United States Office of Water EPA 822-B-01-009

Environmental Protection 4304 December 2001

Agency

Ambient Water Quality

Criteria Recommendations

Information Supporting the Development 

of State and Tribal Nutrient Criteria

Lakes and Reservoirs in 

Nutrient Ecoregion IV
EPA-822-B-01-009

AMBIENT WATER QUALITY CRITERIA RECOMMENDATIONS

INFORMATION SUPPORTING THE DEVELOPMENT OF STATE AND TRIBAL

NUTRIENT CRITERIA

FOR

LAKES AND RESERVOIRS IN NUTRIENT ECOREGION IV

Great Plains Grass and Shrublands

including all or parts of the States of:

North Dakota, Montana, Wyoming, Nebraska, Colorado,

Kansas, Oklahoma, New Mexico, and Texas,

and the authorized Tribes within the Ecoregion

U.S. ENVIRONMENTAL PROTECTION AGENCY

OFFICE OF WATER

OFFICE OF SCIENCE AND TECHNOLOGY

HEALTH AND ECOLOGICAL CRITERIA DIVISION

WASHINGTON, DC

DECEMBER 2001
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FOREWORD

This document presents EPA’s nutrient criteria for Lakes and Reservoirs in Nutrient

Ecoregion IV. These criteria provide EPA’s recommendations to States and authorized Tribes

for use in establishing their water quality standards consistent with section 303(c) of the Clean

Water Act (CWA). Under section 303(c) of the CWA, States and authorized Tribes have the

primary responsibility for adopting water quality standards as part of State or Tribal law or

regulation. Federal regulations require State and Tribal standards to contain scientifically

defensible water quality criteria that are protective of designated uses. EPA’s recommended

section 304(a) criteria are not laws or regulations; they are guidance that States and Tribes may

use as a starting point in creating their own water quality standards. 

The term “water quality criteria” is used in two sections of the CWA, section 304(a)(1) and

section 303(c)(2). The term has a different impact in each section. On the one hand, in section

304, the term represents a scientific assessment of ecological and human health effects that EPA

recommends to States and authorized Tribes for establishing water quality standards that

ultimately provide a basis for controlling discharges or releases of pollutants or related

parameters. On the other hand, in section 303, ambient water quality criteria are developed by

States and Tribes as part of their water quality standards, to define the level of a pollutant (or in

the case of nutrients, a condition) necessary to protect designated uses in ambient waters. 

 Quantified water quality criteria contained within State or Tribal water quality standards

are essential to a water quality-based approach to pollution control. Whether expressed

numerically or as quantified translations of narrative criteria within State or Tribal water quality

standards, quantified criteria are critical for assessing attainment of designated uses and

measuring progress toward meeting CWA goals.

EPA is developing section 304(a) water quality criteria for nutrients because States and

Tribes consistently identify excessive levels of nutrients as a major reason that as many as half of

the Nation’s surface waters surveyed do not meet water quality objectives, such as full support of

aquatic life. EPA expects to develop nutrient criteria that cover four major types of

waterbodies—lakes and reservoirs, rivers and streams, estuarine and coastal areas, and

wetlands—across 14 major ecoregions of the United States. EPA’s section 304(a) criteria are

intended to provide for the protection and propagation of aquatic life and recreation. To support

the development of nutrient criteria, EPA has published and will continue to publish technical

guidance manuals that describe a process for assessing nutrient conditions in the four waterbody

types listed above. 

EPA’s section 304(a) water quality criteria for nutrients provide numeric water quality

criteria and procedures to help establish quantified criteria within State or Tribal water quality

standards. In the case of nutrients, EPA section 304(a) criteria establish values for causal

variables (e.g., total nitrogen and total phosphorus) and response variables (e.g., Secchi depth

and chlorophyll a). EPA believes that State and Tribal water quality standards need to include

quantified endpoints for causal and response variables to provide sufficient protection of uses

and to maintain downstream uses. These endpoints will most often be expressed as numeric

water quality criteria or as procedures to translate a State or Tribal narrative criterion into a

quantified endpoint.
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States and authorized Tribes have several options in adopting these criteria. EPA

recommends the following approaches, in order of preference:

1. Wherever possible, develop nutrient criteria that fully reflect local conditions and protect

specific designated uses through the process described in EPA’s technical guidance

manuals for nutrient criteria development. Such criteria may be expressed either as

numeric criteria or as procedures to translate a State or Tribal narrative criterion into a

quantified endpoint in State or Tribal water quality standards.

2. Adopt EPA’s section 304(a) water quality criteria for nutrients, either as numeric criteria or

as procedures to translate a State or Tribal narrative nutrient criterion into a quantified

endpoint.

3. Develop nutrient criteria protective of designated uses using other scientifically defensible

methods and appropriate water quality data.

EPA developed the nutrient criteria recommendations in this document with the intent that

they serve as a starting point for States and Tribes to develop more refined criteria, as

appropriate, to reflect local conditions. The values presented in this document generally

represent nutrient levels that protect against the adverse effects of nutrient overenrichment. They

are based on the information that was available to the Agency at the time of this publication. 

EPA expects States and Tribes may have additional information and data that may be utilized in

the refinement of these criteria. EPA offers to work with States and authorized Tribes to

establish the necessary quantitative endpoints to reduce the excess nutrient inputs into our

nation's waters and to prevent any further impairments.

 

Geoffrey H. Grubbs, Director

Office of Science and Technology
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DISCLAIMER

This document provides technical guidance and recommendations to States, authorized

Tribes, and other authorized jurisdictions to develop water quality criteria and water quality

standards under the Clean Water Act (CWA) to protect against the adverse effects of nutrient

overenrichment. Under the CWA, States and authorized Tribes are to establish water quality

criteria to protect designated uses. State and Tribal decisionmakers retain the discretion to adopt

approaches on a case-by-case basis that differ from this guidance when appropriate and

scientifically defensible. Even though this document contains EPA’s scientific

recommendations regarding ambient concentrations of nutrients that will protect aquatic resource

quality, it does not substitute for the CWA or EPA regulations, nor is it a regulation itself. Thus

it cannot impose legally binding requirements on EPA, States, authorized Tribes, or the regulated

community, and it might not apply to a particular situation or circumstance. EPA may change

this guidance in the future.
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EXECUTIVE SUMMARY

Nutrient Program Goals

EPA developed the National Strategy for the Development of Regional Nutrient Criteria

(National Strategy) in June 1998. The strategy presents EPA’s intentions to develop technical

guidance manuals for four types of waters (lakes and reservoirs, rivers and streams, estuaries and

coastal waters, and wetlands) and produce section 304(a) criteria for specific nutrient Ecoregions

by the end of 2000. In addition, the Agency formed Regional Technical Assistance Groups

(RTAGs), which include State and Tribal representatives working to develop more refined and

localized nutrient criteria based on approaches described in the waterbody guidance manuals. 

This document presents EPA’s current recommended criteria for total phosphorus (TP), total

nitrogen (TN), chlorophyll a, and Secchi for lakes and reservoirs in Nutrient Ecoregion IV,

which were derived using the procedures described in the Lakes and Reservoirs Nutrient Criteria

Technical Guidance Manual (U.S. EPA, 2000a). 

EPA’s ecoregional nutrient criteria address cultural eutrophication—the adverse effects of

excess human-caused nutrient inputs. The criteria are empirically derived to represent surface

waters that are minimally impacted by human activities and protective of aquatic life and

recreational uses. The information contained in this document represents starting points for

States and Tribes to develop (with assistance from EPA) more refined nutrient criteria.

In developing these criteria recommendations, EPA followed a process that included, to the

extent they were readily available, the following critical elements: 

C Historical and recent nutrient data in Nutrient Ecoregion IV. Data sets from Legacy

STORET, EPA Region 8 - Montana and Wyoming, EPA Region 8 - South Dakota, and

EPA Region 8 - North Dakota were used to assess nutrient conditions from 1990 to 2000.

C Reference sites/reference conditions in Nutrient Ecoregion IV. Reference conditions

presented are based on 25th percentiles of all nutrient data, including a comparison of

reference conditions for the Aggregate Ecoregion versus the subecoregions. States and

Tribes are urged to determine their own reference sites for lakes and reservoirs at different

geographic scales and to compare them to EPA’s reference conditions.

C Models employed for prediction or validation. EPA did not identify any specific models

to develop nutrient criteria. States and Tribes are encouraged to identify and apply

appropriate models to support nutrient criteria development. 

C RTAG expert review and consensus. EPA recommends that when States and Tribes

prepare their nutrient criteria, they obtain the expert review and consent of the RTAG.

C Downstream effects of criteria. EPA encourages the RTAG to assess the potential effects

of the proposed criteria on downstream water quality and uses.
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In addition, EPA followed specific QA/QC procedures during data collection and analysis. 

All data were reviewed for duplications. All data were from ambient waters that were not

located directly outside a permitted discharger. The following States indicated that their data

were sampled and analyzed using either standard methods or EPA-approved methods: North

Dakota, South Dakota, Montana, Wyoming, Colorado, Kansas, Oklahoma, and Texas. Nebraska

indicated that standard or EPA-approved methods were used for some specific nutrient

parameters.

The following tables contain a summary of aggregate and level III Ecoregion values for

TN, TP, water column chlorophyll a, and Secchi.

BASED ON 25th PERCENTILES ONLY

Nutrient Parameters Aggregate Nutrient Ecoregion IV

Reference Conditions

Total phosphorus (μg/L) 20

Total nitrogen (mg/L) (reported) 0.44

Chlorophyll a (μg/L) (spectrophotometric

method)

2

Secchi (m) 2

For subecoregions 26, 28, 30, 31, 43, and 44 the ranges of nutrient parameter reference

conditions are as follows:

BASED ON 25th PERCENTILE ONLY

Nutrient Parameters Range of Level III Subecoregions

Reference Conditions

Total phosphorus (μg/L) 10-47*

Total nitrogen (mg/L) (calculated) 0.39-0.68*

Chlorophyll a (μg/L) (spectrophotometric

method)

0.6-9.3*

Secchi (m) 0.4-2.8*

* This value appears inordinately high and may either be a statistical anomaly or reflect a unique condition. In any case, further

regional investigation is indicated to determine the sources, i.e., measurement error, notational error, statistical anomaly,

naturally enriched conditions, or cultural impacts.
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NOTICE OF DOCUMENT AVAILABILITY

This document is available electronically to the public through the Internet at

http://www.epa.gov/OST/standards/nutrient.html. Requests for hard copies of the document

should be made to EPA’s National Service Center for Environmental Publications (NSCEP),

11029 Kenwood Road, Cincinnati, OH 45242; telephone (513) 489-8190 or toll free (800) 490-

9198. Please refer to EPA document number EPA-82-B-01-009.
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ACKNOWLEDGMENTS

The authors thankfully acknowledge the contributions of the following State and Federal

reviewers: EPA Regions 6, 7, and 8; the States of North Dakota, South Dakota, Montana,

Wyoming, Nebraska, Colorado, Kansas, Oklahoma, New Mexico, and Texas; the Tribes within

the Ecoregion; EPA headquarters personnel from the Office of Wetlands, Oceans, and

Watersheds, Office of Wastewater Management, Office of General Counsel, Office of Research

and Development, and Office of Science and Technology. EPA also acknowleges the external

peer review efforts of Paul Garrison, Wisconsin Department of Natural Resources; John Reuter,

University of California, Davis; and Eugene Welch, University of Washington.
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TABLE OF CONTENTS

Foreword . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . iii

Disclaimer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . v

Executive Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . vii

Notice of Document Availability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ix

Acknowledgments .............................................................x

List of Tables and Figures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xiii

1.0 Introduction ..............................................................1

2.0 Best Use of this Information .................................................6

3.0 Area Covered by this Document ..............................................8

3.1 Description of Aggregate Ecoregion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

3.2 Geographical Boundaries of Aggregate Ecoregion IV ........................9

3.3 Level III Subecoregions Within Aggregate Ecoregion IV ......................9

3.4 Suggested Ecoregional Subdivisions or Adjustments . . . . . . . . . . . . . . . . . . . . . . . . . 12

4.0 Data Review for Lakes and Reservoirs in Aggregate Ecoregion IV . . . . . . . . . . . . . . . . . 12

4.1 Data Sources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

4.2 Historical Data from Aggregate Ecoregion IV (TP, TN, Chl a, and Secchi) . . . . . . . 13

4.3 QA/QC of Data Sources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

4.4 Data for All Lakes/Reservoirs Within Aggregate Ecoregion IV . . . . . . . . . . . . . . . . 13

4.5 Statistical Analysis of Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

4.6 Classification of Lake/Reservoir Type . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

4.7 Summary of Data Reduction Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

5.0 Reference Sites and Conditions in Aggregate Ecoregion IV . . . . . . . . . . . . . . . . . . . . . . . 26

6.0 Models Used to Predict or Verify Response Parameters . . . . . . . . . . . . . . . . . . . . . . . . . . 26

7.0 Framework for Refining Recommended Nutrient Criteria for Lakes and Reservoirs in 

Aggregate Ecoregion IV . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

7.1 Example Worksheet for Developing Aggregate Ecoregion and 

Subecoregion Nutrient Criteria . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

7.2 Setting Seasonal Criteria . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

7.3 When Data/Reference Conditions Are Lacking . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

7.4 Site-Specific Criteria Development . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

8.0 Literature Cited . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
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9.0 Appendices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

Appendix A—Descriptive Statistics Data Tables for Aggregate Ecoregion . . . . . . . . . A-1

Appendix B—Descriptive Statistics Data Tables for Level III Subecoregions Within 

Aggregate Ecoregion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . B-1

Appendix C—Quality Control/Quality Assurance Rules . . . . . . . . . . . . . . . . . . . . . . . . . C-1
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LIST OF TABLES AND FIGURES

Tables

Table 1 Lake and reservoir records for Aggregate Ecoregion IV—Great Plains 

Grass and Shrublands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

Table 2 Reference conditions for Aggregate Ecoregion IV lakes and reservoirs . . . . . . 17

Table 3a-f Reference conditions for Ecoregion IV lakes and reservoirs . . . . . . . . . . . . . . . 18

Table 4 Changes in temperate lake attributes according to trophic state . . . . . . . . . . . . . 22

Figures

Figure 1a Fourteen nutrient Ecoregions as delineated by Omernik (2000) ..............4

Figure 1b Level III Ecoregions of the United States ...............................5

Figure 2 Aggregate Ecoregion IV . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

Figure 3 Aggregate Ecoregion IV with level III ecoregions shown . . . . . . . . . . . . . . . . . 11

Figure 4 Map of sampling locations within each level III ecoregion . . . . . . . . . . . . . . . . 14

Figure 5a Illustration of data reduction process for lake data . . . . . . . . . . . . . . . . . . . . . . . 24

Figure 5b Illustration of reference condition calculation . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
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1.0 INTRODUCTION

Background

Nutrients are essential to the health and diversity of surface waters. However, in excessive

amounts nutrients cause eutrophication or hypereutrophication, which results in overgrowth of

plant life and decline of the biological community. Excessive nutrients can also result in human

health risks, such as the growth of harmful algal blooms, most recently manifested in the

Pfiesteria outbreaks on the Gulf and East Coasts. Chronic nutrient overenrichment of a

waterbody can lead to the following consequences: algal blooms, low dissolved oxygen, fish

kills, overabundance of macrophytes, likely increased sedimentation, and species shifts of both

flora and fauna. 

Historically, National Water Quality Inventories have repeatedly shown that nutrients are a

major cause of ambient water quality use impairments. EPA’s 1996 National Water Quality

Inventory report identifies excessive nutrients as the leading cause of impairment in lakes and

the second leading cause of impairment in rivers (behind siltation). In addition, nutrients were

the second leading cause of impairments after siltation reported by the States in their 1998 lists

of impaired waters. Where use impairment is documented, nutrients contribute roughly 25%-

50% of the impairment nationally. The Clean Water Act (CWA) establishes that, wherever

possible, water quality must provide for the protection and propagation of fish, shellfish, and

wildlife, and recreation in and on the water and/or protecting the physical, chemical, and

biological integrity of those waters. In adopting water quality standards, States and Tribes

designate uses for their waters in consideration of these CWA goals, and establish water quality

criteria that contain sufficient parameters to protect that integrity and those uses. To date, EPA

has not published information and recommendations under section 304(a) for nutrients to assist

States and Tribes in establishing numeric nutrient criteria to protect uses when adopting water

quality standards.

In 1995, EPA gathered a set of national experts and asked them how best to deal with the

national nutrient problem. The experts recommended that the Agency not develop single criteria

values for phosphorus (P) or nitrogen (N) applicable to all waterbodies and regions of the

country. Rather, they recommended that EPA put a premium on regionalization, develop

guidance (assessment tools and control measures) for specific waterbodies and ecological

regions across the country, and use reference conditions (conditions that reflect pristine or

minimally impacted waters) as a basis for developing nutrient criteria.

With these suggestions as starting points, EPA developed the National Strategy for the

Development of Regional Nutrient Criteria (National Strategy), published in June 1998. This

strategy presented EPA’s intentions to develop technical guidance manuals for four types of

waters (lakes and reservoirs, rivers and streams, estuaries and coastal waters, and wetlands), and

thereafter to publish section 304(a) criteria recommendations for specific nutrient Ecoregions. 

Technical guidance manuals for lakes/reservoirs and rivers/streams were published in April 2000

and July 2000, respectively. The technical guidance manual for estuaries/coastal waters was

published in fall 2001, and the draft wetlands technical guidance manual will be published by

December 2001. Each manual presents EPA’s recommended approach for developing nutrient
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criteria values for a specific waterbody type. In addition, EPA is committed to working with

States and Tribes to develop more refined and localized nutrient criteria based on approaches

described in the waterbody guidance manuals and this document. 

Overview of the Nutrient Criteria Development Process

For each nutrient Ecoregion, EPA developed a set of recommendations for two causal

variables (total nitrogen and total phosphorus) and two early indicator response variables

(chlorophyll a [chl a] and Secchi). Other indicators such as dissolved oxygen, macrophyte or

benthic algal growth or speciation, and other fauna and flora changes are also useful. However,

the first four variables are considered to be the best suited for protecting designated uses.

The technical guidance manuals describe a process for developing nutrient criteria that

involves consideration of five factors. The first of these is the Regional Technical Assistance

Group (RTAG), which is a body of qualified regional specialists able to objectively evaluate all

of the available evidence and select the value(s) appropriate to nutrient control in the water

bodies of concern. These specialists may come from such disciplines as limnology, biology, or

natural resources management—especially water resource management, chemistry, and ecology. 

The RTAG evaluates and recommends appropriate classification techniques, usually physical,

for criteria determination within an ecoregional construct.

The second factor is the historical information available to establish a perspective of the

resource base. This is usually data and anecdotal information available within the past 10-25

years. This information gives evidence about the background and enrichment trend of the

resource.

The third factor is the existing reference condition, a selection of reference sites chosen to

represent the least culturally impacted waters of the class at the present time. The data from

these sites are combined and a value is selected to represent the reference condition, the best

attainable, most natural condition of the resource base at this time.

The RTAG comprehensively evaluates these three elements to propose a candidate criterion

(initially one each for TP, TN, chl a, and Secchi).

A fourth factor often employed is mechanistic or empirical models of the historical and

reference condition data to better understand the condition of the resource.

The final element of the process is assessment by the RTAG of the likely downstream

effects of the criterion. Will there be a negative, positive, or neutral effect on the downstream

waterbody? If the RTAG judges that a negative effect is likely, then the proposed State/Tribal

water quality criteria should be revised to ameliorate the potential for any adverse downstream

effects.

Although States and authorized Tribes do not necessarily need to incorporate all five

elements into their water quality criteria setting process (e.g., modeling may be significant in
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only some instances), the best assurance of a representative and effective criterion is a balanced

incorporation of all five elements.

Because some parts of the country have naturally different soil and parent material nutrient

content, and different precipitation regimes, the application of the criterion development process

should reflect this regional variation. Therefore, an ecoregional approach was chosen. Initially,

the continental United States was divided into 14 separate Ecoregions of similar geographical

characteristics and similar nutrient condition (Figure 1a). Ecoregions are defined as regions of

relative homogeneity in ecological systems; they depict areas within which the mosaic of

ecosystem components (biotic and abiotic as well as terrestrial and aquatic) is different from

adjacent areas in a holistic sense. Geographic characteristics such as soils, vegetation, climate,

geology, and land cover are relatively similar within each Ecoregion (Omernik, 2000). 

The nutrient Ecoregions are aggregates of EPA’s hierarchical level III Ecoregions (see

Figure 1b for a map of level III Ecoregions). As such, they are more generalized and less

defined than level III Ecoregions. EPA determined that setting ecoregional criteria for the largescale aggregates is not without its drawbacks: variability is high because of the lumping of many

waterbody classes, seasons, and years worth of multipurpose data over a large geographic area. 

For these reasons, the Agency recommends that States and Tribes develop nutrient criteria at the

level III ecoregional scale and at the waterbody-class scale, where those data are readily

available. Data analyses and recommendations on both the large Aggregate Ecoregion scale and

the more refined scales (level III Ecoregions and waterbody classes), where data were available

to make such assessments, are presented for comparison and completeness of analysis. 

Comparison of Nutrient Criteria to Biological Criteria

Biological criteria are quantitative expressions of the desired condition of the aquatic

community. Such criteria can be based on data from sites that represent the least impacted

attainable condition for a particular waterbody type in an Ecoregion, subecoregion, or watershed. 

EPA’s nutrient criteria recommendations and biological criteria recommendations have many

similarities in their basic approaches to development and data requirements. Both are

empirically derived from statistical analysis of field-collected data and expert evaluation of

current reference conditions and historical information. Both use direct measurements from the

environment to integrate the effects of complex processes that vary according to type and

location of waterbody. The resulting criteria recommendations, in both cases, are efficient uses

of existing resources and are holistic indicators of the water quality necessary to protect uses.

States and authorized Tribes can develop and apply nutrient and biological criteria in

tandem, with each providing important and useful information to interpret both the nutrient

enrichment levels and the biological condition of sampled waterbodies. For example, using the

same reference sites for both types of criteria can lead to efficiencies in both sample design and

data analysis. In one effort, environmental managers can obtain information to support

assessment of biological and nutrient condition, either through evaluating existing data sets or

through designing and conducting a common sampling program. The traditional biological

criteria variables of benthic invertebrate and fish sampling can be readily incorporated in a 
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Figure 1a. Fourteen nutrient Ecoregions as delineated by Omernik (2000). Ecoregions were based on ecosystem type, and nutrient conditions.
5

Figure 1b. Level III Ecoregions of the United States.
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nutrient assessment. To investigate the effectiveness of this tandem approach, EPA has initiated

pilot projects in both freshwater and marine environments to pursue the relationship between

nutrient overenrichment and apparent declines in diversity of benthic invertebrates and fish.

2.0 BEST USE OF THIS INFORMATION

EPA recommendations published under section 304(a) of the CWA serve several purposes,

including providing guidance to States and Tribes in adopting water quality standards for

nutrients and ultimately controlling discharges or releases of pollutants. The recommendations

also provide guidance to EPA when it determines that it is necessary to promulgate Federal

water quality standards under section 303(c). Other uses include identification of

overenrichment problems, management planning, project evaluation, and determination of status

and trends of water resources. 

State water quality inventories and listings of impaired waters consistently rank nutrient

overenrichment as a top contributor to use impairments. EPA’s water quality standards

regulations at 40 CFR §131.11(a) require States and Tribes to adopt criteria that contain

sufficient parameters and constituents to protect the designated uses of their waters. In addition,

States and Tribes need quantifiable targets for nutrients to assess attainment of uses, develop

water quality-based permit limits and source control plans, and establish targets for total

maximum daily loads (TMDLs).

EPA expects States and Tribes to address nutrient overenrichment in their water quality

standards and to build on existing State and Tribal efforts where possible. States and Tribes can

address nutrient overenrichment through establishment of numerical criteria or use of narrative

criteria statements (e.g., “free from excess nutrients that cause or contribute to undesirable or

nuisance aquatic life or produce adverse physiological response in humans, animals, or plants”). 

In the case of narrative criteria, EPA expects that States and Tribes will establish procedures to

quantitatively translate these statements for both assessment and source control purposes. 

Ecoregional nutrient criteria are developed to represent surface waters that are minimally

impacted by human activities and thus protect against the adverse effects of nutrient

overenrichment from cultural eutrophication. EPA’s recommended process for developing such

criteria includes physical classification of waterbodies, determination of current reference

conditions, evaluation of historical data and other information (such as published literature), use

of models to simulate physical and ecological processes or determine empirical relationships

among causal and response variables (if necessary), expert judgment, and evaluation of

downstream effects. EPA has used elements of this process to produce the information

contained in this document. The causal (total nitrogen, total phosphorus) and biological and

physical response (chlorophyll a, Secchi) variables represent a set of starting points for States

and Tribes to use in establishing their own criteria.

EPA recommends that States and Tribes establish numerical criteria based on section

304(a) guidance, section 304(a) guidance modified to reflect site-specific conditions, or other

scientifically defensible methods. For many pollutants, such as toxic chemicals, EPA expects

that section 304(a) guidance will provide an appropriate level of protection without further
7

modification. EPA has also published methods for modifying 304(a) criteria, such as the water

effect ratio, on a site-specific basis where conditions warrant modification to achieve the

intended level of protection. For nutrients, however, EPA expects that it will usually be

necessary for States and authorized Tribes to be more precise in identifying the nutrient levels

that protect aquatic life and recreational uses. This can be achieved through criteria modified to

reflect a smaller geographic scale than an Ecoregion, such as a subecoregion, the State or Tribe

level, or a specific class of waterbodies. Criteria can be refined by grouping data or performing

analyses at these smaller geographic scales. Refinement can also occur through further

consideration of other elements such as published literature or models.

EPA expects that the values presented in this document generally represent nutrient levels

that protect against the adverse effects of cultural overenrichment and are based on information

available to the Agency at the time of this publication. However, States and Tribes should

critically evaluate this information in light of the specific uses that need to be protected. For

example, more sensitive uses may require more stringent criteria to ensure adequate protection. 

On the other hand, overly stringent levels of protection against cultural eutrophication may

actually fall below the natural load of nutrients for certain waterbodies. In cases such as these,

the level of nutrients specified may not be sufficient to support a productive fishery. In the

criteria derivation process, it is important to distinguish between the natural load associated with

a specific waterbody using historical data and expert judgment and current reference conditions. 

These elements of the criteria derivation process are best addressed by States and Tribes with

access to information and local expertise. Therefore, EPA strongly encourages States and Tribes

to use the information contained in this document to develop more refined criteria according to

the methods described in EPA’s technical guidance manuals for specific waterbody types.

To assist in further refinement of nutrient criteria, EPA has established 10 RTAGs (experts

from EPA Regional Offices and States/Tribes). In refining criteria, States and authorized Tribes

need to provide documentation of data and analyses, along with a defensible rationale, for any

new or revised nutrient criteria they submit to EPA for review and approval. As part of EPA’s

review of State and Tribal standards, EPA intends to seek assurance from the RTAG that

proposed criteria are sufficient to protect uses.

In using the information and recommendations in this document and elsewhere to develop

numerical criteria or procedures to translate narrative criteria, EPA encourages States and Tribes

to:

C Address both chemical causal variables and early indicator response variables. Causal

variables are necessary to protect uses before impairment occurs and to maintain

downstream uses. Early response variables are necessary to warn of possible impairment

and to integrate the effects of variable and potentially unmeasured nutrient loads.

C Include variables that can be measured to determine if standards are met, and variables that

can be related to the ultimate sources of excess nutrients.

C Identify appropriate periods of duration (how long) and frequency (how often) of

occurrence in addition to magnitude (how much). EPA does not recommend identifying
8

nutrient concentrations that must be met at all times; rather a seasonal or annual averaging

period (e.g., based on weekly or biweekly measurements) is considered appropriate. 

However, these central tendency measures should apply each season or each year, except

under the most extraordinary conditions (e.g., a 100-year flood).

3.0 AREA COVERED BY THIS DOCUMENT

This chapter provides a general description of the Aggregate Ecoregion and its

geographical boundaries. Descriptions of the level III subecoregions contained within the

Aggregate Ecoregion are also provided.

3.1 Description of Aggregate Ecoregion IV

Ecoregion IV is composed of disjunct, grassy, rolling high plains, hills, plateaus, buttes,

stabilized sand dunes, and badlands. Northernmost parts were once glaciated and contain

hummocky moraines that are studded with wetlands. The average annual freeze-free growing

season ranges from only 90 days in the north to 200 days in the south. Rainfall can vary widely

from year to year. The average annual precipitation ranges from 10 inches to 24 inches; overall,

the Great Plains Grass and Shrublands (IV) is drier than adjoining portions of the Western

Forested Mountains (II), Corn Belt - Northern Great Plains (VI), and South Central Cultivated

Great Plains (V). Both intermittent and ephemeral streams are common; perennial streams also

occur but usually originate outside the region in the Western Forested Mountains (II). 

The natural vegetation is dominantly and characteristically short grass prairie with areas of

savanna also occurring such as on the Edwards Plateau of Texas and on the stony hills of eastern

Montana; woodland is commonly found along stream courses. The region’s short grass prairie is

distinct from the forests of Ecoregion II, the sagebrush, shadscale, and creosote bush of

Ecoregion III, and the tall grass prairie of the Corn Belt and Northern Great Plains (VI).

Today, most of Ecoregion IV is rangeland and is not arable. Cropland is much less

common than in the Corn Belt and Northern Great Plains (VI) and the South Central Cultivated

Great Plains (V) because of the low and erratic precipitation, limited opportunities for irrigation,

and limitations imposed by its soils. Cattle, sheep, and horse grazing are common and have

impacted vegetation; when overgrazed, parts of the region are subject to wind erosion. Cattle

feedlots and their affects occur throughout the region but their density is far less than in

Ecoregion V. Although land use is dominated by grazing activities, some cropland agriculture

occurs such as on irrigated land adjacent to rivers and on the till, terraces, and lake plains of

north-central Montana. 

Much of Ecoregion IV is underlain by moderately soluble sandstone, siltstone, and shale

rocks or glacial drift. Rock and soil type have significantly impacted water quality within

Ecoregion IV; together rocks, glacial drift, and soils significantly affect the alkalinity, dissolved

solid sulfate, salt, and suspended sediment concentrations of streams. Some of these contain

easily dissolved minerals and readily contribute dissolved solids to streams. High sulfate

concentrations in stream water occur over broad areas and are the product of soil leaching. High
9

suspended sediment concentrations in stream water are found in steep, sparsely vegetated

watersheds composed of highly erodible, fine-grained material (USGS, 1993).

Throughout the Great Plains Grass and Shrublands (IV), measured nitrogen and phosphorus

levels in streams are generally much lower than in regions dominated by cropland agriculture or

urban-suburban development. Where cropland agriculture does occur, fertilizers are used; in

these places, both runoff and irrigation return flow carry residues of nitrogen and phosphorus to

streams. Locally, however, industries, coal mining, oil production, livestock operations, and

municipalities have affected stream quality (USGS, 1993).

3.2 Geographical Boundaries of Aggregate Ecoregion IV

Ecoregion IV is a fragmented region composed of four separate segments in the central

portion of the United States where Great Plains grass and shrubs prevail (Figure 2). The largest

and most northern segment includes eastern Montana and Wyoming, western North and South

Dakota, and continues southwards to central Nebraska. 

A second segment begins in southeastern Colorado, continues south to northeastern New

Mexico and extends east as a narrow strip running through northern Texas and extending down

to central Texas. From Texas the thin segment continues to stretch north to include small

portions of western Oklahoma and southwestern Kansas. 

A third segment encompasses a portion of eastern Kansas and runs south into a small

section of central Oklahoma. The remaining segment encompasses a portion south central

Texas.

3.3 Level III Subecoregions Within Aggregate Ecoregion IV

There are six level III subecoregions contained within Aggregate Ecoregion IV (Figure 3). 

The following are brief descriptions provided by Omernik (1999) of the climate, vegetative

cover, topography, and other ecological information pertaining to these subecoregions.

26. Southwestern Tablelands

Unlike most adjacent Great Plains ecological regions, little of the Southwestern Tablelands

is in cropland. Much of this elevated tableland is in subhumid grassland and semiarid grazing

land. The potential natural vegetation in this region is grama-buffalo grass with some mesquitebuffalo grass in the southeast and shinnery (midgrass prairie with open low and shrubs) along the

Canadian River.

28. Flint Hills

The Flint Hills is a region of limestone and shale open hills with relatively narrow steep

valleys. In contrast to surrounding ecological regions that are mostly in cropland, most of the

Flint Hills is grazed by beef cattle. Potential natural vegetation in the region is tallgrass prairie.
10

Figure 2. Aggregate Ecoregion IV.
11

Figure 3. Aggregate Ecoregion IV with level III ecoregions shown.
12

30. Edwards Plateau

This Ecoregion is largely a dissected plateau that is hillier in the south and east where it is

easily distinguished from bordering ecological regions by a sharp fault line. The region contains

a sparse network of perennial streams, but they are relatively clear and cool compared to those of

surrounding areas. Originally covered by juniper-oak savanna and mesquite-oak savanna, most

of the region is used for grazing beef cattle, sheep, goats, and wildlife. Hunting leases are a

major source of income.

31. Southern Texas Plains

This rolling to moderately dissected plain was once covered with grassland and savanna

vegetation. Having been subject to long continued grazing, thorny brush is now the predominant

vegetation type. This “brush country,” as it is called locally, has its greatest extent in Mexico

and contains a greater and more distinct diversity of animal life than that found elsewhere in

Texas.

43. Northwestern Great Plains

The Northwestern Great Plains Ecoregion encompasses the Missouri Plateau section of the

Great Plains. It is a semiarid rolling plain of shale and sandstone punctuated by occasional

buttes. Native grasslands, largely replaced on level ground by spring wheat and alfalfa, persist in

rangeland areas on broken topography. Agriculture is restricted by the erratic precipitation and

limited opportunities for irrigation.

44. Nebraska Sandhills

The Nebraska Sandhills comprise one of the most distinct and homogenous Ecoregions in

North America. One of the largest areas of grass stabilized sand dunes in the world, this region

is generally devoid of cropland agriculture, and except for some riparian areas in the north and

east, the region is treeless. Large portions of this Ecoregion contain numerous lakes and

wetlands and have a lack of streams.

3.4 Suggested Ecoregional Subdivisions or Adjustments

EPA recommends that the RTAG evaluate the adequacy of EPA nutrient ecoregional and

subecoregional boundaries and refine them as needed to reflect local conditions. See the paper

by Dale Robertson (USGS, 2001b) for an alternative approach to Ecoregions entitled “An

Alternative Regarding the Scheme for Defining Nutrient Criteria for Rivers and Streams.”

4.0 DATA REVIEW FOR LAKES AND RESERVOIRS IN AGGREGATE

ECOREGION IV

This section describes the nutrient data EPA has collected and analyzed for this Ecoregion,

including an assessment of data quantity and quality. The data tables present the data for each

causal parameter (total phosphorus and total nitrogen, both reported and calculated from TKN
13

and nitrite/nitrate) and the primary response variables (Secchi and chlorophyll a). EPA

considers these parameters essential to nutrient assessment, because the first two are the main

causative agents of enrichment and the two response variables are the early indicators of

enrichment for most surface waters (see Chapter 5 of the Lakes and Reservoirs Nutrient Criteria

Technical Guidance Manual [U.S. EPA, 2000a] for a complete discussion on choosing causal

and response variables). 

4.1 Data Sources

Data sets from Legacy STORET, EPA Region 8 - Montana and Wyoming, EPA 

Region 8 - South Dakota, and EPA Region 8 - North Dakota were used to assess nutrient

conditions from 1990 to 2000. EPA recommends that the RTAGs identify additional data

sources that can be used to supplement the data sets listed above. In addition, the RTAGs may

utilize published literature values to support quantitative and qualitative analyses.

4.2 Historical Data from Aggregate Ecoregion IV (TP, TN, chl a, and Secchi)

EPA recommends that States/Tribes assess long-term trends observed over the past 50

years to assess the relative stability of the systems. This information may be obtained from

scientific literature or documentation of historical trends. To gain additional perspective on

more recent trends, it is recommended that States and Tribes assess nutrient trends over the past

10 years (e.g., what do seasonal variations indicate?).

4.3 QA/QC of Data Sources

An initial quality screen of data was conducted using the rules presented in Appendix C. 

Data remaining after screening for duplications and other QA measures (e.g., poor or unreported

analytical records, sampling errors or omissions, stations associated with outfalls, stormwater

sewers, hazardous waste sites) were used in the statistical analyses.

States within Ecoregion IV were contacted regarding the quality of their data and

information on the methods used to sample and analyze their waters. The following States

indicated standard methods or approved EPA methods were used: North Dakota, South Dakota,

Montana, Wyoming, Colorado, Kansas, Oklahoma, and Texas. Nebraska indicated that standard

or EPA-approved methods were used for some specific nutrient parameters. New Mexico did

not provide information prior to the publication of this document.

4.4 Data for All Lakes and Reservoirs Within Aggregate Ecoregion IV 

Figure 4 shows the location of the sampling stations within each subecoregion. Table 1

presents all data records for all parameters for Aggregate Ecoregion IV and subecoregions within

the Aggregate Ecoregion.
14

Figure 4. Map of sampling locations within each level III ecoregion.
15

Table 1. Lake and Reservoir records* for Aggregate Ecoregion IV—Great Plains Grass and Shrublands

Aggregate

Ecoregion IV Sub ecoR 26 Sub ecoR 28 Sub ecoR 30 Sub ecoR 31 Sub ecoR 

# of lakes 162 29 40 14 2 59 # of lake stations 285 42 54 43 4 124 Key nutrient parameters (listed below)- # of records for Secchidepth 2582 276 202 1486 27 544 - # of records forchlorophyll a (allmethods)

693 104 178 363 28 20 

- # of records for totalKjeldhal nitrogen(TKN)

1,798 106 409 680 19 528 

- # of records for nitrite+ nitrate (NO2+NO3) 1,453 127 342 417 — 508 

- # of records for totalnitrogen (TN) 7 7 ————- # of records for totalphosphorus (TP) 2,247 261 480 788 32 634 Total # of records forkey nutrient parameters 8,780 881 1,611 3,734 106 2,234 

Definitions: (1) # of records refers to the total count of observations for that parameter over the entire decade (1990-1999) for that particular aggregaare counts for all seasons over that decade. (2) # of lake stations refers to the total number of lake and reservoir stations within the aggregate or subedata were collected. Since lakes and reservoirs can cross ecoregional boundaries, it is important to note that only those portions of a lake or reservoir 

those stations) that exist within the Ecoregion are included within this table.

*The number of lakes presented in this table is based on the number of lakes and reservoirs for which nutrient data were provided in the National Nut

imply that this is the total of lakes within the Ecoregion. States and Tribes should determine the representativeness of the tabular data by comparing tadditional material they may have.
16

4.5 Statistical Analysis of Data

EPA’s Technical Guidance Manual for Developing Nutrient Criteria for Lakes and

Reservoirs describes two ways of establishing a reference condition. One method is to choose

the upper 25th percentile (75th percentile) of a reference population of lakes. This is the

preferred method. The 75th percentile is preferred by EPA because it is likely associated with

minimally impacted conditions, will be protective of designated uses, and provides management

flexibility. When reference lakes are not identified, the second method is to determine the lower

25th percentile of the population of all lakes within a region to attempt to approximate the

preferred approach. The 25th percentile of the entire population was chosen by EPA to represent

a surrogate for an actual reference population. Data analyses to date indicate that the lower 25th

percentile from an entire population roughly approximates the 75th percentile for a reference

population (see case studies for Minnesota lakes in the Lakes and Reservoirs Nutrient Criteria

Technical Guidance Document [U.S. EPA, 2000a], the case study for Tennessee streams in the

Rivers and Streams Nutrient Criteria Technical Guidance Document [U.S. EPA, 2000b], the

letter from Tennessee Department of Environment and Conservation to Geoffrey Grubbs

[TNDEC, 2000], the unpublished paper titled “Estimating the Natural Background

Concentrations of Nutrients in Streams and Rivers of the Conterminous United States” [USGS,

2001]), and the letter from Mathew Liebman, U.S. EPA Region 1 Nutrient Criteria Coordinator

to Geoffrey Grubbs [U.S. EPA, 2000c]. New York State has also presented evidence that the

25th percentile and the 75th percentile compare well based on user perceptions of water

resources (NYSDEC, 2000).

Tables 2 and 3a-f present potential reference conditions for both the Aggregate Ecoregion

and the subecoregions using both methods. However, the reference lake column is left blank

because EPA does not have reference data and anticipates that States/Tribes will provide

information on reference lakes. Tables 3a-f present potential reference conditions for lakes and

reservoirs in the level III subecoregions within the Aggregate Ecoregion. Note that the footnotes

for Table 2 apply to Tables 3a-f. Appendixes A and B provide a complete presentation of all

descriptive statistics for both the Aggregate Ecoregion and the level III subecoregions. 

Table 4 is presented for comparison purposes. It allows the reader to determine where, in

the trophic state, the recommended reference conditions fall within traditionally viewed trophic

boundaries.

4.6 Classification of Lake/Reservoir Type

Assessing the data by lake type should further reduce the variability in the data analysis. 

There were no readily available classification data in the national datasets used to develop these

criteria. States and Tribes are strongly encouraged to classify their lakes before developing a

final criterion.
17

Table 2. Reference conditions for Aggregate Ecoregion IV lakes and reservoirs

Parameter No. of lakes

N*

Reported values 25th percentiles based on all

seasons data for the decade Reference lakes§

Min Max P25† all seasons‡ P75 all seasons

TKN (mg/L) 115 0.08 3.90 0.44

NO2+NO3-N (mg/L) 98 — 1.71 0.01

TN (mg/L) - calculated 0.45

TN (mg/L) - reported 2 0.44 0.49 0.44 (zz)

TP (μg/L) 127 2 580 20

Secchi (meters) 113 0.1 4.63 2

Chlorophyll a (μg/L) - F — — — —

Chlorophyll a (μg/L) - S 65 0.2 37.5 2

Chlorophyll a (μg/L) - T 2 2.04 3.04 2.04 (zz)

*N = largest value reported for a decadal season. TN calculated is based on the sum of TKN + NO2+NO3. TN reported is actual

TN value reported in the database for one sample.

† 75th percentile for Secchi.

‡ Median for all seasons’ 25th percentiles, e.g., this value was calculated from four seasons’ 25th percentiles. If the seasonal

25th percentile (P25) TP values are: spring 10 μg/L, summer 15 μg/L, fall 12 μg/L, and winter 5 μg/L, the median value of all

seasons P25 will be 11μg/L. 

§ As determined by the Regional Technical Assistance Groups (RTAGs).

Abbreviations: P25, 25th percentile of all data; P75, 75th percentile of all data; F, Chlorophyll a measured by Fluorometric

method with acid correction; S, Chlorophyll a measured by Spectrophotometric method with acid correction; T, Chlorophyll a b

c measured by Trichromatic method; —, not applicable.

Definitions: (1) Number of Lakes refers to the largest number of lakes and reservoirs for which data existed for a given season

within an aggregate nutrient Ecoregion. (2) Medians. All values (min, max, and 25th percentiles) included in the table are based

on waterbody medians. All data for a particular parameter within a lake for the decade were reduced to one median for that lake. 

This prevents over-representation of individual waterbodies with a great deal of data versus those with fewer data points within

the statistical analysis. (3) 25th percentile for all seasons is calculated by taking the median of the 4 seasonal 25th percentiles. If

a season is missing, the median was calculated with 3 seasons of data. If fewer than 3 seasons were used to derive the median,

the entry is flagged (z). (4) A 25th percentile for a season is best derived with data from a minimum of 4 lakes/season. 

However, this table provides 25th percentiles that were derived with fewer than 4 lakes/season in order to retain all information

for all seasons. In calculating the 25th percentile for a season with fewer than 4 lake medians, the statistical program

automatically used the minimum value within the fewer-than-4 population. If fewer than 4 lakes were used in developing a

seasonal quartile and or all-seasons median, the entry is flagged (zz).

Note: For seasonal values, refer to Appendix A, “Descriptive Statistics Data Tables for Aggregate Ecoregion.”
18

Table 3a. Reference conditions for Ecoregion IV lakes and reservoirs

subecoregion 26

Parameter No. of lakes

N*

Reported values 25th percentiles based on all

seasons data for the decade Reference lakes§

Min Max P25† all seasons‡ P75 all seasons

TKN (mg/L) 17 0.27 3.54 0.38

NO2+NO3-N (mg/L) 12 0.01 0.18 0.01

TN (mg/L) - calculated 0.39

TN (mg/L) - reported 2 0.44 0.49 0.44 (zz)

TP (μg/L) 21 2 145 20

Secchi (meters) 20 0.3 2.9 1.7

Chlorophyll a (μg/L) - F — — — —

Chlorophyll a (μg/L) - S 11 0.7 18.6 1.2

Chlorophyll a (μg/L) - T 2 2 3 2

Table 3b. Reference conditions for Ecoregion IV lakes and reservoirs

subecoregion 28

Parameter No. of lakes

N*

Reported values 25th percentiles based on all

seasons data for the decade Reference lakes§

Min Max P25† all seasons‡ P75 all seasons

TKN (mg/L) 34 0.48 2.37 0.65

NO2+NO3-N (mg/L) 32 0.01 1.40 0.03

TN (mg/L) - calculated 0.68

TN (mg/L) - reported — — — —

TP (μg/L) 36 4 550 31

Secchi (meters) 34 0.2 0.7 0.4

Chlorophyll a (μg/L) - F — — — —

Chlorophyll a (μg/L) - S 34 1 56 5.5

Chlorophyll a (μg/L) - T — — — —
19

Table 3c. Reference conditions for Ecoregion IV lakes and reservoirs

subecoregion 30

Parameter No. of lakes

N*

Reported values 25th percentiles based on all

seasons data for the decade Reference lakes§

Min Max P25† all seasons‡ P75 all seasons

TKN (mg/L) 13 0.26 0.71 0.35

NO2+NO3-N (mg/L) 12 0.03 0.27 0.04

TN (mg/L) - calculated 0.39

TN (mg/L) - reported — — — —

TP (μg/L) 13 2 60 10

Secchi (meters) 14 0.8 3.3 2.1

Chlorophyll a (μg/L) - F — — — —

Chlorophyll a (μg/L) - S 13 0.5 12.1 1.7

Chlorophyll a (μg/L) - T — — — —

Table 3d. Reference conditions for Ecoregion IV lakes and reservoirs

subecoregion 31

Parameter No. of lakes

N*

Reported values 25th percentiles based on all

seasons data for the decade Reference lakes§

Min Max P25† all seasons‡ P75 all seasons

TKN (mg/L) 2 0.88 0.90 0.88 (zz)

NO2+NO3-N (mg/L) — — — —

TN (mg/L) - calculated —

TN (mg/L) - reported — — — —

TP (μg/L) 2 47 75 47 (zz)

Secchi (meters) 2 0.89 0.89 0.89 (zz)

Chlorophyll a (μg/L) - F — — — —

Chlorophyll a (μg/L) - S 2 9.34 10.85 9.34 (zz)

Chlorophyll a (μg/L) - T — — — —
20

Table 3e. Reference conditions for Ecoregion IV lakes and reservoirs

subecoregion 43

Parameter No. of lakes

N*

Reported values 25th percentiles based on all

seasons data for the decade Reference lakes§

Min Max P25† all seasons‡ P75 all seasons

TKN (mg/L) 43 0.24 3.21 0.41

NO2+NO3-N (mg/L) 40 0.03 0.36 0.05

TN (mg/L) - calculated 0.46

TN (mg/L) - reported — — — —

TP (μg/L) 49 16 415 32

Secchi (meters) 37 0.4 4.6 2.8

Chlorophyll a (μg/L) - F — — — —

Chlorophyll a (μg/L) - S 5 (z) 0.5 1.6 0.6

Chlorophyll a (μg/L) - T — — — —
21

Table 3f. Reference conditions for Ecoregion IV lakes and reservoirs

subecoregion 44

Parameter No. of lakes

N*

Reported values 25th percentiles based on all

seasons data for the decade Reference lakes§

Min Max P25† all seasons‡ P75 all seasons

TKN (mg/L) 14 0.46 2.34 0.68

NO2+NO3-N (mg/L) 16 0.00 0.38 0.00

TN (mg/L) - calculated 0.68

TN (mg/L) - reported — — — —

TP (μg/L) 10 5 237 37

Secchi (meters) 13 0.2 2.9 1

Chlorophyll a (μg/L) - F — — — —

Chlorophyll a (μg/L) - S — — — —

Chlorophyll a (μg/L) - T — — — —

* N = largest value reported for a decadal season. TN calculated is based on the sum of TKN+NO2+NO3. TN reported is actual

TN value reported in the database for one sample.

† 75th percentile for Secchi.

‡ Median for all seasons’ 25th percentiles, e.g., this value was calculated from four seasons’ 25th percentiles. If the seasonal

25th percentile (P25) TP values are: spring 10 μg/L, summer 15 μg/L, fall 12 μg/L, and winter 5 μg/L, the median value of all

seasons’ P25 will be 11μg/L. 

§ As determined by the Regional Technical Assistance Groups (RTAGs).

Abbreviations: P25, 25th percentile of all data; P75, 75th percentile of all data; F, Chlorophyll a measured by Fluorometric

method with acid correction; S, Chlorophyll a measured by Spectrophotometric method with acid correction; T, Chlorophyll a b

c measured by Trichromatic method; —, not applicable.

Definitions: (1) Number of Lakes refers to the number of lakes and reservoirs for which data existed for the summer months

since summer is generally when the greatest amount of nutrient sampling is conducted. If another season greatly predominates,

notification is made (s=spring, f=fall, w=winter). (2) Medians. All values (min, max, and 25th percentiles) included in the table

are based on waterbody medians. All data for a particular parameter within a lake for the decade were reduced to one median for

that lake. This prevents over-representation of individual waterbodies with a great deal of data versus those with fewer data

points within the statistical analysis. (3) 25th percentile for all seasons is calculated by taking the median of the 4 seasonal 25th

percentiles. If a season is missing, the median was calculated with 3 seasons of data. If fewer than 3 seasons were used to derive

the median, the entry is flagged (z). (4) A 25th percentile for a season is best derived with data from a minimum of 4

lakes/season. However, this table provides 25th percentiles that were derived with fewer than 4 lakes/season in order to retain all

information for all seasons. In calculating the 25th percentile for a season with fewer than 4 lake medians, the statistical program

automatically used the minimum value within the fewer-than-4 population. If fewer than 4 lakes were used in developing a

seasonal quartile and or all-seasons median, the entry is flagged (zz).

Note: For seasonal and yearly values, refer to Appendix B, “Descriptive Statistics Data Tables for Level III Subecoregions

Within Aggregate Ecoregion.”
22

Table 4. Changes in temperate lake attributes according to trophic state (adapted from

Carlson and Simpson, 1995)

TSI

Value

SD (m) TP

(μg/L)

Attributes Water Supply Recreation Fisheries

<30 >8 <6 Oligotrophy: Clear water,

oxygen throughout the year

in the hypolimnion

Salmonid

fisheries

dominate

30-40 8-4 6-12 Hypolimnia of shallower

lakes may become anoxic

Salmonid

fisheries in

deep lakes

40-50 4-2 12-24 Mesotrophy: Water

moderately clear but

increasing probability of

hypolimnetic anoxia during

summer

Iron and manganese

evident during the

summer. THM

precursors exceed 0.1

mg/L and turbidity >1

NTU

Hypolimnetic

anoxia results

in loss of

salmonids.

Walleye may

predominate

50-60 2-1 24-48 Eutrophy: Anoxic

hypolimnia, macrophyte

problems possible

Iron, manganese, taste,

and odor problems

worsen

Warm-water

fisheries only.

Bass may be

dominant

60-70 0.5-1 48-96 Blue-green algae dominate,

algal scums and

macrophyte problems

Weeds, algal

scums, and

low

transparency

discourage

swimming and 

boating

70-80 0.25-

0.5

96-192 Hypereutrophy (light

limited). Dense algae and

macrophytes

>80 <0.25 192-

384

Algal scums, few

macrophytes

Rough fish

dominate,

summer fish

kills possible

Note: This table is provided to allow the reader to make comparisons between the ecoregional crriteria provided in this

document and traditional nutrient and biological endpoints.
23

4.7 Summary of Data Reduction Methods

All descriptive statistics were calculated using the medians for each lake within Ecoregion

IV for which data existed. For example, if one lake had 300 observations for phosphorus over

the decade or 1 year’s time, one median resulted. Each median from each lake was then used in

calculating the percentiles for phosphorus for the aggregate nutrient Ecoregion/subecoregion

(level III Ecoregion) by season and year (Figures 5a, 5b).

Preferred Data Choices and Recommendations When Data Are Missing

1. Where data are missing or are very low in total records for a given parameter, use 25th

percentiles for parameters within an adjacent, similar subecoregion within the same

aggregate nutrient Ecoregion, or when a similar subecoregion cannot be determined, use

the the 25th percentile for the Aggregate Ecoregion or consider the lowest 25th percentile

from a subecoregion (level III) within the aggregate nutrient Ecoregion. Without data, one

may assume that the subecoregion in question is as sensitive as the most sensitive

subecoregion within the aggregate.

2. TN calculated: When reported total nitrogen (TN) median values are lacking or very low

in comparison to TKN and Nitrate/Nitrite-N values, the medians for TKN and

nitrite/nitrate-N are added, resulting in a calculated TN value. The number of samples (N)

for calculated TN is not filled in because it is represented by two subsamples of data: TKN

and nitrite/nitrate-N. Therefore, N/A is placed in this box. 

3. TN reported: This is the median based on reported values for TN from the database.

4. Chlorophyll a: Medians based on all methods are reported; however, the acid-corrected

medians are preferred to the uncorrected medians. In developing a reference condition

from a particular method, it is recommended that the method with the most observations be

used. Fluorometric and spectrophotometric observations are preferred over all other

methods. However, when no data exist for fluorometric and spectrophotometric methods,

trichromatic values may be used. Data from the various techniques are not interchangeable.

5. Periphyton: Where periphyton data exist, record them separately. For periphytondominated streams, a measure of periphyton chlorophyll is a more appropriate response

variable than planktonic chlorophyll a. See Table 4, page 101, of the Rivers and Streams

Nutrient Technical Guidance Manual (U.S. EPA, 2000b) for values of periphyton and

planktonic chlorophyll a related to eutrophy in streams.

6. Secchi depth: The 75th percentile is reported for Secchi depth because this is the only

variable for which the value of the parameter increases with greater clarity (for lakes and

reservoirs only).
24

Figure 5a. Illustration of data reduction process for lake data.
Figure 5b. Illustration of reference condition calculation.

25
26

7. Turbidity units: Turbidity units from all methods are reported. FTUs and NTUs are

preferred over JCUs. If FTUs and NTUs do not exist, use JCUs. These units are not

interchangeable. Turbidity is chosen as a response variable in streams because it can be an

indicator of increasing algal biomass due to nutrient enrichment. See pages 32-33 of the

Rivers and Streams Nutrient Technical Guidance Manual for a discussion of turbidity and

correlations with algal growth.

8. Lack of data: A dash (—) represents missing, inadequate, or inconclusive data. 

According to EPA statistical analyses, 5% or fewer of the reported observations are “below

detection.” Because of this low incidence, these data were retained and factored into the

statistical analysis as reported according to the protocols described in Appendix C, “Quality

Control/Quality Assurance Rules.” 

5.0 REFERENCE SITES AND CONDITIONS IN AGGREGATE ECOREGION IV

Reference conditions represent the natural, least impacted conditions, or what is considered

to be the most attainable conditions. This chapter compares the different reference conditions

determined from the two methods and establishes which reference condition is most appropriate.

• A priori determination of reference sites. The preferred method for establishing reference

condition is to choose the upper percentile of an a priori population of reference lakes. 

States and Tribes are encouraged to identify reference conditions based on this method.

• Statistical determination of reference conditions (25th percentile of entire database). See

Tables 2 and 3a-f in Section 4.0.

• RTAG discussion and rationale for selection of reference sites and conditions in Ecoregion

IV. The RTAG should compare the results derived from the two methods described above

and present a rationale for the final selection of reference sites. 

6.0 MODELS USED TO PREDICT OR VERIFY RESPONSE PARAMETERS

The RTAG is encouraged to identify and apply relevant models to support nutrient criteria

development. There are three scenarios under which models may be used to derive criteria or

support criteria development:

• Models for predicting correlations between causal and response variables

• Models used to verify reference conditions based on percentiles

• Regression models used to predict reference conditions in impacted areas

Appendix C of the Rivers and Streams Technical Guidance Manual (U.S. EPA, 2000b), and

Chapter 9 of the Lakes and Reservoirs Technical Guidance Manual (U.S. EPA, 2000a) should be

consulted for further details.
27

7.0 FRAMEWORK FOR REFINING RECOMMENDED NUTRIENT CRITERIA FOR

LAKES AND RESERVOIRS IN AGGREGATE ECOREGION IV

Information on each of the following six weight-of-evidence factors is important to refine

the criteria presented in this document. All elements should be addressed in developing criteria,

as is expressed in EPA’s nutrient criteria technical guidance manuals. It is our expectation that

EPA Regions, States, and Tribes (as RTAGs) will consider these elements as States/Tribes

develop their criteria. This section should be viewed as a worksheet (sections are left blank for

this purpose) to assist in the refinement of nutrient criteria. If many of these elements are

ultimately unaddressed, EPA may rely on the proposed reference conditions presented in Tables

3a-f and other literature and information readily available to the EPA Headquarters nutrient team

to develop nutrient water quality recommendations for this Ecoregion.

7.1 Example Worksheet for Developing Aggregate Ecoregion and Subecoregion Nutrient

Criteria 

Literature sources: 

 

 

 

 

 

Historical data and trends: 

 

 

 

 

 

Reference condition: 

 

 

 

 

 

Models: 

 

 

 
28

RTAG expert review and consensus: 

 

 

 

 

 

Downstream effects: 

 

 

 

 

7.2 Setting Seasonal Criteria

The recommendations presented in this document are based in part on medians of all the

25th percentile seasonal data (decadal), and as such reflect all seasons and not one particular

season or year. It is recommended that States and Tribes monitor in all seasons to best assess

compliance with the resulting criterion. States/Tribes may choose to develop criteria that reflect

each particular season or given season or a given year when there is significant variability

between seasons/years or designated uses that are specifically tied to one or more seasons of the

year (e.g., recreation, fishing). Using the tables in Appendix A and B, one can set reference

conditions based on a particular season or year and then develop a criterion based on each

individual season. Obviously, this option is season-specific and would require increased

monitoring within each season to assess compliance. If a case can be made that one season is

more appropriate than another season or more appropriate than the annual median, criteria

should be season specific. For example, in most parts of the country, spring and summer are the

most common growth periods, so criteria for chlorophyll a and Secchi may be set for spring and

summer only. However, caution should be used when developing criteria for TN and TP

because the peak loading of these nutrients may take place in seasons other than summer, such as

winter and spring. For these reasons, EPA developed annual criteria and provided additional

seasonal information in appendices.

7.3 When Data/Reference Conditions Are Lacking

When data are unavailable to develop a reference condition for a particular parameter(s)

within a subecoregion, EPA recommends one of three options: (1) use data from a similar

neighboring subecoregion (e.g., if data are few or nonexistent for the Northern Cascades,

consider using the data and reference conditions developed for the Cascades); (2) use the 25th

percentiles for the Aggregate Ecoregion; or (3) consider using the lowest of the yearly medians

for that parameter calculated for all the subecoregions within the Aggregate Ecoregion.
29

7.4 Site-Specific Criteria Development 

Criteria may be refined in a number of ways. The best way is to follow the critical

elements of criteria development as well as to refer to the Lakes and Reservoirs Nutrient Criteria

Technical Guidance Manual (U.S. EPA, 2000a). The Technical Guidance Manual presents

sections on each of the following factors to consider in setting criteria:

• Refinements to Ecoregions (Chapter 3). See paper by Dale Robertson (USGS, 2001b), an

alternative approach to ecoregions entitled “An Alternative Regarding the Scheme for

Defining Nutrient Criteria for Rivers and Streams.”

• Classification of waterbodies (Chapter 3)

• Setting seasonal criteria to reflect major seasonal climate differences and accounting for

significant or cyclical precipitation events (high-flow/low-flow conditions) (Chapter 7)

• Setting criteria for reservoirs only. (The technical guidance manual recommends that data

be separated for lakes and reservoirs and treated independently if possible because of

differing physical conditions that occur in lakes and reservoirs. In this document all data

from both reservoirs and lakes were considered together since STORET does not allow for

the differentiation of data except by waterbody name.)

8.0 LITERATURE CITED

NYSDEC (New York State Department of Environment and Conservation). 2000. 

Memorandum from Scott Kishbaugh to Jay Bloomfield, September 26, 2000, regarding reference

lakes for nutrient criteria.

Omernik JM. 1999. Primary Distinguishing Characteristics of Level III Ecoregions of the

Continental United States. Draft.

Omernik JM. 2000. Draft Aggregations of Level III Ecoregions for the National Nutrient

Strategy. [http://www.epa.gov/ost/standards/ecomap.htm]

TNDEC (Tennessee Department of Environment and Conservation). 2000. Letter to Geoff

Grubbs, October 5, 2000, containing comments on draft nutrient criteria recommendations.

U.S. EPA. 2000a. Nutrient Criteria Technical Guidance Manual: Lakes and Reservoirs. U.S.

Environmental Protection Agency, Washington, DC. EPA-822-B00-001.

U.S. EPA. 2000b. Nutrient Criteria Technical Guidance Manual: Rivers and Streams. U.S.

Environmental Protection Agency, Washington, DC. EPA-822-B00-002.

U.S. EPA. 2000c. Memorandum from Matthew Liebman to Geoffrey Grubbs, December 15,

2000, regarding comments on draft ambient water quality recommendations for development of

numeric nutrient criteria.

USGS (U.S. Geological Survey). 1993. National Water Summary 1990-1991. Water Supply

Paper 2400. U.S. Government Printing Office, Washington, D.C. 589 pages.
30

USGS (U.S. Geological Survey). 2001a. Unpublished paper titled: “Estimating the Natural

Background Concentrations of Nutrients in Streams and Rivers of the Conterminous United

States.” 34 pages.

USGS. 2001b. An Alternative Regarding the Scheme for Defining Nutrient Criteria for Rivers

and Streams. Dale M. Robertson, David A. Saad, and Ann Wieben. Water Resources

Investigations Report 01-4073.

9.0 APPENDICES

A. Descriptive Statistics Data Tables for Aggregate Ecoregion

B. Descriptive Statistics Data Tables for Level III Subecoregions Within Aggregate Ecoregion

C. Quality Control/Quality Assurance Rules
APPENDIX A

Descriptive Statistics Data Tables for Aggregate Ecoregion
Appendix A—Descriptive Statistics Data Tables for Aggregate Ecoregion 

 Aggregate Nutrient Ecoregion: IV  Lakes and Reservoirs Descriptive Statistics by Decade and Season from 1990 to 1997 Chloro_A_Phyto_Spec_A_ug_L season N MEAN MIN MAX STDDEV STDERR CV P5 P25 MEDIAN P75  FALL 31 19.45 .25000 98.15 27.21 4.89 140 0.25 2.50 11.10 21.25  SPRING 22 4.82 .25000 19.05 5.70 1.22 118 0.25 0.80 1.97 7.20  SUMMER 65 10.11 .50000 56.00 10.77 1.34 107 0.93 2.85 6.73 11.95  WINTER 18 3.08 .25000 7.48 2.11 0.50 69 0.25 1.50 2.30 4.50  Data were not always available for all years.
Appendix A—Descriptive Statistics Data Tables for Aggregate Ecoregion 

 Aggregate Nutrient Ecoregion: IV  Lakes and Reservoirs Descriptive Statistics by Decade and Season from 1992 to 1997 Chloro_A_Trich_unco_ug_L season N MEAN MIN MAX STDDEV STDERR CV P5 P25 MEDIAN P75  FALL 1 3.04 3.0350 3.04 . . . 3.04 3.04 3.04 3.04  SPRING 1 2.04 2.0350 2.04 . . . 2.04 2.04 2.04 2.04  SUMMER 2 2.46 1.3800 3.54 1.52 1.08 62 1.38 1.38 2.46 3.54  Data were not always available for all years.
Appendix A—Descriptive Statistics Data Tables for Aggregate Ecoregion 

 Aggregate Nutrient Ecoregion: IV  Lakes and Reservoirs Descriptive Statistics by Decade and Season from 1991 to 2000 DIP_ug_L season N MEAN MIN MAX STDDEV STDERR CV P5 P25 MEDIAN P75  FALL 8 39.06 8.5000 149.00 45.59 16.12 117 8.50 17.00 24.75 35.75 1 SUMMER 16 52.64 7.0000 132.00 41.79 10.45 79 7.00 17.25 42.75 81.50 1 Data were not always available for all years.
Appendix A—Descriptive Statistics Data Tables for Aggregate Ecoregion 

 Aggregate Nutrient Ecoregion: IV  Lakes and Reservoirs Descriptive Statistics by Decade and Season from 1998 to 1999 Dissolved_Oxygen_percent_sat season N MEAN MIN MAX STDDEV STDERR CV P5 P25 MEDIAN P75  SUMMER 8 89.04 75.600 104.15 8.71 3.08 10 75.60 83.35 88.85 94.08 10 Data were not always available for all years.
Appendix A—Descriptive Statistics Data Tables for Aggregate Ecoregion 

 Aggregate Nutrient Ecoregion: IV  Lakes and Reservoirs Descriptive Statistics by Decade and Season from 1990 to 2000 Dissolved_Oxygen_mg_L season N MEAN MIN MAX STDDEV STDERR CV P5 P25 MEDIAN P75  FALL 40 8.59 4.7000 13.25 1.51 0.24 18 6.13 7.76 8.79 9.50  SPRING 26 8.92 6.3000 11.90 1.28 0.25 14 7.25 8.00 8.65 9.60  SUMMER 61 7.16 .30000 15.00 2.16 0.28 30 3.10 6.50 7.40 8.15  WINTER 33 9.85 2.8000 13.10 1.94 0.34 20 6.60 8.90 9.80 11.30  Data were not always available for all years.
Appendix A—Descriptive Statistics Data Tables for Aggregate Ecoregion 

 Aggregate Nutrient Ecoregion: IV  Lakes and Reservoirs Descriptive Statistics by Decade and Season from 1990 to 2000 Nitrite_Nitrate_NO2_NO3_mg_L season N MEAN MIN MAX STDDEV STDERR CV P5 P25 MEDIAN P75  FALL 54 0.16 .00125 1.44 0.31 0.04 193 0.00 0.01 0.06 0.11  SPRING 27 0.17 .00000 1.37 0.30 0.06 173 0.01 0.01 0.10 0.18  SUMMER 98 0.61 .00000 43.63 4.40 0.44 724 0.00 0.01 0.06 0.16  WINTER 44 0.14 .00000 1.97 0.32 0.05 236 0.00 0.01 0.04 0.10  Data were not always available for all years.
Appendix A—Descriptive Statistics Data Tables for Aggregate Ecoregion 

 Aggregate Nutrient Ecoregion: IV  Lakes and Reservoirs Descriptive Statistics by Decade and Season from 1990 to 2000 Nitrogen_Tot_Kjeldhal_mg_L season N MEAN MIN MAX STDDEV STDERR CV P5 P25 MEDIAN P75  FALL 62 0.97 .24000 5.20 0.87 0.11 90 0.26 0.53 0.74 1.03  SPRING 38 0.83 .00000 3.60 0.74 0.12 88 0.20 0.40 0.55 1.15  SUMMER 115 0.96 .02500 3.60 0.76 0.07 78 0.23 0.47 0.71 1.22  WINTER 44 1.14 .12500 4.19 1.00 0.15 88 0.28 0.40 0.64 1.81  Data were not always available for all years.
Appendix A—Descriptive Statistics Data Tables for Aggregate Ecoregion 

 Aggregate Nutrient Ecoregion: IV  Lakes and Reservoirs Descriptive Statistics by Decade and Season from 1990 to 2000 SECCHI_m season N MEAN MIN MAX STDDEV STDERR CV P5 P25 MEDIAN P75  FALL 64 1.31 .12000 4.42 1.04 0.13 79 0.26 0.53 1.05 1.88  SPRING 41 1.61 .15240 4.50 1.06 0.17 66 0.23 0.75 1.63 2.15  SUMMER 113 1.25 .10000 4.75 0.88 0.08 70 0.21 0.60 1.03 1.71  WINTER 44 1.72 .10160 6.00 1.28 0.19 75 0.20 0.89 1.31 2.33  Data were not always available for all years.
Appendix A—Descriptive Statistics Data Tables for Aggregate Ecoregion 

 Aggregate Nutrient Ecoregion: IV  Lakes and Reservoirs Descriptive Statistics by Decade and Season from 1992 to 1997 Total_Nitrogen_mg_L season N MEAN MIN MAX STDDEV STDERR CV P5 P25 MEDIAN P75  FALL 1 0.09 .08500 0.09 . . . 0.09 0.09 0.09 0.09  SPRING 1 0.49 .48750 0.49 . . . 0.49 0.49 0.49 0.49  SUMMER 2 1.99 .44000 3.53 2.18 1.55 110 0.44 0.44 1.99 3.53  Data were not always available for all years.
Appendix A—Descriptive Statistics Data Tables for Aggregate Ecoregion 

 Aggregate Nutrient Ecoregion: IV  Lakes and Reservoirs Descriptive Statistics by Decade and Season from 1990 to 2000 Total_Phosphorus_ug_L season N MEAN MIN MAX STDDEV STDERR CV P5 P25 MEDIAN P75  FALL 68 92.60 2.5000 530.00 109.61 13.29 118 10.00 30.00 60.00 97.50 3 SPRING 44 65.54 2.5000 570.00 94.46 14.24 144 2.50 17.50 32.50 85.00 1 SUMMER 127 88.79 2.5000 640.00 111.22 9.87 125 8.00 20.00 50.00 120.00 3 WINTER 42 89.24 2.5000 590.00 135.91 20.97 152 5.00 20.00 30.00 110.00 4 Data were not always available for all years.
Appendix A—Descriptive Statistics Data Tables for Aggregate Ecoregion 

 Aggregate Nutrient Ecoregion: IV  Lakes and Reservoirs Descriptive Statistics by Decade and Season from 1991 to 2000 pH_S_U season N MEAN MIN MAX STDDEV STDERR CV P5 P25 MEDIAN P75  FALL 8 8.22 7.7250 8.73 0.28 0.10 3 7.73 8.12 8.25 8.29  SUMMER 18 8.38 7.2208 9.85 0.58 0.14 7 7.22 8.07 8.27 8.70  Data were not always available for all years.
APPENDIX B

Descriptive Statistics Data Tables for Level III Subecoregions Within Aggregate Ecoregion
Appendix B—Descriptive Statistics Data Tables for Level III Subecoregions Within Aggregate Ec

 Aggregate Nutrient Ecoregion: IV  Lakes and Reservoirs Descriptive Statistics by Subecoregion, Decade and Season from 1990 to 1997 Chloro_A_Phyto_Spec_A_ug_L subecoregion season N MEAN MIN MAX STDDEV STDERR CV P5 P25 MEDIAN P75 P 26 FALL 11 14.26 .25000 94.90 27.3 8.23 191 0.25 0.83 4.80 13.4 94 26 SPRING 6 1.21 .25000 3.53 1.32 0.54 109 0.25 0.25 0.64 1.94 3. 26 SUMMER 11 7.59 1.1900 30.00 8.78 2.65 116 1.19 1.98 3.11 10.1 30 26 WINTER 6 2.67 1.4400 7.30 2.28 0.93 85 1.44 1.49 1.85 2.10 7. 28 FALL 10 34.17 1.0000 98.15 34.7 11.0 101 1.00 5.50 23.6 71.3 98 28 SPRING 6 7.21 .80000 14.80 6.69 2.73 93 0.80 1.00 6.75 13.2 14 28 SUMMER 34 14.75 2.7500 56.00 12.1 2.08 82 2.85 6.80 10.8 18.6 42 30 FALL 8 11.65 1.0000 21.25 8.95 3.17 77 1.00 1.81 14.3 19.4 21 30 SPRING 9 4.06 .50000 13.18 4.02 1.34 99 0.50 1.13 3.70 4.10 13 30 SUMMER 13 3.99 .50000 11.13 2.88 0.80 72 0.50 2.35 3.81 5.02 11 30 WINTER 11 2.90 .25000 5.50 1.70 0.51 59 0.25 1.50 2.92 4.50 5. 31 FALL 1 11.20 11.200 11.20 . . . 11.2 11.2 11.2 11.2 11 31 SPRING 1 19.05 19.050 19.05 . . . 19.1 19.1 19.1 19.1 19 31 SUMMER 2 6.62 2.7425 10.50 5.49 3.88 83 2.74 2.74 6.62 10.5 10 31 WINTER 1 7.48 7.4750 7.48 . . . 7.48 7.48 7.48 7.48 7. 43 FALL 1 0.25 .25000 0.25 . . . 0.25 0.25 0.25 0.25 0. 43 SUMMER 5 1.44 .84000 3.00 0.88 0.39 61 0.84 1.03 1.12 1.23 3.

Data were not always available for all years.
Appendix B—Descriptive Statistics Data Tables for Level III Subecoregions Within Aggregate Ec

 Aggregate Nutrient Ecoregion: IV  Lakes and Reservoirs Descriptive Statistics by Subecoregion, Decade and Season from 1992 to 1997 Chloro_A_Trich_unco_ug_L subecoregion season N MEAN MIN MAX STDDEV STDERR CV P5 P25 MEDIAN P75 P 26 FALL 1 3.04 3.0350 3.04 . . . 3.04 3.04 3.04 3.04 3. 26 SPRING 1 2.04 2.0350 2.04 . . . 2.04 2.04 2.04 2.04 2. 26 SUMMER 2 2.46 1.3800 3.54 1.52 1.08 62 1.38 1.38 2.46 3.54 3.

Data were not always available for all years.
Appendix B—Descriptive Statistics Data Tables for Level III Subecoregions Within Aggregate Ec

 Aggregate Nutrient Ecoregion: IV  Lakes and Reservoirs Descriptive Statistics by Subecoregion, Decade and Season from 1991 to 2000 DIP_ug_L subecoregion season N MEAN MIN MAX STDDEV STDERR CV P5 P25 MEDIAN P75 P 43 FALL 8 39.06 8.5000 149.00 45.6 16.1 117 8.50 17.0 24.8 35.8 1 43 SUMMER 16 52.64 7.0000 132.00 41.8 10.4 79 7.00 17.3 42.8 81.5 1

Data were not always available for all years.
Appendix B—Descriptive Statistics Data Tables for Level III Subecoregions Within Aggregate Ec

 Aggregate Nutrient Ecoregion: IV  Lakes and Reservoirs Descriptive Statistics by Subecoregion, Decade and Season from 1998 to 1999 Dissolved_Oxygen_percent_sat subecoregion season N MEAN MIN MAX STDDEV STDERR CV P5 P25 MEDIAN P75 P 43 SUMMER 8 89.04 75.600 104.15 8.71 3.08 10 75.6 83.4 88.9 94.1 1

Data were not always available for all years.
Appendix B—Descriptive Statistics Data Tables for Level III Subecoregions Within Aggregate Ec

 Aggregate Nutrient Ecoregion: IV  Lakes and Reservoirs Descriptive Statistics by Subecoregion, Decade and Season from 1990 to 2000 Dissolved_Oxygen_mg_L subecoregion season N MEAN MIN MAX STDDEV STDERR CV P5 P25 MEDIAN P75 P 26 FALL 3 9.63 9.4000 10.00 0.32 0.19 3 9.40 9.40 9.50 10.0 10 26 SPRING 5 8.70 7.8000 9.60 0.66 0.30 8 7.80 8.50 8.60 9.00 9. 26 SUMMER 8 6.39 1.0000 10.10 2.59 0.91 40 1.00 5.68 7.08 7.28 10 26 WINTER 5 11.46 8.9000 12.90 1.52 0.68 13 8.90 11.6 11.8 12.2 12 28 FALL 2 10.10 6.9500 13.25 4.45 3.15 44 6.95 6.95 10.1 13.3 13 28 SPRING 1 8.70 8.7000 8.70 . . . 8.70 8.70 8.70 8.70 8. 30 FALL 9 6.97 4.7000 7.85 1.02 0.34 15 4.70 6.60 7.20 7.80 7. 30 SPRING 12 8.21 6.3000 9.90 0.92 0.27 11 6.30 7.85 8.20 8.69 9. 30 SUMMER 14 6.70 3.1000 8.30 1.31 0.35 19 3.10 6.25 6.88 7.70 8. 30 WINTER 13 9.26 8.4500 10.55 0.65 0.18 7 8.45 8.78 9.15 9.60 10 31 FALL 1 6.50 6.5000 6.50 . . . 6.50 6.50 6.50 6.50 6. 31 SPRING 1 7.90 7.9000 7.90 . . . 7.90 7.90 7.90 7.90 7. 31 SUMMER 2 4.00 .30000 7.70 5.23 3.70 131 0.30 0.30 4.00 7.70 7. 31 WINTER 1 10.70 10.700 10.70 . . . 10.7 10.7 10.7 10.7 10 43 FALL 17 8.84 6.0650 9.78 0.93 0.23 11 6.07 8.40 8.90 9.50 9. 43 SPRING 7 10.46 9.0000 11.90 0.99 0.37 9 9.00 9.45 10.6 11.2 11 43 SUMMER 34 7.55 1.3000 15.00 2.06 0.35 27 4.41 6.72 7.76 8.35 9. 43 WINTER 14 9.76 2.8000 13.10 2.59 0.69 27 2.80 9.00 10.2 11.4 13 44 FALL 8 9.36 8.2000 10.55 0.89 0.31 9 8.20 8.65 9.20 10.2 10 44 SUMMER 3 8.93 8.5000 9.60 0.59 0.34 7 8.50 8.50 8.70 9.60 9.

Data were not always available for all years.
Appendix B—Descriptive Statistics Data Tables for Level III Subecoregions Within Aggregate Ec

 Aggregate Nutrient Ecoregion: IV  Lakes and Reservoirs Descriptive Statistics by Subecoregion, Decade and Season from 1990 to 2000 Nitrite_Nitrate_NO2_NO3_mg_L subecoregion season N MEAN MIN MAX STDDEV STDERR CV P5 P25 MEDIAN P75 P 26 FALL 11 0.03 .01000 0.19 0.05 0.02 162 0.01 0.01 0.01 0.03 0. 26 SPRING 9 0.06 .01000 0.18 0.07 0.02 122 0.01 0.01 0.01 0.10 0. 26 SUMMER 12 0.09 .00250 0.37 0.10 0.03 111 0.00 0.02 0.07 0.12 0. 26 WINTER 6 0.03 .00438 0.07 0.02 0.01 87 0.00 0.01 0.03 0.03 0. 28 FALL 13 0.41 .00250 1.44 0.54 0.15 134 0.00 0.02 0.15 0.50 1. 28 SPRING 9 0.31 .01000 1.37 0.49 0.16 157 0.01 0.04 0.10 0.15 1. 28 SUMMER 32 0.27 .00250 1.74 0.49 0.09 180 0.00 0.02 0.09 0.22 1. 28 WINTER 2 0.18 .10000 0.27 0.12 0.08 64 0.10 0.10 0.18 0.27 0. 30 FALL 8 0.10 .01750 0.29 0.10 0.04 103 0.02 0.02 0.06 0.16 0. 30 SPRING 5 0.18 .10000 0.25 0.07 0.03 37 0.10 0.12 0.21 0.22 0. 30 SUMMER 8 0.05 .00250 0.14 0.05 0.02 98 0.00 0.01 0.04 0.09 0. 30 WINTER 12 0.13 .04000 0.31 0.09 0.03 72 0.04 0.07 0.08 0.19 0. 43 FALL 13 0.11 .02500 0.53 0.13 0.04 114 0.03 0.06 0.10 0.10 0. 43 SPRING 2 0.16 .14000 0.18 0.03 0.02 18 0.14 0.14 0.16 0.18 0. 43 SUMMER 40 1.22 .00250 43.63 6.88 1.09 566 0.00 0.01 0.06 0.18 0. 43 WINTER 8 0.07 .03000 0.14 0.04 0.01 58 0.03 0.04 0.06 0.10 0. 44 FALL 9 0.09 .00125 0.52 0.17 0.06 184 0.00 0.01 0.01 0.08 0. 44 SPRING 2 0.06 .00000 0.11 0.08 0.06 141 0.00 0.00 0.06 0.11 0. 44 SUMMER 6 0.12 .00000 0.24 0.12 0.05 103 0.00 0.00 0.12 0.23 0. 44 WINTER 16 0.21 .00000 1.97 0.53 0.13 247 0.00 0.00 0.01 0.01 1.

Data were not always available for all years.
Appendix B—Descriptive Statistics Data Tables for Level III Subecoregions Within Aggregate Ec

 Aggregate Nutrient Ecoregion: IV  Lakes and Reservoirs Descriptive Statistics by Subecoregion, Decade and Season from 1990 to 2000 Nitrogen_Tot_Kjeldhal_mg_L subecoregion season N MEAN MIN MAX STDDEV STDERR CV P5 P25 MEDIAN P75 P 26 FALL 17 0.93 .24500 5.20 1.14 0.28 122 0.25 0.40 0.70 0.90 5. 26 SPRING 11 0.77 .20000 3.60 0.95 0.29 124 0.20 0.36 0.50 0.70 3. 26 SUMMER 17 0.84 .30000 3.48 0.75 0.18 89 0.30 0.50 0.68 0.75 3. 26 WINTER 7 0.67 .30000 1.60 0.48 0.18 71 0.30 0.34 0.42 1.00 1. 28 FALL 12 1.23 .60000 3.50 0.81 0.23 66 0.60 0.78 0.97 1.26 3. 28 SPRING 11 1.39 .42500 2.80 0.67 0.20 48 0.43 0.81 1.45 1.70 2. 28 SUMMER 34 0.81 .02500 1.63 0.44 0.08 55 0.03 0.51 0.75 1.18 1. 28 WINTER 2 1.24 .52750 1.95 1.00 0.71 81 0.53 0.53 1.24 1.95 1. 30 FALL 8 0.54 .27500 0.86 0.20 0.07 36 0.28 0.39 0.53 0.68 0. 30 SPRING 11 0.44 .25000 0.70 0.14 0.04 31 0.25 0.30 0.45 0.54 0. 30 SUMMER 13 0.45 .19000 0.71 0.17 0.05 39 0.19 0.30 0.41 0.60 0. 30 WINTER 12 0.45 .30000 0.57 0.08 0.02 18 0.30 0.39 0.46 0.51 0. 31 FALL 1 0.91 .90500 0.91 . . . 0.91 0.91 0.91 0.91 0. 31 SPRING 1 0.85 .85000 0.85 . . . 0.85 0.85 0.85 0.85 0. 31 SUMMER 2 0.69 .21000 1.18 0.68 0.48 99 0.21 0.21 0.69 1.18 1. 31 WINTER 1 0.90 .90000 0.90 . . . 0.90 0.90 0.90 0.90 0. 43 FALL 15 0.79 .24000 2.83 0.66 0.17 83 0.24 0.33 0.61 0.97 2. 43 SPRING 3 0.31 .00000 0.50 0.27 0.16 88 0.00 0.00 0.42 0.50 0. 43 SUMMER 43 1.33 .24525 3.60 0.95 0.15 72 0.30 0.50 1.06 2.02 3. 43 WINTER 8 2.02 .27500 4.19 1.40 0.50 69 0.28 0.88 1.86 3.10 4. 44 FALL 9 1.37 .42250 3.31 1.01 0.34 74 0.42 0.65 0.78 2.02 3. 44 SPRING 1 1.37 1.3700 1.37 . . . 1.37 1.37 1.37 1.37 1. 44 SUMMER 6 0.81 .50000 1.32 0.33 0.13 40 0.50 0.54 0.76 0.99 1. 44 WINTER 14 1.48 .12500 3.44 0.96 0.26 65 0.13 0.71 1.60 2.02 3.

Data were not always available for all years.
Appendix B—Descriptive Statistics Data Tables for Level III Subecoregions Within Aggregate Ec

 Aggregate Nutrient Ecoregion: IV  Lakes and Reservoirs Descriptive Statistics by Subecoregion, Decade and Season from 1990 to 2000 SECCHI_m subecoregion season N MEAN MIN MAX STDDEV STDERR CV P5 P25 MEDIAN P75 P 26 FALL 12 0.89 .30000 2.45 0.58 0.17 65 0.30 0.58 0.75 1.05 2. 26 SPRING 10 1.34 .22860 3.25 1.01 0.32 75 0.23 0.40 1.30 2.15 3. 26 SUMMER 20 0.89 .10000 2.80 0.69 0.15 77 0.12 0.43 0.68 1.22 2. 26 WINTER 6 1.81 .80000 3.05 0.97 0.39 53 0.80 0.90 1.66 2.80 3. 28 FALL 11 0.34 .12000 0.61 0.14 0.04 42 0.12 0.26 0.33 0.40 0. 28 SPRING 5 0.42 .22500 0.78 0.21 0.09 50 0.23 0.30 0.40 0.40 0. 28 SUMMER 34 0.78 .15240 1.90 0.42 0.07 53 0.21 0.50 0.60 1.10 1. 28 WINTER 2 0.25 .20000 0.30 0.07 0.05 28 0.20 0.20 0.25 0.30 0. 30 FALL 9 2.18 1.2192 3.84 1.02 0.34 47 1.22 1.37 1.78 3.28 3. 30 SPRING 12 1.80 .76000 3.06 0.57 0.17 32 0.76 1.49 1.71 2.08 3. 30 SUMMER 14 1.83 .73000 3.60 0.71 0.19 39 0.73 1.40 1.88 2.15 3. 30 WINTER 13 1.63 .92500 2.87 0.58 0.16 35 0.93 1.33 1.53 2.10 2. 31 FALL 1 0.93 .92500 0.93 . . . 0.93 0.93 0.93 0.93 0. 31 SPRING 1 0.75 .75000 0.75 . . . 0.75 0.75 0.75 0.75 0. 31 SUMMER 2 1.39 1.0300 1.75 0.51 0.36 37 1.03 1.03 1.39 1.75 1. 31 WINTER 1 0.85 .85250 0.85 . . . 0.85 0.85 0.85 0.85 0. 43 FALL 22 1.73 .43180 3.75 0.95 0.20 55 0.52 1.04 1.36 2.23 3. 43 SPRING 11 2.46 .38100 4.50 1.11 0.34 45 0.38 1.79 2.59 3.26 4. 43 SUMMER 37 1.64 .20000 4.75 0.97 0.16 59 0.20 0.84 1.50 2.31 3. 43 WINTER 9 3.51 1.1557 6.00 1.37 0.46 39 1.16 2.74 3.65 4.29 6. 44 FALL 9 1.24 .22860 4.42 1.29 0.43 104 0.23 0.53 0.84 1.22 4. 44 SPRING 2 0.56 .15240 0.97 0.57 0.41 103 0.15 0.15 0.56 0.97 0. 44 SUMMER 6 1.36 .63500 4.32 1.45 0.59 107 0.64 0.66 0.81 0.93 4. 44 WINTER 13 0.81 .10160 1.52 0.39 0.11 49 0.10 0.61 0.89 0.99 1.

Data were not always available for all years.
Appendix B—Descriptive Statistics Data Tables for Level III Subecoregions Within Aggregate Ec

 Aggregate Nutrient Ecoregion: IV  Lakes and Reservoirs Descriptive Statistics by Subecoregion, Decade and Season from 1992 to 1997 Total_Nitrogen_mg_L subecoregion season N MEAN MIN MAX STDDEV STDERR CV P5 P25 MEDIAN P75 P 26 FALL 1 0.09 .08500 0.09 . . . 0.09 0.09 0.09 0.09 0. 26 SPRING 1 0.49 .48750 0.49 . . . 0.49 0.49 0.49 0.49 0. 26 SUMMER 2 1.99 .44000 3.53 2.18 1.55 110 0.44 0.44 1.99 3.53 3.

Data were not always available for all years.
Appendix B—Descriptive Statistics Data Tables for Level III Subecoregions Within Aggregate Ec

 Aggregate Nutrient Ecoregion: IV  Lakes and Reservoirs Descriptive Statistics by Subecoregion, Decade and Season from 1990 to 2000 Total_Phosphorus_ug_L subecoregion season N MEAN MIN MAX STDDEV STDERR CV P5 P25 MEDIAN P75 P 26 FALL 19 45.46 11.250 170.00 37.7 8.65 83 11.3 20.0 30.0 60.0 1 26 SPRING 14 30.98 2.5000 100.00 27.3 7.30 88 2.50 11.3 25.0 30.0 1 26 SUMMER 21 79.76 2.5000 640.00 135 29.4 169 10.0 20.0 50.0 75.0 1 26 WINTER 9 34.72 2.5000 120.00 37.4 12.5 108 2.50 20.0 22.5 30.0 1 28 FALL 14 194.82 20.000 530.00 170 45.4 87 20.0 70.0 123 285 5 28 SPRING 14 137.32 2.5000 570.00 141 37.7 103 2.50 40.0 113 150 5 28 SUMMER 36 101.47 2.5000 595.00 125 20.8 123 8.00 22.5 55.0 125 3 28 WINTER 2 50.63 6.2500 95.00 62.8 44.4 124 6.25 6.25 50.6 95.0 95 30 FALL 8 31.72 2.5000 65.00 22.4 7.93 71 2.50 10.0 35.0 48.1 65 30 SPRING 11 22.05 2.5000 65.00 18.2 5.48 82 2.50 10.0 15.0 35.0 65 30 SUMMER 13 21.68 2.5000 55.00 14.7 4.08 68 2.50 10.0 20.0 30.0 55 30 WINTER 12 24.01 2.5000 51.25 13.3 3.83 55 2.50 16.9 20.9 32.5 51 31 FALL 1 45.00 45.000 45.00 . . . 45.0 45.0 45.0 45.0 45 31 SPRING 1 50.00 50.000 50.00 . . . 50.0 50.0 50.0 50.0 50 31 SUMMER 2 55.63 11.250 100.00 62.8 44.4 113 11.3 11.3 55.6 100 1 31 WINTER 1 182.50 182.50 182.50 . . . 183 183 183 183 1 43 FALL 17 89.98 15.625 380.00 85.1 20.6 95 15.6 44.0 70.0 81.0 3 43 SPRING 3 55.00 35.000 80.00 22.9 13.2 42 35.0 35.0 50.0 80.0 80 43 SUMMER 49 105.15 4.5000 450.00 106 15.2 101 8.00 25.0 60.0 158 3 43 WINTER 8 149.53 16.250 470.00 171 60.5 114 16.3 30.0 80.0 245 4 44 FALL 9 97.50 5.0000 295.00 89.7 29.9 92 5.00 35.0 87.5 130 2 44 SPRING 1 70.00 70.000 70.00 . . . 70.0 70.0 70.0 70.0 70 44 SUMMER 6 67.08 5.0000 180.00 61.3 25.0 91 5.00 40.0 45.0 87.5 1 44 WINTER 10 166.75 5.0000 590.00 199 62.9 119 5.00 22.5 105 180 5

Data were not always available for all years.
Appendix B—Descriptive Statistics Data Tables for Level III Subecoregions Within Aggregate Ec

 Aggregate Nutrient Ecoregion: IV  Lakes and Reservoirs Descriptive Statistics by Subecoregion, Decade and Season from 1991 to 2000 pH_S_U subecoregion season N MEAN MIN MAX STDDEV STDERR CV P5 P25 MEDIAN P75 P 43 FALL 8 8.22 7.7250 8.73 0.28 0.10 3 7.73 8.12 8.25 8.29 8. 43 SUMMER 18 8.38 7.2208 9.85 0.58 0.14 7 7.22 8.07 8.27 8.70 9.

Data were not always available for all years.
Appendix B—Descriptive Statistics Data Tables for Level III Subecoregions Within Aggregate Ec

 Aggregate Nutrient Ecoregion: IV  Lakes and Reservoirs Descriptive Statistics by Subecoregion, Year and Season from 1990 to 1997 Chloro_A_Phyto_Spec_A_ug_L subecoregion year season N MEAN MIN MAX STDDEV STDERR CV P5 P25 MEDIAN P75  26 1990 SPRING 1 1.30 1.3000 1.30 . . . 1.30 1.30 1.30 1.30 1 26 1990 SUMMER 7 5.27 1.9000 13.30 4.46 1.69 85 1.90 2.20 3.60 9.80 13 26 1990 WINTER 4 1.73 1.1750 2.10 0.41 0.20 23 1.18 1.44 1.83 2.03 2 26 1991 FALL 6 24.98 4.8000 94.90 34.41 14.05 138 4.80 11.10 12.50 14.10 94 26 1991 SPRING 2 2.88 .25000 5.50 3.71 2.63 129 0.25 0.25 2.88 5.50 5 26 1991 SUMMER 5 3.56 .25000 10.70 4.20 1.88 118 0.25 1.20 1.85 3.80 10 26 1991 WINTER 4 3.70 2.5000 4.80 1.22 0.61 33 2.50 2.65 3.75 4.75 4 26 1992 SPRING 2 0.25 .25000 0.25 0.00 0.00 0 0.25 0.25 0.25 0.25 0 26 1992 SUMMER 5 3.71 .25000 10.70 4.14 1.85 112 0.25 1.19 2.50 3.90 10 26 1992 WINTER 4 2.01 .25000 7.30 3.53 1.76 175 0.25 0.25 0.25 3.78 7 26 1993 FALL 4 1.22 .25000 2.95 1.28 0.64 105 0.25 0.25 0.83 2.18 2 26 1993 SPRING 3 0.94 .25000 1.55 0.65 0.38 69 0.25 0.25 1.03 1.55 1 26 1993 SUMMER 4 2.46 .05000 4.20 1.74 0.87 71 0.05 1.43 2.80 3.50 4 26 1993 WINTER 1 2.00 2.0000 2.00 . . . 2.00 2.00 2.00 2.00 2 26 1994 FALL 1 0.25 .25000 0.25 . . . 0.25 0.25 0.25 0.25 0 26 1994 SUMMER 4 3.91 .25000 10.59 4.57 2.29 117 0.25 1.20 2.41 6.63 10 26 1995 SPRING 1 0.25 .25000 0.25 . . . 0.25 0.25 0.25 0.25 0 26 1995 SUMMER 5 18.76 .25000 46.70 17.39 7.78 93 0.25 9.85 16.70 20.30 46 26 1995 WINTER 2 0.85 .25000 1.44 0.84 0.60 100 0.25 0.25 0.85 1.44 1 26 1996 SPRING 1 0.25 .25000 0.25 . . . 0.25 0.25 0.25 0.25 0 26 1996 SUMMER 3 3.89 .25000 9.50 4.93 2.85 127 0.25 0.25 1.91 9.50 9 26 1996 WINTER 2 1.03 .25000 1.80 1.10 0.78 107 0.25 0.25 1.03 1.80 1 26 1997 FALL 1 2.63 2.6300 2.63 . . . 2.63 2.63 2.63 2.63 2 26 1997 SPRING 1 1.94 1.9400 1.94 . . . 1.94 1.94 1.94 1.94 1 26 1997 SUMMER 1 3.11 3.1050 3.11 . . . 3.11 3.11 3.11 3.11 3 28 1990 FALL 2 25.03 22.200 27.85 4.00 2.83 16 22.20 22.20 25.03 27.85 27 28 1990 SUMMER 5 9.74 3.0000 19.85 6.23 2.79 64 3.00 7.05 9.00 9.80 19 28 1991 FALL 2 26.48 20.900 32.05 7.88 5.58 30 20.90 20.90 26.48 32.05 32 28 1991 SPRING 2 13.30 11.800 14.80 2.12 1.50 16 11.80 11.80 13.30 14.80 14 28 1991 SUMMER 11 20.62 2.5000 76.75 21.26 6.41 103 2.50 3.10 12.55 28.55 76 28 1992 FALL 2 0.65 .50000 0.80 0.21 0.15 33 0.50 0.50 0.65 0.80 0 28 1992 SPRING 3 1.17 .80000 1.70 0.47 0.27 41 0.80 0.80 1.00 1.70 1
Appendix B—Descriptive Statistics Data Tables for Level III Subecoregions Within Aggregate Ec

 Aggregate Nutrient Ecoregion: IV  Lakes and Reservoirs Descriptive Statistics by Subecoregion, Year and Season from 1990 to 1997 Chloro_A_Phyto_Spec_A_ug_L subecoregion year season N MEAN MIN MAX STDDEV STDERR CV P5 P25 MEDIAN P75  28 1992 SUMMER 4 12.58 5.0000 18.60 6.22 3.11 49 5.00 7.55 13.35 17.60 18 28 1993 SUMMER 16 12.16 .20000 42.30 10.92 2.73 90 0.20 3.63 10.68 17.68 42 28 1994 FALL 8 36.63 1.5000 98.15 38.56 13.63 105 1.50 5.55 18.10 73.05 98 28 1994 SPRING 1 13.15 13.150 13.15 . . . 13.15 13.15 13.15 13.15 13 28 1994 SUMMER 7 10.74 1.6000 32.10 12.01 4.54 112 1.60 2.75 5.40 23.40 32 28 1995 SUMMER 4 8.71 6.0500 11.90 2.65 1.32 30 6.05 6.58 8.45 10.85 11 28 1996 SUMMER 8 13.39 3.5000 35.25 12.54 4.43 94 3.50 4.58 7.13 22.48 35 28 1997 SUMMER 7 8.59 4.2000 17.20 4.31 1.63 50 4.20 5.55 8.05 10.15 17 30 1990 FALL 8 14.69 1.0000 31.50 12.12 4.29 83 1.00 2.25 16.00 24.25 31 30 1990 SPRING 7 3.64 .50000 7.00 2.67 1.01 73 0.50 0.50 4.00 6.00 7 30 1990 SUMMER 12 4.29 .50000 16.70 4.29 1.24 100 0.50 1.71 3.75 5.00 16 30 1990 WINTER 8 3.06 .25000 7.00 2.54 0.90 83 0.25 0.63 3.00 5.00 7 30 1991 FALL 8 9.34 .25000 17.50 6.42 2.27 69 0.25 4.00 10.50 14.00 17 30 1991 SPRING 8 4.48 .25000 15.10 4.92 1.74 110 0.25 0.69 3.85 5.70 15 30 1991 SUMMER 10 5.89 .25000 12.00 3.74 1.18 64 0.25 2.00 6.50 8.20 12 30 1991 WINTER 8 9.38 2.0000 31.30 10.57 3.74 113 2.00 2.81 3.50 14.58 31 30 1992 FALL 3 13.82 2.6500 21.20 9.84 5.68 71 2.65 2.65 17.60 21.20 21 30 1992 SPRING 8 5.62 .25000 12.15 4.51 1.59 80 0.25 2.08 4.40 9.80 12 30 1992 SUMMER 13 5.22 .25000 9.20 3.11 0.86 60 0.25 2.80 5.00 7.55 9 30 1992 WINTER 8 4.38 .25000 14.00 4.74 1.68 108 0.25 1.13 2.25 7.00 14 30 1993 FALL 3 3.23 .25000 9.19 5.16 2.98 160 0.25 0.25 0.25 9.19 9 30 1993 SPRING 6 4.85 .50000 14.20 4.97 2.03 102 0.50 1.30 3.85 5.40 14 30 1993 SUMMER 12 4.90 .25000 12.52 3.31 0.96 68 0.25 3.19 4.99 6.49 12 30 1993 WINTER 6 1.93 .25000 4.00 1.27 0.52 66 0.25 1.10 1.90 2.40 4 30 1994 SPRING 1 0.97 .96500 0.97 . . . 0.97 0.97 0.97 0.97 0 30 1994 SUMMER 11 1.92 .25000 11.13 3.19 0.96 166 0.25 0.25 0.93 2.54 11 30 1995 SUMMER 3 1.38 .25000 3.65 1.96 1.13 142 0.25 0.25 0.25 3.65 3 30 1995 WINTER 3 2.77 1.0150 5.70 2.55 1.47 92 1.02 1.02 1.60 5.70 5 30 1996 SPRING 1 4.49 4.4900 4.49 . . . 4.49 4.49 4.49 4.49 4 30 1996 SUMMER 3 12.65 4.1350 22.00 8.96 5.17 71 4.14 4.14 11.80 22.00 22 30 1996 WINTER 3 1.77 .25000 4.82 2.64 1.52 149 0.25 0.25 0.25 4.82 4 31 1990 FALL 1 11.30 11.300 11.30 . . . 11.30 11.30 11.30 11.30 11
Appendix B—Descriptive Statistics Data Tables for Level III Subecoregions Within Aggregate Ec

 Aggregate Nutrient Ecoregion: IV  Lakes and Reservoirs Descriptive Statistics by Subecoregion, Year and Season from 1990 to 1997 Chloro_A_Phyto_Spec_A_ug_L subecoregion year season N MEAN MIN MAX STDDEV STDERR CV P5 P25 MEDIAN P75 P 31 1990 SPRING 1 19.15 19.150 19.15 . . . 19.15 19.15 19.15 19.15 19. 31 1990 SUMMER 2 5.38 .25000 10.50 7.25 5.13 135 0.25 0.25 5.38 10.50 10. 31 1991 FALL 1 12.40 12.400 12.40 . . . 12.40 12.40 12.40 12.40 12. 31 1991 SPRING 1 4.70 4.7000 4.70 . . . 4.70 4.70 4.70 4.70 4. 31 1991 SUMMER 1 12.00 12.000 12.00 . . . 12.00 12.00 12.00 12.00 12. 31 1991 WINTER 1 9.40 9.4000 9.40 . . . 9.40 9.40 9.40 9.40 9. 31 1992 FALL 1 0.25 .25000 0.25 . . . 0.25 0.25 0.25 0.25 0. 31 1992 SPRING 1 19.05 19.050 19.05 . . . 19.05 19.05 19.05 19.05 19. 31 1992 SUMMER 2 5.46 3.2750 7.65 3.09 2.19 57 3.28 3.28 5.46 7.65 7. 31 1992 WINTER 1 5.55 5.5500 5.55 . . . 5.55 5.55 5.55 5.55 5. 31 1993 FALL 1 11.10 11.100 11.10 . . . 11.10 11.10 11.10 11.10 11. 31 1993 SUMMER 1 3.31 3.3100 3.31 . . . 3.31 3.31 3.31 3.31 3. 31 1994 SUMMER 1 2.21 2.2100 2.21 . . . 2.21 2.21 2.21 2.21 2. 43 1991 SUMMER 4 1.06 .84000 1.23 0.17 0.08 16 0.84 0.94 1.08 1.18 1. 43 1992 FALL 1 0.25 .25000 0.25 . . . 0.25 0.25 0.25 0.25 0. 43 1992 SUMMER 1 3.00 3.0000 3.00 . . . 3.00 3.00 3.00 3.00 3.
Appendix B—Descriptive Statistics Data Tables for Level III Subecoregions Within Aggregate Ec

 Aggregate Nutrient Ecoregion: IV  Lakes and Reservoirs Descriptive Statistics by Subecoregion, Year and Season from 1992 to 1997 Chloro_A_Trich_unco_ug_L subecoregion year season N MEAN MIN MAX STDDEV STDERR CV P5 P25 MEDIAN P75 P 26 1992 SUMMER 1 1.38 1.3800 1.38 . . . 1.38 1.38 1.38 1.38 1. 26 1997 FALL 1 3.04 3.0350 3.04 . . . 3.04 3.04 3.04 3.04 3. 26 1997 SPRING 1 2.04 2.0350 2.04 . . . 2.04 2.04 2.04 2.04 2. 26 1997 SUMMER 1 3.54 3.5350 3.54 . . . 3.54 3.54 3.54 3.54 3.
Appendix B—Descriptive Statistics Data Tables for Level III Subecoregions Within Aggregate Ec

 Aggregate Nutrient Ecoregion: IV  Lakes and Reservoirs Descriptive Statistics by Subecoregion, Year and Season from 1991 to 2000 DIP_ug_L subecoregion year season N MEAN MIN MAX STDDEV STDERR CV P5 P25 MEDIAN P75  43 1991 FALL 8 39.06 8.5000 149.00 45.59 16.12 117 8.50 17.00 24.75 35.75 149 43 1991 SUMMER 15 65.70 8.0000 220.50 57.54 14.86 88 8.00 19.00 47.00 98.50 220 43 1992 SUMMER 6 43.33 5.0000 183.00 68.95 28.15 159 5.00 6.50 19.25 27.00 183 43 1993 SUMMER 8 89.06 6.0000 279.00 110.45 39.05 124 6.00 16.00 30.00 167.75 279 43 1994 SUMMER 7 14.14 5.0000 21.50 6.94 2.62 49 5.00 6.00 16.00 20.00 21 43 1995 SUMMER 4 11.75 6.5000 16.00 4.97 2.49 42 6.50 7.50 12.25 16.00 16 43 1998 SUMMER 4 16.00 9.5000 28.50 8.74 4.37 55 9.50 10.00 13.00 22.00 28 43 1999 SUMMER 5 27.10 7.5000 93.50 37.16 16.62 137 7.50 11.50 11.50 11.50 93 43 2000 SUMMER 2 73.00 26.500 119.50 65.76 46.50 90 26.50 26.50 73.00 119.50 119
Appendix B—Descriptive Statistics Data Tables for Level III Subecoregions Within Aggregate Ec

 Aggregate Nutrient Ecoregion: IV  Lakes and Reservoirs Descriptive Statistics by Subecoregion, Year and Season from 1998 to 1999 Dissolved_Oxygen_percent_sat subecoregion year season N MEAN MIN MAX STDDEV STDERR CV P5 P25 MEDIAN P75 P 43 1998 SUMMER 4 85.94 75.600 93.55 7.84 3.92 9 75.60 80.05 87.30 91.83 93. 43 1999 SUMMER 4 92.14 82.200 104.15 9.48 4.74 10 82.20 84.90 91.10 99.38 104.
Appendix B—Descriptive Statistics Data Tables for Level III Subecoregions Within Aggregate Ec

 Aggregate Nutrient Ecoregion: IV  Lakes and Reservoirs Descriptive Statistics by Subecoregion, Year and Season from 1990 to 2000 Dissolved_Oxygen_mg_L subecoregion year season N MEAN MIN MAX STDDEV STDERR CV P5 P25 MEDIAN P75  26 1990 SPRING 1 10.40 10.400 10.40 . . . 10.40 10.40 10.40 10.40 10 26 1990 SUMMER 6 6.73 5.6000 7.40 0.64 0.26 10 5.60 6.60 6.80 7.20 7 26 1990 WINTER 3 11.43 11.400 11.50 0.06 0.03 1 11.40 11.40 11.40 11.50 11 26 1991 SPRING 2 8.65 8.5000 8.80 0.21 0.15 2 8.50 8.50 8.65 8.80 8 26 1991 SUMMER 4 5.23 1.0000 7.60 2.98 1.49 57 1.00 3.15 6.15 7.30 7 26 1991 WINTER 4 12.14 12.100 12.20 0.05 0.02 0 12.10 12.10 12.13 12.18 12 26 1992 SPRING 1 9.00 9.0000 9.00 . . . 9.00 9.00 9.00 9.00 9 26 1992 SUMMER 4 7.23 4.1000 10.10 2.45 1.23 34 4.10 5.70 7.35 8.75 10 26 1992 WINTER 3 13.57 13.400 13.70 0.15 0.09 1 13.40 13.40 13.60 13.70 13 26 1993 FALL 2 9.75 9.5000 10.00 0.35 0.25 4 9.50 9.50 9.75 10.00 10 26 1993 SUMMER 3 4.78 .30000 7.20 3.89 2.24 81 0.30 0.30 6.85 7.20 7 26 1993 WINTER 1 8.90 8.9000 8.90 . . . 8.90 8.90 8.90 8.90 8 26 1994 FALL 1 9.40 9.4000 9.40 . . . 9.40 9.40 9.40 9.40 9 26 1994 SUMMER 5 5.46 1.0000 7.30 2.63 1.18 48 1.00 5.20 6.60 7.20 7 26 1995 SUMMER 3 5.03 .10000 7.80 4.28 2.47 85 0.10 0.10 7.20 7.80 7 26 1995 WINTER 2 10.43 10.150 10.70 0.39 0.28 4 10.15 10.15 10.43 10.70 10 26 1996 SPRING 2 8.20 7.8000 8.60 0.57 0.40 7 7.80 7.80 8.20 8.60 8 26 1996 SUMMER 1 7.10 7.1000 7.10 . . . 7.10 7.10 7.10 7.10 7 26 1996 WINTER 1 11.70 11.700 11.70 . . . 11.70 11.70 11.70 11.70 11 28 1994 FALL 2 10.10 6.9500 13.25 4.45 3.15 44 6.95 6.95 10.10 13.25 13 28 1994 SPRING 1 8.70 8.7000 8.70 . . . 8.70 8.70 8.70 8.70 8 30 1990 FALL 7 5.49 .10000 8.00 3.20 1.21 58 0.10 1.80 7.15 8.00 8 30 1990 SPRING 10 7.78 2.0000 9.80 2.24 0.71 29 2.00 7.80 8.20 9.00 9 30 1990 SUMMER 12 6.23 1.3000 11.20 2.64 0.76 42 1.30 4.90 5.55 7.85 11 30 1990 WINTER 10 8.30 6.7000 9.70 1.15 0.36 14 6.70 7.25 8.20 9.35 9 30 1991 FALL 7 6.16 2.5500 7.80 1.92 0.73 31 2.55 4.70 6.90 7.80 7 30 1991 SPRING 10 7.57 4.8000 10.20 1.70 0.54 22 4.80 5.90 7.70 9.00 10 30 1991 SUMMER 12 5.52 1.6000 7.70 2.08 0.60 38 1.60 4.53 6.15 7.13 7 30 1991 WINTER 9 8.51 8.0000 9.20 0.47 0.16 6 8.00 8.00 8.50 8.90 9 30 1992 FALL 8 7.18 5.5000 8.50 1.15 0.41 16 5.50 6.28 7.30 8.15 8 30 1992 SPRING 10 7.69 4.9000 10.00 1.64 0.52 21 4.90 6.80 7.25 9.10 10 30 1992 SUMMER 13 5.53 .45000 7.90 2.15 0.60 39 0.45 4.00 6.15 7.10 7
Appendix B—Descriptive Statistics Data Tables for Level III Subecoregions Within Aggregate Ec

 Aggregate Nutrient Ecoregion: IV  Lakes and Reservoirs Descriptive Statistics by Subecoregion, Year and Season from 1990 to 2000 Dissolved_Oxygen_mg_L subecoregion year season N MEAN MIN MAX STDDEV STDERR CV P5 P25 MEDIAN P75  30 1992 WINTER 8 9.54 8.4000 10.60 0.89 0.31 9 8.40 8.65 9.65 10.35 10 30 1993 FALL 1 6.90 6.9000 6.90 . . . 6.90 6.90 6.90 6.90 6 30 1993 SPRING 10 8.84 7.9000 10.10 0.69 0.22 8 7.90 8.25 8.73 9.50 10 30 1993 SUMMER 12 7.86 6.7000 9.75 0.89 0.26 11 6.70 7.30 7.75 8.50 9 30 1993 WINTER 10 9.63 8.8000 11.40 0.83 0.26 9 8.80 8.90 9.45 10.10 11 30 1994 FALL 1 7.55 7.5500 7.55 . . . 7.55 7.55 7.55 7.55 7 30 1994 SPRING 5 8.22 8.1000 8.40 0.13 0.06 2 8.10 8.10 8.20 8.30 8 30 1994 SUMMER 11 7.55 6.5000 8.80 0.72 0.22 10 6.50 6.80 7.70 7.90 8 30 1994 WINTER 3 10.00 9.2000 11.10 0.98 0.57 10 9.20 9.20 9.70 11.10 11 30 1995 SPRING 5 8.30 7.3000 9.30 0.75 0.34 9 7.30 8.00 8.20 8.70 9 30 1995 SUMMER 5 7.04 6.0000 8.30 0.83 0.37 12 6.00 6.80 6.90 7.20 8 30 1995 WINTER 5 9.59 8.9000 10.10 0.53 0.24 6 8.90 9.15 9.80 10.00 10 30 1996 FALL 1 6.60 6.6000 6.60 . . . 6.60 6.60 6.60 6.60 6 30 1996 SPRING 4 8.83 7.4000 10.20 1.38 0.69 16 7.40 7.65 8.85 10.00 10 30 1996 SUMMER 5 7.03 4.1500 9.20 1.86 0.83 26 4.15 6.90 7.00 7.90 9 30 1996 WINTER 4 10.00 8.6000 11.00 1.02 0.51 10 8.60 9.30 10.20 10.70 11 30 1997 SPRING 1 8.40 8.4000 8.40 . . . 8.40 8.40 8.40 8.40 8 30 1997 WINTER 1 9.80 9.8000 9.80 . . . 9.80 9.80 9.80 9.80 9 31 1990 FALL 1 1.40 1.4000 1.40 . . . 1.40 1.40 1.40 1.40 1 31 1990 SPRING 1 7.40 7.4000 7.40 . . . 7.40 7.40 7.40 7.40 7 31 1990 SUMMER 1 0.30 .30000 0.30 . . . 0.30 0.30 0.30 0.30 0 31 1991 FALL 1 9.45 9.4500 9.45 . . . 9.45 9.45 9.45 9.45 9 31 1991 SPRING 1 7.90 7.9000 7.90 . . . 7.90 7.90 7.90 7.90 7 31 1991 SUMMER 1 0.25 .25000 0.25 . . . 0.25 0.25 0.25 0.25 0 31 1991 WINTER 1 10.70 10.700 10.70 . . . 10.70 10.70 10.70 10.70 10 31 1992 FALL 1 6.50 6.5000 6.50 . . . 6.50 6.50 6.50 6.50 6 31 1992 SPRING 1 8.70 8.7000 8.70 . . . 8.70 8.70 8.70 8.70 8 31 1992 SUMMER 2 7.50 7.3000 7.70 0.28 0.20 4 7.30 7.30 7.50 7.70 7 31 1993 SUMMER 1 8.80 8.8000 8.80 . . . 8.80 8.80 8.80 8.80 8 31 1994 SUMMER 1 6.80 6.8000 6.80 . . . 6.80 6.80 6.80 6.80 6 43 1990 FALL 2 9.85 8.0000 11.70 2.62 1.85 27 8.00 8.00 9.85 11.70 11 43 1990 SPRING 1 13.70 13.700 13.70 . . . 13.70 13.70 13.70 13.70 13
Appendix B—Descriptive Statistics Data Tables for Level III Subecoregions Within Aggregate Ec

 Aggregate Nutrient Ecoregion: IV  Lakes and Reservoirs Descriptive Statistics by Subecoregion, Year and Season from 1990 to 2000 Dissolved_Oxygen_mg_L subecoregion year season N MEAN MIN MAX STDDEV STDERR CV P5 P25 MEDIAN P75  43 1990 SUMMER 4 5.98 1.3000 8.60 3.31 1.65 55 1.30 3.65 7.00 8.30 8 43 1990 WINTER 6 10.19 7.4000 14.40 2.60 1.06 26 7.40 8.30 9.53 12.00 14 43 1991 FALL 10 9.02 6.0650 10.60 1.28 0.41 14 6.07 8.78 9.50 9.75 10 43 1991 SPRING 1 9.20 9.2000 9.20 . . . 9.20 9.20 9.20 9.20 9 43 1991 SUMMER 21 7.12 4.2150 15.00 2.20 0.48 31 4.41 6.15 7.17 7.60 9 43 1991 WINTER 2 11.68 10.000 13.35 2.37 1.68 20 10.00 10.00 11.68 13.35 13 43 1992 FALL 6 9.15 8.7000 9.90 0.47 0.19 5 8.70 8.80 9.00 9.50 9 43 1992 SUMMER 12 8.16 6.9170 8.90 0.59 0.17 7 6.92 7.77 8.22 8.65 8 43 1992 WINTER 4 9.00 2.8000 13.80 5.22 2.61 58 2.80 4.70 9.70 13.30 13 43 1993 FALL 6 9.60 8.8000 11.80 1.18 0.48 12 8.80 8.90 9.00 10.10 11 43 1993 SPRING 4 10.20 9.6500 10.70 0.48 0.24 5 9.65 9.80 10.23 10.60 10 43 1993 SUMMER 14 7.26 3.2300 9.40 1.95 0.52 27 3.23 7.00 8.10 8.50 9 43 1993 WINTER 4 12.70 11.500 13.30 0.82 0.41 6 11.50 12.20 13.00 13.20 13 43 1994 FALL 4 8.83 8.5000 9.10 0.25 0.13 3 8.50 8.65 8.85 9.00 9 43 1994 SPRING 6 11.33 9.7000 12.00 0.89 0.36 8 9.70 10.90 11.75 11.90 12 43 1994 SUMMER 12 7.65 5.0670 9.00 1.04 0.30 14 5.07 7.24 7.75 8.45 9 43 1994 WINTER 4 11.53 10.500 13.30 1.24 0.62 11 10.50 10.70 11.15 12.35 13 43 1995 FALL 5 8.76 8.3000 10.00 0.71 0.32 8 8.30 8.40 8.40 8.70 10 43 1995 SPRING 3 10.95 10.350 11.90 0.83 0.48 8 10.35 10.35 10.60 11.90 11 43 1995 SUMMER 12 7.79 6.4170 8.70 0.78 0.22 10 6.42 7.05 8.03 8.38 8 43 1995 WINTER 1 11.30 11.300 11.30 . . . 11.30 11.30 11.30 11.30 11 43 1996 FALL 6 8.48 7.5000 9.20 0.62 0.25 7 7.50 8.10 8.55 9.00 9 43 1996 SPRING 3 9.50 9.0000 10.50 0.87 0.50 9 9.00 9.00 9.00 10.50 10 43 1996 SUMMER 5 7.78 6.7000 10.20 1.44 0.64 18 6.70 6.70 7.50 7.80 10 43 1996 WINTER 2 10.15 10.150 10.15 0.00 0.00 0 10.15 10.15 10.15 10.15 10 43 1997 WINTER 4 9.05 8.0000 10.20 0.99 0.49 11 8.00 8.25 9.00 9.85 10 43 1998 SUMMER 4 8.10 7.2800 8.82 0.64 0.32 8 7.28 7.64 8.15 8.56 8 43 1999 SUMMER 4 8.20 7.1500 9.91 1.25 0.62 15 7.15 7.28 7.86 9.11 9 43 2000 SUMMER 2 7.93 7.2400 8.61 0.97 0.68 12 7.24 7.24 7.93 8.61 8 44 1991 FALL 8 9.36 8.2000 10.55 0.89 0.31 9 8.20 8.65 9.20 10.20 10 44 1991 SUMMER 3 8.93 8.5000 9.60 0.59 0.34 7 8.50 8.50 8.70 9.60 9
Appendix B—Descriptive Statistics Data Tables for Level III Subecoregions Within Aggregate Ec

 Aggregate Nutrient Ecoregion: IV  Lakes and Reservoirs Descriptive Statistics by Subecoregion, Year and Season from 1990 to 2000 Nitrite_Nitrate_NO2_NO3_mg_L subecoregion year season N MEAN MIN MAX STDDEV STDERR CV P5 P25 MEDIAN P75  26 1990 FALL 1 0.11 .11000 0.11 . . . 0.11 0.11 0.11 0.11 0 26 1990 SPRING 1 0.26 .26000 0.26 . . . 0.26 0.26 0.26 0.26 0 26 1990 SUMMER 4 0.15 .02500 0.37 0.15 0.08 104 0.03 0.05 0.10 0.25 0 26 1991 FALL 8 0.04 .01000 0.26 0.09 0.03 197 0.01 0.01 0.01 0.02 0 26 1991 SPRING 6 0.05 .01000 0.16 0.06 0.03 131 0.01 0.01 0.01 0.09 0 26 1991 SUMMER 4 0.09 .02000 0.15 0.06 0.03 64 0.02 0.05 0.09 0.13 0 26 1991 WINTER 3 0.03 .02500 0.03 0.00 0.00 0 0.03 0.03 0.03 0.03 0 26 1992 SUMMER 3 0.07 .05000 0.09 0.02 0.01 31 0.05 0.05 0.06 0.09 0 26 1992 WINTER 1 0.07 .07000 0.07 . . . 0.07 0.07 0.07 0.07 0 26 1994 FALL 1 0.02 .02000 0.02 . . . 0.02 0.02 0.02 0.02 0 26 1995 FALL 1 0.03 .02500 0.03 . . . 0.03 0.03 0.03 0.03 0 26 1995 SPRING 2 0.02 .02000 0.03 0.00 0.00 16 0.02 0.02 0.02 0.03 0 26 1995 SUMMER 3 0.05 .00250 0.14 0.08 0.04 152 0.00 0.00 0.01 0.14 0 26 1995 WINTER 2 0.01 .00625 0.01 0.00 0.00 33 0.01 0.01 0.01 0.01 0 26 1996 SPRING 1 0.00 .00250 0.00 . . . 0.00 0.00 0.00 0.00 0 26 1996 WINTER 2 0.01 .00250 0.01 0.01 0.00 85 0.00 0.00 0.01 0.01 0 26 1997 FALL 1 0.03 .02500 0.03 . . . 0.03 0.03 0.03 0.03 0 26 1997 SPRING 1 0.10 .10000 0.10 . . . 0.10 0.10 0.10 0.10 0 26 1997 SUMMER 1 0.03 .02500 0.03 . . . 0.03 0.03 0.03 0.03 0 28 1990 FALL 2 0.01 .01000 0.01 0.00 0.00 0 0.01 0.01 0.01 0.01 0 28 1990 SUMMER 4 0.15 .05000 0.21 0.07 0.03 46 0.05 0.11 0.16 0.19 0 28 1991 FALL 2 0.02 .02000 0.03 0.00 0.00 16 0.02 0.02 0.02 0.03 0 28 1991 SPRING 3 0.03 .01000 0.04 0.02 0.01 58 0.01 0.01 0.04 0.04 0 28 1991 SUMMER 8 0.30 .01000 1.74 0.58 0.21 193 0.01 0.05 0.12 0.17 1 28 1992 SUMMER 2 0.44 .16000 0.72 0.40 0.28 90 0.16 0.16 0.44 0.72 0 28 1993 SUMMER 12 0.20 .00500 0.91 0.29 0.08 149 0.01 0.01 0.04 0.38 0 28 1994 FALL 5 0.39 .00250 1.44 0.63 0.28 161 0.00 0.00 0.00 0.50 1 28 1994 SUMMER 6 0.01 .00250 0.07 0.03 0.01 198 0.00 0.00 0.00 0.00 0 28 1994 WINTER 2 0.17 .08000 0.27 0.13 0.09 76 0.08 0.08 0.17 0.27 0 28 1995 WINTER 1 0.12 .12000 0.12 . . . 0.12 0.12 0.12 0.12 0 28 1996 SUMMER 3 1.37 .51000 1.87 0.74 0.43 55 0.51 0.51 1.72 1.87 1 28 1997 FALL 6 0.55 .07000 1.31 0.56 0.23 101 0.07 0.15 0.29 1.19 1
Appendix B—Descriptive Statistics Data Tables for Level III Subecoregions Within Aggregate Ec

 Aggregate Nutrient Ecoregion: IV  Lakes and Reservoirs Descriptive Statistics by Subecoregion, Year and Season from 1990 to 2000 Nitrite_Nitrate_NO2_NO3_mg_L subecoregion year season N MEAN MIN MAX STDDEV STDERR CV P5 P25 MEDIAN P75  28 1997 SPRING 6 0.45 .02000 1.37 0.56 0.23 124 0.02 0.10 0.15 0.93 1 28 1997 SUMMER 6 0.62 .12500 1.61 0.73 0.30 118 0.13 0.15 0.16 1.53 1 30 1990 FALL 8 0.09 .02000 0.21 0.08 0.03 90 0.02 0.02 0.06 0.16 0 30 1990 SPRING 4 0.14 .10000 0.25 0.08 0.04 55 0.10 0.10 0.10 0.18 0 30 1990 SUMMER 4 0.05 .02500 0.10 0.04 0.02 68 0.03 0.03 0.04 0.08 0 30 1990 WINTER 10 0.08 .02500 0.20 0.06 0.02 74 0.03 0.04 0.06 0.08 0 30 1991 FALL 3 0.13 .01000 0.37 0.21 0.12 154 0.01 0.01 0.02 0.37 0 30 1991 SPRING 5 0.16 .08000 0.22 0.06 0.03 35 0.08 0.14 0.17 0.21 0 30 1991 SUMMER 5 0.06 .01000 0.14 0.05 0.02 89 0.01 0.01 0.07 0.08 0 30 1991 WINTER 4 0.14 .02500 0.35 0.15 0.07 105 0.03 0.05 0.09 0.23 0 30 1992 SPRING 4 0.58 .48000 0.69 0.09 0.04 15 0.48 0.52 0.58 0.65 0 30 1992 SUMMER 5 0.19 .01000 0.38 0.16 0.07 87 0.01 0.05 0.18 0.33 0 30 1992 WINTER 2 0.45 .45000 0.45 0.00 0.00 0 0.45 0.45 0.45 0.45 0 30 1993 SPRING 1 0.08 .08000 0.08 . . . 0.08 0.08 0.08 0.08 0 30 1993 SUMMER 1 0.14 .14000 0.14 . . . 0.14 0.14 0.14 0.14 0 30 1993 WINTER 1 0.13 .13000 0.13 . . . 0.13 0.13 0.13 0.13 0 30 1995 SUMMER 3 0.01 .00250 0.02 0.01 0.00 108 0.00 0.00 0.00 0.02 0 30 1995 WINTER 3 0.26 .12000 0.46 0.18 0.10 68 0.12 0.12 0.20 0.46 0 30 1996 WINTER 2 0.11 .07000 0.15 0.06 0.04 51 0.07 0.07 0.11 0.15 0 43 1990 SUMMER 2 0.01 .00600 0.01 0.00 0.00 28 0.01 0.01 0.01 0.01 0 43 1990 WINTER 4 0.08 .03000 0.14 0.05 0.02 55 0.03 0.06 0.08 0.11 0 43 1991 FALL 8 0.10 .06250 0.10 0.01 0.00 14 0.06 0.10 0.10 0.10 0 43 1991 SUMMER 21 0.35 .00250 1.10 0.33 0.07 94 0.00 0.03 0.50 0.60 0 43 1992 FALL 1 0.07 .06500 0.07 . . . 0.07 0.07 0.07 0.07 0 43 1992 SUMMER 20 0.04 .00150 0.14 0.04 0.01 89 0.00 0.01 0.04 0.06 0 43 1992 WINTER 2 0.04 .03500 0.04 0.00 0.00 9 0.04 0.04 0.04 0.04 0 43 1993 FALL 1 0.17 .17000 0.17 . . . 0.17 0.17 0.17 0.17 0 43 1993 SPRING 1 0.06 .05500 0.06 . . . 0.06 0.06 0.06 0.06 0 43 1993 SUMMER 9 0.10 .04000 0.20 0.05 0.02 47 0.04 0.07 0.10 0.11 0 43 1993 WINTER 1 0.10 .10000 0.10 . . . 0.10 0.10 0.10 0.10 0 43 1994 SPRING 1 0.24 .24000 0.24 . . . 0.24 0.24 0.24 0.24 0 43 1994 SUMMER 8 1.58 .00100 9.90 3.41 1.20 216 0.00 0.10 0.13 1.13 9
Appendix B—Descriptive Statistics Data Tables for Level III Subecoregions Within Aggregate Ec

 Aggregate Nutrient Ecoregion: IV  Lakes and Reservoirs Descriptive Statistics by Subecoregion, Year and Season from 1990 to 2000 Nitrite_Nitrate_NO2_NO3_mg_L subecoregion year season N MEAN MIN MAX STDDEV STDERR CV P5 P25 MEDIAN P75  43 1994 WINTER 1 0.15 .14500 0.15 . . . 0.15 0.15 0.15 0.15 0 43 1995 FALL 3 0.04 .02500 0.06 0.02 0.01 55 0.03 0.03 0.03 0.06 0 43 1995 SPRING 1 0.14 .14000 0.14 . . . 0.14 0.14 0.14 0.14 0 43 1995 SUMMER 8 0.10 .03800 0.25 0.07 0.03 77 0.04 0.04 0.07 0.13 0 43 1996 FALL 1 0.06 .06000 0.06 . . . 0.06 0.06 0.06 0.06 0 43 1996 SUMMER 1 0.09 .09000 0.09 . . . 0.09 0.09 0.09 0.09 0 43 1996 WINTER 1 0.05 .04500 0.05 . . . 0.05 0.05 0.05 0.05 0 43 1997 FALL 1 0.53 .53000 0.53 . . . 0.53 0.53 0.53 0.53 0 43 1997 SPRING 1 0.18 .18000 0.18 . . . 0.18 0.18 0.18 0.18 0 43 1997 SUMMER 1 0.34 .34000 0.34 . . . 0.34 0.34 0.34 0.34 0 43 1998 SUMMER 4 19.40 .05000 77.35 38.63 19.32 199 0.05 0.05 0.10 38.75 77 43 1999 SUMMER 5 0.13 .05000 0.43 0.17 0.08 134 0.05 0.05 0.05 0.05 0 43 2000 SUMMER 2 0.10 .10000 0.10 0.00 0.00 0 0.10 0.10 0.10 0.10 0 44 1990 WINTER 16 0.21 .00000 1.97 0.53 0.13 247 0.00 0.00 0.01 0.01 1 44 1991 FALL 8 0.09 .00250 0.40 0.13 0.05 155 0.00 0.01 0.03 0.11 0 44 1991 SUMMER 3 0.16 .02000 0.24 0.12 0.07 76 0.02 0.02 0.21 0.24 0 44 1992 FALL 2 0.29 .00000 0.57 0.40 0.29 141 0.00 0.00 0.29 0.57 0 44 1992 SPRING 1 0.11 .11000 0.11 . . . 0.11 0.11 0.11 0.11 0 44 1992 SUMMER 2 0.07 .00250 0.13 0.09 0.06 136 0.00 0.00 0.07 0.13 0 44 1993 FALL 1 0.01 .00500 0.01 . . . 0.01 0.01 0.01 0.01 0 44 1993 SUMMER 1 0.00 .00250 0.00 . . . 0.00 0.00 0.00 0.00 0 44 1994 FALL 1 0.00 .00000 0.00 . . . 0.00 0.00 0.00 0.00 0 44 1994 SUMMER 1 0.01 .01000 0.01 . . . 0.01 0.01 0.01 0.01 0 44 1995 FALL 2 0.25 .03000 0.47 0.31 0.22 124 0.03 0.03 0.25 0.47 0 44 1995 SUMMER 2 0.16 .00000 0.32 0.23 0.16 141 0.00 0.00 0.16 0.32 0 44 1996 FALL 1 0.00 .00000 0.00 . . . 0.00 0.00 0.00 0.00 0 44 1996 SPRING 1 0.00 .00000 0.00 . . . 0.00 0.00 0.00 0.00 0 44 1996 SUMMER 1 0.00 .00000 0.00 . . . 0.00 0.00 0.00 0.00 0 44 1997 WINTER 1 0.00 .00000 0.00 . . . 0.00 0.00 0.00 0.00 0
Appendix B—Descriptive Statistics Data Tables for Level III Subecoregions Within Aggregate Ec

 Aggregate Nutrient Ecoregion: IV  Lakes and Reservoirs Descriptive Statistics by Subecoregion, Year and Season from 1990 to 2000 Nitrogen_Tot_Kjeldhal_mg_L subecoregion year season N MEAN MIN MAX STDDEV STDERR CV P5 P25 MEDIAN P75  26 1990 SUMMER 3 0.57 .44000 0.80 0.20 0.12 35 0.44 0.44 0.47 0.80 0 26 1990 WINTER 1 0.36 .35500 0.36 . . . 0.36 0.36 0.36 0.36 0 26 1991 FALL 7 1.46 .60000 5.20 1.66 0.63 114 0.60 0.70 0.90 1.10 5 26 1991 SPRING 5 1.22 .50000 3.60 1.33 0.60 109 0.50 0.60 0.70 0.70 3 26 1991 SUMMER 3 0.41 .30000 0.60 0.16 0.09 39 0.30 0.30 0.34 0.60 0 26 1991 WINTER 4 0.74 .30000 1.60 0.60 0.30 82 0.30 0.33 0.53 1.15 1 26 1992 SUMMER 2 1.89 .30000 3.48 2.25 1.59 119 0.30 0.30 1.89 3.48 3 26 1992 WINTER 1 1.00 1.0000 1.00 . . . 1.00 1.00 1.00 1.00 1 26 1993 FALL 8 0.60 .29000 1.30 0.36 0.13 61 0.29 0.33 0.42 0.85 1 26 1993 SPRING 3 0.38 .36000 0.43 0.04 0.02 11 0.36 0.36 0.36 0.43 0 26 1993 SUMMER 9 1.30 .30000 3.60 1.13 0.38 87 0.30 0.68 0.75 1.30 3 26 1993 WINTER 1 0.34 .34000 0.34 . . . 0.34 0.34 0.34 0.34 0 26 1994 FALL 1 0.42 .42000 0.42 . . . 0.42 0.42 0.42 0.42 0 26 1994 SUMMER 5 0.44 .39000 0.50 0.05 0.02 12 0.39 0.40 0.41 0.48 0 26 1995 FALL 1 0.60 .60000 0.60 . . . 0.60 0.60 0.60 0.60 0 26 1995 SPRING 2 0.45 .30000 0.60 0.21 0.15 47 0.30 0.30 0.45 0.60 0 26 1995 SUMMER 7 0.60 .30000 1.11 0.30 0.11 50 0.30 0.33 0.51 0.85 1 26 1995 WINTER 2 0.34 .33000 0.36 0.02 0.01 5 0.33 0.33 0.34 0.36 0 26 1996 SPRING 2 0.44 .40000 0.48 0.06 0.04 13 0.40 0.40 0.44 0.48 0 26 1996 SUMMER 3 0.85 .34000 1.62 0.68 0.39 80 0.34 0.34 0.59 1.62 1 26 1996 WINTER 2 0.47 .44000 0.50 0.04 0.03 9 0.44 0.44 0.47 0.50 0 26 1997 FALL 1 0.25 .24500 0.25 . . . 0.25 0.25 0.25 0.25 0 26 1997 SPRING 1 0.20 .20000 0.20 . . . 0.20 0.20 0.20 0.20 0 26 1997 SUMMER 1 0.33 .33000 0.33 . . . 0.33 0.33 0.33 0.33 0 28 1990 FALL 2 1.63 1.5500 1.70 0.11 0.07 7 1.55 1.55 1.63 1.70 1 28 1990 SUMMER 2 0.97 .54500 1.40 0.60 0.43 62 0.55 0.55 0.97 1.40 1 28 1991 FALL 2 0.62 .61000 0.62 0.01 0.01 1 0.61 0.61 0.62 0.62 0 28 1991 SPRING 3 1.91 1.4500 2.80 0.77 0.45 41 1.45 1.45 1.47 2.80 2 28 1991 SUMMER 5 1.10 .50000 2.00 0.59 0.26 54 0.50 0.65 1.13 1.22 2 28 1992 SUMMER 2 0.75 .29000 1.20 0.64 0.46 86 0.29 0.29 0.75 1.20 1 28 1993 SUMMER 16 0.80 .02500 1.87 0.62 0.15 78 0.03 0.16 0.80 1.26 1 28 1994 FALL 8 0.88 .60000 1.35 0.29 0.10 33 0.60 0.65 0.81 1.10 1
Appendix B—Descriptive Statistics Data Tables for Level III Subecoregions Within Aggregate Ec

 Aggregate Nutrient Ecoregion: IV  Lakes and Reservoirs Descriptive Statistics by Subecoregion, Year and Season from 1990 to 2000 Nitrogen_Tot_Kjeldhal_mg_L subecoregion year season N MEAN MIN MAX STDDEV STDERR CV P5 P25 MEDIAN P75  28 1994 SPRING 1 1.90 1.9000 1.90 . . . 1.90 1.90 1.90 1.90 1 28 1994 SUMMER 8 0.98 .02500 1.36 0.42 0.15 43 0.03 0.89 1.11 1.24 1 28 1994 WINTER 2 0.99 .02500 1.95 1.36 0.96 138 0.03 0.03 0.99 1.95 1 28 1995 SPRING 1 0.81 .81000 0.81 . . . 0.81 0.81 0.81 0.81 0 28 1995 SUMMER 4 0.63 .02500 1.00 0.42 0.21 67 0.03 0.38 0.74 0.87 1 28 1995 WINTER 1 1.03 1.0300 1.03 . . . 1.03 1.03 1.03 1.03 1 28 1996 SUMMER 11 0.54 .02500 1.18 0.33 0.10 61 0.03 0.25 0.55 0.75 1 28 1997 FALL 6 1.48 .80000 3.50 1.09 0.45 74 0.80 0.80 0.90 2.00 3 28 1997 SPRING 6 1.15 .42500 1.70 0.54 0.22 47 0.43 0.55 1.30 1.60 1 28 1997 SUMMER 13 0.91 .17250 2.50 0.61 0.17 68 0.17 0.44 0.76 1.21 2 30 1990 FALL 8 0.54 .30000 0.71 0.15 0.05 27 0.30 0.44 0.53 0.68 0 30 1990 SPRING 7 0.43 .30000 0.70 0.14 0.05 32 0.30 0.30 0.40 0.50 0 30 1990 SUMMER 7 0.51 .30000 0.70 0.14 0.05 28 0.30 0.40 0.55 0.60 0 30 1990 WINTER 9 0.51 .30000 0.73 0.15 0.05 30 0.30 0.40 0.50 0.57 0 30 1991 FALL 8 0.80 .15000 1.47 0.44 0.15 55 0.15 0.49 0.80 1.09 1 30 1991 SPRING 6 0.44 .30000 0.60 0.10 0.04 23 0.30 0.40 0.43 0.50 0 30 1991 SUMMER 10 0.67 .30000 1.00 0.25 0.08 38 0.30 0.40 0.72 0.90 1 30 1991 WINTER 9 0.48 .30000 0.72 0.12 0.04 25 0.30 0.40 0.48 0.54 0 30 1992 FALL 3 0.70 .44500 0.95 0.25 0.15 36 0.45 0.45 0.70 0.95 0 30 1992 SPRING 10 0.40 .05000 0.71 0.25 0.08 62 0.05 0.20 0.51 0.61 0 30 1992 SUMMER 10 0.52 .05000 1.24 0.38 0.12 73 0.05 0.30 0.47 0.71 1 30 1992 WINTER 8 0.50 .34000 0.60 0.08 0.03 16 0.34 0.48 0.51 0.56 0 30 1993 FALL 3 0.41 .24000 0.49 0.14 0.08 35 0.24 0.24 0.49 0.49 0 30 1993 SPRING 6 0.36 .25000 0.48 0.10 0.04 29 0.25 0.27 0.35 0.46 0 30 1993 SUMMER 12 0.32 .17000 0.52 0.10 0.03 30 0.17 0.26 0.33 0.38 0 30 1993 WINTER 5 0.21 .14000 0.29 0.06 0.03 28 0.14 0.17 0.23 0.24 0 30 1994 SPRING 2 0.22 .12500 0.31 0.13 0.09 60 0.13 0.13 0.22 0.31 0 30 1994 SUMMER 8 0.37 .05000 0.71 0.19 0.07 51 0.05 0.28 0.38 0.45 0 30 1995 SPRING 2 0.18 .05000 0.30 0.18 0.13 101 0.05 0.05 0.18 0.30 0 30 1995 SUMMER 3 0.46 .31000 0.58 0.14 0.08 30 0.31 0.31 0.48 0.58 0 30 1995 WINTER 3 0.42 .25000 0.59 0.17 0.10 40 0.25 0.25 0.42 0.59 0 30 1996 SPRING 2 0.33 .20000 0.46 0.18 0.13 56 0.20 0.20 0.33 0.46 0
Appendix B—Descriptive Statistics Data Tables for Level III Subecoregions Within Aggregate Ec

 Aggregate Nutrient Ecoregion: IV  Lakes and Reservoirs Descriptive Statistics by Subecoregion, Year and Season from 1990 to 2000 Nitrogen_Tot_Kjeldhal_mg_L subecoregion year season N MEAN MIN MAX STDDEV STDERR CV P5 P25 MEDIAN P75  30 1996 SUMMER 4 0.44 .25000 0.62 0.16 0.08 36 0.25 0.32 0.44 0.56 0 30 1996 WINTER 3 0.38 .28000 0.46 0.09 0.05 24 0.28 0.28 0.41 0.46 0 31 1990 FALL 1 1.20 1.2000 1.20 . . . 1.20 1.20 1.20 1.20 1 31 1990 SPRING 1 0.90 .90000 0.90 . . . 0.90 0.90 0.90 0.90 0 31 1990 SUMMER 1 1.25 1.2500 1.25 . . . 1.25 1.25 1.25 1.25 1 31 1991 SPRING 1 0.80 .80000 0.80 . . . 0.80 0.80 0.80 0.80 0 31 1991 SUMMER 1 1.10 1.1000 1.10 . . . 1.10 1.10 1.10 1.10 1 31 1991 WINTER 1 0.90 .90000 0.90 . . . 0.90 0.90 0.90 0.90 0 31 1993 FALL 1 0.61 .61000 0.61 . . . 0.61 0.61 0.61 0.61 0 31 1993 SUMMER 1 0.21 .21000 0.21 . . . 0.21 0.21 0.21 0.21 0 43 1990 SUMMER 1 3.02 3.0200 3.02 . . . 3.02 3.02 3.02 3.02 3 43 1990 WINTER 4 2.34 1.4600 4.19 1.25 0.62 53 1.46 1.62 1.86 3.07 4 43 1991 FALL 8 0.88 .52500 1.33 0.27 0.10 31 0.53 0.65 0.88 1.05 1 43 1991 SUMMER 22 1.25 .39500 3.60 1.00 0.21 80 0.42 0.53 0.85 1.82 3 43 1992 FALL 2 1.56 .30000 2.83 1.79 1.26 114 0.30 0.30 1.56 2.83 2 43 1992 SUMMER 20 1.33 .23500 3.02 0.87 0.19 65 0.25 0.58 1.16 1.98 2 43 1992 WINTER 2 3.10 2.6800 3.51 0.59 0.42 19 2.68 2.68 3.10 3.51 3 43 1993 FALL 1 0.12 .11750 0.12 . . . 0.12 0.12 0.12 0.12 0 43 1993 SPRING 1 0.83 .82500 0.83 . . . 0.83 0.83 0.83 0.83 0 43 1993 SUMMER 9 0.66 .23100 1.49 0.40 0.13 61 0.23 0.36 0.55 0.86 1 43 1993 WINTER 1 0.33 .33000 0.33 . . . 0.33 0.33 0.33 0.33 0 43 1994 FALL 1 0.33 .32500 0.33 . . . 0.33 0.33 0.33 0.33 0 43 1994 SPRING 1 0.42 .42000 0.42 . . . 0.42 0.42 0.42 0.42 0 43 1994 SUMMER 9 0.87 .20700 2.79 0.82 0.27 95 0.21 0.35 0.54 1.20 2 43 1994 WINTER 1 0.28 .28000 0.28 . . . 0.28 0.28 0.28 0.28 0 43 1995 FALL 3 0.30 .24000 0.41 0.09 0.05 31 0.24 0.24 0.26 0.41 0 43 1995 SPRING 1 0.27 .27000 0.27 . . . 0.27 0.27 0.27 0.27 0 43 1995 SUMMER 9 0.39 .25550 0.69 0.13 0.04 35 0.26 0.30 0.34 0.40 0 43 1996 FALL 1 0.24 .24000 0.24 . . . 0.24 0.24 0.24 0.24 0 43 1996 SPRING 1 0.50 .50000 0.50 . . . 0.50 0.50 0.50 0.50 0 43 1996 SUMMER 2 0.54 .47000 0.60 0.09 0.07 17 0.47 0.47 0.54 0.60 0 43 1996 WINTER 1 0.28 .27500 0.28 . . . 0.28 0.28 0.28 0.28 0
Appendix B—Descriptive Statistics Data Tables for Level III Subecoregions Within Aggregate Ec

 Aggregate Nutrient Ecoregion: IV  Lakes and Reservoirs Descriptive Statistics by Subecoregion, Year and Season from 1990 to 2000 Nitrogen_Tot_Kjeldhal_mg_L subecoregion year season N MEAN MIN MAX STDDEV STDERR CV P5 P25 MEDIAN P75  43 1997 FALL 1 0.56 .56000 0.56 . . . 0.56 0.56 0.56 0.56 0 43 1997 SPRING 1 0.00 .00000 0.00 . . . 0.00 0.00 0.00 0.00 0 43 1997 SUMMER 1 0.75 .75000 0.75 . . . 0.75 0.75 0.75 0.75 0 43 1998 SUMMER 4 1.27 .23000 2.68 1.11 0.56 88 0.23 0.39 1.08 2.14 2 43 1999 SUMMER 5 0.92 .30000 1.93 0.65 0.29 71 0.30 0.58 0.61 1.18 1 43 2000 SUMMER 2 1.25 1.2050 1.30 0.06 0.04 5 1.21 1.21 1.25 1.30 1 44 1990 WINTER 14 1.51 .12500 3.44 0.98 0.26 65 0.13 0.71 1.60 2.09 3 44 1991 FALL 8 1.58 .62000 3.31 1.14 0.40 72 0.62 0.66 1.10 2.60 3 44 1991 SUMMER 3 0.54 .50000 0.58 0.04 0.02 7 0.50 0.50 0.54 0.58 0 44 1992 FALL 2 0.13 .12500 0.13 0.00 0.00 0 0.13 0.13 0.13 0.13 0 44 1992 SUMMER 2 1.08 1.0200 1.13 0.08 0.05 7 1.02 1.02 1.08 1.13 1 44 1993 FALL 1 0.62 .62000 0.62 . . . 0.62 0.62 0.62 0.62 0 44 1993 SUMMER 1 0.71 .70750 0.71 . . . 0.71 0.71 0.71 0.71 0 44 1994 FALL 1 1.34 1.3400 1.34 . . . 1.34 1.34 1.34 1.34 1 44 1994 SUMMER 1 1.01 1.0050 1.01 . . . 1.01 1.01 1.01 1.01 1 44 1995 FALL 2 1.00 .72000 1.28 0.40 0.28 40 0.72 0.72 1.00 1.28 1 44 1995 SUMMER 2 0.92 .87000 0.97 0.07 0.05 7 0.87 0.87 0.92 0.97 0 44 1996 FALL 1 1.72 1.7200 1.72 . . . 1.72 1.72 1.72 1.72 1 44 1996 SPRING 1 1.37 1.3700 1.37 . . . 1.37 1.37 1.37 1.37 1 44 1996 SUMMER 1 1.32 1.3200 1.32 . . . 1.32 1.32 1.32 1.32 1 44 1997 WINTER 1 1.47 1.4650 1.47 . . . 1.47 1.47 1.47 1.47 1
Appendix B—Descriptive Statistics Data Tables for Level III Subecoregions Within Aggregate Ec

 Aggregate Nutrient Ecoregion: IV  Lakes and Reservoirs Descriptive Statistics by Subecoregion, Year and Season from 1990 to 2000 SECCHI_m subecoregion year season N MEAN MIN MAX STDDEV STDERR CV P5 P25 MEDIAN P75  26 1990 FALL 2 1.38 1.0000 1.75 0.53 0.38 39 1.00 1.00 1.38 1.75 1 26 1990 SPRING 4 1.45 .30480 3.00 1.13 0.56 78 0.30 0.75 1.25 2.15 3 26 1990 SUMMER 9 1.45 .23000 3.10 0.85 0.28 58 0.23 0.80 1.50 1.75 3 26 1990 WINTER 4 1.99 1.0000 2.90 0.78 0.39 39 1.00 1.45 2.03 2.53 2 26 1991 FALL 2 2.75 .90000 4.60 2.62 1.85 95 0.90 0.90 2.75 4.60 4 26 1991 SPRING 5 1.53 .22860 4.00 1.62 0.72 106 0.23 0.30 0.80 2.30 4 26 1991 SUMMER 6 1.53 .80000 2.80 0.68 0.28 44 0.80 1.15 1.43 1.60 2 26 1991 WINTER 4 1.60 .90000 2.90 0.89 0.45 56 0.90 1.05 1.30 2.15 2 26 1992 FALL 2 2.15 1.2000 3.10 1.34 0.95 62 1.20 1.20 2.15 3.10 3 26 1992 SPRING 4 1.63 .12700 2.30 1.01 0.51 62 0.13 1.04 2.05 2.23 2 26 1992 SUMMER 6 1.17 .10000 2.70 1.04 0.42 89 0.10 0.14 1.13 1.80 2 26 1992 WINTER 3 1.50 .40000 2.80 1.21 0.70 81 0.40 0.40 1.30 2.80 2 26 1993 FALL 10 0.81 .30000 1.60 0.41 0.13 51 0.30 0.60 0.75 1.10 1 26 1993 SPRING 5 1.42 .40000 4.20 1.60 0.72 113 0.40 0.50 0.60 1.40 4 26 1993 SUMMER 10 1.01 .20000 2.80 0.86 0.27 85 0.20 0.40 0.55 1.70 2 26 1993 WINTER 2 1.90 .80000 3.00 1.56 1.10 82 0.80 0.80 1.90 3.00 3 26 1994 FALL 3 1.50 .90000 2.30 0.72 0.42 48 0.90 0.90 1.30 2.30 2 26 1994 SPRING 2 2.65 2.4000 2.90 0.35 0.25 13 2.40 2.40 2.65 2.90 2 26 1994 SUMMER 7 1.26 .13000 2.80 0.97 0.37 77 0.13 0.30 1.15 1.95 2 26 1994 WINTER 1 3.10 3.1000 3.10 . . . 3.10 3.10 3.10 3.10 3 26 1995 FALL 3 1.72 .55000 2.80 1.13 0.65 66 0.55 0.55 1.80 2.80 2 26 1995 SPRING 3 1.53 .40000 3.20 1.47 0.85 96 0.40 0.40 1.00 3.20 3 26 1995 SUMMER 8 1.34 .45000 2.25 0.79 0.28 59 0.45 0.55 1.43 2.03 2 26 1995 WINTER 2 1.53 1.3500 1.70 0.25 0.17 16 1.35 1.35 1.53 1.70 1 26 1996 FALL 1 1.20 1.2000 1.20 . . . 1.20 1.20 1.20 1.20 1 26 1996 SPRING 2 4.85 2.7000 7.00 3.04 2.15 63 2.70 2.70 4.85 7.00 7 26 1996 SUMMER 4 2.30 .70000 3.50 1.17 0.58 51 0.70 1.55 2.50 3.05 3 26 1996 WINTER 2 2.65 1.9000 3.40 1.06 0.75 40 1.90 1.90 2.65 3.40 3 26 1997 FALL 2 1.81 1.0250 2.60 1.11 0.79 61 1.03 1.03 1.81 2.60 2 26 1997 SPRING 3 3.08 1.6250 4.30 1.35 0.78 44 1.63 1.63 3.30 4.30 4 26 1997 SUMMER 3 2.18 1.1250 3.00 0.96 0.55 44 1.13 1.13 2.40 3.00 3 26 1998 SUMMER 1 2.50 2.5000 2.50 . . . 2.50 2.50 2.50 2.50 2
Appendix B—Descriptive Statistics Data Tables for Level III Subecoregions Within Aggregate Ec

 Aggregate Nutrient Ecoregion: IV  Lakes and Reservoirs Descriptive Statistics by Subecoregion, Year and Season from 1990 to 2000 SECCHI_m subecoregion year season N MEAN MIN MAX STDDEV STDERR CV P5 P25 MEDIAN P75  28 1990 FALL 2 0.35 .30000 0.40 0.07 0.05 20 0.30 0.30 0.35 0.40 0 28 1990 SUMMER 7 0.82 .21000 1.34 0.36 0.14 44 0.21 0.50 0.90 1.00 1 28 1990 WINTER 1 0.20 .20000 0.20 . . . 0.20 0.20 0.20 0.20 0 28 1991 FALL 2 0.50 .40000 0.60 0.14 0.10 28 0.40 0.40 0.50 0.60 0 28 1991 SPRING 2 0.59 .40000 0.78 0.27 0.19 45 0.40 0.40 0.59 0.78 0 28 1991 SUMMER 11 0.53 .15240 1.90 0.48 0.15 91 0.15 0.25 0.50 0.50 1 28 1991 WINTER 1 0.40 .40000 0.40 . . . 0.40 0.40 0.40 0.40 0 28 1992 FALL 3 0.21 .12000 0.30 0.09 0.05 43 0.12 0.12 0.21 0.30 0 28 1992 SUMMER 3 0.73 .46000 1.13 0.35 0.20 48 0.46 0.46 0.60 1.13 1 28 1993 SUMMER 16 0.71 .15000 1.30 0.38 0.10 54 0.15 0.45 0.61 1.10 1 28 1994 FALL 7 0.32 .12000 0.61 0.17 0.06 53 0.12 0.15 0.30 0.46 0 28 1994 SPRING 1 0.30 .30000 0.30 . . . 0.30 0.30 0.30 0.30 0 28 1994 SUMMER 7 1.11 .50000 1.90 0.54 0.21 49 0.50 0.50 1.30 1.45 1 28 1994 WINTER 1 0.20 .20000 0.20 . . . 0.20 0.20 0.20 0.20 0 28 1995 SUMMER 3 0.60 .30000 1.10 0.44 0.25 73 0.30 0.30 0.40 1.10 1 28 1996 SUMMER 10 0.61 .12000 1.80 0.49 0.15 80 0.12 0.27 0.53 0.60 1 28 1997 FALL 2 0.36 .32500 0.40 0.05 0.04 15 0.33 0.33 0.36 0.40 0 28 1997 SPRING 2 0.31 .22500 0.40 0.12 0.09 40 0.23 0.23 0.31 0.40 0 28 1997 SUMMER 9 1.06 .35000 1.80 0.53 0.18 50 0.35 0.60 1.10 1.50 1 30 1990 FALL 8 2.19 .93980 3.45 1.09 0.38 50 0.94 1.26 1.98 3.31 3 30 1990 SPRING 10 1.98 .61000 4.15 0.95 0.30 48 0.61 1.30 1.95 2.40 4 30 1990 SUMMER 12 1.87 .65000 3.90 1.02 0.29 55 0.65 1.27 1.66 2.20 3 30 1990 WINTER 10 2.42 .92710 4.39 1.20 0.38 50 0.93 1.52 2.04 3.00 4 30 1991 FALL 8 2.67 1.3716 4.14 0.90 0.32 34 1.37 2.04 2.59 3.28 4 30 1991 SPRING 11 2.40 1.3700 6.10 1.33 0.40 55 1.37 1.50 2.06 2.60 6 30 1991 SUMMER 12 2.26 .82500 4.57 0.89 0.26 39 0.83 1.84 2.13 2.59 4 30 1991 WINTER 9 1.79 1.0160 3.00 0.68 0.23 38 1.02 1.22 1.54 2.34 3 30 1992 FALL 8 2.01 .93980 3.84 1.07 0.38 53 0.94 1.32 1.65 2.67 3 30 1992 SPRING 10 1.84 .91440 3.51 0.82 0.26 44 0.91 1.37 1.52 2.25 3 30 1992 SUMMER 13 1.97 1.1500 3.25 0.63 0.17 32 1.15 1.47 1.83 2.25 3 30 1992 WINTER 8 0.95 .50800 1.52 0.39 0.14 41 0.51 0.65 0.80 1.33 1 30 1993 FALL 2 2.57 1.1400 4.00 2.02 1.43 79 1.14 1.14 2.57 4.00 4
Appendix B—Descriptive Statistics Data Tables for Level III Subecoregions Within Aggregate Ec

 Aggregate Nutrient Ecoregion: IV  Lakes and Reservoirs Descriptive Statistics by Subecoregion, Year and Season from 1990 to 2000 SECCHI_m subecoregion year season N MEAN MIN MAX STDDEV STDERR CV P5 P25 MEDIAN P75  30 1993 SPRING 10 1.98 1.2446 2.62 0.46 0.15 23 1.24 1.52 2.03 2.44 2 30 1993 SUMMER 12 1.74 .85000 4.55 0.93 0.27 53 0.85 1.39 1.48 1.80 4 30 1993 WINTER 10 1.63 .30480 2.79 0.71 0.23 44 0.30 1.37 1.59 2.08 2 30 1994 FALL 2 2.25 1.5200 2.97 1.03 0.72 46 1.52 1.52 2.25 2.97 2 30 1994 SPRING 5 2.02 1.0000 2.70 0.69 0.31 34 1.00 1.80 2.00 2.60 2 30 1994 SUMMER 12 1.96 .85000 3.30 0.83 0.24 42 0.85 1.26 1.95 2.53 3 30 1994 WINTER 3 1.90 1.3000 2.60 0.66 0.38 35 1.30 1.30 1.80 2.60 2 30 1995 SPRING 5 1.51 .61000 2.50 0.80 0.36 53 0.61 0.76 1.80 1.90 2 30 1995 SUMMER 5 1.67 .61000 2.74 0.94 0.42 56 0.61 0.80 1.83 2.35 2 30 1995 WINTER 5 1.68 .75000 2.64 0.83 0.37 50 0.75 0.90 1.80 2.29 2 30 1996 FALL 1 1.98 1.9800 1.98 . . . 1.98 1.98 1.98 1.98 1 30 1996 SPRING 4 1.28 .61000 2.10 0.66 0.33 52 0.61 0.76 1.20 1.80 2 30 1996 SUMMER 5 1.30 .50000 2.20 0.69 0.31 53 0.50 0.73 1.53 1.56 2 30 1996 WINTER 4 1.49 .95000 2.23 0.59 0.29 39 0.95 1.03 1.40 1.96 2 30 1997 SPRING 1 2.01 2.0100 2.01 . . . 2.01 2.01 2.01 2.01 2 30 1997 WINTER 1 0.61 .61000 0.61 . . . 0.61 0.61 0.61 0.61 0 31 1990 FALL 1 1.20 1.2000 1.20 . . . 1.20 1.20 1.20 1.20 1 31 1990 SPRING 1 0.75 .75000 0.75 . . . 0.75 0.75 0.75 0.75 0 31 1990 SUMMER 1 0.30 .30000 0.30 . . . 0.30 0.30 0.30 0.30 0 31 1991 FALL 1 0.85 .85000 0.85 . . . 0.85 0.85 0.85 0.85 0 31 1991 SPRING 1 0.70 .70000 0.70 . . . 0.70 0.70 0.70 0.70 0 31 1991 SUMMER 1 1.03 1.0300 1.03 . . . 1.03 1.03 1.03 1.03 1 31 1991 WINTER 1 0.99 .99000 0.99 . . . 0.99 0.99 0.99 0.99 0 31 1992 FALL 1 0.60 .60000 0.60 . . . 0.60 0.60 0.60 0.60 0 31 1992 SPRING 1 1.20 1.2000 1.20 . . . 1.20 1.20 1.20 1.20 1 31 1992 SUMMER 2 1.41 1.3000 1.53 0.16 0.11 11 1.30 1.30 1.41 1.53 1 31 1992 WINTER 1 0.72 .71500 0.72 . . . 0.72 0.72 0.72 0.72 0 31 1993 FALL 1 1.00 1.0000 1.00 . . . 1.00 1.00 1.00 1.00 1 31 1993 SUMMER 1 1.85 1.8500 1.85 . . . 1.85 1.85 1.85 1.85 1 31 1994 SUMMER 1 1.75 1.7500 1.75 . . . 1.75 1.75 1.75 1.75 1 43 1990 FALL 5 1.66 .22860 3.60 1.46 0.65 88 0.23 0.46 1.30 2.70 3 43 1990 SPRING 4 2.20 .40640 4.00 1.50 0.75 68 0.41 1.10 2.20 3.30 4
Appendix B—Descriptive Statistics Data Tables for Level III Subecoregions Within Aggregate Ec

 Aggregate Nutrient Ecoregion: IV  Lakes and Reservoirs Descriptive Statistics by Subecoregion, Year and Season from 1990 to 2000 SECCHI_m subecoregion year season N MEAN MIN MAX STDDEV STDERR CV P5 P25 MEDIAN P75  43 1990 SUMMER 7 1.93 .06350 3.50 1.22 0.46 63 0.06 1.00 2.20 2.90 3 43 1990 WINTER 3 3.25 .40640 5.00 2.48 1.43 76 0.41 0.41 4.34 5.00 5 43 1991 FALL 15 1.46 .40640 4.00 0.97 0.25 67 0.41 0.64 1.37 2.13 4 43 1991 SPRING 6 1.61 .20000 3.10 1.28 0.52 80 0.20 0.36 1.50 3.00 3 43 1991 SUMMER 24 1.77 .15200 4.00 1.07 0.22 60 0.27 0.99 1.78 2.51 3 43 1991 WINTER 2 3.25 1.5240 4.98 2.44 1.73 75 1.52 1.52 3.25 4.98 4 43 1992 FALL 9 2.30 .30480 6.50 1.78 0.59 78 0.30 1.80 1.98 2.49 6 43 1992 SPRING 5 3.14 2.0000 5.50 1.44 0.64 46 2.00 2.00 2.88 3.30 5 43 1992 SUMMER 16 2.54 .30480 5.00 1.56 0.39 62 0.30 1.10 2.36 3.87 5 43 1992 WINTER 3 3.48 .78740 6.00 2.61 1.51 75 0.79 0.79 3.66 6.00 6 43 1993 FALL 9 2.12 .86360 3.50 0.96 0.32 45 0.86 1.40 2.03 2.90 3 43 1993 SPRING 7 2.70 1.0033 5.00 1.44 0.54 53 1.00 1.50 2.18 4.06 5 43 1993 SUMMER 18 1.58 .16500 5.00 1.28 0.30 81 0.17 0.71 0.97 2.50 5 43 1993 WINTER 4 2.83 .45720 4.37 1.67 0.83 59 0.46 1.80 3.24 3.85 4 43 1994 FALL 6 2.79 1.6764 3.50 0.69 0.28 25 1.68 2.36 2.90 3.40 3 43 1994 SPRING 10 2.52 .43180 5.20 1.33 0.42 53 0.43 1.79 2.32 3.37 5 43 1994 SUMMER 15 2.38 .61000 7.00 1.60 0.41 67 0.61 1.19 2.03 3.50 7 43 1994 WINTER 5 3.95 2.1844 6.00 1.61 0.72 41 2.18 2.59 3.96 5.03 6 43 1995 FALL 8 2.42 .17780 4.00 1.22 0.43 50 0.18 1.83 2.43 3.34 4 43 1995 SPRING 4 3.84 2.1590 5.00 1.20 0.60 31 2.16 3.08 4.11 4.61 5 43 1995 SUMMER 15 2.17 .64770 4.75 0.94 0.24 43 0.65 1.65 2.03 2.60 4 43 1995 WINTER 2 1.94 .38100 3.51 2.21 1.56 114 0.38 0.38 1.94 3.51 3 43 1996 FALL 8 1.49 .55880 3.50 0.93 0.33 62 0.56 0.93 1.20 1.80 3 43 1996 SPRING 4 2.09 .15240 4.00 1.81 0.90 87 0.15 0.58 2.10 3.60 4 43 1996 SUMMER 15 1.59 .20000 4.25 1.09 0.28 69 0.20 0.75 1.47 2.00 4 43 1996 WINTER 2 2.62 2.4892 2.74 0.18 0.13 7 2.49 2.49 2.62 2.74 2 43 1997 FALL 4 1.85 1.0000 3.15 0.97 0.48 52 1.00 1.12 1.62 2.58 3 43 1997 SPRING 5 2.37 1.7000 3.00 0.62 0.28 26 1.70 1.83 2.30 3.00 3 43 1997 SUMMER 7 2.47 .76200 5.50 1.63 0.62 66 0.76 1.42 1.60 3.40 5 43 1997 WINTER 4 3.45 2.5527 4.29 0.76 0.38 22 2.55 2.85 3.48 4.05 4 43 1998 SPRING 2 3.75 1.5000 6.00 3.18 2.25 85 1.50 1.50 3.75 6.00 6 43 1998 SUMMER 7 2.36 1.0070 6.00 1.74 0.66 74 1.01 1.19 1.99 3.00 6
Appendix B—Descriptive Statistics Data Tables for Level III Subecoregions Within Aggregate Ec

 Aggregate Nutrient Ecoregion: IV  Lakes and Reservoirs Descriptive Statistics by Subecoregion, Year and Season from 1990 to 2000 SECCHI_m subecoregion year season N MEAN MIN MAX STDDEV STDERR CV P5 P25 MEDIAN P75  43 1998 WINTER 1 6.50 6.5000 6.50 . . . 6.50 6.50 6.50 6.50 6 43 1999 SUMMER 5 2.31 1.2100 3.10 0.74 0.33 32 1.21 2.06 2.31 2.87 3 43 2000 SUMMER 2 0.82 .80550 0.84 0.02 0.02 3 0.81 0.81 0.82 0.84 0 44 1990 WINTER 13 0.80 .10160 1.52 0.39 0.11 49 0.10 0.61 0.86 0.99 1 44 1991 FALL 8 1.21 .22860 4.42 1.39 0.49 114 0.23 0.41 0.76 1.36 4 44 1991 SUMMER 3 1.87 .63500 4.32 2.12 1.22 113 0.64 0.64 0.66 4.32 4 44 1992 FALL 1 1.73 1.7272 1.73 . . . 1.73 1.73 1.73 1.73 1 44 1992 SPRING 1 0.15 .15240 0.15 . . . 0.15 0.15 0.15 0.15 0 44 1992 SUMMER 2 0.89 .86360 0.91 0.04 0.03 4 0.86 0.86 0.89 0.91 0 44 1993 FALL 1 1.32 1.3208 1.32 . . . 1.32 1.32 1.32 1.32 1 44 1993 SUMMER 1 1.27 1.2700 1.27 . . . 1.27 1.27 1.27 1.27 1 44 1994 FALL 1 0.46 .45720 0.46 . . . 0.46 0.46 0.46 0.46 0 44 1994 SUMMER 1 0.90 .90170 0.90 . . . 0.90 0.90 0.90 0.90 0 44 1995 FALL 1 1.22 1.2192 1.22 . . . 1.22 1.22 1.22 1.22 1 44 1995 SUMMER 2 0.86 .76200 0.95 0.13 0.10 16 0.76 0.76 0.86 0.95 0 44 1996 SPRING 1 0.97 .96520 0.97 . . . 0.97 0.97 0.97 0.97 0 44 1996 SUMMER 1 0.77 .77470 0.77 . . . 0.77 0.77 0.77 0.77 0 44 1997 WINTER 1 1.04 1.0414 1.04 . . . 1.04 1.04 1.04 1.04 1
Appendix B—Descriptive Statistics Data Tables for Level III Subecoregions Within Aggregate Ec

 Aggregate Nutrient Ecoregion: IV  Lakes and Reservoirs Descriptive Statistics by Subecoregion, Year and Season from 1992 to 1997 Total_Nitrogen_mg_L subecoregion year season N MEAN MIN MAX STDDEV STDERR CV P5 P25 MEDIAN P75 P 26 1992 SUMMER 1 3.53 3.5300 3.53 . . . 3.53 3.53 3.53 3.53 3. 26 1997 FALL 1 0.09 .08500 0.09 . . . 0.09 0.09 0.09 0.09 0. 26 1997 SPRING 1 0.49 .48750 0.49 . . . 0.49 0.49 0.49 0.49 0. 26 1997 SUMMER 1 0.44 .44000 0.44 . . . 0.44 0.44 0.44 0.44 0.
Appendix B—Descriptive Statistics Data Tables for Level III Subecoregions Within Aggregate Ec

 Aggregate Nutrient Ecoregion: IV  Lakes and Reservoirs Descriptive Statistics by Subecoregion, Year and Season from 1990 to 2000 Total_Phosphorus_ug_L subecoregion year season N MEAN MIN MAX STDDEV STDERR CV P5 P25 MEDIAN P75  26 1990 FALL 2 40.00 40.000 40.00 0.00 0.00 0 40.00 40.00 40.00 40.00 40 26 1990 SPRING 2 30.00 30.000 30.00 0.00 0.00 0 30.00 30.00 30.00 30.00 30 26 1990 SUMMER 10 31.00 5.0000 80.00 26.23 8.29 85 5.00 5.00 25.00 50.00 80 26 1990 WINTER 4 14.38 5.0000 25.00 10.87 5.44 76 5.00 5.00 13.75 23.75 25 26 1991 FALL 9 57.36 16.250 170.00 50.31 16.77 88 16.25 20.00 30.00 80.00 170 26 1991 SPRING 8 35.00 10.000 100.00 28.78 10.18 82 10.00 20.00 25.00 40.00 100 26 1991 SUMMER 8 43.44 2.5000 160.00 48.31 17.08 111 2.50 27.50 30.00 35.00 160 26 1991 WINTER 7 24.64 2.5000 70.00 22.38 8.46 91 2.50 5.00 20.00 30.00 70 26 1992 FALL 1 170.00 170.00 170.00 . . . 170.00 170.00 170.00 170.00 170 26 1992 SPRING 2 15.63 11.250 20.00 6.19 4.38 40 11.25 11.25 15.63 20.00 20 26 1992 SUMMER 8 116.56 2.5000 640.00 218.41 77.22 187 2.50 5.00 12.50 127.50 640 26 1992 WINTER 4 85.00 40.000 120.00 41.23 20.62 49 40.00 50.00 90.00 120.00 120 26 1993 FALL 9 33.61 12.500 60.00 19.00 6.33 57 12.50 20.00 20.00 50.00 60 26 1993 SPRING 3 30.00 30.000 30.00 0.00 0.00 0 30.00 30.00 30.00 30.00 30 26 1993 SUMMER 10 36.75 2.5000 90.00 32.28 10.21 88 2.50 5.00 25.00 70.00 90 26 1993 WINTER 1 20.00 20.000 20.00 . . . 20.00 20.00 20.00 20.00 20 26 1994 FALL 2 16.25 2.5000 30.00 19.45 13.75 120 2.50 2.50 16.25 30.00 30 26 1994 SUMMER 6 15.42 2.5000 40.00 13.27 5.42 86 2.50 10.00 10.00 20.00 40 26 1995 FALL 2 27.50 25.000 30.00 3.54 2.50 13 25.00 25.00 27.50 30.00 30 26 1995 SPRING 2 45.00 10.000 80.00 49.50 35.00 110 10.00 10.00 45.00 80.00 80 26 1995 SUMMER 8 73.44 2.5000 280.00 91.52 32.36 125 2.50 10.00 50.00 92.50 280 26 1995 WINTER 2 42.50 20.000 65.00 31.82 22.50 75 20.00 20.00 42.50 65.00 65 26 1996 FALL 1 310.00 310.00 310.00 . . . 310.00 310.00 310.00 310.00 310 26 1996 SPRING 2 2.50 2.5000 2.50 0.00 0.00 0 2.50 2.50 2.50 2.50 2 26 1996 SUMMER 4 132.50 10.000 450.00 212.03 106.02 160 10.00 20.00 35.00 245.00 450 26 1996 WINTER 2 20.00 20.000 20.00 0.00 0.00 0 20.00 20.00 20.00 20.00 20 26 1997 FALL 2 55.00 15.000 95.00 56.57 40.00 103 15.00 15.00 55.00 95.00 95 26 1997 SPRING 1 30.00 30.000 30.00 . . . 30.00 30.00 30.00 30.00 30 26 1997 SUMMER 2 107.50 10.000 205.00 137.89 97.50 128 10.00 10.00 107.50 205.00 205 28 1990 FALL 2 62.50 60.000 65.00 3.54 2.50 6 60.00 60.00 62.50 65.00 65 28 1990 SUMMER 7 140.71 10.000 595.00 209.79 79.29 149 10.00 20.00 60.00 195.00 595 28 1991 FALL 2 60.00 60.000 60.00 0.00 0.00 0 60.00 60.00 60.00 60.00 60
Appendix B—Descriptive Statistics Data Tables for Level III Subecoregions Within Aggregate Ec

 Aggregate Nutrient Ecoregion: IV  Lakes and Reservoirs Descriptive Statistics by Subecoregion, Year and Season from 1990 to 2000 Total_Phosphorus_ug_L subecoregion year season N MEAN MIN MAX STDDEV STDERR CV P5 P25 MEDIAN P75  28 1991 SPRING 3 30.00 20.000 40.00 10.00 5.77 33 20.00 20.00 30.00 40.00 40 28 1991 SUMMER 10 155.00 20.000 525.00 154.94 49.00 100 20.00 55.00 95.00 235.00 525 28 1992 SUMMER 2 57.50 55.000 60.00 3.54 2.50 6 55.00 55.00 57.50 60.00 60 28 1993 SUMMER 16 124.69 10.000 360.00 119.32 29.83 96 10.00 42.50 70.00 237.50 360 28 1994 FALL 10 233.00 20.000 655.00 209.30 66.19 90 20.00 80.00 127.50 360.00 655 28 1994 SPRING 4 231.25 90.000 570.00 227.17 113.59 98 90.00 102.50 132.50 360.00 570 28 1994 SUMMER 8 49.44 2.5000 130.00 48.69 17.21 98 2.50 9.00 37.50 85.00 130 28 1994 WINTER 2 52.50 10.000 95.00 60.10 42.50 114 10.00 10.00 52.50 95.00 95 28 1995 SPRING 1 2.50 2.5000 2.50 . . . 2.50 2.50 2.50 2.50 2 28 1995 SUMMER 4 50.63 2.5000 100.00 43.03 21.51 85 2.50 16.25 50.00 85.00 100 28 1995 WINTER 1 2.50 2.5000 2.50 . . . 2.50 2.50 2.50 2.50 2 28 1996 SUMMER 11 221.36 20.000 710.00 235.97 71.15 107 20.00 55.00 125.00 360.00 710 28 1997 FALL 6 283.33 140.00 570.00 168.16 68.65 59 140.00 180.00 202.50 405.00 570 28 1997 SPRING 6 150.83 100.00 240.00 55.17 22.52 37 100.00 110.00 130.00 195.00 240 28 1997 SUMMER 13 182.31 25.000 425.00 122.16 33.88 67 25.00 120.00 140.00 270.00 425 30 1990 FALL 8 31.41 2.5000 70.00 25.46 9.00 81 2.50 6.25 33.13 50.00 70 30 1990 SPRING 10 29.25 2.5000 115.00 32.66 10.33 112 2.50 10.00 20.00 30.00 115 30 1990 SUMMER 13 39.81 15.000 110.00 28.51 7.91 72 15.00 20.00 30.00 50.00 110 30 1990 WINTER 10 25.25 2.5000 100.00 29.28 9.26 116 2.50 10.00 15.63 30.00 100 30 1991 FALL 8 25.00 2.5000 70.00 26.52 9.38 106 2.50 2.50 13.13 48.13 70 30 1991 SPRING 11 32.27 2.5000 120.00 35.65 10.75 110 2.50 10.00 15.00 50.00 120 30 1991 SUMMER 12 18.02 2.5000 55.00 13.19 3.81 73 2.50 10.00 16.25 20.00 55 30 1991 WINTER 9 9.58 2.5000 30.00 10.97 3.66 114 2.50 2.50 2.50 20.00 30 30 1992 FALL 3 68.33 50.000 100.00 27.54 15.90 40 50.00 50.00 55.00 100.00 100 30 1992 SPRING 10 14.25 2.5000 40.00 11.61 3.67 81 2.50 2.50 12.50 20.00 40 30 1992 SUMMER 13 19.04 2.5000 55.00 14.69 4.08 77 2.50 10.00 20.00 30.00 55 30 1992 WINTER 8 29.53 2.5000 60.00 22.12 7.82 75 2.50 9.38 27.50 50.00 60 30 1993 FALL 3 26.67 10.000 40.00 15.28 8.82 57 10.00 10.00 30.00 40.00 40 30 1993 SPRING 6 57.50 10.000 130.00 52.89 21.59 92 10.00 25.00 30.00 120.00 130 30 1993 SUMMER 12 25.63 2.5000 40.00 10.45 3.02 41 2.50 20.00 25.00 32.50 40 30 1993 WINTER 6 127.50 20.000 480.00 174.92 71.41 137 20.00 40.00 62.50 100.00 480 30 1994 SPRING 2 4.38 2.5000 6.25 2.65 1.88 61 2.50 2.50 4.38 6.25 6
Appendix B—Descriptive Statistics Data Tables for Level III Subecoregions Within Aggregate Ec

 Aggregate Nutrient Ecoregion: IV  Lakes and Reservoirs Descriptive Statistics by Subecoregion, Year and Season from 1990 to 2000 Total_Phosphorus_ug_L subecoregion year season N MEAN MIN MAX STDDEV STDERR CV P5 P25 MEDIAN P75  30 1994 SUMMER 11 19.43 2.5000 105.00 30.36 9.15 156 2.50 2.50 10.00 20.00 105 30 1995 SPRING 2 2.50 2.5000 2.50 0.00 0.00 0 2.50 2.50 2.50 2.50 2 30 1995 SUMMER 3 28.33 20.000 40.00 10.41 6.01 37 20.00 20.00 25.00 40.00 40 30 1995 WINTER 3 31.67 20.000 50.00 16.07 9.28 51 20.00 20.00 25.00 50.00 50 30 1996 SPRING 2 6.25 2.5000 10.00 5.30 3.75 85 2.50 2.50 6.25 10.00 10 30 1996 SUMMER 4 36.25 10.000 70.00 28.69 14.34 79 10.00 12.50 32.50 60.00 70 30 1996 WINTER 3 23.33 20.000 30.00 5.77 3.33 25 20.00 20.00 20.00 30.00 30 31 1990 FALL 1 150.00 150.00 150.00 . . . 150.00 150.00 150.00 150.00 150 31 1990 SPRING 1 100.00 100.00 100.00 . . . 100.00 100.00 100.00 100.00 100 31 1990 SUMMER 2 92.50 20.000 165.00 102.53 72.50 111 20.00 20.00 92.50 165.00 165 31 1991 FALL 1 50.00 50.000 50.00 . . . 50.00 50.00 50.00 50.00 50 31 1991 SPRING 1 40.00 40.000 40.00 . . . 40.00 40.00 40.00 40.00 40 31 1991 SUMMER 1 100.00 100.00 100.00 . . . 100.00 100.00 100.00 100.00 100 31 1991 WINTER 1 85.00 85.000 85.00 . . . 85.00 85.00 85.00 85.00 85 31 1992 FALL 1 40.00 40.000 40.00 . . . 40.00 40.00 40.00 40.00 40 31 1992 SPRING 1 50.00 50.000 50.00 . . . 50.00 50.00 50.00 50.00 50 31 1992 SUMMER 2 16.25 2.5000 30.00 19.45 13.75 120 2.50 2.50 16.25 30.00 30 31 1992 WINTER 1 280.00 280.00 280.00 . . . 280.00 280.00 280.00 280.00 280 31 1993 FALL 1 30.00 30.000 30.00 . . . 30.00 30.00 30.00 30.00 30 31 1993 SUMMER 1 20.00 20.000 20.00 . . . 20.00 20.00 20.00 20.00 20 31 1994 SUMMER 1 2.50 2.5000 2.50 . . . 2.50 2.50 2.50 2.50 2 43 1990 FALL 3 100.00 20.000 140.00 69.28 40.00 69 20.00 20.00 140.00 140.00 140 43 1990 SUMMER 5 209.00 25.000 880.00 375.24 167.81 180 25.00 40.00 50.00 50.00 880 43 1990 WINTER 4 137.50 30.000 360.00 152.18 76.09 111 30.00 40.00 80.00 235.00 360 43 1991 FALL 11 70.25 11.250 181.50 51.49 15.52 73 11.25 39.00 60.50 81.00 181 43 1991 SUMMER 23 101.39 20.000 273.00 89.06 18.57 88 20.00 25.00 75.00 169.50 273 43 1992 FALL 3 166.67 15.000 380.00 190.15 109.79 114 15.00 15.00 105.00 380.00 380 43 1992 SUMMER 23 127.07 8.0000 360.00 121.70 25.38 96 10.00 28.50 55.00 227.50 360 43 1992 WINTER 2 300.00 130.00 470.00 240.42 170.00 80 130.00 130.00 300.00 470.00 470 43 1993 FALL 3 11.67 2.5000 20.00 8.78 5.07 75 2.50 2.50 12.50 20.00 20 43 1993 SPRING 1 35.00 35.000 35.00 . . . 35.00 35.00 35.00 35.00 35 43 1993 SUMMER 11 131.09 10.000 672.50 195.06 58.81 149 10.00 27.00 58.00 159.00 672
Appendix B—Descriptive Statistics Data Tables for Level III Subecoregions Within Aggregate Ec

 Aggregate Nutrient Ecoregion: IV  Lakes and Reservoirs Descriptive Statistics by Subecoregion, Year and Season from 1990 to 2000 Total_Phosphorus_ug_L subecoregion year season N MEAN MIN MAX STDDEV STDERR CV P5 P25 MEDIAN P75  43 1993 WINTER 1 20.00 20.000 20.00 . . . 20.00 20.00 20.00 20.00 20 43 1994 FALL 3 51.67 25.000 100.00 41.93 24.21 81 25.00 25.00 30.00 100.00 100 43 1994 SPRING 1 40.00 40.000 40.00 . . . 40.00 40.00 40.00 40.00 40 43 1994 SUMMER 12 39.42 4.5000 100.00 30.05 8.67 76 4.50 19.25 23.50 64.00 100 43 1994 WINTER 1 12.50 12.500 12.50 . . . 12.50 12.50 12.50 12.50 12 43 1995 FALL 5 52.00 20.000 75.00 27.06 12.10 52 20.00 25.00 70.00 70.00 75 43 1995 SPRING 1 10.00 10.000 10.00 . . . 10.00 10.00 10.00 10.00 10 43 1995 SUMMER 10 22.40 4.5000 65.00 17.15 5.42 77 4.50 15.00 20.00 28.00 65 43 1996 FALL 2 22.25 4.5000 40.00 25.10 17.75 113 4.50 4.50 22.25 40.00 40 43 1996 SPRING 1 80.00 80.000 80.00 . . . 80.00 80.00 80.00 80.00 80 43 1996 SUMMER 11 65.91 10.000 185.00 60.70 18.30 92 10.00 20.00 40.00 120.00 185 43 1996 WINTER 1 30.00 30.000 30.00 . . . 30.00 30.00 30.00 30.00 30 43 1997 FALL 2 102.50 80.000 125.00 31.82 22.50 31 80.00 80.00 102.50 125.00 125 43 1997 SPRING 1 50.00 50.000 50.00 . . . 50.00 50.00 50.00 50.00 50 43 1997 SUMMER 6 112.50 5.0000 300.00 122.95 50.20 109 5.00 10.00 75.00 210.00 300 43 1998 SUMMER 4 40.00 23.000 65.00 19.72 9.86 49 23.00 24.25 36.00 55.75 65 43 1999 SUMMER 5 46.60 11.000 136.00 50.66 22.66 109 11.00 26.00 26.00 34.00 136 43 2000 SUMMER 2 147.00 82.000 212.00 91.92 65.00 63 82.00 82.00 147.00 212.00 212 44 1990 WINTER 10 164.50 .00000 590.00 200.95 63.55 122 0.00 20.00 105.00 180.00 590 44 1991 FALL 8 100.63 5.0000 295.00 97.04 34.31 96 5.00 32.50 67.50 152.50 295 44 1991 SUMMER 3 28.33 5.0000 40.00 20.21 11.67 71 5.00 5.00 40.00 40.00 40 44 1992 FALL 2 115.00 60.000 170.00 77.78 55.00 68 60.00 60.00 115.00 170.00 170 44 1992 SUMMER 2 127.50 65.000 190.00 88.39 62.50 69 65.00 65.00 127.50 190.00 190 44 1993 FALL 1 70.00 70.000 70.00 . . . 70.00 70.00 70.00 70.00 70 44 1993 SUMMER 1 85.00 85.000 85.00 . . . 85.00 85.00 85.00 85.00 85 44 1994 FALL 1 140.00 140.00 140.00 . . . 140.00 140.00 140.00 140.00 140 44 1994 SUMMER 1 115.00 115.00 115.00 . . . 115.00 115.00 115.00 115.00 115 44 1995 FALL 2 140.00 130.00 150.00 14.14 10.00 10 130.00 130.00 140.00 150.00 150 44 1995 SUMMER 2 130.00 90.000 170.00 56.57 40.00 44 90.00 90.00 130.00 170.00 170 44 1996 FALL 1 40.00 40.000 40.00 . . . 40.00 40.00 40.00 40.00 40 44 1996 SPRING 1 70.00 70.000 70.00 . . . 70.00 70.00 70.00 70.00 70 44 1996 SUMMER 1 50.00 50.000 50.00 . . . 50.00 50.00 50.00 50.00 50
Appendix B—Descriptive Statistics Data Tables for Level III Subecoregions Within Aggregate Ec

 Aggregate Nutrient Ecoregion: IV  Lakes and Reservoirs Descriptive Statistics by Subecoregion, Year and Season from 1990 to 2000 Total_Phosphorus_ug_L subecoregion year season N MEAN MIN MAX STDDEV STDERR CV P5 P25 MEDIAN P75 P 44 1997 WINTER 1 45.00 45.000 45.00 . . . 45.00 45.00 45.00 45.00 45.
Appendix B—Descriptive Statistics Data Tables for Level III Subecoregions Within Aggregate Ec

 Aggregate Nutrient Ecoregion: IV  Lakes and Reservoirs Descriptive Statistics by Subecoregion, Year and Season from 1991 to 2000 pH_S_U subecoregion year season N MEAN MIN MAX STDDEV STDERR CV P5 P25 MEDIAN P75  43 1991 FALL 8 8.22 7.7250 8.73 0.28 0.10 3 7.73 8.12 8.25 8.29 8 43 1991 SUMMER 16 8.47 7.8550 9.85 0.52 0.13 6 7.86 8.14 8.34 8.80 9 43 1992 SUMMER 6 8.57 8.0800 9.48 0.50 0.20 6 8.08 8.24 8.43 8.77 9 43 1993 SUMMER 8 8.09 7.7600 8.72 0.29 0.10 4 7.76 7.93 8.05 8.14 8 43 1994 SUMMER 8 8.33 8.0150 9.01 0.32 0.11 4 8.02 8.12 8.23 8.44 9 43 1995 SUMMER 4 7.84 7.5800 7.99 0.18 0.09 2 7.58 7.73 7.90 7.95 7 43 1998 SUMMER 4 7.22 5.3000 8.67 1.71 0.86 24 5.30 5.77 7.45 8.66 8 43 1999 SUMMER 4 8.39 8.2550 8.59 0.14 0.07 2 8.26 8.30 8.35 8.48 8 43 2000 SUMMER 2 8.15 8.0400 8.26 0.16 0.11 2 8.04 8.04 8.15 8.26 8
APPENDIX C

Quality Control/Quality Assurance Rules

Appendix C—Quality Control/Quality Assurance Rules

Continued Support for the Compilation and

Analysis of National Nutrient Data

9 Nutrient Ecoregion/Waterbody Type Summary

Chapters

Prepared for:

Steve Potts

Environmental Protection Agency

OW/OST/HECD

Prepared by:

INDUS Corporation

1953 Gallows Road

Vienna, Virginia 22182

Contract Number:68-C-99-226

Task Number:07

Subtask Number:4

August 27, 2001

9 Nutrient Ecoregion/Waterbody Type Summary Chapters, Contract # 68-C-99-226, TO# 07 August 27, 2001

Appendix C—Quality Control/Quality Assurance Rules C-iii

CONTENTS

1.0 BACKGROUND . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C-1

1.1 Purpose . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C-1

1.2 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C-1

2.0 QA/QC PROCEDURES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C-1

2.1 National Data Sets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C-3

2.2 State Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C-3

2.3 Laboratory Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C-4

2.4 Waterbody Name and Class Information . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C-4

2.5 Ecoregion Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C-5

3.0 STATISTICAL ANALYSIS REPORTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C-5

3.1 Data Source Reports . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C-6

3.2 Remark Code Reports . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C-6

3.3 Median of Each Waterbody . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C-7

3.4 Descriptive Statistic Reports . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C-7

3.5 Regression Models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C-7

4.0 TIME PERIOD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C-8

5.0 DATA SOURCES AND PARAMETERS FOR THE AGGREGATE NUTRIENT

ECOREGIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C-8

5.1 Lakes and Reservoirs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C-9

5.1.1 Aggregate Nutrient Ecoregion 3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C-9

5.1.2 Aggregate Nutrient Ecoregion 4 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C-9

5.1.3 Aggregate Nutrient Ecoregion 5 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C-10

5.1.4 Aggregate Nutrient Ecoregion 14 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C-10

5.2 Rivers and Streams . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C-11

5.2.1 Aggregate Nutrient Ecoregion 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C-11

5.2.2 Aggregate Nutrient Ecoregion 4 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C-12

5.2.3 Aggregate Nutrient Ecoregion 5 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C-13

5.2.4 Aggregate Nutrient Ecoregion 8 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C-13

5.2.5 Aggregate Nutrient Ecoregion 10 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C-14

APPENDIX A Process Used to QA/QA the Legacy STORET Nutrient Data Set . . . . . . C-16

APPENDIX B Process for Adding Aggregate Nutrient Ecoregions and Level III 

Ecoregions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C-22

APPENDIX C Glossary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C-23

9 Nutrient Ecoregion/Waterbody Type Summary Chapters, Contract # 68-C-99-226, TO# 07 August 27, 2001

Appendix C—Quality Control/Quality Assurance Rules C-1

1.0 BACKGROUND

The Nutrient Criteria Program initiated the development of a national Nutrient Criteria Database

application that is used to store and analyze nutrient data. The ultimate use of these data is to

derive ecoregion specific nutrient criteria. EPA converted STOrage and RETrieval (STORET)

legacy data, National Stream Quality Accounting Network (NASQAN) data, National WaterQuality Assessment (NAWQA) data, and other relevant nutrient data from universities and

States/Tribes into the database. The data imported into the Nutrient Criteria Database are used

to develop national nutrient criteria recommendations. 

1.1 Purpose

The purpose of this deliverable is to provide EPA with information regarding the database used

to create the statistical reports which will be used to derive ecoregion-specific nutrient criteria

for Level III ecoregions. There are fourteen aggregate nutrient ecoregions. Each aggregate

nutrient ecoregion is divided into smaller ecoregions (subecoregions) referred to as Level III

ecoregions. EPA will determine criteria for the waterbody types and Level III ecoregions within

the following aggregate nutrient ecoregions:

• Lakes and Reservoirs

- Aggregate Nutrient ecoregions: 3, 4, 5, and 14

• Rivers and Streams 

- Aggregate Nutrient ecoregions: 1, 4, 5, 8, and 10

1.2 References

This section lists documents that contain baselines, standards, guidelines, policies, and

references that apply to the data analysis. Listed editions were valid at the time of publication. 

All documents are subject to revision, but these specific editions govern the concepts described

in this document. 

Nutrient Criteria Technical Guidance Document: Lakes and Reservoirs (Draft). EPA, Office of

Water, EPA 822-D-99-001, April 1999.

Nutrient Criteria Technical Guidance Manual: Rivers and Streams (Draft). EPA, Office of

Water, EPA 822-D-99-003, September 1999.

Guidance for Data Quality Assessment: Practical Methods for Data Analysis. EPA, Office of

Research and Development, EPA QA/G-9, January 1998.

2.0 QA/QC PROCEDURES

In order to develop nutrient criteria, EPA needed to obtain nutrient data from the states. EPA

requested nutrient data from the states and forwarded the data sets to INDUS via e-mail and/or

US mail. In addition, EPA tasked INDUS to convert data from three national data sets. EPA
9 Nutrient Ecoregion/Waterbody Type Summary Chapters, Contract # 68-C-99-226, TO# 07 August 27, 2001

C-2 Appendix C—Quality Control/Quality Assurance Rules

provided INDUS with a Legacy STORET extraction to convert into the database. The United

States Geologic Survey (USGS) sent INDUS a CD-ROM with NASQAN data to convert. 

INDUS downloaded NAWQA files from the USGS Web site to convert the data. In total,

INDUS converted and imported the following national and state data sets into the Nutrient

Criteria Database:

• Legacy STORET

• NAWQA

• NASQAN

• EPA Region 1

• EPA Region 2 - Lake Champlain Monitoring Project

• EPA Region 2 - NYSDEC Finger Lakes Monitoring Program

• EPA Region 2 - NY Citizens Lake Assessment Program

• EPA Region 2 - Lake Classification and Inventory Survey

• EPA Region 2 - NYCDEP (1990-1998)

• EPA Region 2 - NYCDEP (Storm Event data)

• EPA Region 2 - New Jersey Nutrient Data ( Tidal Waters)

• EPA Region 5

• EPA Region 3 

• EPA Region 3 - Nitrite Data

• EPA Region 3 - Choptank River files

• EPA Region 4 - Tennessee Valley Authority

• EPA Region 7 - Central Plains Center for BioAssessment (CPCB) 

• EPA Region 7 - REMAP

• EPA Region 2 - Delaware River Basin Commission (1990-1998)

• EPA Region 3 - PA Lake Data

• EPA Region 3 - University of Delaware

• EPA Region 10 

• University of Auburn

• EPA Region 8 - MT and WY

• EPA Region 9

• Suffolk County

• NYCDEC

• NY Lakes Morphometry

• EPA Region 8 - South Dakota

• EPA Region 8 - Colorado Reservoir

• EPA Region 4

• EPA Region 10 - Lake Data

• EPA Region 7 - Central Plains Center for BioAssessment (CPCB) 2

• EPA Region 8 - North Dakota

• EPA Region 8 - Eagle River

• EPA Region 8 - Utah

• Florida
9 Nutrient Ecoregion/Waterbody Type Summary Chapters, Contract # 68-C-99-226, TO# 07 August 27, 2001

Appendix C—Quality Control/Quality Assurance Rules C-3

As part of the conversion process, INDUS performed a number of Quality Assurance/Quality

Control (QA/QC) steps to ensure that the data were properly converted into the Nutrient Criteria

Database. Sections 2.1 and 2.2 explain the steps performed by INDUS to convert the data.

2.1 National Data Sets

INDUS converted three national data sets into the Nutrient Criteria Database: Legacy STORET

data, NASQAN data, and NAWQA data. A previous EPA contractor performed the extraction

of Legacy STORET data and documented the QA/QC procedures used on the data. This

documentation is included in Appendix A. INDUS performed minimal QA/QC on the Legacy

STORET data set because the previous contractor completed the steps outlined in Appendix A. 

INDUS and EPA also agreed to convert the NAWQA and NASQAN data sets with minimal

QA/QC on the assumption that the source agency, the USGS, QA/QC’d the data. 

For each of the three national data sets, INDUS ran queries to determine if 1) samples existed

without results and 2) if stations existed without samples. Per Task Order Project Officer

(TOPO) direction, these records were deleted from the system. For analysis purposes, EPA

determined that there was no need to keep station records with no samples and sample records

with no results. INDUS also confirmed that each data set contained no duplicate records. 

In addition, INDUS deleted all composite results from the Legacy STORET data. Per TOPO

direction, it was decided that composite sample results would not be used in the statistical

analysis.

2.2 State Data

Each state data set was delivered in a unique format. Many of the data sets were delivered to

INDUS without corresponding documentation. INDUS analyzed each state data set in order to

determine which parameters should be converted for analysis. INDUS obtained a master

parameter table from EPA and converted the parameters in the state data sets according to those

that were present in the EPA parameter table. INDUS converted all of the data elements in the

state data sets that mapped directly to the Nutrient Criteria Database; data elements that did not

map to the Nutrient Criteria Database were not converted. In some cases, state data elements

that did not directly map into the Oracle database were inserted into a comment field within the

database. Also, INDUS maintained an internal record of which state data elements were inserted

into the comment field. 

As part of the data clean-up efforts, INDUS determined whether or not there were any duplicate

records in the state data sets and deleted the duplicate records. INDUS checked the waterbody,

station, and sample entities for duplicate records. However, if there was not enough information

provided to determine duplicates such as sampling date, there was no way for INDUS to locate

duplicate records. In addition, INDUS deleted station records with no samples and sample

records with no results. INDUS also deleted waterbody records that were not associated with a

station. In each case, INDUS maintained an internal record of how many records were deleted. 
9 Nutrient Ecoregion/Waterbody Type Summary Chapters, Contract # 68-C-99-226, TO# 07 August 27, 2001

C-4 Appendix C—Quality Control/Quality Assurance Rules

If INDUS encountered referential integrity errors, such as samples that referred to stations that

did not exist, or if INDUS was unsure of whether a record was a duplicate, INDUS contacted the

agency directly via e-mail or phone to resolve any issues that arose. INDUS saved an electronic

copy of each e-mail correspondence with the states to ensure that a record of the decision was

maintained. 

Finally, INDUS examined the remark codes of each result record in the state data sets. INDUS

mapped the remark codes to the STORET remark codes listed in Table 2 of Appendix A. If any

of the state result records were associated with remark codes marked as “Delete” in Table 2 of

Appendix A, the result records were not converted into the database.

2.3 Laboratory Methods 

Many of the state data sets did not contain laboratory method information. In addition,

laboratory method information was not available for the three national data sets. In order to

determine missing laboratory method information, EPA tasked another contractor to contact the

data owners to obtain the laboratory method. In some cases, the data owners responded and the

laboratory methods were added to the database. In other cases, the methods are unknown. 

2.4 Waterbody Name and Class Information

A large percentage of the data did not have waterbody-specific information. The only waterbody

information contained in the three national data sets was the waterbody name, which was

embedded in the station ‘location description’ field. Most of the state data sets contained

waterbody name information; however, much of the data were duplicated throughout the data

sets. Therefore, the waterbody information was cleaned manually. For the three national data

sets, the ‘location description’ field was extracted from the station table and moved to a

temporary table. The ‘location description’ field was sorted alphabetically. Unique waterbodies

were grouped together based on name similarity and whether or not the waterbodies fell within

the same county, state, and waterbody type. Finally, the ‘location description’ field was edited

to include only waterbody name information, not descriptive information. For example, 110

MILE CREEK AT POMONA DAM OUTFLOW, KS PO-2 was edited to 110 MILE CREEK. 

Also, if 100 MILE CREEK was listed ten times in New York, but in four different counties, four

100 MILE CREEK waterbody records were created.

Similar steps were taken to eliminate duplicate waterbody records in the state data sets. If a

number of records had similar waterbody names and fell within the same state, county, and

waterbody type, the records were grouped to create a unique waterbody record.

Most of the waterbody data did not contain depth, surface area, and volume measurements. EPA

needed this information to classify waterbody types. EPA attempted to obtain waterbody class

information from the states. EPA sent waterbody files to the regional coordinators and requested

that certain class information be completed by each state. The state response was poor;

therefore, EPA was not able to perform statistical analysis for the waterbody types by class.
9 Nutrient Ecoregion/Waterbody Type Summary Chapters, Contract # 68-C-99-226, TO# 07 August 27, 2001

Appendix C—Quality Control/Quality Assurance Rules C-5

2.5 Ecoregion Data

Aggregate nutrient ecoregions and Level III ecoregions were added to the database using the

station latitude and longitude coordinates, the county centroid, or HUC (Hydrological Unit

Code) centroid. If a station was lacking latitude and longitude coordinates and county

information, the data were not included in the statistical analysis. Appendix B lists the steps

taken to add the two ecoregion types (aggregate and Level III) to the Nutrient Criteria Database. 

The ecoregion names were pulled from aggregate nutrient ecoregion and Level III ecoregion

Geographical Information System (GIS) coverages. In summary, the station latitude and

longitude coordinates were used to determine the ecoregion under the following circumstances:

• The latitude and longitude coordinates fell within the county/state listed in the station table.

• The county data were missing.

The county centroid was used to determine the ecoregions under the following circumstances:

• The latitude and longitude coordinates were missing, but the state/county information was

available.

• The latitude and longitude coordinates fell outside the county/state/HUC listed in the station

table. The county information was assumed to be correct; therefore, the county centroid was

used.

The HUC centroid was used to determine the ecoregions under the following circumstances:

• The latitude and longitude coordinates and county were missing, but the HUC information

was available.

If the latitude and longitude coordinates fell outside the continental US county coverage file 

(i.e., the point fell in the ocean or Mexico/Canada), the nearest ecoregion was assigned to the

station.

3.0 STATISTICAL ANALYSIS REPORTS

Aggregate nutrient ecoregion tables were created by extracting all observations for a specific

aggregate nutrient ecoregion from the Nutrient Criteria Database. Then, the data were reduced

to create tables containing only the yearly median values. To create these tables, the median

value for each waterbody was calculated using all observations for each waterbody by Level III

ecoregion, state, county, year, and season. Tables of decade median values were created from

the yearly median tables by calculating the median for each waterbody by Level III ecoregion,

state, county, decade and season.

The Data Source and the Remark Code reports were created using all observations (all reported

values). All the other reports were created from either the yearly median tables or the decade

median tables. In other words, the descriptive statistics and regressions were run using the

median values for each waterbody and not the individual reported values.
9 Nutrient Ecoregion/Waterbody Type Summary Chapters, Contract # 68-C-99-226, TO# 07 August 27, 2001

C-6 Appendix C—Quality Control/Quality Assurance Rules

Statistical analyses were performed under the assumption that this data set is a random sample. 

If this assumption cannot be verified, the observations may or may not be valid. Values below

the 1st and 99th percentile were removed from the Legacy STORET database prior to the creation

of the national database. Also, data were treated according to the Legacy STORET remark codes

in Appendix A. 

The following contains a list of each report and the purpose for creating each report: 

• Data Source—Created to provide a count of the amount of data and to identify the source(s).

• Remark Codes—Created to provide a description of the data. 

• Median of Each Waterbody by Year—This was an intermediate step performed to obtain a

median value for each waterbody to be used in the yearly descriptive statistics reports and the

regression models. 

• Median of Each Waterbody by Decade—This was an intermediate step performed to obtain a

median value for each waterbody to be used in the decade descriptive statistics.

• Descriptive Statistics—Created to provide EPA with the desired statistics for setting criteria

levels.

• Regression Models—Created to examine the relationships between biological and nutrient

variables. 

Note: Separate reports were created for each season.

3.1 Data Source Reports

Data source reports were presented in the following formats:

• The number and percentage of data from each data source were summarized in tables for

each aggregate nutrient ecoregion by season and waterbody type.

• The number and percentage of data from each data source were summarized in tables for

each aggregate nutrient ecoregion for all seasons and waterbody type.

• The number and percentage of data from each data source were summarized in tables for

each Level III ecoregion by season and waterbody type.

The ‘Frequency’ represents the number of data values from a specific data source for each

parameter by data source. The ‘Row Pct’ represents the percentage of data from a specific data

source for each parameter. 

3.2 Remark Code Reports

Remark code reports were presented in the following formats:

• The number and percentage of data associated with a particular remark code for each

parameter were summarized in tables by Level III ecoregion by decade and season.
9 Nutrient Ecoregion/Waterbody Type Summary Chapters, Contract # 68-C-99-226, TO# 07 August 27, 2001

Appendix C—Quality Control/Quality Assurance Rules C-7

• The number and percentage of data associated with a particular remark code for each

parameter were summarized in tables by Level III ecoregion by year and season.

The ‘Frequency’ represents the number of data values corresponding to the remark code in the

column. The ‘Row Pct’ represents the percentage of data that was associated with the remark

code in that row.

In the database, remark codes that were entered by the states were mapped to Legacy STORET

remark codes. Prior to the analysis, the data were treated according to these remark codes. For

example, if the remark code was ‘K,’ then the reported value was divided by two. Appendix A

contains a complete list of Legacy STORET remark codes.

Note: For the reports, a remark code of ‘Z’ indicates that no remark codes were recorded. It does

not correspond to Legacy STORET code ‘Z.’

3.3 Median of Each Waterbody

To reduce the data and to ensure heavily sampled waterbodies or years were not over represented

in the analysis, median value tables (described above) were created. The yearly median tables

and decade median tables were delivered to the EPA in electronic format as csv (comma

separated value or comma delimited) files.

3.4 Descriptive Statistic Reports

The number of waterbodies, median, mean, minimum, maximum, 5th, 25th , 75th , 95th percentiles,

standard deviation, standard error, and coefficient of variation were calculated. The tables

(described above) containing the decade median values for each waterbody for each parameter

were used to create descriptive statistics reports for:

• Level III ecoregions by decade and season

• Aggregate nutrient ecoregions by decade and season

In addition, the tables containing the yearly median values for each waterbody for each

parameter were used to create descriptive statistics reports for:

• Level III ecoregions by year and season

3.5 Regression Models

Simple linear regressions using the least squares method were performed to examine the

relationships between biological and nutrient variables in lakes and reservoirs, and rivers and

streams. Regressions were performed using the yearly median tables. Chlorophyll(s) in

micrograms per liter (ug/L), Secchi in meters (m), Dissolved Oxygen in milligrams per liter

(mg/L), Turbidity, and pH were the biological variables in these models. Secchi data were used

in the lake and reservoir models, and Turbidity data were used in the river and stream models. 
9 Nutrient Ecoregion/Waterbody Type Summary Chapters, Contract # 68-C-99-226, TO# 07 August 27, 2001

C-8 Appendix C—Quality Control/Quality Assurance Rules

The nutrient variables in these models include: Total Phosphorus in ug/L, Total Nitrogen in

mg/L, Total Kjeldahl Nitrogen in mg/L, and Nitrate and Nitrite in mg/L. 

4.0 TIME PERIOD

Data collected from January 1990 to December 2000 were used in the statistical analysis reports. 

To capture seasonal differences, the data were classified as follows:

• Aggregate nutrient ecoregions: 6, 7, and 8

S Spring: April to May 

S Summer: June to August 

S Fall: September to October

S Winter: November to March

• Aggregate nutrient ecoregions: 1, 2, 3, 4, 5, 9, 10, 11, 12, 13, and 14

S Spring: March to May

S Summer: June to August 

S Fall: September to November

S Winter: December to February 

5.0 DATA SOURCES AND PARAMETERS FOR THE AGGREGATE NUTRIENT

ECOREGIONS

This section provides information for the nutrient aggregate ecoregions that were analyzed by

waterbody type. Each section lists the data sources for the aggregate nutrient ecoregion

including: 1) the data sources, 2) the parameters included in the analysis, and 3) the Level III

ecoregions within the aggregate nutrient ecoregions.

Note: For analysis purposes, data for the following parameters were grouped together and

reported under Phosphorous, Dissolved Inorganic (DIP):

Phosphorus, Dissolved Inorganic (DIP) 

Phosphorus, Dissolved (DP) 

Phosphorus, Dissolved Reactive (DRP)

Orthophosphate, dissolved, mg/L as P 

Orthophosphate (OPO4_PO4)
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Appendix C—Quality Control/Quality Assurance Rules C-9

5.1 Lakes and Reservoirs

5.1.1 Aggregate Nutrient Ecoregion 3

Data Sources:

Legacy STORET

EPA Region 10

EPA Region 8 - Colorado Reservoir

Parameters:

Chlorophyll A, Fluorometric, corrected (ug/L)

Chlorophyll A, Phytoplankton, spectrophotometric Acid (ug/L)

Chlorophyll A, Trichromatic, uncorrected (ug/L)

Dissolved Inorganic Phosphorus (DIP) (ug/L)

Dissolved Oxygen (DO) (mg/L)

Nitrite and Nitrate, (NO2+NO3) (mg/L)

Nitrogen, Total (TN) (mg/L)

Nitrogen, Total Kjeldhal (TKN) (mg/L)

Phosphorus, Total (TP) (ug/L)

SECCHI (m)

pH

Level III ecoregions:

6, 10, 12, 13, 18, 20, 22, 24, 80, 81

5.1.2 Aggregate Nutrient Ecoregion 4

Data Sources:

Legacy STORET

EPA Region 8 - MT and WY

EPA Region 8 - South Dakota

EPA Region 8 - North Dakota

Parameters:

Chlorophyll A, Phytoplankton, spectrophotometric Acid (ug/L)

Chlorophyll A, Trichromatic, uncorrected (ug/L)

Dissolved Inorganic Phosphorus (DIP) (ug/L)

Dissolved Oxygen (DO) (% Saturated)

Dissolved Oxygen (DO) (mg/L)

Nitrite and Nitrate, (NO2+NO3) (mg/L)

Nitrogen, Total (TN) (mg/L)
9 Nutrient Ecoregion/Waterbody Type Summary Chapters, Contract # 68-C-99-226, TO# 07 August 27, 2001

C-10 Appendix C—Quality Control/Quality Assurance Rules

Nitrogen, Total Kjeldhal (TKN) (mg/L)

Phosphorus, Total (TP) (ug/L)

SECCHI (m)

pH

Level III ecoregions:

26, 28, 30, 31, 43, 44

5.1.3 Aggregate Nutrient Ecoregion 5

Data sources:

Legacy STORET

EPA Region 8 - MT and WY

EPA Region 8 - South Dakota

EPA Region 8 - North Dakota

Parameters:

Chlorophyll A, Phytoplankton, spectrophotometric Acid (ug/L)

Chlorophyll A, Trichromatic, uncorrected (ug/L)

Dissolved Inorganic Phosphorus (DIP) (ug/L)

Dissolved Oxygen (DO) (% Saturated)

Dissolved Oxygen (DO) (mg/L)

Nitrite and Nitrate, (NO2+NO3) (mg/L)

Nitrogen, Total (TN) (mg/L)

Nitrogen, Total Kjeldhal (TKN) (mg/L)

Phosphorus, Total (TP) (ug/L)

SECCHI (m)

pH

Level III ecoregions:

25, 27, 32, 42

5.1.4 Aggregate Nutrient Ecoregion 14

Data sources:

Legacy STORET

Region 2 - NY Citizens Lake Assessment Program

Region 2 - NYCDEP (1990-1998)

EPA Region 1
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Appendix C—Quality Control/Quality Assurance Rules C-11

Parameters:

CHLB (ug/L)

CHLC (ug/L)

Chlorophyll A, Fluorometric, corrected (ug/L)

Chlorophyll A, Phytoplankton, spectrophotometric Acid (ug/L)

Chlorophyll A, Phytoplankton, spectrophotometric, uncorrected (ug/L)

Chlorophyll A, Trichromatic, uncorrected (ug/L)

Dissolved Inorganic Phosphorus (DIP) (ug/L)

Dissolved Oxygen (DO) (mg/L)

Nitrite and Nitrate, (NO2+NO3) (mg/L)

Nitrogen, Total (TN) (mg/L)

Nitrogen, Total Kjeldhal (TKN) (mg/L)

Phosphorus, Total (TP) (ug/L)

SECCHI (m)

pH

Level III ecoregions:

59, 63, 84

5.2 Rivers and Streams

5.2.1 Aggregate Nutrient Ecoregion 1

Data sources:

Legacy STORET

NASQAN

NAWQA

EPA Region 10

Parameters:

Chlorophyll A, Fluorometric, corrected (ug/L)

Chlorophyll A, Periphyton, spectrophotometric, uncorrected (mg/sqm)

Chlorophyll A, Phytoplankton, spectrophotometric Acid (ug/L)

Chlorophyll A, Trichromatic, uncorrected (ug/L)

Dissolved Inorganic Phosphorus (DIP) (ug/L)

Dissolved Oxygen (DO) (mg/L)

Nitrite and Nitrate, (NO2+NO3) (mg/L)

Nitrogen, Total (TN) (mg/L)

Nitrogen, Total Kjeldhal (TKN) (mg/L)

Phosphorus, Total (TP) (ug/L)

Phosphorus, orthophosphate, total, as P(ug/L)

Turbidity (FTU)
9 Nutrient Ecoregion/Waterbody Type Summary Chapters, Contract # 68-C-99-226, TO# 07 August 27, 2001

C-12 Appendix C—Quality Control/Quality Assurance Rules

Turbidity (NTU)

Turbidity (JCU)

pH

Level III ecoregions:

3, 7

5.2.2 Aggregate Nutrient Ecoregion 4

Data sources:

Legacy STORET

NASQAN

NAWQA

EPA Region 7 - Central Plains Center for BioAssessment (CPCB)

EPA Region 7 - Central Plains Center for BioAssessment (CPCB) 2

EPA Region 7 - REMAP

EPA Region 8 - MT and WY

EPA Region 8 - South Dakota

EPA Region 8 - North Dakota

Parameters:

Chlorophyll A, Fluorometric, corrected (ug/L)

Chlorophyll A, Pheophytin, corrected (ug/L)

Chlorophyll A, Phytoplankton, spectrophotometric Acid (ug/L)

Dissolved Inorganic Phosphorus (DIP) (ug/L)

Dissolved Oxygen (DO) (% Saturated)

Dissolved Oxygen (DO) (mg/L)

Nitrite and Nitrate, (NO2+NO3) (mg/L)

Nitrogen, Total (TN) (mg/L)

Nitrogen, Total Kjeldhal (TKN) (mg/L)

Organic_P (ug/L)

Phosphorus, Total (TP) (ug/L)

Phosphorus, orthophosphate, total, as P(ug/L)

Turbidity (FTU)

Turbidity (NTU)

Turbidity (JCU)

pH

Level III ecoregions:

26, 28, 30, 31, 43, 44
9 Nutrient Ecoregion/Waterbody Type Summary Chapters, Contract # 68-C-99-226, TO# 07 August 27, 2001

Appendix C—Quality Control/Quality Assurance Rules C-13

5.2.3 Aggregate Nutrient Ecoregion 5

Data sources:

Legacy STORET

NASQAN

NAWQA

EPA Region 7 - Central Plains Center for BioAssessment (CPCB)

EPA Region 7 - Central Plains Center for BioAssessment (CPCB) 2

EPA Region 7 - REMAP

EPA Region 8 - MT and WY

EPA Region 8 - South Dakota

EPA Region 8 - North Dakota

Parameters:

Chlorophyll A, Fluorometric, corrected (ug/L)

Chlorophyll A, Pheophytin, corrected (ug/L)

Chlorophyll A, Phytoplankton, spectrophotometric Acid (ug/L)

Dissolved Inorganic Phosphorus (DIP) (ug/L)

Dissolved Oxygen (DO) (% Saturated)

Dissolved Oxygen (DO) (mg/L)

Nitrite and Nitrate, (NO2+NO3) (mg/L)

Nitrogen, Total (TN) (mg/L)

Nitrogen, Total Kjeldhal (TKN) (mg/L)

Organic_P (ug/L)

Phosphorus, Total (TP) (ug/L)

Phosphorus, orthophosphate, total, as P (ug/L)

Turbidity (FTU)

Turbidity (NTU)

Turbidity (JCU)

pH

Level III ecoregions:

25, 27, 32, 42

5.2.4 Aggregate Nutrient Ecoregion 8 

Data sources:

Legacy STORET

NASQAN

NAWQA

EPA Region 2 - NYCDEP (1990-1998)

EPA Region 1
9 Nutrient Ecoregion/Waterbody Type Summary Chapters, Contract # 68-C-99-226, TO# 07 August 27, 2001

C-14 Appendix C—Quality Control/Quality Assurance Rules

EPA Region 3

EPA Region 5

Parameters:

Chlorophyll A, Fluorometric, corrected (ug/L)

Chlorophyll A, Phytoplankton, spectrophotometric Acid (ug/L)

Chlorophyll A, Phytoplankton, spectrophotometric, uncorrected (ug/L)

Chlorophyll A, Trichromatic, uncorrected (ug/L)

Dissolved Inorganic Phosphorus (DIP) (ug/L)

Dissolved Oxygen (DO) (% Saturated)

Dissolved Oxygen (DO) (mg/L)

Nitrite and Nitrate, (NO2+NO3) (mg/L)

Nitrogen, Total (TN) (mg/L)

Nitrogen, Total Kjeldhal (TKN) (mg/L)

Phosphorus, Total (TP) (ug/L)

Phosphorus, orthophosphate, total, as P (ug/L)

Turbidity (FTU)

Turbidity (NTU)

pH

Level III ecoregions:

49, 50, 58, 62, 82

5.2.5 Aggregate Nutrient Ecoregion 10

Data sources:

Legacy STORET

NASQAN

EPA Region 7 - Central Plains Center for BioAssessment (CPCB)

EPA Region 7 - Central Plains Center for BioAssessment (CPCB) 2

EPA Region 7 - REMAP

Parameters:

Chlorophyll A, Fluorometric, corrected (ug/L)

Chlorophyll A, Pheophytin, corrected (ug/L)

Chlorophyll A, Phytoplankton, chromotographic- fluorometric (ug/L)

Chlorophyll A, Phytoplankton, spectrophotometric Acid (ug/L)

Chlorophyll A, Trichromatic, uncorrected (ug/L)

Chlorophyll B, Phytoplankton, chromotographic- fluorometric (ug/L)

Dissolved Inorganic Phosphorus (DIP) (ug/L)

Dissolved Oxygen (DO) (mg/L)

Nitrite and Nitrate, (NO2+NO3) (mg/L)

Nitrogen, Total (TN) (mg/L)
9 Nutrient Ecoregion/Waterbody Type Summary Chapters, Contract # 68-C-99-226, TO# 07 August 27, 2001

Appendix C—Quality Control/Quality Assurance Rules C-15

Nitrogen, Total Kjeldhal (TKN) (mg/L)

Organic_P (ug/L)

Phosphorus, Total (TP) (ug/L)

Phosphorus, orthophosphate, total, as P(ug/L)

Turbidity (FTU)

Turbidity (NTU)

Turbidity (JCU)

pH

Level III ecoregions:

34, 73
9 Nutrient Ecoregion/Waterbody Type Summary Chapters, Contract # 68-C-99-226, TO# 07 August 27, 2001

C-16 Appendix C—Quality Control/Quality Assurance Rules

APPENDIX A. Process Used to QA/QC the Legacy STORET Nutrient Data Set

1. STORET water quality parameters and Station and Sample data items were retrieved from

USEPA’s mainframe computer. Table 1 lists all retrieved parameters and data items.

TABLE 1: PARAMETERS AND DATA ITEMS RETRIEVED FROM STORET

Parameters Retrieved

 (STORET Parameter Code) 

Station Data Items Included

(STORET Item Name)

Sample Data Items

Included

(STORET Item Name)

TN - mg/l (600)

TKN - mg/l (625)

Total Ammonia (NH3+NH4) - mg/l (610)

Total NO2+NO3 - mg/l (630)

Total Nitrite - mg/l (615)

Total Nitrate - mg/l (620)

Organic N - mg/L (605)

TP - mg/l (665)

Chlor a - ug/L (spectrophotometric method,

32211) 

Chlor a - ug/L (fluorometric method corrected,

32209)

Chlor a - ug/L (trichromatic method corrected,

32210)

Secchi Transp. - inches (77)

Secchi Transp. - meters (78)

+Turbidity JCUs (70)

+Turbidity FTUs (76)

+Turbidity NTUs field (82078)

+Turbidity NTUs lab (82079)

+DO - mg/L (300)

+Water Temperature (degrees C, 10/degrees F,

11)

Station Type (TYPE)

Agency Code (AGENCY)

Station No. (STATION)

Latitude - std. decimal degrees

(LATSTD)

Longitude - std. decimal degrees 

(LONGSTD)

Station Location (LOCNAME)

County Name (CONAME)

State Name (STNAME)

Ecoregion Name - Level III

(ECONAME)

Ecoregion Code -Level III

(ECOREG)

Station Elevation (ELEV)

Hydrologic Unit Code

(CATUNIT)

RF1 Segment and Mile

(RCHMIL)

RF1ON/OFF tag (ONOFF)

Sample Date (DATE)

Sample Time (TIME)

Sample Depth (DEPTH)

Composite Sample Code 

(SAMPMETH)

+ If data record available at a station included data only for this or other such marked parameters, data record was

deleted from data set. 

The following set of retrieval rules were applied to the retrieval process:

C Data were retrieved for waterbodies specified only as ‘lake’, ‘stream’, ‘reservoir’, or

‘estuary’ under “Station Type” parameter. Any stations specified as ‘well,’ ‘spring,’ or

‘outfall’ were eliminated from the retrieved data set.

• Data were retrieved for station types described as ‘ambient’ (e.g., no pipe or facility

discharge data) under the “Station Type” parameter.

• Data were retrieved that were designated as ‘water’ samples only. This includes ‘bottom’

and ‘vertically integrated’ water samples.

• Data were retrieved that were designated as either ‘grab’ samples and ‘composite’ samples

(mean result only).
9 Nutrient Ecoregion/Waterbody Type Summary Chapters, Contract # 68-C-99-226, TO# 07 August 27, 2001

Appendix C—Quality Control/Quality Assurance Rules C-17

• No limits were specified for sample depths.

• Data were retrieved for all fifty states, Puerto Rico, and the District of Columbia.

• The time period specified for data retrieval was January 1990 to September 1998.

• No data marked as “Retired Data” (i.e., data from a generally unknown source) were

retrieved.

• Data marked as “National Urban Runoff data” (i.e., data associated with sampling conducted

after storm events to assess nonpoint source pollutants) were included in the retrieval. Such

data are part of STORET’s ‘Archived’ data. 

• Intensive survey data (i.e., data collected as part of specific studies) were retrieved.

2. Any values falling below the 1st percentile and any values falling above the 99th

percentile were transformed into ‘missing’ values (i.e., values were effectively removed

from the data set, but were not permanently eliminated).

3. Based on the STORET ‘Remark Code’ associated with each retrieved data point, the

following rules were applied (Table 2): 

TABLE 2: STORET REMARK CODE RULES

STORET Remark Code Keep or Delete Data Point

blank - Data not remarked. Keep

A - Value reported is the mean of two or more determinations. Keep 

B - Results based upon colony counts outside the acceptable ranges. Delete

C -Calculated. Value stored was not measured directly, but was

calculated from other data available.

Keep

D - Field measurement. Keep

E - Extra sample taken in compositing process. Delete

F - In the case of species, F indicates female sex. Delete

G - Value reported is the maximum of two or more determinations. Delete

H - Value based on field kit determination; results may not be accurate. Delete

I - The value reported is less than the practical quantification limit and

greater than or equal to the method detection limit.

Keep, but used one-half the 

reported value as the new value.

J - Estimated. Value shown is not a result of analytical measurement. Delete
9 Nutrient Ecoregion/Waterbody Type Summary Chapters, Contract # 68-C-99-226, TO# 07 August 27, 2001

TABLE 2: STORET REMARK CODE RULES

C-18 Appendix C—Quality Control/Quality Assurance Rules

K - Off-scale low. Actual value not known, but known to be less than

value shown.

Keep, but used one-half the reported 

value as the new value.

L - Off-scale high. Actual value not known, but known to be greater

than value shown.

Keep

M -Presence of material verified, but not quantified. Indicates a positive

detection, at a level too low to permit accurate quantification.

Keep, but used one half the reported

value as the new value. 

N -Presumptive evidence of presence of material. Delete

O -Sample for, but analysis lost. Accompanying value is not meaningful

for analysis.

Delete

P -Too numerous to count. Delete

Q -Sample held beyond normal holding time. Delete

R -Significant rain in the past 48 hours. Delete

S -Laboratory test. Keep

T -Value reported is less than the criteria of detection. Keep, but replaced reported value with

0.

U -Material was analyzed for, but not detected. Value stored is the limit

of detection for the process in use. 

Keep, but replaced reported value with

0.

V -Indicates the analyte was detected in both the sample and associated

method blank.

Delete

W -Value observed is less than the lowest value reportable under remark

“T.”

Keep, but replaced reported value with

0.

X -Value is quasi vertically-integrated sample. No data point with this remark code in 

data set.

Y -Laboratory analysis from unpreserved sample. Data may not be

accurate.

Delete

Z -Too many colonies were present to count. Delete

If a parameter (excluding water temperature) value was less than or equal to zero and no remark code was present,

the value was transformed into a missing value.

Rationale - Parameter concentrations should never be zero without a proper explanation. A method detection limit

should at least be listed
9 Nutrient Ecoregion/Waterbody Type Summary Chapters, Contract # 68-C-99-226, TO# 07 August 27, 2001

Appendix C—Quality Control/Quality Assurance Rules C-19

4. Station records were eliminated from the data set if any of the following descriptors were

present within the “Station Type” parameter:

< MONITR - Source monitoring site, which monitors a known problem or to detect a specific

problem.

< HAZARD - Site of hazardous or toxic wastes or substances.

< ANPOOL - Anchialine pool, underground pools with subsurface connections to watertable

and ocean.

< DOWN - Downstream (i.e., within a potentially polluted area) from a facility which has a

potential to pollute.

< IMPDMT - Impoundment. Includes waste pits, treatment lagoons, and settling and

evaporation ponds.

< STMSWR - Storm water sewer.

< LNDFL - Landfill.

< CMBMI - Combined municipal and industrial facilities.

< CMBSRC - Combined source (intake and outfall).

Rationale - these descriptors potentially indicate a station location that at which an ambient

water sample would not be obtained (i.e., such sampling locations are potentially biased) or the

sample location is not located within one of the designated water body types (i.e, ANPOOL). 

5. Station records were eliminated from data set if the station location did not fall within any

established cataloging unit boundaries based on their latitude and longitude.

6. Using nutrient ecoregion GIS coverage provided by USEPA, all station locations with

latitude and longitude coordinates were tagged with a nutrient ecoregion identifier (nutrient

region identifiers are values 1 - 14) and the associated nutrient ecoregion name. Because no

nutrient ecoregions exist for Alaska, Hawaii, and Puerto Rico, stations located in these states

were tagged with “dummy” nutrient ecoregion numbers (20 = Alaska, 21 = Hawaii, 22 =

Puerto Rico).

7. Using information provided by TVA, 59 station locations that were marked as ‘stream’

locations under the “Station Type” parameter were changed to ‘reservoir’ locations. 

8. The nutrient data retrieved from STORET were assessed for the presence of duplicate data

records. The duplicate data identification process consisted of three steps: 1) identification of

records that matched exactly in terms of each variable retrieved; 2) identification of records

that matched exactly in terms of each variable retrieved except for their station identification

numbers; and 3) identification of records that matched exactly in terms of each variable

retrieved except for their collecting agency codes. The data duplication assessment

procedures were conducted using SAS programs. 

Prior to initiating the data duplication assessment process, the STORET nutrient data set

contained:

41,210 station records

924,420 sample records
9 Nutrient Ecoregion/Waterbody Type Summary Chapters, Contract # 68-C-99-226, TO# 07 August 27, 2001

C-20 Appendix C—Quality Control/Quality Assurance Rules

• Identification of exactly matching records

All data records were sorted to identify those records that matched exactly. For two records

to match exactly, all variables retrieved had to be the same. For example, they had to have

the same water quality parameters, parameter results and associated remark codes, and have

the same station data item and sample data item information. Exactly matching records were

considered to be exact duplicates, and one duplicate record of each identified matching set

were eliminated from the nutrient data set. A total of 924 sample records identified as

duplicates by this process were eliminated from the data set. 

• Identification of matching records with the exception of station identification number

All data records were sorted to identify those records that matched exactly except for their

station identification number (i.e., they had the same water quality parameters, parameter

results and associated remark codes, and the same station and sample data item information

with the exception of station identification number). Although the station identification

numbers were different, the latitude and longitude for the stations were the same indicating a

duplication of station data due to the existence of two station identification numbers for the

same station. For each set of matching records, one of the station identification numbers was

randomly selected and its associated data were eliminated from the data set. A total of 686

sample records were eliminated from the data set through this process. 

• Identification of matching records with the exception of collecting agency codes

All data records were sorted to identify those records that matched exactly except for their

collecting agency codes (i.e., they had the same water quality parameters, parameter results

and associated remark codes, and the same station and sample data item information with the

exception of agency code). The presence of two matching data records each with a different

agency code attached to it suggested that one agency had utilized data collected by the other

agency and had entered the data into STORET without realizing that it already had been

placed in STORET by the other agency. No matching records with greater than two different

agency codes were identified. For determining which record to delete from the data set, the

following rules were developed: 

< If one of the matching records had a USGS agency code, the USGS record was retained

and the other record was deleted. 

< Higher level agency monitoring program data were retained. For example, federal

program data (indicated by a “1" at the beginning of the STORET agency code) were

retained against state (indicated by a “2") and local (indicated by values higher than 2)

program data. 

< If two matching records had the same level agency code, the record from the agency with

the greater number of overall observations (potentially indicating the data set as the

source data set) was retained.

A total of 2,915 sample records were eliminated through this process. 

As a result of the duplicate data identification process, a total of 4,525 sample records and 36

individual station records were removed from the STORET nutrient data set. The resulting
9 Nutrient Ecoregion/Waterbody Type Summary Chapters, Contract # 68-C-99-226, TO# 07 August 27, 2001

Appendix C—Quality Control/Quality Assurance Rules C-21

nutrient data set contains the following:

41,174 station records

919,895 sample records
9 Nutrient Ecoregion/Waterbody Type Summary Chapters, Contract # 68-C-99-226, TO# 07 August 27, 2001

C-22 Appendix C—Quality Control/Quality Assurance Rules

APPENDIX B. Process for Adding Aggregate Nutrient Ecoregions 

and Level III Ecoregions

The flag_id tracks the type of changes that were made to the data. There are a total of eight flags

that are used to describe the changes made to the data. The flags are defined as follows:

1—The latitude and longitude coordinates match the county that was provided. If the HUC was

null, it was updated based on the latitude and longitude coordinates. The ecoregions were

determined by using the latitude and longitude coordinates.

2—The county and HUC are available, but the latitude and/or longitude coordinates are missing. 

Therefore, the centroid of the intersection of the county and HUC was used to determine the

ecoregions and the latitude and longitude coordinates. If the HUC and county did not intersect, 

the county centroid was used to determine the ecoregions and the latitude and longitude

coordinates.

3—The county is available, but the HUC and the latitude and/or longitude coordinates are

missing. Therefore, the county centroid was used to determine the ecoregions, HUC, and the

latitude and longitude coordinates.

4—The HUC is available, but the county is not and the latitude and/or longitude coordinates are

missing. Therefore, the HUC centroid was used to determine the ecoregions, county, and the

latitude and longitude coordinates.

5—The county is missing, but the latitude and longitude coordinates are available. Note: A

county is considered missing if it is invalid. In other words, if the county entered did not exist in

the state, it was considered null. Therefore, the latitude and longitude coordinates were used to

determine the ecoregions, county, and HUC (if it was missing).

6—The latitude and longitude coordinates did not match the county that was provided, but they

did match the HUC. Therefore, the county centroid was used to determine ecoregion values. 

7—The latitude and longitude coordinates did not match the county or the HUC that was

provided (including null HUCs). Therefore, the county centroid was used to determine

ecoregion values.

8—The latitude and longitude coordinates were missing, but the ecoregions were provided by

the state.

The ecoregions provided by the states were used as the ecoregion values.
9 Nutrient Ecoregion/Waterbody Type Summary Chapters, Contract # 68-C-99-226, TO# 07 August 27, 2001

Appendix C—Quality Control/Quality Assurance Rules C-23

APPENDIX C. Glossary

Coefficient of Variation - A measure of variability. The standard deviation divided by the mean

multiplied by 100. 

Maximum - The highest value.

Mean – A measure of central tendency. The arithmetic average.

Median – A measure of central tendency. The value which cuts the distribution in half, such that

half of the values are above the median, and half of the values are below the median. Also called

the 50th percentile or middle value.

Minimum - The lowest value.

Standard Deviation – A measure of variability. The square root of the variance with the variance

defined as the sum of the squared deviations divided by the sample size minus one.

Standard Error - A measure of variability. The standard deviation divided by the square root of

the sample size.

5th % - the 5th percentile

25th % - the 25th percentile, the first quartile.

75th % - the 75th percentile, the third quartile.

95th % - the 95th percentile