U.S. Geological Survey

Water-Resources Investigations Report 

U.S. Department of the Interior

Percentile Distributions of Median Nitrite 

Plus Nitrate as Nitrogen, Total Nitrogen, 

and Total Phosphorus Concentrations in 

Oklahoma Streams, 1973–2001

03–4084

Prepared in cooperation with the 

OKLAHOMA WATER RESOURCES BOARD















  



 





 







 

  



 



 

 





 







 











 

  







 







































 



  











 





  

 







 





 









  

 





































   

   



   













  











  

 





 















 



 



















 










U.S. Geological Survey

Water-Resources Investigations Report

U.S. Department of the Interior

Percentile Distributions of Median Nitrite 

Plus Nitrate as Nitrogen, Total Nitrogen, 

and Total Phosphorus Concentrations in 

Oklahoma Streams, 1973–2001

By Brian E. Haggard1, Jason R. Masoner2, and Carol J. Becker2

03–4084

Prepared in cooperation with the 

OKLAHOMA WATER RESOURCES BOARD

1U.S. Department of Agriculture, Agricultural Research Service 

2U.S. Geological Survey
Haggard, B.E., and others—Percentile Distributions of Median Nitrite Plus Nitrate as Nitrogen, Total Nitrogen, and Total Phosphorus Concentrations in Oklahoma Streams, 1973–2001—USGS/WRIR 03–4084

Printed on recycled paper
U.S. Department of the Interior

GALE A. NORTON, Secretary

U.S. Geological Survey

CHARLES G. GROAT, Director

The use of firm, trade, and brand names in this report is for identification purposes only and does

not constitute endorsement by the U.S. Geological Survey.

For additional information write to: Copies of this report can be purchased from:

U.S. Geological Survey

Information Services

Box 25286

Federal Center

Denver, CO 80225

UNITED STATES GOVERNMENT PRINTING OFFICE: OKLAHOMA CITY 2003

Additional information about water resources in Oklahoma is available on the World Wide Web at 

http://ok.water.usgs.gov

District Chief

U.S. Geological Survey

Water-Resources Division

202 NW 66 St., Bldg. 7

Oklahoma City, OK 73116
CONTENTS III

CONTENTS

Abstract...................................................................................................................................................................................... 1

Introduction ............................................................................................................................................................................... 2

Purpose and scope ........................................................................................................................................................... 2

Acknowledgments .......................................................................................................................................................... 2

Methods of determining nutrient criteria .................................................................................................................................. 2

U.S. Environmental Protection Agency methods for determining nutrient criteria ........................................................ 2

Modification of use support assessment protocols for Oklahoma .................................................................................. 4

Geographic regions and stream category determination ................................................................................................. 4

Stream water-quality data compilation ......................................................................................................................... 10

Statistical distributions .................................................................................................................................................. 10

Percentile distributions of median nitrite plus nitrate, total nitrogen, and total phosphorus concentrations .......................... 10

Summary ................................................................................................................................................................................. 15

Selected references ................................................................................................................................................................. 16

 Appendix................................................................................................................................................................................. 20

Nutrients and Stream Impairment ................................................................................................................................. 21

ILLUSTRATIONS

Figure 1. Dichotomous process of the Use Support Assessment Protocol used in Oklahoma to identify

nutrient threatened streams ....................................................................................................................................................13

Figures 2-3. Maps showing:

2. Water-quality sites grouped according to stream order and stream slope ............................................................... 5

3. Aggregate nutrient ecoregions and nested level III ecoregions for Oklahoma ....................................................... 7

TABLES

1. U.S. Environmental Protection Agency ambient water-quality criteria recommendations, 25th percentile 

of nitrite plus nitrate as nitrogen, total nitrogen and total phosphorus concentrations for nutrient 

ecoregions IV, V, IX, and XI ...................................................................................................................................... 8

2. U.S. Environmental Protection Agency ambient water-quality criteria recommendations, 25th percentile 

for nitrite plus nitrate as nitrogen, total nitrogen, and total phosphorus concentrations for portions 

of level III ecoregions in Oklahoma ........................................................................................................................... 8

3. Geographic regions used to group water-quality sites .......................................................................................................... 9

4. Description of stream categories used to group water-quality sites ..................................................................................... 9

5. Percentile distributions of median concentrations of nitrite plus nitrate as nitrogen, total nitrogen, and total 

phosphorus for water-quality sites in the Ozark Highland ecoregion in Oklahoma, 

geographic region 1 .................................................................................................................................................. 11

6. Percentile distributions of median concentrations of nitrite plus nitrate as nitrogen, total nitrogen, and total 

phosphorus from water-quality sites in the Ouachita Mountains ecoregion in Oklahoma, 

geographic region 2 .................................................................................................................................................. 12

7. Percentile distributions of median concentrations of nitrite plus nitrate as nitrogen, total nitrogen, and total 

phosphorus from water-quality sites in Oklahoma and part of Arkansas, excluding sites within the

Ozark Highland and Ouachita Mountains ecoregions, geographic region 3 ............................................................ 13

8, Percentile distributions of median concentrations of nitrite plus nitrate as nitrogen, total nitrogen, and total 

phosphorus of all water-quality sites in Oklahoma and part of Arkansas, geographic region 4 .............................. 14
IV CONTENTS

CONVERSION FACTORS AND VERTICAL DATUM

Temperature in degrees Celsius (°C) may be converted to degrees Fahrenheit (°F) as follows:

°F=1.8 °C+32

Temperature in degrees Fahrenheit (°F) may be converted to degrees Celsius (°C) as follows:

°C=(°F-32)/1.8

Vertical coordinate information is referenced to the North American Vertical Datum of 1988 (NAVD 88)

Horizontal coordinate information is referenced to the North American Datum of 1983 (NAD 83).

Elevation, as used in this report, refers to distance above or below NAVD 88.

Multiply By To obtain

Length

inch (in) 2.54 centimeter

foot (ft) 0.3048 meter

mile (mi) 1.609 kilometer

Area

square foot (ft2) 0.09290 square meter

square inch (in2) 6.452 square centimeter

square mile (mi2) 2.590 square kilometer 

Volume

gallon (gal) 3.785 liter

cubic foot (ft3) 0.02832 cubic meter 

Mass

ounce, avoirdupois (oz) 28.35 gram 

pound, avoirdupois (lb) 0.4536 kilogram 

Hydraulic gradient

foot per mile (ft/mi) 0.1894 meter per kilometer
 1

Percentile Distributions of Median Nitrite Plus 

Nitrate as Nitrogen, Total Nitrogen, and Total Phosphorous Concentrations in Oklahoma Streams, 

1973-2001

By Brain E. Haggard, Jason R. Masoner, and Carol J. Becker

Abstract

Nutrients are one of the primary causes of 

water-quality impairments in streams, lakes, 

reservoirs, and estuaries in the United States. The 

U.S. Environmental Protection Agency has developed regional-based nutrient criteria using ecoregions to protect streams in the United States from 

impairment. However, nutrient criteria were based 

on nutrient concentrations measured in large 

aggregated nutrient ecoregions with little relevance to local environmental conditions in states. 

The Oklahoma Water Resources Board is using a 

dichotomous process known as Use Support 

Assessment Protocols to define nutrient criteria in 

Oklahoma streams. The Oklahoma Water 

Resources Board is modifying the Use Support 

Assessment Protocols to reflect nutrient information and environmental characteristics relevant to 

Oklahoma streams, while considering nutrient 

information grouped by geographic regions based 

on level III ecoregions and state boundaries. 

Percentile distributions of median nitrite 

plus nitrate as nitrogen, total nitrogen, and total 

phosphorous concentrations were calculated from 

563 sites in Oklahoma and 4 sites in Arkansas 

near the Oklahoma and Arkansas border to facilitate development of nutrient criteria for Oklahoma streams. Sites were grouped into four 

geographic regions and were categorized into 

eight stream categories by stream slope and 

stream order. The 50th percentiles of median 

nitrite plus nitrate as nitrogen, total nitrogen, and 

total phosphorus concentrations were greater in 

the Ozark Highland ecoregion and were less in 

the Ouachita Mountains ecoregion when 

compared to other geographic areas used to group 

sites. The 50th percentiles of median concentrations of nitrite plus nitrate as nitrogen, total 

nitrogen, and total phosphorus were least in first, 

second, and third order streams. The 50th percentiles of median nitrite plus nitrate as nitrogen, 

total nitrogen and total phosphorus concentrations 

in the Ozark Highland and Ouachita Mountains 

ecoregions were least in first, second, and third 

order streams with streams slopes greater than 17 

feet per mile. 

Nitrite plus nitrate as nitrogen and total 

nitrogen criteria determined by the U.S. Environmental Protection Agency for the Ozark Highland 

ecoregion were less than the 25th percentiles of 

median nitrite plus nitrate as nitrogen, total 

nitrogen, and total phosphorus concentrations in 

the Ozark Highland ecoregion calculated for this 

report. Nitrite plus nitrate as nitrogen and total 

nitrogen criteria developed by the U.S. Environmental Protection Agency for the Ouachita Mountains ecoregion were similar to the 25th 

percentiles of median nitrite plus nitrate as 

nitrogen and total nitrogen concentrations in the 

Ouachita Mountains ecoregion calculated for this 

report. Nitrate as nitrogen and total phosphorus 

concentrations currently (2002) used in the Use 

Support Assessment Protocols for Oklahoma 

were greater than the 75th percentiles of median 

nitrite plus nitrate as nitrogen and total phosphorus concentrations calculated for this report. 
2 Percentile Distributions of Median Nitrite Plus Nitrate as Nitrogen, Total Nitrogen, and Total Phosphorous Concentrations in

Oklahoma Streams, 1973-2001

INTRODUCTION

Nutrients are one of the primary causes of 

water-quality impairments in our Nation's streams, 

lakes, reservoirs, and estuaries (U.S. Environmental 

Protection Agency, 1996). Forty percent of impaired 

streams in the United States were impaired due to 

nutrient enrichment; 51 percent of lakes and reservoirs 

and 57 percent of estuaries also were impaired by 

nutrients (U.S. Environmental Protection Agency, 

1996).

The U.S. Environmental Protection Agency 

(USEPA) has developed regional-based nutrient 

criteria using ecoregions to protect streams in the 

United States from impairment (U.S. Environmental 

Protection Agency, 2000a, 2000b, 2000c, 2001a, and 

2001b). However, nutrient criteria were based on 

nutrient concentrations collected in large aggregated 

nutrient ecoregions with little relevance to local environmental conditions in states. Robertson and others 

(2001) presented an alternative regional approach for 

USEPA Regions 5 and 7 by classifying streams based 

on environmental nutrient zones and environmental 

characteristics. Individual states and tribes are encouraged by the USEPA to modify or improve upon the 

ecoregion approach by Robertson and others (2001).

The nutrient criteria approach used by the Oklahoma Water Resources Board (OWRB) is a dichotomous process that stratifies or groups streams using 

environmental characteristics such as stream order, 

stream slope, turbidity, and percent canopy shading to 

identify streams in Oklahoma affected by nutrients 

(Oklahoma Water Resources Board, 2001). This classification process is called the Use Support Assessment Protocols (USAP), title 785, chapter 46, 

subchapter 15. An example of part of the USAP 

process currently (2002) used in Oklahoma is 

provided in figure 1. The USAP is based on nutrient 

information and environmental characteristics developed by a study in the Netherlands (Peeters and 

Gardeniers, 1998). The OWRB wished to modify the 

USAP to reflect nutrient information and environmental characteristics relevant to Oklahoma streams, 

while considering nutrient information grouped into 

four geographic regions based on level III ecoregions 

and state boundaries. 

The U.S. Geological Survey (USGS), in cooperation with OWRB, calculated median percentile distributions from available nutrient data collected at 563 

sites in Oklahoma and 4 sites in Arkansas near the 

Oklahoma and Arkansas border (fig. 2). The percentile 

distributions presented in this report can be used in the 

USAP to facilitate the development of nutrient criteria 

for Oklahoma streams.

Purpose and Scope

This report provides percentile distributions of 

median concentrations of nitrite plus nitrate as 

nitrogen (referred to as nitrate in this report), total 

nitrogen, and total phosphorus in Oklahoma streams 

based on samples collected from 1973 to 2001. Sites 

were grouped into four geographic regions and were 

categorized into eight stream categories by stream 

slope and stream order. Percentile distributions of 

median concentrations were calculated for each stream 

category for four geographic regions: (1) water-quality 

sites in the Ozark Highland ecoregion; (2) waterquality sites in the Ouachita Mountains ecoregion; (3) 

water-quality sites excluding those in the Ozark Highland and Ouachita Mountains ecoregions; and (4) all 

water-quality sites in Oklahoma and part of Arkansas. 

Differences in nutrient concentrations between 

geographic regions and stream categories are briefly 

discussed, but the purpose of this report is to provide 

tables of percentile distributions of median nutrient 

concentrations to the OWRB. A detailed literature 

review of previous studies on stream classification to 

establish nutrient criteria is provided in appendix 1.

Acknowledgments

The authors thank staff at the Oklahoma 

Conservation Commission and the OWRB for 

providing nutrient data bases used in this study; Evan 

Hornig, William Green, Lyn Osburn, and Robert 

Tortorelli of the USGS for their contributions to the 

report.

METHODS OF DETERMING NUTRIENT 

CRITERIA

U.S. Environmental Protection Agency 

Methods for Determining Nutrient Criteria

The USEPA is exploring several methods to define 

nutrient criteria to assess nutrient-threatened or
 3

Stream order

1, 2, or 3?

Stream slope

> 17 feet per mile

> 17 feet per mile

No

Yes

Yes

No

Yes

No

P > 1.0 mg/L or

P > 0.36 mg/L or

NO2+NO3 > 5.0 mg/L

Use Support Assessment

Yes

Yes

No

No

No

No

Equivalent to stream

categories SS3

Equivalent to stream

categories SS2

Equivalent to stream

categories SS4, SS6, and SS8

Equivalent to stream

category SS1

NO2+NO3 > 4.65 mg/L

 

Protocol continues as 

defined in Oklahoma

Not

threatened

Not

threatened

P > 0.24 mg/L or 

NO2+NO3 > 4.95 mg/L

P > 0.15 mg/L or 

NO2+NO3 > 2.4 mg/L

Stream slope

Yes

EXPLANATION

mg/L, milligrams per liter

>, greater than

SS1, stream orders 1, 2, and 3, and stream slope greater than 17 feet per mile

SS2, stream orders 1, 2, and 3, and stream slope less than or equal to 17 feet per mile

SS3, stream orders 4 and 5, and stream slope greater than 17 feet per mile

SS4, stream orders 4 and 5, and stream slope less than or equal to 17 feet per mile 

SS6, stream orders 4 and above, and stream slope less than or equal to 17 feet per mile

SS8, stream orders greater than or equal to 6, and stream slope less than or equal to 17 feet per mile

Water Resources Board

(2001)

Figure 1. Dichotomous process of the Use Support Assessment Protocol used in Oklahoma to identify nutrient threatened 

streams.
4 Percentile Distributions of Median Nitrite Plus Nitrate as Nitrogen, Total Nitrogen, and Total Phosphorous Concentrations in

Oklahoma Streams, 1973-2001

impaired streams using nitrate, total nitrogen, and total 

phosphorous concentrations. One method used by the 

USEPA to define nutrient criteria for streams assigns a 

reference value based on nutrient concentrations in a 

specific geographic area (U.S. Environmental Protection Agency, 2000a). Nutrient data from the USEPA, 

USGS, and various universities were used to create a 

reference value based on the frequency distribution 

using all available nutrient data in a geographic area. 

The lower 25th percentile of all nutrient data represents the reference condition or the minimally 

impaired stream value (U.S. Environmental Protection 

Agency, 2000a). 

A second method to define reference conditions 

is to select the upper 75th percentile of nutrient data 

collected from a subset of streams considered as being 

the least impaired streams in a defined area (U.S. Environmental Protection Agency, 2000a). The USGS 

Hydrologic Benchmark Network represents over 80 

undeveloped basins throughout the United States. The 

75th percentile was calculated using nutrient data 

collected from streams draining the undeveloped 

basins. The 75th percentile concentration for nitrate 

was 0.21 milligram per liter, total nitrogen was 0.50 

milligram per liter, and total phosphorus was 0.04 

milligram per liter (Clarke and others, 2000). 

The USEPA is using these methods to develop 

tables of the cumulative distribution of nutrient 

concentrations for nutrient ecoregions and level III 

ecoregions throughout the United States (U.S. Environmental Protection Agency, 2001a, p. 4 and 5). 

Nutrient ecoregions are aggregations of level III ecoregions as defined by Omernik (2000). Nutrient ecoregions IV, V, IX and XI comprise various portions of 

Oklahoma (fig 3). The USEPA ambient water-quality 

criteria recommendations for nutrient ecoregions IV, 

V, IX, and XI are presented in table 1. The USEPA 

ambient water-quality criteria recommendations for 

portions of the level III ecoregions in Oklahoma are 

presented in table 2. Water-quality criteria from the 

two USEPA methods will be compared to median 

nitrate, total nitrogen, and total phosphorus concentrations calculated for a modified USAP approach 

presented in this report.

Modification of Use Support Assessment 

Protocols for Oklahoma

A modified USAP approach utilizing stream 

characteristics and geographic regions was used in this 

report to reflect nutrient and hydrological conditions in 

Oklahoma streams based on procedures defined by the 

U.S. Environmental Protection Agency (2000a) and 

Robertson and others (2001). 

Geographic Regions and Stream Category Determination

A Geographical Information System (GIS) was 

used to group 563 water-quality sites in Oklahoma and 

4 water-quality sites in Arkansas near the Oklahoma 

and Arkansas border into four geographic regions 

(table 3). Water-quality sites for each geographic 

region were grouped into eight stream categories 

(table 4) by stream order and stream slope. Individual 

stream characteristics for water-quality sites used in 

this report are available in Masoner and others (2002).

A digital stream network for Oklahoma was 

created using a flow accumulation model created from 

the National Elevation Dataset (U.S. Geological 

Survey, 2001) that represented streams at a consistent 

stream density. Stream order was determined from the 

digital stream network using methods described by 

Strahler (1952, 1957); this technique was automated 

using GIS techniques (Masoner and others, 2002). 

Headwater streams were designated as first order; a 

second-order stream was formed at the intersection of 

two first-order streams; a third-order stream was 

formed at the intersection of two second-order 

streams, and so forth. The technique worked well in 

areas with well-defined topography. Stream order estimations for streams draining lands with less 

pronounced topography generally under predicted 

stream order by a value of one.

Stream slope is defined as the change in elevation between stream confluences upstream and downstream from a water-quality site divided by the 

distance between stream confluences. A digital stream 

dataset was created using a hydrologically conditioned 

Digital Elevation Model with 60-meter cell size. The 

Digital Elevation Model was created from elevation 

data (hypsography) and streams (hydrography) from 

digital versions of U.S. Geological Survey 1:100,000-

scale topographic maps (Cederstrand and Rea, 1995). 

The average length of a stream segment used to calculate stream slope was 1.7 miles. The minimum and
Kiamic

River

Beaver River

No rth

Can a dian

R i ver

Cimarron River Arkansas River 

Washita

100 ̊W 98 ̊W 96 ̊W 

S Environmental Protection Agency

S. Census Bureau

NAD83

S Environmental Protection Agency

S. Census Bureau

NAD83

TEXAS

KANSAS

s

Ozark Highlands ecoregion

Ouachita Mountains ecoregion

er

fifth ordersixth orderseventh ordereighth order

60 80 Miles

0 Kilometers

Canadian River

s grouped by

stream slope

2,3 and stream

er mile (SS1)

2,3 and stream

per mile (SS2)

5 and stream

er mile (SS3)

5 and stream

per mile (SS4)

6 and stream

per mile (SS8)

(SS5) (SS7)

d di t t d d t l

 7

27

25

26

35

25

39

29

40

32

28

36

37

38

102 ̊W 100 ̊W 98 ̊W 96 ̊W 94 ̊W

34 ̊N36 ̊N

23 Boundary and number for nested level III ecoregions

EXPLANATION

Nutrient ecoregion IV

Nutrient ecoregion V

Nutrient ecoregion IX

Nutrient ecoregion XI

OuachitaMountains

Ozark Highlands

Base from U.S. Geological Survey digital data 1:100,000, 1983

Ecoregion boundaries from U.S. Environmental Protection Agency digital data, 2002

Albers Equal Area Conic projection, North American Datum 1983

0 100 200 50 

0 100 200 50 Kilometers

OKLAHOMA

Figure 3. Aggregate nutrient ecoregions and nested level III ecoregions for Oklahoma (U.S. Environmental Protection Agency, 2001).
8 Percentile Distributions of Median Nitrite Plus Nitrate as Nitrogen, Total Nitrogen, and Total Phosphorus Concentrations in 

Oklahoma Streams, 1973-2001

Table 1. Environmental Protection Agency ambient water-quality criteria recommendations, 25th percentile of 

nitrite plus nitrate as nitrogen, total nitrogen and total phosphorus concentrations for nutrient ecoregions IV, V, IX, 

and XI

[mg/L, milligrams per liter]

Nutrient 

ecoregions 

Nitrite plus 

nitrate 

as nitrogen

(mg/L)

Total 

nitrogen

(mg/L)

Total 

phosphorus

(mg/L)1

1Total phosphorus values were originally reported in microgram per liter and have been converted to milligram per liter

References

IV 0.050 0.560 0.023 U.S. Environmental Protection 

Agency (2001a)

V 0.260 0.880 0.068 U.S. Environmental Protection 

Agency (2001b)

IX 0.125 0.692 0.037 U.S. Environmental Protection 

Agency (2000c)

XI 0.093 0.305 0.010 U.S. Environmental Protection 

Agency (2000b)

Table 2. U.S. Environmental Protection Agency ambient water-quality criteria recommendations, 25th percentile for nitrite plus 

nitrate as nitrogen, total nitrogen, and total phosphorus concentrations for portions of level III ecoregions in Oklahoma

[mg/L, milligrams per liter]

Nutrient 

ecoregions

Level III

ecoregions

Nitrite plus 

nitrate 

as nitrogen

(mg/L)

Total 

nitrogen

(mg/L)

Total 

phosphorus

(mg/L)1

1

Total phosphorus values were originally reported in microgram per liter and have been converted to milligram per liter

References

IV 26

28

0.04

0.12

0.45

0.36

0.0250

0.060

U.S. Environmental Protection 

Agency (2001a)

V 25

27

0.72

0.19

1.07

0.84

0.060

0.090

U.S. Environmental Protection 

Agency (2001b)

IX 29

35

37

40

0.078

0.067

0.075

0.23

0.68

0.385

0.683

0.712

0.03750

0.0500

0.04250

0.09250

U.S. Environmental Protection 

Agency (2000c)

XI 36 (Ouachita Mountains)

38

39 (Ozark Highlands)

0.01

0.03

0.239

0.30

1.383

0.379

0.01047

0.00563

0.00663

U.S. Environmental Protection 

Agency (2000b)
 9

Table 3. Geographic regions used to group water-quality sites

Geographic

region Description of geographic region Tables in this report containing results

1 Water-quality sites in the Ozark Highland ecoregion 

(referred to as ecoregion 39)

Table 5.

2 Water-quality sites in the Ouachita Mountains ecoregion 

(referred to as ecoregion 36)

Table 6.

3 All water-quality sites in Oklahoma and part of Arkansas 

excluding those in the Ozark Highland and Ouachita 

Mountains ecoregion

Table 7.

4 All water-quality sites in Oklahoma and part of Arkansas Table 8.

Table 4. Description of stream categories used to group water-quality sites

Stream

categories Criteria for stream categories

SS1 Stream orders 1, 2, and 3, and stream slope greater than 17 feet per mile

SS2 Stream orders 1, 2, and 3, and stream slope less than or equal to 17 feet per mile

SS3 Stream orders 4 and 5, and stream slope greater than 17 feet per mile

SS4 Stream orders 4 and 5, and stream slope less than or equal to 17 feet per mile

SS5 Stream orders 4 and 5, without slope criteria

SS6 Stream orders 4 and above, and stream slope less than or equal to 17 feet per mile

SS7 Stream orders 4 and above, without slope criteria

SS8 Stream orders greater than or equal to 6, and stream slope less than or equal to 17 feet per mile
10 Percentile Distributions of Median Nitrite Plus Nitrate as Nitrogen, Total Nitrogen, and Total Phosphorus Concentrations in

Oklahoma Streams, 1973-2001

maximum length used were 0.31 mile to 3.1 miles 

(Masoner and others, 2002).

Stream Water-Quality Data Compilation

Water-quality data compiled included nitrate, 

total nitrogen, and total phosphorus concentrations 

measured in water samples at 563 sites in Oklahoma 

and 4 sites in Arkansas near the Oklahoma and 

Arkansas border. There were 12,124 measurements of 

nitrate, 8,697 measurements of total nitrogen, and 

14,259 measurements of total phosphorus. Waterquality data and site locations were retrieved from the 

OWRB, Oklahoma Conservation Commission (OCC), 

and the USEPA Storage and Retrieval (STORET) 

system database (U.S. Environmental Protection 

Agency, 2001). The data from OWRB were collected 

from 1998 through 2001. Data from OCC were 

collected from 1990 through 1999. The STORET data 

were collected from 1973 through 1995 and include 

data collected by state and federal agencies and the 

USGS.

Most water-quality sites had more than one 

sampling event during which one, two, or all three 

nutrient concentrations were measured; many sites had 

several hundred sampling events over a period of 

years. A median concentration was calculated for sites 

having more than one measurement for a nutrient. The 

large number of analyses used in this report, over 

35,000, did not allow for an assessment of sampling 

techniques, preservation methods, analytical techniques, or method reporting levels. Data were screened 

for quality-assurance samples. Sampling events having 

quality-assurance samples were identified and a 

median concentration was calculated for that sampling 

event. Concentration values less than the detection 

limit were changed to 0 milligram per liter. Concentrations of nitrate and total nitrogen were measured 

directly or computed from the sum of individual 

nitrogen measurements. 

There were 231 water-quality sites where waterquality data were collected at the same geographic 

location, but were maintained separately by multiple 

state and federal agencies. For these cases, one site 

was designated to represent multiple sites. The waterquality data at these locations were grouped and a 

median concentration was calculated.

Statistical Distributions

The 10th, 25th, 33rd, 50th, 67th, 75th, and 90th 

percentiles, minimum, and maximum values were 

calculated for median concentrations of nitrate, total 

nitrogen, and total phosphorus for each stream category (table 4) for four geographical regions (table 3). 

For sites with fewer than seven water-quality sites, 

only the median, mean, minimum, and maximum 

statistics were calculated. Stream categories SS3 and 

SS8 in the Ozark Highland ecoregion (table 5) and 

Ouachita Mountains ecoregion (table 6) had insufficient numbers of sites to calculate 10th, 25th, 33rd, 

50th, 67th, 75th, and 90th percentiles. Stream category 

SS3 in geographic region 3, all sites excluding those in 

the Ozark Highland and Ouachita Mountains ecoregions (table 7), also had an insufficient number of sites 

to calculate percentiles.

PERCENTILE DISTRIBUTIONS OF 

MEDIAN NITRITE PLUS NITRATE AS 

NITROGEN, TOTAL NITROGEN, AND 

TOTAL PHOSPHORUS CONCENTRATIONS

Percentile distributions of median nutrient 

concentrations were calculated for four geographic 

regions categorized by eight stream categories (tables 

5, 6, 7, and 8). Out of the four geographic regions in 

which statistics were calculated, the 50th percentiles 

of median nitrate and total nitrogen concentrations 

were greatest in the Ozark Highland ecoregion (table 

5) and least in the Ouachita Mountains ecoregion 

(table 6); the 50th percentiles of total phosphorus 

concentrations were least in the Ouachita Mountains 

ecoregion (table 6). The 50th percentiles of median 

concentrations for total phosphorus were similar in the 

Ozark Highland ecoregion (table 5) and geographic 

regions 3 and 4 (tables 7 and 8). The 50th percentiles 

of median concentrations of nitrate, total nitrogen, and 

total phosphorus concentrations generally were least 

in first, second, and third order streams (SS1 and SS2) 

in all geographic regions. 

The 50th percentiles of median total nitrogen, 

and total phosphorus concentrations sampled in the 

Ozark Highland and Ouachita Mountains ecoregions 

on first, second, and third order streams with stream
 11

Table 5. Percentile distributions of median concentrations of nitrite plus nitrate as nitrogen, total nitrogen, and total phosphorus for water-quality sites in the Ozark Highland ecoregion in Oklahoma, geographic region 1

[concentrations are in milligrams per liter; mean, arithmetic average of median values; min, minimum median value; max, maximum median value; -, 

insufficient number of sites to calculate statistic]

Stream

Category1

1

SS1, stream orders 1, 2, and 3, and stream slope greater than 17 feet per mile

SS2, stream orders 1, 2, and 3, and stream slope less than or equal to 17 feet per mile

SS3, stream orders 4 and 5, and stream slope greater than 17 feet per mile

SS4, stream orders 4 and 5, and stream slope less than or equal to 17 feet per mile

SS5, stream orders 4 and 5, without slope criteria

SS6, stream orders 4 and above, and stream slope less than or equal to 17 feet per mile

SS7, stream orders 4 and above, without slope criteria

SS8, stream orders greater than or equal to 6, and stream slope less than or equal to 17 feet per mile

Number 

of sites2

2

Number of water-quality sites with median concentration

Mean

Percentilies of Median Concentrations

Min Max

10 25 33 50 67 75 90

Nitrite plus Nitrate as Nitrogen

SS1 57 1.59 0.16 0.59 0.97 1.40 1.85 2.00 3.00 0.00 5.95

SS2 13 2.27 0.70 1.00 1.37 2.00 2.77 3.13 5.04 0.50 6.00

SS3 4 1.54 - - - 1.58 - - - 1.00 2.00

SS4 10 1.92 1.20 1.75 1.96 2.00 2.04 2.11 2.47 1.19 2.50

SS5 14 1.81 1.07 1.28 1.87 2.00 2.00 2.03 2.33 1.00 2.50

SS6 12 1.65 0.24 1.22 1.48 2.00 2.00 2.08 2.40 0.13 2.50

SS7 16 1.62 0.40 1.16 1.26 2.00 2.00 2.00 2.26 0.13 2.50

SS8 2 0.32 ------- 0.13 0.51

Total Nitrogen

SS1 36 1.66 0.27 0.84 1.00 1.63 2.07 2.23 3.13 0.15 4.13

SS2 10 2.38 0.60 1.23 1.53 2.13 2.60 3.07 5.91 0.56 6.18

SS3 3 2.03 - - - 2.34 - - - 1.30 2.45

SS4 10 2.33 1.55 2.15 2.28 2.33 2.54 2.61 2.92 1.52 2.95

SS5 13 2.26 1.38 2.05 2.28 2.33 2.47 2.56 2.82 1.30 2.95

SS6 12 2.08 0.77 1.60 1.96 2.31 2.46 2.58 2.85 0.64 2.95

SS7 15 2.07 0.90 1.52 1.95 2.32 2.42 2.51 2.76 0.64 2.95

SS8 2 0.86 ------- 0.64 1.07

Total Phosphorus

SS1 59 0.068 0.003 0.003 0.010 0.020 0.030 0.040 0.100 0.000 0.770

SS2 17 0.119 0.018 0.035 0.049 0.070 0.101 0.118 0.434 0.010 0.670

SS3 4 0.074 - - - 0.073 - - - 0.003 0.150

SS4 10 0.129 0.044 0.103 0.110 0.118 0.170 0.179 0.189 0.040 0.190

SS5 14 0.113 0.022 0.070 0.104 0.110 0.151 0.168 0.185 0.003 0.190

SS6 14 0.122 0.040 0.088 0.110 0.118 0.158 0.175 0.187 0.040 0.190

SS7 16 0.110 0.029 0.050 0.095 0.110 0.144 0.161 0.183 0.003 0.190

SS8 2 0.090 ------- 0.040 0.140
12 Percentile Distributions of Median Nitrite Plus Nitrate as Nitrogen, Total Nitrogen, and Total Phosphorus Concentrations in

Oklahoma Streams, 1973-2001

Table 6. Percentile distributions of median concentrations of nitrite plus nitrate as nitrogen, total nitrogen, and total phosphorus from water-quality sites in the Ouachita Mountains ecoregion in Oklahoma, geographic region 2

[concentrations are in milligrams per liter; mean, arithmetic average of median values; min, minimum median value; max, maximum median value; -, 

insufficient number of sites to calculate statistic]

Stream

Category1

1

SS1, stream orders 1, 2, and 3, and stream slope greater than 17 feet per mile

SS2, stream orders 1, 2, and 3, and stream slope less than or equal to 17 feet per mile

SS3, stream orders 4 and 5, and stream slope greater than 17 feet per mile

SS4, stream orders 4 and 5, and stream slope less than or equal to 17 feet per mile

SS5 stream orders 4 and 5, without slope criteria

SS6, stream orders 4 and above, and stream slope less than or equal to 17 feet per mile

SS7, stream orders 4 and above, without slope criteria

SS8, stream orders greater than or equal to 6, and stream slope less than or equal to 17 feet per mile

Number 

of sites2

2

Number of water-quality sites with median concentration

Mean

Percentiles of Median Concentrations

Min Max

10 25 33 50 67 75 90

Nitrite plus Nitrate as Nitrogen

SS1 46 0.08 0.00 0.00 0.00 0.01 0.01 0.01 0.07 0.00 3.00

SS2 16 0.05 0.00 0.00 0.01 0.03 0.05 0.09 0.16 0.00 0.19

SS3 3 0.29 - - - 0.13 - - - 0.11 0.62

SS4 14 0.18 0.00 0.00 0.02 0.09 0.13 0.20 0.74 0.00 1.05

SS5 17 0.20 0.00 0.01 0.03 0.11 0.13 0.26 0.71 0.00 1.05

SS6 15 0.19 0.00 0.00 0.02 0.09 0.14 0.36 0.68 0.00 1.05

SS7 18 0.21 0.00 0.01 0.05 0.12 0.14 0.38 0.66 0.00 1.05

SS8 0 ----------

Total Nitrogen

SS1 24 0.26 0.00 0.00 0.00 0.08 0.19 0.28 0.53 0.00 3.30

SS2 7 0.37 0.02 0.15 0.20 0.40 0.55 0.60 0.70 0.02 0.70

SS3 2 0.52 ------- 0.41 0.63

SS4 13 0.52 0.15 0.20 0.28 0.39 0.59 0.70 1.27 0.12 1.45

SS5 15 0.52 0.16 0.20 0.34 0.41 0.61 0.64 1.18 0.12 1.45

SS6 14 0.54 0.15 0.20 0.33 0.46 0.65 0.78 1.22 0.12 1.45

SS7 16 0.54 0.17 0.23 0.35 0.47 0.63 0.72 1.13 0.12 1.45

SS8 0 ----------

Total Phosphorus

SS1 46 0.023 0.008 0.010 0.013 0.020 0.023 0.030 0.040 0.003 0.110

SS2 16 0.042 0.019 0.020 0.20 0.030 0.040 0.048 0.097 0.016 0.160

SS3 4 0.047 - - - 0.045 - - - 0.020 0.080

SS4 17 0.059 0.018 0.021 0.024 0.030 0.040 0.072 0.177 0.010 0.255

SS5 21 0.057 0.020 0.021 0.025 0.030 0.058 0.072 0.156 0.010 0.255

SS6 18 0.061 0.019 0.021 0.026 0.030 0.058 0.078 0.167 0.010 0.255

SS7 22 0.058 0.020 0.021 0.025 0.030 0.065 0.078 0.155 0.010 0.255

SS8 0 ----------
 13

Table 7. Percentile distributions of median concentrations of nitrite plus nitrate as nitrogen, total nitrogen, and total phosphorus from water-quality sites in Oklahoma and part of Arkansas, excluding sites with the Ozark Highland and Ouachita 

Mountains ecoregions, geographic region 3

[concentrations are in milligrams per liter; mean, arithmetic average of median values; min, minimum median value; max, maximum median value; -, 

insufficient number of sites to calculate statistic]

Stream

Category1

1

SS1, stream orders 1, 2, and 3, and stream slope greater than 17 feet per mile

SS2, stream orders 1, 2, and 3, and stream slope less than or equal to 17 feet per mile

SS3, stream orders 4 and 5, and stream slope greater than 17 feet per mile

SS4, stream orders 4 and 5, and stream slope less than or equal to 17 feet per mile

SS5 stream orders 4 and 5, without slope criteria

SS6, stream orders 4 and above, and stream slope less than or equal to 17 feet per mile

SS7, stream orders 4 and above, without slope criteria

SS8, stream orders greater than or equal to 6, and stream slope less than or equal to 17 feet per mile

Number 

of sites2

2

Number of water-quality sites with median concentration

Mean

Percentiles of Median Concentrations

Min Max

10 25 33 50 67 75 90

Nitrite plus Nitrate as Nitrogen

SS1 76 0.45 0.00 0.00 0.03 0.11 0.27 0.45 1.22 0.00 5.00

SS2 127 0.35 0.00 0.00 0.00 0.07 0.14 0.25 1.00 0.00 7.00

SS3 4 0.32 - - - 0.31 - - - 0.13 0.55

SS4 84 0.63 0.11 0.13 0.13 0.32 0.50 0.80 1.73 0.00 6.30

SS5 88 0.61 0.11 0.13 0.13 0.32 0.49 0.78 1.71 0.00 6.30

SS6 149 0.53 0.09 0.13 0.13 0.32 0.50 0.64 1.38 0.00 6.30

SS7 153 0.52 0.09 0.13 0.13 0.32 0.50 0.60 1.28 0.00 6.30

SS8 65 0.41 0.07 0.13 0.15 0.34 0.50 0.60 0.86 0.00 1.64

Total Nitrogen

SS1 44 1.07 0.17 0.40 0.49 0.72 1.31 1.57 2.52 0.00 3.66

SS2 74 1.06 0.22 0.35 0.45 0.60 0.95 1.37 2.76 0.00 7.49

SS3 4 0.88 - - - 0.85 - - - 0.64 1.20

SS4 72 1.20 0.51 0.64 0.70 1.00 1.30 1.50 2.14 0.31 3.96

SS5 76 1.18 0.53 0.62 0.70 0.93 1.26 1.49 2.12 0.31 3.96

SS6 135 1.18 0.55 0.69 0.78 0.99 1.28 1.48 2.08 0.31 3.96

SS7 139 1.17 0.56 0.69 0.78 0.97 1.27 1.45 2.07 0.31 3.96

SS8 63 1.16 0.59 0.73 0.80 0.99 1.27 1.41 1.93 0.34 3.11

Total Phosphorus

SS1 87 0.077 0.006 0.020 0.028 0.040 0.065 0.080 0.168 0.003 1.315

SS2 133 0.083 0.025 0.040 0.041 0.060 0.085 0.100 0.168 0.006 0.476

SS3 5 0.107 - - - 0.060 - - - 0.030 0.290

SS4 68 0.140 0.030 0.055 0.065 0.088 0.138 0.158 0.331 0.003 0.850

SS5 91 0.138 0.030 0.055 0.062 0.086 0.136 0.158 0.320 0.003 0.850

SS6 151 0.156 0.030 0.060 0.075 0.110 0.155 0.190 0.333 0.003 0.850

SS7 156 0.154 0.030 0.060 0.075 0.110 0.155 0.190 0.329 0.003 0.850

SS8 65 0.176 0.045 0.071 0.099 0.133 0.186 0.223 0.352 0.021 0.790
14 Percentile Distributions of Median Nitrite Plus Nitrate as Nitrogen, Total Nitrogen, and Total Phosphorus Concentrations in

Oklahoma Streams, 1973-2001

Table 8. Percentile distributions of median concentrations of nitrite plus nitrate as nitrogen, total nitrogen, and total phosphorus 

of all water-quality sites in Oklahoma and part of Arkansas, geographic region 4

[concentrations are in milligrams per liter; mean, arithmetic average of median values; min, minimum median value; max, maximum median value; -, insufficient number of sites to calculate statistic]

Stream

category1

1SS1, stream orders 1, 2, and 3, and stream slope greater than 17 feet per mile

SS2, stream orders 1, 2, and 3, and stream slope less than or equal to 17 feet per mile

SS3, stream orders 4 and 5, and stream slope greater than 17 feet per mile

SS4, stream orders 4 and 5, and stream slope less than or equal to 17 feet per mile

SS5 stream orders 4 and 5, without slope criteria

SS6, stream orders 4 and above, and stream slope less than or equal to 17 feet per mile

SS7, stream orders 4 and above, without slope criteria

SS8, stream orders greater than or equal to 6, and stream slope less than or equal to 17 feet per mile

Number 

of 

sites2

2

Number of water-quality sites with median concentration

Mean

Percentiles of median concentrations

Min Max

10 25 33 50 67 75 90

Nitrite plus Nitrate as Nitrogen

SS1 179 0.72 0.00 0.01 0.01 0.13 0.73 1.00 2.00 0.00 5.95

SS2 156 0.48 0.00 0.00 0.00 0.08 0.16 0.49 1.82 0.00 7.00

SS3 11 0.75 0.11 0.13 0.13 0.55 1.01 1.15 2.00 0.11 2.00

SS4 108 0.69 0.06 0.13 0.13 0.33 0.70 0.91 2.00 0.00 6.30

SS5 119 0.69 0.08 0.13 0.13 0.36 0.70 0.95 2.00 0.00 6.30

SS6 176 0.58 0.07 0.13 0.13 0.34 0.50 0.70 1.69 0.00 6.30

SS7 187 0.59 0.08 0.13 0.13 0.35 0.51 0.71 1.71 0.00 6.30

SS8 68 0.40 0.08 0.13 0.15 0.35 0.50 0.59 0.80 0.00 1.64

Total Nitrogen

SS1 104 1.09 0.00 0.22 0.38 0.69 1.42 1.76 2.74 0.00 4.13

SS2 91 1.15 0.21 0.40 0.45 0.61 1.19 1.55 3.00 0.00 7.49

SS3 9 1.18 0.41 0.63 0.68 0.91 1.27 1.82 2.45 0.41 2.45

SS4 95 1.22 0.39 0.60 0.68 1.00 1.45 1.53 2.38 0.12 3.96

SS5 104 1.22 0.40 0.61 0.68 0.97 1.43 1.53 2.40 0.12 3.96

SS6 161 1.19 0.48 0.65 0.75 0.99 1.32 1.51 2.31 0.12 3.96

SS7 170 1.19 0.47 0.64 0.75 0.98 1.31 1.51 2.32 0.12 3.96

SS8 66 1.15 0.59 0.73 0.75 0.98 1.22 1.40 1.88 0.34 3.11

Total Phosphorus

SS1 192 0.061 0.003 0.010 0.018 0.026 0.040 0.050 0.121 0.000 1.315

SS2 166 0.083 0.020 0.030 0.040 0.055 0.080 0.100 0.162 0.006 0.670

SS3 13 0.079 0.010 0.027 0.036 0.060 0.090 0.110 0.234 0.003 0.290

SS4 113 0.127 0.024 0.047 0.060 0.084 0.129 0.156 0.228 0.003 0.850

SS5 126 0.122 0.023 0.040 0.055 0.080 0.118 0.151 0.227 0.003 0.850

SS6 68 0.172 0.043 0.069 0.095 0.132 0.179 0.214 0.339 0.021 0.790

SS7 181 0.144 0.030 0.055 0.066 0.106 0.150 0.178 0.292 0.003 0.850

SS8 68 0.172 0.043 0.069 0.095 0.132 0.179 0.214 0.339 0.021 0.790
 15

slopes greater than 17 feet per mile (SS1) were less 

than those with stream slopes less than or equal to 17 

feet per mile (SS2). There was little difference for 

nitrate and total nitrogen concentrations on fourth 

order streams or higher (SS3 through SS8). 

The U.S. Environmental Protection Agency 

(2000a) reports the lower 25th percentile nutrient 

concentrations analyzed from all nutrient data available for a geographic area as being representative of 

the minimally impaired reference condition. Nutrient 

criteria for median nitrate, total nitrogen, and total 

phosphorus concentrations provided by the USEPA for 

the Ozark Highland ecoregion (table 2) were less than 

the 25th percentiles of median concentrations calculated for this report (table 5). The USEPA criteria for 

the Ouachita Mountains ecoregion (table 2) for nitrate 

and total nitrogen were similar to the 25th percentiles 

of median concentrations calculated for this report 

(table 6). The 25th percentiles of median total phosphorus concentrations calculated for this report were 

similar to those reported by the USEPA for the 

Ouachita Mountains ecoregion. 

The 75th percentiles of median nitrate and total 

phosphorus concentrations in geographic region 4 

(table 8) calculated for this report were less than the 

nutrient criteria currently (2002) used in the Oklahoma 

USAP (fig. 1). The 90th percentiles of median total 

phosphorus concentrations in geographic region 4 

(table 8) were less than the nutrient criteria currently 

(2002) used in the Oklahoma USAP except for stream 

category SS2. 

The percentile distributions of median concentrations of nitrate, total nitrogen, and total phosphorus 

presented in tables 5, 6,7, and 8 will facilitate the 

development of nutrient criteria for use in the USAP 

for Oklahoma. Using nutrient information collected 

from streams in Oklahoma and grouping these data by 

geographic regions and stream characteristics will 

allow nutrient criteria to be established that represent 

local water-quality conditions relevant to Oklahoma.

SUMMARY

The U.S. Environmental Protection Agency has 

developed regional-based nutrient criteria using ecoregions to protect streams in the United States from 

impairment. However, nutrient criteria were based on 

nutrient concentrations collected in large aggregated 

nutrient ecoregions with little relevance to local environmental conditions in states. The Oklahoma Water 

Resources Board is using a dichotomous process 

known as Use Support Assessment Protocols to define 

nutrient criteria in Oklahoma streams. The Oklahoma 

Water Resources Board is modifying the Use Support 

Assessment Protocols to reflect nutrient information 

and environmental characteristics relevant to Oklahoma streams, while considering nutrient information 

grouped by geographic regions based on level III 

ecoregions and state boundaries. 

This report provides percentile distributions of 

median nitrite plus nitrate as nitrogen (referred to as 

nitrate), total nitrogen, and total phosphorus concentrations measured in water samples collected from 

1973-2001 at 563 sites in Oklahoma and 4 sites in 

Arkansas near the Oklahoma and Arkansas border. 

Sites were grouped into eight stream categories by 

stream slope and stream order. Percentile distributions 

were calculated for four geographic regions in Oklahoma and part of Arkansas: (1) water-quality sites in 

the Ozark Highland ecoregion; (2) water-quality sites 

in the Ouachita Mountains ecoregion; (3) waterquality sites excluding those in the Ozark Highland 

and Ouachita Mountains ecoregions; and (4) all waterquality sites in Oklahoma and part of Arkansas. 

The 50th percentiles of median nitrate, total 

nitrogen, and total phosphorus concentrations were 

greatest in the Ozark Highland ecoregion and least in 

the Ouachita Mountains ecoregion compared to the 

other geographic regions. The 50th percentiles of 

median concentrations of nitrate, total nitrogen, and 

total phosphorus generally were less in first, second, 

and third order streams (SS1 and SS2). The 50th 

percentiles of median total nitrogen, and total phosphorus concentrations in the Ozark Highland and 

Ouachita Mountains ecoregions in first, second, and 

third order streams with stream slopes greater than 17 

feet per mile (SS1) were less than streams with stream 

slopes less than or equal to 17 feet per mile (SS2).

Nutrient criteria reported by the U.S. Environmental Protection Agency for the Ozark Highland 

ecoregion were lower than the 25th percentiles of 

median nitrate, total nitrogen, and total phosphorus 

concentrations calculated for this report. Nitrate and 

total nitrogen criteria developed by the U.S. Environmental Protection Agency for the Ouachita Mountains 

ecoregion were similar to the 25th percentiles of 

median values for the Ouachita Mountains ecoregion 

calculated for this report. The 75th percentiles of 

median nitrate and total phosphorus concentrations in 
16 Percentile Distributions of Median Nitrite Plus Nitrate as Nitrogen, Total Nitrogen, and Total Phosphorus Concentrations in

Oklahoma Streams, 1973-2001

geographic region 4 were less than the nutrient criteria 

currently (2002) used in the Oklahoma Use Support 

Assessment Protocols. The 90th percentiles of median 

total phosphorus concentrations in geographic region 

4 were less than the nutrient criteria currently (2002) 

used in the Oklahoma Use Support Assessment Protocols except for stream category SS2. 

The percentile distributions of median nitrate, 

total nitrogen, and total phosphorus concentrations 

calculated in this report will facilitate the development 

of nutrient criteria that can be used to modify the Use 

Support Assessment Protocols for Oklahoma. By 

using nutrient information collected in Oklahoma 

streams and grouping nutrient data by local 

geographic regions and stream characteristics will 

facilitate nutrient criteria that represents water-quality 

conditions in Oklahoma.

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Report to Congress: Office of Water, accessed July 

2002, at URL: http://www.epa.gov/305b/98report.

———2000a, Nutrient criteria technical guidance manual: 

rivers and streams: EPA- 822-B-00-002, p. 141.

———2000b, Ambient water quality criteria recommendations: rivers and streams in nutrient ecoregion XI: 

EPA- 822-B-00-020, p. 99.

———2000c, Ambient water quality criteria recommendations: rivers and streams in nutrient ecoregion IX: 

EPA- 822-B-00-019, p. 108.

———2001a, Ambient water quality criteria recommendations: rivers and streams in nutrient ecoregion IV: EPA822-B-01-013, p. 159.

———2001b, Ambient water quality criteria recommendations: rivers and streams in nutrient ecoregion V: EPA822-B-01-4, p. 152.

———2001, U.S. Environmental Protection Agency 

Storage and Retrieval system database, accessed on 

July, 2001 at URL http://www.epa.gov/waterscience/criteria/nutrient/database/index.html
18 Percentile Distributions of Median Nitrite Plus Nitrate as Nitrogen, Total Nitrogen, and Total Phosphorus Concentrations in

Oklahoma Streams, 1973-2001

U.S. Geological Survey, 2001, National Elevation Dataset 

Homepage: accessed November 14, 2001, at URL 

http://gisdata.usgs.gov/ned

Van Nieuwenhuyse, E.E., and Jones, J.R., 1996, Phosphorus-chlorophyll relationship in temperate streams 

and its variation with stream catchment area: Canadian 

Journal of Fisheries and Aquatic Sciences, v. 53, p. 99-

105.

Welch, E.B., Jacob, J.M., Horner, R.R., and Seely, M.R., 

1988, Nuisance biomass levels of periphytic algae in 

streams: Hydrobiologia, v. 157, p. 161-168.

———, Quinn, J.M., and Hickey, C.W., 1992, Periphyton 

biomass related to point-source nutrient enrichment in 

seven New Zealand streams: Water Research, v. 26, no. 

5, p. 669-675.

———, Horner, R.R., and Patmont, C.R., 1989, Prediction 

of nuisance periphytic biomass: a management 

approach: Water Research, v. 23, no. 4, p. 401-405.

Winter, J.G., and Duthie, H.C., 2000, Epilithic diatoms as 

indicators of stream total N and total P concentrations: 

Journal of the North American Benthological Society, 

v. 19, no. 1, p. 32-49.

Winterbourn, M.J., Hildrew, A.G., and Orton, Stuart, 1992, 

Nutrients, algae and grazers in some British streams of 

contrasting pH: Freshwater Biology, v. 28, p. 173-182.
APPENDIX
20 Percentile Distributions of Median Nitrite Plus Nitrate as Nitrogen, Total Nitrogen, and Total Phosphorus Concentrations in 

Oklahoma Streams, 1973-2001
 21

Nutrients and Stream Impairment

Establishing nutrient concentrations for potentially 

impaired streams has been a slower process compared to 

lakes (Welch and others, 1989; Dodds and others, 1997; 

Dodds and others 1998; Robertson and others, 2001). 

Eutrophic nutrient concentrations in lakes have been defined 

by relations between algal biomass or indirect biomass 

assessment by chlorophyll-a concentrations and the limiting 

nutrient (nitrogen or phosphorus) of the aquatic system 

(Carlson, 1977; Kratzner and Brezonik, 1981). Similar to 

lakes or lentic systems, the aesthetic, ecological, and 

economic value of streams or lotic systems is often impaired 

by excessive nutrient concentrations and algal growth. 

Excessive algal growth impedes boat traffic, obstructs 

fishing, and creates unsightly and malodorous masses 

(Caffrey, 2002). Welch and others (1989) report that 

nuisance (peak) algal biomass can be present in streams 

when benthic chlorophyll-a concentrations exceed 100 to 

150 milligrams of chlorophyll-a per meter squared (10-15 

micrograms per centimeter squared). Few studies have 

established water column nutrient concentrations characterizing excessive algal growth and eutrophic or potentially 

impaired stream ecosystems.

Nutrient and algal concentrations generally are the 

basis of trophic state classification in lakes and reservoirs 

(Carlson, 1977; Kratzner and Brezonik, 1981). Correlation 

between nutrient concentrations and algal productivity 

(chlorophyll) is expected and significant in lakes and reservoirs (Dillon and Rigler, 1974; Canfield and Bachman, 

1981). However, the correlation between nutrient concentrations and chlorophyll is more variable in streams, because 

of the effects of floods, water velocity, turbidity, grazing, 

shading, and catchment area among other factors (Biggs, 

1996; 2000; Delong and Brusven, 1992; Dodds, 1991; 

Lohman and Jones, 1999; Lohman and others, 1992; Jones 

and others, 1984; Van Nieuwenhuyse and Jones, 1996; and 

Winterbourn and others, 1992). Physical factors not only 

affect chlorophyll concentrations, but also shift the 

taxonomy of algal species in streams (Carpenter and Waite, 

2000). Thus, deriving chlorophyll concentrations, based on 

stream nutrient concentrations has a higher degree of uncertainty compared to the predictions made for lakes and reservoirs.

Increasing nutrient concentrations in streams generally produce increased chlorophyll concentrations and algal 

production in the water column and in the stream-bottom 

environment. Stream algae respond to the addition of nutrients as commonly observed in nutrient limitation experiments using in situ artificial substrata and nutrient 

enrichment (Chessman and others, 1992; Lohman and 

others, 1992; Matlock and others, 1999a). The uptake 

kinetics of algae also indicates that algal production is stimulated by enrichment of nutrients; however, the concentrations at which algal cellular growth is saturated are quite 

low. For example, Bothwell (1985) reported that diatom 

saturation occurred at dissolved phosphorus concentration 

less than 5 micrograms per liter. Horner and others (1983) 

reported that an average dissolved phosphorus concentration of 15-25 micrograms per liter provided apparent saturation for chlorophyll accrual. However, a dissolved 

phosphorus concentration of 50 micrograms per liter was 

needed to achieve maximum biomass accrual (Bothwell, 

1989). These concentrations are typical of many streams, 

especially in Oklahoma, yet nuisance biomass accrual is 

limited by disturbance factors such as flooding and grazing. 

Where disturbances inhibit the production of 

nuisance algal growth, conditions can favor the development of excessive algal biomass; the summer season may be 

the most likely period of sustained algal growth because 

fewer episodic flood events occur. Management of stream 

nutrient concentrations and algae require the ability to 

predict the occurrence of nuisance algae. Biggs (1995) 

reported the following range in chlorophyll values for 

streams: 1) 0.5-3 milligrams of chlorophyll-a per meter 

squared (0.05-0.3 micrograms per centimeter squared) for 

unenriched, forested streams, 2) 3-60 milligrams of chlorophyll-a per meter squared (0.3-6 microgram per centimeter 

squared) for moderately enriched stream draining catchments with moderate agricultural land use, and 3) 25-260 

milligrams of chlorophyll-a per meter squared (2.5-26 

micrograms per centimeter squared) for highly enriched 

streams draining catchments with highly developed agricultural land use, and underlain by nutrient-rich bedrock. 

Welch and others (1988) reported that nuisance algal 

biomass might be present in streams when benthic chlorophyll-a concentrations exceed 100-150 milligrams of chlorophyll-a per meter squared (10-15 micrograms per 

centimeter squared). Below these measurements, the 

proportion of filamentous periphytic coverage was less than 

20 percent. Nuisance levels of algal biomass affect the beneficial use of stream systems. Biggs (1985) summarized 

water-quality problems associated with nuisance biomass 

accrual. For example, high biomass accrual may impair 

water withdrawals for irrigation and municipal water 

supply. The large amount of autochthonous algal production 

also may contribute to oxygen demand and produce 

substantial diurnal variations in dissolved oxygen concentrations resulting in fish kills; similar diurnal variations in 

pH also are observed. Increased coverage of filamentous 

species further alters ecosystems by changing the habitat 

and macro invertebrate community structure. 

Nuisance production of algal biomass represents a 

management issue for streams, an unacceptable degree of 

eutrophication. Simple statistical models have predicted 

algal biomass as a function of dissolved nutrient concentrations, total nutrient concentrations, flooding frequency (days 

of biomass accrual), water velocity, and temperature (Welch 

and others, 1989, 1992; Dodds and others, 1997; Winter and 

Duthie, 2000). Statistical models were improved after 

seasonal variation was removed using mean water chemistry 
22 Percentile Distributions of Median Nitrite Plus Nitrate as Nitrogen, Total Nitrogen, and Total Phosphorus Concentrations in 

Oklahoma Streams, 1973-2001

and chlorophyll concentrations (Winter and Duthie, 2000); 

statistical models also more closely predicted maximum 

chlorophyll concentrations (Biggs, 2000). Statistical models 

using total nutrient concentrations can perform better than 

those using dissolved nutrients (Dodds and others, 1997) 

because dissolved nutrient concentrations are determined by 

the balance between uptake and regeneration, similar to 

lakes. Dodds and others (1997) reported that using 

dissolved nutrient concentrations as predictors of chlorophyll concentrations is unwarranted without site-specific 

models for uptake and regeneration. However, Biggs (2000) 

and Welch and others (1992) have shown significant correlation between dissolved nutrient concentrations and chlorophyll concentrations.

Other statistical models relate total nutrient concentrations, catchment area, and land use to sestonic chlorophyll concentrations (Lohman and Jones, 1999; Jones and 

others, 1984; Van Nieuwenhuyse and Jones, 1996); the 

sestonic algae were assumed to result from sloughing of 

periphytic algae. These reports indicate that sestonic chlorophyll is related to total nutrient concentrations, but also may 

vary with some physical factors, similar to benthic chlorophyll concentrations. Sestonic chlorophyll concentrations in 

Iowa streams averaged 55 milligrams of chlorophyll-a per 

meters cubed (55 micrograms per liter) whereas benthic 

chlorophyll concentrations averaged 160 milligrams of 

chlorophyll-a per meter squared (16 micrograms per centimeter squared) (Swanson and Bachman, 1976). These averages were 2 to 10 times greater than those observed in 

Missouri Ozark streams (Jones and others, 1984). Because 

of the complexity and variability involved in measuring 

benthic chlorophyll concentrations, measurements of 

sestonic chlorophyll may be an easier method to monitor the 

occurrence of nuisance biomass accrual provided some relation between sestonic and benthic chlorophyll concentrations exist.

Statistical models developed between total nutrient 

concentrations and chlorophyll concentrations have been 

used to estimate total nutrient concentrations relating to 

nuisance biomass accrual (Dodds and others, 1997), where 

streams with total nutrient concentrations greater than those 

required to limit nuisance biomass accrual may represent 

eutrophic conditions. Dodds and others (1997) used a 

second complementary approach identifying reference 

portions or stream reaches where mean and maximum chlorophyll does not exceed desirable levels. The two techniques used produced similar results for the Clark Fork 

River in Montana. Maintaining mean total nitrogen and total 

phosphorus below 350 micrograms per liter and 30 micrograms per liter will result in mean chlorophyll concentrations less than 100 milligrams of chlorophyll-a per meter 

squared (10 micrograms per centimeter squared) (Dodds 

and others, 1997).

Statistical models have been developed relating total 

nutrient concentrations and sestonic chlorophyll concentrations. Van Nieuwenhuyse and Jones (1996) confirmed a 

positive relation between sestonic chlorophyll concentrations and total phosphorus concentrations; the standard 

error of regression for this stream model was within the 

range reported in large-scale lake models. The U.S. Environmental Protection Agency (2000a) reported a sestonic 

chlorophyll concentration of 8 micrograms of chlorophyll-a

per liter as the mesoeutrophic-eutrophic boundary based on 

this investigation. This sestonic chlorophyll concentration 

corresponded to a total phosphorus concentration of 42 

micrograms per liter; the Redfield ratio (Smith and others, 

1997) was used to estimate the total nitrogen concentration 

of 300 micrograms per liter. The source of sestonic chlorophyll concentrations is from the proliferation of stream 

phytoplankton or pseudo-phytoplankton; the source of 

pseudo-phytoplankton could be sloughing of benthic algae.

An alternative approach to identifying eutrophic 

streams is to use cumulative distributions of total nutrient 

and chlorophyll concentrations (Dodds and others, 1998; 

U.S. Environmental Protection Agency, 2000a); it was 

suggested that the upper third of the distribution be considered eutrophic. The cumulative distribution of trophic staterelated variables collected in temperate streams of North 

America and New Zealand (Dodds and others, 1997; Van 

Nieuwenhuyse and Jones, 1996; Omernik, 1977) were used 

to compute the oligotrophic, mesotrophic and eutrophic 

boundaries (U.S. Environmental Protection Agency, 2000a). 

The oligotrophic-mesotrophic boundary for mean benthic 

chlorophyll, maximum benthic chlorophyll, sestonic chlorophyll, total nitrogen, and total phosphorus are 20 milligrams 

of chlorophyll-a per meter squared (2 micrograms per centimeter squared), 60 milligrams of chlorophyll-a per meter 

squared (6 micrograms per centimeter squared), 10 micrograms of chlorophyll-a per liter, 700 micrograms of total 

nitrogen per liter, and 25 micrograms of total phosphorus 

per liter. The mesotrophic-eutrophic boundary for mean 

benthic chlorophyll, maximum benthic chlorophyll, 

sestonic chlorophyll, total nitrogen, and total phosphorus 

was 70 milligrams of chlorophyll-a per meter squared (7 

micrograms per centimeter squared), 200 milligrams of 

chlorophyll-a per meter squared (20 micrograms per centimeter squared), 30 micrograms of chlorophyll-a per liter, 

1,500 micrograms per liter, and 75 micrograms per liter. 

Streams with trophic state-related variables with concentrations greater than the mesotrophic-eutrophic boundary may 

represent eutrophic systems. These data were compiled 

from different sources; the data source used for the cumulative distribution of total nitrogen and total phosphorus 

concentrations was different than the data source used for 

the benthic and sestonic chlorophyll concentrations. These 

boundaries were used to provide a source of comparison 

when developing nutrient criteria.

Another approach uses the ratio of periphytic growth 

on artificial substrate with and without nutrient enrichment 

(Matlock and others, 1999a; 1999b); this method is referred 
 23

to as the Lotic Ecosystem Trophic Status Index (LETSI). 

LETSI is simply the ratio of periphytic growth (chlorophylla concentration) on the control artificial substrate to periphytic growth (chlorophyll-a concentration) on a nutrient 

enriched artificial substrate; thus, LETSI theoretically 

ranges from 0 to 1. A LETSI of 1 indicates that a stream 

exceeds the nutrient assimilative capacity of the stream 

biota or a factor other than nutrients is limiting periphytic 

growth. Therefore, the stream has reduced nutrient retention 

efficiency, and nutrients are transported downstream 

without substantial biotic uptake. Thus, a long nutrient 

uptake length (Stream Solute Workshop, 1990) would be 

expected in streams with a LETSI approaching 1. LETSI 

has been used to define eutrophic conditions in tributaries of 

the Illinois River in northeastern Oklahoma (Matlock and 

others, 1999a) and for the Bosque River in central Texas 

(Matlock and others, 1999b), LETSI was used in the 

Bosque River to determine a potential criterion for phosphorus in the streams draining the Lake Waco watershed 

(Kiesling and others, 2001); this study assumed a LETSI of 

0.5 represented the boundary between mesotrophic and 

eutrophic conditions. A dissolved phosphorus concentration 

of about 40 micrograms per liter was the phosphorus target 

using an asymptotic relation between LETSI and dissolved 

phosphorus concentration in the water column of these 

streams (Kiesling and others, 2001). As with any measure 

of periphytic growth (especially on artificial substrate), 

there are many sources of variability to be recognized and 

addressed when using the LETSI method (Matlock and 

others, 1999a)
24 Percentile Distributions of Median Nitrite Plus Nitrate as Nitrogen, Total Nitrogen, and Total Phosphorus Concentrations in 

Oklahoma Streams, 1973-2001