United States Office of Water EPA 822-B-01-016

Environmental Protection 4304 December 2001

Agency

Ambient Water Quality

Criteria Recommendations

Information Supporting the Development 

of State and Tribal Nutrient Criteria

Rivers and Streams in 

Nutrient Ecoregion X
EPA-822-B-01-016

AMBIENT WATER QUALITY CRITERIA RECOMMENDATIONS

INFORMATION SUPPORTING THE DEVELOPMENT OF STATE AND TRIBAL

NUTRIENT CRITERIA

FOR

RIVERS AND STREAMS IN NUTRIENT ECOREGION X

Texas-Louisiana Coastal and Mississippi Alluvial Plains

including all or parts of the States of:

Texas, Louisiana, Mississippi, Arkansas, Tennessee, Kentucky, Missouri, and Illinois,

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 Rivers and Streams in Nutrient

Ecoregion X. 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., turbidity 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 turbidity for rivers and streams in Nutrient Ecoregion X, which

were derived using the procedures described in the Rivers and Streams Nutrient Criteria

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

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 X. Data sets from Legacy

STORET, NASQAN, EPA Region 7 - Central Plains Center for BioAssessment (CPCB),

EPA Region 7 - CPCB 2, and EPA Region 7 - REMAP were used to assess nutrient

conditions from 1990 to 2000.

C Reference sites/reference conditions in Nutrient Ecoregion X. 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 rivers and streams 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: Texas,

Louisiana, Tennessee, Kentucky, and Illinois. Mississippi indicated that standard or EPAapproved methods were used for some specific parameters.

The following tables contain a summary of aggregate and level III ecoregion values for TN,

TP, water column chlorophyll a, and turbidity.

BASED ON 25th PERCENTILES ONLY

Nutrient Parameters Aggregate Nutrient Ecoregion X

Reference Conditions

Total phosphorus (μg/L) 128*

Total nitrogen (mg/L) (reported) 0.76

Chlorophyll a (μg/L) 

(spectrophotometric method)

2.10

Turbidity (FTU) 17.5

* 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.

For subecoregions 34 and 73, 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) 125-126*

Total nitrogen (mg/L) (reported) 0.71-0.86

Chlorophyll a (μg/L) (fluorometric method) N/A

Turbidity (FTU) 12.27-19

* 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-822-B-01-016.
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ACKNOWLEDGMENTS

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

reviewers: EPA Regions 4, 5, 6, and 7; the States of Texas, Louisiana, Mississippi, Arkansas,

Missouri, Tennessee, Kentucky, and Illinois; 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 acknowledges the external peer review efforts of Nina

Caraco, Institute of Ecosystem Studies; Amy Parker, University of Georgia; Jan Stevenson,

University of Michigan.
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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 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xii

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 X . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

3.2 Geographical Boundaries of Aggregate Ecoregion X . . . . . . . . . . . . . . . . . . . . . . . . 10

3.3 Level III Subecoregions Within Aggregate Ecoregion X . . . . . . . . . . . . . . . . . . . . . 10

3.4 Suggested Ecoregional Subdivisions or Adjustments . . . . . . . . . . . . . . . . . . . . . . . . 13

4.0 Data Review for Rivers and Streams in Aggregate Ecoregion X . . . . . . . . . . . . . . . . . . . . 13

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

4.2 Historical Data from Aggregate Ecoregion X (TP, TN, chl a, turbidity) . . . . . . . . . . 13

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

4.4 Data for All Rivers and Streams Within Aggregate Ecoregion X . . . . . . . . . . . . . . . 14

4.5 Statistical Analysis of Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

4.6 Classification of River/Stream Type . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

4.7 Summary of Data Reduction Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

5.0 Reference Sites and Conditions in Aggregate Ecoregion X . . . . . . . . . . . . . . . . . . . . . . . . 24

6.0 Models Used to Predict or Verify Response Parameters . . . . . . . . . . . . . . . . . . . . . . . . . . 24

7.0 Framework for Refining Recommended Nutrient Criteria for Rivers and Streams in 

Aggregate Ecoregion X . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

7.1 Example Worksheet for Developing Aggregate Ecoregion and 

Subecoregion Nutrient Criteria . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

7.2 Setting Seasonal Criteria . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

7.3 When Data/Reference Conditions Are Lacking . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

7.4 Site-Specific Criteria Development . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

8.0 Literature Cited . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

9.0 Appendices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

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 River and stream records for Aggregate Ecoregion X—Texas-Louisiana 

Coastal and Mississippi Alluvial Plains . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

Table 2 Reference conditions for Aggregate Ecoregion X streams . . . . . . . . . . . . . . . . . 17

Table 3a-b Reference conditions for Ecoregion X streams . . . . . . . . . . . . . . . . . . . . . . . . . 18

Table 4 Suggested boundaries for trophic classification of streams from cumulative

frequency distributions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

Table 5 Nutrient (:g/L) and algal biomass criteria limits recommended to prevent

nuisance conditions and water quality degradation in streams based either on

nutrient-chlorophyll a relationships or preventing risks to stream impairment as

indicated . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

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 X . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

Figure 3 Aggregate Ecoregion X with level III Ecoregions shown . . . . . . . . . . . . . . . . . 12

Figure 4 Sampling locations within each level III Ecoregion . . . . . . . . . . . . . . . . . . . . . . 15

Figure 5a Illustration of data reduction process for stream data . . . . . . . . . . . . . . . . . . . . . 22

Figure 5b Illustration of reference condition calculation . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
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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
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December 2001. Each manual presents EPA’s recommended approach for developing nutrient

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 some measure of turbidity). 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 some measure of turbidity).

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.
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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

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 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 large-scale

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 
4

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.
6

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

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, turbidity) variables represent a set of starting points for States

and Tribes to use in establishing their own criteria.
7

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

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.
8

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

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 X

Ecoregion X is nearly level and is composed of alluvial plains in the Mississippi River

Valley and coastal plains along the Gulf of Mexico. It is well known for producing large

amounts of cotton and rice and has concentrations of industrial and urban activity. The

Mississippi alluvial plain has nutrient-rich soils, a favorable climate, and is one of the most

important cash crop areas in the United States; soils in the more irregular Southeastern

Temperate Forested Plains and Hills (IX) are not as naturally fertile. The coastal plain is

dominated by range, pasture, marshy wildlife habitat, woodland, cropland, and both recreational

and urban development. Cotton, soybeans, rice, sorghum, corn, sugarcane, hay, and wheat are

grown in Ecoregion X. A higher percentage of the land is in cropland than in bordering nutrient

Ecoregions. Wet soils are common and must be artificially drained to be farmed. The wettest

areas that are not artificially drained remain in forests and wetlands and are important wildlife

habitat. Urban and industrial areas are found in the region and population is increasing. 

Urbanization, industrial activity, and agricultural runoff have affected the region’s water quality. 

The Mississippi River Valley portion of Ecoregion X extends from its junction with the Ohio

River to the Mississippi Delta. It is mostly a broad, flat floodplain that contains many oxbow

lakes and bayous; it is physiographically distinct from the neighboring Southeastern Temperate

Forested Plains and Hills (IX) and the Central and Eastern Forested Uplands (XI). River terraces

and levees provide the main elements of relief. Winters are mild, summers are hot, and both

temperature and precipitation increase from north to south. The mean annual temperature is very

warm, freeze-free growing season is long, and mean annual precipitation is plentiful and are,

respectively, 57-70°F, 200-340 days, and 45-65 inches. Soils are nutrient-rich and have a

thermic temperature regime and an aquic or udic moisture regime. The potential natural

vegetation of the Mississippi River Valley is mostly southern floodplain forest (dominants:

tupelo, oak, bald cypress) with some areas of oak-hickory forest mixed in; southern cordgrass

prairie is found on the outer Mississippi Delta. Most of the original bottomland deciduous forest

has been cleared and drained for cultivation and, today, the Valley is an important agricultural

area. Overall, about 55% of the Mississippi River Valley portion of Ecoregion X is used as
9

cropland, 35% is used as woodland, and 7% is used as pastureland. Cropland comprises about

75% of the north but, in the south, pasture, woodland, forest, marshland, and swamp are more

common. The Valley s favorable climate and its productive, nutrient-rich soils support soybean,

cotton, rice, sorghum, corn, hay, wheat, and, in Louisiana, sugarcane farming; land in cotton

increased by almost a million acres between 1987 and 1992, by far the largest such increase in

the United States. Most of the northern and central parts of the region receive heavy treatments

of insecticides, herbicides, and fertilizers and have a high potential for pesticide, phosphate

fertilizer, and nitrogen fertilizer runoff. Soils are poorly-drained and must be artificially drained

to be farmed. Controlling surface water is a major concern of management. Agricultural runoff

containing fertilizers, herbicides, pesticides, and livestock waste and channelization have

affected surficial water quality. Total phosphorus, total ammonia, and nitrite plus nitrate

concentrations are often high in rivers, streams, and ditches due to extensive fertilizer use and

discharge from municipal sewage treatment plants (U.S. Geological Survey, 1993). Turbidity is

a problem in channelized areas. Pesticide residues are found in the surface waters (USGS,

1993).

The coastal plain portion of Ecoregion X extends westward from the Mississippi River

Delta to Brownsville, Texas along the Gulf of Mexico. It is comprised of nearly level coastal

lowlands with barrier islands, bays, marshes, and some low dunes in sandy areas. Elevations

typically range from sea level to 150 feet but, in some areas, they reach 325 feet. Lowest parts

of the area are marshy and are covered by high tides and storm tides. Farther inland, are the

higher and more irregular plains of Ecoregions IV, V, and IX. Winters are mild, summers are

hot, and precipitation increases from west to east. The mean annual temperature is very warm,

freeze-free growing season is long, and mean annual precipitation is plentiful and are,

respectively, 66-73°F, 250-350 days, and 25-60 inches. The potential natural vegetation is sea

oats prairie on barrier islands , southern cordgrass prairie in the near-shore zone, and seacoast

bluestem-coastal sacahuista prairie in higher areas away from the coast to an elevation of about

150 feet; farther inland in eastern Texas and western Louisiana, pine-hardwood forest is mapped

to about 325 feet elevation sweetgum, blackgum, post oak, blackjack oak, and southern red oak]

and both mesquite-live oak savanna and mesquite-acacia savanna occur in drier areas of southern

Texas. Today, near shore areas are mostly native rangeland and wildlife habitat; in addition,

there is expanding urban and recreation development especially along the barrier beaches but

freshwater is limited and often must be piped in. Farther inland, land-use changes. On the

former seacoast bluestem-coastal sacahuista prairie, about 40% of the acreage is now cropland

producing rice, soybeans, grain sorghum, cotton, corn, and hay, 30% is range or pasture, and

hardwood forest borders the rivers and streams; locally, urban development is rapidly expanding

onto agricultural land. Most soils are artificially drained for general farm crops and rice is

irrigated from streams and wells. In eastern Texas and western Louisiana, about 75% remains in

pine-hardwood forest and produces lumber and pulpwood; the remainder is mostly used as

pasture or as sites for small subdivisions. Present-day land use significantly impacts surface and

subsurface water quality. Elevated concentrations of nitrate and phosphorus are significant

water quality problems in many basins and can be a byproduct of agricultural runoff. Dissolved

oxygen depletion has occurred in some sluggish, warm rivers affected by nutrient-laden

agricultural runoff. Sulfate concentrations are locally high where streams are impacted by

industrial activity but are usually lower than in nutrient regions underlain by gypsum or salt
10

beds. Urban runoff, municipal wastewater effluent, and livestock impact the level of nutrient

and fecal coliform bacteria in streams and rivers. Industrial activity has released contaminants

including heavy metals to surface waters. In addition, groundwater overdraft has caused saline

intrusion into fresh water aquifers.

3.2 Geographical Boundaries of Aggregate Ecoregion X

The northern boundary of Ecoregion X begins along the borders of Illinois, Missouri, and

Kentucky at the junction of the Ohio River and Mississippi River (Figure 2). The region

continues south along the Mississippi River Valley, encompassing portions of Missouri,

Tennessee, Arkansas, Mississippi, and Louisiana.

At the junction of the Mississippi River and Red River, the region continues northwest up

the Red River through Louisiana and into Arkansas.

The coastal plain portion of Ecoregion X extends westward from the Mississippi River

Delta along the Gulf of Mexico through Louisiana and down to the southern tip of Texas.

3.3 Level III Subecoregions Within Aggregate Ecoregion X

There are two level III subecoregions contained within Aggregate Ecoregion X (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.

34. Western Gulf Coastal Plain

The principal distinguishing characteristics of the Western Gulf Coastal Plain are its

relatively flat coastal plain topography and mainly grassland potential natural vegetation.

Inland from this region the plains are more irregular and have mostly forest or savanna-type

vegetation potentials. Largely because of these characteristics, a higher percentage of the land is

in cropland than in bordering ecological regions. Recent urbanization and industrialization have

become concerns in this region.

73. Mississippi Alluvial Plain

This riverine ecoregion extends from southern Illinois, at the confluence of the Ohio River with

the Mississippi River, south to the Gulf of Mexico. It is mostly a flat, broad floodplain with

river terraces and levees providing the main elements of relief. Soils tend to be poorly drained,

except for the areas of sandy soils. Winters are mild and summers are hot, with temperatures and

precipitation increasing from north to south. Bottomland deciduous forest vegetation covered

the region before much of it was cleared for cultivation. Presently, most of the northern and

central parts of the region are in cropland and receive heavy treatments of insecticides and

herbicides. Soybeans, cotton, and rice are the major crops.
11

Figure 2. Aggregate Ecoregion X.
12

Figure 3. Aggregate Ecoregion X with level III Ecoregions shown.
13

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 RIVERS AND STREAMS IN AGGREGATE ECOREGION X

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

and nitrite/nitrate) and the primary response variables (some measure of turbidity 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 3 of the Rivers and Streams

Nutrient Criteria Technical Guidance Manual [U.S. EPA, 2000b] for a complete discussion on

choosing causal and response variables). 

4.1 Data Sources

Data sets from Legacy STORET, NASQAN, EPA Region 7 - Central Plains Center for

BioAssessment (CPCB), EPA Region 7 - CPCB 2, and EPA Region 7 - REMAP 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 X (TP, TN, chl a, and turbidity)

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 X 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: Texas, Louisiana, Tennessee,
14

Kentucky, and Illinois. Mississippi indicated that standard or EPA-approved methods were used

for some specific nutrient parameters. Arkansas and Missouri did not provide information prior

to the publication of this document. 

4.4 Data for All Rivers and Streams Within Aggregate Ecoregion X 

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

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

the aggregate ecoregion.

4.5 Statistical Analysis of Data

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

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

upper 25th percentile (75th percentile) of a reference population of streams. 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 streams are not identified, the second method is to determine the lower 25th

percentile of the population of all streams 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 Matthew 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-b present potential reference conditions for both the aggregate ecoregion

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

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

information on reference streams. Tables 3a-b present potential reference conditions for rivers

and streams in the level III subecoregions within the aggregate ecoregion. Note that the

footnotes for Table 2 apply to Tables 3a-b. Appendixes A and B provides a complete

presentation of all descriptive statistics for both the aggregate ecoregion and the level III

subecoregions.

Tables 4 and 5 are presented for comparison purposes. They allow the reader to determine

where, in the trophic state, the recommended reference conditions fall within traditionally

viewed trophic boundaries.
15

Figure 4. Sampling locations within each level III Ecoregion.
16

Table 1. River and stream records* for Aggregate Ecoregion X—Texas-Louisiana Coastal

and Mississippi Alluvial Plains

Aggregate

Ecoregion X

Sub ecoR

34

Sub ecoR

73

# of named streams 321 73 248

# of stream stations 624 128 496

Key nutrient parameters (listed below)

- # of records for turbidity (all methods) 10,370 1,535 8,835

- # of records for chlorophyll a (all methods) 1,382 731 638

- # of records for total Kjeldhal nitrogen 

(TKN) 10,335 2,545 7,790

- # of records for nitrite + nitrate (NO2+NO3) 12,399 1,855 10,544

- # of records for total nitrogen (TN) 834 202 632

- # of records for total phosphorus (TP) 14,419 2,877 11,542

Total # of records for 

key nutrient parameters 49,739 9,745 39,981

*The number of rivers and streams presented in this table is based on the number of rivers and streams for which nutrient data

were provided in the National Nutrient database. This does not imply that this is the total of rivers and streams within the

Ecoregion. States and Tribes should determine the representativeness of the tabular data by comparing this information with any

additional material they may have.

Definitions: (1) # of records refers to the total count of observations for that parameter over the entire decade (1990-1999) for

that particular aggregate or subecoregion. These are counts for all seasons over that decade. (2) # of stream stations refers to

the total number of river and stream stations within the aggregate or subecoregion from which nutrient data was collected. Since

streams and rivers can cross ecoregional boundaries, it is important to note that only those portions of a river or stream (and data

associated with those stations) that exist within the ecoregion are included within this table.
17

Table 2. Reference conditions for Aggregate Ecoregion X streams

Parameter

No. of

streams

N*

Reported values 25th percentiles based on all

seasons data for the decade

Reference

streams‡

Min Max P25 all seasons† P75 all seasons

TKN (mg/L) 220 0.03 2.95 0.71

NO2+NO3-N (mg/L) 253 0.00 6.77 0.14

TN (mg/L) - calculated 0.85

TN (mg/L) - reported 36 0.16 5.01 0.76

TP (μg/L) 295 4 1,610 128

Turbidity (NTU) 170 2.50 140.50 19.50

Turbidity (FTU) 84 2.84 145 17.50

Turbidity (JCU) 3 10.75 11 10.75 (zz)

Chlorophyll a (μg/L) - F 6 1.1 1.4 1.1

Chlorophyll a (μg/L) - S 36 0.2 35.7 2.1

Chlorophyll a (μg/L) - T 15 1.3 40.5 6.4

Periphyton Chl a (mg/m2

) — — ——

* N = largest value reported for a decade/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.

† 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 Streams refers to the largest number of streams and rivers 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 stream for the decade were reduced to one median for that

stream. 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

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

retain all information for all seasons. In calculating the 25th percentile for a season with fewer than 4 stream medians, the

statistical program automatically used the minimum value within the fewer-than-4 population. If fewer than 4 streams 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 X streams

subecoregion 34

Parameter

No. of

streams

N*

Reported values 25th percentiles based on all

seasons data for the decade

Reference

streams‡

Min Max P25 all seasons† P75 all seasons

TKN (mg/L) 64 0.26 2.05 0.74

NO2+NO3-N (mg/L) 51 0.01 6.77 0.14

TN (mg/L) - calculated 0.88

TN (mg/L) - reported 9 0.50 5.01 0.86

TP (μg/L) 67 16 1,455 126

Turbidity (NTU) 16 10.25 91.88 37.75

Turbidity (FTU) 16 4.49 99.25 12.27

Turbidity (JCU) 3 (z) 7.80 10 7.80 (zz)

Chlorophyll a (μg/L) - F — — — —

Chlorophyll a (μg/L) - S 36 0.2 35.7 2.1

Chlorophyll a (μg/L) - T — — — —

Periphyton Chl a (mg/m2) — — ——
19

Table 3b. Reference conditions for Ecoregion X streams

subecoregion 73

Parameter

No. of

streams

N*

Reported values 25th percentiles based on all

seasons data for the decade

Reference

streams‡

Min Max P25 all seasons† P75 all seasons

TKN (mg/L) 158 0.06 2.95 0.69

NO2+NO3-N (mg/L) 203 — 2.90 0.13

TN (mg/L) - calculated 0.82

TN (mg/L) - reported 27 0.16 2.68 0.71

TP (μg/L) 228 5 1,325 125

Turbidity (NTU) 154 2.50 140.50 18.69

Turbidity (FTU) 68 3 145 19

Turbidity (JCU) 1 (z) 12 12 12

Chlorophyll a (μg/L) - F 6 1.1 1.4 1.1

Chlorophyll a (μg/L) - S — — — —

Chlorophyll a (μg/L) - T 15 1.3 40.5 6.4

Periphyton Chl a (mg/m2) — — ——

* N = largest value reported for a decade/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.

† 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 Streams refers to the number of streams and rivers 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 stream for the decade were reduced to one median

for that stream. 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

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

retain all information for all seasons. In calculating the 25th percentile for a season with fewer than 4 stream medians, the

statistical program automatically used the minimum value within the fewer-than-4 population. If fewer than 4 streams 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.”
20

Table 4. Suggested boundaries for trophic classification of streams from cumulative

frequency distributions. The boundary between oligotrophic and mesotrophic systems

represents the lowest third of the distribution and the boundary between mesotrophic and

eutrophic marks the top third of the distribution.

Variable (units)

Oligotrophicmesotrophic 

boundary

Mesotrophic-eutrophic 

boundary Sample size

(N)

mean benthic chlorophyll (mg m-2)

a 20 70 286

maximum benthic chlorophyll (mg m-2)

a 60 200 176

sestonic chlorophyll 

(:g L-1)

b 10 30 292

TN (:g L-1)

a,c 700 1,500 1,070

TP (:g L-1)

a,b,c 25 75 1,366

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

and traditional nutrient and biological endpoints.

a

Data from Dodds et al. (1998); bdata from Van Nieuwenhuyse and Jones (1996); cdata from Omernik (1977).

Table 5. Nutrient (μg/L) and algal biomass criteria limits recommended to prevent

nuisance conditions and water quality degradation in streams based either on nutrientchlorophyll a relationships or preventing risks to stream impairment as indicated.

Periphyton Maximum in mg/m2

TN TP DIN SRP Chlorophyll a Impairment Risk Source

100-200 nuisance growth Welch et al. 1988, 1989

275-650 38-90 100-200 nuisance growth Dodds et al. 1997

1,500 75 200 eutrophy Dodds et al. 1998

300 20 150 nuisance growth Clark Fork River Tri-State

Council, MT

 20 Cladophora

nuisance growth

Chetelat et al. 1999

 10-20 Cladophora

nuisance growth

Stevenson unpubl. data

 430 60 eutrophy UK Environ. Agency 1988

 100a 10a 200 nuisance growth Biggs 2000

 25 3 100 reduced invertebrate

diversity

Nordin 1985

15 100 nuisance growth Quinn 1991

1,000 10b ~100 eutrophy Sosiak pers. comm.

Plankton Mean in μg/L

TN TP DIN SRP Chlorophyll a Impairment Risk Source

 300c 42 8 eutrophy Van Nieuwenhuyse and

Jones 1996

70 15 chlorophyll action level OAR 2000

 250c 35 8 eutrophy OECD 1992 (for lakes) a

30-day biomass accrual time.

b

Total dissolved P.

c

Based on Redfield ratio of 7.2N:1P (Smith et al. 1997).
21

4.6 Classification of River/Stream Type

Assessing the data by stream 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 streams before developing a

final criterion.

4.7 Summary of Data Reduction Methods

All descriptive statistics were calculated using the medians for each stream within

Ecoregion X for which data existed. For example, if one stream had 300 observations for

phosphorus over the decade or 1 year’s time, one median resulted. Each median from each

stream 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 
22

Figure 5a. Illustration of data reduction process for stream data.
23

Figure 5b. Illustration of reference condition calculation.
24

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).

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 X

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 streams. 

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-b in Section 4.0.

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

X. 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
25

• 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.

7.0 FRAMEWORK FOR REFINING RECOMMENDED NUTRIENT CRITERIA FOR

RIVERS AND STREAMS IN AGGREGATE ECOREGION X

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-b 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: 

 

 

 

 

 
26

Models: 

 

 

 

 

 

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.
27

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.

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 Rivers and Streams Nutrient Criteria

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

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

• Refinements to ecoregions (Section 2.3). 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 2)

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

significant or cyclical precipitation events (high-flow/low-flow conditions) (Chapter 4)

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.
28

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 1990-1994. Water Supply

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

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: X  Rivers and Streams Descriptive Statistics by Decade and Season from 1999 to 2000 Chloro_A_Fluor_cor_ug_L season N MEAN MIN MAX STDDEV STDERR CV P5 P25 MEDIAN P75  FALL 4 1.25 1.1000 1.40 0.17 0.09 14 1.10 1.10 1.25 1.40  SPRING 3 5.85 3.1000 7.57 2.41 1.39 41 3.10 3.10 6.88 7.57  SUMMER 6 0.93 .70000 1.10 0.19 0.08 20 0.70 0.70 1.00 1.10  Data were not always available for all years.
Appendix A—Descriptive Statistics Data Tables for Aggregate Ecoregion 

 Aggregate Nutrient Ecoregion: X  Rivers and Streams Descriptive Statistics by Decade and Season from 1999 to 2000 Chloro_A_Pheo_cor_ug_L season N MEAN MIN MAX STDDEV STDERR CV P5 P25 MEDIAN P75  FALL 2 0.90 .60000 1.20 0.42 0.30 47 0.60 0.60 0.90 1.20  SPRING 3 2.85 2.0200 3.85 0.93 0.53 32 2.02 2.02 2.69 3.85  SUMMER 3 1.10 .90000 1.40 0.26 0.15 24 0.90 0.90 1.00 1.40  Data were not always available for all years.
Appendix A—Descriptive Statistics Data Tables for Aggregate Ecoregion 

 Aggregate Nutrient Ecoregion: X  Rivers and Streams Descriptive Statistics by Decade and Season from 1990 to 1995 Chloro_A_Phyto_Chro_flu_ug_L season N MEAN MIN MAX STDDEV STDERR CV P5 P25 MEDIAN P75  FALL 2 2.70 2.2000 3.20 0.71 0.50 26 2.20 2.20 2.70 3.20  SPRING 2 1.64 1.4500 1.83 0.27 0.19 16 1.45 1.45 1.64 1.83  SUMMER 2 2.44 1.7250 3.15 1.01 0.71 41 1.73 1.73 2.44 3.15  WINTER 2 1.28 1.0625 1.50 0.31 0.22 24 1.06 1.06 1.28 1.50  Data were not always available for all years.
Appendix A—Descriptive Statistics Data Tables for Aggregate Ecoregion 

 Aggregate Nutrient Ecoregion: X  Rivers and Streams Descriptive Statistics by Decade and Season from 1990 to 1996 Chloro_A_Phyto_Spec_A_ug_L season N MEAN MIN MAX STDDEV STDERR CV P5 P25 MEDIAN P75  FALL 31 5.63 .25000 17.45 4.65 0.84 83 0.25 2.20 4.60 8.41  SPRING 33 8.64 .25000 66.90 13.01 2.27 151 0.25 2.00 4.79 8.60  SUMMER 36 10.06 .25000 33.00 9.01 1.50 90 1.90 3.46 7.41 14.78  WINTER 30 5.13 .25000 38.40 9.14 1.67 178 0.25 0.25 1.49 2.80  Data were not always available for all years.
Appendix A—Descriptive Statistics Data Tables for Aggregate Ecoregion 

 Aggregate Nutrient Ecoregion: X  Rivers and Streams Descriptive Statistics by Decade and Season from 1990 to 1993 Chloro_A_Trich_unco_ug_L season N MEAN MIN MAX STDDEV STDERR CV P5 P25 MEDIAN P75  FALL 10 26.05 4.9500 68.90 24.65 7.80 95 4.95 8.00 13.48 55.40  SPRING 13 11.02 1.3000 27.50 8.56 2.38 78 1.30 4.71 7.05 15.90  SUMMER 15 17.18 1.3000 46.70 12.75 3.29 74 1.30 8.70 14.27 26.10  WINTER 11 12.82 1.1000 34.23 12.97 3.91 101 1.10 1.85 6.73 25.93  Data were not always available for all years.
Appendix A—Descriptive Statistics Data Tables for Aggregate Ecoregion 

 Aggregate Nutrient Ecoregion: X  Rivers and Streams Descriptive Statistics by Decade and Season from 1990 to 1995 Chloro_B_Phyto_Chro_flu_ug_L season N MEAN MIN MAX STDDEV STDERR CV P5 P25 MEDIAN P75  FALL 2 0.15 .10000 0.20 0.07 0.05 47 0.10 0.10 0.15 0.20  SPRING 2 0.07 .05000 0.09 0.03 0.02 39 0.05 0.05 0.07 0.09  SUMMER 2 0.14 .10000 0.18 0.05 0.04 39 0.10 0.10 0.14 0.18  WINTER 2 0.06 .05000 0.08 0.02 0.01 28 0.05 0.05 0.06 0.08  Data were not always available for all years.
Appendix A—Descriptive Statistics Data Tables for Aggregate Ecoregion 

 Aggregate Nutrient Ecoregion: X  Rivers and Streams Descriptive Statistics by Decade and Season from 1990 to 1999 DIP_ug_L season N MEAN MIN MAX STDDEV STDERR CV P5 P25 MEDIAN P75  FALL 30 119.70 5.0000 1350.00 240.11 43.84 201 12.50 40.00 65.00 95.00 2 SPRING 28 78.62 10.000 420.00 90.74 17.15 115 18.75 36.25 47.50 73.75 3 SUMMER 32 108.91 7.5000 1000.00 171.69 30.35 158 12.50 47.50 78.75 98.75 2 WINTER 28 93.26 12.500 650.00 139.20 26.31 149 16.25 42.50 52.50 72.50 4 Data were not always available for all years.
Appendix A—Descriptive Statistics Data Tables for Aggregate Ecoregion 

 Aggregate Nutrient Ecoregion: X  Rivers and Streams 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 248 6.43 1.1000 14.10 2.04 0.13 32 3.05 5.25 6.60 7.70  SPRING 265 6.93 1.3000 13.28 1.84 0.11 27 3.60 5.85 7.00 8.30  SUMMER 267 5.52 1.0000 12.80 1.91 0.12 35 2.30 4.20 5.78 6.80  WINTER 256 8.57 2.5000 12.80 1.96 0.12 23 4.70 7.45 8.60 9.98  Data were not always available for all years.
Appendix A—Descriptive Statistics Data Tables for Aggregate Ecoregion 

 Aggregate Nutrient Ecoregion: X  Rivers and Streams 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 234 0.37 .00000 7.44 0.83 0.05 227 0.01 0.05 0.12 0.29  SPRING 243 0.54 .00000 6.83 0.84 0.05 154 0.04 0.16 0.26 0.57  SUMMER 253 0.49 .00000 5.90 0.74 0.05 152 0.02 0.13 0.24 0.47  WINTER 241 0.53 .00250 6.72 0.89 0.06 167 0.05 0.15 0.25 0.50  Data were not always available for all years.
Appendix A—Descriptive Statistics Data Tables for Aggregate Ecoregion 

 Aggregate Nutrient Ecoregion: X  Rivers and Streams Descriptive Statistics by Decade and Season from 1990 to 1998 Nitrogen_Tot_Kjeldhal_mg_L season N MEAN MIN MAX STDDEV STDERR CV P5 P25 MEDIAN P75  FALL 216 0.85 .01000 3.24 0.41 0.03 48 0.37 0.60 0.76 1.06  SPRING 215 1.05 .30000 4.40 0.51 0.03 48 0.47 0.71 0.93 1.27  SUMMER 220 0.98 .05000 2.65 0.40 0.03 41 0.43 0.72 0.95 1.24  WINTER 215 0.95 .01000 2.48 0.39 0.03 41 0.44 0.70 0.90 1.15  Data were not always available for all years.
Appendix A—Descriptive Statistics Data Tables for Aggregate Ecoregion 

 Aggregate Nutrient Ecoregion: X  Rivers and Streams Descriptive Statistics by Decade and Season from 1999 to 2000 Organic_P_ug_L season N MEAN MIN MAX STDDEV STDERR CV P5 P25 MEDIAN P75  FALL 4 30.51 11.920 49.09 21.46 10.73 70 11.92 11.92 30.51 49.09 4 SPRING 3 197.51 38.300 383.16 173.94 100.43 88 38.30 38.30 171.08 383.16 38 SUMMER 6 45.32 25.480 61.37 16.32 6.66 36 25.48 25.48 49.12 61.37 6 Data were not always available for all years.
Appendix A—Descriptive Statistics Data Tables for Aggregate Ecoregion 

 Aggregate Nutrient Ecoregion: X  Rivers and Streams Descriptive Statistics by Decade and Season from 1990 to 1995 Phosph_Ortho_Tot_as_P_ug_L season N MEAN MIN MAX STDDEV STDERR CV P5 P25 MEDIAN P75  FALL 29 105.83 5.0000 590.00 116.63 21.66 110 10.00 54.00 80.00 105.00 3 SPRING 29 106.55 10.000 457.50 81.86 15.20 77 16.25 65.00 95.00 125.00 1 SUMMER 29 149.48 15.000 1475.00 263.11 48.86 176 30.00 80.00 90.00 130.00 3 WINTER 29 147.86 27.500 1115.00 210.91 39.17 143 30.00 70.00 100.00 115.00 5 Data were not always available for all years.
Appendix A—Descriptive Statistics Data Tables for Aggregate Ecoregion 

 Aggregate Nutrient Ecoregion: X  Rivers and Streams Descriptive Statistics by Decade and Season from 1990 to 2000 Total_Nitrogen_mg_L season N MEAN MIN MAX STDDEV STDERR CV P5 P25 MEDIAN P75  FALL 33 1.02 .03000 4.80 0.89 0.15 87 0.03 0.60 0.76 1.20  SPRING 32 1.46 .10000 5.23 0.97 0.17 66 0.50 0.87 1.19 1.86  SUMMER 36 1.28 .21000 4.20 0.99 0.17 77 0.21 0.66 0.95 1.75  WINTER 29 1.58 .53000 6.60 1.33 0.25 84 0.58 0.90 1.20 1.83  Data were not always available for all years.
Appendix A—Descriptive Statistics Data Tables for Aggregate Ecoregion 

 Aggregate Nutrient Ecoregion: X  Rivers and Streams 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 264 207.75 5.0000 1650.00 208.76 12.85 100 50.00 110.00 155.00 220.00 6 SPRING 281 273.99 2.5000 2100.00 239.34 14.28 87 50.00 135.00 210.00 340.00 6 SUMMER 295 233.58 2.5000 1400.00 217.01 12.63 93 50.00 120.00 185.00 242.50 6 WINTER 267 260.47 5.0000 1570.00 216.57 13.25 83 65.00 135.00 200.00 310.00 6 Data were not always available for all years.
Appendix A—Descriptive Statistics Data Tables for Aggregate Ecoregion 

 Aggregate Nutrient Ecoregion: X  Rivers and Streams Descriptive Statistics by Decade and Season from 1990 to 1997 Turbidity_FTU season N MEAN MIN MAX STDDEV STDERR CV P5 P25 MEDIAN P75  FALL 58 27.93 3.9000 83.75 19.42 2.55 70 4.10 13.00 25.00 38.00  SPRING 80 54.04 2.7750 150.00 36.39 4.07 67 7.10 21.00 52.50 70.75 1 SUMMER 69 36.75 2.7000 140.00 30.57 3.68 83 6.20 15.00 28.50 51.75 1 WINTER 84 59.72 2.9000 190.00 41.57 4.54 70 4.30 20.00 57.50 88.38 1 Data were not always available for all years.
Appendix A—Descriptive Statistics Data Tables for Aggregate Ecoregion 

 Aggregate Nutrient Ecoregion: X  Rivers and Streams Descriptive Statistics by Decade and Season from 1993 to 1997 Turbidity_JCU season N MEAN MIN MAX STDDEV STDERR CV P5 P25 MEDIAN P75  FALL 1 18.00 18.000 18.00 . . . 18.00 18.00 18.00 18.00 1 SPRING 1 12.00 12.000 12.00 . . . 12.00 12.00 12.00 12.00 1 SUMMER 1 9.50 9.5000 9.50 . . . 9.50 9.50 9.50 9.50  WINTER 3 9.27 7.8000 10.00 1.27 0.73 14 7.80 7.80 10.00 10.00 1 Data were not always available for all years.
Appendix A—Descriptive Statistics Data Tables for Aggregate Ecoregion 

 Aggregate Nutrient Ecoregion: X  Rivers and Streams Descriptive Statistics by Decade and Season from 1990 to 2000 Turbidity_NTU season N MEAN MIN MAX STDDEV STDERR CV P5 P25 MEDIAN P75  FALL 161 22.09 .00000 83.75 14.76 1.16 67 4.20 10.90 20.00 30.00  SPRING 165 57.40 2.8000 435.00 52.98 4.12 92 12.75 23.00 38.00 75.00 1 SUMMER 170 32.90 3.2500 111.00 21.47 1.65 65 7.00 16.00 28.00 44.50  WINTER 162 54.01 2.2000 170.00 36.60 2.88 68 11.60 24.50 47.00 75.00 1 Data were not always available for all years.
Appendix A—Descriptive Statistics Data Tables for Aggregate Ecoregion 

 Aggregate Nutrient Ecoregion: X  Rivers and Streams Descriptive Statistics by Decade and Season from 1999 to 2000 pH_S_U season N MEAN MIN MAX STDDEV STDERR CV P5 P25 MEDIAN P75  FALL 4 8.50 7.8400 9.15 0.76 0.38 9 7.84 7.84 8.50 9.15  SPRING 3 8.32 7.7100 9.06 0.69 0.40 8 7.71 7.71 8.18 9.06  SUMMER 6 8.13 7.8900 8.43 0.20 0.08 2 7.89 8.02 8.07 8.32  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: X  Rivers and Streams Descriptive Statistics by Subecoregion, Decade and Season from 1999 to 2000 Chloro_A_Fluor_cor_ug_L subecoregion season N MEAN MIN MAX STDDEV STDERR CV P5 P25 MEDIAN P75 P 73 FALL 4 1.25 1.1000 1.40 0.17 0.09 14 1.10 1.10 1.25 1.40 1. 73 SPRING 3 5.85 3.1000 7.57 2.41 1.39 41 3.10 3.10 6.88 7.57 7. 73 SUMMER 6 0.93 .70000 1.10 0.19 0.08 20 0.70 0.70 1.00 1.10 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: X  Rivers and Streams Descriptive Statistics by Subecoregion, Decade and Season from 1999 to 2000 Chloro_A_Pheo_cor_ug_L subecoregion season N MEAN MIN MAX STDDEV STDERR CV P5 P25 MEDIAN P75 P 73 FALL 2 0.90 .60000 1.20 0.42 0.30 47 0.60 0.60 0.90 1.20 1. 73 SPRING 3 2.85 2.0200 3.85 0.93 0.53 32 2.02 2.02 2.69 3.85 3. 73 SUMMER 3 1.10 .90000 1.40 0.26 0.15 24 0.90 0.90 1.00 1.40 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: X  Rivers and Streams Descriptive Statistics by Subecoregion, Decade and Season from 1990 to 1995 Chloro_A_Phyto_Chro_flu_ug_L subecoregion season N MEAN MIN MAX STDDEV STDERR CV P5 P25 MEDIAN P75 P 73 FALL 2 2.70 2.2000 3.20 0.71 0.50 26 2.20 2.20 2.70 3.20 3. 73 SPRING 2 1.64 1.4500 1.83 0.27 0.19 16 1.45 1.45 1.64 1.83 1. 73 SUMMER 2 2.44 1.7250 3.15 1.01 0.71 41 1.73 1.73 2.44 3.15 3. 73 WINTER 2 1.28 1.0625 1.50 0.31 0.22 24 1.06 1.06 1.28 1.50 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: X  Rivers and Streams Descriptive Statistics by Subecoregion, Decade and Season from 1990 to 1996 Chloro_A_Phyto_Spec_A_ug_L subecoregion season N MEAN MIN MAX STDDEV STDERR CV P5 P25 MEDIAN P75 P 34 FALL 31 5.63 .25000 17.45 4.65 0.84 83 0.25 2.20 4.60 8.41 15 34 SPRING 33 8.64 .25000 66.90 13.0 2.27 151 0.25 2.00 4.79 8.60 35 34 SUMMER 36 10.06 .25000 33.00 9.01 1.50 90 1.90 3.46 7.41 14.8 32 34 WINTER 30 5.13 .25000 38.40 9.14 1.67 178 0.25 0.25 1.49 2.80 26 Data were not always available for all years.
Appendix B—Descriptive Statistics Data Tables for Level III Subecoregions Within Aggregate Ec

 Aggregate Nutrient Ecoregion: X  Rivers and Streams Descriptive Statistics by Subecoregion, Decade and Season from 1990 to 1993 Chloro_A_Trich_unco_ug_L subecoregion season N MEAN MIN MAX STDDEV STDERR CV P5 P25 MEDIAN P75 P 73 FALL 10 26.05 4.9500 68.90 24.7 7.80 95 4.95 8.00 13.5 55.4 68 73 SPRING 13 11.02 1.3000 27.50 8.56 2.38 78 1.30 4.71 7.05 15.9 27 73 SUMMER 15 17.18 1.3000 46.70 12.7 3.29 74 1.30 8.70 14.3 26.1 46 73 WINTER 11 12.82 1.1000 34.23 13.0 3.91 101 1.10 1.85 6.73 25.9 34 Data were not always available for all years.
Appendix B—Descriptive Statistics Data Tables for Level III Subecoregions Within Aggregate Ec

 Aggregate Nutrient Ecoregion: X  Rivers and Streams Descriptive Statistics by Subecoregion, Decade and Season from 1990 to 1995 Chloro_B_Phyto_Chro_flu_ug_L subecoregion season N MEAN MIN MAX STDDEV STDERR CV P5 P25 MEDIAN P75 P 73 FALL 2 0.15 .10000 0.20 0.07 0.05 47 0.10 0.10 0.15 0.20 0. 73 SPRING 2 0.07 .05000 0.09 0.03 0.02 39 0.05 0.05 0.07 0.09 0. 73 SUMMER 2 0.14 .10000 0.18 0.05 0.04 39 0.10 0.10 0.14 0.18 0. 73 WINTER 2 0.06 .05000 0.08 0.02 0.01 28 0.05 0.05 0.06 0.08 0. Data were not always available for all years.
Appendix B—Descriptive Statistics Data Tables for Level III Subecoregions Within Aggregate Ec

 Aggregate Nutrient Ecoregion: X  Rivers and Streams Descriptive Statistics by Subecoregion, Decade and Season from 1990 to 1999 DIP_ug_L subecoregion season N MEAN MIN MAX STDDEV STDERR CV P5 P25 MEDIAN P75 P 34 FALL 9 236.67 5.0000 1350.00 427 142 180 5.00 40.0 90.0 175 13 34 SPRING 9 143.75 18.750 420.00 140 46.6 97 18.8 50.0 105 168 4 34 SUMMER 9 181.25 12.500 1000.00 316 105 174 12.5 26.3 67.5 125 10 34 WINTER 9 180.83 12.500 650.00 227 75.7 126 12.5 45.0 85.0 160 6 73 FALL 21 69.58 12.500 175.07 42.8 9.34 62 25.0 40.0 60.0 80.0 1 73 SPRING 19 47.76 10.000 115.00 23.4 5.37 49 10.0 35.0 45.0 57.5 1 73 SUMMER 23 80.61 7.5000 247.02 47.9 9.98 59 15.0 52.5 80.0 95.0 1 73 WINTER 19 51.78 16.250 120.00 21.7 4.97 42 16.3 40.0 50.0 57.5 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: X  Rivers and Streams 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  34 FALL 56 6.39 1.9000 10.60 1.96 0.26 31 2.85 4.78 6.83 7.83 9 34 SPRING 61 6.83 1.3000 11.70 1.85 0.24 27 3.90 5.85 7.10 7.95 9 34 SUMMER 60 5.89 1.7750 11.60 1.88 0.24 32 2.74 4.75 6.00 7.14 8 34 WINTER 60 8.25 2.7000 12.80 1.74 0.23 21 5.46 7.20 8.40 9.40 1 73 FALL 192 6.45 1.1000 14.10 2.07 0.15 32 3.05 5.29 6.60 7.63 9 73 SPRING 204 6.96 1.4500 13.28 1.84 0.13 26 3.60 5.86 7.00 8.30 9 73 SUMMER 207 5.42 1.0000 12.80 1.91 0.13 35 2.15 4.20 5.70 6.70 8 73 WINTER 196 8.67 2.5000 12.60 2.02 0.14 23 4.55 7.53 8.80 10.2 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: X  Rivers and Streams 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  34 FALL 47 0.84 .01000 7.44 1.63 0.24 194 0.02 0.06 0.20 0.57 4 34 SPRING 51 1.10 .01500 6.83 1.48 0.21 135 0.05 0.22 0.40 1.32 4 34 SUMMER 50 0.87 .00250 5.90 1.31 0.19 150 0.01 0.14 0.37 0.71 4 34 WINTER 51 1.02 .03000 6.72 1.62 0.23 158 0.06 0.15 0.28 0.75 4 73 FALL 187 0.25 .00000 3.11 0.38 0.03 152 0.01 0.05 0.11 0.26 1 73 SPRING 192 0.40 .00000 2.69 0.46 0.03 115 0.04 0.15 0.24 0.42 1 73 SUMMER 203 0.39 .00000 2.25 0.47 0.03 119 0.02 0.11 0.24 0.41 1 73 WINTER 190 0.40 .00250 3.73 0.48 0.04 120 0.04 0.15 0.24 0.45 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: X  Rivers and Streams Descriptive Statistics by Subecoregion, Decade and Season from 1990 to 1998 Nitrogen_Tot_Kjeldhal_mg_L subecoregion season N MEAN MIN MAX STDDEV STDERR CV P5 P25 MEDIAN P75  34 FALL 58 0.81 .22500 1.80 0.27 0.04 34 0.37 0.65 0.79 1.00 1 34 SPRING 63 1.15 .30000 2.71 0.50 0.06 43 0.47 0.80 1.10 1.45 1 34 SUMMER 64 0.95 .05000 1.90 0.38 0.05 40 0.30 0.68 0.97 1.17 1 34 WINTER 58 1.00 .34500 2.20 0.33 0.04 33 0.53 0.80 0.98 1.15 1 73 FALL 158 0.87 .01000 3.24 0.45 0.04 52 0.31 0.60 0.76 1.10 1 73 SPRING 152 1.00 .30000 4.40 0.51 0.04 50 0.51 0.70 0.86 1.20 1 73 SUMMER 156 1.00 .10000 2.65 0.41 0.03 41 0.48 0.73 0.93 1.25 1 73 WINTER 157 0.94 .01000 2.48 0.41 0.03 43 0.38 0.67 0.85 1.13 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: X  Rivers and Streams Descriptive Statistics by Subecoregion, Decade and Season from 1999 to 2000 Organic_P_ug_L subecoregion season N MEAN MIN MAX STDDEV STDERR CV P5 P25 MEDIAN P75 P 73 FALL 4 30.51 11.920 49.09 21.5 10.7 70 11.9 11.9 30.5 49.1 49 73 SPRING 3 197.51 38.300 383.16 174 100 88 38.3 38.3 171 383 3 73 SUMMER 6 45.32 25.480 61.37 16.3 6.66 36 25.5 25.5 49.1 61.4 61 Data were not always available for all years.
Appendix B—Descriptive Statistics Data Tables for Level III Subecoregions Within Aggregate Ec

 Aggregate Nutrient Ecoregion: X  Rivers and Streams Descriptive Statistics by Subecoregion, Decade and Season from 1990 to 1995 Phosph_Ortho_Tot_as_P_ug_L subecoregion season N MEAN MIN MAX STDDEV STDERR CV P5 P25 MEDIAN P75 P 34 FALL 9 173.89 10.000 590.00 192 63.9 110 10.0 50.0 110 160 5 34 SPRING 9 134.17 10.000 457.50 134 44.8 100 10.0 56.3 113 150 4 34 SUMMER 9 266.11 30.000 1475.00 465 155 175 30.0 42.5 87.5 140 14 34 WINTER 9 270.33 30.000 1115.00 358 119 132 30.0 80.0 110 200 11 73 FALL 20 75.20 5.0000 170.00 37.5 8.38 50 22.5 54.5 70.0 90.0 1 73 SPRING 20 94.13 32.500 175.00 41.8 9.35 44 32.5 67.5 87.5 108 1 73 SUMMER 20 97.00 15.000 180.00 39.9 8.91 41 27.5 80.0 90.0 128 1 73 WINTER 20 92.75 27.500 190.00 37.1 8.29 40 33.8 70.0 91.3 110 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: X  Rivers and Streams Descriptive Statistics by Subecoregion, Decade and Season from 1990 to 2000 Total_Nitrogen_mg_L subecoregion season N MEAN MIN MAX STDDEV STDERR CV P5 P25 MEDIAN P75 P 34 FALL 9 1.16 .35000 4.80 1.38 0.46 119 0.35 0.60 0.74 0.88 4. 34 SPRING 9 1.80 .62500 5.23 1.40 0.47 78 0.63 1.18 1.35 2.05 5. 34 SUMMER 9 1.55 .37000 4.20 1.42 0.47 92 0.37 0.79 1.00 1.16 4. 34 WINTER 9 2.22 .67000 6.60 2.18 0.73 98 0.67 0.92 1.20 1.90 6. 73 FALL 24 0.97 .03000 2.10 0.65 0.13 67 0.03 0.58 0.76 1.50 2. 73 SPRING 23 1.33 .10000 2.95 0.74 0.15 56 0.50 0.84 1.10 1.68 2. 73 SUMMER 27 1.19 .21000 2.90 0.82 0.16 69 0.21 0.46 0.95 1.80 2. 73 WINTER 20 1.30 .53000 2.45 0.58 0.13 45 0.56 0.87 1.12 1.66 2. Data were not always available for all years.
Appendix B—Descriptive Statistics Data Tables for Level III Subecoregions Within Aggregate Ec

 Aggregate Nutrient Ecoregion: X  Rivers and Streams 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  34 FALL 59 277.42 30.000 1065.00 257 33.5 93 40.0 105 195 320  34 SPRING 64 369.18 2.5000 1510.00 326 40.7 88 60.0 148 269 498 1 34 SUMMER 67 340.58 2.5000 1400.00 368 44.9 108 50.0 105 200 420 1 34 WINTER 60 350.96 30.000 1570.00 342 44.1 97 50.0 148 215 419 1 73 FALL 205 187.69 5.0000 1650.00 189 13.2 100 50.0 110 150 200  73 SPRING 217 245.91 5.0000 2100.00 200 13.5 81 50.0 130 205 320  73 SUMMER 228 202.13 10.000 1000.00 132 8.72 65 50.0 120 180 230  73 WINTER 207 234.24 5.0000 970.00 155 10.8 66 70.0 130 195 300  Data were not always available for all years.
Appendix B—Descriptive Statistics Data Tables for Level III Subecoregions Within Aggregate Ec

 Aggregate Nutrient Ecoregion: X  Rivers and Streams Descriptive Statistics by Subecoregion, Decade and Season from 1990 to 1997 Turbidity_FTU subecoregion season N MEAN MIN MAX STDDEV STDERR CV P5 P25 MEDIAN P75 P 34 FALL 12 20.84 6.2000 83.75 23.3 6.72 112 6.20 6.48 11.3 23.0 83 34 SPRING 16 48.32 2.7750 110.00 38.5 9.63 80 2.78 14.0 39.7 89.9 1 34 SUMMER 16 23.62 2.7000 88.50 21.1 5.28 89 2.70 10.5 19.0 28.3 88 34 WINTER 16 49.48 10.000 110.00 28.9 7.22 58 10.0 26.0 40.5 75.5 1 73 FALL 46 29.78 3.9000 77.50 18.1 2.67 61 4.10 15.7 27.8 40.0 62 73 SPRING 64 55.48 2.9000 150.00 36.0 4.50 65 9.60 22.8 55.0 70.0 1 73 SUMMER 53 40.72 3.1000 140.00 32.0 4.40 79 7.20 20.5 32.0 55.0 1 73 WINTER 68 62.13 2.9000 190.00 43.9 5.32 71 3.50 17.5 61.3 91.3 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: X  Rivers and Streams Descriptive Statistics by Subecoregion, Decade and Season from 1993 to 1997 Turbidity_JCU subecoregion season N MEAN MIN MAX STDDEV STDERR CV P5 P25 MEDIAN P75 P 34 WINTER 3 9.27 7.8000 10.00 1.27 0.73 14 7.80 7.80 10.0 10.0 10 73 FALL 1 18.00 18.000 18.00 . . . 18.0 18.0 18.0 18.0 18 73 SPRING 1 12.00 12.000 12.00 . . . 12.0 12.0 12.0 12.0 12 73 SUMMER 1 9.50 9.5000 9.50 . . . 9.50 9.50 9.50 9.50 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: X  Rivers and Streams Descriptive Statistics by Subecoregion, Decade and Season from 1990 to 2000 Turbidity_NTU subecoregion season N MEAN MIN MAX STDDEV STDERR CV P5 P25 MEDIAN P75  34 FALL 13 25.23 6.2500 49.75 13.5 3.74 53 6.25 18.5 26.0 31.0 4 34 SPRING 13 89.04 14.000 150.00 43.7 12.1 49 14.0 57.0 97.8 115  34 SUMMER 16 34.69 6.5000 78.75 20.5 5.11 59 6.50 16.8 31.9 51.5 7 34 WINTER 13 65.42 15.500 105.00 29.1 8.07 44 15.5 57.5 70.0 78.0  73 FALL 148 21.82 .00000 83.75 14.9 1.22 68 4.20 10.7 19.8 30.0 4 73 SPRING 152 54.69 2.8000 435.00 52.9 4.29 97 12.6 21.4 36.1 66.0  73 SUMMER 154 32.72 3.2500 111.00 21.6 1.74 66 7.00 16.0 27.9 44.0 7 73 WINTER 149 53.01 2.2000 170.00 37.1 3.04 70 10.9 24.0 42.7 71.3  Data were not always available for all years.
Appendix B—Descriptive Statistics Data Tables for Level III Subecoregions Within Aggregate Ec

 Aggregate Nutrient Ecoregion: X  Rivers and Streams Descriptive Statistics by Subecoregion, Decade and Season from 1999 to 2000 pH_S_U subecoregion season N MEAN MIN MAX STDDEV STDERR CV P5 P25 MEDIAN P75 P 73 FALL 4 8.50 7.8400 9.15 0.76 0.38 9 7.84 7.84 8.50 9.15 9. 73 SPRING 3 8.32 7.7100 9.06 0.69 0.40 8 7.71 7.71 8.18 9.06 9. 73 SUMMER 6 8.13 7.8900 8.43 0.20 0.08 2 7.89 8.02 8.07 8.32 8. Data were not always available for all years.
Appendix B—Descriptive Statistics Data Tables for Level III Subecoregions Within Aggregate Ec

 Aggregate Nutrient Ecoregion: X  Rivers and Streams Descriptive Statistics by Subecoregion, Year and Season from 1999 to 2000 Chloro_A_Fluor_cor_ug_L subecoregion year season N MEAN MIN MAX STDDEV STDERR CV P5 P25 MEDIAN P75 P 73 1999 FALL 4 1.25 1.1000 1.40 0.17 0.09 14 1.10 1.10 1.25 1.40 1. 73 1999 SUMMER 6 0.93 .70000 1.10 0.19 0.08 20 0.70 0.70 1.00 1.10 1. 73 2000 SPRING 3 5.85 3.1000 7.57 2.41 1.39 41 3.10 3.10 6.88 7.57 7.
Appendix B—Descriptive Statistics Data Tables for Level III Subecoregions Within Aggregate Ec

 Aggregate Nutrient Ecoregion: X  Rivers and Streams Descriptive Statistics by Subecoregion, Year and Season from 1999 to 2000 Chloro_A_Pheo_cor_ug_L subecoregion year season N MEAN MIN MAX STDDEV STDERR CV P5 P25 MEDIAN P75 P 73 1999 FALL 2 0.90 .60000 1.20 0.42 0.30 47 0.60 0.60 0.90 1.20 1. 73 1999 SUMMER 3 1.10 .90000 1.40 0.26 0.15 24 0.90 0.90 1.00 1.40 1. 73 2000 SPRING 3 2.85 2.0200 3.85 0.93 0.53 32 2.02 2.02 2.69 3.85 3.
Appendix B—Descriptive Statistics Data Tables for Level III Subecoregions Within Aggregate Ec

 Aggregate Nutrient Ecoregion: X  Rivers and Streams Descriptive Statistics by Subecoregion, Year and Season from 1990 to 1995 Chloro_A_Phyto_Chro_flu_ug_L subecoregion year season N MEAN MIN MAX STDDEV STDERR CV P5 P25 MEDIAN P75 P 73 1990 FALL 2 2.25 2.2000 2.30 0.07 0.05 3 2.20 2.20 2.25 2.30 2. 73 1990 SPRING 2 1.60 1.5000 1.70 0.14 0.10 9 1.50 1.50 1.60 1.70 1. 73 1990 SUMMER 2 1.05 1.0000 1.10 0.07 0.05 7 1.00 1.00 1.05 1.10 1. 73 1990 WINTER 2 1.65 1.5000 1.80 0.21 0.15 13 1.50 1.50 1.65 1.80 1. 73 1991 FALL 2 4.75 4.3000 5.20 0.64 0.45 13 4.30 4.30 4.75 5.20 5. 73 1991 SPRING 2 1.05 .90000 1.20 0.21 0.15 20 0.90 0.90 1.05 1.20 1. 73 1991 SUMMER 2 4.15 2.4000 5.90 2.47 1.75 60 2.40 2.40 4.15 5.90 5. 73 1991 WINTER 2 0.65 .60000 0.70 0.07 0.05 11 0.60 0.60 0.65 0.70 0. 73 1992 FALL 2 3.23 3.2000 3.25 0.04 0.02 1 3.20 3.20 3.23 3.25 3. 73 1992 SPRING 2 3.88 2.3500 5.40 2.16 1.53 56 2.35 2.35 3.88 5.40 5. 73 1992 SUMMER 2 2.53 1.6500 3.40 1.24 0.88 49 1.65 1.65 2.53 3.40 3. 73 1992 WINTER 2 1.36 1.2250 1.50 0.19 0.14 14 1.23 1.23 1.36 1.50 1. 73 1993 FALL 2 2.40 2.2000 2.60 0.28 0.20 12 2.20 2.20 2.40 2.60 2. 73 1993 SPRING 2 1.88 1.8000 1.95 0.11 0.08 6 1.80 1.80 1.88 1.95 1. 73 1993 SUMMER 2 2.20 1.2000 3.20 1.41 1.00 64 1.20 1.20 2.20 3.20 3. 73 1993 WINTER 2 2.08 1.8000 2.35 0.39 0.28 19 1.80 1.80 2.08 2.35 2. 73 1994 FALL 2 2.85 .70000 5.00 3.04 2.15 107 0.70 0.70 2.85 5.00 5. 73 1994 SPRING 2 1.10 .80000 1.40 0.42 0.30 39 0.80 0.80 1.10 1.40 1. 73 1994 SUMMER 2 2.05 1.8500 2.25 0.28 0.20 14 1.85 1.85 2.05 2.25 2. 73 1994 WINTER 2 0.63 .55000 0.70 0.11 0.08 17 0.55 0.55 0.63 0.70 0. 73 1995 SPRING 2 1.85 1.2000 2.50 0.92 0.65 50 1.20 1.20 1.85 2.50 2. 73 1995 SUMMER 2 2.45 1.8000 3.10 0.92 0.65 38 1.80 1.80 2.45 3.10 3. 73 1995 WINTER 1 0.90 .90000 0.90 . . . 0.90 0.90 0.90 0.90 0.
Appendix B—Descriptive Statistics Data Tables for Level III Subecoregions Within Aggregate Ec

 Aggregate Nutrient Ecoregion: X  Rivers and Streams Descriptive Statistics by Subecoregion, Year and Season from 1990 to 1996 Chloro_A_Phyto_Spec_A_ug_L subecoregion year season N MEAN MIN MAX STDDEV STDERR CV P5 P25 MEDIAN P75  34 1990 FALL 16 5.03 .25000 14.40 4.86 1.22 97 0.25 1.50 3.05 7.55 14 34 1990 SPRING 18 13.50 .25000 66.90 17.29 4.08 128 0.25 2.00 4.80 23.10 66 34 1990 SUMMER 21 9.87 .25000 32.00 9.95 2.17 101 0.25 3.20 4.00 16.00 29 34 1990 WINTER 15 8.88 .25000 25.60 8.20 2.12 92 0.25 2.90 5.10 12.20 25 34 1991 FALL 24 5.33 .25000 22.70 5.74 1.17 108 0.25 1.81 4.00 7.40 21 34 1991 SPRING 20 9.54 .25000 66.90 16.35 3.66 171 0.25 1.23 3.90 6.65 51 34 1991 SUMMER 23 12.72 .25000 40.80 11.64 2.43 92 0.25 2.00 12.60 22.50 33 34 1991 WINTER 21 4.45 .25000 19.40 5.69 1.24 128 0.25 1.20 2.40 5.55 19 34 1992 FALL 17 17.86 .25000 91.10 22.44 5.44 126 0.25 4.90 9.40 25.45 91 34 1992 SPRING 26 11.46 .25000 50.50 13.45 2.64 117 0.25 0.25 7.45 17.00 48 34 1992 SUMMER 26 13.97 .25000 61.70 17.06 3.35 122 0.25 3.00 6.46 20.90 60 34 1992 WINTER 22 3.61 .25000 26.70 5.66 1.21 157 0.25 0.25 2.51 4.40 8 34 1993 FALL 24 8.39 .25000 61.00 13.34 2.72 159 0.25 0.25 4.30 8.79 29 34 1993 SPRING 16 14.16 .25000 108.00 26.16 6.54 185 0.25 2.00 7.62 11.64 108 34 1993 SUMMER 22 16.26 .25000 54.20 14.77 3.15 91 0.25 4.50 9.73 23.45 40 34 1993 WINTER 23 8.47 .25000 52.90 13.02 2.71 154 0.25 0.25 2.80 11.30 38 34 1994 FALL 17 3.54 .25000 19.80 5.21 1.26 147 0.25 0.25 0.25 4.81 19 34 1994 SPRING 25 9.69 .25000 60.90 14.20 2.84 147 0.25 0.25 3.78 14.50 28 34 1994 SUMMER 16 8.08 .25000 33.10 9.63 2.41 119 0.25 1.09 2.66 15.35 33 34 1994 WINTER 19 3.48 .25000 19.10 5.06 1.16 145 0.25 0.25 1.34 4.15 19 34 1995 FALL 20 6.85 .25000 32.10 7.83 1.75 114 0.25 0.25 5.31 10.23 23 34 1995 SPRING 21 6.87 .25000 58.90 14.23 3.11 207 0.25 0.25 1.60 4.33 33 34 1995 SUMMER 21 2.70 .25000 15.90 4.53 0.99 168 0.25 0.25 0.25 2.27 12 34 1995 WINTER 19 3.80 .25000 40.00 9.28 2.13 244 0.25 0.25 0.25 2.22 40 34 1996 FALL 2 10.40 8.0000 12.80 3.39 2.40 33 8.00 8.00 10.40 12.80 12 34 1996 SPRING 20 10.06 .25000 35.20 9.82 2.20 98 0.25 2.85 6.95 15.35 30 34 1996 SUMMER 19 16.34 .25000 73.00 21.24 4.87 130 0.25 4.01 6.86 22.00 73 34 1996 WINTER 16 14.38 .25000 69.90 22.42 5.61 156 0.25 0.25 1.02 27.85 69
Appendix B—Descriptive Statistics Data Tables for Level III Subecoregions Within Aggregate Ec

 Aggregate Nutrient Ecoregion: X  Rivers and Streams Descriptive Statistics by Subecoregion, Year and Season from 1990 to 1993 Chloro_A_Trich_unco_ug_L subecoregion year season N MEAN MIN MAX STDDEV STDERR CV P5 P25 MEDIAN P75  73 1990 FALL 10 33.13 4.9500 96.15 36.29 11.48 110 4.95 8.00 13.48 68.90 96 73 1990 SPRING 13 8.33 1.3000 20.70 5.50 1.53 66 1.30 4.71 6.72 12.45 20 73 1990 SUMMER 15 15.30 1.3000 52.85 12.32 3.18 81 1.30 8.70 13.40 17.70 52 73 1990 WINTER 10 9.19 1.0000 22.85 8.49 2.69 92 1.00 1.75 6.20 18.95 22 73 1991 FALL 4 24.89 10.350 34.60 10.31 5.16 41 10.35 18.38 27.30 31.40 34 73 1991 WINTER 11 15.43 1.1000 58.50 19.60 5.91 127 1.10 1.80 3.20 29.40 58 73 1992 FALL 4 52.28 26.400 69.10 18.23 9.12 35 26.40 40.90 56.80 63.65 69 73 1992 SPRING 4 22.98 15.900 28.80 6.60 3.30 29 15.90 17.35 23.60 28.60 28 73 1992 SUMMER 4 40.25 34.400 46.70 6.35 3.18 16 34.40 34.80 39.95 45.70 46 73 1992 WINTER 4 26.89 11.600 45.60 14.72 7.36 55 11.60 15.65 25.18 38.13 45 73 1993 SPRING 4 23.10 19.200 27.50 3.75 1.88 16 19.20 20.05 22.85 26.15 27 73 1993 SUMMER 4 33.10 21.700 42.50 8.62 4.31 26 21.70 27.35 34.10 38.85 42 73 1993 WINTER 4 25.68 14.050 48.50 15.85 7.93 62 14.05 14.98 20.08 36.38 48
Appendix B—Descriptive Statistics Data Tables for Level III Subecoregions Within Aggregate Ec

 Aggregate Nutrient Ecoregion: X  Rivers and Streams Descriptive Statistics by Subecoregion, Year and Season from 1990 to 1995 Chloro_B_Phyto_Chro_flu_ug_L subecoregion year season N MEAN MIN MAX STDDEV STDERR CV P5 P25 MEDIAN P75 P 73 1990 FALL 2 0.13 .05000 0.20 0.11 0.08 85 0.05 0.05 0.13 0.20 0. 73 1990 SPRING 2 0.05 .05000 0.05 0.00 0.00 0 0.05 0.05 0.05 0.05 0. 73 1990 SUMMER 2 0.05 .05000 0.05 0.00 0.00 0 0.05 0.05 0.05 0.05 0. 73 1990 WINTER 2 0.05 .05000 0.05 0.00 0.00 0 0.05 0.05 0.05 0.05 0. 73 1991 FALL 2 0.05 .05000 0.05 0.00 0.00 0 0.05 0.05 0.05 0.05 0. 73 1991 SPRING 2 0.08 .05000 0.10 0.04 0.03 47 0.05 0.05 0.08 0.10 0. 73 1991 SUMMER 2 0.20 .10000 0.30 0.14 0.10 71 0.10 0.10 0.20 0.30 0. 73 1991 WINTER 2 0.05 .05000 0.05 0.00 0.00 0 0.05 0.05 0.05 0.05 0. 73 1992 FALL 2 0.23 .20000 0.25 0.04 0.03 16 0.20 0.20 0.23 0.25 0. 73 1992 SPRING 2 0.14 .07500 0.20 0.09 0.06 64 0.08 0.08 0.14 0.20 0. 73 1992 SUMMER 2 0.29 .10000 0.48 0.27 0.19 92 0.10 0.10 0.29 0.48 0. 73 1992 WINTER 2 0.10 .10000 0.10 0.00 0.00 0 0.10 0.10 0.10 0.10 0. 73 1993 FALL 2 0.15 .10000 0.20 0.07 0.05 47 0.10 0.10 0.15 0.20 0. 73 1993 SPRING 2 0.10 .10000 0.10 0.00 0.00 0 0.10 0.10 0.10 0.10 0. 73 1993 SUMMER 2 0.15 .10000 0.20 0.07 0.05 47 0.10 0.10 0.15 0.20 0. 73 1993 WINTER 2 0.15 .10000 0.20 0.07 0.05 47 0.10 0.10 0.15 0.20 0. 73 1994 FALL 2 0.35 .05000 0.65 0.42 0.30 121 0.05 0.05 0.35 0.65 0. 73 1994 SPRING 2 0.08 .05000 0.10 0.04 0.03 47 0.05 0.05 0.08 0.10 0. 73 1994 SUMMER 2 0.14 .12500 0.15 0.02 0.01 13 0.13 0.13 0.14 0.15 0. 73 1994 WINTER 2 0.05 .05000 0.05 0.00 0.00 0 0.05 0.05 0.05 0.05 0. 73 1995 SPRING 2 0.05 .05000 0.05 0.00 0.00 0 0.05 0.05 0.05 0.05 0. 73 1995 SUMMER 2 0.08 .05000 0.10 0.04 0.03 47 0.05 0.05 0.08 0.10 0. 73 1995 WINTER 1 0.13 .12500 0.13 . . . 0.13 0.13 0.13 0.13 0.
Appendix B—Descriptive Statistics Data Tables for Level III Subecoregions Within Aggregate Ec

 Aggregate Nutrient Ecoregion: X  Rivers and Streams Descriptive Statistics by Subecoregion, Year and Season from 1990 to 1999 DIP_ug_L subecoregion year season N MEAN MIN MAX STDDEV STDERR CV P5 P25 MEDIAN P75  34 1990 FALL 8 172.19 5.0000 550.00 207.87 73.49 121 5.00 31.25 100.00 280.00 550 34 1990 SPRING 8 207.81 17.500 670.00 232.64 82.25 112 17.50 50.00 92.50 345.00 670 34 1990 SUMMER 5 153.00 25.000 350.00 137.32 61.41 90 25.00 45.00 110.00 235.00 350 34 1990 WINTER 5 266.00 5.0000 705.00 306.38 137.02 115 5.00 70.00 80.00 470.00 705 34 1991 FALL 6 140.83 5.0000 400.00 146.44 59.78 104 5.00 40.00 95.00 210.00 400 34 1991 SPRING 8 123.75 5.0000 455.00 146.84 51.92 119 5.00 37.50 65.00 162.50 455 34 1991 SUMMER 5 141.50 12.500 370.00 144.20 64.49 102 12.50 45.00 90.00 190.00 370 34 1991 WINTER 5 238.00 55.000 635.00 244.76 109.46 103 55.00 60.00 130.00 310.00 635 34 1992 FALL 7 77.86 5.0000 275.00 97.59 36.89 125 5.00 10.00 40.00 135.00 275 34 1992 SPRING 8 111.25 25.000 410.00 127.75 45.17 115 25.00 35.00 65.00 127.50 410 34 1992 SUMMER 8 73.44 7.5000 310.00 99.33 35.12 135 7.50 20.00 40.00 75.00 310 34 1992 WINTER 8 108.44 12.500 485.00 153.63 54.32 142 12.50 47.50 60.00 77.50 485 34 1993 FALL 7 281.07 12.500 1400.00 501.94 189.72 179 12.50 40.00 85.00 290.00 140 34 1993 SPRING 9 113.33 15.000 420.00 130.53 43.51 115 15.00 45.00 60.00 100.00 420 34 1993 SUMMER 8 218.75 12.500 1300.00 440.84 155.86 202 12.50 26.25 45.00 147.50 130 34 1993 WINTER 4 163.75 30.000 365.00 143.66 71.83 88 30.00 67.50 130.00 260.00 365 34 1994 FALL 5 255.00 5.0000 900.00 371.37 166.08 146 5.00 5.00 175.00 190.00 900 34 1994 SPRING 6 125.83 7.5000 210.00 88.38 36.08 70 7.50 27.50 150.00 210.00 210 34 1994 SUMMER 6 235.00 27.500 1000.00 378.63 154.57 161 27.50 40.00 96.25 150.00 100 34 1994 WINTER 4 140.00 15.000 290.00 116.26 58.13 83 15.00 55.00 127.50 225.00 290 34 1995 FALL 3 512.50 12.500 1350.00 729.83 421.37 142 12.50 12.50 175.00 1350.0 135 34 1995 SPRING 5 261.00 5.0000 935.00 383.62 171.56 147 5.00 40.00 145.00 180.00 935 34 1995 SUMMER 5 192.00 25.000 670.00 272.78 121.99 142 25.00 40.00 60.00 165.00 670 34 1995 WINTER 3 393.33 20.000 935.00 480.16 277.22 122 20.00 20.00 225.00 935.00 935 73 1990 FALL 19 52.89 5.0000 120.00 31.50 7.23 60 5.00 25.00 50.00 65.00 120 73 1990 SPRING 16 44.53 12.500 125.00 29.41 7.35 66 12.50 22.50 40.00 52.50 125 73 1990 SUMMER 18 64.58 7.5000 110.00 29.56 6.97 46 7.50 40.00 67.50 90.00 110 73 1990 WINTER 16 51.88 7.5000 130.00 31.58 7.89 61 7.50 37.50 47.50 60.00 130 73 1991 FALL 15 58.17 12.500 95.00 25.57 6.60 44 12.50 40.00 60.00 80.00 95 73 1991 SPRING 18 50.31 15.000 155.50 31.88 7.51 63 15.00 30.00 45.00 55.00 155 73 1991 SUMMER 18 81.94 7.5000 135.00 42.09 9.92 51 7.50 50.00 97.50 115.00 135 73 1991 WINTER 16 59.38 12.500 140.00 38.28 9.57 64 12.50 30.00 55.00 77.50 140
Appendix B—Descriptive Statistics Data Tables for Level III Subecoregions Within Aggregate Ec

 Aggregate Nutrient Ecoregion: X  Rivers and Streams Descriptive Statistics by Subecoregion, Year and Season from 1990 to 1999 DIP_ug_L subecoregion year season N MEAN MIN MAX STDDEV STDERR CV P5 P25 MEDIAN P75  73 1992 FALL 16 63.44 5.0000 185.00 46.19 11.55 73 5.00 35.00 60.00 85.00 185 73 1992 SPRING 14 44.29 12.500 95.00 22.63 6.05 51 12.50 27.50 37.50 65.00 95 73 1992 SUMMER 19 71.58 7.5000 145.00 36.58 8.39 51 7.50 50.00 60.00 95.00 145 73 1992 WINTER 16 50.16 7.5000 120.00 26.87 6.72 54 7.50 32.50 50.00 65.00 120 73 1993 FALL 12 73.54 12.500 125.00 32.41 9.36 44 12.50 50.00 75.00 100.00 125 73 1993 SPRING 15 58.50 7.5000 115.00 29.79 7.69 51 7.50 40.00 55.00 75.00 115 73 1993 SUMMER 18 72.08 12.500 135.00 32.77 7.72 45 12.50 50.00 75.00 90.00 135 73 1993 WINTER 18 61.25 7.5000 120.00 27.39 6.46 45 7.50 40.00 57.50 75.00 120 73 1994 FALL 4 70.00 50.000 100.00 22.73 11.37 32 50.00 52.50 65.00 87.50 100 73 1994 SPRING 12 50.63 7.5000 115.00 29.47 8.51 58 7.50 40.00 45.00 62.50 115 73 1994 SUMMER 15 68.33 10.000 165.00 38.62 9.97 57 10.00 35.00 75.00 90.00 165 73 1994 WINTER 13 40.38 30.000 70.00 11.81 3.27 29 30.00 30.00 40.00 45.00 70 73 1995 FALL 1 70.00 70.000 70.00 . . . 70.00 70.00 70.00 70.00 70 73 1995 SPRING 6 59.17 20.000 95.00 28.88 11.79 49 20.00 45.00 52.50 90.00 95 73 1995 SUMMER 7 49.64 7.5000 70.00 21.43 8.10 43 7.50 35.00 60.00 60.00 70 73 1995 WINTER 5 35.00 20.000 45.00 11.73 5.24 34 20.00 25.00 40.00 45.00 45 73 1999 FALL 2 158.05 141.03 175.07 24.07 17.02 15 141.03 141.03 158.05 175.07 175 73 1999 SUMMER 3 147.16 82.340 247.02 87.75 50.66 60 82.34 82.34 112.13 247.02 247
Appendix B—Descriptive Statistics Data Tables for Level III Subecoregions Within Aggregate Ec

 Aggregate Nutrient Ecoregion: X  Rivers and Streams 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  34 1990 FALL 36 7.21 2.6000 11.50 2.20 0.37 31 2.80 5.98 7.50 8.58 10 34 1990 SPRING 38 6.27 1.4000 9.60 1.96 0.32 31 2.30 4.90 6.50 7.85 9 34 1990 SUMMER 39 6.28 1.7500 14.20 2.22 0.35 35 2.80 4.90 6.10 7.20 12 34 1990 WINTER 35 8.19 2.6000 12.00 1.96 0.33 24 5.00 7.20 8.00 9.60 11 34 1991 FALL 33 6.62 2.0000 11.80 2.00 0.35 30 2.40 5.15 6.85 7.80 9 34 1991 SPRING 47 6.37 1.3000 12.00 2.29 0.33 36 2.80 4.50 6.60 7.90 9 34 1991 SUMMER 46 5.23 1.2500 11.00 2.24 0.33 43 1.85 3.50 5.30 6.40 9 34 1991 WINTER 34 9.23 5.4000 13.60 1.75 0.30 19 5.80 8.20 9.10 10.00 13 34 1992 FALL 29 7.81 3.0000 10.40 1.57 0.29 20 4.60 7.00 8.00 8.90 10 34 1992 SPRING 49 6.60 2.3000 11.40 1.78 0.25 27 3.10 5.90 6.80 7.60 8 34 1992 SUMMER 51 6.01 1.6000 11.60 2.04 0.29 34 2.70 4.80 6.10 7.40 9 34 1992 WINTER 46 8.63 5.2500 14.70 1.82 0.27 21 5.80 7.40 8.60 9.95 10 34 1993 FALL 41 6.73 2.3000 12.70 2.04 0.32 30 4.00 5.30 6.90 7.90 9 34 1993 SPRING 32 6.74 2.8000 11.20 1.81 0.32 27 4.00 5.68 6.88 8.10 8 34 1993 SUMMER 37 5.69 1.7000 10.00 2.08 0.34 37 2.10 4.30 5.80 7.30 9 34 1993 WINTER 34 8.06 3.1000 13.30 2.44 0.42 30 3.80 6.50 7.80 9.60 13 34 1994 FALL 37 5.91 1.1000 10.00 2.26 0.37 38 1.50 4.40 6.00 7.40 9 34 1994 SPRING 35 7.18 3.9000 10.20 1.74 0.29 24 4.60 5.60 7.30 8.50 10 34 1994 SUMMER 35 5.36 2.1000 9.80 1.69 0.29 31 2.10 4.50 5.30 6.40 8 34 1994 WINTER 34 8.01 4.9000 12.20 2.00 0.34 25 5.40 6.30 7.65 9.70 11 34 1995 FALL 38 6.12 1.3000 10.60 2.29 0.37 37 1.60 4.60 6.45 8.00 9 34 1995 SPRING 37 7.03 3.6000 10.30 1.65 0.27 23 4.00 6.30 6.90 8.40 9 34 1995 SUMMER 37 5.54 2.0000 8.80 1.75 0.29 32 2.50 4.70 5.80 6.70 8 34 1995 WINTER 36 7.58 2.1000 11.15 2.01 0.33 26 4.40 6.00 7.70 8.98 10 34 1996 FALL 27 5.05 .25000 8.20 2.34 0.45 46 0.30 3.00 5.40 7.15 7 34 1996 SPRING 34 6.89 3.1000 11.50 2.27 0.39 33 3.50 5.30 7.05 8.70 11 34 1996 SUMMER 35 5.29 .25000 11.80 2.48 0.42 47 0.60 3.50 5.50 6.90 9 34 1996 WINTER 38 8.49 3.3000 14.70 1.94 0.31 23 5.70 7.30 8.45 9.50 11 34 1997 FALL 13 4.71 2.5000 7.60 1.51 0.42 32 2.50 3.50 4.60 5.90 7 34 1997 SPRING 13 4.54 2.3500 7.30 1.59 0.44 35 2.35 3.55 4.40 5.80 7 34 1997 SUMMER 13 3.85 1.0000 6.90 1.93 0.54 50 1.00 1.90 4.00 5.50 6 34 1997 WINTER 14 6.95 2.8000 9.80 2.04 0.55 29 2.80 5.85 7.40 8.20 9
Appendix B—Descriptive Statistics Data Tables for Level III Subecoregions Within Aggregate Ec

 Aggregate Nutrient Ecoregion: X  Rivers and Streams 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  34 1998 SPRING 13 5.88 3.5000 10.10 1.90 0.53 32 3.50 4.20 5.50 6.70 10 34 1998 WINTER 13 7.27 5.4500 9.20 1.16 0.32 16 5.45 6.30 7.40 8.05 9 73 1990 FALL 116 6.93 .95000 12.35 2.19 0.20 32 2.60 5.50 7.08 8.40 10 73 1990 SPRING 123 6.97 1.1000 10.70 1.91 0.17 27 3.20 5.80 7.10 8.30 9 73 1990 SUMMER 108 5.59 1.0625 10.90 2.00 0.19 36 2.20 4.25 5.85 7.00 8 73 1990 WINTER 111 8.43 3.1000 12.70 1.99 0.19 24 3.70 7.10 8.60 9.90 11 73 1991 FALL 63 8.03 .50000 12.30 1.98 0.25 25 5.40 7.25 8.40 9.40 10 73 1991 SPRING 121 6.54 .90000 10.80 2.20 0.20 34 2.70 5.25 6.70 8.30 9 73 1991 SUMMER 123 5.06 .40000 9.20 2.26 0.20 45 1.40 3.20 5.10 6.90 8 73 1991 WINTER 79 9.54 2.5000 12.60 1.84 0.21 19 5.40 8.60 9.90 10.70 11 73 1992 FALL 77 6.91 .20000 11.20 2.46 0.28 36 2.20 5.60 7.45 8.60 10 73 1992 SPRING 130 7.16 .60000 12.00 1.87 0.16 26 4.30 6.00 7.30 8.40 9 73 1992 SUMMER 130 5.36 .20000 9.50 1.93 0.17 36 1.80 4.20 5.68 6.70 8 73 1992 WINTER 135 8.86 2.2000 12.50 2.39 0.21 27 4.20 7.30 9.30 10.75 12 73 1993 FALL 111 6.51 .55000 10.80 2.10 0.20 32 2.60 5.15 6.70 8.00 9 73 1993 SPRING 115 6.80 1.4000 13.70 2.24 0.21 33 3.20 5.30 6.80 8.50 10 73 1993 SUMMER 128 5.37 .70000 8.70 1.86 0.16 35 1.80 4.45 5.70 6.80 7 73 1993 WINTER 133 8.68 1.1000 13.20 2.41 0.21 28 4.30 6.90 9.00 10.30 12 73 1994 FALL 139 5.67 .45000 9.50 1.90 0.16 34 1.70 4.65 5.80 7.10 8 73 1994 SPRING 108 6.97 1.4000 11.20 1.96 0.19 28 3.45 5.83 7.05 8.48 9 73 1994 SUMMER 141 5.25 .75000 12.80 2.01 0.17 38 1.90 3.80 5.30 6.70 8 73 1994 WINTER 112 8.45 1.5000 13.40 2.37 0.22 28 3.20 7.55 8.80 10.00 12 73 1995 FALL 137 6.17 .50000 10.00 1.94 0.17 31 2.65 5.00 6.30 7.50 9 73 1995 SPRING 137 6.81 2.5000 13.70 1.89 0.16 28 3.80 5.60 6.80 8.10 9 73 1995 SUMMER 113 5.11 .20000 11.90 2.11 0.20 41 1.90 3.75 5.20 6.70 7 73 1995 WINTER 134 8.81 1.6000 13.00 2.13 0.18 24 4.30 7.90 9.30 10.35 11 73 1996 FALL 135 5.70 .95000 9.60 1.93 0.17 34 1.70 4.50 6.00 7.00 8 73 1996 SPRING 111 7.49 2.2000 11.40 1.79 0.17 24 4.20 6.50 7.40 8.70 10 73 1996 SUMMER 109 5.14 .50000 9.20 1.79 0.17 35 1.45 4.10 5.30 6.45 8 73 1996 WINTER 135 8.52 4.0000 13.80 2.10 0.18 25 4.80 7.10 8.40 10.10 12 73 1997 FALL 89 6.16 1.4000 10.10 1.77 0.19 29 2.80 5.25 6.40 7.40 8 73 1997 SPRING 97 6.34 1.2500 10.00 1.92 0.19 30 2.30 5.35 6.50 7.75 9
Appendix B—Descriptive Statistics Data Tables for Level III Subecoregions Within Aggregate Ec

 Aggregate Nutrient Ecoregion: X  Rivers and Streams 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  73 1997 SUMMER 95 5.29 .75000 10.65 1.86 0.19 35 1.90 4.10 5.50 6.60 7 73 1997 WINTER 97 7.76 .20000 11.80 2.39 0.24 31 3.60 6.70 8.40 9.25 10 73 1998 SPRING 45 7.46 2.5000 10.80 1.98 0.30 27 3.40 6.80 7.40 8.60 10 73 1998 WINTER 69 7.36 2.9000 11.25 1.88 0.23 26 3.90 6.20 7.70 8.55 10 73 1999 FALL 4 11.79 9.4700 14.10 2.67 1.34 23 9.47 9.47 11.79 14.10 14 73 1999 SUMMER 6 8.40 7.3800 9.67 1.04 0.43 12 7.38 7.38 8.14 9.67 9 73 2000 SPRING 3 10.49 6.2700 13.28 3.72 2.15 35 6.27 6.27 11.91 13.28 13 73 2000 SUMMER 3 5.27 4.2000 6.30 1.05 0.61 20 4.20 4.20 5.30 6.30 6
Appendix B—Descriptive Statistics Data Tables for Level III Subecoregions Within Aggregate Ec

 Aggregate Nutrient Ecoregion: X  Rivers and Streams 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  34 1990 FALL 29 0.88 .02000 5.90 1.44 0.27 163 0.03 0.07 0.30 0.84 4 34 1990 SPRING 24 0.85 .02500 3.20 0.95 0.19 113 0.09 0.23 0.44 0.97 3 34 1990 SUMMER 26 1.48 .00500 7.40 2.34 0.46 159 0.03 0.07 0.40 1.10 7 34 1990 WINTER 24 1.97 .02500 7.40 2.46 0.50 125 0.05 0.16 0.24 3.40 6 34 1991 FALL 15 0.56 .01000 2.70 0.82 0.21 148 0.01 0.11 0.13 0.45 2 34 1991 SPRING 34 1.13 .01000 5.65 1.54 0.26 137 0.05 0.20 0.37 1.29 5 34 1991 SUMMER 32 1.15 .05000 6.40 1.78 0.31 155 0.09 0.22 0.26 1.31 5 34 1991 WINTER 19 2.32 .10000 5.90 1.93 0.44 83 0.10 0.46 1.50 4.70 5 34 1992 FALL 16 0.81 .01000 3.30 1.24 0.31 153 0.01 0.02 0.27 0.81 3 34 1992 SPRING 32 1.31 .07000 8.00 1.90 0.34 145 0.09 0.29 0.60 1.20 6 34 1992 SUMMER 33 0.81 .00500 4.70 1.12 0.19 138 0.03 0.13 0.37 0.81 3 34 1992 WINTER 30 0.50 .05000 3.70 0.77 0.14 152 0.07 0.14 0.26 0.52 2 34 1993 FALL 17 0.91 .00500 6.30 1.67 0.41 183 0.01 0.09 0.19 0.83 6 34 1993 SPRING 22 0.63 .00500 5.30 1.19 0.25 189 0.08 0.16 0.24 0.38 2 34 1993 SUMMER 20 0.74 .00500 4.60 1.18 0.26 160 0.02 0.12 0.34 0.74 4 34 1993 WINTER 17 0.26 .03000 1.11 0.32 0.08 122 0.03 0.09 0.15 0.21 1 34 1994 FALL 23 0.91 .00500 7.44 1.91 0.40 211 0.01 0.04 0.20 0.56 4 34 1994 SPRING 18 0.62 .07000 3.45 0.75 0.18 121 0.07 0.32 0.39 0.60 3 34 1994 SUMMER 17 0.75 .02000 5.00 1.22 0.30 163 0.02 0.21 0.38 0.66 5 34 1994 WINTER 25 0.49 .04000 4.70 0.94 0.19 192 0.05 0.13 0.23 0.44 1 34 1995 FALL 34 0.57 .01000 6.83 1.41 0.24 248 0.01 0.04 0.10 0.25 3 34 1995 SPRING 37 0.75 .01000 4.75 1.04 0.17 138 0.03 0.19 0.36 0.80 3 34 1995 SUMMER 37 0.46 .00250 2.60 0.58 0.10 127 0.00 0.10 0.34 0.48 2 34 1995 WINTER 32 0.85 .02000 6.60 1.60 0.28 189 0.03 0.12 0.23 0.56 4 34 1996 FALL 13 0.15 .02000 0.31 0.09 0.03 61 0.02 0.08 0.14 0.22 0 34 1996 SPRING 17 0.81 .02000 4.38 1.13 0.27 139 0.02 0.16 0.52 0.74 4 34 1996 SUMMER 16 0.56 .01500 3.19 0.87 0.22 155 0.02 0.10 0.15 0.71 3 34 1996 WINTER 18 0.83 .09000 6.84 1.76 0.41 212 0.09 0.16 0.27 0.37 6 34 1997 FALL 13 0.24 .03000 0.83 0.24 0.07 100 0.03 0.10 0.16 0.21 0 34 1997 SPRING 13 0.21 .02000 0.55 0.14 0.04 68 0.02 0.13 0.18 0.24 0 34 1997 SUMMER 13 0.29 .06000 0.61 0.19 0.05 67 0.06 0.11 0.25 0.38 0 34 1997 WINTER 13 0.16 .00500 0.60 0.15 0.04 94 0.01 0.09 0.14 0.19 0
Appendix B—Descriptive Statistics Data Tables for Level III Subecoregions Within Aggregate Ec

 Aggregate Nutrient Ecoregion: X  Rivers and Streams 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  34 1998 SPRING 13 0.15 .02000 0.24 0.06 0.02 44 0.02 0.09 0.15 0.19 0 34 1998 WINTER 13 0.09 .02000 0.15 0.04 0.01 47 0.02 0.07 0.09 0.10 0 73 1990 FALL 113 0.37 .00500 1.70 0.46 0.04 126 0.02 0.06 0.17 0.36 1 73 1990 SPRING 105 0.36 .02250 1.20 0.29 0.03 80 0.05 0.17 0.28 0.49 1 73 1990 SUMMER 108 0.43 .00500 2.65 0.63 0.06 146 0.01 0.08 0.21 0.34 2 73 1990 WINTER 106 0.37 .00500 1.20 0.31 0.03 82 0.05 0.17 0.25 0.56 1 73 1991 FALL 59 0.28 .00500 1.41 0.31 0.04 110 0.01 0.04 0.17 0.49 0 73 1991 SPRING 122 0.44 .00500 2.51 0.56 0.05 126 0.04 0.10 0.20 0.52 1 73 1991 SUMMER 124 0.45 .00500 2.95 0.70 0.06 157 0.01 0.03 0.20 0.38 2 73 1991 WINTER 70 0.39 .00500 1.50 0.35 0.04 89 0.07 0.17 0.27 0.46 1 73 1992 FALL 75 0.36 .00500 1.30 0.44 0.05 124 0.01 0.05 0.14 0.47 1 73 1992 SPRING 121 0.48 .00500 2.68 0.63 0.06 131 0.03 0.12 0.21 0.48 2 73 1992 SUMMER 126 0.42 .00500 2.07 0.45 0.04 108 0.01 0.11 0.25 0.59 1 73 1992 WINTER 133 0.51 .00250 5.26 0.70 0.06 139 0.01 0.15 0.27 0.53 2 73 1993 FALL 121 0.46 .00500 9.10 0.94 0.09 205 0.01 0.08 0.17 0.37 1 73 1993 SPRING 115 0.45 .00500 4.64 0.58 0.05 130 0.02 0.15 0.29 0.49 1 73 1993 SUMMER 133 0.51 .00500 2.20 0.60 0.05 117 0.01 0.10 0.26 0.65 2 73 1993 WINTER 128 0.34 .00500 2.10 0.41 0.04 122 0.02 0.10 0.19 0.34 1 73 1994 FALL 147 0.25 .00500 2.89 0.36 0.03 146 0.01 0.04 0.12 0.29 0 73 1994 SPRING 119 0.43 .00500 2.80 0.45 0.04 104 0.02 0.14 0.26 0.60 1 73 1994 SUMMER 151 0.45 .00500 1.83 0.39 0.03 87 0.01 0.18 0.32 0.64 1 73 1994 WINTER 119 0.37 .00500 7.50 0.73 0.07 197 0.01 0.12 0.21 0.35 1 73 1995 FALL 138 0.20 .00500 3.11 0.38 0.03 194 0.01 0.01 0.06 0.20 0 73 1995 SPRING 148 0.34 .00500 2.88 0.47 0.04 140 0.01 0.09 0.19 0.32 1 73 1995 SUMMER 147 0.37 .00500 2.53 0.44 0.04 117 0.01 0.07 0.22 0.49 1 73 1995 WINTER 145 0.33 .00500 1.90 0.36 0.03 108 0.03 0.14 0.23 0.36 1 73 1996 FALL 137 0.25 .00500 3.75 0.44 0.04 174 0.01 0.05 0.13 0.25 0 73 1996 SPRING 144 0.47 .00500 3.32 0.47 0.04 101 0.05 0.18 0.32 0.55 1 73 1996 SUMMER 104 0.51 .00500 2.80 0.64 0.06 126 0.02 0.11 0.25 0.51 1 73 1996 WINTER 142 0.43 .00500 6.33 0.67 0.06 156 0.03 0.14 0.24 0.42 1 73 1997 FALL 94 0.28 .00250 2.12 0.36 0.04 128 0.00 0.05 0.13 0.40 0 73 1997 SPRING 105 0.36 .00500 2.96 0.47 0.05 130 0.03 0.12 0.20 0.38 1
Appendix B—Descriptive Statistics Data Tables for Level III Subecoregions Within Aggregate Ec

 Aggregate Nutrient Ecoregion: X  Rivers and Streams 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  73 1997 SUMMER 102 0.46 .01000 3.87 0.54 0.05 116 0.04 0.17 0.26 0.56 1 73 1997 WINTER 102 0.32 .00500 3.57 0.48 0.05 149 0.01 0.10 0.20 0.31 1 73 1998 SPRING 48 0.38 .02000 2.09 0.51 0.07 134 0.07 0.10 0.16 0.37 1 73 1998 WINTER 69 0.36 .02000 3.81 0.65 0.08 178 0.02 0.08 0.14 0.27 1 73 1999 FALL 4 0.00 .00000 0.00 0.00 0.00 . 0.00 0.00 0.00 0.00 0 73 1999 SUMMER 6 0.03 .00000 0.07 0.03 0.01 94 0.00 0.00 0.03 0.07 0 73 2000 SPRING 3 0.11 .00000 0.32 0.18 0.11 173 0.00 0.00 0.00 0.32 0
Appendix B—Descriptive Statistics Data Tables for Level III Subecoregions Within Aggregate Ec

 Aggregate Nutrient Ecoregion: X  Rivers and Streams Descriptive Statistics by Subecoregion, Year and Season from 1990 to 1998 Nitrogen_Tot_Kjeldhal_mg_L subecoregion year season N MEAN MIN MAX STDDEV STDERR CV P5 P25 MEDIAN P75  34 1990 FALL 28 0.89 .25000 2.15 0.40 0.08 46 0.35 0.62 0.83 1.14 1 34 1990 SPRING 26 1.12 .25000 2.90 0.64 0.13 57 0.40 0.60 1.06 1.56 1 34 1990 SUMMER 31 1.04 .10000 1.90 0.39 0.07 37 0.40 0.80 1.00 1.35 1 34 1990 WINTER 28 1.20 .40000 2.60 0.55 0.10 46 0.46 0.79 1.15 1.38 2 34 1991 FALL 15 0.94 .30000 1.80 0.46 0.12 50 0.30 0.65 0.87 1.35 1 34 1991 SPRING 34 1.20 .35000 2.40 0.45 0.08 37 0.50 0.84 1.22 1.43 2 34 1991 SUMMER 32 1.20 .75000 1.76 0.26 0.05 22 0.90 1.00 1.20 1.38 1 34 1991 WINTER 19 1.08 .59500 2.00 0.44 0.10 41 0.60 0.70 0.82 1.50 2 34 1992 FALL 19 0.68 .20000 1.30 0.34 0.08 49 0.20 0.36 0.71 0.90 1 34 1992 SPRING 32 1.04 .30000 2.07 0.50 0.09 48 0.40 0.60 0.98 1.43 1 34 1992 SUMMER 33 0.85 .05000 1.75 0.40 0.07 48 0.30 0.54 0.85 1.05 1 34 1992 WINTER 31 1.06 .50000 3.80 0.62 0.11 59 0.50 0.70 0.94 1.14 2 34 1993 FALL 42 0.80 .05000 1.83 0.32 0.05 41 0.37 0.62 0.79 0.96 1 34 1993 SPRING 33 0.98 .40000 2.12 0.43 0.07 43 0.40 0.75 0.97 1.10 1 34 1993 SUMMER 40 0.93 .05000 3.54 0.62 0.10 66 0.13 0.50 0.84 1.17 1 34 1993 WINTER 38 1.06 .26000 3.21 0.59 0.10 55 0.27 0.66 1.00 1.28 2 34 1994 FALL 34 0.81 .40000 1.40 0.23 0.04 29 0.50 0.62 0.80 0.98 1 34 1994 SPRING 42 1.19 .39000 3.02 0.57 0.09 48 0.40 0.76 1.07 1.47 2 34 1994 SUMMER 33 1.01 .30000 2.38 0.45 0.08 44 0.40 0.77 0.96 1.17 1 34 1994 WINTER 39 1.11 .20000 1.88 0.37 0.06 33 0.50 0.86 1.02 1.44 1 34 1995 FALL 37 0.82 .26000 1.60 0.31 0.05 38 0.30 0.58 0.82 1.01 1 34 1995 SPRING 40 1.25 .30000 2.85 0.62 0.10 50 0.44 0.77 1.20 1.66 2 34 1995 SUMMER 41 0.71 .05000 1.44 0.36 0.06 50 0.11 0.49 0.75 0.94 1 34 1995 WINTER 37 0.99 .32000 1.83 0.41 0.07 42 0.34 0.65 0.93 1.33 1 34 1996 FALL 19 1.11 .65000 1.86 0.32 0.07 29 0.65 0.87 1.06 1.19 1 34 1996 SPRING 34 1.21 .32000 2.71 0.65 0.11 54 0.48 0.79 1.06 1.47 2 34 1996 SUMMER 36 1.31 .44000 2.79 0.58 0.10 45 0.47 0.85 1.24 1.69 2 34 1996 WINTER 31 0.85 .11000 1.73 0.39 0.07 46 0.17 0.60 0.83 1.17 1 34 1997 FALL 13 0.83 .39000 1.72 0.36 0.10 44 0.39 0.57 0.66 1.00 1 34 1997 SPRING 16 1.99 .85000 4.08 0.88 0.22 44 0.85 1.40 1.85 2.46 4 34 1997 SUMMER 13 1.09 .53000 1.99 0.42 0.12 38 0.53 0.85 1.09 1.21 1 34 1997 WINTER 19 1.23 .77000 1.77 0.27 0.06 22 0.77 1.00 1.24 1.49 1
Appendix B—Descriptive Statistics Data Tables for Level III Subecoregions Within Aggregate Ec

 Aggregate Nutrient Ecoregion: X  Rivers and Streams Descriptive Statistics by Subecoregion, Year and Season from 1990 to 1998 Nitrogen_Tot_Kjeldhal_mg_L subecoregion year season N MEAN MIN MAX STDDEV STDERR CV P5 P25 MEDIAN P75  34 1998 SPRING 13 1.66 .49000 2.98 0.68 0.19 41 0.49 1.32 1.46 2.10 2 34 1998 WINTER 13 1.34 .54000 2.29 0.51 0.14 38 0.54 0.93 1.31 1.67 2 73 1990 FALL 100 1.02 .07500 3.82 0.62 0.06 61 0.38 0.64 0.87 1.26 2 73 1990 SPRING 99 1.00 .30000 2.20 0.37 0.04 37 0.50 0.74 0.93 1.20 1 73 1990 SUMMER 100 1.04 .10000 3.80 0.53 0.05 51 0.52 0.74 0.87 1.23 2 73 1990 WINTER 87 1.12 .17500 2.70 0.46 0.05 41 0.63 0.81 0.98 1.31 2 73 1991 FALL 37 0.65 .07500 1.10 0.22 0.04 34 0.18 0.50 0.66 0.80 1 73 1991 SPRING 97 0.98 .30000 2.60 0.46 0.05 47 0.50 0.69 0.83 1.10 2 73 1991 SUMMER 99 0.99 .05000 1.91 0.32 0.03 32 0.54 0.75 0.94 1.19 1 73 1991 WINTER 47 0.98 .20000 2.45 0.49 0.07 50 0.29 0.71 0.90 1.05 2 73 1992 FALL 57 0.86 .07500 3.15 0.58 0.08 68 0.20 0.50 0.70 0.93 2 73 1992 SPRING 100 0.89 .20000 2.45 0.42 0.04 47 0.42 0.65 0.79 0.99 1 73 1992 SUMMER 106 1.01 .10000 2.90 0.51 0.05 50 0.39 0.70 0.89 1.28 2 73 1992 WINTER 109 0.92 .07500 5.09 0.62 0.06 67 0.35 0.59 0.76 1.11 1 73 1993 FALL 94 0.82 .37000 2.34 0.34 0.03 41 0.48 0.61 0.76 0.93 1 73 1993 SPRING 99 0.87 .25000 2.80 0.35 0.03 40 0.47 0.64 0.79 1.06 1 73 1993 SUMMER 102 0.87 .07500 1.90 0.36 0.04 41 0.40 0.62 0.84 1.08 1 73 1993 WINTER 108 0.84 .02500 2.10 0.38 0.04 45 0.40 0.59 0.80 1.01 1 73 1994 FALL 86 0.74 .29000 2.75 0.38 0.04 51 0.37 0.51 0.61 0.86 1 73 1994 SPRING 92 0.94 .15000 2.61 0.44 0.05 47 0.42 0.62 0.80 1.17 1 73 1994 SUMMER 92 0.92 .07500 2.88 0.41 0.04 44 0.52 0.68 0.84 1.11 1 73 1994 WINTER 95 0.93 .30000 4.30 0.51 0.05 54 0.43 0.62 0.80 1.13 1 73 1995 FALL 84 0.99 .31000 2.61 0.50 0.05 51 0.42 0.64 0.83 1.23 2 73 1995 SPRING 91 1.16 .39000 3.80 0.60 0.06 52 0.58 0.82 1.04 1.28 2 73 1995 SUMMER 91 0.91 .26000 2.95 0.43 0.05 47 0.39 0.65 0.77 1.11 1 73 1995 WINTER 89 0.91 .21000 3.84 0.52 0.06 58 0.42 0.61 0.76 1.00 1 73 1996 FALL 91 0.90 .22000 2.39 0.48 0.05 53 0.33 0.58 0.75 1.14 2 73 1996 SPRING 95 1.12 .21000 5.20 0.76 0.08 68 0.50 0.70 0.83 1.40 2 73 1996 SUMMER 90 1.17 .09000 3.56 0.71 0.08 61 0.49 0.76 0.95 1.36 2 73 1996 WINTER 93 0.87 .19000 2.64 0.47 0.05 54 0.29 0.54 0.75 1.09 1 73 1997 FALL 89 0.81 .01000 3.66 0.52 0.05 64 0.23 0.50 0.70 1.03 1 73 1997 SPRING 85 0.99 .30000 2.23 0.38 0.04 38 0.56 0.71 0.94 1.20 1
Appendix B—Descriptive Statistics Data Tables for Level III Subecoregions Within Aggregate Ec

 Aggregate Nutrient Ecoregion: X  Rivers and Streams Descriptive Statistics by Subecoregion, Year and Season from 1990 to 1998 Nitrogen_Tot_Kjeldhal_mg_L subecoregion year season N MEAN MIN MAX STDDEV STDERR CV P5 P25 MEDIAN P75  73 1997 SUMMER 80 1.06 .17000 3.31 0.53 0.06 50 0.48 0.70 0.95 1.28 2 73 1997 WINTER 101 0.98 .01000 2.38 0.45 0.04 46 0.44 0.69 0.87 1.15 1 73 1998 SPRING 48 0.96 .56000 1.93 0.31 0.04 32 0.59 0.75 0.88 1.11 1 73 1998 WINTER 69 1.04 .14000 3.31 0.47 0.06 45 0.35 0.77 1.01 1.22 1
Appendix B—Descriptive Statistics Data Tables for Level III Subecoregions Within Aggregate Ec

 Aggregate Nutrient Ecoregion: X  Rivers and Streams Descriptive Statistics by Subecoregion, Year and Season from 1999 to 2000 Organic_P_ug_L subecoregion year season N MEAN MIN MAX STDDEV STDERR CV P5 P25 MEDIAN P75 P 73 1999 FALL 4 30.51 11.920 49.09 21.46 10.73 70 11.92 11.92 30.51 49.09 49. 73 1999 SUMMER 6 45.32 25.480 61.37 16.32 6.66 36 25.48 25.48 49.12 61.37 61. 73 2000 SPRING 3 197.51 38.300 383.16 173.94 100.43 88 38.30 38.30 171.08 383.16 383.
Appendix B—Descriptive Statistics Data Tables for Level III Subecoregions Within Aggregate Ec

 Aggregate Nutrient Ecoregion: X  Rivers and Streams Descriptive Statistics by Subecoregion, Year and Season from 1990 to 1995 Phosph_Ortho_Tot_as_P_ug_L subecoregion year season N MEAN MIN MAX STDDEV STDERR CV P5 P25 MEDIAN P75  34 1990 FALL 9 176.11 5.0000 590.00 199.78 66.59 113 5.00 50.00 130.00 130.00 590 34 1990 WINTER 1 110.00 110.00 110.00 . . . 110.00 110.00 110.00 110.00 110 34 1991 FALL 6 146.25 12.500 390.00 141.98 57.96 97 12.50 45.00 107.50 215.00 390 34 1991 SPRING 9 155.00 10.000 485.00 151.69 50.56 98 10.00 12.50 170.00 215.00 485 34 1991 SUMMER 7 532.14 30.000 2700.00 963.88 364.31 181 30.00 90.00 160.00 420.00 270 34 1991 WINTER 7 470.71 55.000 1800.00 622.60 235.32 132 55.00 60.00 290.00 660.00 180 34 1992 FALL 7 101.43 10.000 380.00 134.47 50.82 133 10.00 10.00 50.00 170.00 380 34 1992 SPRING 8 126.25 20.000 430.00 128.86 45.56 102 20.00 57.50 92.50 130.00 430 34 1992 SUMMER 9 110.56 30.000 325.00 105.70 35.23 96 30.00 40.00 55.00 120.00 325 34 1992 WINTER 9 174.56 30.000 510.00 175.43 58.48 101 30.00 60.00 90.00 171.00 510 73 1990 FALL 16 74.69 5.0000 260.00 54.57 13.64 73 5.00 50.00 65.00 85.00 260 73 1990 SPRING 3 126.67 100.00 160.00 30.55 17.64 24 100.00 100.00 120.00 160.00 160 73 1990 SUMMER 3 73.33 30.000 120.00 45.09 26.03 61 30.00 30.00 70.00 120.00 120 73 1990 WINTER 10 74.50 10.000 150.00 44.75 14.15 60 10.00 55.00 75.00 110.00 150 73 1991 FALL 17 72.35 5.0000 150.00 33.31 8.08 46 5.00 55.00 80.00 80.00 150 73 1991 SPRING 20 103.50 40.000 280.00 55.92 12.50 54 45.00 65.00 95.00 135.00 220 73 1991 SUMMER 17 97.21 12.500 200.00 50.64 12.28 52 12.50 55.00 95.00 130.00 200 73 1991 WINTER 18 95.83 15.000 190.00 46.69 11.01 49 15.00 65.00 87.50 120.00 190 73 1992 FALL 14 79.18 30.000 170.00 44.27 11.83 56 30.00 40.00 71.75 100.00 170 73 1992 SPRING 15 72.33 15.000 185.00 39.64 10.23 55 15.00 50.00 70.00 90.00 185 73 1992 SUMMER 20 99.25 20.000 180.00 36.39 8.14 37 45.00 80.00 90.00 117.50 175 73 1992 WINTER 18 93.89 10.000 180.00 45.65 10.76 49 10.00 70.00 95.00 130.00 180 73 1993 FALL 6 95.17 41.000 140.00 38.96 15.91 41 41.00 65.00 100.00 125.00 140 73 1993 SPRING 3 84.00 50.000 102.00 29.46 17.01 35 50.00 50.00 100.00 102.00 102 73 1993 SUMMER 6 127.50 30.000 280.00 85.07 34.73 67 30.00 70.00 125.00 135.00 280 73 1993 WINTER 3 74.00 60.000 82.00 12.17 7.02 16 60.00 60.00 80.00 82.00 82 73 1994 FALL 7 63.00 15.000 100.00 29.41 11.11 47 15.00 36.00 65.00 90.00 100 73 1994 SPRING 7 63.79 15.000 100.00 29.05 10.98 46 15.00 45.00 60.00 90.00 100 73 1994 SUMMER 8 62.75 15.000 90.00 29.53 10.44 47 15.00 36.00 77.50 85.00 90 73 1994 WINTER 7 95.00 45.000 180.00 44.44 16.80 47 45.00 70.00 80.00 120.00 180 73 1995 FALL 2 40.00 40.000 40.00 0.00 0.00 0 40.00 40.00 40.00 40.00 40 73 1995 SPRING 5 99.00 70.000 130.00 28.81 12.88 29 70.00 70.00 100.00 125.00 130
Appendix B—Descriptive Statistics Data Tables for Level III Subecoregions Within Aggregate Ec

 Aggregate Nutrient Ecoregion: X  Rivers and Streams Descriptive Statistics by Subecoregion, Year and Season from 1990 to 1995 Phosph_Ortho_Tot_as_P_ug_L subecoregion year season N MEAN MIN MAX STDDEV STDERR CV P5 P25 MEDIAN P75 P 73 1995 SUMMER 6 61.67 40.000 90.00 20.41 8.33 33 40.00 40.00 60.00 80.00 90. 73 1995 WINTER 4 81.25 65.000 100.00 14.36 7.18 18 65.00 72.50 80.00 90.00 100.
Appendix B—Descriptive Statistics Data Tables for Level III Subecoregions Within Aggregate Ec

 Aggregate Nutrient Ecoregion: X  Rivers and Streams Descriptive Statistics by Subecoregion, Year and Season from 1990 to 2000 Total_Nitrogen_mg_L subecoregion year season N MEAN MIN MAX STDDEV STDERR CV P5 P25 MEDIAN P75  34 1990 FALL 9 1.43 .35000 5.70 1.69 0.56 118 0.35 0.60 0.80 1.10 5 34 1990 SPRING 9 1.73 .40000 3.80 1.12 0.37 65 0.40 0.95 1.10 2.30 3 34 1990 SUMMER 7 1.79 .60000 4.20 1.65 0.62 92 0.60 0.80 0.95 4.20 4 34 1990 WINTER 7 2.79 .40000 7.30 2.55 0.96 91 0.40 1.10 2.00 5.30 7 34 1991 FALL 6 1.35 .35000 4.05 1.35 0.55 100 0.35 0.74 0.90 1.15 4 34 1991 SPRING 9 1.84 .50000 5.25 1.36 0.45 74 0.50 1.40 1.55 1.80 5 34 1991 SUMMER 7 2.40 1.0500 7.50 2.41 0.91 101 1.05 1.10 1.20 3.50 7 34 1991 WINTER 7 2.89 .90000 6.60 2.19 0.83 76 0.90 1.15 2.10 5.35 6 34 1992 FALL 7 1.29 .30000 4.40 1.42 0.54 110 0.30 0.30 0.90 1.30 4 34 1992 SPRING 8 1.77 .58000 5.20 1.48 0.52 84 0.58 1.02 1.28 1.90 5 34 1992 SUMMER 8 1.18 .37000 3.95 1.17 0.41 99 0.37 0.48 0.89 1.20 3 34 1992 WINTER 9 1.43 .67000 4.50 1.19 0.40 83 0.67 0.91 1.00 1.20 4 34 1993 FALL 5 1.50 .40000 4.80 1.86 0.83 124 0.40 0.50 0.80 1.00 4 34 1993 SPRING 8 1.80 .40000 6.35 1.89 0.67 105 0.40 0.85 1.33 1.65 6 34 1993 SUMMER 5 1.55 .68000 3.95 1.36 0.61 88 0.68 0.88 1.10 1.12 3 34 1993 WINTER 3 1.40 1.4000 1.40 0.00 0.00 0 1.40 1.40 1.40 1.40 1 34 1994 FALL 4 1.85 .50000 5.00 2.11 1.06 114 0.50 0.65 0.95 3.05 5 34 1994 SPRING 5 2.00 .60000 4.65 1.63 0.73 81 0.60 0.95 1.40 2.40 4 34 1994 SUMMER 5 1.28 .40000 3.40 1.24 0.55 97 0.40 0.40 1.00 1.20 3 34 1994 WINTER 4 2.50 1.0000 5.70 2.16 1.08 87 1.00 1.20 1.65 3.80 5 34 1995 FALL 2 0.64 .40000 0.87 0.33 0.24 52 0.40 0.40 0.64 0.87 0 34 1995 SPRING 4 2.30 .65000 5.65 2.27 1.14 99 0.65 0.95 1.45 3.65 5 34 1995 SUMMER 4 1.65 .70500 3.70 1.38 0.69 84 0.71 0.85 1.10 2.45 3 34 1995 WINTER 2 3.50 .80000 6.20 3.82 2.70 109 0.80 0.80 3.50 6.20 6 73 1990 FALL 18 1.31 .40000 3.20 0.79 0.19 60 0.40 0.70 1.08 2.00 3 73 1990 SPRING 17 1.23 .45000 2.40 0.49 0.12 40 0.45 0.90 1.20 1.40 2 73 1990 SUMMER 18 1.64 .30000 3.30 1.01 0.24 62 0.30 0.95 1.20 2.80 3 73 1990 WINTER 17 1.27 .45000 2.10 0.49 0.12 39 0.45 1.00 1.20 1.55 2 73 1991 FALL 15 1.01 .47000 1.50 0.33 0.09 33 0.47 0.74 0.94 1.30 1 73 1991 SPRING 19 1.70 .67000 3.60 0.87 0.20 51 0.67 0.92 1.45 2.30 3 73 1991 SUMMER 19 1.87 .61500 4.30 1.12 0.26 60 0.62 0.98 1.50 2.40 4 73 1991 WINTER 17 1.34 .59000 2.45 0.54 0.13 40 0.59 0.97 1.30 1.50 2
Appendix B—Descriptive Statistics Data Tables for Level III Subecoregions Within Aggregate Ec

 Aggregate Nutrient Ecoregion: X  Rivers and Streams Descriptive Statistics by Subecoregion, Year and Season from 1990 to 2000 Total_Nitrogen_mg_L subecoregion year season N MEAN MIN MAX STDDEV STDERR CV P5 P25 MEDIAN P75  73 1992 FALL 17 1.10 .20000 2.00 0.63 0.15 57 0.20 0.57 0.80 1.70 2 73 1992 SPRING 15 1.36 .55000 3.35 0.91 0.23 67 0.55 0.74 0.81 2.00 3 73 1992 SUMMER 19 1.26 .60000 2.15 0.50 0.11 39 0.60 0.83 1.10 1.65 2 73 1992 WINTER 17 1.48 .56000 2.80 0.80 0.19 54 0.56 0.90 1.30 2.30 2 73 1993 FALL 13 1.53 .71000 2.40 0.63 0.18 41 0.71 1.10 1.30 2.20 2 73 1993 SPRING 16 1.34 .58000 2.70 0.58 0.14 43 0.58 0.96 1.10 1.80 2 73 1993 SUMMER 17 1.65 .44500 3.40 0.98 0.24 59 0.45 0.80 1.62 2.55 3 73 1993 WINTER 19 1.32 .49000 2.50 0.66 0.15 50 0.49 0.72 1.10 1.80 2 73 1994 FALL 5 1.10 .82000 1.30 0.19 0.09 17 0.82 1.00 1.20 1.20 1 73 1994 SPRING 13 1.22 .47000 2.40 0.55 0.15 45 0.47 0.80 1.00 1.50 2 73 1994 SUMMER 14 1.29 .70000 2.20 0.49 0.13 38 0.70 0.86 1.25 1.70 2 73 1994 WINTER 13 1.26 .57000 2.20 0.55 0.15 43 0.57 0.83 1.20 1.70 2 73 1995 FALL 3 0.63 .51000 0.74 0.12 0.07 18 0.51 0.51 0.64 0.74 0 73 1995 SPRING 7 1.53 .58000 3.00 0.84 0.32 55 0.58 0.88 1.40 2.20 3 73 1995 SUMMER 8 1.32 .40000 2.30 0.69 0.24 52 0.40 0.73 1.30 1.90 2 73 1995 WINTER 6 1.14 .74500 1.80 0.41 0.17 36 0.75 0.81 1.04 1.40 1 73 1999 FALL 4 0.07 .03000 0.10 0.04 0.02 62 0.03 0.03 0.07 0.10 0 73 1999 SUMMER 6 0.27 .21000 0.38 0.08 0.03 30 0.21 0.21 0.23 0.38 0 73 2000 SPRING 3 0.56 .10000 1.09 0.50 0.29 88 0.10 0.10 0.50 1.09 1
Appendix B—Descriptive Statistics Data Tables for Level III Subecoregions Within Aggregate Ec

 Aggregate Nutrient Ecoregion: X  Rivers and Streams 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  34 1990 FALL 38 334.08 20.000 1500.00 325.30 52.77 97 45.00 140.00 235.00 410.00 102 34 1990 SPRING 39 685.64 50.000 12000.0 1907.1 305.38 278 50.00 130.00 280.00 470.00 186 34 1990 SUMMER 43 449.42 50.000 2300.00 557.26 84.98 124 80.00 145.00 270.00 530.00 200 34 1990 WINTER 35 500.14 40.000 2000.00 544.42 92.02 109 50.00 170.00 260.00 770.00 191 34 1991 FALL 33 465.53 17.500 2250.00 474.29 82.56 102 40.00 140.00 355.00 655.00 172 34 1991 SPRING 45 434.00 40.000 1680.00 349.85 52.15 81 70.00 170.00 340.00 615.00 119 34 1991 SUMMER 48 431.15 30.000 2700.00 520.08 75.07 121 50.00 160.00 272.50 410.00 160 34 1991 WINTER 33 600.45 50.000 2400.00 649.41 113.05 108 50.00 190.00 370.00 690.00 230 34 1992 FALL 30 411.83 30.000 2290.00 548.37 100.12 133 30.00 90.00 235.00 510.00 213 34 1992 SPRING 49 370.71 60.000 1800.00 368.45 52.64 99 65.00 150.00 300.00 390.00 152 34 1992 SUMMER 51 340.69 20.000 1830.00 359.85 50.39 106 60.00 125.00 200.00 380.00 120 34 1992 WINTER 47 342.45 40.000 1740.00 306.43 44.70 89 60.00 170.00 245.00 410.00 895 34 1993 FALL 42 269.52 50.000 1340.00 285.16 44.00 106 60.00 120.00 170.00 240.00 930 34 1993 SPRING 33 306.52 40.000 1010.00 217.24 37.82 71 60.00 155.00 240.00 400.00 905 34 1993 SUMMER 40 338.75 50.000 1920.00 464.28 73.41 137 50.00 100.00 200.00 310.00 171 34 1993 WINTER 38 343.42 60.000 1565.00 310.70 50.40 90 60.00 180.00 255.00 420.00 140 34 1994 FALL 36 222.22 30.000 1650.00 267.46 44.58 120 50.00 85.00 170.00 240.00 440 34 1994 SPRING 44 327.84 2.5000 1650.00 300.23 45.26 92 50.00 120.00 285.00 420.00 940 34 1994 SUMMER 37 238.24 30.000 1210.00 218.58 35.93 92 50.00 90.00 180.00 280.00 580 34 1994 WINTER 37 277.03 10.000 1200.00 277.98 45.70 100 30.00 110.00 190.00 310.00 116 34 1995 FALL 35 220.86 30.000 1500.00 304.09 51.40 138 30.00 80.00 130.00 190.00 101 34 1995 SPRING 38 269.41 2.5000 1000.00 206.43 33.49 77 40.00 130.00 237.50 370.00 690 34 1995 SUMMER 41 184.02 2.5000 630.00 141.95 22.17 77 30.00 90.00 145.00 250.00 460 34 1995 WINTER 36 237.50 30.000 940.00 202.73 33.79 85 40.00 100.00 200.00 310.00 800 34 1996 FALL 19 272.37 100.00 480.00 118.08 27.09 43 100.00 190.00 250.00 390.00 480 34 1996 SPRING 34 381.32 40.000 2125.00 485.06 83.19 127 50.00 100.00 177.50 420.00 155 34 1996 SUMMER 35 194.21 2.5000 520.00 132.13 22.33 68 40.00 100.00 150.00 270.00 490 34 1996 WINTER 31 326.29 20.000 2110.00 474.04 85.14 145 30.00 90.00 160.00 280.00 143 34 1997 FALL 13 155.77 80.000 240.00 57.80 16.03 37 80.00 100.00 150.00 200.00 240 34 1997 SPRING 16 413.13 65.000 870.00 205.02 51.25 50 65.00 300.00 412.50 482.50 870 34 1997 SUMMER 13 198.08 80.000 310.00 76.69 21.27 39 80.00 130.00 180.00 270.00 310 34 1997 WINTER 19 232.89 30.000 720.00 151.06 34.66 65 30.00 170.00 200.00 270.00 720
Appendix B—Descriptive Statistics Data Tables for Level III Subecoregions Within Aggregate Ec

 Aggregate Nutrient Ecoregion: X  Rivers and Streams 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  34 1998 SPRING 13 252.69 70.000 520.00 147.12 40.80 58 70.00 150.00 210.00 330.00 520 34 1998 WINTER 13 188.08 60.000 400.00 96.15 26.67 51 60.00 120.00 185.00 265.00 400 73 1990 FALL 125 197.26 10.000 1430.00 174.04 15.57 88 60.00 110.00 150.00 240.00 420 73 1990 SPRING 126 246.12 20.000 660.00 147.24 13.12 60 50.00 120.00 230.00 350.00 540 73 1990 SUMMER 125 225.54 30.000 1810.00 187.95 16.81 83 60.00 130.00 200.00 270.00 450 73 1990 WINTER 111 278.46 3.7500 1400.00 225.78 21.43 81 60.00 120.00 225.00 340.00 670 73 1991 FALL 66 171.48 2.5000 1700.00 223.49 27.51 130 45.00 100.00 140.00 175.00 250 73 1991 SPRING 125 259.57 16.250 1370.00 225.80 20.20 87 80.00 130.00 210.00 300.00 700 73 1991 SUMMER 127 221.85 10.000 1640.00 219.77 19.50 99 60.00 120.00 180.00 260.00 410 73 1991 WINTER 80 234.13 40.000 720.00 142.68 15.95 61 85.00 145.00 200.00 290.00 580 73 1992 FALL 86 174.28 22.500 1100.00 152.30 16.42 87 50.00 90.00 130.00 190.00 410 73 1992 SPRING 129 202.50 20.000 730.00 141.86 12.49 70 40.00 100.00 170.00 260.00 470 73 1992 SUMMER 134 235.99 2.5000 1800.00 202.76 17.52 86 60.00 140.00 187.50 260.00 610 73 1992 WINTER 139 210.30 26.250 970.00 154.20 13.08 73 60.00 105.00 170.00 260.00 480 73 1993 FALL 124 218.76 2.5000 1650.00 239.59 21.52 110 45.00 105.00 160.00 250.00 475 73 1993 SPRING 123 233.29 20.000 1420.00 204.50 18.44 88 45.00 105.00 190.00 300.00 510 73 1993 SUMMER 142 203.24 2.5000 1400.00 192.54 16.16 95 50.00 100.00 167.50 250.00 510 73 1993 WINTER 140 211.45 2.5000 940.00 130.19 11.00 62 60.00 120.00 200.00 270.00 425 73 1994 FALL 147 196.13 2.5000 1710.00 218.08 17.99 111 35.00 90.00 140.00 230.00 530 73 1994 SPRING 120 235.25 2.5000 1430.00 206.71 18.87 88 30.00 120.00 170.00 307.50 645 73 1994 SUMMER 152 243.41 3.7500 860.00 160.50 13.02 66 60.00 135.00 205.00 312.50 570 73 1994 WINTER 123 230.47 21.250 2070.00 254.58 22.95 110 60.00 100.00 160.00 260.00 580 73 1995 FALL 139 179.42 5.0000 1735.00 205.18 17.40 114 40.00 80.00 130.00 190.00 610 73 1995 SPRING 151 260.97 20.000 1900.00 244.52 19.90 94 40.00 135.00 200.00 320.00 610 73 1995 SUMMER 149 200.43 20.000 1580.00 176.30 14.44 88 45.00 100.00 165.00 240.00 485 73 1995 WINTER 147 187.87 2.5000 1190.00 154.21 12.72 82 40.00 100.00 160.00 220.00 425 73 1996 FALL 142 198.59 5.0000 1865.00 202.02 16.95 102 60.00 100.00 150.00 220.00 425 73 1996 SPRING 139 242.01 5.0000 2300.00 269.03 22.82 111 30.00 90.00 150.00 335.00 710 73 1996 SUMMER 111 221.82 2.5000 2350.00 263.20 24.98 119 40.00 105.00 170.00 240.00 570 73 1996 WINTER 147 239.06 5.0000 950.00 176.89 14.59 74 50.00 110.00 190.00 315.00 610 73 1997 FALL 94 177.61 10.000 1680.00 194.92 20.10 110 60.00 100.00 127.50 195.00 455 73 1997 SPRING 106 219.20 5.0000 1220.00 166.33 16.16 76 70.00 110.00 187.50 260.00 490
Appendix B—Descriptive Statistics Data Tables for Level III Subecoregions Within Aggregate Ec

 Aggregate Nutrient Ecoregion: X  Rivers and Streams 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  73 1997 SUMMER 101 226.68 50.000 1000.00 165.70 16.49 73 70.00 120.00 180.00 260.00 525 73 1997 WINTER 104 260.24 5.0000 2020.00 268.80 26.36 103 50.00 112.50 180.00 277.50 695 73 1998 SPRING 47 176.81 5.0000 750.00 119.24 17.39 67 40.00 100.00 170.00 210.00 355 73 1998 WINTER 69 220.58 40.000 870.00 129.66 15.61 59 80.00 130.00 190.00 300.00 390 73 1999 FALL 4 188.56 186.99 190.12 1.81 0.90 1 186.99 186.99 188.56 190.12 190 73 1999 SUMMER 6 192.49 107.82 296.14 85.49 34.90 44 107.82 107.82 173.50 296.14 296 73 2000 SPRING 3 319.41 205.35 509.16 165.44 95.52 52 205.35 205.35 243.73 509.16 509
Appendix B—Descriptive Statistics Data Tables for Level III Subecoregions Within Aggregate Ec

 Aggregate Nutrient Ecoregion: X  Rivers and Streams Descriptive Statistics by Subecoregion, Year and Season from 1990 to 1997 Turbidity_FTU subecoregion year season N MEAN MIN MAX STDDEV STDERR CV P5 P25 MEDIAN P75  34 1990 FALL 10 34.25 6.4500 120.00 45.78 14.48 134 6.45 7.00 10.75 29.00 120 34 1990 SPRING 8 40.86 9.1000 130.00 38.75 13.70 95 9.10 18.00 30.00 45.88 130 34 1990 SUMMER 10 19.66 1.6500 56.50 15.70 4.96 80 1.65 11.60 16.50 25.00 56 34 1990 WINTER 11 31.85 2.4000 75.00 22.19 6.69 70 2.40 16.00 24.00 47.00 75 34 1991 FALL 9 32.71 3.9000 113.50 36.39 12.13 111 3.90 7.85 17.00 49.00 113 34 1991 SPRING 14 41.25 3.1500 108.00 40.01 10.69 97 3.15 7.35 24.25 85.00 108 34 1991 SUMMER 13 36.19 9.0000 130.00 33.94 9.41 94 9.00 11.00 28.50 36.00 130 34 1991 WINTER 12 45.25 10.000 125.00 34.34 9.91 76 10.00 19.50 32.25 70.00 125 34 1992 FALL 7 22.93 3.0000 61.00 24.83 9.39 108 3.00 3.10 15.00 56.00 61 34 1992 SPRING 11 58.52 2.4000 120.00 42.66 12.86 73 2.40 16.00 52.00 98.00 120 34 1992 SUMMER 14 29.18 4.5000 99.50 23.36 6.24 80 4.50 15.00 24.50 36.00 99 34 1992 WINTER 14 71.79 24.000 150.00 34.67 9.26 48 24.00 42.00 70.00 97.00 150 34 1993 FALL 6 28.75 6.2000 75.00 30.65 12.51 107 6.20 6.30 12.50 60.00 75 34 1993 SPRING 9 56.77 6.7500 110.00 35.86 11.95 63 6.75 27.70 61.00 88.50 110 34 1993 SUMMER 9 39.98 .40000 100.00 37.66 12.55 94 0.40 12.00 21.00 71.00 100 34 1993 WINTER 4 46.00 27.000 76.00 21.02 10.51 46 27.00 33.00 40.50 59.00 76 34 1994 FALL 4 31.30 1.2000 92.00 41.58 20.79 133 1.20 4.60 16.00 58.00 92 34 1994 SPRING 5 57.30 14.500 102.00 37.20 16.64 65 14.50 25.00 62.50 82.50 102 34 1994 SUMMER 5 36.44 6.2000 88.50 32.84 14.69 90 6.20 13.50 28.00 46.00 88 34 1994 WINTER 4 49.28 1.1000 100.00 47.25 23.62 96 1.10 9.55 48.00 89.00 100 34 1995 FALL 3 17.77 7.3000 25.00 9.28 5.36 52 7.30 7.30 21.00 25.00 25 34 1995 SPRING 4 74.65 18.000 130.00 51.57 25.79 69 18.00 32.05 75.30 117.25 130 34 1995 SUMMER 4 54.38 22.500 93.00 35.50 17.75 65 22.50 24.25 51.00 84.50 93 34 1995 WINTER 2 57.00 25.000 89.00 45.25 32.00 79 25.00 25.00 57.00 89.00 89 34 1996 FALL 2 58.38 41.250 75.50 24.22 17.13 41 41.25 41.25 58.38 75.50 75 34 1996 SPRING 2 64.75 30.500 99.00 48.44 34.25 75 30.50 30.50 64.75 99.00 99 34 1996 SUMMER 2 54.75 22.500 87.00 45.61 32.25 83 22.50 22.50 54.75 87.00 87 34 1997 WINTER 2 52.00 29.000 75.00 32.53 23.00 63 29.00 29.00 52.00 75.00 75 73 1990 FALL 38 35.91 2.9000 150.00 28.67 4.65 80 4.60 14.50 26.75 55.00 78 73 1990 SPRING 35 60.77 6.8000 150.00 42.47 7.18 70 18.00 25.00 46.00 100.00 140 73 1990 SUMMER 20 36.47 9.3500 93.00 21.89 4.90 60 9.43 22.75 33.50 41.00 83 73 1990 WINTER 47 68.23 3.5000 180.00 50.83 7.41 75 5.70 24.50 55.00 97.50 170
Appendix B—Descriptive Statistics Data Tables for Level III Subecoregions Within Aggregate Ec

 Aggregate Nutrient Ecoregion: X  Rivers and Streams Descriptive Statistics by Subecoregion, Year and Season from 1990 to 1997 Turbidity_FTU subecoregion year season N MEAN MIN MAX STDDEV STDERR CV P5 P25 MEDIAN P75  73 1991 FALL 29 26.62 2.5000 77.50 14.75 2.74 55 5.00 17.50 25.00 36.00 42 73 1991 SPRING 32 62.47 2.9000 180.00 39.14 6.92 63 5.30 37.75 65.00 77.00 130 73 1991 SUMMER 32 33.56 3.1000 130.00 27.14 4.80 81 4.50 16.00 23.50 47.50 79 73 1991 WINTER 40 71.74 3.1000 164.00 42.79 6.77 60 4.83 27.00 80.00 100.00 138 73 1992 FALL 38 25.69 3.9000 93.00 19.17 3.11 75 4.10 13.00 21.00 36.00 61 73 1992 SPRING 33 56.12 5.8000 150.00 32.25 5.61 57 12.00 34.00 60.00 70.00 106 73 1992 SUMMER 39 52.91 11.000 140.00 31.77 5.09 60 12.00 27.00 58.00 70.00 120 73 1992 WINTER 30 69.03 2.5000 170.00 48.18 8.80 70 2.90 23.00 78.75 97.50 170 73 1993 FALL 20 66.43 5.0000 190.00 44.53 9.96 67 16.25 41.50 56.25 65.25 175 73 1993 SPRING 28 59.04 9.6000 165.00 33.91 6.41 57 11.15 35.23 56.25 79.75 110 73 1993 SUMMER 32 39.85 4.9000 110.00 35.21 6.22 88 5.00 11.58 22.00 76.00 98 73 1993 WINTER 34 66.13 7.5000 190.00 39.33 6.75 59 10.00 37.50 65.00 92.50 140 73 1994 FALL 12 15.56 2.5000 77.00 20.31 5.86 131 2.50 5.50 8.38 17.00 77 73 1994 SPRING 19 40.69 9.5000 90.00 22.59 5.18 56 9.50 23.00 36.00 50.00 90 73 1994 SUMMER 22 37.31 4.0000 150.00 35.20 7.51 94 4.10 16.00 26.58 41.00 94 73 1994 WINTER 21 55.23 2.5000 160.00 35.09 7.66 64 16.00 37.00 49.00 68.00 120 73 1995 FALL 8 10.91 2.4000 29.00 10.98 3.88 101 2.40 3.65 5.35 18.93 29 73 1995 SPRING 14 83.29 17.000 140.00 40.56 10.84 49 17.00 38.50 93.00 120.00 140 73 1995 SUMMER 12 41.63 9.0000 110.00 27.81 8.03 67 9.00 20.03 42.25 52.75 110 73 1995 WINTER 16 44.49 4.8500 120.00 31.00 7.75 70 4.85 24.00 36.00 54.50 120 73 1996 FALL 14 42.21 15.000 96.50 20.80 5.56 49 15.00 26.00 42.50 51.00 96 73 1996 SPRING 12 40.88 5.1000 73.50 22.77 6.57 56 5.10 25.00 42.50 62.75 73 73 1996 SUMMER 13 29.03 1.7000 60.00 16.68 4.63 57 1.70 16.00 31.00 40.00 60 73 1996 WINTER 11 63.09 17.000 130.00 38.98 11.75 62 17.00 18.00 60.00 84.00 130 73 1997 SPRING 3 42.83 23.500 55.00 16.93 9.77 40 23.50 23.50 50.00 55.00 55 73 1997 WINTER 6 119.50 7.5000 180.00 72.10 29.44 60 7.50 79.50 135.00 180.00 180
Appendix B—Descriptive Statistics Data Tables for Level III Subecoregions Within Aggregate Ec

 Aggregate Nutrient Ecoregion: X  Rivers and Streams Descriptive Statistics by Subecoregion, Year and Season from 1993 to 1997 Turbidity_JCU subecoregion year season N MEAN MIN MAX STDDEV STDERR CV P5 P25 MEDIAN P75 P 34 1993 WINTER 3 9.27 7.8000 10.00 1.27 0.73 14 7.80 7.80 10.00 10.00 10. 73 1996 FALL 1 18.00 18.000 18.00 . . . 18.00 18.00 18.00 18.00 18. 73 1996 SPRING 1 12.00 12.000 12.00 . . . 12.00 12.00 12.00 12.00 12. 73 1997 SUMMER 1 9.50 9.5000 9.50 . . . 9.50 9.50 9.50 9.50 9.
Appendix B—Descriptive Statistics Data Tables for Level III Subecoregions Within Aggregate Ec

 Aggregate Nutrient Ecoregion: X  Rivers and Streams Descriptive Statistics by Subecoregion, Year and Season from 1990 to 2000 Turbidity_NTU subecoregion year season N MEAN MIN MAX STDDEV STDERR CV P5 P25 MEDIAN P75  34 1990 FALL 11 19.77 5.0000 44.00 13.89 4.19 70 5.00 5.80 18.00 32.00 44 34 1990 SPRING 11 74.41 12.000 119.00 38.14 11.50 51 12.00 32.00 80.00 107.00 119 34 1990 SUMMER 14 38.52 6.5000 96.00 32.44 8.67 84 6.50 14.00 24.50 56.00 96 34 1990 WINTER 11 64.95 18.000 110.00 27.98 8.44 43 18.00 31.50 70.00 80.00 110 34 1991 FALL 1 32.00 32.000 32.00 . . . 32.00 32.00 32.00 32.00 32 34 1991 SPRING 13 78.85 9.8000 175.00 50.82 14.10 64 9.80 56.00 63.00 82.50 175 34 1991 SUMMER 13 37.85 16.500 76.00 20.21 5.61 53 16.50 25.50 31.00 38.00 76 34 1992 FALL 4 23.13 6.0000 64.00 27.53 13.77 119 6.00 6.75 11.25 39.50 64 34 1992 SPRING 13 89.85 14.000 175.00 54.32 15.07 60 14.00 40.00 76.50 124.50 175 34 1992 SUMMER 13 37.39 7.0500 100.00 25.76 7.15 69 7.05 26.00 30.00 39.00 100 34 1992 WINTER 13 73.90 14.000 170.00 48.13 13.35 65 14.00 32.50 64.00 103.00 170 34 1993 FALL 13 25.52 5.2000 50.00 15.63 4.34 61 5.20 11.00 22.75 37.00 50 34 1993 SPRING 12 76.48 9.3000 168.00 50.31 14.52 66 9.30 44.00 67.50 98.75 168 34 1993 SUMMER 13 31.08 11.000 80.00 21.22 5.89 68 11.00 16.50 20.00 38.00 80 34 1993 WINTER 13 68.04 10.000 140.00 36.59 10.15 54 10.00 60.00 68.00 78.00 140 34 1994 FALL 13 24.94 5.0000 55.00 14.84 4.11 59 5.00 17.00 25.50 30.00 55 34 1994 SPRING 12 95.04 22.000 190.00 50.56 14.59 53 22.00 48.75 100.00 125.00 190 34 1994 SUMMER 13 52.30 10.000 128.00 33.65 9.33 64 10.00 35.00 50.00 68.00 128 34 1994 WINTER 13 74.22 7.9000 129.00 43.56 12.08 59 7.90 38.00 70.00 110.00 129 34 1995 FALL 13 27.33 9.3500 60.00 15.86 4.40 58 9.35 16.00 26.00 30.00 60 34 1995 SPRING 12 90.29 14.000 170.00 48.37 13.96 54 14.00 57.50 75.00 136.25 170 34 1995 SUMMER 13 39.52 9.5000 80.00 22.70 6.30 57 9.50 20.00 45.00 50.00 80 34 1995 WINTER 12 69.58 11.000 120.00 42.12 12.16 61 11.00 27.50 82.50 107.50 120 34 1996 FALL 13 25.77 4.0000 42.00 13.24 3.67 51 4.00 11.00 29.00 34.00 42 34 1996 SPRING 12 63.35 6.7000 160.00 45.12 13.02 71 6.70 27.00 53.50 93.50 160 34 1996 SUMMER 13 30.86 6.1500 80.00 19.43 5.39 63 6.15 18.00 28.00 35.00 80 34 1996 WINTER 13 65.46 17.000 105.00 30.98 8.59 47 17.00 48.00 75.00 80.00 105 34 1997 FALL 13 31.95 7.4000 65.00 18.78 5.21 59 7.40 19.00 27.00 49.50 65 34 1997 SPRING 11 79.31 9.4000 136.00 44.68 13.47 56 9.40 25.00 92.50 120.00 136 34 1997 SUMMER 13 54.69 13.500 130.00 33.40 9.26 61 13.50 26.00 60.00 70.00 130 34 1997 WINTER 12 59.83 21.000 100.00 24.52 7.08 41 21.00 43.50 62.00 75.00 100 34 1998 SPRING 10 119.15 14.000 190.00 50.59 16.00 42 14.00 90.00 130.00 160.00 190
Appendix B—Descriptive Statistics Data Tables for Level III Subecoregions Within Aggregate Ec

 Aggregate Nutrient Ecoregion: X  Rivers and Streams Descriptive Statistics by Subecoregion, Year and Season from 1990 to 2000 Turbidity_NTU subecoregion year season N MEAN MIN MAX STDDEV STDERR CV P5 P25 MEDIAN P75  34 1998 WINTER 13 66.35 17.000 153.50 36.50 10.12 55 17.00 39.00 67.00 80.50 153 73 1990 FALL 69 24.54 4.0000 170.00 23.39 2.82 95 5.90 10.00 20.00 29.00 59 73 1990 SPRING 69 55.18 6.4000 185.00 47.96 5.77 87 12.80 21.00 40.00 64.00 175 73 1990 SUMMER 68 34.69 6.8000 121.00 27.53 3.34 79 8.20 15.00 23.50 51.75 102 73 1990 WINTER 68 57.80 3.5000 160.00 37.29 4.52 65 7.70 28.50 52.25 80.00 140 73 1991 FALL 25 32.93 3.5000 107.00 29.97 5.99 91 4.40 7.80 25.00 44.00 95 73 1991 SPRING 81 44.74 5.0000 196.00 39.83 4.43 89 11.50 20.00 31.00 49.25 133 73 1991 SUMMER 93 29.26 1.4500 128.00 21.76 2.26 74 7.00 13.00 23.00 41.00 66 73 1991 WINTER 20 42.54 4.3000 160.00 33.11 7.40 78 7.15 24.50 33.50 49.50 120 73 1992 FALL 39 21.40 3.0000 68.50 15.94 2.55 74 3.60 8.00 15.00 32.00 58 73 1992 SPRING 97 40.64 4.1000 167.00 32.80 3.33 81 6.60 18.50 28.60 53.00 124 73 1992 SUMMER 95 37.81 3.0000 157.00 32.92 3.38 87 5.80 16.00 24.00 50.00 115 73 1992 WINTER 97 42.79 4.9000 180.00 36.02 3.66 84 6.40 19.00 30.00 51.00 126 73 1993 FALL 99 26.90 3.0000 100.95 20.35 2.05 76 5.60 13.00 21.00 34.00 77 73 1993 SPRING 91 52.40 2.0000 190.00 46.02 4.82 88 10.00 20.00 36.25 65.50 165 73 1993 SUMMER 104 32.17 1.9000 138.00 27.00 2.65 84 4.60 13.00 22.50 48.00 82 73 1993 WINTER 102 49.20 5.4500 198.00 33.03 3.27 67 12.00 23.00 44.00 66.00 107 73 1994 FALL 130 25.88 1.0000 120.00 20.44 1.79 79 5.00 12.00 20.00 34.00 70 73 1994 SPRING 94 46.16 3.4000 193.00 39.21 4.04 85 8.30 20.00 30.00 65.00 145 73 1994 SUMMER 127 41.85 3.0500 133.00 32.00 2.84 76 7.20 18.00 31.00 55.00 111 73 1994 WINTER 97 43.09 3.0500 188.00 35.59 3.61 83 8.00 20.80 33.00 50.00 130 73 1995 FALL 132 20.88 2.2000 109.50 18.04 1.57 86 4.10 9.88 16.50 25.00 49 73 1995 SPRING 130 46.96 1.0000 160.00 35.96 3.15 77 10.00 20.00 36.50 65.00 120 73 1995 SUMMER 133 30.82 1.5000 180.00 29.96 2.60 97 5.50 14.00 21.00 36.00 92 73 1995 WINTER 127 45.37 2.8000 188.00 41.31 3.67 91 6.60 17.00 32.50 57.00 160 73 1996 FALL 132 27.58 1.7000 170.00 28.59 2.49 104 3.50 8.80 19.75 35.00 78 73 1996 SPRING 121 44.47 3.8500 198.00 40.62 3.69 91 7.20 16.50 33.00 60.00 130 73 1996 SUMMER 97 29.54 2.3000 175.00 27.87 2.83 94 3.40 11.00 20.00 41.00 83 73 1996 WINTER 134 54.47 1.6000 195.00 45.63 3.94 84 7.80 20.00 36.00 80.00 165 73 1997 FALL 94 23.81 2.0000 105.00 19.76 2.04 83 5.10 10.00 18.50 29.00 75 73 1997 SPRING 100 36.93 1.1000 162.00 32.71 3.27 89 7.13 15.00 25.00 51.50 114 73 1997 SUMMER 99 40.51 3.1500 150.00 31.10 3.13 77 5.20 19.00 33.00 54.00 110
Appendix B—Descriptive Statistics Data Tables for Level III Subecoregions Within Aggregate Ec

 Aggregate Nutrient Ecoregion: X  Rivers and Streams Descriptive Statistics by Subecoregion, Year and Season from 1990 to 2000 Turbidity_NTU subecoregion year season N MEAN MIN MAX STDDEV STDERR CV P5 P25 MEDIAN P75  73 1997 WINTER 93 45.51 3.6000 180.00 42.65 4.42 94 6.50 18.00 30.00 62.00 156 73 1998 SPRING 47 63.87 10.000 170.00 39.33 5.74 62 18.00 32.00 50.50 90.00 135 73 1998 WINTER 65 65.68 5.3000 198.00 46.40 5.76 71 13.00 31.50 59.00 89.00 166 73 1999 FALL 4 1.00 .00000 2.00 1.15 0.58 115 0.00 0.00 1.00 2.00 2 73 1999 SUMMER 6 12.00 5.0000 16.00 5.44 2.22 45 5.00 5.00 15.00 16.00 16 73 2000 SPRING 3 218.33 15.000 435.00 210.32 121.43 96 15.00 15.00 205.00 435.00 435 73 2000 SUMMER 3 41.67 17.000 57.00 21.57 12.45 52 17.00 17.00 51.00 57.00 57
Appendix B—Descriptive Statistics Data Tables for Level III Subecoregions Within Aggregate Ec

 Aggregate Nutrient Ecoregion: X  Rivers and Streams Descriptive Statistics by Subecoregion, Year and Season from 1999 to 2000 pH_S_U subecoregion year season N MEAN MIN MAX STDDEV STDERR CV P5 P25 MEDIAN P75 P 73 1999 FALL 4 8.50 7.8400 9.15 0.76 0.38 9 7.84 7.84 8.50 9.15 9. 73 1999 SUMMER 6 8.19 8.0500 8.43 0.19 0.08 2 8.05 8.05 8.08 8.43 8. 73 2000 SPRING 3 8.32 7.7100 9.06 0.69 0.40 8 7.71 7.71 8.18 9.06 9. 73 2000 SUMMER 3 7.96 7.7300 8.20 0.24 0.14 3 7.73 7.73 7.95 8.20 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)
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-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
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-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).
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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