and adopting nutrient criteria is to protect and maintain the quality of 

our national waters. Protecting and maintaining water quality may include restoration of impaired systems, 

conservation of high quality waters, and protection of systems at high risk for future impairment. The specific 

goals of a State water quality program may be defined differently based on the needs of each State, but should, 

at a minimum, be established to protect the designated uses for the waterbodies within State lands. In addition, 

as numeric nutrient criteria are developed for the nation’s waters, States should revisit their goals for water 

quality and revise their water quality standards as needed. 

1.5 ROADMAP TO THE DOCUMENT

As set out in Figure 1.1, the process of developing numeric nutrient criteria begins with defining the goals of 

criteria development and water quality standards adoption. Those goals are pertinent to the classification of 

systems, the development of a monitoring program, and the application of numeric nutrient criteria to permit 

limits and water quality protection. These goals therefore should be determined with the intent of revising and 

adapting them as new information is obtained and the paths to achieving those goals are clarified. Defining 

the goals for criteria development is the first step in the process. The summaries below describe each chapter 

in this document. The document is written to provide recommendations for a stepwise procedure for criteria 

development. Some chapters contain information that is not needed by some readers; the descriptions below 

should serve as a guide to the most relevant information for each reader. 

Chapter Two describes many of the functions of wetland systems and their role in the landscape with respect 

to nutrients. This chapter is intended to familiarize the reader with some basic scientific information about 

wetlands that will provide a better understanding of how nutrients move within a wetland and the importance 

of wetland systems in the landscape. 

Chapter Three discusses wetland classification and presents the reader with options for classifying wetlands 

based on system characteristics. This chapter introduces the scientific rationale for classifying wetlands, 

reviews some common classification schemes, and discusses their role in establishing nutrient criteria for
Nutrient Criteria–Wetlands 1-7

 wetlands. The classification of these systems is important to identifying their nutrient status and their 

condition in relation to similar wetlands. 

Chapter Four provides technical guidance on designing effective sampling programs for State wetland water 

quality monitoring programs. Most States should begin wetland monitoring programs to collect water quality 

and biological data in order to develop nutrient criteria protective of wetland systems. The best monitoring 

programs are designed to assess wetland condition with statistical rigor and maximize effective use of available 

resources. The sampling protocol selected, therefore, should be determined based on the goals of the monitoring 

program, and the resources available. 

Chapter Five gives an overview of candidate variables that could be used to establish nutrient criteria for 

wetlands. Primary variables are expected to be most broadly useful in characterizing wetland conditions with 

respect to nutrients, and include nutrient loading rates, soil nutrient concentrations, and nutrient content of 

wetland vegetation. Supporting variables provide information useful for normalizing causal and response 

variables. The candidate variables suggested here are not the only parameters that can be used to determine 

wetland nutrient condition, but rather identify those variables that are thought to be most likely to identify the 

current nutrient condition and of the greatest utility in determining a change in nutrient status. 

A database of relevant water quality information can be an invaluable tool to States as they develop nutrient 

criteria. If little or no data are available for most regions or parameters, it may be necessary for States to create 

a database of newly gathered data. Chapter Six provides the basic information on how to develop a database of 

nutrient information for wetlands, and supplies links to ongoing database development efforts at the State and 

national levels. 

The purpose of Chapter Seven is to explore methods for analyzing data that can be used to develop nutrient 

criteria. The quality of the analysis and interpretation of data generally determines whether the criteria will 

be scientifically defensible and effective. This chapter describes recommended approaches to data analysis for 

developing numeric nutrient criteria for wetlands. Included are techniques to evaluate metrics, to examine or 

compare distributions of nutrient exposure or response variables, and to examine nutrient exposure-response 

relationships. 

Chapter Eight describes the details of establishing scientifically defensible criteria in wetlands. Several 

approaches are presented that water quality managers can use to derive numeric criteria for wetland systems 

in their State waters. They include: (1) the use of the reference conditions concept to characterize natural or 

minimally impaired wetland systems with respect to causal and response variables; (2) applying predictive 

relationships to select nutrient concentrations that will protect wetland function; and, (3) developing criteria 

from established nutrient exposure-response relationships (as in the peer-reviewed, published literature). This 

chapter provides recommendations regarding how to determine the appropriate numeric criterion based on the 

data collected and analyzed. 

The appendices include a glossary of terms and acronyms and case study examples of wetland nutrient 

enrichment and management.
1-8 Nutrient Criteria–Wetlands

MONITOR

IDENTIFY

GOALS

CLASSIFY

WETLANDS

SELECT

VARIABLES

DETERMINE

SAMPLING

DESIGN

BUILD

DATABASE

ANALYZE

DATA

DEVELOP

CRITERIA

Figure 1.: Flowchart identifying the steps of the recommended process to develop wetland nutrient 

criteria.
Nutrient Criteria–Wetlands 2-1

Chapter 2: Overview of Wetland Science

2.1 INTRODUCTION

Wetlands exist at the interface between terrestrial and aquatic environments. They serve as sources, sinks, 

and transformers of materials. (Figure 2.1) Wetlands serve as sites for transformation of nutrients such as 

nitrogen (N) and phosphorus (P). Dissolved inorganic forms of N and P are assimilated by microorganisms 

and vegetation and incorporated into organic compounds. Nitrate in surface- and ground-water is reduced 

to gaseous forms of N (NO, N2O, N2) by microorganisms, a process known as denitrification, and returned 

to the atmosphere. Phosphorus undergoes a variety of chemical reactions with iron (Fe), aluminum (Al), and 

calcium (Ca) that depend on the pH of the soil, availability of sorption sites, redox potential, and other factors. 

These biogeochemical reactions are important in evaluating the nutrient condition (oligotrophic, mesotrophic, 

eutrophic) of the wetland and its susceptibility to nutrient enrichment.

Wetlands also generally are sinks for sediment, and wetlands that are connected to adjacent aquatic ecosystems 

(e.g., rivers, estuaries) may trap more sediment as compared to wetlands that lack such connectivity (Fryirs 

et al. 2007; Mitsch and Gosselink 2000; Dunne and Leopold 1978). Wetlands also may be sources of organic 

carbon (C) (Bouchard 2007; Raymond and Bauer 2001) and nitrogen (N) (Mitsch and Gosselink 2000; 

Mulholland and Kuenzler 1979) to aquatic ecosystems. Production of plant biomass (leaves, wood, and roots) 

from riparian, alluvial, and floodplain forests and from fringe wetlands such as tidal marshes and mangroves 

provide organic matter to support heterotrophic foodwebs of streams, rivers, estuaries, and nearshore waters 

(Mitsch and Gosselink 2000; Day et al. 1989). 

Figure 2.1: Schematic of nutrient transfer among potential system sources and sinks. 
2-2 Nutrient Criteria–Wetlands

2.2 COMPONENTS OF WETLANDS

Wetlands are distinguished by three primary components: hydrology, soils, and vegetation. Wetland hydrology 

is the driving force that determines soil development, the assemblage of plants and animals that inhabit the 

site, and the type and intensity of biochemical processes. Wetland soils may be either organic or mineral, but 

share the characteristic that they are saturated or flooded at least some of the time during the growing season. 

Wetland vegetation consists of many species of algae, rooted plants that may be herbaceous and emergent, such 

as cattail (Typha sp.) and arrowhead (Saggitaria sp.), or submergent, such as pondweeds (Potamogeton sp.), 

or may be woody such as bald cypress (Taxodium distichum) and tupelo (Nyssa aquatica). Depending on the 

duration, depth, and frequency of inundation or saturation, wetland plants may be either obligate (i.e., species 

found almost exclusively in wetlands) or facultative (i.e., species found in wetlands but which also may be 

found in upland habitats). The discussion that follows provides an overview of wetland hydrology, soils, and 

vegetation, as well as aspects of biogeochemical cycling in these systems. 

Hydrology

Hydrology is characterized by water source, hydroperiod (depth, duration, and frequency of inundation or 

soil saturation), and hydrodynamics (direction and velocity of water movement). The hydrology of wetlands 

differs from that of terrestrial ecosystems in that wetlands are inundated or saturated long enough during 

the growing season to produce soils that are at least periodically deficient in oxygen. Wetlands differ from 

other aquatic ecosystems by their shallow depth of inundation that enables rooted vegetation to become 

established, in contrast to deep water aquatic ecosystems, where the depth and duration of inundation can be 

too great to support emergent vegetation. Anaerobic soils promote colonization by vegetation adapted to low 

concentrations of oxygen in the soil. 

Wetlands primarily receive water from three sources: precipitation, surface flow, and groundwater (Figure 2.2). 

The relative proportion of these hydrologic inputs influences the plant communities that develop, the types 

of soils that form, and the predominant biogeochemical processes. Wetlands that receive mostly precipitation 

tend to be “closed” systems with little exchange of materials with adjacent terrestrial or aquatic ecosystems. 

Examples of precipitation-driven wetlands include “ombrotrophic” bogs and depressional wetlands such as 

cypress domes and vernal pools. Wetlands that receive water mostly from surface flow tend to be “open” 

systems with large exchanges of water and materials between the wetland and adjacent non-wetland 

ecosystems. Examples include floodplain forests and fringe wetlands such as lakeshore marshes, tidal marshes, 

and mangroves. Wetlands that receive primarily groundwater inputs tend to have more stable hydroperiods 

than precipitation- and surface water-driven wetlands, and, depending on the underlying bedrock or parent 

material, high concentrations of dissolved inorganic constituents such as calcium (Ca) and magnesium (Mg). 

Fen wetlands and seeps are examples of groundwater-fed wetlands.

FENS,

SEEPS

BOGS,

POCOSINS

RIVERINE,

FRINGE,

TIDAL

GROUND WATER PRECIPITATION

SURFACE FLOW

Figure 2.2: Relationship between water source and wetland vegetation. Modified from Brinson (1993)
Nutrient Criteria–Wetlands 2-3

Hydroperiod is highly variable depending on the type of wetland. Some wetlands that receive most of their 

water from precipitation (e.g., vernal pools) have very short duration hydroperiods. Wetlands that receive 

most of their water from surface flooding (e.g., floodplain swamps) often are flooded longer and to a greater 

depth than precipitation-driven wetlands. Fringe wetlands such as tidal marshes and mangroves are frequently 

flooded (up to twice daily) by astronomical tides but the duration of inundation is relatively short. In 

groundwater-fed wetlands, hydroperiod is more stable and water levels are relatively constant as compared to 

precipitation- and surface water-driven wetlands, because groundwater provides a less variable input of water 

throughout the year.

Hydrodynamics is especially important in the exchange of materials between wetlands and adjacent 

terrestrial and aquatic ecosystems. In fact, the role of wetlands as sources, sinks, and transformers of material 

depends, in large part, on hydrodynamics. For example, many wetlands are characterized by lateral flow of 

surface- or ground-water. Flow of water can be unidirectional or bidirectional. An example of a wetland with 

unidirectional flow is a floodplain forest where surface water spills over the river bank, travels through the 

floodplain, and re-enters the river channel some distance downstream. In fringe wetlands such as lakeshore 

marshes, tidal marshes, and mangroves, flow is bidirectional as wind-driven or astronomical tides transport 

water into, then out of the wetland. These wetlands have the ability to intercept sediment and dissolved 

inorganic and organic materials from adjacent systems as water passes through them. In precipitation-driven 

wetlands, flow may occur more in the vertical direction as rainfall percolates through the wetland soils to 

underlying aquifers or nearby streams. Wetlands with lateral surface flow may be important in maintaining 

water quality of adjacent aquatic systems by trapping sediment and other pollutants. Surface flow wetlands 

also may be an important source of organic C to aquatic ecosystems as detritus, particulate C, and dissolved 

organic C are transported out of the wetland into rivers and streams down gradient or to adjacent lakes, 

estuaries, and nearshore waters.

Soils

Wetland soils, also known as hydric soils, are defined as “soils that formed under conditions of saturation, 

flooding, or ponding long enough during the growing season to develop anaerobic conditions in the upper part” 

(NRCS 1998). Anaerobic conditions result because the rate of oxygen diffusion through water is approximately 

10,000 times less than in air. Wetland soils may be composed mostly of mineral constituents (sand, silt, clay) or 

they may contain large amounts of organic matter. Because anaerobic conditions slow or inhibit decomposition 

of organic matter, wetland soils typically contain more organic matter than terrestrial soils of the same 

region or climatic conditions. Under conditions of near continuous inundation or saturation, organic soils 

(histosols) may develop. Histosols are characterized by high organic matter content, 20-30% (12-18% organic 

C depending on clay content) with a thickness of at least 40 cm (USDA 1999). Because of their high organic 

matter content, histosols possess physical and chemical properties that are very different from mineral wetland 

soils. For example, organic soils generally have lower bulk densities, higher porosity, greater water holding 

capacity, lower nutrient availability, and greater cation exchange capacity than many mineral soils. 

Mineral wetland soils, in addition to containing greater amounts of sand, silt, and clay than histosols, are 

distinguished by changes in soil color that occur when elements such as Fe and manganese (Mn) are reduced 

by microorganisms under anaerobic conditions. Reduction of Fe leads to the development of grey or “gleyed” 

soil color as oxidized forms of Fe (ferric Fe, Fe3+) are converted to reduced forms (ferrous Fe, Fe2+). In sandy 

soils, development of a dark-colored, organic-rich surface layer is used to distinguish hydric soil from nonhydric (terrestrial) soil. An organic-rich surface layer, indicative of periodic inundation or saturation, is not 

sufficiently thick (<40 cm) to qualify as a histosol, which forms under near-continuous inundation.

Wetland soils serve as sites for many biogeochemical transformations. They also provide long and short 

term storage of nutrients for wetland plants. Wetland soils are typically anaerobic within a few millimeters 

of the soil-water interface. Water column oxygen concentrations are often depressed due to the slow rate of 

oxygen diffusion through water. However, even when water column oxygen concentrations are supported by 
2-4 Nutrient Criteria–Wetlands

advective currents, high rates of oxygen consumption lead to the formation of a very thin oxidized layer at 

the soil-water interface. Similar oxidized layers can also be found surrounding roots of wetland plants. Many 

wetland plants are known to transport oxygen into the root zone, thus creating aerobic zones in predominantly 

anaerobic soil. The presence of these aerobic (oxidizing) zones within the reducing environment in saturated 

soils allows for the occurrence of oxidative and reductive transformations to occur in close proximity to each 

other. For example, ammonia is oxidized to nitrate within the aerobic zone surrounding plant roots in a process 

called nitrification. Nitrate then readily diffuses into adjacent anaerobic soil, where it is reduced to molecular 

nitrogen via denitrification or may be reduced to ammonium in certain conditions through dissimilatory 

nitrate reduction (Mitsch and Gosselink 2000; Ruckauf et al. 2004; Reddy and Delaune 2005). The anaerobic 

environment hosts the transformations of N, P, sulfur (S), Fe, Mn, and C. Most of these transformations are 

microbially mediated. The oxidized soil surface layer also is important to the transport and translocation 

of transformed constituents, providing a barrier to translocation of some reduced constituents. These 

transformations will be discussed in more detail below in Biogeochemical Cycling.

Vegetation

Wetland plants consist of macrophytes and microphytes. Macrophytes include free-floating, submersed, 

floating-leaved, and rooted emergent plants. Microphytes are algae that may be free floating or attached 

to macrophyte stems and other surfaces. Plants require oxygen to meet respiration demands for growth, 

metabolism, and reproduction. In macrophytes, much (about 50%) of the respiration occurs below ground in 

the roots. Wetland macrophytes, however, live in periodically to continuously-inundated and saturated soils 

and, therefore, use specialized adaptations to grow in anaerobic soils (Cronk and Fennessy 2001). Adaptations 

consist of morphological/anatomical adaptations that result in anoxia avoidance, and metabolic adaptations 

that result in true tolerance to anoxia. Morphological/anatomic adaptations include shallow roots systems, 

aerenchyma, buttressed trunks, pneumatophores (e.g., black mangrove (Avicennia germinans)), and lenticles 

on the stem. These adaptations facilitate oxygen transport from the shoots to the roots where most respiration 

occurs. Many wetland plants also possess metabolic adaptations, such as anaerobic pathways of respiration, 

that produce non-toxic metabolites such as malate to mitigate the adverse effects of oxygen deprivation, 

instead of toxic compounds like ethanol (Mendelssohn and Burdick 1988). 

Species best adapted to anaerobic conditions are typically found in areas inundated for long periods, whereas 

species less tolerant of anaerobic conditions are found in areas where hydroperiod is shorter. For example, 

in southern forested wetlands, areas such as abandoned river channels (oxbows) are dominated by obligate 

species such as bald cypress (Taxodium distichum) and tupelo gum (Nyssa aquatica) (Wharton et al. 1982). 

Areas inundated less frequently are dominated by hardwoods such as black gum (Nyssa sylvatica), green ash 

(Fraxinus pennsylvanicus), and red maple (Acer rubrum), and the highest, driest wetland areas are dominated 

by facultative species such as sweet gum (Liquidambar styraciflua) and sycamore (Platanus occidentalis) 

(Wharton et al. 1982). Herbaceous-dominated wetlands also exhibit patterns of zonation controlled by 

hydroperiod (Mitsch and Gosselink 2000). 

In estuarine wetlands such as salt- and brackish-water marshes and mangroves, salinity and sulfides also 

adversely affect growth and reproduction of vegetation (Webb and Mendelssohn 1996; Mitsch and Gosselink 

2000). Inundation with seawater brings dissolved salts (NaCl) and sulfate. Salt creates an osmotic imbalance in 

vegetation, leading to desiccation of plant tissues. However, many plant species that live in estuarine wetlands 

possess adaptations to deal with salinity (Winchester et al. 1985; Whipple et al. 1981; Zheng et al. 2004). These 

adaptations include salt exclusion at the root surface, salt secreting glands on leaves, schlerophyllous (thick, 

waxy) leaves, low transpiration rates, and other adaptations to reduce uptake of water and associated salt. 

Sulfate carried in by the tides undergoes sulfate reduction in anaerobic soils to produce hydrogen sulfide (H2

S) 

that, at high concentrations, is toxic to vegetation. At sub-lethal concentrations, H2S inhibits nutrient uptake 

and impairs plant growth.
Nutrient Criteria–Wetlands 2-5

Sources of Nutrients

Point Sources

Point source discharges of nutrients to wetlands may come from municipal or industrial discharges, including 

stormwater runoff from municipalities or industries, or in some cases from large animal feeding operations. 

Nutrients from point source discharges may be controlled through the National Pollutant Discharge 

Elimination System (NPDES) permits, most of which are administered by States authorized to issue them. In 

general, point source discharges that are not stormwater related are fairly constant with respect to loadings.

Nonpoint Sources

Nonpoint sources of nutrients are commonly discontinuous and can be linked to seasonal agricultural 

activity or other irregularly occurring events such as silviculture, non-regulated construction, and storm 

events. Nonpoint nutrient pollution from agriculture is most commonly associated with row crop agriculture 

and livestock production that tend to be highly associated with rain events and seasonal land use activities. 

Nonpoint nutrient pollution from urban and suburban areas is most often associated with climatological events 

(rain, snow, and snowmelt), when pollutants are most likely to be transported to aquatic resources. 

Urban and agricultural runoff is generally thought to be the largest source of nonpoint source pollution; 

however, growing evidence suggests that atmospheric deposition may have a significant influence on nutrient 

enrichment, particularly from nitrogen (Jaworski et al. 1997). Gases released through fossil fuel combustion 

and agricultural practices are two major sources of atmospheric N that may be deposited in waterbodies 

(Carpenter et al. 1998). Nitrogen and nitrogen compounds formed in the atmosphere return to the earth as acid 

rain or snow, gas, or dry particles. Atmospheric deposition, like other forms of pollution, may be determined 

at different scales of resolution. More information on national atmospheric deposition can be found at: http://

www.arl.noaa.gov/research/programs/airmon.html and http://nadp.sws.uiuc.edu/. These national maps may 

provide the user with information about regional areas where atmospheric deposition, particularly of nitrogen, 

may be of concern. However, these maps are generally low resolution when considered at the local and sitespecific scale and may not reflect areas of high local atmospheric deposition, such as local areas in a downwind 

plume from an animal feedlot operation. 

Other nonpoint sources of nutrient pollution may include certain silviculture and mining operations; these 

activities generally constitute a smaller fraction of the national problem, but may be locally significant nutrient 

sources. Control of nonpoint source pollutants focuses on land management activities and regulation of 

pollutants released to the atmosphere (Carpenter et al. 1998). 

2.3 WETLAND NUTRIENT COMPONENTS

Nutrient Budgets

Wetland nutrient inputs mirror wetland hydrologic inputs (e.g., precipitation, surface water, and ground water), 

with additional loading associated with atmospheric dry deposition and nitrogen transformation (Figures 2.5 

and 2.6). Total atmospheric deposition (wet and dry) may be the dominant input for precipitation-dominated 

wetlands, while surface- or ground-water inputs may dominate other wetland systems. 

The total annual nutrient load (mg-nutrients/yr) into a wetland is the sum of the dissolved and particulate loads. 

The dissolved load (mg-nutrients/s) can be estimated by multiplying the instantaneous inflow (L/s) by the 

nutrient concentration (mg-nutrients/L). EPA recommends calculating the annual load by the summation of this 

function over the year—greater loads may be found during periods of increased flow and EPA recommends 

monitoring during these intervals. Where continuous data are unavailable, average flows and concentrations 

may be used if a bias factor (Cohn et al. 1989) is included to account for unmeasured loads during high flows. 
2-6 Nutrient Criteria–Wetlands

Particulate loads (mg-nutrients/yr) can be estimated using the product of suspended and bedload inputs (kgsediments/yr) and the mass concentrations (mg-nutrients/kg-sediment).

Surface-water nutrient inputs are associated with flows from influent streams, as well as diffuse sources from 

overland flow through the littoral zone. Ground-water inputs can also be concentrated at points (e.g., springs), 

or diffuse (such as seeps). The influence of allochthonous sources is likely to be greatest in those zones closest 

to the source. 

Because wetlands generally tend to be low-velocity, depositional environments, they often sequester sediments 

and their associated nutrients. These sediment inputs generally accumulate at or near the point of entry into the 

wetland, forming deltas or levees near tributaries, or along the shoreline for littoral inputs. Coarser fractions 

(e.g., gravels and sands) tend to settle first; the finer fractions (silts, clays, and organic matter) tend to settle 

further from the inlet point. Particulate input from ground-water sources can usually be neglected, while 

particulate inputs from atmospheric sources may be important if local or regional sources are present.

Wetland nutrient outputs again mirror hydrologic outputs (e.g., surface- and ground-water), and loads are 

again estimated as the product of the flow and the concentration of nutrients in the flow. While evaporation 

losses from wetlands may be significant, there are no nutrient losses associated with this loss. Instead, loss 

of nutrients to the atmosphere may occur as a result of ammonia volatilization, as well as N2

O losses from 

incomplete denitrification. Because sediment outputs from wetlands may be minor, nutrient exports by this 

mechanism may not be important. 

Nutrient accumulation in wetlands occurs when nutrient inputs exceed outputs. Net nutrient loads can be 

estimated as the difference between these inputs and outputs. It is important, therefore, to have some estimate 

of net accumulation by taking the difference between upstream and downstream loads. Sampling ground-water 

nutrient concentrations in wells located upstream and downstream of the wetland can provide some sense of 

net nutrient sequestration, while sampling wetland nutrient inflows and outflows is needed for determining the 

additional sequestration for this pathway.

Biochemical Cycling

Biogeochemical cycling of nutrients in wetlands is governed by physical, chemical, and biological processes in 

the soil and water column. Biogeochemical cycling of nutrients is not unique to wetlands, but the aerobic and 

anaerobic interface generally found in saturated soils of wetlands creates unique conditions that allow both 

aerobic and anaerobic processes to operate simultaneously. The hydrology and geomorphology of wetlands 

(Johnston et al. 2001) influences biogeochemical processes and constituent transport and transformation within 

the systems (e.g., water-sediment exchange, plant uptake, and export of organic matter). Interrelationships 

among hydrology, biogeochemistry, and the response of wetland biota vary among wetland types (Mitsch and 

Gosselink 2000; Reddy and Delaune 2005). 

Biogeochemical processes in the soil and water column are key drivers of several ecosystem functions 

associated with wetland values (e.g., water quality improvement through denitrification, long-term nutrient 

storage in the organic matter) (Figure 2.3). The hub for biogeochemistry is organic matter and its cycling in 

the soil and water column. Nutrients such as N, P, and S are primary components of soil organic matter, and 

cycling of these nutrients is always coupled to C cycling. Many processes occur within the carbon, nitrogen, 

phosphorus, and sulfur (C, N, P, or S) cycles; microbial communities mediate the rate and extent of these 

reactions in soil and the water column. 

Aerobic-anaerobic interfaces are more common in wetlands than in upland landscapes and may occur at the 

soil water interface, in the root zones of aquatic macrophytes, and at surfaces of detrital tissue and benthic 

periphyton mats. The juxtaposition of aerobic and anaerobic zones in wetlands supports a wide range of 

microbial populations and associated metabolic activities, with oxygen reduction occurring in the aerobic 
Nutrient Criteria–Wetlands 2-7

interface of the substrate, and reduction of alternate electron acceptors occurring in the anaerobic zone 

(D’Angelo and Reddy 1994a or b). Under continuously saturated soil conditions, vertical layering of different 

metabolic activities can be present, with oxygen reduction occurring at and just below the soil floodwater 

interface. Substantial aerobic decomposition of plant detritus occurs in the water column; however, the supply 

of oxygen may be insufficient to meet demands and drive certain microbial groups to utilize alternate electron 

acceptors (e.g., nitrate, oxidized forms of iron (Fe) and manganese (Mn), sulfate, and bicarbonate (HCO3)). 

Soil drainage adds oxygen to the soil, while other inorganic electron acceptors may be added through hydraulic 

loading to the system. Draining wetland soil accelerates organic matter decomposition due to the introduction 

of oxygen deeper into the profile. In many wetlands, the influence of NO3 and oxidized forms of Mn and Fe 

on organic matter decomposition is minimal. This is because the concentrations of these electron acceptors 

are usually low as a result of the fact that they have greater reduction potential than other alternate electron 

acceptors, so they generally are depleted rapidly from systems. Long-term sustainable microbial activity is 

then supported by electron acceptors of lower reduction potentials (sulfate and HCO3

). Methanogenesis is often 

viewed as the terminal step in anaerobic decomposition in freshwater wetlands, whereas sulfate reduction is 

viewed as the dominant process in coastal wetlands. However, both processes can function simultaneously in 

the same ecosystem and compete for available substrates (Capone and Kiene 1988).

A simple way to characterize wetlands for aerobic and anaerobic zones is to determine the oxidation-reduction 

potential or redox potential (Eh) of the soil-water column (Figure 2.4). Redox potential is expressed in units of 

millivolts (mV) and is measured using a voltmeter coupled to a platinum electrode and a reference electrode. 

Typically, wetland soils with Eh values >300 mV are considered aerobic and typical of drained soil conditions, 

while soils with Eh values <300 mV are considered anaerobic and are devoid of molecular oxygen (Figure 2.4).

Figure 2.3: Schematic showing basic nutrient cycles in soil-water column of a wetland. 

Water

Soil

C Bioavailable

nutrients

Periphyton

Emergent

macrophyte

Submerged 

macrophyte

N P

S
2-8 Nutrient Criteria–Wetlands

Flooded Soil Drained Soil

Anaerobic Aerobic

Oxidation-Reduction Potential (mV)

Highly

Reduced

Reduced Moderately

Reduced

Oxidized

Wetlands, as low-lying areas in the landscape, receive inputs from all hydrologically connected uplands. 

Many wetlands are open systems receiving inputs of carbon (C) and nutrients from upstream portions of the 

watershed that can include agricultural and urban areas. 

Prolonged nutrient loading to wetlands can result in distinct gradients in water and soil. Mass loading and 

hydraulic retention time determine the degree and extent of nutrient enrichment. Continual nutrient loading 

to an oligotrophic wetland can result in a zone of high nutrient availability near the input, and low nutrient 

availability and possibly nutrient limiting conditions further from the input point. This enrichment effect can 

be seen in many freshwater wetlands, most notably in the sub-tropical Everglades where light is abundant and 

temperatures are high (Davis 1991; Reddy et al. 1993; Craft and Richardson 1993 a, b; DeBusk et al. 1994), and 

in some estuarine marshes (Morris and Bradley 1999). Between these two extremes, there can exist a gradient 

in quality and quantity of organic matter, nutrient accumulation, microbial and macrobiotic communities, 

composition, and biogeochemical cycles. 

Compared to terrestrial ecosystems, most wetlands show an accumulation of organic matter, and therefore 

wetlands function as global sinks for carbon. Accumulation of organic C in wetlands is primarily a result of 

the balance of C fixation through photosynthesis and losses through decomposition. Rates of photosynthesis in 

wetlands are typically higher than in other ecosystems, and rates of decomposition are typically lower due to 

anaerobic conditions, hence organic matter tends to accumulate. In addition to maintaining proper functioning 

of wetlands, organic matter storage also plays an important role in regulating other ecosystems and the 

biosphere. For example, organic matter contains substantial quantities of N, P, and S; therefore, accumulation 

of organic matter in wetlands decreases transport of these nutrients to downstream aquatic systems.

Figure 2.4: Range of redox potentials in wetland soils (Reddy and Delaune 2007). 
Nutrient Criteria–Wetlands 2-9

Nitrogen (N)

Nitrogen enters wetlands in organic and inorganic forms, with the relative proportion of each depending on 

the input source. Organic forms are present in dissolved and particulate fractions, while inorganic N (NH4-N, 

NO3-N and NO2-N) is present in dissolved fractions (Figure 2.5) or bound to suspended sediments (NH4-N). 

Particulate fractions are removed through settling and burial, while the removal of dissolved forms is regulated 

by various biogeochemical reactions functioning in the soil and water column. Relative rates of these processes 

are affected by physico-chemical and biological characteristics of plants, algae, and microorganisms.

Nitrogen reactions in wetlands effectively process inorganic N through nitrification and denitrification, 

ammonia volatilization, and plant uptake. These processes aid in lowering levels of inorganic N in the water 

column. A significant portion of dissolved organic N assimilated by plants is returned to the water column 

during breakdown of detrital tissue or soil organic matter, and the majority of this dissolved organic N is 

resistant to decomposition. Under these conditions, water leaving wetlands may contain elevated levels of 

N in organic form. Exchange of dissolved nitrogen species between soil and water column support several 

nitrogen reactions. For example, nitrification in the aerobic soil layer is supported by ammonium flux from 

the anaerobic soil layer. Similarly, denitrification in the anaerobic soil layer is supported by nitrate flux 

from the aerobic soil layer and water column. Relative rates of these reactions will, however, depend on the 

environmental conditions present in the soil and water column (Reddy and Delaune 2007). 

Phosphorus (P)

Phosphorus retention by wetlands is regulated by physical (sedimentation and entrainment), chemical 

(precipitation and flocculation), and biological mechanisms (uptake and release by vegetation, periphyton, 

and microorganisms). Phosphorus in the influent water is found in soluble and particulate fractions, with both 

fractions containing a certain proportion of inorganic and organic forms. Relative proportions of these pools 

depend on the input source. For example, municipal wastewater may contain a large proportion (>75%) as 

inorganic P in soluble forms, as compared to effluents from agricultural watersheds where a greater percentage 

of P loading may be in the particulate fraction.

Organic

Plant

uptake

Adsorbed NH4

+

NO3

-

Litterfall

Organic N

Plant biomass 

[NH4

+

]s

NH4

+

[NH4

+

]s

NO3 NH4

NN2, N2O (g)

N2 N2O 

Microbial 

Biomass N 

N2 NH3

Nitrogen 

 Fixation 

Atmospheric 

Deposition

Inflow Outflow 

Soil - ANAEROBIC 

Soil - AEROBIC 

Nitrification Mineralization

Peat 

accretion

Volatilization

Water Column

Denitrification

Figure 2.5: Schematic of the nitrogen cycle in wetlands.
2-10 Nutrient Criteria–Wetlands

Water Column

Soil - ANAEROBIC

[Fe, Al or

Ca-bound 

P] 

Adsorbed

IP

DOP

Plant biomass P

DIP PIP POP

DIP 

POP DOP

DOP

DIP 

DIP 

Outflow

Periphyton 

Atmospheric

Deposition

Inflow 

Soil - AEROBIC 

Litterfall

Peat

accretion

Figure 2.6: Schematic of the phosphorous cycle in wetlands.

Phosphorus forms that enter a wetland are grouped into: (i) dissolved inorganic P (DIP); (ii) dissolved organic 

P (DOP); (iii) particulate inorganic P (PIP); and, (iv) particulate organic P (POP) (Figure 2.6). The particulate 

and soluble organic fractions may be further separated into labile and refractory components. Dissolved 

inorganic P is generally bioavailable, whereas organic and particulate P forms generally must be transformed 

into inorganic forms before becoming bioavailable. Both biotic and abiotic mechanisms regulate relative pool 

sizes and transformations of P compounds within the water column and soil. Alterations in these fractions can 

occur during flow through wetlands and depend on the physical, chemical, and biological characteristics of the 

systems. Thus, both biotic and abiotic processes should be considered when evaluating P retention capacities 

of wetlands. Biotic processes include assimilation by vegetation, plankton, periphyton, and microorganisms. 

Abiotic processes include sedimentation, adsorption by soils, precipitation, and exchange processes between 

soil and the overlying water column (Reddy et al. 1999, 2005; Reddy and Delaune 2007). The processes 

affecting phosphorus exchange at the soil/sediment water interface include: (i) diffusion and advection due 

to wind-driven currents; (ii) diffusion and advection due to flow and bioturbation; (iii) processes within the 

water column (mineralization, sorption by particulate matter, and biotic uptake and release); (iv) diagenetic 

processes (mineralization, sorption, and precipitation dissolution) in bottom sediments; (v) redox conditions 

(O2 content) at the soil/sediment-water interface; and, (vi) phosphorus flux from water column to soil mediated 

by evapotranspiration by vegetation.

The key biogeochemical services provided by wetlands include nutrient transformation and removal by 

decreasing concentrations of nutrients and other contaminants, and sequestration of carbon and nutrients into 

stable pools (Kadlec and Knight 1996). The biogeochemical processes regulating water quality improvement 

are well established, and are made use of in treatment wetlands. Increased nutrient loading to oligotrophic 

wetlands results in increased primary productivity and nutrient enrichment. This resulting eutrophication can 

have both positive and negative impacts to the environment. Higher rates of primary productivity increase 

rates of organic matter accumulation, thus increasing carbon sequestration. However, eutrophication may lead 

to increased periodic and episodic export of DIP (Kadlec and Knight 1996; Reddy et al. 1995, 1996, 2005; 

Reddy and Delaune 2007).
Nutrient Criteria–Wetlands 3-1

Chapter 3: Classification of Wetlands

3.1 Introduction

Developing individual, site-specific nutrient criteria is not practical for every wetland. Instead, criteria for 

groups of similar wetlands in a region are needed. To this end, a means of grouping or classifying wetlands 

is required. This chapter introduces the scientific rationale for classifying wetlands, reviews some common 

classification schemes, and discusses their implications for establishing nutrient criteria for wetlands. Use 

of a common scheme across State boundaries should facilitate collaborative efforts in describing reference 

condition for biota or water quality and in developing assessment methods, indices of biotic integrity (IBI) 

(USEPA 1993b, http://www.epa.gov/emap/remap/index.html), nutrient-response relationships, and nutrient 

criteria for wetlands. This chapter describes a series of national classification systems that could be used to 

provide a common framework for development of nutrient criteria for wetlands, and suggests ways in which 

these classification schemes could be combined in a hierarchical fashion. Many existing classification schemes 

may be relevant and should be considered for use or modification, even if they were not originally derived 

for wetland nutrient criteria because: (1) they incorporate key factors that control nutrient inputs and cycling; 

(2) they have already been mapped; and, (3) they have been incorporated into sampling, assessment, and 

management strategies for wetland biology or for other surface water types, thus facilitating integration of 

monitoring strategies. Adoption of any classification scheme should be an iterative process, whereby initial 

results of biological or water quality sampling are used to test for actual differences in reference condition 

for nutrients or nutrient-response relationships among proposed wetland classes. Wetland classes that behave 

similarly can be combined, and apparent outliers in distributions of nutrient concentrations from reference 

sites or in nutrient-response relationships can be examined for additional sources of variability that may need 

to be considered. In addition, new classification schemes can be derived empirically through many multivariate 

statistical methods designed to determine factors that can discriminate among wetlands based on nutrient 

levels or nutrient-response relationships. 

The overall goal of classification is to reduce variability within classes due to differences in natural condition 

related to factors such as geology, hydrology, and climate. This will minimize the number of classes for 

which reference conditions must be defined. For example, we would expect different conditions for water 

quality or biological community composition for wetland classes in organic soils (histosols), compared to 

wetlands in mineral soils. In assessing impacts to wetlands, comparing a wetland from within the same class 

would increase the precision of assessments, enable more sensitive detection of change, and reduce errors in 

characterizing the status of wetland condition.

Reference Concept

Reference conditions “describe the characteristics of waterbody segments least impaired by human activities 

and are used to define attainable biological or habitat conditions” (USEPA 1990; Stoddard et al. 2006). At least 

two general approaches have been defined to establish reference condition—the site-specific and the regional 

(U.S. EPA 1990b, http://www.epa.gov/bioindicators/). The current approach to developing water quality 

criteria for nutrients also emphasizes the identification of expected ranges of nutrients by waterbody type 

and ecoregion for the least-impaired reference conditions (U.S. EPA 1998; http://www.epa.gov/waterscience/

standards/nutrient.html).

Although different concepts of reference condition have been used in other programs (e.g., for evaluation of 

wetland mitigation projects (Smith et al. 1995; http://el.erdc.usace.army.mil/wetlands/pdfs/wrpde9.pdf)), for 

the purposes of this document, the term “reference condition” refers to wetlands that are minimally or least 

impacted by human activities. Most, if not all, wetlands in the U.S. are affected to some extent by human 

activities such as acid precipitation, global climate change, or other atmospheric deposition of nitrogen and 

mercury, and changes in historic fire regime. “Minimally impacted” is therefore operationally defined by 
3-2 Nutrient Criteria–Wetlands

choosing sites with fewer stressors or fewer overall impacts as described by indicators of stressors, such as 

land-use or human activities within the watershed or buffer area surrounding a wetland and source inputs. 

Identifying reference wetlands in areas of high local or regional atmospheric deposition of nitrogen should also 

be carefully considered because indicators such as local land use activities may not be sufficient to indicate 

nutrient enrichment from dry or wet air deposition. 

 

3.2 EXISTING WETLAND CLASSIFICATION SCHEMES

There are two different approaches for classification of aquatic resources. One is geographically-based, 

and the other is independent of geography but relies on environmental characteristics that determine 

aquatic ecosystem status and vulnerability at the region-, watershed-, or ecosystem-scale (Detenbeck et al. 

2000). Ecoregions (including “nutrient ecoregions”) and Ecological Units represent geographically-based 

classification schemes that have been developed and applied nation-wide (Omernik 1987; Keys et al. 1995). 

The goal of geographically-based classification schemes is to reduce variability in reference condition based 

on spatial co-variance in climate and geology, along with topography, vegetation, hydrology, and soils. 

Geographically-independent or environmentally-based classification schemes include those derived using 

watershed characteristics such as land-use and/or land-cover (Detenbeck et al. 2000), hydro geomorphology 

(Brinson 1993), vegetation type (Grossman et al. 1998), or some combination of these (Cowardin et al. 1979). 

Both geographically- and environmentally-based schemes have been developed for wetland classification. 

These approaches can be applied individually or combined within a hierarchical framework (Detenbeck et al. 

2000).

Geographically-based Classification Schemes

Regional classification systems were first developed specifically for the United States by land management 

agencies. The U.S. Department of Agriculture (USDA) has described a hierarchical system of Land Resource 

Regions and Major Land Resource Areas based mainly on soil characteristics for agricultural management 

(USDA SCS 1981). Ecoregions were then refined for USDA and the U.S. Forest Service based on a hierarchical 

system in which each of several environmental variables such as climate, landform, and potential natural 

vegetation were applied to define different levels of classification (Bailey 1976). Subsequently, Omernik and 

colleagues developed a hierarchical, nationwide ecoregion system to classify streams using environmental 

features they expected would influence aquatic resources, as opposed to terrestrial resources (Hughes and 

Omernik 1981; Omernik et al. 1982). The latter was based on an overlay of “component maps” for land use, 

potential natural vegetation, land-surface form, and soils, along with a subjective evaluation of the spatial 

congruence of these factors as compared to the hierarchical approach used by Bailey, which relied only on 

natural features (not land-use). Omernik has produced a national map of 84 ecoregions defined at a scale 

of 1:7,500,000 (Figure 3.1; Omernik 1987, http://water.usgs.gov/GIS/metadata/usgswrd/XML/ecoregion.

xml). More detailed, regional maps have been prepared at a scale of 1:2,500,000 in which the most “typical” 

areas within each ecoregion are defined. Cowardin et al., (1979) have suggested an amendment to Bailey’s 

ecoregions to include coastal and estuarine waters (Figure 3.2a). In practice, Omernik’s scheme has been more 

widely used for geographic classification of aquatic resources such as streams, but few examples to verify the 

appropriateness of this grouping to wetland nutrients are available.

Finally, an attempt has been made to integrate approaches across Federal agencies to produce regional 

boundaries termed Ecological Units (Keys et al. 1995). Information has been combined on climate, landform, 

geomorphology, geology, soils, hydrology, and potential vegetation to produce a nested series of boundaries for 

the eastern U.S. Different combinations of environmental parameters are emphasized at each hierarchical level 

of classification. This scheme was developed to explain variation in both terrestrial and aquatic systems, and 

is consistent with a more comprehensive strategy to classify lotic systems down to the level of stream reaches 

(Maxwell et al. 1995). The mapped system for the eastern U.S. includes classification at the following levels:

domain (n=2) > divisions (n=5) > provinces (n=14) > sections (n=78) > subsections (n=xxx),
Nutrient Criteria–Wetlands 3-3

where Sections are roughly half the size of Omernik ecoregions (Figure 3.3). For lotic systems, additional 

spatial detail can be added by defining watersheds (at the level of land type associations), subwatersheds (at the 

level of land types), valley segments, stream reaches, and, finally, channel units (Maxwell et al. 1995). In reality, 

not all watersheds nest neatly within subsections, and may cross-subsection boundaries.

Figure 3.1: Map of Omernik aquatic ecoregions.

Figure 3.2a: Map of Bailey ecoregions with coastal and estuarine provinces. (Cowardin et al. 1979).
3-4 Nutrient Criteria–Wetlands

Figure 3.2b: Legend for Bailey ecoregion map shown in Figure 3.2a.

a

 Domains, Divisions, Provinces, and Sections used on Bailey’s (1976) map and described in detail in Bailey 

(1978). Highland ecoregions are designated M mountain, P plateau, and A altiplano.

1000 Polar

 1200 Tundra

 1210 Arctic Tundra

 1220 Bering Tundra

 M1210 Brooks Range

 1300 Subarctic

 1310 Yukon Parkland

 1320 Yukon Forest

 M1310 Alaska Range

2000 Humid Temperate

 2100 Warm Continental

 2110 Laurentian Mixed Forest

 2111 Spruce-Fir Forest

 2112 Northern Hardwoods-Fir Forest

 2113 Northern Hardwoods Forest

 2114 Northern Harwoods-Spruce Forest

 M2110 Columbia Forest

 M2111 Douglas-fir Forest

 M2112 Cedar-Hemlock-Douglas-fir Forest

2200 Hot Continental

 2210 Eastern Deciduous Forest

 2211 Mixed Mesophytic Forest

 2212 Beech-Maple Forest

 2213 Maple-Basswood Forest + Oak Savanna

 2214 Appalachian Oak Forest

 2215 Oak-Hickory Forest

2300 Subtropical

 2310 Outer Coastal Plain Forest

 2311 Beech-Sweetgum-Magnolia-Pine-Oak

 2312 Southern Floodplain Forest

 2320 Southeastern Mixed Forest

2400 Marine

 2410 Willamette-Puget Forest

 M2410 Pacific Forest (in conterminous U.S.)

 M2411 Sitka Spruce-Cedar-Hemlock Forest

 M2412 Redwood Forest

 M2413 Cedar-Hemlock-Douglas-fir Forest

 M2414 California Mixed Evergreen Forest

 M2415 Silver fir-Douglas-fir Forest

 M2410 Pacific Forest (in Alaska)

2500 Prairie

 2510 Prairie Parkland

 2511 Oak-Hickory-Bluestem Parkland

 2512 Oak+Bluestem Parkland

 2520 Prairie Brushland

 2521 Mesquite-Buffalo Grass

 2522 Juniper-Oak-Mesquite

 2523 Mesquite-Acacia

 2530 Tall-Grass Prairie

 2531 Bluestem Prairie

 2532 Whestgrass-Bluestem-Needlegrass

 2533 Bluestem-Gamma Prairie

2600 Mediterranean (Dry-summer Subtropical)

 2610 California Grassland

 M2610 Sierran Forest

 M2620 California Chaparral

3000 Dry 

 3100 Steppe

 3110 Great Planins-Shortgrass Prairie

 3111 Gramma-Needlegrass-Wheatgrass

 3112 Wheatgrass-Needlegrass

 3113 Grama-Buffalo Grass

 M3110 Rocky Mountain Forest

 M3111 Grand-fir-Douglas-fir Forest

 M3112 Douglas-fir Forest

 M3113 Ponderosa Pine-Douglas-fir Forest

 3120 Palouse Grassland

 M3120 Upper Gila Mountains Forest

 3130 Intermountain Sagebrush

 3131 Sagebrush-Wheatgrass

 3132 Lahontan Saltbush-Greasewood

 3133 Great Basin Sagebrush

 3134 Bonneville Saltbush-Greasewood

 3135 Ponderosa Shrub Forest

 P3130 Colorado Plateau

 P3131 Juniper-Pinyon Woodland + Sagebrush Saltbush 

 Mosaic

 P3132 Grama-Galleta Steppe + Juniper-Pinyon 

 Woodland Mosaic

 3140 Mexican Highland Shrub Steppe

A3140 Wyoming Basin

A3141 Wheatgrass-Needlegrass-Sagebrush

A3142 Sagebrush-Wheatgrass

 3200 Desert 3210 Chihuahuan Desert

 3211 Grama-Tobosa

 3212 Tarbush-Creosote Bush

 3220 American Desert

 3221 Creosote Bush

 3222 Creosote Bush-Bur Sage

4000 Humid Tropical

 4100 Savanna

 4110 Everglades

 4200 Rainforest

 M4210 Hawaiian Islands

 
Nutrient Criteria–Wetlands 3-5

Figure 3.3: Examples of first four hierarchical levels of Ecological Units: domain, division, province, 

and section, from USEPA Environmental Atlas.

Some States have chosen to refine the spatial resolution of Omernik’s ecoregional boundaries for management 

of aquatic resources (e.g., Region 3 and Florida). For example, the State of Florida has defined subecoregions 

for streams based on analysis of macroinvertebrate data from 100 minimally-impacted sites. Efforts are currently 

underway to define ecoregions for Florida wetlands based on variables influencing the water budget and plant 

community composition (Dougherty et al. 2000; Lane 2000).

a. Domain b. Division

c. Province d. Section
3-6 Nutrient Criteria–Wetlands

Environmentally-based Classification Systems

Wetland habitat types are described very simply but coarsely by Shaw and Fredine (1956, Circular 39), ranging 

from temporarily-flooded systems to ponds. A more refined hierarchical classification system is available 

based on vegetation associations; for example, the system developed by the Nature Conservancy for terrestrial 

vegetation includes some wetland types (Grossman et al. 1998). Vegetation associations have also been used to 

classify Great Lakes coastal wetlands within coastal geomorphic type (Michigan Natural Features Inventory 

1997).

Cowardin Classification System

The Cowardin classification system (Cowardin et al. 1979) was developed for the U.S. Fish and Wildlife Service 

(FWS) as a basis for identifying, classifying, and mapping wetlands, special aquatic sites, and deepwater aquatic 

habitats. The Cowardin system combines a number of approaches incorporating landscape position, hydrologic 

regime, and habitat (vegetative) type (http://www.nwi.fws.gov) (Figure 3.4). Wetlands are categorized first by 

landscape position (tidal, riverine, lacustrine, and palustrine), then by cover type (e.g., open water, submerged 

aquatic bed, persistent emergent vegetation, shrub wetlands, and forested wetlands), and then by hydrologic 

regime (ranging from saturated or temporarily-flooded to permanently flooded). Modifiers can be added for 

different salinity or acidity classes, soil type (organic vs. mineral), or disturbance activities (impoundment, 

beaver activity). Thus, the Cowardin system includes a mixture of geographically-based factors, proximal 

forcing functions (hydrologic regime, acidity), anthropogenic disturbance regimes, and vegetative outcomes. 

In practice, the Cowardin system can be aggregated by combination of hydrogeomorphic (HGM) type and 

predominant vegetation cover if digital coverages are available (Ernst et al. 1995).

Figure 3.4a: Cowardin hierarchy of habitat types for estuarine systems; from Cowardin et al. 1979.
Nutrient Criteria–Wetlands 3-7

Figure 3.4b: Cowardin hierarchy of habitat types for Palustrine systems; from Cowardin et al. 1979.

Hydrogeomorphic Classification System(s)

Brinson (1993) has defined a hydrogeomorphic classification system for wetlands based on geomorphic setting, 

dominant water source (Figure 3.5), and dominant hydrodynamics (Figure 3.6; http://www.wes.army.mil/el/

wetlands). Seven classes have been described: depressional, lacustrine fringe, tidal fringe, slope, riverine, 

mineral soil flats, and organic soil flats (Smith et al. 1995). Also see Hydrogeomorphic Classification in http://

www.epa.gov/waterscience/criteria/wetlands/7Classification.pdf.

Depressional systems, as the name implies, are located in topographic depressions where surface water can 

accumulate. Depression wetlands can be further classified based on presence of inlets or outlets and primary 

water source as closed, open/groundwater, or open/surface water subclasses. 

Lacustrine fringe wetlands are located along lake shores where the water elevation of the lake determines the 

water table of the adjacent wetland. Great Lakes coastal wetlands represent one important region of lacustrine 

fringe wetlands. These coastal systems are strongly influenced by coastal forming processes, and, as such, 

have been further classified by geomorphic type through various schemes (Jaworski and Raphael 1979, and 

others summarized in Michigan Natural Features Inventory 1997). These geomorphic coastal positions 

will further influence the predominant source of water and the degree and type of energy regime (riverine 

vs. seiche and wave activity). Tidal fringe wetlands occupy a similar position relative to marine coasts and 

estuaries, where water level is influenced by sea level. Tidal fringe wetlands can be broken down further 

based on salinity into euhaline vs. mixohaline subclasses. Slope wetlands occur on slopes where groundwater 

discharges to the land surface, but typically do not have the capacity for surface water storage (Figure 3.7). 

Riverine wetlands are found in floodplains and riparian zones associated with stream channels. Riverine 

systems can be broken down based on watershed position (and, thus, hydrologic regime) into tidal, lower 

perennial, upper perennial, and nonperennial subclasses. Mineral soil flats are in areas of low topographic 

relief (e.g., interfluves, relic lake bottoms, and large floodplain terraces) with precipitation as the main source of 

water. The topography of organic soil flats (e.g., peatlands), in contrast, is controlled by the vertical accretion of 

organic matter.
3-8 Nutrient Criteria–Wetlands

Figure 3.5: Dominant water sources to 

wetlands, from Brinson 1993. Cowardin 

hierarchy of habitat types for estuarine 

systems.

Figure 3.6: Dominant hydrodynamic 

regimes for wetlands based on flow 

pattern (Brinson 1993).

The HGM classification system is being further refined to the subclass level for different regions or States 

and classes (Cole et al. 1997, http://www.wes.army.mil/el/wetlands). In addition to the classification factors 

described above, Clairain (2002) suggests using parameters such as the degree of connection between the 

wetland and other surface waters (depressional wetlands), salinity gradients (tidal), degree of slope or channel 

gradient (slope and riverine wetlands), position in the landscape (riverine, slope), and a scaling factor (stream 

Figure 3.7: Interaction with break in slope with 

groundwater inputs to slope wetlands (Brinson 1993).
Nutrient Criteria–Wetlands 3-9

order, watershed size or floodplain width for riverine subclasses). In some cases, existing regional schemes 

have been used as the basis for subclass definition (e.g., Stewart and Kantrud 1971; Golet and Larson 1974; 

Wharton et al. 1982; Weakley and Schafale 1991; Keough et al. 1999). 

The HGM classification system has been applied primarily to assess wetland functions related to hydrology, 

biological productivity, biogeochemical cycling, and habitat (Smith et al. 1995, http://www.wes.army.mil/

el/wetlands/pdfs/wrpde9.pdf). The same environmental parameters that influence wetland functions also 

determine hydrologic characteristics and background water quality, which in turn drive wetland habitat 

structure and community composition and the timing of biotic events. Thus, the HGM classification system 

can serve as a basis for partitioning variability in reference trophic status and biological condition, as well as 

defining temporal strategies for sampling.

Comparison of Environmentally-based Classification Systems

If an integrated assessment of aquatic resources within a watershed or region is desired, it may be useful to 

consider intercomparability of classification schemes for wetlands, lakes, and riverine systems to promote 

cost-effective sampling and ease of interpretation. The HGM approach could integrate readily with a finer 

level of classification for lake type because lentic systems are separated out as lacustrine fringe or depressional 

wetlands based on lake or pond size and influence of water level on the adjacent wetland. Lacustrine 

classification systems for water quality have included geography (climate + bedrock characteristics, Gorham 

et al. 1983) or hydrologic setting (Winter 1977; Eilers et al. 1983) as factors for categorization. McKee et al., 

(1992) suggest a modification of Cowardin’s system for Great Lakes coastal wetlands incorporating landscape 

position (system), depth zone (littoral vs. limnetic subsystems), vegetative or substrate cover (class and 

subclass), and modifiers of ecoregions, water level regimes, fish community structure, geomorphic structure, 

and human modification. In contrast, the Michigan Natural Features Inventory (1997) categorizes Great Lakes 

coastal wetlands by Great Lake, then nine unique geomorphic types within lakes, then vegetative association.

For lotic systems, Brinson et al., (1995) describes an approach to further classify riverine classes into 

subclasses based on watershed position and stream size/permanence. This strategy is consistent with current 

monitoring efforts to develop stream IBIs (Indices of Biotic Integrity), which typically use stream order as 

a surrogate for watershed size in explaining additional background variation in IBI scores (USEPA 1996). 

A more detailed classification of stream reach types, based on hydrogeomorphic character, is described by 

Rosgen (1996). This classification scheme has been predominantly applied to assessments of channel stability 

and restoration options, and not to development of criteria. Gephardt et al., (1990) described a cross-walk 

between riparian and wetland classification and description procedures.

Combinations of Geographic and Environmentally-based Approaches

It is possible to combine geographically-based classification with hydrogeomorphic and/or habitat-based 

approaches. For example, a scheme could be defined that nests Cowardin (Cowardin et al. 1979) vegetative 

cover class within HGM class within ecoregion. Maxwell et al., (1995) have defined a scheme for linking 

geographically-based units based on geoclimatic setting (domains => divisions => provinces => sections => 

subsections) to watersheds and subwatersheds, and thus to riverine systems composed of valley segments, 

stream reaches, and channel units, or to lacustrine systems composed of lakes, lake depth zones, and lake sites/

habitat types. 

Maxwell et al., (1995) also define a series of fundamental hydrogeomorphic criteria for classifying wetlands 

based on Brinson (1993) and Winter (1992), including physiography (landscape position), water source, 

hydrodynamics, and climate. The first three of these are similar to the HGM classification system (see 

summary tables in Keys et al. 1995). Finer scale variation in landforms is also discussed and may be of use in 

determining the dominance of different hydrogeomorphic classes of wetlands and associated surface waters 

(lakes and rivers). Characteristics and relative advantages and disadvantages of different classification systems 

are summarized in Table 2.
3-10 Nutrient Criteria–Wetlands

3.3 SOURCES OF INFORMATION FOR MAPPING WETLAND CLASSES

In order to select wetlands for sampling in a random- or random-stratified design (described in Chapter 4), it 

is important to have a record of wetland locations to choose from, preferably categorized by the classification 

system of interest. For some, but not all portions of the country, wetlands have been mapped from aerial 

photography through the National Wetlands Inventory (NWI) maintained by the U.S. Fish and Wildlife 

Service (http://www.fws.gov/nwi/; Dahl 2005). In other cases, individual States have developed inventories, or 

researchers have developed lists for specific types of wetlands within a given region, e.g., Great Lakes coastal 

wetlands (Herdendorf et al. 1981). In order to sample these mapped wetland areas in a random fashion, it is 

important to have a list of each wetland that occurs within each class and its associated area. A geographic 

information system (GIS) allows one to automatically produce a list of all wetland polygons by type within a 

specified geographic region. Sources of digital information for mapping and/or classifying wetlands in a GIS 

are presented in the Land-Use Characterization for Nutrient and Sediment Risk Assessment Module (http://

www.epa.gov/waterscience/criteria/wetlands/17LandUse.pdf). 

In areas for which digital NWI maps do not yet exist, potential wetland areas can be mapped using GIS tools 

to predict relative wetness (e.g., Phillips 1990) or soil survey maps with hydric soil series can be used. It 

should be noted that in areas in which hydrology has been significantly altered (e.g., through ditching, tiling, 

or construction of urban stormwater systems), areas of potential wetlands could have been removed already. 

Similarly, although there are no current maps of wetlands by hydrogeomorphic class, these could be derived 

through GIS techniques using a combination of wetland coverages, hydrography (adjacency to large lakes 

and rivers), and digital elevation models to derive landforms (mineral and organic soil flats) and/or landscape 

position (slope and depressional wetlands).

3.4 DIFFERENCES IN NUTRIENT REFERENCE CONDITION OR SENSITIVITY TO 

 NUTRIENTS AMONG WETLAND CLASSES

Very few studies to verify classification systems for wetland nutrient monitoring have been completed, 

although a number of monitoring strategies have been implemented based on pre-selected strata. Monitoring 

efforts to develop or assess biological criteria generally have used a combination of geographic region and 

hydrogeomorphic class or subclass (e.g., Cole et al. 1997; Bennett 1999; Apfelbeck 1999; Michigan Natural 

Features Inventory 1997). Analysis of plant associations has been used to derive empirical classifications based 

on factors such as landscape position, water source, climate, bedrock, and sediment hydraulic conductivity 

(Weakley and Schafale 1991; Nicholson 1995; Halsey et al. 1997; Michigan Natural Features Inventory 1997). 

Only one case of classification based on wetland macroinvertebrate composition was found. For Australian 

wetlands, wetland classes grouped by macroinvertebrate communities were distinguished by water chemistry 

extremes (low pH, high salinity), degree of nutrient enrichment, and water color (Growns et al. 1992).

In some cases (e.g., northern peatlands) classification criteria derived on the basis of plant associations are 

less powerful in discriminating among nutrient regimes (e.g., Nicholson 1995); this may be particularly 

true where variation in vegetation type is related to differences in major ion chemistry and pH, rather than 

nutrients. The same is true in southern pocosins, where short and tall pocosins differ in seasonal hydrology 

but not soil chemistry. However, when contrasting pocosins and swamp forests, soil nutrients differed strongly 

(Bridgham and Richardson 1993). For some potential indicators of nutrient status such as vegetation nitrogen 

to phosphorus ratios, indicator thresholds will be consistent across species (Koerselman and Meuleman 1996), 

while response thresholds for other indicators of plant nutrient status vary across functional plant groupings 

with different life history strategies. These differences may indicate potential differences in sensitivity to 

excess nutrient loading (McJannet et al. 1995). Thus, vegetation community types are not always a good 

predictor of background nutrient concentrations (reference condition) or sensitivity to nutrient loading.

Sensitivity to nutrient loading (as evidenced by differences in nutrient cycling and availability) may also be 
Nutrient Criteria–Wetlands 3-11

related to differences in hydroperiod among wetlands. Wetland mesocosms exposed to pulse discharges had 

higher nutrient loss from the water column than those exposed to continuous flow regimes (Busnardo et al. 

1992). Depending on the predominant mechanism for nutrient loss (e.g., plant uptake versus denitrification), 

nutrient-controlled primary production could be either stimulated or reduced. Mineralization rates of carbon, 

nitrogen, and phosphorus differ significantly among soils from northern Minnesota wetlands, related to an 

ombrotrophic to minerotrophic gradient (i.e., degree of groundwater influence), and aeration status (Bridgham 

et al. 1998).

In general, very few definitive tests of alternative classification schemes for wetlands are available with 

respect to describing reference condition for either nutrient criteria or biocriteria. However, evidence from the 

literature suggests that in many cases both geographic factors (e.g., climate, geologic setting) and landscape 

setting (hydrogeomorphic type) are expected to affect water quality and biotic communities. 

3.5 RECOMMENDATIONS

Classification strategies for nutrient criteria development should incorporate factors affecting background 

nutrient levels and wetland sensitivity to nutrient loading at several spatial scales. 

• Classification of physiographic regions eliminates background variation in lithology and soil 

 texture (affecting background nutrient levels and sorption capacity), in climate (affecting 

 seasonality, productivity, decomposition, and peat formation), and in landforms, which 

 determines the predominance of different hydrogeomorphic classes. 

• Classification by hydrogeomorphic class reduces background variation in predominant water 

 and nutrient sources, water depth and dynamics, hydraulic retention time, assimilative 

 capacity, and interactions with other surface water types (Table 3). 

• Classification by water depth and duration (which may or may not be incorporated 

 into hydrogeomorphic classes) helps to explain variation in internal nutrient cycling, 

 dissolved oxygen level and variation, and the ability of wetlands to support some higher 

 trophic levels such as fish and amphibians. 

• Classification by vegetation type or zone, whether to inform site selection or to etermine

 sampling strata within a site, helps to explain background variation in predominant primary 

 producer form (which will affect endpoint selection), as well as turnover and growth rates 

 (which will affect rapidity of response to nutrient loadings). 

In general, the choice of specific alternatives among the classification schemes listed above depends on 

their intrinsic value as well as practical considerations, e.g., whether a classification scheme is available in 

mapped digital form or can be readily derived from existing map layers, whether a hydrogeomorphic or other 

classification scheme has been refined for a particular region and wetland type, and whether classification 

schemes are already in use for monitoring and assessment of other waterbody types in a state or region. 

Revisiting classification decisions once data from a sufficient number of sites have been sampled may be useful 

to ensure the original classification was correct.
3-12 Nutrient Criteria–Wetlands

Classification 

Scheme

Scale Hierarchical? Levels of 

Strata

Advantages Disadvantages Potential Links with 

Other Schemes

Bailey’s ecoregions Nationwide Yes Domains

Divisions

Provinces

Sections

Only natural 

attributes included

Digital maps

Terrestrial basis

Untested for wetlands

No hydrology

Could form first strata for any 

of the schemes below ecological 

units

Omernik ecoregions Nationwide No Ecoregions

Subecoregions

Digital maps Combines land-use 

with natural attributes

Untested for most 

wetlands

No hydrology

Could form first strata for any 

of the schemes below ecological 

units

Ecological units 

(Maxwell et al. 1995)

Nationwide Yes Domain 

Divisions

Provinces

Sections 

Subsections

Digital maps Greater number of 

strata and units than 

for ecoregions

Untested for wetlands

Could form first strata for any 

of the schemes below ecological 

units

Ties to classification schemes 

already defined within 

hydrogeomorphic types

US ACE 

Hydrogeomorphic 

Classes

Nationwide 

at class level; 

regionalized 

at subclass 

level

Yes - limited Class

Subclass

Specific for 

wetlands

Subclasses not 

comparable across 

different regions

Intermediate strata between 

geographic and habitat-scale

Rosgen channel types Nationwide Yes Level I

Level II

Captures 

differences in 

hydrologic regime 

for riverine 

wetlands

More focused on 

instream channel 

form than riparian 

characteristics

Riverine only

Not mapped

Intermediate strata between 

hydro-geomorphic type and 

habitat-scale

Anderson land-cover 

classes

Nationwide Yes Level I

Level II

Level III

Common basis for 

land-use/land-cover 

mapping

Not functionally based Cross-walk w NWI system 

possible

Circular 39 classes Nationwide No Class Popular recognition Mixture of criteria 

used to distinguish 

classes

Not mapped

Strata below geographic 

but contains mixture of 

hydrogeomorphic type and habitat 

type

National Wetland 

Inventory

Nationwide Yes System

Subsystem

Class

Subclass

Hydrologic 

modifier

Other modifiers

Digital maps 

available for much 

of nation (but 

smallest wetlands 

omitted)

Inconsistencies in 

mapping water quality 

modifiers

Limited consideration 

of hydrogeomorphic 

type

Strata below geographic

Hydrogeomorphic class could be 

improved by link w HGM system

Vegetation 

associations

International Yes System

Formation class

Formation 

subclass 

Formation 

group

Formation 

subgroup 

Formation

alliance

Association

Consistency across 

terrestrial and 

aquatic systems

Not functionally based

No digital maps

Taxa specific

Could be used as lowest level 

within other schemes

Table 2: Comparison of landscape and wetland classification schemes.
Nutrient Criteria–Wetlands 3-13

Table 3: Features of the major hydrogeomorphic classes of wetlands that may influence background 

nutrient concentrations, sensitivity to nutrient loading, nutrient storage forms and assimilative capacity, designated use, and choice of endpoints.

HGM Class Organic 

Flats

Mineral Flats Depressional Riverine Fringe Slope

Predominant Nutrient 

Source(S)

Atmospheric 

Deposition

Atmospheric 

Deposition, 

Groundwater

Runoff 

(Particulate 

and Dissolved), 

Surface and 

Groundwater

Runoff 

(Particulate), 

Overbank Flooding 

(Particulate, 

Dissolved)

Adjacent Lake, Possible 

Stream or Riverine Source, 

Groundwater

Groundwater

Landscape Position Adjacent to Rivers Adjacent to Lakes Slope, Toe of Slope

Hydrologic Regime Saturated, 

Little 

Standing 

Water

Saturated, Little 

Standing Water

Depth and 

Duration Vary 

from Saturated 

to Temporary 

to Seasonal to 

Semi-Permanent 

to Permanent 

Inundation

Depth, Duration 

Vary With River 

Flooding Regime

Standing Water In 

Emergent and Submerged 

Aquatic Zones, Short-Term 

Fluctuation Related to 

Seiche Activity, Long-Term 

to Wet-Dry Cycles

Saturated

Hydraulic Retention 

Time

Decades Decades Varies With 

Inflows/Outflows, 

Landscape 

Position

<Day to Few Days < Day < Day

Nutrient Assimilation 

Capacity

Low High Sorption 

Capacity

High Sorption, 

Plant Uptake, 

(Limited) 

Sediment Storage

High Sorption,

Sediment Trapping,

Plant Uptake In 

Floodplain

Some Sediment Trapping

Nutrient Transformer

High Sorption Capacity

Predominant 

Vegetation Growth 

Form

Mosses

Sedges

Sedges Varies With Zone 

And Duration of 

Flooding:

Wooded

Grass/Sedge

Emergents

Submerged 

Aquatics*

Wooded,

Emergent 

Vegetation 

Submerged 

Aquatics*

Varies With Zone:

Grass/Sedge

Emergents

Submerged Aquatics*

Wooded

Grasses

Sedges

Top Trophic Level Mammals

Birds

Amphibians

Invertebrates

Mammals

Birds

Amphibians

Invertebrates

Mammals

Birds

Mudminnows

Amphibians

Invertebrates

Fish

Birds

Mammals

Fish

Birds

Mammals

Mammals

Birds

Amphibians

Invertebrates

CommerciallyImportant Fish/

Wildlife

Waterfowl Fish* Waterfowl

Fish*

Recreational Use 

Likely

Yes Yes Yes

Drinking Water Source 

Downstream

Possible Likely Possible

Nutrient Criteria–Wetlands 4-1

Chapter 4: Sampling Design for Wetland Monitoring

4.1 INTRODUCTION

This chapter provides technical guidance on designing effective sampling programs for State wetland water 

quality monitoring programs. EPA recommends that States begin wetland monitoring programs to collect 

water quality and biological data in order to characterize the condition of existing wetlands as they develop 

nutrient criteria that will protect their wetlands. The best monitoring programs are designed to assess wetland 

conditions with statistical rigor while maximizing available resources. 

At the broadest level, monitoring data should:

1. Detect and characterize the condition of existing wetlands.

2. Describe whether wetland conditions are improving, degrading, or staying the same.

3. Define seasonal patterns, impairments, and deviations in status of wetland conditions.

Water quality monitoring programs should collect a sufficient number of samples over time and space to 

identify changes in system condition or estimate average conditions with statistical rigor. Three approaches to 

study design for assessing water quality and biological and ecological condition, and identifying degradation 

in wetlands are described in this chapter. Specific issues to consider in designing monitoring programs for 

wetland systems are also discussed in this chapter. The study designs presented here can be tailored to fit the 

goals of specific monitoring programs. 

The three approaches described below (Section 4.3) (probabilistic sampling, targeted/tiered, and Before/AfterControl/Impact [BACI]), present study designs that allow one to obtain a significant amount of information 

with relatively minimal effort. Probabilistic sampling begins with a large-scale, random monitoring design 

that is reduced as the wetland system conditions are characterized. This approach is used to find the average 

condition of each wetland class in a specific region. Probabilistic sampling design is frequently used for 

new large-scale monitoring programs at the State and Federal level (e.g., Environmental Monitoring and 

Assessment Program (EMAP), Regional Environmental Monitoring and Assessment Program (REMAP), 

State programs [e.g., Maine, Montana, Wisconsin]). The tiered or targeted approach to monitoring begins 

with coarse screening and proceeds to more detailed monitoring protocols as impaired and high-risk systems 

are identified and targeted for further investigation. Targeted sampling design provides a triage approach to 

more thoroughly assess condition and diagnose stressors in wetland systems in need of restoration, protection, 

and intensive management. Several State pilot projects use this method or a modification of this method for 

wetland assessment (e.g., Florida, Ohio, Oregon, and Minnesota). The synoptic approach described in Kentula 

et al., (1993) uses a modified targeted sampling design. The BACI design and its modifications are frequently 

used to assess the success of restoration efforts or other management experiments. BACI design allows for 

comparisons in similar systems over time to determine the rate of change in relation to the management 

activity, e.g., to assess the success of a wetland hydrologic restoration. The BACI design, in particular, is 

included to assist States in evaluating ongoing management actions, and may provide less statistical rigor if 

adopted as a general monitoring program design. This design, however, is of considerable value in assessing 

restoration success and has been included at the request of States with ongoing wetland restoration. Detenbeck 

et al., (1996) used BACI design for monitoring water quality of wetlands in the Minneapolis/St. Paul, 

Minnesota metro area. 

Monitoring programs should be designed to describe what the current conditions are and to answer under what 

conditions impairment may occur. A well-designed monitoring program can contribute to determining those 

conditions.

Sampling design is dependent on the management question being asked. Sampling efforts should be designed 
4-2 Nutrient Criteria–Wetlands

to collect information that will answer the management question. For example, probabilistic sampling might 

be good for ambient (synoptic) monitoring programs, BACI for evaluating management actions such as 

restoration, and targeted sampling/stratified and random sampling for developing Index of Biotic Integrity 

(IBIs) or nutrient criteria thresholds. In practice, some State programs likely will need to use a combination of 

approaches.

4.2 CONSIDERATIONS FOR SAMPLING DESIGN

Describing the Management Question

Clearly defining the question being asked (identifying the hypothesis) encourages the use of appropriate 

statistical analyses, reduces the occurrence of Type I (false positive) errors, and increases the efficient 

use of management resources (Suter 1993; Leibowitz et al. 1992; Kentula et al. 1993). Beginning a study 

or monitoring program with carefully defined questions and objectives helps to identify the statistical 

analyses most appropriate for the study and reduces the chance that statistical assumptions will be violated. 

Management resources are optimized because resources are directed at monitoring that which is most likely 

to answer management questions. In addition, defining the specific hypotheses to be tested, carefully selecting 

reference sites, and identifying the most useful sampling interval can help reduce the uncertainty associated 

with the results of any sampling design and further conserve management resources (Kentula et al. 1993). 

Protecting or improving the quality of a wetland system often depends on the ability of the monitoring 

program to identify cause-response relationships, for example, the relationship of nutrient concentration 

(causal variable) to nutrient content of vegetation or vegetation biomass (response variable). Cause-response 

relationships can be identified using large sample sizes and systems that span the gradient (low to high) of 

wetland quality. All ranges of response should be observed along the causal gradient from minimally disturbed 

to high levels of human disturbance.

Monitoring efforts often are prioritized to best utilize limited resources. For example, the Oregon case study 

chose not to monitor depressional wetlands due to funding constraints. They further tested the degree of 

independence of selected sites (and thus the need to monitor all of those sites) using cluster analysis and other 

statistical tests (http://www.epa.gov/owow/wetlands/bawwg/case/or.html). Frequency of monitoring should 

be determined by the management question being asked and the intensity of monitoring necessary to collect 

enough information to answer the question. In addition, monitoring should identify the watershed level 

activities that are likely to result in ecological degradation of wetland systems (Suter et al. 1993).

Site Selection

Site selection is one of many important tasks in developing a monitoring program (Kentula et al. 1993). Site 

selection for a monitoring program is based on the need to sample a sufficiently large number of wetlands to 

establish the range of wetland quality in a specific regional setting. Wetland monitoring frequently includes an 

analysis of both watershed/landscape characteristics and wetland specific characteristics (Kentula et al. 1993; 

Leibowitz et al. 1992). Therefore, wetland sampling sites should be selected based on land use in the region 

so that watersheds range from minimally impaired with few expected stressors to high levels of development 

(e.g., agriculture, forestry, or urban) with multiple expected stressors (see the Land-Use Characterization 

for Nutrient and Sediment Risk Assessment). There is often a lag in time between the causal stress and the 

response in the wetland system. This time lag between stress and response and the duration of this lag depends 

on many factors, including the type of stressor, climate, and system hydrology; these factors should be 

considered when selecting sites to establish the range of wetland quality within a region. 

Landscape Characterization

The synoptic approach described in Liebowitz et al., (1992) provides a method of rapid assessment of 

wetlands at the regional and watershed levels that can help identify the range of wetland quality within a 

region. Liebowitz et al., (1992) recommend an initial assessment for site selection based on current knowledge 

of watershed and landscape level features; modification of such an assessment can be made as more data 
Nutrient Criteria–Wetlands 4-3

are collected. Assessing watershed characteristics through aerial photography and the use of geographical 

information systems (GIS) linked to natural resource and land-use databases can aid in identifying reference 

and degraded systems (see the Land-Use Characterization for Nutrient and Sediment Risk Assessment); 

Johnston et al. 1988; 1990; Gwin et al. 1999; Palik et al. 2000; Brown and Vivas 2004). Some examples of 

watershed characteristics which can be evaluated using GIS and aerial photography include land use, land 

cover (including riparian vegetation), soils, bedrock, hydrography, and infrastructure (e.g., roads or railroads). 

Changes in point sources can be monitored through the NPDES permit program (USEPA 2000). Changes in 

nonpoint sources can be evaluated through the identification and tracking of wetland loss and/or degradation, 

increased residential development, urbanization, increased tree harvesting, shifts to more intensive 

agriculture with greater fertilizer use or increases in livestock numbers, and other land use changes. Local 

planning agencies should be informed of the risk of increased anthropogenic stress and encouraged to guide 

development accordingly.

Identifying and Characterizing Reference Wetlands

The term “reference” in this document refers to those systems that are least impaired by anthropogenic 

effects. The use of the term reference is confusing because of the different meanings that are currently in use 

in different classification methods, particularly its use in hydrogeomorphic (HGM) wetland classification. A 

discussion of the term reference and its multiple meanings is provided in Chapter 3. 

Watersheds with little or no development that receive minimal anthropogenic inputs could potentially 

contain wetlands that may serve as minimally impaired reference sites. Watersheds with a high percentage 

of the drainage basin occupied by urban areas, agricultural land, and altered hydrology are likely to contain 

wetlands that are impaired or could potentially be considered “at risk” for developing problems. Wetland 

loss in the landscape also should be considered when assessing watershed characteristics for reference 

wetland identification. Biodiversity can become impoverished due to wetland fragmentation or decreases 

in regional wetland density even in the absence of site-specific land-use activities. Reference wetlands may 

be more difficult to locate if fragmentation of wetland habitats is significant and may no longer represent 

the biodiversity of minimally disturbed wetlands in the region. The continued high rate of wetland loss in 

most States dictates that multiple reference sites be selected to ensure some consistency in reference sites for 

multiple year sampling programs (Liebowitz et al. 1992; Kentula et al. 1993). Once the watershed level has 

been considered, a more site-specific investigation can be initiated to better assess wetland condition. 

The ideal reference site will have similar soils, vegetation, hydrologic regime, and landscape setting to other 

wetlands in the region (Adamus 1992; Liebowitz et al. 1992; Kentula et al. 1993; Detenbeck et al. 1996). 

Classification of wetlands, as discussed in Chapter 3, may aid in identifying appropriate reference wetlands for 

specific regions and wetland types. Wetland classification should be supplemented with information on wetland 

hydroperiod to assure that the selected reference wetlands are truly representative of wetlands in the region, 

class, or subclass of interest. Reference wetlands may not be available for all wetland classes. In that case, 

data from systems that are as close as possible to the assumed unimpaired state of wetlands in the wetland 

class of interest should be sought from States within the same geologic province. Development of a conceptual 

reference may be important if appropriate reference sites cannot be found in the local region or geologic 

province. Techniques for defining a conceptual reference are discussed at some length in Harris et al., (1995), 

Trexler (1995), and Toth et al., (1995). 

Reference wetlands should be selected based on low levels of human alteration in their watersheds (Liebowitz 

et al. 1992; Kentula et al. 1993; USEPA 2000). Selecting reference wetlands usually involves assessment of 

land-use within watersheds and visits to individual wetland systems to ground-truth expected land-use and 

check for unsuspected impacts. Ground-truthing visits to reference wetlands are crucial for identification of 

ecological impairment that may not be apparent from land-use and local habitat conditions. Again, sufficient 

sample size is important to characterize the range of conditions that can be expected in the least impacted 

systems of the region (Detenbeck et al. 1996). Reference wetlands should be identified for each ecoregion or 
4-4 Nutrient Criteria–Wetlands

geological province in the State lands and then characterized with respect to ecological integrity. A minimum 

of three low impact reference systems is recommended for each wetland class for statistical analyses. However, 

power analysis can be performed to determine the degree of replication necessary to detect an impact to the 

systems being investigated (Detenbeck et al. 1996; Urquhart et al. 1998). Highest priority should be given to 

identifying reference systems for those wetland types considered to be at the greatest risk from anthropogenic 

stress. 

When to Sample

Sampling may be targeted to the periods when effects are most likely to be detected – the index period. The 

appropriate index period should be defined by what the investigator is trying to investigate and what taxonomic 

assemblage or parameters are being used for that investigation (Barbour et al. 1999). For example, increased 

nutrient concentrations and sedimentation from non-point sources may occur following periods of high runoff 

during spring and fall, while point sources of nutrient pollutants may cause plankton blooms and/or increased 

water and soil nutrient concentrations in wetland pools during times of low rainfall. Hence, different index 

periods may be needed to detect effects from point source and nonpoint source nutrients, respectively. Each 

taxonomic assemblage studied also should have an appropriate index period—usually in the growing season 

(see assemblage methods in the Maine case study: http://www.epa.gov/waterscience/criteria/wetlands/).

The index period window may be early in the growing season for amphibians and algae. Other assemblages, 

such as vegetation and birds, may benefit from a different sampling window for the index period; see the 

assemblage specific modules for recommendations. Once wetland condition has been characterized, onetime annual sampling during the appropriate index period may be adequate for multiple year monitoring of 

indicators of nutrient status, designated use, and biotic integrity. However, criteria and ecological indicator 

development may benefit from more frequent sampling to define conditions that relate to the stressor or 

perturbor of interest (Karr and Chu 1999; Stevenson 1996; Stevenson 1997). Regardless of the frequency of 

sampling, selection of index periods and critical review of the data gathered and analyzed should be done to 

scientifically validate the site characterization and index periods for data collection.

Ideally, water quality monitoring programs produce long-term data sets compiled over multiple years 

to capture the natural, seasonal, and year-to-year variations in biological communities and constituent 

concentrations (e.g., Tate 1990; Dodds et al. 1997; McCormick et al. 1999; Craft 2001; Craft et al. 2003; Zheng 

et al. 2004). Multiple-year data sets can be analyzed with statistical rigor to identify the effects of seasonality 

and variable hydrology. Once the pattern of natural variation has been described, the data can be analyzed to 

determine the ecological state of the wetland. Long-term data sets have also been important in influencing 

management decisions about wetlands, most notably in the Everglades, where long-term data sets have induced 

Federal, State, and Authorized Tribal actions for conservation and restoration of the largest wetland system in 

the U.S. (see Davis and Ogden 1994; Everglades Interim Report, South Florida Water Management District 

[SFWMD 1999]; Everglades Consolidated Report [SFWMD 2000, 2001]; 1994 Everglades Forever Act, 

Florida Statute § 373.4592). 

In spite of the documented value of long-term data sets, there is a tendency to intensively study a wetland 

for one year before and one year after treatment. A more cost-effective approach may be to measure only the 

indices most directly related to the stressor of interest (i.e., those parameters or indicators that provide the 

best information to answer the specific management question), but to double or triple the monitoring period. 

Multiple years (two or more) of data are often needed to identify the effects of years with extreme climatic 

or hydrologic conditions. Comparisons over time between reference and at risk or degraded systems can help 

describe biological response and annual patterns in the presence of changing climatic conditions. Multi-year 

data sets also can help describe regional trends. Flooding or drought may significantly affect wetland biological 

communities and the concentrations of water column and soil constituents. Effects of uncommon climatic 

events can be characterized to discern the overall effect of management actions (e.g., nutrient reduction, water 

diversion) if several years of data are available to identify the long-term trends. 
Nutrient Criteria–Wetlands 4-5

At the very minimum, two years of data before and after specific management actions, but preferably three 

or more each, are recommended to evaluate the cost-effectiveness of management actions with some degree 

of certainty (USEPA 2000). If funds are limited, restricting sampling frequency and/or numbers of indices 

analyzed should be considered to preserve a longer-term data set. Reducing sampling frequency or numbers of 

parameters measured will allow for effectiveness of management approaches to be assessed against the high 

annual variability that is common in most wetland systems. Wetlands with high hydrological variation from 

year to year may benefit from more years of sampling both before and after specific management activities to 

identify the effects of the natural hydrologic variability (Kadlec and Knight 1996).

Characterizing Precision of Estimates

Estimates of cause-response relationships, nutrient and biological conditions in reference systems, and wetland 

conditions in a region are based on sampling; hence, precision should be assessed. Precision is defined as 

the “measure of the degree of agreement among the replicate analyses of a sample, usually expressed as the 

standard deviation” (APHA 1999). Determining precision of measurements for one-time assessments from 

single samples in a wetland is often important. The variation associated with one-time assessments from single 

samples can be determined by re-sampling a specific number of wetlands during the survey. Measurement 

variation among replicate samples then can be used to establish the expected variation for one-time assessment 

of single samples. Re-sampling does not establish the precision of the assessment process, but rather identifies 

the precision of an individual measurement (Kentula et al. 1993).

Re-sampling frequency is often conducted for one wetland site in every block of 10 sites. However, 

investigators should adhere to the objectives of re-sampling (often considered an essential element of Quality 

Assurance/Quality Control (QA/QC)) to establish an assessment of the variation in a one-time/sample 

assessment. Often, more than one in 10 samples should be replicated in monitoring programs to provide a 

reliable estimate of measurement precision (Barbour et al. 1999). The reader should understand that this is a 

very brief description of the concerns about precision, and that any monitoring program or study involving 

monitoring should include consultation with a professional statistician before the program begins and regularly 

during the course of the monitoring program to assure statistical rigor.

4.3 SAMPLING PROTOCOL

Approaches to Sampling Design

The following sections discuss three different approaches to sampling design, probabilistic, targeted, and 

BACI. These approaches have advantages and disadvantages that under different circumstances warrant the 

choice of one approach over the other (Table 4). The decision as to the best approach for sample design in a 

new monitoring program should be made by the water quality resource manager or management team after 

careful consideration of the different approaches. For example, justification of a dose-response relationship is 

confounded by lack of randomization and replication and should be considered in choosing a sampling design 

for a monitoring program. 

Probalistic Sampling Design for Assessing Condition

Probabilistic sampling – a sampling process wherein randomness is requisite (Hayek 1994) – can be used to 

characterize the status of water quality conditions and biotic integrity in a region’s wetland system. This type 

of sampling design is used to describe the average conditions of a wetland population, identify the variability 

among sampled wetlands, and help determine the range of wetland system conditions in a region. Data 

collected from a probabilistic random sample design generally will be characteristic of the dominant class or 

type of wetland in the region, but rare wetlands may be under-represented or absent from the probabilistically 

sampled wetlands. Additional sampling sites may need to be added to precisely characterize the complete 

range of wetland conditions and types in the region. 
4-6 Nutrient Criteria–Wetlands

Probabilistic designs are often modified by stratification (such as classification). Stratified random sampling 

is a type of probabilistic sampling where a target population is divided into relatively homogenous groups or 

classes (strata) prior to sampling based on factors that influence variability in that population (Hayek 1994). 

Stratification by wetland size and class or types ensures more complete information about different types of 

wetlands within a region. Sample statistics from random selection alone would be most characteristic of the 

dominant wetland type in a region if the population of wetlands is not stratified.

Many State 305(b) and watershed monitoring programs utilize stratified random sampling designs, and we 

will further discuss this type of probabilistic sampling. Pilot projects in Maine, Montana, and Wisconsin all 

use stratified random sampling design. Details of these monitoring designs can be found in the Case Studies 

Module #14 and on the Web at http://www.epa.gov/waterscience/criteria/wetlands/index.html. 

Stratification is based on identifying wetland systems in a region (or watershed) and then selecting an 

appropriate sample of systems from the defined population. The determination of an appropriate sample 

population usually is dependent on the management questions being asked. A sample population of isolated 

depressional wetlands could be identified as a single stratum, but investigations of these wetlands would not 

provide any information on riparian wetlands in the same region. If the goal of the monitoring program is to 

identify wetland condition for all wetland classes within a region, then a sample population of wetlands should 

be randomly selected from all wetlands within each class. In practice, most State programs stratify random 

populations by size, wetland class (see Chapter 3), and landscape characteristics or location (see http://www.

epa.gov/waterscience/criteria/wetlands/, http://www.epa.gov/waterscience/criteria/wetlands/17LandUse.pdf). 

Once the wetlands for each stratum have been identified, the list of wetlands can be used to select a spatiallybalanced stratified random sample. Spatial-balance will ensure spatial coverage over the assessment region, 

usually increase the types of wetlands sampled (assuming classes of wetlands vary spatially), and reduce 

spatial autocorrelation among the sampled wetlands. For example, EMAP implements spatially-balanced 

samples using Generalized Random Tessellation Stratified (GRTS) designs applied to GIS coverages of 

wetlands within the assessment region. GRTS using a hierarchical grid randomization process to ensure the 

sites are spatially distributed (Paulsen et al. 1991; Stevens and Olsen 2004). Estimates of ecological conditions 

from these kinds of modified probabilistic sampling designs can be used to characterize the water quality 

conditions and biological integrity of wetland systems in a region, and over time, to distinguish trends in 

ecological condition within a region. (See http://www.epa.gov/owow/wetlands/bawwg/case/mtdev.html and 

http://www.epa.gov/owow/wetlands/bawwg/case/fl1.html). 

Targeted Design

A targeted approach to sampling design may be more appropriate when resources are limited (Stern 2004). The 

example of targeted sampling described here involves defining a gradient of impairment. Once the gradient has 

been defined and systems have been placed in categories of impairment, investigators focus the greatest efforts 

on identifying and characterizing wetland systems or sites likely to be impacted by anthropogenic stressors, 

and on relatively undisturbed wetland systems or sites (see Identifying and Characterizing Reference Systems, 

Chapter 3), that can serve as regional, sub-regional, or watershed examples of natural biological integrity. 

Florida Department of Environmental Protection (FDEP) uses a targeted sampling design for developing 

thresholds of impairment with macroinvertebrates (http://www.epa.gov/owow/wetlands/bawwg/case/fl2.html). 

Choosing sampling stations that best allow comparison of ecological integrity at reference wetland sites of 

known condition can conserve financial resources. A sampling design that tests specific hypotheses (e.g., the 

FDEP study tested the effect of elevated water column phosphorus on macroinvertebrate species richness) 

generally can be analyzed with statistical rigor and can conserve resources by answering specific questions. 

Furthermore, identification of systems with problems and reference conditions eliminates the need for 

selecting a random sample of the population for monitoring. 
Nutrient Criteria–Wetlands 4-7

Targeted sampling assumes some knowledge of the systems sampled (Stern 2004; Kentula et al. 1993). Systems 

based on independent variables with evidence of degradation are compared to reference systems that are 

similar in their physical structure (i.e., in the same class of wetlands). Wetland systems should be viewed along 

a continuum from reference to degraded. An impaired or degraded wetland is a system in which anthropogenic 

impacts exceed acceptable levels or interfere with beneficial uses. Comparison of the monitoring data to that 

collected from reference wetlands will allow characterization of the sampled systems. Wetlands identified as 

“at risk” should be evaluated through a sampling program to characterize the degree of degradation. Once 

characterized, the wetlands should be placed in one of the following categories:

1. Degraded wetlands—wetlands in which the level of anthropogenic perturbance interferes 

 with designated uses.

2. High-risk wetlands—wetlands where anthropogenic stress is high but does not significantly 

 impair designated uses. In high risk systems, impairment is prevented by one or a few 

 factors that could be changed by human actions, though characteristics of ecological integrity are 

 already marginal.

3. Low-risk wetlands—wetlands where many factors prevent impairment, stressors are maintained 

 below problem levels, and/or no development is contemplated that would change these conditions.

4. Reference wetlands—wetlands where the ecological characteristics most closely represent the 

 pristine or minimally impaired condition.

Once wetland systems have been classified based on their physical structure (see Chapter 3) and placed into the 

above categories, specific wetlands need to be selected for monitoring. At this point, randomness is introduced; 

wetlands should be randomly selected within each class and risk category for monitoring. An excellent 

example of categorizing wetlands in this manner is given in the Ohio Environmental Protection Agency’s (OH 

EPA) case study, available at: http://www.epa.gov/owow/wetlands/bawwg/case/oh1.html. They used the Ohio 

Rapid Assessment Method to categorize wetlands by degree of impairment. The Minnesota Pollution Control 

Agency (MPCA) also used a targeted design for monitoring wetlands (http://www.epa.gov/owow/wetlands/

bawwg/case/mn1.html). They used the best professional judgment of local resource managers to identify 

reference sites and those with known impairment from identified stressors (agriculture and stormwater runoff). 

Targeted sampling design involves monitoring identified degraded systems and comparable reference systems 

most intensively. Low risk systems are monitored less frequently (after initial identification) unless changes in 

the watershed indicate an increased risk of degradation. 

Activities surrounding impaired wetland systems may be used to help identify which actions negatively affect 

wetlands, and therefore may initiate more intensive monitoring of at-risk wetlands. Monitoring should focus on 

factors likely to identify ecological degradation and anthropogenic stress and on any actions that might alter 

those factors. State water quality agencies should encourage adoption of local watershed protection plans to 

minimize ecological degradation of natural wetland systems. Development plans in the watershed should be 

evaluated to identify potential future stressors. Ecological degradation often gradually increases due to many 

growing sources of anthropogenic stress. Hence, frequent monitoring may be warranted for high-risk wetlands 

if sufficient resources remain after meeting the needs of degraded wetlands. Whenever development plans 

appear likely to alter factors that maintain ecological integrity in a high-risk wetland (e.g., vegetated buffer 

zones), monitoring should be initiated at a higher sampling frequency in order to enhance the understanding of 

baseline conditions (USEPA 2000).

Before/After Control/Impact (BACI) Design

An ideal before/after impact survey has several features: (1) the type of impact, time of impact, and place of 

occurrence should be known in advance; (2) the impact should not have occurred yet; and, (3) control areas 
4-8 Nutrient Criteria–Wetlands

should be available (Green 1979). The first feature allows the surveys to be efficiently planned to account for 

the probable change in the environment. The second feature allows a baseline study to be established and 

extended as needed. The last feature allows the surveyor to distinguish between temporal effects unrelated 

to the impact and changes related to the impact. In practice however, advance knowledge of specific impacts 

is rare, and the ideal impact survey is rarely conducted. BACI designs modified to monitor impacts during or 

after their occurrence still can provide information, but there is an increase in the uncertainty associated with 

the results and the likelihood of finding a statistically significant change due to the impact is less probable. 

In addition, other aspects of survey design are dependent on the study objectives, e.g., the sampling interval, 

the length of time the survey is conducted (i.e., sampling for acute versus chronic effects), and the statistical 

analyses appropriate for analyzing the data (Suter 1993). 

The best interval for sampling is determined by the objectives of the study (Kentula et al. 1993). If the 

objective is to detect changes in trends (e.g., regular monitoring for detection of changes in water quality 

or biotic integrity), regularly spaced intervals are preferred because the analysis is easier. On the other 

hand, if the objective is to assess differences before and after impact, then samples at random time points 

are advantageous. Random sample intervals reduce the likelihood that cyclic differences unforeseen by the 

sampler will influence the size of the difference before and after the impact. For example, surveys taken 

every summer for a number of years before and after a clear-cut may show little difference in system quality; 

however, differences may exist that can only be detected in the winter and therefore may go undetected if 

sampling occurs only during summer.

The simplest impact survey design involves taking a single survey before and after the impact event (Green 

1979). This type of design has the obvious pitfall that there may be no relationship between the observed event 

and the changes in the response variable—the change may be entirely coincidental. This pitfall is addressed 

in BACI design by comparing before and after impact data to data collected from a similar control system 

nearby. Data are collected before and after a potential disturbance in two areas (treatment and a control), with 

measurements on biological and environmental variables in all combinations of time and area (Green 1979). 

We will use a clear-cut adjacent to a wetland as an example to illustrate the BACI design. The sampling design 

is developed to identify the effects of clear-cutting on adjacent wetland systems. In the simplest BACI design, 

two wetlands would be sampled. One wetland would be adjacent to the clear-cut (the treatment wetland); 

the second wetland would be adjacent to a control site that is not clear-cut. The control site should have 

characteristics (soil, vegetation, structure, functions) similar to the treatment wetland and is exposed to climate 

and weather similar to the first wetland. Both wetlands are sampled at the same time points before the clear-cut 

occurs and at the same time point after the clear-cut takes place. This design is technically known as an areaby-time factorial design. Evidence of an impact is found by comparing the control site samples (before and 

after) with the treatment site before and after samples. Area-by-time factorial design allows for both natural 

wetland-to-wetland variation and coincidental time effects. If there is no effect of the clear-cut, then change in 

system quality between the two time points should be the same. If there is an effect of the clear-cut, the change 

in system quality between the two time points should be different. 

Considerations for BACI Design

There are some potential problems with BACI design. First, because the control and impact sites are not 

randomly assigned, observed differences between sites may be related solely to some other factor that differs 

between the two sites. One could argue that it is unfair to ascribe the effect to the impact (Hurlbert 1984; 

Underwood 1991). However, as pointed out by Stewart-Oaten et al., (1986), the survey is concerned about 

a particular impact in a particular place, not in the average of the impact when replicated in many different 

locations. Consequently, it may be possible to detect a difference between these two specific sites. Even so, if 

there are no randomized replicate treatments, the results of the study cannot be generalized to similar events 

at different wetlands. In any case, the likelihood that the differences between sites are due to factors other than 

the impact can be reduced by monitoring several control sites (Underwood 1991) because multiple control sites 

provide some information about potential effects of other factors. 
Nutrient Criteria–Wetlands 4-9

The second and more serious concern with the simple Before-After design with a single sampling point before 

and after the impact is that it fails to recognize that there may be natural fluctuations in the characteristic 

of interest that are unrelated to any impact (Hurlbert 1984; Stewart-Oaten 1986). Single samples before and 

after impact would be sufficient to detect the effects of the impact if there were no natural fluctuations over 

time. However, if the population also has natural fluctuations over and above the long-term average, then 

it is impossible to distinguish between cases where there is no effect from cases where there is an impact. 

Consequently, measured differences in system quality may be artifacts of the sampling dates and natural 

fluctuations may obscure differences or lead one to believe differences are present when they are not. 

The simple BACI design was extended by Stewart-Oaten et al., (1986) by pairing surveys at several selected 

time points before and after the impact to help resolve the issue of psuedoreplication (Hulbert 1984). This 

modification of the BACI design is referred to as BACI-PS (Before-After, Control-Impact Paired Series 

design). The selected sites are measured at the same time points. The rationale behind this paired design is 

that repeated sampling before the impact gives an indication of the pattern of differences of potential change 

between the two sites. BACI-PS study design provides information both on the mean difference in the wetland 

system quality before and after impact and on the natural variability of the system quality measurements. 

The resource manager has detected an effect if the changes in the mean difference are large relative to natural 

variability. Considerations for sampling at either random or regularly spaced intervals also apply here. 

Replication of samples should also be included if resources allow in order to improve certainty of analytical 

results. 

Violation of the BACI assumptions may invalidate conclusions drawn from the data. Enough data should 

be collected before the impact to identify the trends in the communities of each sampling site if the BACI 

assumptions are to be met. Clearly defining the objectives of the study and identifying a statistically testable 

model of the relationships the investigator is studying can help resolve these issues (Suter 1993). 

The designs described above are suitable for detecting longer-term chronic effects in the mean level of the 

variable of interest. However, the impact may have an acute effect (i.e., effects only last for a short while) 

or may change the variability in response (e.g., seasonal changes become more pronounced) in some cases. 

The sampling schedule can be modified so that it occurs at two temporal scales (enhanced BACI-PS design) 

that encompass both acute and chronic effects (Underwood 1991). The modified temporal design introduces 

randomization by randomly choosing sampling occasions in two periods (Before and After) in the control or 

impacted sites. The two temporal scales (sampling periods vs. sampling occasions) allow the detection of a 

change in mean and of a change in variability after impact. For example, groups of surveys could be conducted 

every year with five surveys one week apart randomly located within each group. The analysis of such a design 

is presented in Underwood (1991). Again, multiple control sites should be used to counter the argument that 

detected differences are specific to the sampled site. The September 2000 issue of the Journal of Agricultural, 

Biological, and Environmental Statistics discusses many of the advantages and disadvantages of the BACI 

design and provides several examples of appropriate statistical analyses for evaluation of BACI studies.
4-10 Nutrient Criteria–Wetlands

Table 4: Comparison of Probabilistic, Targeted, and BACI Sampling Designs.

Probabilistic Targeted BACI

Random selection of wetland systems from entire 

population within a region.

Targeted selection of wetlands based on problematic 

(wetland systems known to have problems) and 

reference wetlands.

Selection of wetlands based on a known impact.

This design requires minimal prior knowledge of 

wetlands within the sample population for stratification.

This design requires prior knowledge of wetlands 

within the sample population.

This design requires knowledge of a specific 

impact to be analyzed.

This design may use more resources (time and money) to 

randomly sample wetland classes because more wetlands 

may need to be sampled.

This design utilizes fewer resources because only 

targeted systems are sampled.

This design may use fewer resources because 

only wetlands with known impacts and 

associated control systems are sampled.

System characterization for a class of wetlands is more 

statistically robust.

System characterization for a class of wetlands is less 

statistically robust, although characterization of a 

targeted wetland may be statistically robust.

Characterization of the investigated systems is 

statistically robust.

Rare wetlands may be under- represented or absent from 

the sampled wetlands.

This design may miss important wetland systems if 

they are not selected for the targeted investigation.

The information gained in this type of 

investigation is not transferable to wetland 

systems not included in the study.

This design is potentially best for regional 

characterization of wetland classes, especially if water 

quality conditions are not known.

This design is potentially best for site-specific and 

watershed-specific criteria development when water 

quality conditions for the wetland of interest are 

known.

This design is potentially best for monitoring 

restoration or creation of wetlands and systems 

that have specific known stressors.

4.4 SUMMARY

State monitoring programs should be designed to assess wetland condition with statistical rigor while 

maximizing available management resources. The three approaches described in this module—probabilistic 

sampling, targeted/tiered approach, and BACI (Before/After, Control/Impact)—present study designs that 

allow one to obtain a significant amount of information for statistical analyses. The sampling design selected 

for a monitoring program should depend on the management question being asked. Sampling efforts should 

be designed to collect information that will answer management questions in a way that will allow robust 

statistical analysis. In addition, site selection, characterization of reference sites or systems, and identification 

of appropriate index periods are all of particular concern when selecting an appropriate sampling design. 

Careful selection of sampling design will allow the best use of financial resources and will result in the 

collection of high quality data for evaluation of the wetland resources of a State. Examples of different sampling 

designs currently in use for State wetland monitoring are described in the Case Study Module #14 on the Web 

site http://www.epa.gov/waterscience/criteria/wetlands/. Well-designed monitoring programs tend to produce 

data that managers can use in nutrient criteria development, such as in developing reference networks or 

utilizing distribution-based approaches.
Nutrient Criteria–Wetlands 5-1

Chapter 5: Candidate Variables for Establishing Nutrient Criteria

5.1 OVERVIEW OF CANDIDATE VARIABLES

This chapter provides an overview of candidate variables that could be used to establish nutrient criteria for 

wetlands. A more detailed discussion of sampling methods and laboratory analysis with useful references can 

be found in the Methods for Evaluating Wetland Condition module series for sampling wetlands4,5 at: http://

www.epa.gov/waterscience/criteria/wetlands/. 

A good place to start with selecting candidate variables is by developing a conceptual model of how human 

activities affect nutrients and wetlands. These conceptual models may vary from complex to very simple 

models, such as relating nitrogen concentrations in sediments and plant biomass or species composition. 

Conceptual models establish the detail and scope of the project and the most important variables to select. In 

addition, they define the cause-effect relationships that should be documented to determine whether a problem 

occurs and what is causing the problem. 

In general, for the purposes of numeric nutrient criteria development, it is helpful to develop an understanding 

of the relationships among human activities, nutrients and habitat alterations, and attributes of ecosystem 

structure and function to establish a simple causal pathway among three basic elements in a conceptual 

model. These three basic groups of variables are important to distinguish because we use them differently 

in environmental management (Stevenson et al. 2004a). A fourth group of variables is important in order to 

account for variation in expected condition of wetlands due to natural variation in landscape setting.

The overview of candidate variables in this chapter follows the outline provided in the conceptual model 

in Figure 5.1. Historically, variables in conceptual models have been grouped many ways with a variety of 

group names (Paulsen et al. 1991; Stevenson 1998; Stevenson 2004a, b). In this document, three groups and 

group names are used to emphasize cause-effect relationships, simplify their presentation and discussion for a 

diversity of audiences, and maintain some continuity between their use in the past and their use here. The three 

groups are: supporting variables, causal variables, and response variables. 

Supporting variables provide information useful in normalizing causal and response variables and categorizing 

wetlands. (These are in addition to characteristics used to define wetland classes as described in Chapter 3.) 

Causal variables characterize pollution or habitat alterations.

Causal variables are intended to characterize nutrient availability in wetlands and could include nutrient 

loading rates and soil nutrient concentrations. Response variables are direct measures or indicators of 

ecological properties. Response variables are intended to characterize biotic response and could include 

community structure and composition of vegetation and algae. The actual grouping of variables is much less 

important than understanding relationships among variables.

It is important to recognize the complex temporal and spatial structure of wetlands when measuring or 

interpreting causal and response variables with respect to nutrient condition. The complex interaction of 

climate, geomorphology, soils, and internal interactions has led to a diverse array of wetland types ranging 

from infrequently flooded, isolated depressional wetlands such as seasonal prairie potholes and playa lakes, to 

very large, complex systems such as the Everglades and the Okefenokee Swamp. In addition, most wetlands 

are complex temporal and spatial mosaics of habitats with distinct structural and functional characteristics 

illustrated most visibly by patterns in vegetation structure. 

4

EPA is developing and revising additional modules as a part of the Methods for Evaluating Wetland Conditions Module Series–Biogeochemical Indicators, Wetland Hydrology, and Nutrient Loading Estimation.

5

The references for these modules can be found in the Supplementary References following the References section.
5-2 Nutrient Criteria–Wetlands

Agriculture Urban Dev.

No-till Row Crops

Channelization

Human Activities

Causal Response

Climate Geology

Soils

Topography

Hydrology

Natural Factors

Nitrogen Phosphorus

Invasive Species

Hydrologic 

Alterations

Stressors: Contaminants & Habitat Alterations

Ecosystem Structure

•Biomass

•Biodiversity

Flora & Fauna

Ecosystem Function

•Productivity

•Nutrient Retention

•Hydrologic Regulation

Responses: Valued Ecological Attributes

supporting

Riparian Buffers

Figure 5.1: Conceptual model of causal pathway between human activities and ecological attributes

This conceptual model illustrates the casual pathway between human activities and valued ecological attributes. It includes 

the role of nutrients in a broader context that includes natural variation among wetlands. The relationship between different 

approaches of grouping variables is illustrated to emphasize the importance of cause/effect relationships. Here, natural factors 

and human activities regulate the physical, chemical and biological attributes of wetlands. Some wetland attributes are more 

valued than others and provide the endpoints of assessment and management. Some physical, chemical, and biological attributes 

are stressors, (i.e., contaminants and habitat alterations) caused by human activities that negatively affect valued ecological 

attributes. The overview of variables in Chapter 5 is organized in three sections: supporting, causal, and response variables. 

Supporting variables are natural landscape level factors that classify expected conditions of wetlands. Causal factors “cause” 

effects in response variables.

Horizontal zonation is a common feature of wetland ecosystems, and in most wetlands, relatively distinct 

bands of vegetation develop in relation to water depth. Bottomland hardwood forests and prairie pothole 

wetlands provide excellent illustrations of zonation in two very divergent wetland types. However, vegetation 

zones are not static. Seasonal and long-term changes in vegetation structure are a common characteristic of 

most wetland ecosystems. Wetlands may exhibit dramatic shifts in vegetation patterns in response to changes 

in hydrology, with entire wetlands shifting between predominantly emergent vegetation to completely open 

water within only a year or two. Such temporal patterns in fact are important features of many wetlands 

and should be considered in interpreting any causal or response variable. For example, seasonal cycles are 

an essential feature of floodplain forests, which are typically flooded during high spring flows but dry by 

mid to late summer. Longer-term cycles are similarly essential features of prairie pothole wetlands, which 

exhibit striking shifts in vegetation in response to water level fluctuations over periods of a few years in 

smaller wetlands to decades in larger, more permanent wetlands (van der Valk 2000). Vegetation patterns can 

significantly affect the physical and chemical characteristics of sediments and overlying waters and are likely 

to control major aspects of wetland biogeochemistry and trophic dynamics (Rose and Crumpton 1996).
Nutrient Criteria–Wetlands 5-3

The complex temporal and spatial structure of wetlands should influence the selection of variables to measure 

and methods for measuring them. Most wetlands are characterized by extremely variable hydrologic and 

nutrient loading rates and close coupling of soil and water column processes. As a result, estimates of 

nutrient loading may prove more useful than direct measurements of water column nutrient concentrations as 

causal variables for establishing the nutrient condition of wetlands. In addition, soil nutrients that integrate a 

wetland’s variable nutrient history over a period of years may provide the most useful metric against which to 

evaluate wetland response.

5.2 SUPPORTING VARIABLES

Supporting variables are not intended to characterize nutrient availability or biotic response but, rather, to 

provide information that can be useful in normalizing causal and response variables. Below is a brief overview 

of supporting variables that might be useful for categorizing wetlands and for normalizing and interpreting 

causal and response variables. 

Conductivity

Conductivity (also called electrical conductance or specific conductance) is an indirect measure of total 

dissolved solids. This is due to the ability of water to conduct an electrical current when there are dissolved 

ions in solution—water with higher concentrations of dissolved inorganic compounds have higher 

conductivity. Conductivity is commonly measured in situ using a handheld probe and conductivity meter 

(APHA 1999) or using automated conductivity loggers. Because the conductivity changes with temperature, 

the raw measurement should be adjusted to a reference temperature of 25oC. A multiplier of 0.7 is commonly 

applied to estimate the total dissolved solids concentration (mg/L) in fresh water when the conductivity is 

measured in units of microsiemens per centimeter (μS/cm), although this multiplier varies with the types of 

dissolved ions and should be adjusted for local chemical conditions.

Conductivity is a useful tool for characterizing wetland inputs and interpreting nutrient condition because of 

its sensitivity to changes in these inputs. Rainfall tends to have lower conductivity than surface water, with 

ground water often having higher values due to the longer residence time of water in the subsurface. Coastal 

and marine waters—as well as water in terminal lakes and wetlands—have even higher conductivity due to 

the influence of salinity. Municipal and industrial discharges often have higher conductivity than their intake 

waters due to the addition of soluble wastes. Wetland hydrologic inputs can be identified by comparing the 

measured input conductivity with the conductivity of potential local sources. 

Soil pH 

Soil pH can be important for categorizing wetland soils and interpreting soil nutrient variables. The pH 

of wetland soils and water varies over a wide range of values. Many ombrotrophic organic wetland soils 

(histosols) such as bogs and non-limestone based wetlands are often acidic, and mineral wetland soils are 

frequently neutral or alkaline. Flooding a soil results in consumption of electrons and protons. In general, 

flooding acidic soils results in an increase in pH, and flooding alkaline soils decreases pH (Mitsch and 

Gosselink 2000). The increase in pH of low pH (acidic) wetland soils is largely due to the reduction of iron and 

manganese oxides. However, the initial decrease in pH of alkaline wetland soils is due to rapid decomposition 

of soil organic matter and accumulation of CO2

. The decrease in pH that generally occurs when alkaline soils 

are flooded results from the buildup of CO2 and carbonic acid. In addition, the pH of alkaline soils is highly 

sensitive to changes in the partial pressure of CO2. Carbonates of iron and manganese also can buffer the pH 

of soil to neutrality. Soil pH determinations should be made on wet soil samples. Once the soils are air-dried, 

oxidation of various reduced compounds results in a decrease in pH and the values may not represent ambient 

conditions.
5-4 Nutrient Criteria–Wetlands

Soil pH is measured using commercially available combination electrodes on soil slurries. If air dry or moist 

soil is used, a 1:1 soil to water ratio should be used. For details on methodology, the reader is referred to 

Thomas (1996).

Soil pH can explain the availability and retention capacity of phosphorus. For example, phosphorus 

bioavailability is highest at soil pH near neutral conditions. For mineral soils, phosphorus adsorption capacity 

has been directly linked to extractable iron and aluminum. For details, the reader is referred to Supplementary 

References.

Soil Bulk Density

Soil bulk density is the mass of dry solids per unit volume of soil, which includes the volume of solids plus 

air- and water-filled pore space. Bulk density is a useful parameter for expressing the concentration of nutrients 

on a volume basis, rather than mass basis. For example, concentration of nutrients in organic wetland soils can 

be high when expressed on a mass basis (mg/kg or μg/g of dry soil), as compared to mineral wetland soils. 

However, the difference in concentration may not be as high when expressed on a volume (cm3) basis, which is 

calculated as the product of bulk density and nutrient concentration per gram of soil. Expressing soil nutrient 

concentrations on a volume basis is especially relevant to uptake by vegetation since plant roots explore 

a specific volume, not mass, of soil. Expressing nutrients on a volume basis also helps in calculating total 

nutrient storage in a defined soil layer. 

Bulk density is measured by collecting an intact soil core of known volume at specific depths in the soil (Blake 

and Hartge 1986). Cores are oven-dried at 70oC and weighed. Bulk density is calculated as follows:

Bulk density (dry) (g/cm3) = mass dry weight (grams)/volume (cm3)

Bulk densities of wetland organic soils range from 0.1 to 0.5 g/cm3, whereas bulk densities of mineral wetland 

soils range from 0.5 to 1.5 g/cm3. Soil bulk densities are directly related to soil organic matter content, as bulk 

densities decrease with increases in soil organic matter content.

Soil Organic Matter Content

Soil organic matter can be important for categorizing wetland soils and interpreting soil nutrient variables. 

Wetland soils often are characterized by the accumulation of organic matter because rates of primary 

production often exceed rates of decomposition. Some wetlands accumulate thick layers of organic matter that, 

over time, form peat soil. Organic matter provides nutrient storage and supply, increases the cation exchange 

capacity of soils, enhances adsorption or deactivation of organic chemicals and trace metals, and improves 

overall soil structure, which results in improved air and water movement. A number of methods are now 

routinely used to estimate soil organic matter content expressed as total organic carbon or loss on ignition 

(APHA 1999; Nelson and Sommers 1996).

Soil organic matter content represents the soil organic carbon content of soils. Typically, soil organic matter 

content is approximately 1.7 to 1.8 times that of total organic carbon. The carbon to nitrogen and carbon to 

phosphorus ratios of soils can provide an indication of nutrient availability in soils. 

Hydrolic Condition

Wetland hydrologic condition is important for characterizing wetlands and for normalizing many causal and 

response variables. Hydrologic conditions can directly affect the chemical and physical processes governing 

nutrient and suspended solids dynamics within wetlands (Mitsch and Gosselink 2000). Detailed, site-specific 

hydrologic information available is best, but at a minimum, some estimate of water level fluctuation should 

be made. A defining characteristic of wetlands is oxygen deficiency in the soil caused by flooding or soil 

saturation. These conditions influence vegetation dynamics through differential growth and survival of plant 

species and also exert significant control over biogeochemical processes involved in carbon flow and nutrient 
Nutrient Criteria–Wetlands 5-5

cycling within wetlands. Spatial and temporal patterns in hydrology can create complex patterns in soil and 

water column oxygen availability, including alternating aerobic and anaerobic conditions in wetland soils, with 

obvious implications for plant response and biogeochemical process dynamics. Water levels in wetlands can 

be determined using a staff gauge when surface water is present. A staff gauge measures the depth of surface 

flooding relative to a reference point such as the soil surface. Other methods to assess past water levels when 

standing water is not present include moss collars, staining, and cypress knee heights. While surface flooding 

may be rare or absent in a wetland, high water tables may still cause soil saturation in the rooting zone. In 

wetlands where soils are saturated, water level can be measured with a small diameter perforated tube installed 

in the soil to a specified depth (Amoozegar and Warrick 1986). Automated water level recorders using floats, 

capacitance probes, or pressure transducers are suitable for measuring water levels both above- and belowground. The reader is referred to the Supplementary References for details.

5.3 CAUSAL VARIABLES

Causal variables are intended to characterize nutrient availability in wetlands. Most wetlands are characterized 

by extremely variable nutrient loading rates and close coupling of soil and water column processes. As a 

result, estimates of nutrient loading and measurements of soil nutrients may prove more useful than direct 

measurements of water column nutrient concentrations as causal variables for establishing the nutrient 

condition of wetlands. Nutrient loading history and soil nutrient measures can integrate a wetland’s variable 

nutrient history over a period of years and may provide especially useful metrics against which to evaluate 

nutrient condition. Wetlands exhibit a high degree of spatial heterogeneity in chemical composition of soil 

layers, and areas impacted by nutrients may exhibit more variability than unimpacted areas of the same 

wetland. Thus, sampling protocols should capture this spatial variability. Developing nutrient criteria and 

monitoring the success of nutrient management programs involves important considerations for sampling 

designed to capture spatial and temporal patterns. 

Below is a brief overview of the use of nutrient loading and soil and water column nutrient measures for 

estimating nutrient condition of wetlands. Please refer to Supplementary References for a list of references on 

both nutrient load estimation and biogeochemical indicators, with a focus on soil and water column nutrient 

measures.

Nutrient Loading

External nutrient loads to wetlands are determined primarily by surface and subsurface transport from the 

contributing landscape, and vary significantly as a function of weather and landscape characteristics such as 

soils, topography, and land use. Most wetlands are characterized by extremely variable hydrologic and nutrient 

loading rates, which present considerable obstacles to obtaining adequate direct measurement of nutrient 

inputs. Adequate measurement of loads may require automated samplers capable of providing flow-weighted 

samples when loading rates are highly variable. In many cases, nonpoint source loads simply may not be 

adequately sampled. The more detailed the loading measurements the better, but it is not reasonable to expect 

adequate direct measurement of loads for most wetlands. In the absence of sufficient, direct measurements, it 

may be possible to estimate nutrient loading using an appropriate loading model or at least to provide a relative 

ranking of wetlands based on expected nutrient load. One advantage of loading models is that nutrient loading 

can be integrated over the appropriate time scale for characterizing wetland nutrient condition and, in some 

cases, historical loading patterns can be reconstructed. Loading models also can provide hydrologic loading 

rates to calculate critical supporting variables such as hydroperiod and residence times.

Loading function models are based on empirical or semi-empirical relationships that provide estimates of 

pollutant loads on the basis of long-term measurements of flow and contaminant concentration. Generally, 

loading function models contain procedures for estimating pollutant load based on empirical relationships 

between landscape physiographic characteristics and phenomena that control pollutant export. McElroy et 

al., (1976) and Mills (1985) described loading functions employed in screening models developed by the 
5-6 Nutrient Criteria–Wetlands

USEPA to facilitate estimation of nutrient loads from point and nonpoint sources. The models contain simple 

empirical expressions that relate the magnitude of nonpoint pollutant load to readily available or measurable 

input parameters such as soils, land use and cover, land management practices, and topography. Preston 

and Brakebill (1999) described a spatial regression model that relates the water quality conditions within a 

watershed to sources of nutrients and to those factors that influence transport of the nutrients. The regression 

model, Spatially-Referenced Regressions on Watersheds (SPARROW), involves a statistical technique that 

utilizes spatially referenced information and data to provide estimates of nutrient load (Smith et al. 1997; Smith 

et al. 2003; http://water.usgs.gov/nawqa/sparrow/).

In general, the SPARROW methodology was designed to provide statistically based relationships between 

stream water quality and anthropogenic factors such as contaminant sources within the contributing 

watersheds, land surface characteristics that influence the delivery of pollutants to the stream, and in-stream 

contaminant losses via chemical and biological process pathways. The Generalized Watershed Loading 

Functions (GWLF) model (Haith and Shoemaker 1987; Haith et al. 1992) uses daily time steps, and to some 

extent, both can be used to examine seasonal variability and the response to landscape characteristics of 

specific watersheds. The GWLF model was developed to evaluate the point and nonpoint loading of nitrogen 

and phosphorus in urban and rural watersheds. The model enhances assessment of effectiveness of certain 

land use management practices and makes extensive use of readily available watershed data. The GWLF also 

provides an analytical tool to identify and rank critical areas of a watershed and evaluate alternative land 

management programs.

Process-oriented simulation models attempt to explicitly represent biological, chemical, and physical processes 

controlling hydrology and pollutant transport. These models are at least partly mechanistic in nature and 

are built from equations that contain directly definable, observable parameters. Examples of processoriented simulation models that have been used to predict watershed hydrology and water quality include the 

Agricultural Nonpoint Source model (AGNPS), the Hydrologic Simulation Program-Fortran (HSPF), and 

the Soil and Water Assessment Tool (SWAT). AGNPS (Young et. al. 1987) is a distributed parameter, eventbased and continuous simulation model that predicts the behavior of runoff, sediment, nutrients, and pesticide 

transport from watersheds that have agriculture as the primary land use. Because of its simplicity and ease 

of use, AGNPS is probably one of the most widely used hydrologic and water quality models of watershed 

assessment. HSPF (Johansen et al. 1984; Bicknell et al. 1993; Donigian et al. 1995a) is a lumped parameter, 

continuous simulation model developed during the mid-1970s to predict watershed hydrology and water 

quality for both conventional and toxic organic pollutants. HSPF is one of the most comprehensive models 

available for simulating nonpoint source nutrient loading. The capability, strengths, and weaknesses of HSPF 

have been demonstrated by its application to many urban and rural watersheds and basins (e.g., Donigian et al. 

1990; Moore et al. 1992; and Ball et al. 1993). SWAT (Arnold et al. 1995) is a lumped parameter, continuous 

simulation model developed by USDA-Agricultural Research Services that provides long-term simulation of 

impact of land management practices on water, sediment, and agricultural chemical yields in large complex 

watersheds. Because of its lumped parameter nature, coupled with its extensive climatic, soil, and management 

databases, the SWAT model is one of the most widely used hydrologic and water quality models for large 

watersheds and basins, and the model has found widespread application in many modeling studies that involve 

systemic evaluation of impact of agricultural management on water quality.

These loading models address only gross, external nutrient inputs. It is important to consider the overall 

mass balance for the receiving wetland in developing measures of nutrient loading against which to evaluate 

wetland nutrient condition. This requires some estimate of nutrient export, storage, and transformation. In the 

absence of sufficient, direct measurements from which to calculate nutrient mass balance, it may be possible 

to estimate nutrient mass balances using an appropriate wetland model. Strictly empirical, regression models 

can be used to estimate nutrient retention and export in wetlands but these regressions are of little value 

outside the data domain in which they are developed. When developed for a diverse set of systems, the scatter 

in these regressions can be quite large. In contrast to strictly empirical regressions, mass balance models 
Nutrient Criteria–Wetlands 5-7

incorporate principles of mass conservation. These models integrate external loading to the wetland, nutrient 

transformation and retention within the wetland, and nutrient export from the wetland. Mass balance models 

allow time varying hydrologic and nutrient inputs and can provide estimates of spatial nutrient distribution 

within the wetland. The most difficult problem is developing removal rate equations which adequately 

represent nutrient transformation and retention across the range of conditions for which estimates are needed. 

Land Use

Identifying land uses in regions surrounding wetlands is important for characterizing reference condition, 

identifying reference wetlands, and providing indicators of nutrient loading rates for criteria development. 

Most simply, the percentage of natural area or the percentage of agricultural and urban lands can be used to 

characterize land uses around wetlands. More detailed quantitative data can be gathered from GIS analysis, 

which provides higher resolution identification of land use types such as pastures, row crops, and confined 

animal feeding operations for agriculture. Ideally these characterizations should be done for the entire source 

shed, including both air and water, in the regions around wetlands. Air-sheds should incorporate potential 

atmospheric sources of nutrients, and watersheds should incorporate potential aquatic sources. However, in 

practice, land use around wetlands is typically used for defining reference wetlands and also in most nutrient 

loading models to characterize groundwater and surface water sources. Land use in buffer zones, one kilometer 

zones around wetlands and wetland watersheds (delineated by elevation), has been used to characterize human 

activities that could be affecting wetlands (Brooks et al. 2004). 

Extractable Soil Nitrogen and Phosphorus

Ammonium is the dominant form of inorganic N in wetland soils, and unlike total soil N (Craft et al. 1995; 

Chiang et al. 2000), soil extractable NH4-N increases in response to N loadings. Enrichment leads to enhanced 

cycling of N between wetland biota (Valiela and Teal 1974; Broome et al. 1975; Chalmers 1979; Shaver et al. 

1998), greater activity of denitrifying bacteria (Johnston 1991; Groffman 1994; White and Reddy 1999), and 

accelerated organic matter and N accumulation in soil (Reddy et al. 1993; Craft and Richardson 1998). In 

most cases, extractable soil N should be measured in the surface soil where roots and biological activity are 

concentrated.

Extractable N is measured by extraction of inorganic (NH4-N) N with 2 M KCl (Mulvaney 1996). Ten to 

20 grams of field moist soil is equilibrated with 100 ml of 2 M KCl for one hour on a reciprocating shaker, 

followed by filtration through Whatman No. 42 filter paper. Ammonium-N in soil extracts is determined 

colorimetrically using the phenate or salicylate method (APHA 1999, Method 350.2; USEPA 1993a). 

Extractable P is often a reliable indicator of the P enrichment of soils, and in wetlands, extractable P is strongly 

correlated with surface water P concentration and P enrichment from external sources (Reddy et al. 1995; 

1998). Selected methods used to extract P are described below (Kuo 1996). Many soil testing laboratories 

perform these analyses on a routine basis. Historically, these methods have been used to determine nutrient 

needs of agronomic crops, but the methods have been used more recently to estimate P impacts in upland and 

wetland soils (Sharpley et al. 1992; Nair et al. 1995; Reddy et al. 1995, 1998). 

The Mehlich I method is typically used in the Southeast and Mid-Atlantic regions on mineral soils with pH of 

< 7.0 (Kuo 1996). The extractant consists of dilute concentrations of strong acids. Many plant nutrients such as 

P, K, Ca, Mg, Fe, Zn, and Cu extracted with Mehlich I methods have been calibrated for production of crops 

in agricultural ecosystems. This solvent extracts some Fe and Al- bound P, and some Ca-bound P. Soil (dry) 

to extractant ratio is set at 1:4 for mineral soils, while wider ratios are used for organic soils. Soil solutions are 

equilibrated for a period of five minutes on a mechanical shaker and then filtered through a Whatman No. 42 

filter. Filtered solutions are analyzed for P and other nutrients using standard methods (Method 365.1, USEPA 

1993a).
5-8 Nutrient Criteria–Wetlands

The Bray P-1 method has been widely used as an index of available P in soils (Kuo 1996). The combination of 

dilute concentration of strong acid (HCl at 0.025 M) and ammonium fluoride (NH4F at 0.03 M) is designed to 

easily remove acid extractable soluble P forms such as Ca-bound P, and some Fe and Al-bound P. Soil (dry) to 

extractant ratio is set at 1:7 for mineral soils with wider ratios used for highly organic soils, then shaken for five 

minutes and filtered through a Whatman No. 42 filter. Filtered solutions are analyzed for P and other nutrients 

using the same methods used for the Mehlich I extraction (Method 365.1, USEPA 1993a).

Bicarbonate Extractable P is a suitable method for calcareous soils. Soil P is extracted from the soil with 0.5 

M NaHCO3 at a nearly constant pH of 8.5 (Kuo 1996). In calcareous, alkaline, or neutral soils containing Cabound P, this extractant decreases the concentration of Ca in solution by causing precipitation of Ca as CaCO3

. 

As a result, P concentration in soil solution increases. Soil (dry) to extraction ratio is set at 1:20 for mineral 

soils and 1:100 for highly organic soils. Soil solutions are equilibrated for a period of 30 minutes on a shaker, 

filtered through a Whatman No. 42 filter paper, and analyzed for P using standard methods (Method 365.1, 

USEPA 1993a).

Total Soil Nitrogen and Phosphorus

Nutrient enrichment leads to enrichment of total soil P (Craft and Richardson 1993; Reddy et al. 1993; 

Bridgham et al. 2001). In contrast, soil total N usually does not increase in response to nutrient enrichment 

(Craft et al. 1995; Chiang et al. 2000). Rather, enrichment leads to enhanced cycling of N between wetland 

biota that is reflected in greater N uptake and net primary production (NPP) of wetland vegetation (Valiela 

and Teal 1974; Broome et al. 1975; Chalmers 1979; Shaver et al. 1998), greater activity of denitrifying bacteria 

(Johnston 1991, Groffman 1994; White and Reddy 1999), and accelerated organic matter and N accumulation 

in soil (Reddy et al. 1993; Craft and Richardson 1998). In most cases, total N and P should be measured in at 

least the surface soil where most roots and biological activity are concentrated.

Since ammonium N is the dominant form of inorganic nitrogen in saturated wetland soils with very little 

nitrate (NO3) present, total Kjeldahl nitrogen (TKN) can generally be taken as a measure of total N in such 

soils. The difference between TKN and ammonium N provides information on soil organic N. The soil organic 

carbon to soil organic nitrogen ratio can provide an indication of the soil’s capacity to mineralize organic N 

and provide ammonium N to vegetation. TKN in soils is determined by converting organic forms of N to 

NH4-N by digestion with concentrated H2SO4 at temperatures of 300-350oC (Bremner 1996). The NH4-N in 

digested samples is analyzed using colorimetric (e.g., phenate, salicylate) methods (APHA 1999; Mulvaney 

1996).

Total P in soils is determined by oxidation of organic forms of P and acid (nitric-perchloric acid) dissolution 

of minerals at temperatures of <300oC (Kuo 1996). Digested solutions are analyzed for P using colorimetric 

methods (e.g., ascorbic acid-molybdate) (APHA 1999; Kuo 1996). Many laboratories may not have access 

to perchloric acid fume-hoods. Alternatively, soil total phosphorus can be determined using the ashing 

method (Anderson 1976). Results obtained from this method are reliable and comparable to total phosphorus 

measurements made using perchloric acid digestion method.

Water Column Nitrogen and Phosphorus

Nutrient inputs to wetlands are highly variable across space and time, hence, single measurements of water 

column N and P represent only a “snap-shot” of nutrient condition and may or may not reflect the long-term 

pattern of nutrient inputs that alter biogeochemical cycles and affect wetland biota. The best use of water 

column N and P concentrations for nutrient criteria development will be based on frequent monitoring of 

nutrient concentrations over time (e.g., weekly or monthly measurements). Of course, in wetlands that are 

seldom flooded, measurements of water column N and P may not be practical or even relevant for assessing 

impacts. Whenever water samples are obtained, it is important that the water depth is recorded because 

nutrient concentration is related to water depth. In the case of tidal estuarine or freshwater wetlands, it is also 

important to record flow and the point in the tidal cycle that the samples were collected. 
Nutrient Criteria–Wetlands 5-9

Methodologies to monitor N in surface waters are well developed for other ecosystems and can be readily adopted 

for wetlands. The most commonly monitored N species are total Kjeldahl nitrogen (TKN), ammonium N, and 

nitrate plus nitrite N (APHA 1999). The TKN analysis includes both organic and ammonium N, but does not 

include nitrate plus nitrite N. Organic N is determined as the difference between TKN and NH4-N. Forms of N 

in surface water are measured by standard methods, including phenol-hypochlorite for ammonium N, cadmium 

reduction of nitrate to nitrite for nitrate N, and Kjeldahl digestion of total N to ammonium for analysis of total 

N (APHA 1999). Dissolved organic N is primarily used by heterotrophic microbes, whereas plants and various 

microorganisms take up inorganic forms of N (ammonium N and nitrate N) to support metabolism and new 

growth.

Methodologies to monitor P in surface waters are well developed for aquatic ecosystems and can be readily 

adopted for wetlands (APHA 1999). The most commonly measured forms of P in surface water are total P, 

dissolved inorganic P (i.e., PO4-P), and total dissolved P. To trace the transport and transformations of P in 

wetlands, it might be useful to distinguish four forms of P: (1) dissolved inorganic P (DIP, also referred to as 

dissolved reactive P (DRP) or soluble reactive phosphorous (SRP)); (2) dissolved organic P (DOP); (3) particulate 

inorganic P (PIP); and, (4) particulate organic P (POP). Dissolved inorganic P (PO4-P) is considered bioavailable 

(e.g., available for uptake and use by microorganisms, algae, and vegetation), whereas organic and particulate 

P forms generally must be transformed into inorganic forms before being considered bioavailable. In P limited 

wetlands, a significant fraction of DOP can be hydrolyzed by phosphatases and utilized by bacteria, algae, and 

macrophytes.

5.4 RESPONSE VARIABLES 

Biotic measures that can integrate a wetland’s variable nutrient history over a period of months to years may 

provide the most useful measures of wetland response to nutrient enrichment. Microorganisms, algae, and 

macrophytes respond to nutrient enrichment by: (1) increasing the concentration of nutrients (P, N) in their tissues; 

(2) increasing growth and biomass production; and, (3) shifts in species composition. The biotic response to 

nutrient enrichment generally occurs in a sequential manner as nutrient uptake occurs first, followed by increased 

biomass production, followed by a shift in species composition as some species disappear and other species 

replace them. Macroinvertebrates respond to nutrient enrichment indirectly as a result of changes in food sources, 

habitat structure, and dissolved oxygen. Because of their short life cycle, microorganisms and algae respond more 

quickly to nutrient enrichment than macrophytes. However, biotic measures that can integrate a wetland’s variable 

nutrient history over a period of months to years may provide the most useful measures of wetland response.

Below is a brief overview of the use of macrophytes, algae, and macroinvertebrates to assess nutrient condition of 

wetlands. Please refer to the relevant modules in the EPA series “Methods for Evaluating Wetland Condition” for 

details on using vegetation (http://www.epa.gov/waterscience/criteria/wetlands/15Indicators.pdf; 

http://www.epa.gov/waterscience/criteria/wetlands/16Vegetation.pdf), algae (http://www.epa.gov/waterscience/

criteria/wetlands/11Algae.pdf); and, macroinvertebrates (http://www.epa.gov/waterscience/criteria/

wetlands/9Invertebrate.pdf) to assess wetland condition, including nutrients. 

Macrophyte Nitrogen and Phosphorus

Wetland macrophytes respond to nutrient enrichment by increasing uptake and storage of N and P (Verhoeven and 

Schmitz 1991, Shaver et al. 1998; Chiang et al. 2000). In wetlands where P is the primary limiting nutrient, the 

P content of vegetation increases almost immediately (within a few months) in response to nutrient enrichment 

(Craft et al. 1995). Increased P uptake by plants is known as “luxury uptake” because P is stored in vacuoles 

and used later (Davis 1991). Like P, leaf tissue N may increase in response to N enrichment (Brinson et al. 1984; 

Shaver et al. 1998). However, most N is directly used to support new plant growth so that luxury uptake of N is not 

usually observed (Verhoeven and Schmitz 1991). Tidal marsh grasses, however, do appear to store nitrogen in both 

living and dead tissues that can be accessed by living plant tissue. A discussion of conservation and translocation 

of N in saltwater tidal marshes can be found in Hopkinson and Schubauer (1980) and Thomas and Christian 

(2001). 
5-10 Nutrient Criteria–Wetlands

Nutrient content of macrophyte tissue holds promise as a means to assess nutrient enrichment of wetlands. 

However, several caveats should be kept in mind when using this diagnostic tool (Gerloff 1969; Gerloff and 

Krombholz 1966; EPA 2002c). 

1. The most appropriate plant parts to sample and analyze should be determined. It is 

 generally recognized that the plant or plant parts should be of the same physiological age.

2. Samples from the same species should be collected and analyzed. Different species assimilate 

 and concentrate nutrients to different levels.

3. Tissue nutrient concentrations vary with (leaf) position, plant part, and age. It is important

 to sample and analyze leaves from the same position and age (e.g., third leaf from the terminal 

 bud on the plant) to ensure comparability of results from sampling of different wetlands.

4. Tissue P may be a more reliable indicator of nutrient condition than N. This is because N is

 used to increase production of aboveground biomass, whereas excess P is stored via luxury 

 uptake.

Another promising macrophyte-based tool is the measurement of nutrient resorption of N and P prior to 

leaf senescence and dieback. Nutrient resorption is an important strategy used by macrophytes to conserve 

nutrients (Hopkinson and Schubauer 1984; Shaver and Melillo 1984). In nutrient-poor environments, 

macrophytes resorb N and P from green leaves prior to senescence, leading to low concentrations of N and P 

in senesced leaves. In nutrient-rich environments, resorption becomes less important so that senesced leaves 

retain much of the N and P that was present when the leaves were green. 

Nitrogen and phosphorus should be measured in green leaves of the same approximate age collected from 

the dominant wetland plant species. Samples also should be collected throughout the wetland to account for 

spatial variability. If an environmental gradient is known or suspected to exist within the wetland, then sites 

along this gradient should be sampled separately. At each sampling location, approximately five green leaves 

are collected from each of the dominant plant species. Leaves are collected from the middle portion of the 

stem, avoiding very young leaves at the top of the stem and very old leaves at the bottom of the stem. At each 

location, leaf samples by species are combined for analysis, oven-dried at 70o

C, and ground.

Nitrogen is measured by dry combustion using a CHN analyzer. Phosphorus is measured colorimetrically 

after digestion in strong acid (H2SO4-H2O2) (Allen et al. 1986). Many land-grant universities, State agricultural 

testing laboratories, and environmental consulting laboratories perform these analyses. Contact your local U.S. 

Department of Agriculture office or land-grant agricultural extension office for information on laboratories 

that perform plant tissue nutrient analyses. 

Please see the EPA Module, Vegetation-based Indicators of Wetland Nutrient Enrichment (http://www.epa.gov/

waterscience/criteria/wetlands/16Indicators.pdf) for a detailed description of indicators derived from N and P 

content of macrophytes.

Above Ground Biomass and Stem Height

Wetland macrophytes also respond to nutrient enrichment by increased net primary production (NPP) and 

growth if other factors such as light are not limiting growth (Chiang et al. 2000). Net primary production 

is the amount of carbon fixed during photosynthesis that is incorporated into new leaves, stems, and roots. 

Most techniques to measure NPP focus on aboveground biomass and discount root production because it is 

difficult to measure, even though root production may account for 50% of NPP. The simplest way to measure 

aboveground biomass is by harvesting all of the standing material (biomass) at the end of the growing season 

(Broome et al. 1986). The harvest method is useful for measuring NPP of herbaceous emergent vegetation, 

especially in temperate climates where there is a distinct growing season. If root production desired, it can be 
Nutrient Criteria–Wetlands 5-11

determined by sequentially harvesting roots at monthly intervals during the year (Valiela et. al. 1976).

Enhanced NPP often is reflected by increased height and, sometimes, stem density of herbaceous emergent 

vegetation (Broome et. al. 1983). Because increased stem density may reflect other factors like vigorous clonal 

growth, it is not recommended as an indicator of nutrient enrichment. 

Aboveground biomass of herbaceous vegetation may be determined by end-of-season harvest of aboveground 

plant material in small 0.25 m2 quadrats stratified by macrophyte species or inundation zone (Broome et al. 

1986). Stem height of individuals of dominant species is measured in each plot. Height of the five to ten tallest 

stems in each plot has been shown to be a reliable indicator of NPP (Broome et al. 1986) that saves time as 

compared to height measurements of all stems in the plot. Aboveground biomass is clipped at the end of the 

growing season, in late summer or fall. Clipped material is separated into live (biomass) versus dead material, 

then dried at 70oC to a constant weight. For stem height and biomass sampling, five to ten plots per vegetation 

zone are collected. In forested sites, biomass production is defined as the sum of the leaf and fruit fall and 

aboveground wood production (Newbould 1967). Please see the EPA module Vegetation-based Indicators 

of Wetland Nutrient Enrichment (http://www.epa.gov/waterscience/criteria/wetlands/16Indicators.pdf) for a 

detailed description of sampling aboveground biomass in wetlands. 

Algal Nitrogen and Phosphorus

In some cases, measurements of algal N and P can provide a useful complement to vegetation and soil nutrient 

analyses that integrate nutrient history over a period of months in the case of vegetation (Craft et al. 1995), to 

years in the case of soils (Craft and Richardson 1998; Chiang et al. 2000). Nutrient concentrations in algae can 

integrate variation in water column N and P bioavailability over a time scale of weeks, potentially providing 

an indication of the recent nutrient status of a wetland (Fong et al. 1990; Stevenson et al. 2001). Caution is 

warranted for this method because it is not useful in all wetlands; for example, in wetlands where surface 

inundation occurs intermittently or for short periods of time, where the water surface is severely shaded as in 

some forested wetlands, or under other circumstances where unrelated environmental factors exert primary 

control over algal growth.

Algae should be sampled by collecting grab samples from different locations in the wetland to account for 

spatial variability in the wetland. If an environmental gradient is known or suspected (i.e., decreasing canopy 

or impacted land uses) or exists within the wetland as a result of specific source discharges, then sites along 

this gradient should be sampled separately. Comparisons among wetlands or locations within a wetland 

should be done on a habitat-specific basis (e.g., phytoplankton vs. periphyton). Samples are processed in the 

same manner as wetland plants to determine N and P content. Nitrogen is determined using a CHN analyzer, 

whereas P is measured colorimetrically after acid digestion. 

Please see the EPA module Using Algae to Assess Environmental Conditions in Wetlands (http://www.epa.

gov/waterscience/criteria/wetlands/11Algae.pdf) for a detailed description of indicators derived from to N and 

P content of algae.

Macrophyte Community Structure and Composition

The composition of the plant community and the changes that result from human activities can be used 

as sensitive indicators of the biological integrity of wetland ecosystems. In particular, aggressive, fastgrowing species such as cattail (Typha spp.), giant reed (Phragmites communis), reed canarygrass (Phalaris 

arundincea), and other clonal species invade and may eventually come to dominate the macrophyte 

community. Data collection methods and analyses for using macrophyte community structure and composition 

as an indicator of nutrient enrichment and ecosystem integrity for wetlands are described in Vegetation-based 

Indicators of Wetland Nutrient Enrichment (http://www.epa.gov/waterscience/criteria/wetlands/16Indicators.

pdf) and Using Vegetation to Assess Environmental Conditions in Wetlands (http://www.epa.gov/waterscience/

criteria/wetlands/10Vegetation.pdf), respectively. 
5-12 Nutrient Criteria–Wetlands

Algal Community Structure and Composition

Algae can be used as a valuable indicator of biological and ecological condition of wetlands. Structural 

and functional attributes of algae can be measured including diversity, biomass, chemical composition, 

productivity, and other metabolic functions. Species composition of algae, particularly of the diatoms, is 

commonly used as an indicator of biological integrity and physical and chemical conditions of wetlands. 

Discussions of sampling, data analyses, and interpretation are included in Using Algae to Assess 

Environmental Conditions in Wetlands (http://www.epa.gov/waterscience/criteria/wetlands/11Algae.pdf). 

Invertebrate Community Structure and Composition

Aquatic invertebrates can be used to assess the biological and ecological condition of wetlands. The approach 

for developing an Index of Biological Integrity for wetlands based on aquatic invertebrates is described in 

Developing an Invertebrate Index of Biological Integrity for Wetlands (http://www.epa.gov/waterscience/

criteria/wetlands/9Invertebrate.pdf).

5.5 SUMMARY

Candidate variables to use in determining nutrient condition of wetlands and to help identify appropriate 

nutrient criteria for wetlands consist of supporting variables, causal variables, and response variables. 

Supporting variables provide information useful in normalizing causal and response variables and categorizing 

wetlands. Causal variables are intended to characterize nutrient availability (or assimilation) in wetlands 

and could include nutrient loading rates and soil nutrient concentrations. Response variables are intended to 

characterize biotic response and could include community structure and composition of macrophytes and 

algae. 

The complex temporal and spatial structure of wetlands will influence the selection of variables to measure 

and methods for measuring them. The information contained in this chapter is a brief summary of suggested 

analyses that can be used to determine wetland condition with respect to nutrient status. The authors recognize 

that the candidate variables and analytical methods described here will generally be the most useful for 

identifying wetland nutrient condition, while other methods and analyses may be more appropriate in certain 

systems. 
Nutrient Criteria–Wetlands 6-1

Chapter 6: Database Development and New Data Collection

6.1 INTRODUCTION

A database of relevant water quality information can be an invaluable tool to States as they develop nutrient 

criteria. In some cases, existing data are available and can provide additional information that is specific to 

the region where criteria are to be set. However, little or no data are available for most regions or parameters 

and creating a database of newly gathered data is strongly recommended. In the case of existing data, the 

data should be located and their suitability (type and quality and sufficient associated metadata) ascertained. 

It is also important to determine how the data were collected to ensure that future monitoring efforts are 

compatible with earlier approaches. 

Databases operate much like spreadsheet applications but have greater capabilities. Databases store and 

manage large quantities of data and allow viewing and exporting of data sorted in a variety of ways, while 

spreadsheets analyze and graphically display small quantities of data. Databases can be used to organize 

existing information, store newly gathered monitoring data, and manipulate data for water quality criteria 

development. Databases can sort data for export into statistical analyses programs, spreadsheets, and graphics 

programs. This chapter will discuss the role of databases in nutrient criteria development and provide a brief 

review of existing sources of nutrient-related water quality information for wetlands.

6.2 DATABASES AND DATABASE MANAGEMENT

A database is a collection of information related to a particular subject or purpose. Databases are arranged so 

that individual values are kept separate, yet can be linked to other values based on some common denominator 

(such as association of time or location). Geographic Information Systems (GIS) are geo-referenced relational 

databases that have a geographical component (i.e., spatial platform) in the user interface. Spatial platforms 

associated with a database allow geographical display of sets of sorted data. GIS platforms such as ArcViewTM, 

ArcInfoTM, and MapInfoTM are frequently used to integrate spatial data with monitoring data for watershed 

analysis. Data stored in simple tables, relational databases, or geo-reference databases can also be located, 

retrieved, and manipulated using queries. A query allows the user to find and retrieve only the data that meets 

user-specified conditions. Queries can also be used to update or delete multiple records simultaneously and to 

perform built-in or custom calculations of data. Data in tables can be analyzed and printed in specific layouts 

using reports. Data can be analyzed or presented in a specific way in print by creating a report. The most 

effective use of these tools requires a certain amount of training, expertise, and software support, especially 

when using geo-referenced data. 

To facilitate data storage, manipulation, and calculations, it is highly recommended that historical and presentday data be transferred to a relational database (i.e., AccessTM). Relational databases store data in tables 

as sets of rows and columns and are powerful tools for data manipulation and initial data reduction. They 

allow selection of data by specific, multiple criteria and definition and redefinition of linkages among data 

components. Data queries can also be exported to GIS, provided that the data is related to some geo-referenced 

coordinate system. 
6-2 Nutrient Criteria–Wetlands

Potential Data Sources

EPA Water Quality Data

STORET

EPA has many programs of national scope that focus on collection and analysis of water quality data. The 

following presents information on several of the databases and national programs that may be useful to water 

quality managers as they compile data for criteria development. STORET STOrage and RETrieval system 

(STORET) is EPA’s national database for water quality and biological data. 

Environmental Monitoring and Assessment Program (EMAP)

The Environmental Monitoring and Assessment Program is an EPA research program designed to develop 

the tools necessary to monitor and assess the status and trends of national ecological resources (see EMAP 

Research Strategy on the EMAP Web site: http://www.epa.gov/emap). EMAP’s goal is to develop the scientific 

understanding for translating environmental monitoring data from multiple spatial and temporal scales into 

assessments of ecological condition and forecasts of future risks to the sustainability of the Nation’s natural 

resources. Data from the EMAP program can be downloaded directly from the EMAP Web site (http://www.

epa.gov/emap/html/data/index.html). The EMAP Data Directory contains information on available data sets, 

including data and metadata (language that describes the nature and content of data). Current status of the data 

directory, as well as composite data and metadata files, are available on this Web site.

U.S. Geological Survey (USGS) Water Data

The USGS has national and distributed databases on water quantity and quality for waterbodies across the 

nation. Much of the data for rivers and streams are available through the National Water Information System 

(NWIS). These data are organized by State, Hydrologic Unit Codes (HUCs), latitude and longitude, and other 

descriptive attributes. Most water quality chemical analyses are associated with an instantaneous streamflow 

at the time of sampling and can be linked to continuous streamflow to compute constituent loads or yields. The 

most convenient method of accessing the local databases is through the USGS State representative. Every State 

office can be reached through the USGS home page at: http://www.usgs.gov. 

HBN and NASQAN

USGS data from several national water quality programs covering large regions offer highly controlled and 

consistently collected data that may be particularly useful for nutrient criteria analysis. Two programs, the 

Hydrologic Benchmark Network (HBN) and the National Stream Quality Accounting Network (NASQAN), 

include routine monitoring of rivers and streams over the past 30 years. The HBN consists of 63 relatively 

small, minimally disturbed watersheds. HBN data were collected to investigate naturally-induced changes in 

streamflow and water quality and the effects of airborne substances on water quality. The NASQAN program 

consists of 618 larger, more culturally influenced watersheds. NASQAN data provides information for tracking 

water-quality conditions in major U.S. rivers and streams. The watersheds in both networks include a diverse 

set of climatic, physiographic, and cultural characteristics. Data from the networks have been used to describe 

geographic variations in water-quality concentrations, quantify water-quality trends, estimate rates of chemical 

flux from watersheds, and investigate relations of water quality to the natural environment and anthropogenic 

contaminant sources. 

WEBB

The Water, Energy, and Biogeochemical Budgets (WEBB) program was developed by USGS to study water, 

energy, and biogeochemical processes in a variety of climatic/regional scenarios. Five ecologically diverse 

watersheds, each with an established data history, were chosen. This program may prove to be a rich data 

source for ecoregions in which the five watersheds are located. Many publications on the WEBB project are 

available. See the USGS Web site for more details (http://water.usgs.gov/nrp/webb/about.html).
Nutrient Criteria–Wetlands 6-3

US Department of Agriculture (USDA) Agricultural Research Service (ARS)

The USDA ARS houses the Natural Resources and Sustainable Agricultural Systems Scientific Directory 

(http://hydrolab.arsusda.gov/arssci.html), which has seven national programs to examine the effect of 

agriculture on the environment. The program on Water Quality and Management addresses the role of 

agriculture in nonpoint source pollution through research on Agricultural Watershed Management and 

Landscape Features, Irrigation and Drainage Management Systems, and Water Quality Protection and 

Management Systems. Research is conducted across the country and several models and databases have been 

developed. Information on research and program contacts is listed on the Web site (http://www.nps.ars.usda.

gov/programs/nrsas.htm).

Forest Service

The Forest Service has designated research sites across the country, many of which are Long Term Ecological 

Research (LTER) sites. Many of the data from these experiments are available in the USFS databases located 

on the Web site (http://www.fs.fed.us/research/). Most of the data are forest-related but may be of use for 

determining land uses and questions on silviculture runoff.

National Science Foundation (NSF) 

The National Science Foundation (NSF) funds projects for the Long Term Ecological Research (LTER) 

Network. The Network is a collaboration of over 1,100 researchers investigating a wide range of ecological 

topics at 24 different sites nationwide. The LTER research programs are not only an extremely rich data 

source, but also a source of data available to anyone through the Network Information System (NIS), the NSF 

data source for LTER sites. Data sets from sites are highly comparable due to standardization of methods and 

equipment. 

U.S. Army Corps of Engineers (COE)

The U.S. Army Corps of Engineers (COE) is responsible for many federal wetland jurisdiction issues. 

Although a specific network of water quality monitoring data does not exist, specific studies on wetlands by 

the COE may provide suitable data. The COE focuses more on water quantity issues than on water quality 

issues. As a result, much of the wetland system data collected by the COE does not include nutrient data. 

Nonetheless, the COE does have a large water sampling network and supports USGS and EPA monitoring 

efforts in many programs. A list of the water quality programs that the COE actively participates in can be 

found at http://www.usace.army.mil/public.html. 

U.S. Department of the Interior, Bureau of Reclamation (BuRec)

The Bureau of Reclamation of the U.S. Department of the Interior manages many irrigation and water supply 

reservoirs in the West, some of which may have wetland applicable data available. These data focus on water 

supply information and limited water quality data. However, real time flow data are collected for rivers 

supplying water to BuRec, which may be useful if a flow component of criteria development is chosen. These 

data can be gathered on a site-specific basis from the BuRec Web site: http://www.usbr.gov. 

State Monitoring Programs

Some States may have wetland water quality data as part of a research study, use attainability analysis (UAA), 

or to assess mitigation or nutrient related impacts. Most of this data is collected by State natural resources or 

environmental protection agencies, or by regional water management authorities. Data collected by State water 

quality monitoring programs can be used for nutrient criteria development and may provide pertinent data 

sources, although they may be regionally limited. These data should be available from the agencies responsible 

for monitoring. 
6-4 Nutrient Criteria–Wetlands

Volunteer Monitoring Programs

State and local agencies may use volunteer data to screen for water quality problems, establish trends in 

waters that would otherwise be unmonitored, and make planning decisions. Volunteers benefit from learning 

more about their local water resources and identifying what conditions or activities might contribute to 

pollution problems. As a result, volunteers frequently work with clubs, environmental groups, and State or 

local governments to address problem areas. The EPA supports volunteer monitoring and local involvement in 

protecting our water resources. 

Academic and Literature Sources

Most of the data available on water and soil quality in wetlands is the result of research studies conducted 

by academic institutions. Much of the research conducted by the academic community, however, was not 

conducted for the purpose of spatial or long-term biogeochemical characterization of the nation’s wetlands; 

instead, water quality information was often collected to characterize the environmental conditions under 

which a particular study or experiment was conducted. Infrequently, spatial studies of limited extent or 

duration were conducted. Data collected from these sources, therefore, may not be sufficiently representative 

of the population of wetlands within an ecoregion. However, this limited data may be the only information 

available and therefore could be useful for identifying reference conditions or determining where to begin a 

more comprehensive survey to support development of nutrient criteria. Academic research data is available 

from researchers and the scientific literature. 

6.3 QUALITY OF HISTORICAL AND COLLECTED DATA

The value of older historical data is a recurrent problem because data quality is often unknown. Knowledge 

of data quality is also problematic for long-term data repositories such as STORET and long-term State 

databases, where objectives, methods, and investigators may have changed many times over the years. 

The most reliable data tend to be those collected by a single agency using the same protocol. Supporting 

documentation should be examined to determine the consistency of sampling and analytical protocols. The 

suitability of data in large, heterogeneous data repositories for establishing nutrient criteria are described 

below. These same factors need to be taken into account when developing a new database such that future 

investigators will have sufficient information necessary to evaluate the quality of the database. 

Location

Geo-referenced data is extremely valuable in that it allows for aggregating and summarizing data according 

to any GIS coverage desired, whether the data was historically related to a particular coverage theme or not. 

However, many studies conducted prior to the availability and accuracy of hand held Global Positioning 

System (GPS) units relied on narrative and less definitive descriptions of location such as proximity to 

transportation corridor, county, or nearest municipal center. This can make comparison of data, depending 

upon desired spatial resolution, difficult. Knowledge of the rationale and methods of site selection from 

the original investigators may supply valuable information for determining whether inclusion of the site or 

study in the database is appropriate based on potential bias relative to overall wetland data sources. STORET 

and USGS data associated with the National Hydrography Dataset (NHD) are geo-referenced with latitude, 

longitude, and Reach File 3 (RF3) codes (http://nhd.usgs.gov/). In addition, STORET often contains a site 

description to supplement location information. Metadata of this type, when known, is frequently stored within 

large long-term databases. 

Variables and Analytical Methods

Each separate analytical method yields a unique variable. For example, five ways of measuring TP result 

in five unique variables. Data generated using different analytical methods should not be combined in data 

analyses because methods differ in accuracy, precision, and detection limits. Data generated from one 

method may be too limited, making it important to select the most frequently used analytical methods in the 

database. Data that were generated using the same analytical methods may not always be obvious because of 
Nutrient Criteria–Wetlands 6-5

synonymous names or analytical methods. Consistency in taxonomic conventions and indicator measurements 

is likewise important for biological variables and multimetric indices comparisons. Review of recorded data 

and analytical methods by knowledgeable personnel is important to ensure that there are no problems with 

data sets developed from a particular database. 

Laboratory Quality Control

Data generated by agencies or laboratories with known quality control/quality assurance protocols are most 

reliable. Laboratory Quality Control (QC) data (blanks, spikes, replicates, known standards) are infrequently 

reported in larger data repositories. Records of general laboratory quality control protocols and specific 

quality control procedures associated with specific data sets are valuable in evaluating data quality. However, 

premature elimination of lower quality data can be counterproductive because the increase in variance caused 

by analytical laboratory error may be negligible compared to natural variability or sampling error, especially 

for nutrients and related water quality parameters. However, data of uncertain and undocumented quality 

should not be accepted.

Water column nutrient data can be reported in different units (e.g., ppm, mg/L, mmoles). Reporting of nutrient 

data from other strata such as soils, litter, and vegetation can further expand the list of reporting units (e.g., 

mg/kg, g/kg, %, mg/cm3

). In many instances, conversion of units is possible; however, in other instances unit 

conversion is not possible or is lacking support information for conversion. Consistency in reporting units and 

the need to provide conversion tables cannot be overemphasized.

Data Collecting Agencies

Selecting data from particular agencies with known, consistent sampling and analytical methods and known 

quality will reduce variability due to unknown quality problems. Requesting data review for quality assurance 

from the collecting agency will reduce uncertainty about data quality. 

Time Period

Long-term records are critically important for establishing trends. Determining if trends exist in the time 

series database is also important for characterizing reference conditions for nutrient criteria. Length of time 

series data needed for analyzing nutrient data trends is discussed in Chapter 7. 

Index Period

An index period—the time period most appropriate for sampling—for estimating average concentrations 

can be established if nutrient and water quality variables were measured through seasonal cycles. The 

index period may be the entire year or the summer growing season. The best index period is determined by 

considering wetland characteristics for the region, the quality and quantity of data available, and estimates of 

temporal variability (if available). Consideration of the data available relative to longer-term oscillations in 

environmental conditions (e.g., dry years, wet years) should also be taken into account such that the data is 

representative and appropriate. Additional information and considerations for establishing an index period are 

discussed in Chapter 7. 

Representativeness

Data may have been collected for specific purposes. Data collected for toxicity analyses, effluent limit 

determinations, or other pollution problems may not be useful for developing nutrient criteria. Further, data 

collected for specific purposes may not be representative of the region or wetland classes of interest. The 

investigator should determine if all wetlands or a subset of the wetlands in the database are representative 

of the population of wetlands to be characterized. If a sufficient sample of representative wetlands cannot be 

found, then a new survey is strongly recommended.
6-6 Nutrient Criteria–Wetlands

6.4 COLLECTING NEW DATA

New data should be collected when no data presently exist or the data available are not suitable, and should 

be gathered following the sampling design protocols discussed in Chapter 4. New data collection activities 

for developing nutrient criteria should focus on filling in gaps in the database and collecting spatially 

representative regional monitoring data. In many cases, this may mean starting from scratch because no 

data presently exists or the data available are not suitable. Data gathered under new monitoring programs 

should be imported into databases or spreadsheets and, if comparable, merged with existing data for criteria 

development. It is best to archive the data with as much data-unique information (meta-data) as possible. It 

is always possible to aggregate at a later time, but impossible to separate lumped data without having the 

parameter needed to partition the data set. Redundancy may also be a problem but can more easily be avoided 

when common variables or parameters are kept in each database (i.e., dates may be very important). The 

limitations and qualifications of each data set should be known and data ‘tagged’, if possible, before combining 

them. The following five factors should be considered when collecting new data and before combining new 

data with existing data sets: representativeness, completeness, comparability, accuracy, and precision. 

Representativeness 

Sampling program design (when, where, and how you sample) should produce samples that are representative 

or typical of the regional area being described and the classes of wetlands present. Sampling designs for 

developing nutrient criteria are addressed in Chapter 4. Databases populated by data from the literature or 

historical studies will not likely provide sufficient spatial or class representation of a region. Data interpretation 

should recognize these gaps and be limited until gaps are filled using additional survey information. 

Completeness

A QA/QC plan should describe how to complete the data set in order to answer questions posed (with a 

statistical test of given power and confidence) and the precautions being taken to ensure that completeness. 

Data collection procedures should document the extent to which these conditions have been met. Incomplete 

data sets may not invalidate the collected data but may reduce the rigor of statistical analyses. Precautions to 

ensure completeness may include collecting extra samples, having back-up equipment in the field, copying 

field notebooks after each trip, and/or maintaining duplicate sets of data in two locations. 

Comparability

In order to compare data collected under different sampling programs or by different agencies, sampling 

protocols and analytical methods should demonstrate comparable data. The most efficient way to produce 

comparable data is to use sampling designs and analytical methods that are widely used and accepted and 

examined for compatibility with other monitoring programs prior to initiation of a survey. Comparability 

should be assessed for field sample collection, sample preservation, sample preparation and analysis, and 

among laboratories used for sample analyses. 

Accuracy

To assess the accuracy of field instruments and analytical equipment, a standard (a sample with a known value) 

should be analyzed and the measurement error or bias determined. Internal standards should periodically be 

checked with external standards provided by acknowledged sources. At Federal, State, and local government 

levels, the National Institute of Standards and Technology (NIST) provides advisory and research services to 

all agencies by developing, producing, and distributing standard reference materials for vegetation, soils, and 

sediments. Standards and methods of calibration are typically included with turbidity meters, pH meters DO 

meters, and DO testing kits. The U.S. EPA, USGS, and some private companies provide reference standards or 

QC samples for nutrients. 
Nutrient Criteria–Wetlands 6-7

Variability

The variability in field measurements and analytical methods should be demonstrated and documented to 

identify the source and magnitude of variability when possible. EPA QA/QC guidance provides an explanation 

and protocols for measuring sampling variability (USEPA 1998c). 

Data Reduction

For data reduction, it is important to have a clear idea of the analysis that will be performed and a clear 

definition of the sample unit for analysis. For example, a sample unit might be defined as “a wetland during 

July-August.” For each variable measured, a mean value would then be estimated for each wetland during the 

July-August index period on record. Analyses are then conducted on the observations (estimated means) for 

each sample unit, not with the raw data. Steps recommended for reducing the data include:

1. Selecting the long-term time period for analysis;

2. Selecting an index period;

3. Selecting relevant variables of interest;

4. Identifying the quality of analytical methods;

5. Identifying the quality of the data recorded; and,

6. Estimating values for analysis (mean, median, minimum, maximum) based on the reduction 

 selected.

6.5 QUALITY ASSURANCE / QUALITY CONTROL

The validity and usefulness of data depend on the care with which they were collected, analyzed, and 

documented. EPA provides guidance on data quality assurance and quality control (USEPA 1998c) to assure 

the quality of data. Factors that should be addressed in a QA/QC plan are elaborated below. The QA/QC plan 

should state specific goals for each factor and should describe the methods and protocols used to achieve the 

goals. 

1. Who will use the data?

2. What the project’s goals/objectives/questions or issues are?

3. What decision(s) will be made from the information obtained?

4. How, when, and where project information will be acquired or generated?

5. What possible problems may arise and what actions can be taken to mitigate their impact on the 

 project?

6. What type, quantity, and quality of data are specified?

7. How “good” those data have to be to support the decision to be made?

8. How the data will be analyzed, assessed, and reported?

Nutrient Criteria–Wetlands 7-1

Chapter 7: Data Analysis

7.1 INTRODUCTION

 

Data analysis is critical to nutrient criteria development. Proper analysis and interpretation of data determine 

the scientific defensibility and effectiveness of the criteria. Therefore, it is important to evaluate short- and 

long-term goals for wetlands of a given class within the region of concern. These goals should be addressed 

when analyzing and interpreting nutrient and response data. Specific objectives to be accomplished through 

use of nutrient criteria should be identified and revisited regularly to ensure that goals are being met. The 

purpose of this chapter is to explore methods for analyzing data that can be used to develop nutrient criteria 

consistent with these goals. Included are techniques to evaluate metrics, to examine or compare distributions 

of nutrient exposure or response variables, and to examine nutrient exposure-response relationships.

Statistical analyses are used to interpret monitoring data for criteria development. Statistical methods are datadriven and range from very simple descriptive statistics to more complex statistical analyses. Generally, the 

type of statistical analysis used for criteria development is determined by the source, quality, and quantity of 

data available.

Biological Condition

Increasing Levels of Stressors

Natural structure & function of biotic community maintained

Minimal changes in structure & function

Evident changes in structure and 

minimal changes in function

Moderate changes in structure & 

minimal changes in function

Major changes in structure & 

moderate changes in function

Severe changes in structure & function

1

2

3

4

5

6

BCG Model: Snap Shot

Figure 7.1: Biological condition gradient model describing biotic community condition as levels of 

stressors increase.
7-2 Nutrient Criteria–Wetlands

7.2 FACTORS AFFECTING ANALYSIS APPROACH

Wetland systems should be appropriately classified a priori for nutrient criteria development to minimize 

natural background variation (see Chapter 3). This section discusses some of the factors that should be 

considered when classifying wetland systems and in determining the choice of predictor (causal) and response 

variables to include in the analysis. 

Wetland hydrogeomorphic type http://el.erdc.usace.army.mil/wrap/wrap.html may determine the sensitivity 

of wetlands to nutrient inputs, as well as the interaction of nutrients with other driving factors in producing 

an ecological response. Hydrogeomorphic types differ in landscape position, predominant water source, 

and hydrologic exchanges with adjacent water bodies (Brinson 1993). These factors, in turn, influence water 

residence time, hydrologic regime, and disturbance regime. In general, isolated depressional wetlands will 

have greater residence times than fringe wetlands, which, in turn, will have greater residence times than 

riverine wetlands. Systems with long residence times are likely to behave more like lakes than flow-through 

systems and may show a greater response to cumulative loadings. Thus, nutrient loading rates or indicators 

thereof are likely to be a more sensitive predictor of ecological effects for depressional wetlands, while nutrient 

water column or sediment concentrations are likely to be a more sensitive predictor of responses for riverine 

wetlands. Water column concentrations will influence the response of algal communities, while macrophytes 

derive nutrients from both the water column and sediments. Fringe wetlands are likely to be influenced both 

by concentration of nutrients in the adjacent lake or estuary as well as the accumulation of nutrients within 

these systems from groundwater inflow and, in some cases, riverine inputs. The relative influence of these 

two sources will depend on the exchange rate with the adjacent lake, e.g., through seiche activity (Keough et 

al. 1999; Trebitz et al. 2002). In practice, it is difficult to measure loadings from multiple sources including 

groundwater and exchange with adjacent water bodies. If sediment concentrations are shown to be a good 

indicator of recent loading rates, then sediment concentrations might be the best predictor to use across 

systems.

It may be important to control for ancillary factors when teasing out the relationship between nutrients and 

vegetation community response, particularly if those factors interact with nutrients in eliciting responses. 

For example, riverine and fringe wetlands differ from basin wetlands in the frequency and intensity of 

disturbance from flooding events or ice. Day et. al. (1988) describe a fertility-disturbance gradient model for 

riverine wetlands describing how the relative dominance of plant guilds with different growth forms and life 

history strategies depends on the interactive effects of productivity, fertility, disturbance, and water level. 

In depressional wetlands, the model could be simplified to include only the interaction of fertility with the 

hydrologic regime. Disturbance regimes and water level could be incorporated into analysis of cause-effect 

relationships either as categorical factors or as covariates.

The selection of assessment and measurement of response attributes for determining ecological response 

to nutrient loadings should depend, in part, on designated uses assigned to wetlands as part of standards 

development. Designated uses such as recreation (aesthetics and contact) or drinking water are not typically 

assigned to wetlands; thus, defining nuisance algal blooms in terms of taste or odor problems or aesthetic 

considerations may not be appropriate for wetlands. Guidance for the definition of aquatic life use is currently 

being refined to describe six stages of impact along a human disturbance gradient, from pristine reference 

condition to heavily degraded sites (Figure 7.1, Stevenson and Hauer 2002; Davies and Jackson 2006). The 

relative abundance of sensitive native taxa is expected to shift with relatively minor impacts, while organism 

condition or functional attributes are relatively robust to altered loadings. However, if maintenance of 

ecological integrity of sensitive downstream systems is of concern, then it may be important to measure 

some functional attributes related to nutrient retention. Stevenson and Hauer (2002) have suggested a series 

of “resource condition tiers” analogous to those defined for biological condition but related to ecosystem 

functions. Tier 1 requirements are proposed as “Native structure and function of the hydrologic and 
Nutrient Criteria–Wetlands 7-3

geomorphic regimes and processes are in the natural range of variation in time and space.” Thus maintenance 

of structure and function of upstream processes should be protective of downstream biological conditions.

7.3 DISTRIBUTION-BASED APPROACHES

Frequency distributions can aid in the setting of criteria by describing central tendency and variability among 

wetlands. Approaches to numeric nutrient criteria development based on frequency distributions do not require 

specific knowledge of individual wetland condition prior to setting criteria. Criteria are based on and, in a sense, 

developed relative to the conditions of the population of wetlands of a given class.

The simplest statistic describing the shape of distributions refers to quartiles, or the 25th and the 75th percentile. 

These can be defined as the observation which has 25% of the observations on one side and 75% on the 

other side in the case of the first quartile (25th percentile), or vice versa in the case of the third quartile (75th 

percentile). In the same manner, the median is the second quartile or the 50th percentile. Graphically, this is 

depicted in boxplots as the box length, the lower extreme represents the first quartile, and the upper extreme 

represents the third quartile, the area inside the box encompassing 50% of the data. 

Distributions of nutrient exposure metrics or response variables can be developed to represent either an 

entire population of wetlands or only a subset of those considered to be minimally impacted. In either case, a 

population of wetlands should be defined narrowly enough through classification so that the range in attributes 

due to natural variability does not equal or exceed the range in attributes related to anthropogenic effects. 

The effects of natural variability can be minimized by classifying wetlands by type and/or region. Nutrient 

ecoregions define one potential regional classification system (USEPA 2000). Alternatively, thresholds in 

landscape or watershed attributes defining natural breakpoints in nutrient concentrations can be determined 

objectively through procedures such as classification and regression tree (CART) analysis (Robertson et. al. 

2001). If a distribution-based approach is used, periodic reviews using empirical data that relate a measured 

value to an ecological attribute or ecosystem function can validate the assumptions of the chosen percentiles. 

7.4 RESPONSE-BASED APPROACHES

Indicators characterized as “response” or “condition” metrics should be distinguished from “stressor” or 

“causal” indicators, such as nutrient concentrations (Paulsen et al. 1991; USEPA 1998a; Stevenson 2004a). 

While both “response“ and “causal” indicators could be used in a single multimetric index, it is recommended 

that separate multimetric indices be used for “response” and “causal” assessment. Distinguishing between 

“response” and “causal” indices can be accomplished utilizing a risk assessment approach with separate 

hazard and exposure assessments that are linked to response-stressor relationships (USEPA 1996, 1998a; 

Stevenson 1998; Stevenson et al. 2004a, b). A multimetric index that specifically characterizes “responses” 

can be used to clarify goals of management (maintenance or restoration of ecological attributes) and to 

measure whether goals have been attained with nutrient management strategies. Response-based multimetric 

indices can also be used more directly for natural resource damage assessments than multimetric indices with 

response and causal variables.

Factors that should be considered in selecting indicators include conceptual relevance (relevance to the 

assessment and ecological function), feasibility of implementation (data collection logistics, information 

management, quality assurance, cost), response variability (measurement error, seasonal variability, 

interannual variability, spatial variability, discriminatory ability), and interpretation and utility (data quality 

objectives, assessment thresholds, link to management actions) (Jackson et al. 2000). Of these factors, cost, 

response variability, and ability to meet data quality objectives can be assessed through quantitative methods. 

An analytical understanding of the factors that affect wetlands the most will also help States develop the most 

effective monitoring and assessment strategies. 
7-4 Nutrient Criteria–Wetlands

Designated uses such as contact recreation and drinking water may not be applicable to wetlands, hence, it 

may not be readily apparent what the relative significance of changes in different primary producers is for 

organisms at higher trophic levels. Wetland food webs have traditionally been considered to be detritus-based 

(Odum and de la Cruz 1967; Mann 1972; 1988). However, more recent research on wetland food webs utilizing 

stable isotope analysis have identified the importance of phytoplankton, periphyton, or benthic algae as the 

base of the food chain for higher trophic levels (Fry 1984; Kitting et al. 1984; Sullivan and Moncreiff 1990; 

Hamilton et al. 1992; Newell et al. 1995; Keough et al. 1996); in these cases, it would be particularly important 

to monitor shifts in algal producers.

Empirical relationships can be derived directly between water quality parameters such as total P or 

transparency and wetland biological responses. Unlike lakes or streams, the level of algal biomass 

corresponding to aesthetic problems or ecological degradation in wetlands is not readily defined, so that 

defining a TP-chlorophyll a relationship based on water column measurements is not likely to be useful. 

However, in some wetlands such as coastal Great Lakes, the loss of submerged aquatic vegetation biomass 

and/or diversity with increased eutrophication provides an ecologically significant endpoint (Lougheed et al. 

2001). Reductions in submerged plant species diversity was associated with increases in turbidity, total P, total 

N, and chlorophyll a, suggesting that a trophic state index incorporating multiple parameters might be a better 

predictor than a single variable such as total P (Carlson 1977).

Models describing empirical relationships can include linear or nonlinear univariate forms with a single 

response metric, multivariate with multiple response metrics, a series of linked relationships, and simulation 

models. The simplest forms of linear univariate approaches are correlation and regression analyses; these 

approaches have the advantage that they are simple to perform and transparent to the general public. When 

assessment thresholds can be determined based on severity of effect or difference from reference conditions 

such that associated exposure criteria can be derived, linear forms should be adequate. In the case of nonlinear 

relationships, data can generally be transformed to linearize the relationship. However, if it is desired to 

identify the inflection point in a curvilinear relationship as an indicator of rapid ecological change, alternative 

data analysis methods are available, including changepoint analysis (Richardson and Qian 1999) and piecewise 

iterative regression techniques (Wilkinson 1999).

Multivariate models are useful for relating nutrient exposure metrics to community-level responses. Both 

parametric and nonparametric (nonmetric dimensional scaling or NMDS) ordination procedures can be used to 

define axes or gradients of variation in community composition based on relative density, relative abundance, 

or simple presence-absence measures (Gauch 1982; Beals 1984; Heikkila 1987; Growns et al. 1992). Ordination 

scores then can be regressed against nutrient exposure metrics as an indicator of a composite response 

(McCormick et al. 1996). Direct gradient analysis techniques such as canonical correspondence analysis can be 

used to determine which combination of nutrient exposure variables predict a combination of nutrient response 

variables as a first step in deriving multimetric exposure and response variables (Cooper et al. 1999). Indicator 

analysis can be used to determine which subset of species best discriminate between reference sites with low 

nutrient loadings versus potentially impacted sites with high loadings, or weighted averaging techniques can 

be used to infer nutrient levels from species composition (McCormick et al. 1996; Cooper et al. 1999; Jensen 

et al. 1999). In the latter case, paleoecological records can be examined to infer historic changes in total P 

levels from macrophyte pollen or diatom frustrules, which will be particularly valuable in the absence of sites 

representing reference condition (Cooper et al. 1999; Jensen et. al. 1999).

Some ecohydrological models have been derived that incorporate the effect of multiple stressors (hydrology, 

eutrophication, acidity) on wetland vegetation, thus providing a link between process-based models and 

community level response (see Olde Venterink and Wassen 1997 for review). These models are based on: (1) a 

combination of expert opinion to estimate species sensitivities, supplemented by multivariate classification of 

vegetation and environmental data to determine boundaries of species guilds; or, (2) field measurements used 

to derive logistic models to quantify dose-response. These approaches could be used to derive wetland nutrient 
Nutrient Criteria–Wetlands 7-5

criteria for the U.S. provided that models could be calibrated using species and response curves developed 

using data for the U.S. Most multiple-stressor models for wetland vegetation have been calibrated using data 

from Western Europe (Olde Venterink and Wassen 1997). Latour and colleagues (Latour and Reiling 1993; 

Latour et al. 1994) have suggested a mechanism for setting nutrient standards using the occurrence probability 

of species along a trophic gradient to extrapolate maximum tolerable concentrations that protect 95% of 

species.

A series of linked empirical relationships for wetlands may be most effective for developing nutrient criteria. 

Linked empirical relationships may be most useful in cases where integrative exposure measurements such as 

sediment nutrient concentrations are more sensitive predictors of shifts in community composition, or algal 

P limitation, or other ecological responses (phosphatase enzyme assays; Qian et al. 2003) than are spatially 

and temporally heterogeneous water column nutrient concentrations. In these cases, it may be important 

to develop one set of relationships between nutrient loading and exposure indicators for a subset of sites at 

which intensive monitoring is done, and another set of relationships between nutrient exposure and ecological 

response indicators for a larger sample population (Qian et al. 2003).

7.5 PARTITIONING EFFECTS AMONG MULTIPLE STRESSORS

Changes in nutrient concentrations within or loadings to wetlands often co-occur with other potential stressors 

such as changes in hydrologic regime and sediment loading. In a few cases, researchers have been able to 

separate the simple effects of nutrient addition through manipulations of mesocosms (Busnardo et al. 1992; 

Gabor et al. 1994; Murkin et al. 1994; McDougal et al. 1997; Hann and Goldsborough 1997), segments of 

natural systems (Richardson and Qian 1999; Thormann and Bayley 1997), or whole wetlands (Spieles and 

Mitsch 2000). In other cases, both simple and interactive effects have been examined experimentally, e.g., to 

separate effects of hydrologic regime from nutrient loading (Neill 1990a, b; Neill 1992; Bayley et al. 1985). 

If nutrient effects are examined by comparing condition of natural wetlands along a loading or concentration 

gradient, effects of other driving factors can be minimized by making comparisons among wetlands of similar 

hydrogeomorphic type and climatic regime within a well-defined sampling window. In addition, multivariate 

techniques for partitioning effects among multiple factors can be used, such as partial CCA or partial 

redundancy analysis (Cooper et al. 1999; Jensen et al. 1999). 

7.6 STATISTICAL TECHNIQUES

Quantitative methods can be used to assess metric cost, evaluation, response variability, and ability to 

meet data quality objectives. The most appropriate method varies with respect to the indicator or variable 

being considered. In general, statistical techniques are aimed at making conjectures or inferences about 

a population’s values or relationships between variables in a sample randomly taken from the population 

of interest. In these terms, population is defined as all possible values that a certain parameter may take. 

For example, in the case of total phosphorus levels present in marsh sediments in nutrient ecoregion VII, 

the total population would be determined if all the marshes in that ecoregion were sampled, which would 

negate the need for data analysis. Practically, a sample is taken from the population and the characteristics 

associated with that sample (mean, standard deviation) are “transferred” to the entire population. Many of 

the basic statistical techniques are designed to quantify the reliability of this transferred estimate by placing 

a confidence interval over the sample-derived parameter. More complex forms of data analysis involve 

comparisons of these parameters from different populations (for example, comparison between sites) or the 

establishment of complex data models that are thought to better describe the original population structure 

(for example, regression). They are still basic inference techniques that utilize sample characteristics to make 

conjectures about the original population. 

A basic and typical issue facing any type of sampling design is the number of samples that should be taken 

to be confident in the translation from samples to population. The degree of confidence required should 
7-6 Nutrient Criteria–Wetlands

be defined as data quality objectives by the end-user and identify the expected statistical rigor for those 

objectives to be met. There are extensive texts on types and manners of sampling schemes; these will not 

be discussed here. This section is geared to determining the minimum data set recommended to work with 

subsequent sections of the data analysis chapter. In interpreting the results of various forms of data analysis, 

an acceptable level of statistical error is formulated; this is called Type I error, or alpha (α). Type I error can 

be defined as the probability of rejecting the null hypothesis (H0) when this is actually true. In setting the 

Type I error rate, the Type II error rate is also specified. The Type II error rate, or beta (β), is defined as failing 

to reject the null hypothesis when it is actually false, i.e., declaring that no significant effect exists when in 

reality this is the case. In setting the Type I error rate, an acceptable level of risk is recommended; the risk of 

concluding that a significance exists when this is not the case in reality, i.e., the risk of a “false positive”(Type 

I error) or “false negative” (Type II error). The concepts of Type I and Type II errors are introduced in Chapter 

4 with reference to sampling design and monitoring, and more fully discussed in Chapter 8 with reference to 

criteria development. 

In experimental or sampling design, of greater interest is a statistic associated with beta (β), specifically 1 – β, 

which is the power of a statistical test. Power is the ability of the statistical test to indicate significance based 

on the probability that it will reject a false null hypothesis. Statistical power depends on the level of acceptable 

statistical significance (usually expressed as a probability 0.05 – 0.001 (5% -1%) and termed the α level); the 

level of power dictates the probability of “success,” or identifying the effect. Statistical power is a function of 

three factors: effect size, alpha (α), and sample size, the relationship between the three factors being relatively 

complex.

1. Effect size is defined as the actual magnitude of the effect of interest. This could be the difference 

 between two means or the actual correlation between the variables. The relationship between the 

 effect size and power is intuitive; if the effect size is large (for example, a large difference between 

 means this results in a concomitantly large power.

2. Alpha is related to power; to achieve a higher level of significance, power decreases if other factors 

 are kept constant. 

3. Sample size. Generally, this is the easiest factor to control. If the two preceding factors are set, 

 increased sample sizes will always result in a greater power. 

As indicated before, the relationship between these three factors is complex and depends on the nature of the 

intended statistical analysis. An online guide for selecting appropriate statistical procedures is available at: 

http://www.socialresearchmethods.net/. Software packages for performing power analysis have been reviewed 

by Thomas and Krebs (1997). Online power calculations have been made available by several statistical faculty 

and are available at these Web sites: http://www.yorku.ca/isr/scs/, http://www.surveysystem.com/sscalc.htm, 

http://www.stat.ohio-state.edu/~jch/ssinput.html, and http://www.stat.uiowa.edu. Additional Web sites are 

listed in Chapter 4 that emphasize designs for monitoring with statistical rigor. 

Metric response variability can be evaluated by examining the signal to noise ratio along a gradient of nutrient 

concentrations or loading rates (Reddy et al. 1999). The power of regression analyses can be determined 

using the power function for a t-test. Optimization of the design, such as the spacing, number of levels of 

observations, and replication at each level, depend on the purpose of the regression analysis (Neter et al. 1983).

Multiple correlation analysis can compound uncertainty and in some instances misidentify correlations due 

to chance as relevant. Appropriate corrections (e.g., Bonferroni) should be applied to avoid these errors (Rice 

1989).

Multimetric Indices 

Multimetric indices are valuable for summarizing and communicating results of environmental assessments. 

Use of multimetric indices is one approach in developing criteria. Furthermore, preservation of the biotic 
Nutrient Criteria–Wetlands 7-7

integrity of algal assemblages, as well as fish and macroinvertebrate assemblages, may be an objective 

for establishing nutrient criteria. Multimetric indices for stream macroinvertebrates and fish are common 

(e.g., Kerans and Karr 1994; Barbour et al. 1999), and multimetric indices with benthic algae have recently 

been developed and tested on a relatively limited basis (Kentucky Division of Water 1993; Hill et al. 2000). 

Efforts are underway to develop multi-metric indices of biotic integrity for wetlands, and methods modules 

are available for characterizing wetland algal, plant, macroinvertebrate, amphibian, and bird communities 

(http://www.epa.gov/waterscience/criteria/wetlands/). Methods for multi-metric indices are well developed for 

streams and are readily transferable to wetlands. However, higher trophic levels do not often directly respond 

to nutrients and therefore may not be as sensitive to relatively small changes in nutrient concentrations as algal 

assemblages. It is recommended that relations between biotic integrity of algal or vegetation assemblages 

and nutrients be defined and then related to biotic integrity of macroinvertebrate and fish assemblages in a 

stepwise, mechanistic fashion. The practitioner should realize, however, that wetlands with a history of high 

nutrient loadings have often lost the most sensitive species and in these cases higher trophic level species may 

prove to be the best indicators of current nutrient loadings and wetland nutrient condition. 

This section provides an overview for developing a multimetric index that will indicate shifts in primary 

producers that are associated with trophic status in wetlands. The first step in developing a multimetric 

index of trophic status is to select a set of ecological attributes that respond to human changes in nutrient 

concentrations or loading. Attributes that respond to an increase in human disturbance are referred to as 

metrics. Six to 10 metrics should be selected for the index based on their sensitivity to human activities that 

increase nutrient availability (loading and concentrations), their precision, and their transferability among 

regions and habitat types. Selected metrics also should respond to the breadth of biological responses to 

nutrient conditions (see discussion of metric properties in McCormick and Cairns 1994). 

Effects of nutrients on primary producers and effects of primary producers on the biotic integrity of 

macroinvertebrates and fish should be characterized to aid in developing nutrient criteria that will protect 

designated uses related to aquatic life (e.g., Miltner and Rankin 1998; King and Richardson 2002). 

Another approach for characterizing biotic integrity of assemblages as a function of trophic status is to 

calculate the deviation in species composition or growth forms at assessed sites from composition in the 

reference condition. Similarity or dissimilarity indices can be used for the determining the differences in biotic 

integrity of a wetland in comparison to the reference condition. Multivariate similarity or dissimilarity indices 

need to be calculated for multivariate attributes such as taxonomic composition (Stevenson 1984; Raschke 

1993) as defined by relative abundance of different growth forms or species, or species presence/absence. One 

standard form of these indices is percent community similarity (PSc) (Whittaker 1952):

PSc= Σi=1,smin(ai,bi)

Here ai is the percentage of the ith species in sample a, and bi is the percentage of the same ith species in a 

subsequent sample, sample b. 

A second common community similarity measurement is based on a distance measurement (which is actually 

a dissimilarity measurement, rather than similarity measurement, because the index increases with greater 

dissimilarity, Stevenson 1984; Pielou 1984). Euclidean distance (ED) is a standard distance dissimilarity index, 

where: 

ED= √(Σi=1(ai-bi)

2)

Log-transformation of species relative abundances in these calculations can increase precision of metrics 

by reducing variability in the most abundant taxa. However, the practitioner should also be aware that 

transformation, while reducing variability, often decreases sensitivity and the ability to distinguish true fine 
7-8 Nutrient Criteria–Wetlands

scale changes in community and species composition. Theoretically and empirically, we expect to find that 

multivariate attributes based on taxonomic composition more precisely and sensitively respond to nutrient 

conditions than do univariate attributes, for instance multimetric algal assemblages (see discussions in 

Stevenson and Pan 1999).

To develop the multimetric index, metrics should be selected and their values normalized to a standard range 

such that they all increase with trophic status. Criteria for selecting metrics can be found in McCormick and 

Cairns (1994) or many other references. Basically, sensitive and precise metrics should be selected for the 

multimetric index and selected metrics should represent a broad range of impacts and, perhaps, designated 

uses. Values can be normalized to a standard range using many techniques. For example, if 10 metrics are used 

and the maximum value of the multimetric index is defined as 100, all 10 metrics should be normalized to the 

range of 10 so that the sum of all metrics would range between 0 and 100. The multimetric index is calculated 

as the sum of all metrics measured in a system. A high value of this multimetric index of trophic status 

would indicate high impacts of nutrients and should be a robust (certain and transferable) and moderately 

sensitive indicator of nutrient impacts in a stream. A 1-3-5 scaling technique is commonly used with aquatic 

invertebrates (Barbour et al. 1999; Karr and Chu 1999) and could be used with a multimetric index of trophic 

status as well. Using the 95th percentile when developing metrics is an approach that may decrease the 

influence of outliers (Mack 2004).

7.7 LINKING NUTRIENT AVAILABILITY TO PRIMARY PRODUCER RESPONSE

When evaluating the relationships between nutrients and primary producer response within wetland systems, 

it is important to first understand which nutrient is limiting. Once the limiting nutrient is defined, critical 

nutrient concentrations can be specified and nutrient-response relationships developed. 

Defining the Limiting Nutrient

The first step in identifying nutrient-producer relationships should be to define the limiting nutrient. Limiting 

nutrients will control biomass and productivity within a system. However, non-limiting nutrients may 

have other impacts, e.g., toxicological effects related to ammonia concentrations in sediments or effects on 

competitive interactions that determine vegetation community composition (Guesewell et al. 2003). A review 

of fertilization studies indicated that vegetation N:P mass ratios are a good predictor of the nature of nutrient 

limitation in wetlands, with N:P ratios > 16 indicating P limitation at a community level, and N:P ratios < 14 

indicative of N limitation (Koerselman and Meuleman 1996). Guesewell et al. (2003) found that vegetation 

N:P ratios were a good predictor of community-level biomass response to fertilization by N or P, but for 

individual species were only predictive of P-limitation and could not distinguish between N-limitation, colimitation, or no limitation. Likewise, N, P, and K levels in wet meadow and fen vegetation were found to be 

correlated with estimated supply rates or extractable fractions in soils (Odle Venterink et al. 2002). A survey of 

literature values of vegetation and soil total N:P ratios by Bedford et al., (1999) indicated that many temperate 

North American wetlands are either P-limited or co-limited by N and P, especially those with organic soils. 

Only marshes have N:P ratios in both soils and plants indicative of N limitation, while soils data suggest that 

most swamps are also N-limited. 

Many experimental procedures are used to determine which nutrient (N, P, or carbon) limits algal growth. 

Algal growth potential (AGP) bioassays are very useful for determining the limiting nutrient (USEPA 1971). 

Yet, results from such assays usually agree with what would have been predicted from N:P biomass ratios, and 

in some cases N:P ratios in the water. Limiting nutrient-potential biomass relationships from AGP bottle tests 

are useful in projecting maximum potential biomass in standing or slow-moving water bodies. However, they 

are not as useful in fast-flowing, and/or gravel or cobble bed environments. Also, the AGP bioassay utilizes a 

single species, which may not be representative of the response of the natural species assemblage.
Nutrient Criteria–Wetlands 7-9

Limitation may be detected by other means, such as alkaline-phosphatase activity, to determine if phosphorus 

is limiting. Alkaline phosphatase is an extracellular enzyme excreted by some algal species and from roots 

in some macrophytes in response to P limitation. This enzyme hydrolyzes phosphate ester bonds, releasing 

orthophosphate (PO4) from organic phosphorus compounds (Mullholland et al. 1991). Therefore, the 

concentration of alkaline phosphatase in the water can be used to assess the degree of P limitation. Alkaline 

phosphatase activity, monitored over time in a wetland, can be used to assess the influence of P loads on the 

growth limitation of algae (Richardson and Qian 1999).

There have been no empirical relationships published relating nutrient concentrations or inputs to wetland 

chlorophyll a or productivity levels as there have been for streams and lakes. This is likely due to the large 

number of factors interacting with nutrients that determine net ecological effects in wetlands. For example, 

eutrophication of Great Lakes coastal wetlands and increases in agricultural area in upstream watersheds 

have been correlated with decreases in diversity of submerged aquatic vegetation, yet researchers were unable 

to uncouple the effects of nutrients from those of turbidity (Lougheed et al. 2001). Even in experimentally 

controlled settings, where it is possible to separate increased suspended solids loadings from nutrient loadings, 

effects of nutrients depend heavily on other factors such as periodicity of nutrient additions (pulse vs. press 

loadings; Gabor et al. 1994; Murkin et al. 1994; Hann and Goldsborough 1997; McDougal et al. 1997), water 

regime (Neill 1990a, b; Thormann and Bayley 1997), food web structure (Goldsborough and Robinson 1996), 

and time lags (Neill 1990a, b). It is important in experimental settings to utilize adequate controls for water 

additions that may accompany nutrients (Bayley et al. 1985); in empirical comparisons from field data, it 

may be difficult if not impossible to separate out these effects. Day et al., (1988) propose a general conceptual 

model describing responses of different wetland plant guilds in riverine wetlands based on a combination of 

disturbance regime, hydrologic regime, and nutrients. In the latter case, proper classification of sites based 

on disturbance and hydrologic regime prior to describing reference condition help to adequately separate out 

nutrient-related effects and explain differences in response.

The significance of food web structure in determining nutrient effects does not preclude deriving predictive 

nutrient-primary producer relationships or minimize the importance of describing significant impacts. 

However, it does highlight the importance of adequately characterizing the trophic structure of wetlands prior 

to comparison, especially the number of trophic levels (e.g., presence or absence of planktivorous fish), and 

examining interactive effects on multiple classes of primary producers: phytoplankton, epipelon, epiphytic 

algae, metaphyton, and macrophytes (Goldsborough and Robinson 1996; McDougal et al. 1997). In some cases, 

addition of nutrients may have little or no effect on some components such as benthic algae, but can create 

significant shifts in primary productivity among others, such as a loss of macrophytes and associated epiphytes 

with an increase in inedible filamentous metaphyton and shading of the water column (McDougal et al. 1997).

Nutrient Criteria–Wetlands 8-1

Chapter 8: Criteria Development

8.1 INTRODUCTION

This chapter describes recommendations for setting scientifically defensible criteria for nutrients in wetlands 

by using data that address causal and biotic response variables. Causal variables (external nutrient loading, 

soil extractable P, soil extractable N, total soil N and P, and water column N and P), and biotic response 

variables (vegetation N and P, biomass, species composition, and algal N and P) and the supporting variables 

(hydrologic condition, conductivity, soil pH, soil bulk density, particle size distribution, and soil organic 

matter), as described in Chapter 5 provide an overview of environmental conditions and nutrient status of the 

wetland; these parameters are considered critical to nutrient assessment in wetlands. Several recommended 

approaches that water quality managers can use to derive numeric criteria in combination with other biological 

response variables are presented. These recommended approaches can be used alone, in combination, or 

may be modified for use by State water quality managers to derive criteria for wetlands that are scientifically 

defensible and protective of the designated use. Criteria developed from multiple lines of evidence using 

combined approaches will provide the greatest scientific defensibility. Recommended approaches for numeric 

nutrient criteria development presented here include: 

• the use of reference conditions to characterize natural or minimally impaired wetland systems 

 with respect to causal and exposure indicator variables; 

• applying predictive relationships to select nutrient concentrations that will protect wetland 

 structure and/or function; and,

• developing criteria from established nutrient exposure-response relationships (as in the peer-

 reviewed published literature). 

The first approach is based on the assumption that maintaining nutrient levels within the range of values 

measured for reference systems will maintain the biological integrity of wetlands. This presumes that a 

sufficient number of reference systems can be identified. The second two approaches are response-based; 

hence, the level of nutrients associated with biological impairment should be used to identify criteria. Ideally, 

both kinds of information (background variability and exposure-response relationships) will be available for 

criteria development. Recommendations are also presented for deriving criteria based on the potential for 

effects to downstream receiving waters (i.e., the lake, reservoir, stream, or estuary influenced by wetlands). 

States should consider relating these measures to metrics of ecological integrity and periodically assessing 

measures to verify assumptions made in criteria development. The chapter concludes with a recommended 

process for evaluating proposed criteria, suggestions of how to interpret and apply criteria, considerations for 

sampling for comparison to criteria, potential modifications to established criteria, and adoption of criteria into 

water quality standards. 

The Regional Technical Assistance Group (RTAG) is composed of State and Regional specialists who will 

help the Agency and States establish nutrient criteria for adoption into their water quality standards. Expert 

evaluations are important throughout the criteria development process. The data upon which criteria are based 

and the analyses performed to arrive at criteria should be assessed for veracity and applicability. 

8.2 METHODS FOR DEVELOPING NUTRIENT CRITERIA

The following discussions focus on three general methods that can be used in developing nutrient criteria. 

First, identification of reference or control systems for each established wetland type and class should be based 

on either best professional judgment or percentile selections of data plotted as frequency distributions. The 

second method uses refinement of classification systems, models, and/or examination of system biological 

attributes to assess the relationships among nutrients, vegetation or algae, soil, and other variables. Finally, the 

third method identifies published nutrient and vegetation, algal, and soil relationships and values that may be 
8-2 Nutrient Criteria–Wetlands

used (or modified for use) as criteria. A weight of evidence approach with multiple attributes that combines one 

or more of these three approaches should produce criteria of greater scientific validity. 

Using Reference Condition to Establish Criteria

One approach to consider in setting criteria is the concept of reference condition. This approach involves using 

relatively undisturbed wetlands as reference systems to serve as examples for the natural or least disturbed 

ecological conditions of a region. These approaches are most useful for estimating reference conditions 

appropriate to the specific designated use for a class of wetlands. Three recommended ways of using reference 

condition to establish criteria are: 

1. Characterize reference systems for each class within a region using best professional judgment 

 and use these reference conditions to define criteria.

2. Identify the 75th to 95th percentile of the frequency distribution for a class of reference wetlands 

 as defined in Chapter 3 and use this percentile to define the criteria.

3. Calculate a 5th to 25th percentile of the frequency distribution of the general population of a class 

 of wetlands and use the selected percentile to define the criteria. 

Defining the nutrient condition of wetlands within classes will allow the manager to identify protective criteria 

and determine which systems may benefit from management action. Criteria that are identified using reference 

condition approaches may require comparisons to similar systems in other States that share the ecoregion 

so that reference condition and developed criteria can be validated. Furthermore, the 95th percentile of the 

reference population and the 5th percentile of the general population are best used to define the criteria when 

there is great confidence that the group of reference waters truly reflects reference conditions as opposed, for 

example, to best available condition.

Reference wetlands should be identified for each class of wetland within a State or ecoregion and then 

characterized with respect to external nutrient loading, water column N and P, biotic response variables 

(macrophytes, algae, soils) and supporting environmental conditions. Wetlands classified as reference quality 

should be verified by comparing the data from the reference systems to general population data for each 

wetland class. Reference systems should be minimally disturbed and should have biotic response values that 

reflect this condition. 

Conditions at reference sites may be characterized using either of two frequency distribution approaches (see 

2 and 3 above). In both approaches, an optimal reference condition value is selected from the distribution of 

an available set of wetland data for a given wetland class. This approach may be of limited value at this time 

because few States currently collect wetland monitoring data. However, as more wetlands are monitored and 

more data become available, this approach may become more viable. 

In the first frequency distribution approach, a percentile (75th to 95th is recommended) is selected from the 

distribution of causal and biotic response variables of reference systems selected a priori based on very specific 

criteria (i.e., highest quality or least impacted wetlands for that wetland class within a region). The values 

for variables at the selected quartile may be used as the basis for nutrient criteria. The selection of a specific 

percentile as the basis for the criterion should be determined by the uses designated for that water. 

If reference wetlands of a given class are rare within a given region or if inadequate information is available to 

assign wetlands with historic nutrient data as “reference” versus “impacted” wetlands, another approach may 

be appropriate. The second frequency distribution approach involves selecting a percentile of: (1) all wetland 

data in the class (reference and non-reference); or, (2) a random sample distribution of all wetland data within a 

particular class. Due to the random selection process, a lower percentile should be selected because the sample 

distribution is expected to contain some degraded systems. This option is most useful in regions where the 

number of legitimate “natural” reference wetlands is usually very small, such as in highly developed land use 
Nutrient Criteria–Wetlands 8-3

Figure 8.1: Use of frequency distributions of nutrient concentration for establishing criteria (left 

graphic), and use of effects thresholds with nutrient concentration for establishing criteria (right 

graphic).

areas (e.g., the agricultural lands of the Midwest and the urbanized east or west coasts). EPA’s recommendation 

in this case is the 5th to 25th percentile depending upon the number of “natural” reference systems available. 

If almost all systems are impaired to some extent, then a lower percentile, generally the 5th percentile, is 

recommended for selection of reference wetlands.

Both the 75th percentile for the subset of reference systems and the 5th to 25th percentile from a representative 

random sample distribution are only recommendations. The actual distribution of the observations should 

be the major determinant of the threshold point chosen. For example, a bi-modal distribution of sediment 

or water-column nutrients might indicate a natural breakpoint between reference and enriched systems. To 

illustrate, Figure 8.1 shows both options and illustrates the presumption that these two alternative methods 

should approach a common reference condition along a continuum of data points. In this illustration, the 75th 

percentile of the reference data distribution produces an extractable soil P reference condition that corresponds 

to the 25th percentile of the random sample distribution. 

The choice of a distribution cut-off to define the upper range of reference wetland nutrient levels is analogous 

to defining an acceptable level of Type I error, the frequency for rejecting wetlands as members of the 

“unimpacted” class when in fact they are part of the reference wetland population (a false designation of 

impairment). If a distribution cut-off of 25% is chosen, the rate of falsely designating wetlands as impaired will 

be higher than if a distribution cutoff of 5% is chosen; however, the frequency of committing Type II errors 

(failing to identify anthropogenically-enriched wetlands) will be lower. As described in Chapter 7, there is a 

trade-off between Type I and Type II errors. When additional information is available, it may be possible to 

justify a range of values that are representative of least-impaired wetlands that would reduce Type I errors on a 

system by system basis. 

75th

25th

Frequency

Concentration

75th percentile of

reference population is

the starting point for 

criteria.

Nutrient data from reference waters (blue) or

from all waters (gold) similar physical

characteristics.

Measurable Effect

Concentration

Effects thresholds

can help justify 

criterion value.

Paired nutrient and effects data from waters

with similar physical characteristics.

75th

25th
8-4 Nutrient Criteria–Wetlands

State water quality managers also may consider analyzing wetlands data based on designated use 

classifications. Using this approach, frequency distributions for specific designated uses, as opposed to 

frequency distributions of reference or general populations, could be examined and criteria proposed based 

on maintenance of high quality systems that are representative of each designated use. For example, one 

criterion could be derived that protects superior quality wetland habitat (SWLH), and a second criterion could 

be identified that maintains good quality wetland habitat (function maintained but some loss of sensitive 

species (Figure 8.2); see Office of Water tiered aquatic life use training module (PDF): (http://www.epa.gov/

waterscience/biocriteria/modules/wet101-05-alus-monitoring.pdf). This recommended approach is designated 

as the Tiered Aquatic Life Use (TALU) and is being developed by the EPA Office of Water in a more detailed 

publication. Using this approach, a criterion range is created and a greater number of wetland systems will 

likely be considered protective of the designated use. In this case, emphasis may be shifted from managing 

wetland systems based on a central tendency toward more pristine systems associated with Tiers I and II. This 

approach also will aid in prioritizing systems for protection and restoration. Subsequent management efforts 

using this approach should focus on improving wetland conditions so that, over time, plots of wetland data 

shift to the left (i.e., improved nutrient condition) of their initial position. 

Applying Predictive Relationships

Two fundamental reasons are commonly considered for using biological attributes in developing nutrient 

criteria. The concepts basically promote the use of biotic responses or biocriteria to nutrient enrichment, 

i.e., both rationales support evaluation of physical and chemical conditions in conjunction with biological 

parameters when establishing water quality criteria. The first reason is that the primary goal of environmental 

assessment and management is to protect and restore ecosystem services and ecological attributes, which are 

often closely related to biological features and functions in ecosystems. Therefore, it is the effects of nutrients 

on the living components of ecosystems that should become the critical determinant of nutrient criteria, rather 

than the actual nutrient concentrations. The second reason for using biocriteria is that attributes of biological 

Figure 8.2: Tiered Aquatic Life Use model used in Maine. Biological Condition 

Low Increasing Effects of Stressors High

1

Native or natural condition

2 Minimal loss of species; some

density changes may occur

3

Some replacement of

sensitive-rare species; 

functions fully

maintained 4

Some sensitive species 

maintained; altered

distributions; functions 

largely maintained

5

6

Tolerant species show 

increasing dominance; 

sensitive species are rare; 

functions altered Severe alteration of

structure and function

Natural

Degraded

NA

B

C

AA

A

MAINE TALU
Nutrient Criteria–Wetlands 8-5

assemblages usually vary less in space and time than most physical and chemical characteristics measured 

in environmental assessments. Thus, fewer mistakes in assessment may occur if biocriteria are employed in 

addition to physical and chemical criteria. In those environments where biological attributes change fairly 

rapidly, such as in Louisiana’s coastal wetland environment where salinity can vary dramatically in response 

to wet versus drought years, other techniques will need to be developed. Information on some other techniques 

can be found at The Louisiana State University School of the Coast and Environment (http://www.wbi.lsu.edu/

wbi/web-content/index.html) and also in interagency efforts through the Los Angeles Department of Natural 

Resources) to assess coastal area ecology. (http://data.lca.gov/Ivan6/app/app_c_ch9.pdf).

Multimetric indices are a special form of indicators of biological condition in which several metrics are used to 

summarize and communicate in a single number the state of a complex ecological system. Multimetric indices 

for macroinvertebrates and fish are used successfully to establish biocriteria for aquatic systems in many 

States, and several States are developing multimetric indices for wetlands (see http://www.epa.gov/owow Web 

site). 

Another recommended approach is to identify threshold or non-linear biotic responses to nutrient enrichment. 

Some biological attributes respond linearly with increasing nutrient concentrations, whereas some attributes 

change in a non-linear manner. Non-linear changes in metrics indicate thresholds along environmental 

gradients where small changes in environmental conditions cause relatively great changes in a biological 

attribute. In an example from the Everglades, a specific level of P concentration and loadings was associated 

with a dramatic shift in algal composition and loss of the calcareous algal mats typical of this system (Figure 

8.3). Overall, metrics or indices that change linearly (typically higher-level community attributes such as 

diversity or a multimetric index) provide better variables for establishing biocriteria because they respond 

to environmental change along the entire gradient of human disturbance. However, metrics that change in a 

non-linear manner along environmental gradients are valuable for determining where along the environmental 

gradient the physical and chemical criteria should be set and, correspondingly, how to interpret other biotic 

response variables of interest (Stevenson et al. 2004a).

Figure 8.3: Percent calcareous algal mat cover in relation to distance from the P source showing the 

loss of the calcareous algal mat in those sites closer to the source (Stevenson et al. 2002).
8-6 Nutrient Criteria–Wetlands

Using Data Published in the Literature 

Values from the published literature may be used to develop nutrient criteria if a strong rationale is presented 

that demonstrates the suitability of these data to the wetland of interest (i.e., the system of interest should 

share the same characteristics with the systems used to derive the published values). Published data, if there 

is enough of it, could be used to develop criteria for: (1) reference condition; (2) predictive (cause and effect) 

relationships between nutrients and biotic response variables; (3) tiered criteria; or, (4) criteria that exhibit 

a threshold response to nutrients. However, published data from similar wetlands should not substitute for 

collection and analysis of data from the wetland or wetlands of interest. 

Considerations for Downstream Receiving Waters

More stringent nutrient criteria may be appropriate for wetlands that drain into lentic or standing waters. 

For example, it is proposed that 35 μg/L TP concentration and a mean concentration of 8 μg/L chlorophyll 

a constitute the dividing line between eutrophic and mesotrophic lakes (OECD 1982). Natural nutrient 

concentrations in some wetlands may be higher than downstream lakes. In addition, assimilative capacity 

for nutrients without changes in valued attributes may also be higher in wetlands than lakes. Nutrient criteria 

for wetlands draining into lakes may need to be lower than typically would be set if only effects on wetlands 

were considered. This is because EPA’s regulations require States to take into consideration the water quality 

standards of downstream waters when designating uses of a water body and adopting appropriate criteria to 

protect those uses. (See 40 CFR 131.10(b).) Therefore, when adopting nutrient criteria for wetlands draining 

into lakes, States should take into account the protection of the downstream waters of receiving lakes in 

addition to wetlands. 

8.3 EVALUATION OF PROPOSED CRITERIA

Following criteria derivation, an expert assessment of the proposed criteria and their applicability to all 

wetlands within the class of interest is encouraged. Criteria should be verified in many cases by comparing 

criteria values for a wetland class within an ecoregion across State boundaries. In fact, development of 

interstate criteria should be an integral part of a State’s water quality standards program. In addition, prior 

to recommending any proposed criterion, it is recommended that States take into consideration the water 

quality standards of downstream waters to ensure that their water quality standards provide for attainment and 

maintenance of the water quality standards of downstream waters. (see 40 CFR 131.10(b)). Load estimating 

models, such as those recommended by EPA (USEPA 1999), can assist in this determination (see External 

Nutrient Loading in Chapter 5.3). Water quality managers responsible for downstream receiving waters also 

should be consulted.
Nutrient Criteria–Wetlands R-1

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Nutrient Criteria–Wetlands A-1

APPENDIX A. 

ACRONYM LIST AND GLOSSARY

Acronym List

ACOE/ACE/COE - Army Corps of Engineers

AGNPS - Agricultural Nonpoint Source Pollution 

model

AGP - Algal Growth Potential

ARS - Agricultural Research Service

BACI - Before/After, Control/Impact

BMP - Best Management Practice

BPJ - Best Professional Judgement

BuRec - Bureau of Reclamation

CART - Classification and Regression Tree

CCC - Commodity Credit Corporation

CENR - Committee for the Environment and Natural 

Resources

CGP - Construction General Permit

CHN - Carbon-Hydrogen-Nitrogen

COE - U.S. Army Corps of Engineers

CPGL - Conservation of Private Grazing Land

CPP - Continuing Planning Process

CREP - Conservation Reserve Enhancement Program

CRP - Conservation Reserve Program

CSFFCP - Central Florida Flood Control Project

CSO - Combined Sewer Overflow

CWA - Clean Water Act

CZARA - Coastal Zone Act Reauthorization 

Amendment

DIP - Dissolved inorganic phosphorus

DO - Dissolved oxygen

DOP - Dissolved organic phosphorus

DRP - Dissolved reactive phosphorus

ECARP - Environmental Conservation Acreage 

Reserve Program

ED - Euclidean Distance

EDAS - Ecological Data Application System

Eh - Redox potential

EMAP - Environmental Monitoring and Assessment 

Program

EPA - U.S. Environmental Protection Agency

EQIP - Environmental Quality Incentive Program

FDEP - Florida Department of Environmental 

Protection

FIP - Forestry Incentive Program

GIS - Geographic Information System

GPS - Geospatial Positioning System

GRTS - Generalized Random Tessellation Stratified

GWLF - Generalized Watershed Loading Function

HBN - Hydrologic Benchmark Network

HEL - Highly erodible land

HGM - Hydrogeomorphic approach

HSPF - Hydrologic Simulation Program - Fortran

HUC - Hydrologic Unit Codes

IBI - Index of Biotic Integrity

ICI - Invertebrate Community Index

LNWR - Loxahachee National Wildlife Refuge

LTER - Long Term Ecological Research

MPCA - Minnesota Pollution Control Agency

N - Nitrogen

NAAQS - National Ambient Air Quality Standard

NASQAN - National Stream Quality Assessment 

Network

NAWQA - National Water Quality Assessment

NHD - National Hydrology Dataset

NIS - Network Information System

NIST - National Institute of Standards and 

Technology

NOAA - National Oceanic and Atmospheric 

Administration

NPDES - National Pollution Discharge Elimination 

System

NPP - Net primary production

NRCS - Natural Resources Conservation Service

NSF - National Science Foundation

NWI - National Wetlands Inventory

NWIS - National Water Information Systems

OH EPA - Ohio EPA

ONRW - Outstanding Natural Resource Waters

P - Phosphorus

PCB - Polychlorinated biphenyls

PCS - Permit Compliance System

PIP - Particulate inorganic phosphorus

POP - Particulate organic phosphorus

PSA - Particle size analysis

PSc - Percent Community Similarity

QA - Quality Assurance

QA/QC - Quality Assurance/Quality Control

QC - Quality Control

REMAP - Regional Environmental Monitoring and 

Assessment Program

RF3 - Reach File 3

RTAG - Regional Technical Assistance Group

SCS - Soil Conservation Service
A-2 Nutrient Criteria–Wetlands

SPARROW - Spatially Referenced Regressions on Watersheds

SRP - Soluble reactive phosphorus

STORET - Storage and Retrieval System

SWAT - Soil and Water Assessment Tool

SWLH - Superior Quality Wetland Habitat

TALU - Tiered Aquatic Life Use

TKN - Total Kjeldahl Nitrogen

TMDL - Total Maximum Daily Load

TP - Total Phosphorus

UAA - Use Attainability Analysis

USDA - United States Department of Agriculture

USEPA - United States Environmental Protection Agency

USFWS - United States Fish and Wildlife Service

USGS - United States Geological Survey

WCA - Water Conservation Area

WEBB - Water, Energy, and Biogeochemical Budgets

WHIP - Wildlife Habitat Incentive Program

WLA - Wasteload Allocation

WQBEL - Water Quality Based Effluent Limit

WQS - Water Quality Standard

WRP - Wetlands Reserve Program
Nutrient Criteria–Wetlands A-3

Glossary

aquatic ecoregion

Level II ecoregions defined by Omernik according to expected similarity in attributes affecting nutrient supply. 

biocriteria

(biological criteria) Narrative or numeric expressions that describe the desired biological condition of aquatic 

communities inhabiting particular types of waterbodies and serve as an index of aquatic community health. 

(USEPA 1994).

cluster analysis

An exploratory multivariate statistical technique that groups similar entities in an hierarchical structure.

criteria

Elements of State water quality standards, expressed as constituent concentrations, levels, or narrative statements, 

representing a quality of water that supports a particular use. When criteria are met, water quality will generally 

protect the designated use (40 CFR 131.3(b)).

designated use

Uses defined in water quality standards for each waterbody or segment whether or not the use is being attained 

(USEPA 1994).

detritus

Unconsolidated sediments comprised of both inorganic and dead and decaying particulate organic matter inhabited 

by decomposer microorganisms (Wetzel 1983).

ecological unit

Mapped units that are delineated based on similarity in climate, landform, geomorphology, geology, soils, 

hydrology, potential vegetation, and water.

ecoregion

A region defined by similarity of climate, landform, soil, potential natural vegetation, hydrology, and other 

ecologically relevant variables.

emergent vegetation

“Erect, rooted herbaceous angiosperms that may be temporarily to permanently flooded at the base but do not 

tolerate prolonged inundation of the entire plant; e.g., bulrushes (Scirpus spp.), saltmarsh cordgrass” (Cowardin 

et al. 1979).

eutrophic

Abundant in nutrients and having high rates of productivity frequently resulting in oxygen depletion below the 

surface layer (Wetzel 1983).

eutrophication

The increase of nutrients in [waterbodies] either naturally or artificially by pollution (Goldman and Horne 

1983).

GIS (Geographical Information Systems)

A computerized information system that can input, store, manipulate, analyze, and display geographically 

referenced data to support decision-making processes. (NDWP Water Words Dictionary)
A-4 Nutrient Criteria–Wetlands

HGM, hydrogeomorphic

Land form characterized by a specific origin, geomorphic setting, water source, and hydrodynamic (NDWP Water 

Words Dictionary)

index of biotic integrity (IBI) 

An integrative expression of the biological condition that is composed of multiple metrics. Similar to economic 

indexes used for expressing the condition of the economy.

interfluve

An area of relatively unchannelized upland between adjacent streams flowing in approximately the same 

direction.

lacustrine

“Includes wetlands and deepwater habitats with all of the following characteristics: (1)

situated in a topographic depression or a dammed river channel; (2) lacking trees, persistent emergents, emergent 

mosses or lichens with greater than 30% areal coverage; and, (3) total area exceeds 8 ha (20 acres). Similar 

wetland and deepwater habitats totaling less than 8 ha are also included in the Lacustrine System if an active 

wave-formed or bedrock shoreline feature makes up all or part of the boundary, or if the water depth in the deepest 

part of the basin exceeds 2 m (6.6 feet) at low water...may be tidal or nontidal, but ocean-derived salinity is always 

less than 0.5%” (Cowardin et al. 1979).

lentic

Relatively still-water environment (Goldman and Horne 1983).

limnetic

The open water of a body of fresh water.

littoral

Region along the shore of a non-flowing body of water.

lotic

Running-water environment (Goldman and Horne 1983).

macrophyte

(Also known as SAV-Submerged Aquatic Vegetation) Larger aquatic plants, as distinct from the microscopic 

plants, including aquatic mosses, liverworts, angiosperms, ferns, and larger algae as well as vascular plants; no 

precise taxonomic meaning (Goldman and Horne 1983).

μg/L

micrograms per liter, 10-6 grams per liter

mg/L

milligrams per liter, 10-3 grams per liter

mineral soil flats

Level wetland landform with predominantly mineral soils

minerotrophic

Receiving water inputs from groundwater, and thus higher in salt content (major ions) and pH than ombrotrophic 

systems.
Nutrient Criteria–Wetlands A-5

mixohaline

Water with salinity of 0.5 to 30%, due to ocean salts.

molarity

Molarity, moles of an element as concentration

multivariate

Type of statistics that relates one or more independent (explanatory) variables with multiple dependent (response) 

variables.

oligotrophic

Trophic status of a waterbody characterized by a small supply of nutrients (low nutrient release from sediments), 

low production of organic matter, low rates of decomposition, oxidizing hypolimnetic condition (high DO) 

(Wetzel 1983).

palustrine

“Nontidal wetlands dominated by trees, shrubs, persistent emergents, emergent mosses orlichens, and all such 

wetlands that occur in tidal areas where salinity due to ocean-derived salts is below 0.5%. It also includes wetlands 

lacking such vegetation, but with all of the following four characteristics: (1) area less than 8 ha (20 acres); (2) 

active wave-formed or bedrock shoreline features lacking; (3) water depth in the deepest part of basin less than 2 

m at low water; and, (4) salinity due to ocean-derived salts less than 0.5%” (Cowardin et al. 1979).

peatland

“A type of wetland in which organic matter is produced faster than it is decomposed, resulting in the accumulation 

of partially decomposed vegetative material called Peat. In some mires peat never accumulates to the point where 

plants lose contact with water moving through mineral soil. Such mires, dominated by grasslike sedges, are called 

Fens. In other mires peat becomes so thick that the surface vegetation is insulated from mineral soil. These plants 

depend on precipitation for both water and nutrients. Such mires, dominated by acid forming sphagnum moss, are 

called Bogs.” (NDWP Water Words Dictionary)

periphyton

Associated aquatic organisms attached or clinging to stems and leaves of rooted plants or other surfaces projecting 

above the bottom of a waterbody (USEPA 1994).

pocosin

Evergreen shrub bog, found on Atlantic coastal plain.

riverine wetland

A hydrogeomorphic class of wetlands found in floodplains and riparian zones associated with stream or river 

channels.

slope wetland

A wetland typically formed at a break in slope where groundwater discharges to the surface. Typically there is no 

standing water.

trophic status

Degree of nutrient enrichment of a waterbody.
A-6 Nutrient Criteria–Wetlands

waters of the U.S.

Waters of the United States is defined at 40 CFR § 230.3(s)) as including:

a. All waters that are currently used, were used in the past, or may be susceptible to use in interstate or 

 foreign commerce, including all waters that are subject to the ebb and flow of the tide;

b. All interstate waters, including interstate wet lands; and,

c. All other waters such as interstate lakes, rivers, streams (including intermittent streams), mudflats, 

 sandflats, wetlands, sloughs, prairie potholes, wet meadows, playa lakes, or natural ponds the use, 

 degradation, or destruction of which would affect or could affect interstate or foreign commerce 

 including any such waters:

1. That are or could be used by interstate or foreign travelers for recreational or other purposes;

2. From which fish or shellfish are or could be taken and sold in interstate or foreign commerce; or,

3. That are used or could be used for industrial purposes by industries in interstate commerce;

d. All impoundments of waters otherwise defined as waters of the United States under this definition;

e. Tributaries of waters identified in paragraphs (a) through (d) of this definition;

f. The territorial sea; and,

g. Wetlands adjacent to waters (other than waters that are themselves wetlands) identified in 

 paragraphs (a) through (f) of this definition.

For further information regarding the scope of ‘waters of the U.S.’ in light of the U.S. Supreme Court’s 2006 

decision in Rapanos v. United States, see “Clean Water Act Jurisdiction Following the U.S. Supreme Court’s 

Decision in Rapanos v. United States & Carabell v. United States,” which was jointly issued by the U.S. 

Environmental Protection Agency and the Army Corps of Engineers and is available at: http://www.epa.gov/

owow/wetlands/pdf/RapanosGuidance6507.pdf, http://www.epa.gov/owow/wetlands/. 

wetland(s)

Those areas that are inundated or saturated by surface or groundwater at a frequency and duration sufficient to 

support, and that under normal circumstances do support, a prevalence of vegetation typically adapted for life in 

saturated soil conditions [EPA, 40 CFR§ 230.3 (t)/USACE,33 CFR § 328.3 (b)]. 
Nutrient Criteria–Wetlands B-1

APPENDIX B. 

CASE STUDY: DERIVING A PHOSPHORUS CRITERION FOR THE FLORIDA 

EVERGLADES

INTRODUCTION

The Everglades (Figure B1.1) is the largest subtropical wetland in North America and is widely recognized 

for its unique ecological character. It has been affected for more than a century by rapid population growth 

in south Florida. Roughly half of the ecosystem has been drained and converted to agricultural and urban 

uses. Among other changes, the conversion of 500,000 acres of the northern Everglades to agriculture (the 

Everglades Agricultural Area or EAA) and the subsequent diking of the southern rim of Lake Okeechobee 

eliminated the normal seasonal flow of water southward from Lake Okeechobee. Furthermore, the construction 

of a complex network of internal canals and levees disrupted the natural sheetflow of water through the system 

and created a series of impounded wetlands known as “Water Conservation Areas” or WCAs. This conversion 

from a hydrologically open to a highly managed wetland occurred gradually, beginning with the excavation of 

four major canals during the 1900-1910 period and culminating with the construction of the Central and South 

Florida Flood Control Project (CSFFCP) during the 1950s and 60s (Light and Dineen 1994).

The remnant Everglades is managed for multiple and often conflicting uses including water supply, flood 

control, and the hydrologic needs of the natural ecosystem. Water management operations have altered the 

quantity, quality, timing, and delivery of flows to the Everglades relative to the pre-disturbance system; 

some parts of the system have been damaged by overdrainage, excessive flooding in other areas has stressed 

native vegetation communities. Changes to the seasonal pattern of flooding and drying have influenced 

many ecological processes, including changes in the dominant micro- and macro-phytic vegetation, declines 

in critical species, and the nesting success of wading bird populations that rely on drawdowns during a 

narrow window of time to concentrate fish prey. Canal inputs containing runoff from agricultural and urban 

lands contribute roughly 50% of flows to the managed system and have increased loads of nutrients and 

contaminants. In particular, phosphorus (P) has been identified as a key limiting nutrient in the Everglades and 

increased inputs of this nutrient have been identified as a significant factor affecting ecological processes and 

communities. 

The primary source of P to the pre-disturbance Everglades was rainfall, although seasonal flows from Lake 

Okeechobee likely contributed significant P to the northern fringe of the wetland. Prior to the implementation 

of P control efforts in the late 1990s, canal flows were estimated to contribute more than half of the P load 

to the managed Everglades (SFWMD 1992). Discharge from the EAA is the main source of water to the 

Everglades, with approximately 500,000 acres of farmland draining southward via SFWMD canals, and is the 

major source of anthropogenic P. Significant inputs also come from Lake Okeechobee, a naturally mesotrophic 

lake that has also been enriched by agricultural runoff. Several other agricultural and urban catchments 

contribute smaller amounts of P via canal discharges into various parts of the Everglades. However, in general, 

canal P concentrations and loads (and associated wetland concentrations) decline from north to south.

The history of P enrichment and associated ecological impacts is not well documented but probably occurred 

at a limited scale for much of the last century. Early reports by the South Florida Water Management District 

(e.g., Gleason et al. 1975; Swift and Nicholas 1987) showed an expansion of cattail and changes in the 

periphyton community in portions of the northern Everglades receiving EAA runoff. The severity and extent 

of P impacts were more fully recognized by 1988 when the Federal Government sued the State of Florida for 

allowing P-enriched discharges and associated impacts to occur in the Everglades. Settlement of this lawsuit 

eventually resulted in the enactment of the Everglades Forever Act by the Florida Legislature in 1994, which 

required the Florida Department of Environmental Protection (FDEP) to derive a numeric water quality 

criterion for P that would “prevent ecological imbalances in natural populations of flora or fauna” in the 
B-2 Nutrient Criteria–Wetlands

Appendix B1. Figure B1.1: Major hydrologic units of the remnant Florida Everglades (shaded region) 

including (from north to south) the A.R.M. Loxahacthee National Wildlife Refuge (LNWR), Water 

Conservation Area (WCA) 2A, WCA 3A, and Everglades National Park. Shaded lines represent 

the regional canal and levee system that conveys water southward from Lake Okeechobee and the 

Everglades Agricultural Area to the Everglades and urban areas along the coast.

florida

area of 

enlargement

lake

okeechobe

everglades

agricultural lnw

wca

wca

everglades

national

n
Nutrient Criteria–Wetlands B-3

Everglades. These legal and legislative events provided the basis for numerous research and monitoring efforts 

designed to better understand the effects of P enrichment and to determine levels of enrichment that produced 

undesirable ecosystem changes.

Research and monitoring were initiated by the State of Florida (the Florida Department of Environmental 

Protection and the South Florida Water Management District) and other university research groups (e.g., Duke 

University, Florida International University, University of Florida) to better understand ecological responses to 

anthropogenic P inputs and to identify a P concentration or range of concentrations that result in unacceptable 

degradation of the Everglades ecosystem. This case study reviews research and monitoring conducted by the 

State to derive a P criterion for the Everglades. This criterion was proposed by the FDEP in 2001 and approved 

in 2003. This process is divided into three parts:

1. Define the reference (i.e., historical) conditions for P and the oligotrophic ecology of the 

 Everglades;

2. Determine the types of ecological impacts caused by P enrichment; and,

3. Identify wetland P concentrations that produce these impacts, and determine a criterion that 

 will protect the resource from those impacts.

Defining the Reference Condition

Several sources of information were used to characterize reference conditions across the Everglades. Sampling 

in minimally impacted locations (i.e., reference sites) believed to best reflect historical conditions provided 

the quantitative basis for establishing reference conditions with respect to P concentrations and associated 

ecological conditions. Where possible, this characterization was augmented by historical evidence. Written 

accounts of surveys conducted during the 1800s and early 1900s provided useful qualitative data on past 

ecological conditions. Early scientific literature contained substantial information on large-scale vegetation 

patterns (e.g., Davis 1943; Loveless 1959). Paleoecological assessments, including the dating and analysis of 

soil cores with respect to nutrient content and preserved materials such as pollen provided, further information 

(e.g., Cooper and Goman 2001, Willard et al. 2001).

Predisturbance Everglades exhibited significant spatial and temporal variation, and, while its conversion to a 

smaller, more managed wetland resulted in the loss of some of this heterogeneity, the legacy of past variations 

in hydrology, chemistry, and biology remain in many areas. Legislation mandating the development of a P 

criterion stipulated that natural variation in P concentrations and ecological conditions within the remnant 

ecosystem be considered. This required that sampling efforts encompass the expected range of background 

variability in the remnant ecosystem. To ensure that spatial variation in P conditions were considered, 

sampling was conducted in all four major hydrologic units: The Loxahatchee National Wildlife Refuge 

(LNWR), WCA-2A, WCA-3A, and Everglades National Park (see Figure B1.1). 

Water Column Phosphorus

Nutrient inputs to the Everglades were historically derived primarily from atmospheric deposition (rainfall and 

dry fallout), which is typically low in P. Historical loading rates have been estimated from annual atmospheric 

P inputs in south Florida and reconstructions of P accumulation in Everglades soils and probably averaged 

less than 0.1 g P m-2 y-1 (SFWMD 1992). Atmospheric inputs of P were augmented by inflows from Lake 

Okeechobee, which was connected by surface-water flows to the northern Everglades during periods of high 

water (Parker et al. 1955). While inflows from this historically eutrophic lake were undoubtedly enriched in 

P compared with the Everglades, the influence of these inputs were likely limited to wetlands along the lake’s 

southern fringe (Snyder and Davidson 1994) as is demonstrated by the limited extent of pond apple and other 

vegetation that require more nutrients for growth than the sawgrass (Cladium jamaicense) that dominates most 

of the Everglades.
B-4 Nutrient Criteria–Wetlands

Interior areas of the Everglades generally retain the oligotrophic characteristics of the predrainage ecosystem 

and, thus, provide the best contemporary information on historical P concentrations. Water chemistry data 

were available for several interior locations that had been sampled by the State for many years. Median 

water-column TP concentrations at these stations ranged between 4 and 10 μg L-1, with lowest concentrations 

occurring in southern areas that have been least affected by anthropogenic P loads (Figure B1.2). Phosphorus 

concentrations >10 μg L-1 were measured periodically at many of these sites. Isolated high P concentrations 

at reference stations were attributed to P released as a result of oxidation of exposed soils, increased fire 

frequency during droughts, and difficulties in collecting water samples that are not contaminated by flocculent 

wetland sediments when water depths are low. Data from reference sites may represent an upper estimate of 

historical TP concentrations in the Everglades since several stations are located in areas that either have been 

overdrained, a condition which promotes soil oxidation and P release, or so heavily exposed to canal inflows 

(e.g., WCA 2A) that some P inputs have likely intruded even into interior areas. However, in the absence of 

reliable historical data these values were deemed as best available for defining reference condition.

Soil Phosphorus

Extensive soil mapping projects across interior portions of the central and northern Everglades indicate a 

reference range for soil TP in the surface 0-10 cm of soil of between 200 and 500 mg kg-1 on a mass basis 

(DeBusk et al. 1994; Reddy et al. 1994a, Newman et al. 1997; Richardson et al. 1997a, Newman et al. 1998). 

Fewer data are available from ENP, but available evidence indicates background concentrations of < 400 mg 

kg-1 (Doren et al. 1997). Soil P content also varies volumetrically as a function of changing soil bulk density. 

The typical bulk density of flooded Everglades peat soils is approximately 0.08 g cm-3, whereas soils that have 

been subjected to extended dry out and oxidation can have bulk densities greater than 0.2 g cm-3 (Newman 

et al. 1998). Increases in volumetric nutrient concentrations resulting from increased bulk density can have 

a stimulatory effect on plant growth even in the absence of external P inputs (see Chapter 2). Following 

correction for the varying bulk densities in the peat soils of the Everglades, a historical TP concentration of 

<40 μg cm-3 may be applicable for most regions (DeBusk et al. 1994; Reddy et al. 1994a, Newman et al. 1997; 

Newman et al. 1998; Reddy et al. 1998). In the LNWR, most of the interior area has soil TP < 20 μg TP cm-3

(Newman et al. 1997).

Appendix B1. figure B1.2: Box plots showing surface-water P concentrations at long-term monitoring 

stations in each major hydrologic unit that illustrate the minimally impacted (i.e., reference) condition 

of the Everglades with respect to P. The top, mid-line, and bottom of each box represents the 75th, 

50th (median), and 25th percentile of data, respectively; the error bars represent the 90th and 10th 

percentiles; open circles are data outside the 90th percentile; the dashed line is the analytical limit for 

TP (4 μg L-1). 

Hydrologic Unit

Water-column TP (μg/L)

1

10

100

1000

 LNWR WCA2A WCA3A ENP
Nutrient Criteria–Wetlands B-5

Reference Ecological Conditions

The Everglades is perhaps the most intensively studied wetland in the world and, therefore, the ecological 

attributes that defined the predisturbance structure and function of this ecosystem are well understood 

compared with most wetlands. Clearly, not all of the valued ecological attributes of this or any other wetland 

are affected directly by P enrichment. Thus, in order to define the reference condition of the ecosystem with 

respect to the role of P, this assessment focused on those processes and communities that are most sensitive 

to P enrichment. Based on available information and preliminary scoping studies, five ecological features 

were selected as biotic response variables. These features included three indicators of ecosystem structure, 

one indicator of ecosystem function, and one indicator of landscape change. Structural indicators included the 

periphyton community, dominant macrophyte populations, and the benthic macroinvertebrate community. 

Diel fluctuations in water column DO provided an important indicator of shifts in aquatic metabolism. The 

landscape indicator of change was the loss of open-water slough-wet prairie habitats—areas of high natural 

diversity and productivity.

Periphyton

Aquatic vegetation and other submerged surfaces in the oligotrophic Everglades interior are covered with 

periphyton, a community of algae, bacteria and other microorganisms. Periphyton accounts for a significant 

portion of primary productivity in sloughs and wet prairies (Wood and Maynard 1974; Browder et al. 1982; 

McCormick et al. 1998), and floating and attached periphyton mats provide an important habitat and food 

source for invertebrates and small fish (Browder et al. 1994; Rader 1994). These mats store large amounts 

of P (approaching 1 kg TP m-2 in some locations) and, thus, may play a critical role in maintaining low P 

concentrations in reference areas (McCormick et al. 1998; McCormick and Scinto 1999). Periphyton biomass 

and productivity peak towards the end of the wet season (August through October) and reach a minimum 

during the colder months of the dry season (January through March). Periphyton biomass in open-water 

habitats can exceed 1 kg m-2 during the wet season (Wood and Maynard 1974; Browder et al.1982; McCormick 

et al. 1998) when floating mats can become so dense as to cover the entire water surface. Aerobic conditions in 

slough-wet prairie habitats is maintained by the high productivity of this community and the capacity of dense 

algal mats to trap oxygen released during photosynthesis (McCormick and Laing 2003).

Two types of periphyton communities occur in reference areas of the Everglades. Mineral-rich waters, such 

as those found WCA 2A and Taylor Slough (ENP), support a periphyton assemblage dominated by a few 

species of calcium-precipitating cyanobacteria and diatoms, while the soft-water interior of LNWR contain a 

characteristic assemblage of desmid green algae and diatoms. Waters across much of the southern Everglades 

(WCA-3A and portions of ENP) tend to be intermediate with respect to mineral content and contain some taxa 

from both assemblages. 

The chemical composition of periphyton in the oligotrophic Everglades is indicative of severe P limitation. 

Periphyton samples from reference areas of major hydrologic units within the Everglades are characterized 

by an extremely low P content (generally <0.05%) and extremely high N:P ratios (generally >60:1 w:w). This 

observational evidence for P limitation is supported by experimental fertilization studies that have shown 

that: 1) periphyton responds more strongly to P enrichment than to enrichment with other commonly limiting 

nutrients such as nitrogen (Scheidt et al. 1989; Vymazal et al. 1994); and, 2) periphyton changes in response 

to experimental P enrichment mimic those that occur along field nutrient gradients (McCormick and O’Dell 

1996). Thus, it is well-established that periphyton is strongly P-limited in reference areas of the Everglades.

Dissolved Oxygen

Interior Everglades habitats exhibit characteristic diel fluctuations in water-column dissolved oxygen (DO), 

although aerobic conditions are generally maintained throughout much or all of the diel cycle (Belanger et 

al. 1989; McCormick et al. 1997; McCormick and Laing 2003). High daytime concentrations in open-water 

habitats (i.e., sloughs, wet prairies) are a product of photosynthesis by periphyton and other submerged 

vegetation. These habitats may serve as oxygen sources for adjacent sawgrass stands, where submerged 
B-6 Nutrient Criteria–Wetlands

productivity is low (Belanger et al. 1989). Oxygen concentrations decline rapidly during the night due to 

periphyton and sediment microbial respiration and generally fall below the 5 mg L-1 standard for Class III 

Florida waters (Criterion 17-302.560(21), F.A.C.). However, these diurnal excursions are characteristic of 

reference areas throughout the Everglades (McCormick et al. 1997) and are not considered a violation of the 

Class III standard (Nearhoof 1992). In fact, a site specific criterion for DO has been adopted by the State; a 

PDF copy of the technical support document (Weaver 2004) can be found at: http://www.dep.state.fl.us/water/

wqssp/everglades/docs/DOTechSupportDOC2004.pdf. 

Vegetation 

The vegetation communities characteristic of the pristine Everglades are dominated by species adapted to 

low P, seasonal patterns of wetting and drying, and periodic natural disturbances such as fire, drought, and 

occasional freezes (Duever et al. 1994; Davis 1943; Steward and Ornes 1983; Parker 1974). Major aquatic 

vegetation habitats in oligotrophic areas include sawgrass wetlands, wet prairies, and sloughs (Loveless 1959; 

Gunderson 1994). The spatial arrangement of these habitats is dynamic and controlled by environmental 

factors such as fire, water depth, nutrient availability, and local topography (Loveless 1959). 

Sawgrass (Cladium jamaicense) is the dominant macrophyte in the Everglades, and stands of this species 

compromise approximately 65 to 70% of the total vegetation cover of the Everglades (Loveless 1959). Wet 

prairies include a collection of low-stature, graminoid communities occurring on both peat and marl soils 

(Gunderson 1994). Dominant macrophyte taxa in these habitats include Rhynchospora, Panicum, and 

Eleocharis (Loveless 1959; Craighead 1971). Sloughs are deeper water habitats that remain wet most or all of 

the year and are characterized by floating macrophytes such as fragrant white water lily (Nymphaea odorata), 

floating hearts (Nymphoides Aquaticum), and spatterdock (Nuphar advena) (Loveless 1959; Gunderson 1994). 

Submerged aquatic plants, primarily bladderworts (Utricularia foliosa and U. purpurea in particular), also can 

be abundant in these habitats and, in the case of U. purpurea, provide a substrate for the formation of dense 

periphyton mats.

Several studies have concluded that macrophyte communities in the Everglades are P-limited. Sawgrass is 

adapted to the low-P conditions indicative of the pristine Everglades (Steward and Ornes 1975b, Steward 

and Ornes 1983). During field and greenhouse manipulations, sawgrass responded to P enrichment either by 

increasing the rate of growth or P uptake (Steward and Ornes 1975a, Steward and Ornes 1983; Craft et al. 1995; 

Miao et al. 1997; Daoust and Childers 1999). Furthermore, additions of N alone had no effect on sawgrass 

or cattail growth under low-P conditions (Steward and Ornes 1983; Craft et al. 1995). Recent experimental 

evidence in the Everglades National Park (Daoust and Childers 1999) has shown that other native vegetation 

associations such as wet prairie communities are also limited by P. 

Historically, cattail (Typha spp.) was one of several minor macrophyte species native to the Everglades (Davis 

1943; Loveless 1959). In particular, cattail is believed to have been associated largely with areas of disturbance 

such as alligator holes and recent burns (Davis 1994). Analyses of Everglades peat deposits reveal no evidence 

of cattail peat, although the presence of cattail pollen indicates its presence historically in some areas (Gleason 

and Stone 1994; Davis et al. 1994; Bartow et al. 1996). Findings such as these confirm the historical presence 

of cattail in the Everglades but provide no evidence for the existence of dense cattail stands covering large 

areas (Wood and Tanner 1990) as now occurs in the northern Everglades. In contrast, sawgrass and water lily 

peats have been major freshwater Everglades soils for approximately 4,000 years (McDowell et al. 1969). 

Macroinvertebrates

Aquatic invertebrates (e.g., insects, snails, and crayfish) represent a key intermediate position in energy flow 

through the Everglades food web as these taxa are the direct consumers of primary production and, in turn, 

are consumed by vertebrate predators. Invertebrates occupy several functional niches within the Everglades 

food web; however, most taxa are direct consumers of periphyton and/or plant detritus (e.g., Rader and 

Richardson 1994; McCormick et al. 2004). Rader (1994) sampled both periphyton and macrophyte habitats in 
Nutrient Criteria–Wetlands B-7

this same area and, based on the proportional abundance of different functional groups, suggested that grazer 

(periphyton) and detrital (plant) pathways contributed equally to energy flow in low-nutrient areas of the 

Everglades. 

The macroinvertebrate fauna of the Everglades is fairly diverse (approximately 200 taxa identified) and is 

dominated by Diptera (49 taxa), Coleoptera (48 taxa), Gastropoda (17 taxa), Odonata (14 taxa), and Oligochaeta 

(11 taxa) (Rader 1999). Most studies have focused on a few conspicuous species (e.g., crayfish and apple 

snails) considered to be of special importance to vertebrate predators, and relatively little is known about 

the distribution and environmental tolerances of most taxa. An assemblage of benthic microinvertebrates 

(meiofauna) dominated by Copepoda and Cladocera is also present in the Everglades (Loftus et al. 1986), but 

even less is known about the distribution and ecology of these organisms.

Invertebrates are not distributed evenly among Everglades habitats but, instead, tend to be concentrated in 

periphyton-rich habitats such as sloughs. In an early study, Reark (1961) noted that invertebrate densities 

in ENP were higher in periphyton habitats compared with sawgrass stands. Rader (1994) reported similar 

findings in the northern Everglades and found mean annual invertebrate densities to be more than six-fold 

higher in sloughs than in sawgrass stands. Invertebrate assemblages in sloughs were more species-rich 

and contained considerably higher densities of most dominant invertebrate groups. Functionally, slough 

invertebrate assemblages contained similar densities of periphyton grazers and detritivores compared with a 

detritivore-dominated assemblage in sawgrass stands. Higher invertebrate densities in sloughs were attributed 

primarily to abundant growths of periphyton and submerged vegetation, which provide oxygen and a source of 

high-quality food.

Quantifying Impacts

A targeted design (see Chapter 4) was used to quantify changes in key ecological attributes in response to 

P enrichment. Discharges of canal waters through fixed water-control structures are the primary source of 

anthropogenic P for the Everglades and produce P gradients that extend several kilometers into the wetland 

in several locations. These gradients have existed for several decades and provided the clearest example of 

the long-term ecological impacts associated with P enrichment. Monitoring was conducted along gradients in 

different parts of the Everglades to assess ecological responses to P enrichment. Fixed sampling stations were 

located along the full extent of each gradient to document ecological conditions associated with increasing 

levels of P enrichment. Intensive monitoring was performed along gradients in two northern Everglades 

wetlands, WCA 2A and the LNWR. WCA 2A is a mineral-rich, slightly basic peatland and contains the most 

pronounced and well studied P gradient in the Everglades, whereas LNWR is a soft-water, slightly acidic 

peatland. These two wetlands represent the most extreme natural water chemistry conditions in the Everglades 

and support distinct periphyton assemblages and macrophyte populations while sharing dominant species such 

as sawgrass and water lily. Less intensive sampling along gradients in other parts of the Everglades (WCA 3A 

and ENP) to confirm that P relationships were consistent across the wetland.

 

Chemical and biological conditions were measured at each sampling station along the two intensively sampled 

gradients. Repeated sampling, sometimes over several years, was performed to ensure that temporal variation 

in each metric was considered in the final data analysis. Monthly surface-water sampling and less frequent 

soil sampling were performed to quantify P gradients in each area. Diel DO regimes, periphyton, and benthic 

macroinvertebrates were sampled quarterly when surface water was present. Macrophyte sampling included 

ground-based methods to document shifts in species composition and remote sensing to determine changes 

in landscape patterns. The hydrology of each site was characterized to determine whether P gradients were 

confounded with hydrologic gradients, which can also exert a strong influence on ecological patterns. 

Numerous field experiments have been conducted to quantify ecological responses to P enrichment and to 

better understand how interactions between P enrichment and other factors such as hydrology may affect 

these responses. The design of these experiments varied in complexity with respect to size and dosing regimen 
B-8 Nutrient Criteria–Wetlands

depending on the specific objective of each study and has included enclosed fertilizer plots (e.g., Craft et al. 

1995), semi-permeable mesocosms receiving periodic P additions to achieve fixed loading rates in the form of 

periodic additions (e.g., McCormick and O’Dell 1996), flumes receiving semi-continuous enrichment at a fixed 

rate (Pan et al. 2000), and flumes receiving flow-adjusted dosing to achieve constant inflow concentrations 

(Childers et al. 2002). These experiments were useful in establishing the causal nature of responses to P 

enrichment documented along the P gradients described above. 

Gradient Phosporous Concentrations

Strong gradients in P concentrations were documented downstream of canal discharges into most Everglades 

wetlands (Figure B1.3). Inflow TP concentrations in from 1996-1999 have averaged as high as 100 μg L-1 as 

compared with reference and pre-disturbance concentrations < 10 μg L-1 . The degree and spatial extent of 

P enrichment varies among areas depending on the source and magnitude of inflows. The most extensive 

enrichment has occurred in the northern Everglades near EAA inflows, while southern areas (e.g., ENP) have 

been relatively less affected. The most extensive enrichment has occurred in WCA-2A, which, unlike other 

areas, receives most of its water from canal discharges. Soil TP was strongly correlated with surface-water 

concentrations and exceeded 1500 mg kg-1 at the most enriched locations as compared with concentrations < 

500 mg kg-1 in reference areas. In general, this enrichment effect is limited to the surface 30 cm of soil depth 

(Reddy et al. 1998). 

Ecological Responses to Phosphorous Enrichment

Periphyton

Periphyton responses to P enrichment include changes in productivity, biomass, and species composition. 

Periphyton rapidly accumulates P from the water (McCormick et al. 2001, Noe et al. 2003), and, thus, a 

strong relationship between P concentrations in the water and periphyton is maintained along the P gradients 

Appendix B1. Figure B1.3: Mean water-column TP concentrations (1996-1999) at long-term 

monitoring stations downstream of canal discharges in two northern Everglades wetlands, WCA 2A 

(circles connected by solid line) and LNWR (squares connected by dashed line). Error bars are + 1 SE. 

Distance from canal (km)

02468 10 12 14 16

Water-column TP (μg/L)

0

30

60

90

120

150
Nutrient Criteria–Wetlands B-9

(Grimshaw et al. 1993; McCormick et al. 1996). In fact, increases in periphyton P may provide one of the 

earliest signals of P enrichment (e.g., Gaiser et al. 2004). Rapid increases in periphyton photosynthetic 

activity and growth rates occur in response to P enrichment (e.g., Swift and Nicholas 1987; McCormick et al. 

1996; McCormick et al. 2001). All of these responses are consistent with the P-limited nature of Everglades 

periphyton. 

Paradoxically, these physiological responses are associated with sharply lower periphyton biomass in 

P-enriched areas due to the loss of the abundant community of calcareous cyanobacteria and diatoms that 

is indicative of mineral-rich reference areas. This community is replaced by a eutrophic community of 

filamentous cyanobacteria, filamentous green algae, and diatoms in areas having even slightly elevated 

P concentrations. For example, McCormick and O’Dell (1996) found that the calcareous assemblage that 

existed at low water-column P concentrations (TP = 5 to 7 μg L-1) was replaced by a filamentous green algal 

assemblage at moderately elevated concentrations (TP = 10 to 28 μg L-1) and by eutrophic cyanobacteria and 

diatoms species at even higher concentrations (TP = 42 to 134 μg L-1). These results are representative of those 

documented by other investigators (e.g., Swift and Nicholas 1987; Pan et al. 2000). Taxonomic changes in 

response to controlled P enrichment in field experiments have been shown to be similar to those documented 

along field enrichment gradients (Figure B1.4), thereby providing causal evidence that changes in the 

periphyton assemblage were largely a product of P enrichment (McCormick and O’Dell 1996; Pan et al. 2000). 

Community Metabolism and Dissolved Oxygen Concentrations

Phosphorus enrichment causes a shift in the balance between autotrophy and heterotrophy in the water 

column as a result of contrasting effects on periphyton productivity and microbial respiration. Rates of aquatic 

primary productivity (P) and respiration (R) are approximately balanced (P:R ratio = 1) across the diel cycle 

in minimally impacted sloughs throughout the Everglades (Belanger et al. 1989; McCormick et al. 1997). In 

contrast, respiration rates exceed productivity by a considerable margin (P:R ratio << 1) at enriched locations. 

This change is related primarily to a large reduction in areal periphyton productivity as a result of shading 

by dense stands of cattail (Typha domingensis) that form a nearly continuous cover in the most enriched 

areas (McCormick and Laing, 2003). Increased cattail production also stimulates microbial respiration (e.g., 

sediment oxygen demand) (e.g., Belanger et al. 1989) due to an increase in the quantity and decomposability 

of macrophyte litter.

The shift towards dominance of heterotrophic processes with P enrichment, in turn, affects dissolved oxygen 

(DO) concentrations in enriched areas. For example, DO concentrations at an enriched site in WCA 2A rarely 

exceeded 2 mg L-1 compared with concentrations as high as 12 mg L-1 at reference locations (McCormick et 

al. 1997). Depressed water-column DO concentrations have subsequently been documented in enriched areas 

of WCA 2A and the LNWR (Figure B1.5) and confirmed in experimental P-enrichment studies (McCormick 

and Laing 2003). Declines in DO along field P gradients were steepest within a range of water-column TP 

concentrations roughly between 10 and 30 μg L-1. Lower DO in enriched areas are associated with other 

changes including an increase in anaerobic microbial processes and a shift in invertebrate species composition 

toward species tolerant of low DO, described later in this study. 

Macrophytes

Nutrient enrichment initially stimulates the growth of existing vegetation as evidenced by increased plant P 

content, photosynthesis, and biomass production, as it does for periphyton. Persistent enrichment eventually 

produces a shift in vegetation composition toward species better adapted to rapid growth and expansion under 

conditions of high P availability. Two major shifts in Everglades plant communities have been documented 

along P gradients, including: (1) the replacement of sawgrass stands by cattail; and, (2) the replacement of 

slough-wet prairie habitat by cattail. 

Sawgrass populations in the Everglades have life-history characteristics indicative of plants adapted to lownutrient environments (Davis 1989; Davis 1994; Miao and Sklar 1998). Sawgrass responses to P enrichment 
B-10 Nutrient Criteria–Wetlands

Appendix B1. figure B1.4: Changes in percent biomass (as biovolume) of major algal groups in 

field enclosures dosed weekly with different P loads (left panel) and along a P enrichment gradient 

downstream of canal discharges (right panel) in WCA 2A. From McCormick and O’Dell (1996). 

Loading rate

(g P m-2 yr-1)

0.1 1 10

Percent Chlorophyta

0

20

40

60

80

100

Water Column P

(μg TP L-1)

10 100

Mesocosms Transects

0.1 1 10

Percent Cyanobacteria

0

20

40

60

80

100

10 100

0.1 1 10

Percent Bacillariophyta

0

10

20

30

40

50

10 100

Appendix B1. Figure B1.5: Relationship between water-column DO metrics and TP concentration 

at several stations and time intervals along P gradients downstream of canal discharges into two 

northern Everglades wetlands (see Figure 1 for map). Total P concentrations are mean values for all 

samples (n = 3 to 6) collected during the three-month period preceding DO measurements, which were 

typically collected over 3-4 diel cycles using data loggers. Correlation coefficients are Spearman rank 

coefficients based on all data in the plot. Adapted from McCormick and Laing (2003). 

WCA-2A

Mean DO (mg/L)

0

2

4

6

8

0 10 20 30 40

r

2

 = 0.489

p < 0.001

Water-column TP (μg/L)

0 30 60 90 120

Frequency <1 mg/L (%)

0

25

50

75

100 r

2

 = 0.692

p < 0.001 

r

2

 = 0.436

p < 0.001 

LNWR

r

2

 = 0.314

p = 0.004 

r

2

 = 0.712

p < 0.001 

Minimum DO (mg/L)

0

2

4

6

8 r

2

 = 0.615

p < 0.001 
Nutrient Criteria–Wetlands B-11

include an increase in tissue P, plant biomass, P storage, annual leaf production and turnover rates, and seed 

production (e.g., Davis 1989; Craft and Richardson 1997; Miao and Sklar 1998). Cattail is characterized by 

a high growth rate, a short life cycle, high reproductive output, and other traits that confer a competitive 

advantage under enriched conditions (Davis 1989; Davis 1994; Goslee and Richardson 1997; Miao and Sklar 

1998).

Measurements and controlled enrichment experiments have shown that cattail growth rates exceed those 

of sawgrass under enriched conditions (Davis 1989; Newman et al. 1996; Miao and DeBusk 1999). The 

replacement of sawgrass by cattail in P enriched areas may be facilitated by disturbances such as flooding or 

severe fires that weaken or kill sawgrass plants and create openings. Consequently, sawgrass distributional 

patterns were not as clearly related to P gradients as were other ecological indicators of enrichment.

Sloughs and wet prairies appear to be particularly sensitive to replacement by cattail under P-enriched 

conditions, possibly due to the sparser vegetation cover in these habitats. The process of slough enrichment 

and replacement by cattail as shown in satellite imagery is supported by ground-based sampling methods 

(McCormick et al. 1999) that documented changes in slough vegetation and encroachment of these habitats 

by cattail in areas where soil TP concentrations averaged between 400 and 600 mg kg-1 and water-column TP 

in recent years averaged > 10 μg L-1. Eleocharis declined in response to increased soil P, and Nymphaea was 

stimulated by enrichment and was dominant in slightly enriched sloughs. Increased occurrence of cattail in 

sloughs was associated with a decline in Nymphaea, probably as a result of increased shading of the water 

surface. These findings are consistent with those of Vaithiyanathan et al., (1995) who documented a decline in 

slough habitats along this same nutrient gradient and the loss of sensitive taxa such as Eleocharis at locations 

where soil TP exceeded 700 mg kg-1. As discussed by McCormick et al., (2002), loss of these open-water areas 

is a sensitive landscape indicator of P enrichment (Figure B1.6).

Benthic Macroinvertebrates

Macroinvertebrates are the most widely used biological indicator of water quality impacts, and several changes 

that occur in this community along P enrichment gradients in the Everglades are similar to those documented 

in response to eutrophication in other aquatic ecosystems. Several studies have documented an overall increase 

in macroinvertebrate abundance with increasing P enrichment (Rader and Richardson 1994; Trexler and 

Turner et al. 1999; McCormick et al. 2004). However, differences in sampling methodology have apparently 

produced conflicting results with respect to changes in species richness and diversity. For example, Rader and 

Richardson (1994) documented an increase in both macroinvertebrate species richness and diversity with P 

enrichment in open-water (i.e., low emergent macrophyte cover) habitats and concluded that enrichment had 

not impacted this community. McCormick et al., (2004), however, using a landscape approach that involved 

habitat-weighted sampling, found little change in either species richness or diversity in response to enrichment. 

This latter study accounted for the decline in the cover of habitats such as sloughs and wet prairies, which 

contain the most diverse and abundant macroinvertebrate communities (Rader 1994). McCormick et al., 

(2004) also documented a pronounced shift in community composition with increasing P enrichment as taxa 

characteristic of the oligotrophic interior of the wetland are replaced by common pollution-tolerant taxa of 

oligochaetes and chironomids. These changes were indicative of habitat degradation as determined using biotic 

indices derived by the Florida DEP to assess stream condition based on macroinvertebrate composition (results 

available at http://www.epa.gov/owow/wetlands/bawwg/case/fl2.html). 

As for many other P-induced biological changes, the greatest change in the macroinvertebrate community 

occurred in response to relatively small increases in P concentration. Along field enrichment gradients, 

community shifts were associated with increases in water-column TP above approximately 10 μg L-1

(McCormick et al. 2004). Similarly, Qian et al., (2004) documented several shifts in community structure and 

function in response to long-term experimental dosing at average concentrations of approximately 10-15 μg 

L-1.
B-12 Nutrient Criteria–Wetlands

Establishing a Phosphorous Criterion

The FDEP was charged with reviewing and analyzing available P and ecological data collected throughout 

the Everglades to establish a numeric P criterion. A brief summary of this process is provided here, and more 

detailed information can be found in Payne et al., (2000, 2001a,b; available at http://www.dep.state.fl.us/water/

wqssp/everglades/pctsd.htm; and, 2002, 2003, available at http://www.sfwmd.gov/sfer/previous_ecr.html). 

The narrative nutrient standard for Class III Florida waters such as the Everglades states that “in no case shall 

nutrient concentrations of a body of water be altered so as to cause an imbalance in natural populations of 

aquatic flora or fauna.” The FDEP approach to detecting violations of this standard with respect to surfacewater P concentrations in the Everglades was to test for statistically significant departures in ecological 

conditions from those at reference sites (i.e., interior sampling locations with background P concentrations). 

Biological and chemical data collected along anthropogenic P gradients throughout the Everglades were 

analyzed to determine P concentrations associated with such departures. Results showed that sampling sites 

with average (geometric mean) surface-water TP concentrations significantly greater than 10 ppb consistently 

exhibited significant departures in ecological condition from that of reference sites. A key finding supporting 

this concentration as the standard was the fact that multiple changes in each of the major indicator groups—

periphyton, dissolved oxygen, macrophytes, and macroinvertebrates—all occurred at or near this same 

concentration (e.g., Payne et al. 2001). 

Data from field and laboratory experiments conducted by various research groups provided valuable 

supporting information for understanding responses to P enrichment. While such experiments were not 

used directly to derive the P criterion, they established cause-effect relationships between P enrichment 

and ecological change that supported correlative relationships documented along field P gradients. For 

example, McCormick and O’Dell (1996) and Pan et al., (2000) showed that major shifts in periphyton species 

composition documented along field P gradients matched those elicited by controlled P dosing in field 

enrichment experiments. McCormick and Laing (2003) confirmed that controlled P enrichment produced 

declines in water-column DO similar to those measured along the gradients. Macroinvertebrate community 

changes were documented experimentally, Qian et al., (2004).

While the criterion established a surface-water concentration of 10 μg L-1 TP as protective of native flora 

and fauna, the methodology used to measure compliance with the criterion needed to normalize background 

fluctuations in concentration. Additional analyses of P data collected over several years at reference sites was 

used to set both a longer-term average concentration and a shorter-term maximum concentration for each 

site. Based on these analyses, the FDEP concluded that annual maximum concentrations at a given sampling 

location should not exceed 15 μg L-1 TP over the long-term, while five-year average concentrations should not 

exceed 10 μg L-1 TP. These limits would be applied to reference areas to ensure no further degradation and 

to areas already impacted by P enrichment to gauge the rate and extent of recovery in response to a suite of P 

control measures, including agricultural BMPs and the construction of treatment wetlands to remove P from 

surface runoff prior to being discharged into the Everglades. Additional information on Florida’s progress 

in assessing and implementing the adopted standard can be found on the South Florida Water Management 

District Web site: www.sfwmd.gov.
Nutrient Criteria–Wetlands B-13

Appendix B1. figure B1.6: Changes in the percentage of open-water (i.e., sloughs, wet prairies, or 

other opening caused by natural disturbance or airboats) cover at 94 locations along a P enrichment 

gradient in WCA 2A as determined using aerial 

Distance from canal (km)

0 2 4 6 8 10 12 14 16

Percent open water

0

10

20

30

40

50

60

70

80

Water-Column TP (μg/L)

0

20

40

60

80

100

120

140

160

180

Nutrient Criteria–Wetlands L-1

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