U.S. GEOLOGICAL SURVEY

Water-Resources Investigations Report 

Benthic Invertebrate Assemblages and 

Their Relation to Physical and Chemical 

Characteristics of Streams in the 

Eastern Iowa Basins, 1996–98

By Allison R. Brigham and Eric M. Sadorf

Iowa City, Iowa

2001

00–4256
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

U.S. Geological Survey

Information Services

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from:

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U.S. Geological Survey

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400 South Clinton Street

P.O. Box 1230

Iowa City, IA 52244
FOREWORD

III

The U.S. Geological Survey (USGS) is committed to serve the Nation with accurate and timely scientific 

information that helps enhance and protect the overall quality of life, and facilitates effective management of water, 

biological, energy, and mineral resources (http://www.usgs.gov/). Information on the quality of the Nation’s water 

resources is of critical interest to the USGS because it is so integrally linked to the long-term availability of water 

that is clean and safe for drinking and recreation, and that is suitable for industry, irrigation, and habitat for fish 

and wildlife. Escalating population growth and increasing demands for the multiple water uses make water 

availability, now measured in terms of quantity and quality, even more critical to the long-term sustainability of 

our communities and ecosystems.

The USGS implemented the National Water-Quality Assessment (NAWQA) Program to support national, 

regional, and local information needs and decisions related to water-quality management and policy 

(http://water.usgs.gov/nawqa). Shaped by and coordinated with ongoing efforts of other Federal, State, and local 

agencies, the NAWQA Program is designed to answer: What is the condition of our Nation’s streams and ground 

water? How are the conditions changing over time? How do natural features and human activities affect the quality 

of streams and ground water, and where are those effects most pronounced? By combining information on water 

chemistry, physical characteristics, stream habitat, and aquatic life, the NAWQA Program aims to provide sciencebased insights for current and emerging water issues and priorities. NAWQA results can contribute to informed 

decisions that result in practical and effective water-resource management and strategies that protect and restore 

water quality.

Since 1991, the NAWQA Program has implemented interdisciplinary assessments in more than 50 of the Nation’s 

most important river basins and aquifers, referred to as Study Units. 

(http://water.usgs.gov/nawqa/nawqamap.html). Collectively, these Study Units account for more than 60 percent 

of the overall water use and population served by public water supply, and are representative of the Nation’s major 

hydrologic landscapes, priority ecological resources, and agricultural, urban, and natural sources of contamination. 

Each assessment is guided by a nationally consistent study design and methods of sampling and analysis. The 

assessments thereby build local knowledge about water-quality issues and trends in a particular stream or aquifer 

while providing an understanding of how and why water quality varies regionally and nationally. The consistent, 

multi-scale approach helps to determine if certain types of water-quality issues are isolated or pervasive, and 

allows direct comparisons of how human activities and natural processes affect water quality and ecological health 

in the Nation’s diverse geographic and environmental settings. Comprehensive assessments on pesticides, 

nutrients, volatile organic compounds, trace metals, and aquatic ecology are developed at the national scale 

through comparative analysis of the Study-Unit findings (http://water.usgs.gov/nawqa/natsyn.html). 

The USGS places high value on the communication and dissemination of credible, timely, and relevant science so 

that the most recent and available knowledge about water resources can be applied in management and policy 

decisions. We hope this NAWQA publication will provide you the needed insights and information to meet your 

needs, and thereby foster increased awareness and involvement in the protection and restoration of our Nation’s 

waters. 

The NAWQA Program recognizes that a national assessment by a single program cannot address all waterresource issues of interest. External coordination at all levels is critical for a fully integrated understanding of 

watersheds and for cost-effective management, regulation, and conservation of our Nation’s water resources. The 

Program, therefore, depends extensively on the advice, cooperation, and information from other Federal, State, 

interstate, Tribal, and local agencies, non-government organizations, industry, academia, and other stakeholder 

groups. The assistance and suggestions of all are greatly appreciated.

Robert M. Hirsch

Associate Director for Water
CONTENTS V

CONTENTS

Foreword................................................................................................................................................................................ III

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

Introduction............................................................................................................................................................................ 1

Purpose and Scope....................................................................................................................................................... 3

Description of the Eastern Iowa Basins....................................................................................................................... 3

Acknowledgments ....................................................................................................................................................... 3

Methods ................................................................................................................................................................................. 5

Site Selection and Description..................................................................................................................................... 5

Water-Quality Variables............................................................................................................................................... 5

Habitat.......................................................................................................................................................................... 5

Benthic Invertebrate Collection and Data Preparation ................................................................................................ 8

Field Sampling................................................................................................................................................... 8

Laboratory Processing ....................................................................................................................................... 8

Data Preparation ................................................................................................................................................ 8

Statistical Analyses and Other Calculations ................................................................................................................ 14

Distribution of Benthic Invertebrates .................................................................................................................................... 18

Spatial and Temporal Variability ................................................................................................................................. 18

Spatial Variability .............................................................................................................................................. 20

Temporal Variability .......................................................................................................................................... 20

Differences in Benthic Invertebrates Among Site Groups .......................................................................................... 28

Influence of Physical and Chemical Characteristics of Streams on Benthic Invertebrate Assemblages .............................. 30

Identification of Important Environmental Variables .................................................................................................. 31

Distinctions Among Site Groups ................................................................................................................................. 32

Responses of Benthic Invertebrates to Nutrients and Organic Enrichment................................................................. 33

Summary................................................................................................................................................................................ 34

References Cited.................................................................................................................................................................... 35

Supplemental Data................................................................................................................................................................. 39

FIGURES

1. Map showing location of benthic invertebrate sampling sites and landforms in the Eastern Iowa Basins, 

Iowa and Minnesota .................................................................................................................................................. 2

2–5 Graphs showing:

2. Discharge at the three multiple-reach, multiple-year sampling sites in the Eastern Iowa Basins, 

Iowa and Minnesota .......................................................................................................................................... 4

3. Detrended correspondence analysis (DCA) biplot of benthic invertebrate taxa in relation to 31 basic fixed 

and synoptic sites in the Eastern Iowa Basins, Iowa and Minnesota, 1996−97 ................................................. 19

4. Linear regression of Ephemeroptera, Plecoptera, and Trichoptera (EPT) taxa and Chironomidae 

as relative abundance in 37 samples from 31 basic fixed and synoptic sites in the Eastern Iowa Basins,

Iowa and Minnesota, 1996−97 ........................................................................................................................... 20

5. Detrended correspondence analysis (DCA) biplot of benthic invertebrates in relation to 12 basic 

fixed sites in the Eastern Iowa Basins, Iowa and Minnesota, 1996 .................................................................. 22

TABLES

1. Location and basin characteristics of 31 stream sites in the Eastern Iowa Basins, Iowa and Minnesota ............... 6

2. Environmental variables used to describe 12 basic fixed sites in the Eastern Iowa Basins, Iowa and 

Minnesota, 1996−98 ................................................................................................................................................. 7

3. Distribution of benthic invertebrates collected at 31 sites in the Eastern Iowa Basins, Iowa and Minnesota, 

1996−98, and the number of sites where each taxon was found .............................................................................. 9

4. Distribution of samples among site groups, and mean and cumulative relative abundance of benthic 

invertebrates in the Eastern Iowa Basins, Iowa and Minnesota, 1996−97 ............................................................... 15
VI CONTENTS

5. Values of metrics and functional-feeding-group data associated with benthic invertebrates in site groups 

identified in the Eastern Iowa Basins, Iowa and Minnesota, 1996−97 .................................................................... 17

6. Jaccard coefficient of community between reaches in percent similarity of taxonomic composition and 

the number of taxa at sites 5, 19, and 21 in the Eastern Iowa Basins, Iowa and Minnesota, 1997......................... 18

7. Jaccard coefficient of community between years in percent similarity of taxonomic composition and 

the number of taxa at sites 5, 19, and 21 in the Eastern Iowa Basins, Iowa and Minnesota, 1996−98.................... 19

8. Correlations of detrended correspondence analysis (DCA) axis scores and environmental variables from 

12 basic fixed sites in the Eastern Iowa Basins, Iowa and Minnesota, 1996.......................................................... 21

9. Mean concentration or value of environmental variables in four site groups used to describe 12 basic 

fixed sites in the Eastern Iowa Basins, Iowa and Minnesota, 1996 ........................................................................ 23

10. Responses of benthic invertebrates and associated metrics to concentrations of organic carbon, nitrogen, 

and phosphorus, and reported tolerance values for each taxon............................................................................... 25

11. Percentages of land use/land cover and landform at 31 stream-site basins in the Eastern Iowa Basins, Iowa 

and Minnesota ......................................................................................................................................................... 41

12. Pesticide and fertilizer applications in stream-site basins, and physical variables at 12 basic fixed sites in 

the Eastern Iowa Basins, Iowa and Minnesota........................................................................................................ 42

13. Streambank characteristics and canopy shading at 12 basic fixed sites in the Eastern Iowa Basins, Iowa 

and Minnesota ......................................................................................................................................................... 43

14. Streambed materials and stream characteristics at 12 basic fixed sites in the Eastern Iowa Basins, Iowa 

and Minnesota ......................................................................................................................................................... 44

CONVERSION FACTORS, VERTICAL DATUM, AND ABBREVIATIONS

Multiply inch-pound units By To obtain metric units

centimeter (cm) 0.3937 inch

meter (m) 3.281 foot (ft)

kilometer (km) 0.6214 mile (m)

square kilometer (km2) 0.3861 square mile (mi2)

kilogram (kg) 0.4536 pound (lb)

liter (L) 0.2642 gallon (gal)

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

° F = [1.8(°C)] + 32

Sea level: In this report, “sea level” refers to the National Geodetic Vertical Datum of 1929 (NGVD of 1929)—a geodetic 

datum derived from a general adjustment of the first-order level nets of the United States and Canada, formerly called Sea 

Level Datum of 1929.

Abbreviated water-quality units: Chemical concentrations are given in metric units of milligrams per liter (mg/L) and 

micrograms per liter (μg/L). Milligrams per liter and micrograms per liter are units expressing the concentration of chemical 

constituents in solution as mass (milligrams or micrograms) of solute per unit volume (liter) of water. For concentrations less

than 7,000 mg/L, the numerical value of milligrams per liter is the same as for concentrations in parts per million. The 

numerical value of micrograms per liter is the same as for concentrations in parts per billion.
CONTENTS VII

These additional abbreviations and symbols are used in this report:

Abbreviation Description

ANOVA Analysis of variance

DCA Detrended correspondence analysis

EIWA Eastern Iowa Basins

EPT Ephemeroptera, Plecoptera, Trichoptera

GIS Geographic Information System

NAWQA National Water-Quality Assessment Program

NWQL National Water Quality Laboratory

TWINSPAN Two-Way Indicator Species Analysis

USGS U.S. Geological Survey

± plus or minus

> greater than

< less than

≥ greater than or equal to

≤ less than or equal to

% percent

μm micrometer

g O2/m3/hr grams of oxygen per cubic meter per hour
Abstract 1

Benthic Invertebrate Assemblages and Their 

Relation to Physical and Chemical Characteristics 

of Streams in the Eastern Iowa Basins, 1996–98

By Allison R. Brigham and Eric M. Sadorf

Abstract

Over 250 benthic invertebrate taxa were 

identified from snags and woody debris in streams 

and rivers of the Wapsipinicon, Cedar, Iowa, and 

Skunk River Basins in the Eastern Iowa Basins 

(EIWA) study unit of the U.S. Geological Survey 

National Water-Quality Assessment Program. The 

composition, distribution, and abundance of 

74 predominant taxa were related to environmental conditions in the study unit, using habitat, 

hydrologic, and water-quality data. Four groups 

of sites were defined, based on the distribution 

and relative abundance of taxa. Detrended correspondence analysis was used to identify relations 

in the structure of the invertebrate assemblages, 

and the correspondence of taxa and sites in the 

groups was related to habitat, hydrologic, and 

water-quality information. Responses of invertebrate assemblages were explained by natural 

factors, such as surficial geology or physical 

habitat conditions, as well as human influences, 

such as agriculture or high-density hog-feeding 

operations.

Mayflies, caddisflies, and true flies were 

well represented in streams and rivers of the 

EIWA study unit. The mayflies Tricorythodes and 

Baetis intercalaris, the net-spinning caddisflies 

Hydropsyche bidens and H. simulans, and the 

Chironomidae Glyptotendipes, Polypedilum, and 

Rheotanytarsus predominated. Spatial variation in 

benthic invertebrate assemblages within a site was 

less than that observed among sites. Assemblages 

from 3 years of sampling generally were grouped 

by site, with exceptions related to differences in 

discharge among years.

The benthic invertebrate assemblages associated with the four groups of sites reflected the 

cumulative effects of agricultural and urban land 

use, sources of nutrient and organic enrichment, 

and longitudinal stream succession—the natural 

sequence of communities in streams from headwaters to large rivers. These factors, especially the 

natural changes from upstream to downstream, 

were influential in characterizing the benthic 

invertebrate assemblages of the site groups. 

Stream size, a reflection of basin area, was 

a principal influence in categorizing the benthic 

invertebrate assemblages, with sites that have the 

largest basin areas forming a separate group. 

Although it is difficult to distinguish among the 

contributions of large basin area, increased 

concentrations of nutrients and pesticides, and 

decreasing instream habitat diversity, the resulting 

invertebrate assemblage described was distinct. 

The remaining sites were headwater or tributary 

streams that reflected conditions more common to 

smaller streams, such as higher gradients and the 

potential for more diverse or extensive riparian 

habitat, but were distinguished by landform. 

Following basin area in importance, landform 

contributed to the differences observed among the 

benthic invertebrate communities at the remaining 

sites.

INTRODUCTION

The Eastern Iowa Basins (EIWA) study unit 

(fig. 1) drains about 50,500 km2

 (19,500 mi2), 
2 Benthic Invertebrate Assemblages and Their Relation to Physical and Chemical Characteristics of Streams in the Eastern Iowa 

Basins, 1996–98

EXPLANATION

Basic fixed site and number

Synoptic site and number

Combined synoptic and basic fixed site

 and number

Site group 1

Site group 2

Site group 3

Site group 4

Des Moines Lobe

Iowan Surface

Iowan Karst

Southern Iowa Drift Plain

Landforms

0 30

0 30 60 MILES 

60 KILOMETERS

41°

42°

43°

93°

92°

91°

West Fork Cedar River

Base from U.S. Geological Survey, 1:250,000

Albers Equal-Area Conic Projection

Standard parallels 41°30' and 44°30'

Landforms modified

from Prior (1991)

2

1

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

21

22

23

29

30 31

25

24

26

27

28

Shell Rock River

Cedar Ri ver

Wapsipinicon

R

English River

Cedar River

Iowa

River

Skunk

River

South

Skunk Ri ver

North Skunk River

Iowa River

Cedar River

Iowa River

Eastern Iowa

Basins study unit

MINNESOTA

IOWA

Wolf Cr

Figure 1. Location of benthic invertebrate sampling sites and landforms in the Eastern Iowa Basins, Iowa and Minnesota.
INTRODUCTION 3

including the Wapsipinicon, Cedar, Iowa, and Skunk 

River Basins. Presettlement, the area was primarily tall 

grass prairie with wooded stream valleys (Prior, 1991); 

now more than 90 percent of the area is in intensive 

agricultural production (Schnoebelen and others, 

1999). Nutrients and pesticides used to produce row 

crops, such as corn and soybeans, enter streams by 

surface runoff and discharges from ground-water flow 

and tile drains. The effects of these chemicals on 

benthic invertebrates and how assemblages respond to 

differences in surficial geology, riparian cover, and 

streamflow characteristics are poorly known.

The National Water-Quality Assessment 

(NAWQA) Program was established to describe waterquality conditions for a large part of the Nation’s 

streams, rivers, and aquifers; to describe how water 

quality is changing over time; and to improve understanding of natural and human factors that affect 

water-quality conditions (Hirsch and others, 1988; 

Leahy and others, 1990; Gilliom and others, 1995). 

One of 59 water-quality investigations in large river 

basins or aquifer systems (study units) throughout the 

United States, the EIWA study unit represents a hydrologic system in an intensive agricultural area (Akers 

and others, 1999). The integrated assessment objectives of the program incorporated physical, chemical, 

and biological components (Gurtz, 1994; Meador and 

Gurtz, 1994) that used basic data-collection activities 

to determine the status and trends in water quality. In 

addition, sites in the EIWA study unit were included in 

a regional, low-flow synoptic study that assessed the 

condition of wooded riparian zones and the influence 

of basin soil-drainage characteristics on water quality 

and biological-community responses (Sorenson and 

others, 1999).

Purpose and Scope

This report describes the benthic invertebrates 

present in the streams and rivers of the Wapsipinicon, 

Cedar, Iowa, and Skunk River Basins during 1996–98. 

The composition, distribution, and abundance of 

benthic invertebrates were related to physical and 

chemical characteristics of streams, using habitat, 

hydrologic, and water-quality data. The quality of the 

aquatic resources was assessed by evaluating the influence of natural factors, such as differences in surficial 

geology or physical habitat conditions, and human 

influences, including row-crop agriculture and 

confined, high-density hog-feeding operations, on the 

benthic invertebrate assemblages.

Description of the Eastern Iowa Basins 

The major landforms present in the study unit 

are the Des Moines Lobe, Iowan Karst, Iowan Surface, 

and Southern Iowa Drift Plain (fig. 1). The Des 

Moines Lobe is a region of the most recent glaciation 

in Iowa and is characterized by knob and kettle terrain 

with areas of low relief. This area contains natural 

wetlands caused by poor surface drainage. The Iowan 

Surface is characterized by a gently rolling terrain, and 

consists of thin loess or loam over glacial drift, with 

bedrock near the land surface. The Iowan Karst is a 

subunit of the Iowan Surface defined for this study and 

not described by Prior (1991). It is an area with a thin 

layer of unconsolidated material overlying limestone 

and dolomite that contains karst features, recognized 

by sinkholes on the land surface. The Southern Iowa 

Drift Plain is similar to the Iowan Surface except it has 

relatively thick glacial drift with a thick loess cover. 

Streams have eroded deeply into the glacial drift to 

produce steeply rolling hills. 

Agriculture, the dominant land use, is present 

across about 93 percent of the study unit. Forests cover 

about 5 percent of the study unit, and urban land 

accounts for about 2 percent. Water, wetland, and 

barren land each account for less than 1 percent. 

Average annual precipitation from 1961 through 

1990 ranged from 76 cm in the northwestern part of 

the study unit to 91 cm in the southeastern part (Wendland and others, 1992), with most of the precipitation 

occurring during spring and summer. Snowfall has 

been recorded from September to May, with accumulations rarely exceeding 25 cm in 1 day.

Streamflow in eastern Iowa generally increases 

from snowmelt or rain in spring, often as early as 

February, and may remain high into early summer 

(fig. 2). In a typical year, streamflow during late 

summer is fed primarily through ground-water 

discharge. This low-flow condition persists through 

autumn and into winter.

Acknowledgments

The authors acknowledge the contributions and 

assistance of the EIWA NAWQA team and the many 
4 Benthic Invertebrate Assemblages and Their Relation to Physical and Chemical Characteristics of Streams in the Eastern Iowa 

Basins, 1996–98

F M A M J J A S O N D J F M A M J J A S O N D J F M A M J J A S O N

1996 1997 1998

0

1,000

200

400

600

800

DISCHARGE, IN CUBIC

METERS PER SECOND

F M A M J J A S O N D J F M A M J J A S O N D J F M A M J J A S O N

1996 1997 1998

0

60

10

20

30

40

50

DISCHARGE, IN CUBIC

METERS PER SECOND

DISCHARGE, IN CUBIC

METERS PER SECOND

F M A M J J A S O N D J F M A M J J A S O N D J F M A M J J A S O N

1996 1997 1998

0

60

10

20

30

40

50

Middle 50 percent Site 5, Iowa River at Wapello

of historical discharge (1914Ð98)

Mean monthly discharge

Invertebrate sample collected

Middle 50 percent

of historical discharge (1940Ð98)

Mean monthly discharge

Invertebrate sample collected

Mean monthly discharge

Invertebrate sample collected

Site 21, Iowa River near Rowan

Site 19, Wolf Creek near Dysart

Figure 2. Discharge at the three multiple-reach, multiple-year sampling sites in the Eastern Iowa Basins, Iowa and 

Minnesota. Discharge during sampling years is shown with historical discharge for two of the three sites.
METHODS 5

U.S. Geological Survey (USGS) staff members who 

participated in field collection and analyses, including 

Kymm Akers, Kent Becher, James Cerveny, David 

Connell, Jeff Copa, Joshua Eash, Debra Sneck-Fahrer, 

Demarius George, Jeffery Harms, Sienna Hill, Sandy 

Kautz, James Klaus, Patrick Lustgraaf, Denise Montgomery, Julie Noe, Stephen Porter, Nancy Reilly, 

Pamela Smith, Anna Sojka, Angie Stortz, Patrick 

Sweeney, Scott Thul, Jennifer Tobias, and Scott Yess. 

Linda Roberts, the former study-unit biologist for the 

EIWA study unit, planned, organized, and participated 

in the sampling. Stephen Porter, Jonathan Kennen, 

Jeffrey Pritt (USGS), and John Kingston (University 

of Minnesota–Duluth, Natural Resources Research 

Institute) provided guidance and assistance with statistical analyses and interpretation of data. Robin Brightbill, Stephen Kalkhoff, Stephen Porter, and Jon Raese 

(USGS) provided technical review of the manuscript 

and valuable insight.

METHODS

Site Selection and Description

Thirty-one sites within the EIWA study unit 

were sampled for benthic invertebrates. Twelve of 

these were basic fixed sites, chosen for the NAWQA 

network of surface-water-quality sampling. These sites 

represented either large basins, where the stream or 

river might be affected by a combination of land-use 

types, point sources of chemical contributions, and 

natural factors that affect water quality or small basins 

that reflected a specific combination of land use and 

physiographic condition (table 1, fig. 1) (Gilliom and 

others, 1995; Akers and others, 1999). Twenty-five 

sites within the study unit, including six of the basic 

fixed sites, were chosen as part of a low-flow synoptic 

water-quality study in the upper Midwest that was 

conducted during August 1997 (Sorenson and others, 

1999). 

Environmental variables used to describe the 

basic fixed sites at which benthic invertebrate data 

were collected are listed in table 2. Variables associated with site characterization include agriculturalchemical use, crop data, geology and land-use/landcover information, and population data. Various 

geographic information system (GIS) coverages were 

used to quantify the variables used for describing the 

synoptic (Sorenson and others, 1999) and basic fixed 

sites. Data for population density, land use/land cover, 

and landform are listed in table 11. Application rates 

for pesticides, nitrogen (inorganic, organic, and atmospheric sources) and phosphorus (inorganic and 

organic sources), and results of field measurements are 

listed in table 12. Tables 11 and 12 are in the "Supplemental Data" section of this report. Data derived from 

the synoptic study are tabulated in Sorenson and others 

(1999).

Water-Quality Variables 

Surface water was sampled monthly, with extra 

high- or low-flow samples collected as needed. Standard protocols were used to ensure consistency 

(Edwards and Glysson, 1988; Ward and Harr, 1990; 

Shelton, 1994; Capel and Larson, 1996). General 

water-quality variables monitored and used for this 

study include field measurements of water temperature, dissolved oxygen, pH, and specific conductance 

and concentrations of nutrients, major ions, organic 

carbon, and dissolved pesticides (tables 2 and 12). 

Field data and data from water-quality sampling 

closest to the time of biological sampling were used in 

the analyses. Estimates of algal stream productivity 

were calculated using the method described in 

Sorenson and others (1999).

The design of the surface-water-quality 

sampling program, and the methods used for surfacewater sample collection and analyses are summarized 

in Akers and others (1999, 2000). These comprehensive reports include descriptions, analytical techniques, and minimum reporting limits for data 

collected from September 1995 through September 

1998. Schnoebelen and others (1999) reported historical nutrient and pesticide concentrations. 

Habitat 

Reach selection and characterization at a site 

followed the NAWQA Level-1 habitat-assessment 

protocol (Meador and others, 1993). Habitat variables 

(table 2) were measured at each of the main stream 

reaches. Habitat features were quantified at multiple 

spatial scales—basin, stream segment, and stream 

reach. Segment and basin features included sinuosity, 

gradient, drainage area, and land use/land cover. 
6 Benthic Invertebrate Assemblages and Their Relation to Physical and Chemical Characteristics of Streams in the Eastern Iowa

Basins, 1996–98

Table 1. Location and basin characteristics of 31 stream sites in the Eastern Iowa Basins, Iowa and Minnesota [km2, square kilometers; IA, Iowa; MN, Minnesota; 42o 24' 57", location in degrees, minutes, seconds; X, basic fixed or synoptic site; –, not applicable]  Site U.S. Geological Location Basin Site type  number Site name Survey station Latitude Longitude area Basin Basic Syn- (fig. 1) identification (km2) fixed optic 1 Cedar River at Gilbertville, IA 05464020 42o 24' 57" 92o 13' 07" 13,564 Cedar (large-river site) X – 

 2 Iowa River at Marengo, IA 05453100 41o 48' 48" 92o 03' 51" 7,239 Iowa (large-river site) X –  3 Wapsipinicon River near DeWitt, IA 05422000 41o 46' 01" 90o 32' 05" 6,050 Wapsipinicon (large-river site) X –  4 Cedar River near Conesville, IA 05465000 41o 24' 36" 91o 17' 06" 20,153 Cedar (large-river site) X –  5 Iowa River at Wapello, IA 05465500 41o 10' 48" 91o 10' 57" 32,365 Iowa (large-river site) X –  6 Skunk River at Augusta, IA 05474000 40o 45' 13" 91o 16' 40" 11,163 Skunk (large-river site) X –  7 Wapsipinicon River near Tripoli, IA 05420680 42o 50' 10" 92o 15' 26" 896 Wapsipinicon X X  8 East Fork Wapsipinicon River near Tripoli, IA 05420720 42o 50' 51" 92o 13' 48" 373 Wapsipinicon – X  9 Little Wapsipinicon River at Littleton, IA 05420900 42o 32' 27" 92o 01' 30" 544 Wapsipinicon – X  10 Buffalo Creek near Stone City, IA 05421700 42o 08' 33" 91o 20' 44" 603 Wapsipinicon – X  11 Mud Creek near Donahue, IA 05421870 41o 44' 18" 90o 41' 27" 308 Wapsipinicon – X  12 Turtle Creek at Austin, MN 05456510 43o 40' 25" 93o 01' 11" 396 Cedar – X  13 Little Cedar River near Floyd, IA 05457950 43o 11' 54" 92o 41' 15" 648 Cedar – X  14 Maynes Creek near Kesley, IA 05458870 42o 41' 47" 92o 54' 27" 352 Cedar – X  15 Winnebago River near Fertile, IA 05459300 43o 14' 49" 93o 26' 16" 761 Cedar – X  16 Flood Creek near Powersville, IA 05461390 42o 54' 26" 92o 43' 14" 389 Cedar X X  17 Beaver Creek near Parkersburg, IA 05462770 42o 35' 15" 92o 48' 37" 376 Cedar – X  18 Black Hawk Creek at Waterloo, IA 05463510 42o 27' 24" 92o 25' 22" 847 Cedar – X  19 Wolf Creek near Dysart, IA 05464220 42o 15' 06" 92o 17' 55" 847 Cedar X X  20 East Branch Iowa River at Belmond, IA 05449200 42o 51' 48" 93o 36' 47" 505 Iowa – X  21 Iowa River near Rowan, IA 05449500 42o 45' 36" 93o 37' 23" 1,083 Iowa X X  22 South Fork Iowa River near New Providence, IA 05451210 42o 18' 54" 93o 09' 08" 580 Iowa X X  23 Salt Creek at Belle Plaine, IA 05452020 41o 53' 31" 92o 18' 00" 518 Iowa – X  24 Old Mans Creek near Iowa City, IA 05455100 41o 36' 23" 91o 36' 56" 521 Iowa X X  25 English River near Kalona, IA 05455500 41o 28' 11" 91o 42' 52" 1,487 Iowa – X  26 Long Creek near Columbus Junction, IA 05465310 41o 13' 36" 91o 16' 31" 399 Iowa – X  27 South Skunk River near Story City, IA 05469980 42o 08' 14" 93o 34' 01" 554 Skunk – X  28 East Branch Indian Creek near Iowa Center, IA 05471120 41o 57' 08" 93o 24' 21" 332 Skunk – X  29 Crooked Creek near Coppock, IA 05473060 41o 09' 31" 91o 42' 30" 736 Skunk – X  30 Cedar Creek near Oakland Mills, IA 05473400 40o 55' 20" 91o 40' 10" 1,380 Skunk – X  31 Big Creek near Lowell, IA 05473550 40o 51' 37" 91o 28' 51" 433 Skunk – X 
METHODS 7

Table 2. Environmental variables used to describe 12 basic fixed sites in the Eastern Iowa Basins, Iowa and Minnesota, 

1996–98 

[km2, square kilometer; %, percent; population data, see Sorenson and others, 1999; kg/km2, kilogram per square kilometer; mg/L, milligrams per liter; 

CaCO3, calcium carbonate; N, nitrogen; P, phosphorus; S, sulfur; μg/L, micrograms per liter; ESA, ethanesulfonic acid; °C, degrees Celsius; μS/cm, 

microsiemens per centimeter at 25 degrees Celsius; m, meter; m/km, meters per kilometer; m/s, meters per second; g O2/m3

/hr, grams of oxygen per 

cubic meter per hour] 

Site Characterization 

General 

 Basin area (km2

) 

 Geology and Land Use/Land 

 Cover (% of basin area) 

 Des Moines Lobe 

 Iowan Surface 

 Iowan Karst 

 Southern Iowa Drift Plain 

 Agriculture 

 Barren 

 Grassland 

 Row crop 

 Urban 

 Wetland 

 Wooded 

Population Data (1990) 

 Human population density 

 (number/km2

) 

 Human population of basin

 Agricultural Use of Nitrogen, 

 Phosphorus, and Pesticides 

 (kg/km2

 in basin) 

 Pesticides applied per basin, total 

 Nitrogen from: 

 atmosphere 

 inorganic fertilizer 

 manure 

 Phosphorus from: 

 inorganic fertilizer 

 manure 

Water-Column Chemistry 

 Water Quality (mg/L) 

 Alkalinity (as CaCO3) 

 Bicarbonate (as CaCO3) 

 Carbonate (as CaCO3) 

 Calcium, dissolved 

 Carbon, organic 

 dissolved 

 suspended 

 Chloride, dissolved 

 Fluoride, dissolved 

 Hardness, total (as CaCO3) 

 Iron, dissolved 

 Magnesium, dissolved 

Manganese, dissolved 

 Nitrogen (as N): 

 ammonia + organic, dissolved 

 ammonia + organic, total 

 ammonia, dissolved 

 nitrite + nitrate, dissolved 

 nitrite, dissolved 

 Phosphorus (as P): 

 dissolved 

 ortho, dissolved 

 total 

 Potassium, dissolved 

 Silicon, dissolved 

 Sodium, dissolved 

 Sulfate, dissolved (as S) 

 Herbicides and Degradation 

 Products (μg/L)

 Fonofos 

 Chlorpyrifos 

 Diazinon 

 Carbofuran 

 Triazine 

 Atrazine 

 Deethylatrazine 

 Deisopropylatrazine 

 Atrazine + metabolites 

 Cyanazine 

 Cyanazine amide 

 Hydroxylatrazine 

 Prometon 

 Simazine 

Chloroacetamide 

 Acetochlor 

 Acetochlor ESA 

 Acetochlor oxanilic acid 

 Acetochlor + metabolites 

 Alachlor 

 Alachlor ESA 

 Alachlor oxanilic acid 

 Alachlor + metabolites 

 Metolachlor 

 Metolachlor ESA 

 Metolachlor oxanilic acid 

 Metolachlor + metabolites 

 Total acetanilide herbicides 

Single Measurements and 

 Continuous-Monitoring Data 

 Water temperature (°C): 

 time of sampling 

 4-month low 

 Specific conductance (μS/cm): 

 time of sampling 

 4-month low 

 pH: 

 time of sampling 

 4-month low 

 Dissolved oxygen: 

 time of sampling (mg/L) 

 4-month low (mg/L) 

 saturation, 4-month low (%) 

Habitat 

 Bank height (m): 

 mean 

 maximum 

 Bank vegetation stability 

 mean 

 maximum 

 Bank width (m): 

 mean 

 maximum 

 Bottom material (lab analysis): 

 % clay 

 % sand 

 % silt 

 Canopy angle (degrees): 

 mean (sum both sides) 

 maximum 

 Gradient, stream segment (m/km) 

 Sinuosity, stream segment 

 Stream depth (m): 

 mean 

 maximum 

 Stream width (m): 

 wetted channel 

 maximum 

 Velocity (m/s): 

 mean 

 maximum 

Stream Productivity 

 g O2/m3

/hr by calculation 
8 Benthic Invertebrate Assemblages and Their Relation to Physical and Chemical Characteristics of Streams in the Eastern Iowa 

Basins, 1996–98

Stream-reach information was gathered from multiple 

transects at a site as described in Meador and others 

(1993). Data were collected to quantify the riparian 

and instream habitats. Stream depth, velocity, and 

particle size of the bed material were measured at the 

stream-reach level.

Streambank characteristics and canopy shading 

at basic fixed sites in the EIWA study unit are listed in 

table 13, streambed material and stream characteristics 

in table 14. These tables are in the "Supplemental 

Data" section of this report. 

Benthic Invertebrate Collection 

and Data Preparation

Field Sampling

Samples of benthic invertebrates were collected 

as part of basic ecological studies to meet occurrence 

and distribution objectives of the NAWQA Program 

(1996–98) (Meador and Gurtz, 1994; Gilliom and 

others, 1995), and as part of a low-flow, synoptic 

water-quality study in the upper Midwest (Sorenson 

and others, 1999). Cuffney and others (1993) describe 

the benthic invertebrate sampling protocol used. 

Quantitative and qualitative samples of benthic 

invertebrates were collected during the seasonal lowflow period between July and September 1996 in the 

main reach of each of the 12 basic fixed sites (fig. 1, 

table 1). Three of these sites (Iowa River near Rowan, 

Iowa River at Wapello, and Wolf Creek near Dysart) 

were selected for more intensive quantitative and qualitative sampling in July and August 1997 when 

samples were collected at multiple reaches (the main 

reach and two additional reaches located upstream and 

downstream from the main reach). Only the main 

reach was sampled in August 1998 (fig. 2). These data 

were used to assess the degree of spatial (reach-toreach) and temporal (year-to-year) variability. Quantitative samples were collected in August 1997 from 

25 sites on medium-sized streams throughout the 

study unit (synoptic sites in fig. 1 and table 1) as part 

of the upper Midwest low-flow synoptic study 

(Sorenson and others, 1999). 

In 1996, quantitative samples were taken from 

woody snags that appeared to have been submerged in 

flowing water and available for colonization by 

benthic invertebrates for an extended period. Organisms were gently dislodged with a brush from five 

separate areas of submerged woody debris within each 

stream reach, caught, and composited into a single 

sample using a Slack sampler equipped with a 

425-μm-mesh sieve. The sample was transferred to a 

labeled jar and preserved with 10-percent buffered 

formalin. In 1997 and 1998, 30- to 50-cm sections of 

snags were cut under water and caught in the Slack 

sampler before removing the organisms. The total area 

of the woody snags associated with each sample was 

measured. 

A D-frame net fitted with a 210-μm-mesh net 

was used to collect invertebrates for a qualitative 

sample during a 1-hour period at basic fixed sites from 

all accessible instream habitats, supplemented by 

hand-picking from large rocks, woody debris, and 

leaves. Specimens were composited into a single 

sample container, labeled, and preserved with 

10-percent buffered formalin. 

Laboratory Processing

All invertebrate samples were sent to the USGS 

National Water Quality Laboratory’s (NWQL) Biological Group in Lakewood, Colo., for processing. Identification to the lowest reasonable taxonomic level and 

quantification of organisms followed the standard 

NWQL protocol that included a fixed-count method 

for quantitative samples (no more than 500 organisms 

sorted and identified for a sample) and a visual-sort 

method for qualitative samples. The visual-sort 

method only records a taxon as present (Moulton and 

others, 2000). A voucher collection of invertebrates is 

kept at the NWQL. Taxonomic nomenclature and hierarchy follow the recommendations of the Biological 

Group (S.R. Moulton and J.P. Slusark, U.S. Geological 

Survey, written commun., 2000;1 Moulton and others, 

2000).

Data Preparation

Invertebrate taxa found at 31 sites sampled in 

the EIWA study unit are listed in table 3. All available 

1

 Moulton, S.R., II, and Slusark, J.P., 2000, 

Taxonomic identification of benthic macroinvertebrates: unpublished U.S. Geological Survey document, National Water Quality Laboratory Standard 

Operating Procedure—Laboratory Analytical 

Method or Procedure BS0335.0, 7 April 2000, 

20 p.
M

ETHODS

9

Table 3. Distribution of benthic invertebrates collected at 31 sites in the Eastern Iowa Basins, Iowa and Minnesota, 1996–98, and the number of sites where each 

taxon was found 

[All invertebrate data available for a site (quantitative, qualitative, multiple-reach and multiple-year samples) were included in the summary. Data are unedited and uncensored. The number of sites sampled in each watershed or group is in parentheses under the name of the site group]  Number of sites with taxon Number of sites with taxon 

 Headwater or tributary sites Headwater or tributary sites 

 Taxon Wapsi- Large All Taxon Wapsi- Large All  pinicon Cedar Iowa Skunk river sites pinicon Cedar Iowa Skunk river sites  (5) (8) (7) (5) (6) (31) (5) (8) (7) (5) (6) (31) 

Platyhelminthes  Turbellaria 0 4 1 0 0 5 Nematoda 4 1 0 0 2 7 

Bryozoa 2 1 2 1 2 8 Mollusca  Gastropoda  Ancylidae 2 0 0 0 0 2 

Ferrissia 1 0 0 0 0 1 

 Lymnaeidae 1 0 1 1 0 3  Physidae 0 0 3 0 0 3 

Physella 1 2 2 1 2 8 

 Planorbidae 0 0 1 0 0 1  Pleuroceridae 0 1 0 0 0 1  Bivalvia  Sphaeriidae 0 0 1 1 0 2 Annelida  Oligochaeta  Naididae 5 8 6 5 4 28  Tubificidae 1 4 4 0 3 12  Hirudinea  Glossiphoniidae 1 1 2 0 2 6 Arthropoda  Chelicerata  Acari  Hydrachnidia 5 6 7 5 3 26  Malacostraca  Amphipoda  Gammaridae 

Gammarus 0 0 0 0 1 1 

 Hyalellidae 

Hyalella azteca (Saussure) 0 1 2 0 1 4 

 Decapoda  Cambaridae 1 1 2 0 1 5 

Cambarus 0 0 1 0 0 1 

Orconectes 1 2 2 1 3 9 

 Isopoda  Asellidae 

Caecidotea 0 0 0 1 0 1 

 Insecta  Ephemeroptera 1 1 5 0 3 10  Baetidae 5 8 7 5 5 30 

Acentrella turbida (McDunnough) 0 0 1 0 0 1 

Acentrella 3 5 3 4 0 15 

Baetis intercalaris McDunnough 4 7 5 4 3 23 

Baetis 0 0 3 0 2 5 Centroptilum/Procloeon 3 2 3 2 1 11 

Fallceon quilleri (Dodds) 2 3 5 4 4 18 Paracloeodes minutus (Daggy) 0 1 0 0 1 2 Plauditus parvulus (McDunnough) 0 1 1 0 0 2 Plauditus punctiventris

 (McDunnough) 0 0 1 0 0 1 

Plauditus 1 1 1 0 0 3 Pseudocloeon dardanum

 (McDunnough) 0 2 2 0 1 5 

Pseudocloeon ephippiatum (Traver) 0 0 1 0 0 1 Pseudocloeon longipalpus 

 (Morihara and McCafferty) 0 0 0 0 3 3 

Pseudocloeon propinquum (Walsh) 0 1 1 0 0 2 Pseudocloeon 1 3 3 2 1 10 

 Baetiscidae 

Baetisca 0 0 2 1 0 3 

 Caenidae 0 0 1 0 1 2 

Amercaenis ridens (McDunnough) 0 1 1 0 4 6 

Brachycercus 0 0 1 0 1 2 Caenis hilaris (Say) 0 1 1 0 1 3 

Caenis 4 5 5 4 5 23 Cercobrachys 0 1 1 0 0 2 

 Ephemeridae 0 1 0 0 1 2 

Ephemera simulans Walker 0 0 0 0 2 2 Ephemera 0 0 0 0 1 1
10 Benthic Invertebrate Assemblages and Their Relation to Physical and Chemical Characteristics of Streams in the Eastern Iowa 

Basins, 1996–98

Table 3. Distribution of benthic invertebrates collected at 31 sites in the Eastern Iowa Basins, Iowa and Minnesota, 1996–98, and the number of sites where each 

taxon was found—Continued 

 Number of sites with taxon Number of sites with taxon 

Headwater or tributary sites Headwater or tributary sites 

 Taxon Wapsi- Large All Taxon Wapsi- Large All  pinicon Cedar Iowa Skunk river sites pinicon Cedar Iowa Skunk river sites 

(5) (8) (7) (5) (6) (31) (5) (8) (7) (5) (6) (31) 

Hexagenia atrocaudata

 McDunnough 0 0 1 0 0 1 

Hexagenia limbata (Serville) 0 1 1 0 0 2 Hexagenia 2 0 2 1 0 5 

Pentagenia vittigera (Walsh) 0 0 0 0 1 1 

 Heptageniidae 4 7 6 5 3 25 

Heptagenia diabasia Burks 2 6 5 2 6 21 

Heptagenia flavescens (Walsh) 2 5 3 3 6 19 

Heptagenia 2 2 3 0 4 11 

Leucrocuta 0 2 2 0 0 4 Stenacron candidum (Traver) 0 1 0 0 0 1 

Stenacron interpunctatum (Say) 0 1 1 0 0 2 

Stenacron 2 1 6 2 1 12 

Stenonema exiguum Traver 5 5 5 2 4 21 

Stenonema mexicanum (Ulmer) 1 4 2 2 6 15 

Stenonema pulchellum (Walsh) 0 1 0 1 0 2 

Stenonema terminatum (Walsh) 3 5 5 3 1 17 

Stenonema 5 7 6 4 4 26 

 Isonychiidae 

Isonychia 4 4 5 3 5 21 

 Leptohyphidae 

Tricorythodes 5 8 7 5 6 31 

 Oligoneuriidae 2 1 0 1 0 4 

Homoeoneuria ammophila (Spieth) 0 0 0 0 1 1 

 Polymitarcyidae 

Ephoron album (Say) 0 1 2 0 2 5 

Ephoron 1 0 0 0 0 1 Tortopus primus (McDunnough) 0 0 0 0 1 1 

 Potamanthidae 

Anthopotamus 0 0 1 0 0 1 

 Odonata  Aeshnidae 0 1 0 0 1 2 

Aeshna 0 1 0 0 0 1 Anax 0 0 1 0 0 1 Boyeria vinosa (Say) 1 2 1 0 0 4 Boyeria 0 2 0 0 0 2 

 Calopterygidae 0 0 1 0 0 1 

Hetaerina americana (Fabricius) 0 1 2 0 2 5 

Hetaerina titia (Drury) 0 1 0 0 0 1 Hetaerina 1 1 0 0 0 2 

 Coenagrionidae 3 1 4 4 0 12 

Argia 0 1 0 1 2 4 

 Corduliidae 

Neurocordulia molesta (Walsh) 0 1 0 0 1 2 

 Gomphidae 1 1 2 3 2 9 

Arigomphus 0 0 1 0 0 1 Gomphus 0 1 0 0 2 3 Progomphus obscurus (Rambur) 0 1 0 1 0 2 Progomphus 0 0 0 1 0 1 

Stylurus 0 1 0 0 3 4 

 Macromiidae 

Didymops transversa (Say) 0 0 0 0 1 1 Macromia illinoiensis Walsh 0 0 0 1 0 1 Macromia 0 0 0 0 1 1 

 Plecoptera  Perlidae 0 1 1 0 1 3 

Acroneuria 1 4 3 2 2 12 Agnetina 0 0 1 0 0 1 Neoperla 0 0 0 0 1 1 Paragnetina 2 0 0 0 0 2 

Perlesta placida complex 0 2 0 0 0 2 

Perlesta 0 2 0 0 0 2 

 Pteronarcyidae 

Pteronarcys 3 5 3 2 3 16 

 Heteroptera  Belostomatidae 

Belostoma 1 0 1 0 0 2 

 Corixidae 1 3 4 3 5 16 

Palmacorixa gillettei Abbott 1 1 2 0 0 4 Palmacorixa 1 1 2 0 0 4 

Sigara 0 2 0 0 0 2 Trichocorixa 0 1 4 0 6 11 

 Gerridae 0 1 2 0 0 3 

Metrobates hesperius Uhler 0 1 0 0 1 2 Metrobates 3 1 1 0 3 8 Rheumatobates 1 1 3 2 3 10 
METHODS 11

Table 3. Distribution of benthic invertebrates collected at 31 sites in the Eastern Iowa Basins, Iowa and Minnesota, 1996–98, and the number of sites where each 

taxon was found—Continued 

 Number of sites with taxon Number of sites with taxon 

Headwater or tributary sites Headwater or tributary sites 

 Taxon Wapsi- Large All Taxon Wapsi- Large All  pinicon Cedar Iowa Skunk river sites pinicon Cedar Iowa Skunk river sites 

(5) (8) (7) (5) (6) (31) (5) (8) (7) (5) (6) (31) 

 Mesoveliidae 

Mesovelia 0 0 0 1 0 1 

 Naucoridae 

Pelocoris 0 0 0 0 1 1 

 Nepidae 

Ranatra 0 1 0 0 0 1 

 Pleidae 

Neoplea striola (Fieber) 0 1 1 0 0 2 Neoplea 1 0 2 0 0 3 

 Saldidae 

Saldula 0 0 0 0 1 1 

 Veliidae 0 0 1 2 1 4 

Rhagovelia oriander Parshley 0 1 1 0 0 2 

Rhagovelia 3 1 3 1 5 13 

 Megaloptera  Corydalidae 0 0 1 1 0 2 

Chauliodes rastricornis Rambur 0 0 0 0 1 1 

Corydalus cornutus (Linnaeus) 0 1 1 1 3 6 

Corydalus 0 0 1 0 0 1 

 Sialidae 

Sialis 2 0 0 0 0 2 

 Trichoptera 0 2 1 0 0 3  Brachycentridae 

Brachycentrus numerosus (Say) 2 5 1 0 1 9 

 Hydropsychidae 4 7 7 5 6 29 

Ceratopsyche morosa (Hagen) 0 1 0 0 0 1 

Ceratopsyche slossonae (Banks) 0 1 0 0 0 1 

Ceratopsyche 3 5 5 4 1 18 

Cheumatopsyche aphanta Ross 0 0 0 1 0 1 

Cheumatopsyche 5 5 5 4 2 21 

Hydropsyche betteni Ross 0 0 0 1 0 1 

Hydropsyche bidens Ross 3 5 6 5 6 25 

Hydropsyche bidens Ross/orris Ross 0 1 0 0 0 1 Hydropsyche orris Ross 0 2 0 0 4 6 

Hydropsyche rossi Flint, Voshell, 

 and Parker/simulans Ross 0 1 1 0 1 3 

Hydropsyche simulans Ross 4 6 7 5 6 28 

Hydropsyche 3 8 6 5 5 27 

Potamyia flava (Hagen) 0 3 2 1 5 11 

 Hydroptilidae 1 6 2 1 1 11 

Hydroptila ajax Ross 0 1 0 0 0 1 Hydroptila 2 5 2 2 0 11 

Mayatrichia ayama Mosely 0 4 0 1 1 6 Mayatrichia 0 0 1 2 0 3 

 Leptoceridae 0 1 2 0 0 3 

Ceraclea maculata (Banks) 0 0 0 1 0 1 

Ceraclea 2 0 0 1 0 3 

Nectopsyche candida (Hagen) 0 1 4 4 6 15 

Nectopsyche candida (Hagen)/ 

spiloma (Ross) 0 1 1 0 1 3 

Nectopsyche diarina (Ross) 2 2 2 2 0 8 

Nectopsyche exquisita (Walker) 0 1 2 0 0 3 

Nectopsyche 1 2 5 0 2 10 

Oecetis 0 0 0 1 0 1 

 Limnephilidae 1 0 0 0 0 1 

Pycnopsyche 2 0 3 1 0 6 

 Polycentropodidae 

Neureclipsis 1 0 1 0 1 3 

Paranyctiophylax 0 3 2 2 0 7 

 Lepidoptera 1 0 0 0 1 2  Pyralidae 

Crambus 0 1 0 0 0 1 

 Coleoptera 0 1 1 0 1 3  Dryopidae 

Helichus lithophilus (Germar) 0 4 4 2 4 14 

Helichus striatus LeConte 1 1 1 0 1 4 

Helichus 0 1 0 1 0 2 

 Dytiscidae 

Laccophilus maculosus Say 0 0 2 0 5 7 

Laccophilus 0 0 0 0 1 1 

Liodessus 0 0 1 0 0 1 

 Elmidae 5 4 4 2 0 15 

Ancyronyx variegata (Germar) 2 3 1 0 0 6 

Dubiraphia minima Hilsenhoff 0 1 0 0 1 2 Dubiraphia vittata (Melsheimer) 0 0 1 0 0 1 Dubiraphia 2 3 6 4 2 17 
12 Benthic Invertebrate Assemblages and Their Relation to Physical and Chemical Characteristics of Streams in the Eastern Iowa 

Basins, 1996–98

Table 3. Distribution of benthic invertebrates collected at 31 sites in the Eastern Iowa Basins,, Iowa and Minnesota, 1996–98, and the number of sites where 

each taxon was found—Continued 

 Number of sites with taxon Number of sites with taxon 

Headwater or tributary sites Headwater or tributary sites 

 Taxon Wapsi- Large All Taxon Wapsi- Large All  pinicon Cedar Iowa Skunk river sites pinicon Cedar Iowa Skunk river sites 

(5) (8) (7) (5) (6) (31) (5) (8) (7) (5) (6) (31) 

Macronychus glabratus Say 5 6 7 4 4 26 

Stenelmis crenata (Say) 0 1 1 0 0 2 

Stenelmis decorata Sanderson 0 2 0 0 2 4 Stenelmis grossa Sanderson 2 3 4 4 3 16 Stenelmis 2 2 3 4 4 15 

 Gyrinidae 

Dineutus assimilis (Kirby) 0 0 0 0 2 2 

Dineutus discolor Aubè 1 0 0 0 0 1 

Dineutus 1 0 0 0 2 3 Gyrinus analis Say 0 0 0 0 1 1 Gyrinus fraternus Couper 0 1 0 0 0 1 

 Haliplidae 

Haliplus 0 0 1 0 0 1 Peltodytes 0 1 0 0 1 2 

 Helophoridae 

Helophorus 0 1 0 0 0 1 

 Hydrochidae 

Hydrochus 0 0 1 0 0 1 

 Hydrophilidae 0 0 2 1 1 4 

Berosus 0 0 1 0 1 2 

Crenitis 0 0 1 0 0 1 

Cymbiodyta 0 1 0 0 0 1 Enochrus 0 0 0 0 1 1 

Paracymus 0 1 0 0 0 1 Sperchopsis tessellata (Ziegler) 0 1 1 0 0 2 

Tropisternus lateralis (Fabricius) 0 0 0 0 1 1 Tropisternus 0 0 2 0 3 5 

 Scirtidae 0 2 2 1 1 6  Diptera  Nematocera  Ceratopogonidae 1 0 2 0 0 3 

Atrichopogon 0 2 2 2 1 7 

Bezzia/Palpomyia 1 0 1 0 0 2 Culicoides 1 0 1 0 0 2 

 Chironomidae 3 4 5 1 5 18  Chironominae 5 8 7 5 6 31  Chironomini 4 2 4 0 1 11 

Axarus 0 0 1 2 2 5 

Chernovskiia 0 0 1 0 1 2 Chironomus 1 4 4 2 4 15 

Cryptochironomus 1 3 4 1 1 10 Cryptotendipes 0 2 1 2 0 5 

Dicrotendipes 3 6 4 4 1 18 Endochironomus 0 1 1 0 0 2 

Endotribelos 0 0 1 0 0 1 

Glyptotendipes 2 5 3 4 6 20 

Harnischia 0 1 1 0 0 2 Lipiniella 0 0 0 0 1 1 Microtendipes 1 1 1 0 0 3 Parachironomus 3 1 2 1 5 12 Paracladopelma 1 2 1 0 1 5 Paralauterborniella 0 1 1 0 0 2 Phaenopsectra 0 3 1 2 1 7 

Polypedilum 5 7 7 5 6 30 

Robackia 1 0 1 1 1 4 Saetheria 2 1 3 1 0 7 Stelechomyia perpulchra (Mitchell) 0 0 0 2 0 2 

Stenochironomus/Xestochironomus 0 0 1 0 0 1 

Stenochironomus 4 7 7 4 6 28 

Stictochironomus 0 1 0 0 0 1 Tribelos 0 1 1 0 1 3 

 Pseudochironomini 

Pseudochironomus 0 1 0 0 0 1 

 Tanytarsini 4 4 3 3 2 16 

Cladotanytarsus 1 3 3 2 2 11 Micropsectra 0 2 1 0 0 3 Micropsectra/Tanytarsus 0 1 0 0 0 1 Paratanytarsus 2 3 0 0 0 5 

Rheotanytarsus 5 8 7 5 6 31 Stempellinella 1 2 0 0 0 3 Sublettea 0 1 0 0 0 1 Tanytarsus 4 7 6 5 3 25 

 Orthocladiinae 5 6 5 3 3 22  Corynoneurini 

Corynoneura 2 1 1 0 3 7 Thienemanniella 5 4 4 3 1 17 
METHODS 13

Table 3. Distribution of benthic invertebrates collected at 31 sites in the Eastern Iowa Basins, Iowa and Minnesota, 1996–98, and the number of sites where each 

taxon was found—Continued 

 Number of sites with taxon Number of sites with taxon 

Headwater or tributary sites Headwater or tributary sites 

 Taxon Wapsi- Large All Taxon Wapsi- Large All  pinicon Cedar Iowa Skunk river sites pinicon Cedar Iowa Skunk river sites 

(5) (8) (7) (5) (6) (31) (5) (8) (7) (5) (6) (31) 

 Orthocladiini/Metriocnemini 

Brillia 1 5 3 0 0 9 Cricotopus bicinctus group 3 4 6 2 2 17 

Cricotopus trifascia group 0 1 2 0 0 3 

Cricotopus/Orthocladius 5 8 5 2 3 23Cricotopus 1 3 2 0 3 9 Eukiefferiella 1 1 0 0 0 2 

Lopescladius 0 0 0 0 1 1 

Nanocladius 3 3 2 1 4 13 

Parakiefferiella 4 6 4 2 1 17 

Parametriocnemus 0 1 1 0 0 2 Rheocricotopus 4 5 4 1 0 14 

Tvetenia 1 1 0 0 0 2 

 Tanypodinae 3 2 6 3 4 18  Pentaneurini 0 2 1 0 3 6 

Ablabesmyia 1 4 3 1 4 13 Labrundinia 2 3 1 2 1 9 

Larsia 1 2 2 0 1 6 Nilotanypus 4 4 3 2 0 13 Paramerina 0 1 0 0 0 1 

Thienemannimyia group 5 7 7 5 6 30 

 Procladiini 

Procladius 1 2 3 0 1 7 

 Culicidae 0 1 0 0 0 1 

Aedes 0 1 0 0 0 1 Anopheles 0 0 1 0 0 1 

 Simuliidae 3 6 5 4 4 22 

Simulium 1 4 5 1 5 16 

 Tipulidae 0 0 1 0 2 3 

Hexatoma 1 0 0 0 0 1 Limonia 0 1 0 0 1 2 

 Brachycera 3 2 0 0 2 7  Athericidae 

Atherix variegata Walker 4 6 3 0 0 13 

 Dolichopodidae 0 0 2 1 1 4  Empididae 1 1 0 0 1 3 

Chelifera 0 0 0 0 2 2 Chelifera/Hemerodromia 2 2 4 2 3 13 Hemerodromia 4 5 4 5 4 22 

 Tabanidae 2 0 0 0 0 2
14 Benthic Invertebrate Assemblages and Their Relation to Physical and Chemical Characteristics of Streams in the Eastern Iowa 

Basins, 1996–98

quantitative, qualitative, multiple-reach, and multipleyear data were used to prepare the composite list of 

taxa and their distribution within the study unit. 

Entries in table 3 reflect the number of headwater or 

tributary streams within a watershed (Wapsipinicon, 

Cedar, Iowa, Skunk) or large river sites (basic fixed 

sites with a basin area greater than 6,000 km2

) within 

the study unit where a taxon was found. Invertebrate 

taxa in table 3 were later edited and censured as 

described below.

Qualitative and quantitative benthic invertebrate 

data were formatted into a species-by-sample data 

matrix and ambiguous taxa were removed. This step 

avoids an overestimation of taxa richness and diversity 

that might result from problems associated with 

benthic invertebrate identification (Cuffney and others, 

1997). There are many reasons why benthic invertebrates cannot be identified to consistent and low taxonomic levels. The most commonly cited reasons 

include specimens too immature or damaged for identification and the absence of appropriate keys or 

descriptions for the life stage of the specimens or a 

particular region of the country. Subsets of the edited 

data matrix were used in subsequent analyses.

Three techniques were applied to the data 

matrix to minimize the presence of ambiguous taxa. 

The commonly used technique was proportioning the 

abundances of higher level (parental) taxa among 

lower level (child) taxa in accordance with the abundances of the children. In some situations child taxa 

were combined with the parent taxon if the child abundances were low or about equal to that of the parent, or 

the parent taxon was eliminated if its abundance were 

low compared to that of the child taxa. Cuffney and 

others (1997) concluded that such data-editing techniques ensured consistency in the level of identification for all sites and increased the validity of intersite 

comparisons. 

Benthic invertebrate data are often censured by 

eliminating rare taxa to emphasize more common or 

abundant taxa. Rare species are eliminated for the 

following reasons: (1) frequency or abundance did 

not meet a predetermined level or value [for example, 

all taxa whose combined abundance does not exceed 

0.02 percent of the total (Leland, 1995)]; (2) taxa were 

not present at a minimum number of sites [for 

example, taxa found at less than 5 of 25 sites (Cuffney 

and others, 1997)]; or (3) taxa did not meet a combination of criteria [for example, taxa present at only one 

site and not comprising at least 0.5 percent of the total 

abundance (Tate and Heiny, 1995)]. Terrestrial taxa 

were removed. No additional taxa were eliminated 

from the basic-fixed-site data set initially, but species 

whose combined abundance was less than 0.05 percent 

were eliminated from the synoptic-site data set. Prior 

to detrended correspondence analysis (DCA), taxa 

found in fewer than five samples were eliminated to 

create data sets of a more manageable and interpretable size. Organism abundances in the edited data set 

were converted to relative abundance by sample and 

octave-transformed prior to data analysis (Gauch, 

1982; Mohler, 1987). 

Statistical Analyses and 

Other Calculations

The data distribution of each environmental 

variable was examined and transformed (log or 

square-root transformation), if necessary, to achieve 

approximate normal distribution prior to analysis.

Species composition among instream reaches, 

collection years, and all sites was examined using the 

two-way indicator species analysis (TWINSPAN), a 

divisive classification method based on correspondence analysis (Hill, 1979). TWINSPAN was used 

to identify four groups of sites based on the affinities 

of the benthic invertebrate assemblages identified 

from quantitative samples collected at the 12 basic 

fixed sites in 1996 and the 25 synoptic sites in 1997 

(table 4).

Taxa represented in the final species-by-sample 

data matrix were assigned to one of five functional 

feeding groups on the basis of available trophic information in Merritt and Cummins (1996). These groups 

describe how invertebrates obtain food. The groups 

include shredders (that feed on live or dead plant material, including leaves and wood), scrapers (that graze 

on periphyton and associated material on organic and 

mineral substrates), collector-filterers (that feed on 

suspended fine particulate organic material), collectorgatherers (that feed on sediment or loose surface 

films), and predators (that feed on living animal 

tissues, including carnivores as well as those that 

pierce tissues and cells to suck body fluids). The relative abundance of each functional feeding group in 

each of the four TWINSPAN site groups and all 

samples combined is listed in table 5.

Metrics (data summaries) based on functional 

feeding groups and other aspects of the benthic inver-
METHODS 15

Table 4. Distribution of samples among site groups, and mean and cumulative relative abundance of benthic invertebrates in the 

Eastern Iowa Basins, Iowa and Minnesota, 1996–97 

[Taxa ranked by mean relative abundance among all samples. Site groups identified using TWINSPAN analysis (Hill, 1979). The number of sites sampled in 

each indicated site group is in parentheses under the number of the site group; %, percent; p-value, attained level of significance determined from one-way 

ANOVA (SAS Institute, Inc., 1999a, 1999b) with site group as independent variable; –, not significantly different among site groups (p-value greater than 0.05); 

<, less than] 

 

 Number of samples Mean relative abundance Cumulative 

 Variable in indicated site group by indicated site group or total (%) relative p-value 

 1 2 3 4 All All abundance 

 (16) (6) (9) (6) (37) 1 2 3 4 samples (%) 

Tricorythodes 16 6 9 4 35 12.8 13.2 13.3 0.6 11.0 11.0 – 

Polypedilum 15 6 9 6 36 5.9 6.2 10.4 17.7 8.9 19.9 <0.01 

Hydropsyche bidens 14 3 8 6 31 12.1 1.6 1.5 18.7 8.9 28.8 .01 

Rheotanytarsus 15 6 9 6 36 8.0 2.0 5.0 21.6 8.5 37.3 .01 

Baetis intercalaris 16 4 7 2 29 6.3 9.1 2.6 2.9 5.3 42.6 – 

Hydropsyche simulans 14 5 8 6 33 4.9 5.9 2.2 2.3 4.0 46.6 – 

Glyptotendipes 4 2 7 6 19 .1 .5 12.2 5.1 3.9 50.5 – 

Naididae 13 6 8 4 31 6.0 1.9 3.2 1.6 3.9 54.4 – 

Macronychus glabratus 14 5 6 3 28 4.0 3.1 1.2 .2 2.6 57.0 – 

Cricotopus/Orthocladius 15 4 6 4 29 3.8 1.2 .9 1.5 2.3 59.3 – 

Stenonema terminatum 11 6 5 1 23 3.2 4.5 .6 .1 2.3 61.6 .05 

Fallceon quilleri 8 4 5 2 19 1.6 7.8 .7 .6 2.2 63.8 – 

Stenonema 15 5 2 0 22 3.7 3.0 <.1 .0 2.1 65.9 .02 

Thienemannimyia group 13 6 9 5 33 1.1 3.6 1.9 .8 1.6 67.6 <.01 

Acentrella 11 2 3 0 16 3.3 .9 .2 .0 1.6 69.1 – 

Tanytarsus 11 5 9 2 27 1.0 1.6 3.0 .5 1.5 70.7 .05 

Ceratopsyche 13 4 4 1 22 3.0 .6 .2 <.1 1.5 72.1 – 

Stenochironomus 13 6 7 6 32 .8 2.9 .9 2.4 1.4 73.5 .02 

Cricotopus bicinctus group 10 2 6 1 19 .8 4.1 1.0 <.1 1.2 74.8 – 

Caenis 10 4 8 5 27 .8 .3 1.9 1.7 1.1 75.9 – 

Corixidae 1 2 6 2 11 <.1 .1 4.3 .2 1.1 77.0 .05 

Dicrotendipes 8 3 7 1 19 .2 .8 3.7 .1 1.1 78.1 <.01 

Simuliidae 13 4 5 4 26 1.3 .8 .6 1.0 1.0 79.1 – 

Hydrachnidia 11 5 6 2 24 .9 1.7 1.2 .1 1.0 80.1 – 

Hemerodromia 9 6 7 4 26 .6 1.1 .7 2.2 1.0 81.1 .02 

Stenelmis grossa 8 3 4 0 15 1.8 .4 .5 .0 1.0 82.0 – 

Atherix variegata 13 1 3 0 17 2.0 .1 .1 .0 .9 82.9 .04 

Cheumatopsyche 10 4 5 2 21 1.2 1.3 .3 .2 .8 83.8 – 

Cladotanytarsus 3 0 6 2 11 .1 .0 3.1 .3 .8 84.6 .03 

Centroptilum/Procloeon 5 2 3 1 11 .4 .7 1.7 .2 .7 85.3 – 

Parakiefferiella 10 4 3 1 18 .7 1.7 .4 .1 .7 86.0 – 

Pseudocloeon 4 3 3 1 11 .3 .8 .7 1.3 .7 86.7 – 

Chironomus 1 0 7 3 11 <.1 .0 2.1 .8 .7 87.4 – 

Heptagenia diabasia 9 6 2 1 18 .8 1.1 .5 <.1 .6 88.0 – 

Stenacron 2 3 5 0 10 .1 2.3 .8 .0 .6 88.6 – 

Potamyia flava 3 0 2 4 9 .1 .0 .2 3.2 .6 89.2 <.01 

Coenagrionidae 2 4 6 1 13 <.1 2.0 .7 <.1 .5 89.7 <.01 

Rheocricotopus 9 5 1 0 15 .5 1.1 .4 .0 .5 90.2 – 

Phaenopsectra 1 1 4 1 7 <.1 .1 1.6 .1 .4 90.6 – 

Stenonema mexicanum 5 3 3 2 13 .3 .3 .6 .6 .4 91.1 – 

Hydropsyche orris 1 0 1 4 6 <0.1 0.0 <0.1 2.3 0.4 91.5 <.01 

Hydroptila 10 5 2 0 17 .5 .8 .1 .0 .4 91.9 .02 

Thienemanniella 9 3 5 0 17 .5 .6 .2 .0 .4 92.2 – 

Heptagenia flavescens 7 4 5 4 20 .3 .6 .4 .2 .4 92.6 – 

Isonychia 9 3 4 2 18 .3 .2 .3 .7 .3 92.9 – 

Amercaenis ridens 0 1 1 4 6 .0 .2 .1 1.6 .3 93.3 .01 

Tubificidae 4 1 3 2 10 .2 <.1 .6 .1 .3 93.5 – 

Saetheria 0 2 3 0 5 .0 .5 .7 .0 .3 93.8 – 

Parachironomus 4 2 1 3 10 .2 .1 <.1 .9 .2 94.0 .05 

Nanocladius 4 2 4 4 14 .1 .2 .4 .3 .2 94.3 – 
16 Benthic Invertebrate Assemblages and Their Relation to Physical and Chemical Characteristics of Streams in the Eastern Iowa 

Basins, 1996–98

each of the four TWINSPAN site groups and all 

samples combined is listed in table 5.

Metrics (data summaries) based on functional 

feeding groups and other aspects of the benthic invertebrate assemblage are used in assessments of biological impairment as part of the rapid bioassessment 

protocol of the U.S. Environmental Protection Agency 

(USEPA) (Plafkin and others, 1989). Seven of the 

USEPA's invertebrate assemblage metrics were calculated, including taxa richness (the number of distinct 

taxa in a sample); EPT taxa richness [the number of 

taxa in the orders Ephemeroptera (mayflies), Plecoptera (stoneflies) and Trichoptera (caddisflies)]; the 

ratio of shredders to the total abundance of organisms 

in a sample; the ratio of scrapers to collector-filterers; 

the percent contribution of the dominant three taxa in a 

sample; the percent contribution of EPT taxa; and the 

percent contribution of Chironomidae (midges) (table 

5). These metrics and their application to interpretations of benthic invertebrate data are discussed by 

Resh and Jackson (1993) and Plafkin and others 

(1989). 

Differences in quantitative taxonomic and 

derived assemblage metric and functional-feedinggroup data among the four TWINSPAN site groups 

defined by the 37 samples in the combined basicfixed- and synoptic-site data set were examined for 

significance (p ≤0.05) using one-way analysis of variance (ANOVA) in StatView version 5 (SAS Institute, 

Inc., 1999a, 1999b). Results are included in tables 4 

and 5.

Jaccard coefficients of community were calculated to assess the degree of similarity in taxonomic 

composition between pairs of data by using presence 

or absence of taxa to differentiate between collections 

(Jaccard, 1912) from multiple reaches (table 6) or 

multiple years (table 7) at three basic fixed sites 

(1996–98). This is a conventional method used to 

assess community similarity (Plafkin and others, 1989; 

Table 4. Distribution of samples among site groups, and mean and cumulative relative abundance of benthic invertebrates in the 

Eastern Iowa Basins, Iowa and Minnesota, 1996–97—Continued 

 

 Number of samples Mean relative abundance Cumulative 

 Variable in indicated site group by indicated site group or total (%) relative p-value 

 1 2 3 4 All All abundance 

 (16) (6) (9) (6) (37) 1 2 3 4 samples (%) 

Paratanytarsus 0 2 3 0 5 .0 .4 .6 .0 .2 94.5 – 

Dubiraphia 3 3 7 0 13 <.1 .2 .6 .0 .2 94.7 <0.01 

Nilotanypus 9 3 1 0 13 .3 .3 <.1 .0 .2 94.9 .04 

Brillia 3 1 3 0 7 .1 .3 .4 .0 .2 95.1 – 

Brachycentrus numerosus 7 0 0 0 7 .4 .0 .0 .0 .2 95.2 – 

Mayatrichia ayama 7 2 2 1 12 .2 .3 .1 .1 .2 95.4 – 

Hexagenia 3 2 2 0 7 .1 .4 .2 .0 .1 95.5 – 

Ablabesmyia 1 1 5 0 7 <.1 <.1 .5 .0 .1 95.7 <.01 

Nectopsyche candida 3 3 4 1 11 .1 .2 .3 <.1 .1 95.8 – 

Corynoneura 1 1 1 2 5 <.1 .1 .1 .5 .1 95.9 .03 

Nematoda 2 2 1 2 7 <.1 .1 <.1 .5 .1 96.0 – 

Veliidae 1 4 2 0 7 <.1 .4 .1 .0 .1 96.1 .01 

Paranyctiophylax 3 1 3 0 7 <.1 .1 .3 .0 .1 96.2 – 

Labrundinia 2 2 4 0 8 <.1 .1 .2 .0 .1 96.3 – 

Ancyronyx variegata 5 0 0 0 5 .2 .0 .0 .0 .1 96.4 – 

Atrichopogon 3 2 1 1 7 .1 .1 <.1 .1 .1 96.5 – 

Cryptochironomus 1 2 3 0 6 <.1 .1 .2 .0 .1 96.5 – 

Helichus lithophilus 3 1 4 2 10 <.1 <.1 .2 <.1 .1 96.6 – 

Gomphidae 1 1 4 1 7 <.1 <.1 .2 <.1 .1 96.7 <.01 

Nectopsyche diarina 3 4 3 0 10 <.1 .1 .1 .0 .1 96.7 – 

Acroneuria 7 2 1 0 10 .1 .1 <.1 .0 .1 96.8 – 

Pteronarcys 11 2 2 1 16 .1 <.1 <.1 <.1 .1 96.8 – 

Bryozoa 3 0 2 0 5 <.1 .0 <.1 .0 <.1 96.9 – 

Pycnopsyche 3 2 0 0 5 <.1 .1 .0 .0 <.1 96.9 – 
METHODS 17

The analysis of data from all sites (ordination of 

74 taxa and 12 derived metrics from 37 samples from 

31 stream sites) was used to illustrate the four site 

groups (fig. 3) associated with the differences in 

benthic invertebrate assemblages identified in the 

TWINSPAN analysis. Ordination by DCA displays 

sites in a biplot whereby sites with similarities in taxonomic composition, assemblage metrics, and functional feeding groups tend to cluster.

Axis or site scores for ordination axes 1 and 2 

derived from the DCA were correlated to the taxa in 

the original data set to identify those that contributed 

significantly to grouping of the sites in the biplot 

(fig. 3). Paired comparisons for this and other subsequent analyses were performed using the z-test with a 

significance level of 0.05 in StatView version 5 (SAS 

Institute, Inc., 1999a, 1999b). The z-test uses Fisher's 

R to z transformation to test the hypothesis that the 

correlation between two variables is equal to a specified value.

A linear regression was performed to assess the 

significance of the relation between two invertebrate 

measures (fig. 4) because the relative abundances of 

EPT taxa and Chironomidae were important in interpreting the gradient along DCA axis 1 (the x axis). 

Quantitative taxonomic data from basic fixed 

sites in 1996 were examined using DCA (ordination of 

33 taxa and 12 sites). Correlation coefficients between 

DCA site scores and environmental data (table 8) were 

used to identify significant physical, chemical, and 

land-use variables in the study unit that influenced the 

grouping of sites in the biplot (fig. 5) from the benthic 

invertebrate data.

Environmental variables from the basic fixed 

sites were assigned to the appropriate TWINSPAN site 

group and analyzed by using the Kruskal-Wallis 

ANOVA in Systat version 8.0 (SPSS, Inc., 1998). 

Basic-fixed-site data were used to represent environmental conditions among the four site groups (table 9).

The relative abundances of 74 benthic invertebrate taxa were correlated with the concentrations of 

forms of organic carbon, nitrogen, and phosphorus 

measured at basic fixed and synoptic sites when the 

benthic invertebrates were collected. This was done to 

Table 5. Values of metrics and functional-feeding-group data associated with benthic invertebrates in site 

groups identified in the Eastern Iowa Basins, Iowa and Minnesota, 1996–97 

[Site groups identified using TWINSPAN analysis. p-value, attained level of significance determined from one-way ANOVA 

with site group as independent variable; –, not significantly different among site groups (p-value greater than 0.05); %, 

percent; <, less than; EPT, Ephemeroptera, Plecoptera, and Trichoptera taxa. The sum of the five invertebrate functional 

feeding groups by site group is not 100 percent because some taxa could not be classified.] 

 

 Mean value of metric 

 by indicated site group or all samples 

 Variable All p-value

 1 2 3 4 samples 

Invertebrate assemblage metrics 

Taxa richness 45.4 50.8 52.0 36.3 46.4 0.02 

EPT taxa richness 0.4 22.7 17.3 15.2 19.2 –

Shredders/total abundance .1 .1 .2 .2 .2 .05 

Scrapers/collector-filterers .4 1.0 .5 .0 .4 –

Dominant taxa abundance (%) 47.3 40.1 45.2 55.9 47.0 –

EPT abundance (%) 50.0 57.8 30.8 39.8 48.0 <.01 

Chironomidae abundance (%) 25.2 29.7 52.9 53.6 37.3 <.01 

 

Invertebrate functional feeding groups (% total abundance) 

Shredder 10.3 12.0 25.0 21.7 16.0 .05 

Scraper 6.7 8.2 4.2 .8 5.4 .01 

Collector-filterer 31.9 14.2 13.4 50.5 27.5 <.01 

Collector-gatherer 45.1 54.4 44.5 22.0 42.7 <.01 

Predator 5.6 10.3 11.5 4.8 7.7 .03 
18 Benthic Invertebrate Assemblages and Their Relation to Physical and Chemical Characteristics of Streams in the Eastern Iowa 

Basins, 1996–98

The relative abundances of 74 benthic invertebrate taxa were correlated with the concentrations of 

forms of organic carbon, nitrogen, and phosphorus 

measured at basic fixed and synoptic sites when the 

benthic invertebrates were collected. This was done to 

identify taxa that might be tolerant or intolerant of 

organic enrichment or exhibit no response to ambient 

concentrations. A taxon with a significant (p ≤0.05) 

positive correlation to any of the forms of carbon, 

nitrogen, or phosphorus measured was listed as positive for that variable in table 10. A taxon with a significant negative correlation was listed as negative. 

DISTRIBUTION OF BENTHIC 

INVERTEBRATES

Over 250 benthic invertebrate taxa were identified from snags and woody debris in streams and 

rivers of the Wapsipinicon, Cedar, Iowa, and Skunk 

River Basins. After data editing and censuring, 74 taxa 

remained to represent the more widely distributed and 

abundant taxa. Mayflies, caddisflies, and true flies 

were well represented in streams and rivers of the 

EIWA study unit. The mayflies Tricorythodes and 

Baetis intercalaris; the net-spinning caddisflies 

Hydropsyche bidens and H. simulans; and the 

Chironomidae Glyptotendipes, Polypedilum, and 

Rheotanytarsus accounted for about 50 percent of the 

cumulative mean relative abundance of organisms in 

collections from 1996 and 1997 (table 4). The distribution of individuals followed the recognizable pattern 

of a few taxa represented at many sites by many individuals, with most taxa collected infrequently and 

represented by only a few individuals (Johnson and 

others, 1993).

Spatial and Temporal Variability

Data used to describe the benthic invertebrate 

assemblages of the EIWA study unit were collected on 

single visits to each of 12 basic fixed sites in 1996 and 

25 synoptic sites in 1997. Additional samples were 

taken from three basic fixed sites (sites 5, 19, and 21) 

(table 1, fig. 1) to assess the amount of spatial (reachTable 6. Jaccard coefficient of community between reaches in percent similarity of 

taxonomic composition and the number of taxa at sites 5, 19, and 21 in the Eastern Iowa 

Basins, Iowa and Minnesota, 1997 

[QT, quantitative; QT + QL; combined quantitative and qualitative; –, no comparison made; main, stream reach 

sampled 1996–98; upper, stream reach located upstream main reach; lower, stream reach located downstream 

main reach; QA, an additional sample for quality assurance from the lower reach] 

 Variable Site 5 Site 19 Site 21 

 QT QT+QL QT QT+QL QT QT+QL 

 Pairs of Percent similarity 

 reaches 

 main and upper 75 48 46 53 65 45 

 main and lower 62 53 62 70 50 43 

 upper and lower 67 65 41 51 43 47 

 main and QA – – 53 55 – – 

 upper and QA – – 38 42 – – 

 lower and QA – – 77 56 – – 

 Reach Number of taxa 

 main 17 29 23 39 11 33 

 upper 18 30 31 48 17 38 

 lower 17 26 24 41 16 37 

 QA – – 26 37 – – 
DISTRIBUTION OF BENTHIC INVERTEBRATES 19

Collector-filterers (percent total abundance)

Potamyia flava

Rheotanytarsus

Collector-gatherers (percent total abundance)

Scrapers:collector-filterers

Ablabesmyia

Labrundinia

Thienemannimyia

Phaenopsectra

Brillia

Group 1

Group 2

Group 3

Group 4

River Sites

EXPLANATION

Chironomidae (percent total abundance)

Hydropsyche orris

Polypedilum

Shredders (percent total abundance)

DCA AXIS 1

+1.5

DCA AXIS 2

0

0

30

2

30 24B

8

27

28

10

13

14

9

17 15

21A

23

7B

21B

22B

16B

19B,22A

20 18

1

4 5

6

29

25

31

26

7A

16A 12

19A

+1.5

24A

11

3

EPT (percent total abundance)

Hydroptila

Nilotanypus









    

 











 









 







 







 

 



 



River. Six sites among the 25 chosen for the 1997 lowflow, synoptic water quality in the upper Midwest 

duplicated basic fixed sites, thus providing an additional opportunity to evaluate temporal variability. 

These included sites 7, 16, 19, 21, 22, and 24. These 

six duplicated sites are distinguished in figure 3 by the 

suffix “A” or “B,” which represent basic-fixed-site 

sampling and synoptic-site sampling, respectively.

TWINSPAN and Jaccard’s coefficient of 

community were used to assess spatial variability 

among the reaches sampled at the three basic fixed 

sites in 1997 and temporal variability among the sites 

Table 7. Jaccard coefficient of community between years in percent similarity of 

taxonomic composition and the number of taxa at sites 5, 19, and 21 in the Eastern Iowa 

Basins, Iowa and Minnesota, 1996–98 

[QT, quantitative; QT + QL; combined quantitative and qualitative] 

 Variable Site 5 Site 19 Site 21 

 QT QT+QL QT QT+QL QT QT+QL

 Pairs of Percent similarity 

 years 

1996 and 1997 38 45 33 37 22 29 

1996 and 1998 36 35 16 33 32 36 

1997 and 1998 43 33 23 33 28 37 

 Year Number of taxa 

 1996 25 38 50 64 33 48 

 1997 17 30 22 36 11 32 

 1998 13 27 15 44 21 58 

Figure 3. Detrended correspondence analysis (DCA) biplot of benthic invertebrate taxa in relation to 

31 basic fixed and synoptic sites in the Eastern Iowa Basins, Iowa and Minnesota, 1966–97.
20 Benthic Invertebrate Assemblages and Their Relation to Physical and Chemical Characteristics of Streams in the Eastern Iowa 

Basins, 1996–98

0.2

0.4

0.6

0.8

1.0

0 0.2 0.4 0.6 0.8 1.0

EPT TAXA (RELATIVE ABUNDANCE)

CHIRONOMIDAE (RELATIVE ABUNDANCE)

0

EPT = 0.804 Ð 0.872 (Chironomidae)

 p < 0.001

River Sites

Group 1

Group 2

Group 3

Group 4

EXPLANATION

sampled in 1996–98. The biplot of the ordination of 

86 invertebrate measures (74 taxa and 12 derived 

metrics) and 37 samples (12 basic fixed and 

25 synoptic sites) was used to assess temporal variability among the six sites duplicated in the two 

studies.

Spatial Variability

TWINSPAN analysis of quantitative, multiplereach data for 1997 included 53 taxa and three reaches 

at sites 5, 19, and 21. Ten samples were analyzed that 

included an additional sample from a fourth reach at 

site 19 that was submitted for field-collection and 

laboratory-processing quality control. Samples in the 

TWINSPAN analysis segregated by site, with the variation within a site less than that observed among sites. 

A considerable range in percent similarity on the basis 

of taxonomic composition was present within each site 

for both the quantitative data and the combined quantitative and qualitative data (table 6). Overall, Jaccard 

coefficients of community for pairs of reaches ranged 

from 38 to 77 percent, with mean values from 52.7 at 

site 21 and 52.8 at site 19 to 68.0 at site 5.

Jaccard coefficients of community for pairs of 

reaches generally decreased with the inclusion of taxa 

collected in qualitative samples at sites 5 and 21, but 

increased for nearly all pairs at site 19. Although the 

number of taxa per reach for the combined quantitative 

and qualitative samples increased in every instance 

over that observed for only the quantitative samples 

(table 6), the taxa added by each qualitative sample 

varied considerably from reach to reach.

Only three of the 53 taxa included in the TWINSPAN analysis were present in all reaches sampled. 

These were the mayfly Baetis intercalaris, the netspinning caddisfly Hydropsyche simulans, and the 

chironomid Polypedilum. Nine additional taxa were 

found at all three sites, but not in all reaches sampled. 

These included small oligochaete worms in the family 

Naididae; the mayflies Heptagenia diabasia, 

Heptagenia flavescens, and Stenonema; the net-spinning caddisfly Hydropsyche bidens; the riffle beetle 

Stenelmis; the chironomids Stenochironomus and 

Thienemannimyia group; and the dance fly Hemerodromia. Ubiquitous and predominant taxa often 

possess opportunistic characteristics, such as high 

reproductive rates or successful dispersal mechanisms, 

rather than tolerance to pollution (Johnson and others, 

1993).

Variation in the presence of taxa among multiple 

samples from the same reach or site is common. Typically, the abundance of a few taxa is high while most 

are represented by only a few individuals (Johnson and 

others, 1993). Benthic invertebrate taxa are patchy in 

distribution (Williams and Feltmate, 1992). This result 

is expressed in multiple collections from the same site 

that exhibit considerable differences in numbers and 

kinds of taxa, and abundance. Large numbers of 

samples are required to overcome these localized 

differences in populations. Canton and Chadwick 

(1988) and Voshell and others (1989), as reported by 

Resh and McElravy (1993), concluded that six 

samples would typically provide estimates of only 

± 40 percent of the mean total number of organisms.

Temporal Variability

TWINSPAN analysis of quantitative, multipleyear data included a total of 82 taxa in nine samples 

collected at sites 5, 19, and 21 during 1996–98. 

Samples from the 3 years generally segregated by site 

with two exceptions—site 19 in 1998 grouped with 

site 21 for 1997–98, and site 21 in 1996 grouped with 

site 19 for 1996–97. Differences in discharge (fig. 2) 

support these observations. Mean discharge (January 

through September) at site 19 in 1998 (14.7 m3

/s) was 

Figure 4. Linear regression of Ephemeroptera, 

Plecoptera, and Trichoptera (EPT) taxa and Chironomidae as relative abundance in 37 samples from 31 

basic fixed and synoptic sites in the Eastern Iowa 

Basins, Iowa and Minnesota, 1966–97.
DISTRIBUTION OF BENTHIC INVERTEBRATES 21

Table 8. Correlations of detrended correspondence analysis (DCA) axis scores and environmental variables from 12 basic fixed sites in 

the Eastern Iowa Basins, Iowa and Minnesota, 1996 

[r-value, correlation coefficient from Fisher's R to z test; p-value, attained level of significance from Fisher's R to z test; p-value in boldface type, 

attained level of significance less than or equal to 0.05; km2

, square kilometer; <, less than; %, percent; population data, see Sorenson and others, 1999; 

kg/km2

, kilograms per square kilometer; mg/L, milligrams per liter; CaCO3, calcium carbonate; N, nitrogen; P, phosphorus; S, sulfur; μg/L, micrograms 

per liter; ESA, ethanesulfonic acid; °C, degrees Celsius; ; μS/cm, microsiemens per centimeter at 25 degrees Celsius; m, meter; m/km, meters per 

kilometer; m/s, meters per second; g O2/m3

/hr, grams of oxygen per cubic meter per hour] 

 Environmental DCA Axis 1 DCA Axis 2 Environmental DCA Axis 1 DCA Axis 2 

 variable r- p- r- p- variable r- p- r- p- 

 value value value value value value value value

Site Characterization 

 General 

 basin area (km2

) -0.94 <0.01 0.01 0.98 

 Geology & Land Use/ 

 Land Cover (% of basin area) 

 Des Moines Lobe -.32 .32 -.29 .38 

 Iowan Surface -.50 .10 .64 .02

 Iowan Karst -.50 .10 .25 .44 

 Southern Iowa Drift Plain -.56 .06 -.40 .20 

 Agriculture .81 <.01 -.23 .49 

 Barren -.76 <.01 .24 .46 

 Grassland -.52 .08 -.13 .70 

 Row crop .65 .02 .16 .62 

 Urban -.88 <.01 -.01 .97 

 Wetland -.17 .60 .59 .04

 Wooded -.84 <.01 -.16 .63 

 Population Data (1990) 

 Human population density 

 (number/km2

) -.92 <.01 -.10 .75 

 Agricultural Use of Nitrogen, 

 Phosphorus, and Pesticides 

 (kg/km2

 in basin) 

 Pesticides applied, total .15 .64 .08 .53 

 Nitrogen from: 

 atmosphere <.01 .99 .18 .42 

 inorganic fertilizer .15 .64 .55 .06 

 manure .30 .35 <.01 .98 

 Phosphorus from: 

 inorganic fertilizer .15 .64 .54 .07 

 manure .32 .32 -.04 .91 

Water-Column Chemistry 

 Water Quality (mg/L) 

 Alkalinity (as CaCO3) .62 .03 -.48 .12 

 Bicarbonate (as CaCO3) .65 .02 -.45 .15 

 Carbonate (as CaCO3) -.63 .03 .18 .58 

 Calcium, dissolved .64 .02 -.46 .14 

 Carbon, organic: 

 dissolved -.21 .52 .05 .88 

 suspended -.93 <.01 .21 .53 

 Chloride, dissolved -.75 <.01 .14 .67 

 Fluoride, dissolved -.08 .81 -.57 .06 

 Hardness, total (as CaCO3) -.63 .03 .18 .58 

 Iron, dissolved -.29 .37 .65 .02

 Magnesium, dissolved .03 .92 -.64 .02

 Manganese, dissolved .76 <.01 -.08 .81 

 Nitrogen (as N): 

 ammonia + organic, 

 dissolved -.01 .99 -.16 .62 

 ammonia + organic, total -.73 <.01 .31 .33 

 ammonia, dissolved .21 .53 -.05 .88 

 nitrite + nitrate, dissolved .53 .08 -.58 .05

 nitrite, dissolved .23 .47 -.14 .67 

 Phosphorus (as P): 

 dissolved .62 .03 -.48 .12 

 ortho, dissolved .61 .03 -.44 .16 

 total -0.63 0.03 -0.01 0.97 

 Potassium, dissolved -.27 .41 .14 .67 

 Silicon, dissolved .68 .01 -.49 .11 

 Sodium, dissolved -.73 .01 .04 .91 

 Sulfate, dissolved (as S) -.68 .01 -.22 .50 

Herbicides and Degradation 

 Products (μg/L) 

 Fonofos .19 .56 -.42 .18 

 Chlorpyrifos .28 .38 -.31 .34 

 Diazinon -.31 .33 .32 .32 

 Carbofuran .16 .63 .54 .07 

 Triazine

 Atrazine -.31 .34 -.37 .24 

 Deethylatrazine -.15 .66 -.02 .95 

 Deisopropylatrazine -.56 .06 -.57 .05

 Atrazine + metabolites -.40 .20 -.51 .09 

 Cyanazine -.46 .14 -.30 .35 

 Cyanazine amide -.55 .06 -.55 .06 

 Hydroxylatrazine -.26 .42 -.11 .73 

 Prometon .37 .25 .18 .59 

 Simazine -.43 .17 -.16 .63 

 Chloroacetamide

 Acetochlor .18 .59 -.27 .42 

 Acetochlor ESA -.14 .66 -.66 .02

 Acetochlor oxanilic acid -.30 .36 -.09 .78 

 Acetochlor + metabolites -.07 .83 -.15 .65 

 Alachlor .29 .37 -.03 .93 

 Alachlor ESA -.16 .64 -.08 .80 

 Alachlor oxanilic acid .40 .20 .04 .90 

 Alachlor + metabolites -.11 .75 -.08 .81 

 Metolachlor .17 .61 -.33 .30 

 Metolachlor ESA .39 .22 -.11 .74 

 Metolachlor oxanilic acid .11 .74 -.42 .18 

 Metolachlor + metabolites .36 .26 -.24 .47 

Single Measurement and 

 Continuous-Monitoring Data 

 Water temperature (°C): 

 time of sampling -.45 .15 -.19 .57 

 4-month low -.48 .12 -.06 .85 

 Specific conductance (μS/cm): 

 time of sampling .53 .07 -.63 .03

 4-month low .38 .22 -.63 .03

 pH: 

 time of sampling -.50 .10 .20 .54 

 4-month low -.65 .02 -.07 .83 

 Dissolved oxygen: 

 time of sampling (mg/L) -.45 .14 -.12 .72 

 4-month low (mg/L) -.23 .49 -.06 .86 

 saturation, 4-month low (%) -.42 .18 -.05 .88 

Habitat 

 Bank height (m): 

 mean -.40 .20 -.50 .10 

 maximum -.35 .27 -.45 .14 

 Bank vegetation stability: 

 mean .60 .04 -.15 .65 
22 Benthic Invertebrate Assemblages and Their Relation to Physical and Chemical Characteristics of Streams in the Eastern Iowa 

Basins, 1996–98

Table 8. Correlations of detrended correspondence analysis (DCA) axis scores and environmental variables from 12 basic fixed sites in 

the Eastern Iowa Basins, Iowa and Minnesota, 1996—Continued 

 Environmental DCA Axis 1 DCA Axis 2 Environmental DCA Axis 1 DCA Axis 2 

 variable r- p- r- p- variable r- p- r- p- 

 value value value value value value value value

 maximum 0.38 0.23 0.08 0.82 

 Bank width (m): 

 mean -.40 .20 -.38 .23 

 maximum -.21 .52 .14 .68

 Bottom material: 

 % clay -.48 .12 -.25 .44 

 % sand -.27 .40 .14 .66 

 % silt .44 .15 .11 .73 

 Canopy angle (degrees): 

 mean (sum both sides) .75 <.01 .08 .82 

 maximum .82 <.01 .12 .72 

 Gradient, stream segment (m/km) .63 .03 .14 .67 

 Sinuosity, stream segment -0.02 0.96 0.14 0.67 

 Stream depth (m): 

 mean -.71 .01 .13 .69 

 maximum -.79 <.01 .22 .51 

 Stream width (m): 

 wetted channel -.90 <.01 -.03 .92 

 maximum -.88 <.01 -.05 .89 

 Velocity (m/s): 

 mean -.57 .05 .11 .74 

 maximum -.55 .06 .02 .95 

Stream Productivity 

 g O2/m3

/hr by calculation -.53 .08 -.09 .79 

B

B

J

H

H

H

F

F

F

F

F

F

B

J

H

F

River Sites

Group 1

Group 2

Group 3

Group 4

Iowan Surface

Wetlands

Iron, dissolved

Bank width

Magnesium, dissolved

Nitrite + nitrate, dissolved

Herbicide degradation products

Specific conductance

0 +2.0

DCA AXIS 1

7A

16A

19A 22A

21A

24A

2

1 3

4

5

6

Basin area

Land cover (nonagricultural)

Population density

Human-influenced water-quality variables

Stream depth, width, velocity

Land cover (agricultural)

Lithologic water-quality variables

Bank vegetation stability

Canopy angle

Stream-segment gradient

DCA AXIS 2

+1.5

0

EXPLANATION

several times greater than measured during the same 

period in 1996 and 1997; this value was comparable to 

those observed at site 21 in 1997 and 1998 (11.3 and 

11.2 m3/s, respectively). Similarly, in 1996, the 

discharge at site 21 was 57 percent (6.4 m3

/s) of the 

values measured for the same period in 1997 and 

1998; this value was comparable to those observed at 

site 19 in 1996 and 1997 (3.5 and 5.7 m3/s, respectively). Although discharge at site 5, a large-river site, 

varied among the years (244 to 431 m3

/s), discharges 

at that site were considerably greater and more similar 

to each other among years compared to the other two 

sites. 

Over the 3-year period, temporal differences in 

benthic invertebrate populations at a site exceeded 

spatial differences within a site (tables 6 and 7). 

Jaccard coefficients of community for pairs of years 

ranged from 16 to 43 percent, with mean values of 

24.0 (site 19) and 27.3 (site 21) to 39.0 (site 5) (table 

6). None of the 82 taxa included in the TWINSPAN 

analysis was found at all sites in all years sampled 

although 14 taxa were collected at all three sites. Eight 

of these were among the predominant taxa present in 

the EIWA study unit (table 4), including the mayflies 

Baetis intercalaris, Stenonema, and Tricorythodes; the 

net-spinning caddisflies Hydropsyche bidens and H. 

Figure 5. Detrended correspondence analysis (DCA) biplot of benthic invertebrates in relation 

to 12 basic fixed sites in the Eastern Iowa Basins, Iowa and Minnesota, 1996.
D

ISTRIBUTIONOFBENTHICINVERTEBRATES

23

Table 9. Mean concentration or value of environmental variables in four site groups used to describe 12 basic fixed sites in the Eastern Iowa Basins, Iowa and Minnesota, 1996 [Site groups identified using TWINSPAN analysis. p-value, attained level of significance from Kruskal-Wallis test; p-value in boldface type, attained level of significance less than or equal to 0.05; parentheses in header (2), number of sites in group; km2, square kilometer; %, percent; population data, see Sorenson and others, 1999; x103, multiply mean value by 1,000; kg/km2, kilogram per square 

kilometer; mg/L, milligrams per liter; CaCO3, calcium carbonate; N, nitrogen; P, phosphorus; S, sulfur; μg/L, micrograms per liter; <, less than; °C, degrees Celsius; μS/cm, microsiemens per centimeter at 

25 degrees Celsius; m, meter; m/km, meters per kilometer; m/s, meters per second; g O2/m3/hr, grams of oxygen per cubic meter per hour] 

 Mean concentration or value Mean concentration or value  Environmental by indicated site group or total Environmental by indicated site group or total 

 variable 1 2 3 4 All p-value variable 1 2 3 4 All p-value

 sites sites  (2) (1) (3) (6) (12) (2) (1) (3) (6) (12) 

Site Characterization 

General 

 basin area (km2) 832 522 664 15,089 7,893 0.03

Geology and Land Use/Land 

 Cover (% of basin area)  Des Moines Lobe 10.0 0.0 0.0 23.1 28.2 .04

 Iowan Surface .0 .0 66.7 35.2 34.3 .22  Iowan Karst .0 .0 33.3 13.8 15.3 .63  Southern Iowa Drift Plain .0 10.0 .0 27.9 22.3 .05

 Agriculture 97.4 95.8 96.0 93.4 94.9 .24  Barren .1 .2 .8 1.0 .7 .07  Grassland 12.1 38.1 2.0 25.9 23.1 .06  Row crop 84.6 56.1 76.2 64.8 7.2 .10  Urban .7 1.4 .8 2.0 1.4 .04

 Wetland .4 .0 .4 .3 .3 .50  Wooded 2.5 4.8 2.4 6.6 4.7 .06 

Population Data (1990) 

 Human population density  (number/km2) 7.3 12.9 8.5 20.4 14.6 .03

 Human population of basin (x103) 6.9 6.7 5.9 341 174 .04

Agricultural Use of Nitrogen, 

 Phosphorus, and Pesticides  (kg/km2 in basin) 

 Pesticides applied, total 161 107 142 139 141 .02

 Nitrogen from:  atmosphere 569 604 552 578 573 .49  inorganic fertilizer 6,839 4,659 6,078 5,949 6,022 .03

 manure 3,031 1,837 1,639 1,910 2,023 .16  Phosphorus from:  inorganic fertilizer 924 620 819 800 810 .02

 manure 1,507 754 695 845 910 .09 Water-Column Chemistry  Water Quality (mg/L)  Alkalinity (as CaCO3) 269 172 183 176 193 .19 

 Bicarbonate (as CaCO3) 328 210 223 208 232 .19 

 Carbonate (as CaCO3) 0 0 0 9 5 0.15 

 Calcium, dissolved 82.5 56.0 66.7 56.7 63.4 .27  Carbon, organic:  dissolved 4.0 2.9 2.4 3.8 3.4 .16  suspended 1.2 .8 1.4 3.9 2.6 .04

 Chloride, dissolved 18.0 17.0 18.0 26.2 22.0 .16  Fluoride, dissolved .30 .30 .20 .28 .26 .45  Hardness, total (as CaCO3) 325 230 230 228 245 .19 

 Iron, dissolved 2.8 3.0 8.5 15.3 1.5 .38  Magnesium, dissolved 28.5 21.0 16.0 21.3 21.2 .13  Manganese, dissolved .28 .96 .31 .03 .22 .06  Nitrogen (as N):  ammonia + organic, dissolved .45 .20 .17 .32 .29 .10  ammonia + organic, total .70 .30 .63 2.27 1.43 .03

 ammonia, dissolved .04 .04 .03 .04 .04 .71  nitrite + nitrate, dissolved 6.50 9.40 7.77 4.61 6.11 .27  nitrite, dissolved .05 .03 .04 .03 .04 .39  Phosphorus (as P):  dissolved .10 .07 .07 .06 .07 .65  ortho, dissolved .11 .09 .08 .07 .08 .71  total .13 .08 .10 .28 .19 .05

 Potassium, dissolved 2.8 1.3 2.0 2.5 2.3 .34  Silicon, dissolved 22.5 13.0 11.2 8.5 11.9 .15  Sodium, dissolved 7.9 12.0 9.6 16.2 12.8 .19  Sulfate, dissolved (as S) 3.0 23.0 25.3 39.7 33.1 .07  Herbicides and Degradation  Products (μg/L)  Fonofos <.01 .01 <.01 <.01 <.01 .01

 Chlorpyrifos <.01 .01 <.01 <.01 <.01 .17  Diazinon <.01 <.01 <.01 <.01 <.01 .80  Carbofuran <.01 <.01 .01 <.01 <.01 .39 

Triazine

 Atrazine .19 .48 .27 .59 .43 .23  Deethylatrazine .13 .20 .12 .20 .17 .28  Deisopropylatrazine .05 .14 .05 .12 .09 .19 
24 Benthic Invertebrate Assemblages and Their Relation to Physical and Chemical Characteristics of Streams in the Eastern Iowa 

Basins, 1996–98

Table 9. Mean concentration or value of environmental variables in four site groups used to describe 12 basic fixed sites in the Eastern Iowa Basins, Iowa and Minnesota, 1996— 

Continued

 Mean concentration or value Mean concentration or value  Environmental by indicated site group or total Environmental by indicated site group or total 

 variable 1 2 3 4 All p-value variable 1 2 3 4 All p-value 

 sites sites  (2) (1) (3) (6) (12) (2) (1) (3) (6) (12) 

 Atrazine + metabolites 0.36 0.82 0.44 0.91 0.69 0.06  Cyanazine .00 .11 .02 .04 .03 .19  Cyanazine amide .03 .23 .03 .09 .07 .05

 Hydroxylatrazine .37 .45 .23 .31 .31 .86  Prometon .06 .01 .02 .02 .03 .33  Simazine .00 .00 .00 .01 .00 .56 

Chloroacetamide

 Acetochlor .01 .02 .01 .01 .01 .45  Acetochlor ethanesulfonic acid .19 1.25 .16 .32 .34 .33  Acetochlor oxanilic acid .10 .29 .10 .19 .16 .33  Acetochlor + metabolites .29 1.56 .82 .90 .83 .63  Alachlor .01 .01 .01 .01 .01 .60  Alachlor ethanesulfonic acid 1.89 1.74 1.36 1.64 1.62 .95  Alachlor oxanilic acid .10 .28 .24 .10 .15 .07  Alachlor + metabolites 2.00 2.03 1.61 1.75 1.78 .95  Metolachlor .29 .11 .08 .19 .17 .26  Metolachlor ethanesulfonic acid 7.07 2.58 4.37 3.16 4.06 .19  Metolachlor oxanilic acid 1.84 .74 .34 .62 .76 .10  Metolachlor + metabolites 9.20 3.43 4.79 3.97 5.00 .18 

Single Measurements and 

 Continuous-Monitoring Data  Water temperature (°C):  time of sampling 24.3 21.4 20.7 24.5 23.3 .13  4-month low 23.6 18.6 20.4 23.9 22.5 .16  Specific conductance (μS/cm):  time of sampling 698 504 481 443 500 .16  4-month low 664 517 502 453 506 .15  pH:  time of sampling 8.0 8.0 7.7 8.3 8.1 .28  4-month low 8.3 8.0 8.0 8.4 8.2 .06  Dissolved oxygen:  time of sampling (mg/L) 8.6 8.1 8.0 11.0 9.6 .20 

 4-month low (mg/L) 8.5 9.9 8.1 8.8 8.7 0.39  saturation, 4-month low (%) 104 107 96 109 105 .56 Habitat  Bank height (m):  mean 1.4 6.2 1.6 3.2 2.7 .14  maximum 2.2 7.5 2.3 4.2 3.7 .16  Bank vegetation stability  mean 1.9 2.8 2.6 1.8 2.1 .18  maximum 3.0 4.0 4.0 3.3 3.5 .29  Bank width (m):  mean 3.4 6.6 4.0 5.7 4.9 .23  maximum 8.0 9.9 9.5 14.6 11.8 .73  Bottom material (lab analysis):  % sand 58 45 59 40 48 .67  % silt 26 41 25 42 35 .67  % clay 16 14 16 19 17 .84  Canopy angle (degrees):  mean (sum both sides) 57 65 62 22 41 .12  maximum 70 78 79 30 53 .08  Gradient, stream segment (m/km) .68 .57 .57 .31 .46 .41  Sinuosity, stream segment 1.23 1.26 1.22 1.28 1.26 .90  Stream depth (m):  mean .48 .21 .28 1.00 .67 .04

 maximum .80 .59 .69 2.10 1.40 .03

 Stream width (m):  wetted channel 22.4 16.3 17.8 123 7.9 .04

 maximum 31.9 19.8 2.8 156 9.1 .03

 Velocity (m/s):  mean .29 .34 .26 .44 .36 .04

 maximum .51 .49 .41 .62 .54 .15 Stream Productivity  g O2/m3/hr by calculation .30 .26 .20 .36 .30 .02
DISTRIBUTION OF BENTHIC INVERTEBRATES 25

Table 10. Responses of benthic invertebrates and associated metrics to concentrations of organic carbon, nitrogen, and phosphorus, and reported tolerance values for each taxon [Taxa ranked by mean relative abundance among all samples (table 4). Taxa and chemical variables collected at 12 basic fixed sites in 1996 and 25 synoptic sites in 1997. Tolerance value, a water-quality rating between 0 (intolerant) and 10 (tolerant) derived from Iowa Department of Natural Resources data (T. Wilton, Iowa Department of Natural Resources, written commun., 1998); –, correlation between relative abundance of a taxon or value of a metric and the concentration of a variable not significant (p greater than 0.05) from Fisher's R to z test; positive, significant (p less than or equal to 0.05) positive 

correlation between relative abundance of a taxon or value of a metric and the concentration of a variable; negative, significant (p less than or equal to 0.05) negative correlation between relative abundance 

of a taxon or value of a metric and the concentration of a variable; N/A, tolerance value not available; EPT, Ephemeroptera, Plecoptera, and Trichoptera taxa; %, percent] 

 Organic carbon Nitrogen Phosphorus Tolerance 

 Dissolved Sus- Ammonia Ammonia Ammonia, Nitrite, Nitrate Total Dissolved Ortho, value  Variable pended plus plus dissolved dissolved plus dissolved from  organic, organic, nitrite, literature  dissolved total dissolved 

Invertebrate taxa 

Tricorythodes – – – – – – – – – – 4 Polypedilum – positive positive – – – – – – – 6 Hydropsyche bidens – positive positive – – – – – negative – 3 Rheotanytarsus – – – – – – – – – – 6 Baetis intercalaris – – – – – – – – – – 6 Hydropsyche simulans – – – – – – – – – – 7 

Glyptotendipes – – – – – – – – – – 10

Naididae – – – – – positive – negative – negative 7.6 Macronychus glabratus – negative negative – – – – negative – – 4 

Cricotopus/Orthocladius – negative – – – – – negative – – 7/6 Stenonema terminatum – negative – – – – – – – – 4

Fallceon quilleri – – – – – – – – – – 6

Stenonema – – – – – – – – – – 3.7

Thienemannimyia group – – – positive – positive – – – – 6 

Acentrella negative negative negative negative – – – – – – 6

Tanytarsus – – – – – – – – – – 6Ceratopsyche – – – – – – – – – – 4.5

Stenochironomus – – – – – – – – – – 5Cricotopus bicinctus group – – – – – – positive – – – 6 Caenis – – – – – – – – – – 7 Corixidae positive – – positive positive – – – – – 5 

Dicrotendipes positive – – positive positive – negative positive positive positive 8 

Simuliidae – – – – – – positive – – – 6 Hydrachnidia – – – – – – negative – – – N/A 

Hemerodromia – – positive – – – – positive – – 6

Stenelmis grossa – – – – – – – – – – 5

Atherix variegata – – – – – – – – – – 2

Cheumatopsyche – negative – – – positive – – – – 5Cladotanytarsus – – – – – – – – – – 7
26 Benthic Invertebrate Assemblages and Their Relation to Physical and Chemical Characteristics of Streams in the Eastern Iowa 

Basins, 1996–98

Table 10. Responses of benthic invertebrates and associated metrics to concentrations of organic carbon, nitrogen, and phosphorus, and reported tolerance values for each taxon— 

Continued 

 Organic carbon Nitrogen Phosphorus Tolerance 

 Dissolved Sus- Ammonia Ammonia Ammonia, Nitrite, Nitrate Total Dissolved Ortho, value  Variable pended plus plus dissolved dissolved plus dissolved from  organic, organic, nitrite, literature  dissolved total dissolved 

Invertebrate taxa–Continued 

Centroptilum/Procloeon – – – positive positive – – positive positive positive 2/6Parakiefferiella – negative – – – – – – – – 6Pseudocloeon – – – – – – positive – positive positive 4

Chironomus – – – – – – – – – – 10

Heptagenia diabasia negative – – – – – – – – – 3

Stenacron – – – positive – – – – positive – 7

Potamyia flava – positive – – – – – – – – 5

Coenagrionidae positive – – positive – – – – – – 8 

Rheocricotopus negative – negative – – – positive – – – 6Phaenopsectra – – – – – – – – – – 7

Stenonema mexicanum – – – – – – – – – – 4

Hydropsyche orris – positive – – – – – – – – 5Hydroptila – negative negative – – – – negative – – 6

Thienemanniella negative negative negative negative – – positive – – – 6

Heptagenia flavescens – – – – – – – – – – 4Isonychia – – – – – – – – positive positive 2Amercaenis ridens – – – – – – positive – – positive 7

Tubificidae positive – – – – – – – – – 10 Saetheria – – – – – – – – – – 4

Parachironomus – – – – – – – – – – 10Nanocladius – – – – – – – – – – 3Paratanytarsus – – – – – positive – – – – 6Dubiraphia positive – positive – positive – negative positive positive positive 6Nilotanypus – negative – – – – – – – – 6Brillia negative – – – – – – – – – 5Brachycentrus numerosus – – – – – positive – negative – – 1Mayatrichia ayama – – – – – – – – – – 6Hexagenia – – – – – – – – – – 6Ablabesmyia – – – – – – – – – – 8Nectopsyche candida positive – – positive positive – negative positive positive positive 3

Corynoneura – – – – – – positive – – – 7

Nematoda – – – – – – – – – – N/A 

Veliidae – – – – – – negative – – – N/A
DISTRIBUTION OF BENTHIC INVERTEBRATES 27

Table 10. Responses of benthic invertebrates and associated metrics to concentrations of organic carbon, nitrogen, and phosphorus, and reported tolerance values for each taxon— 

Continued 

 Organic carbon Nitrogen Phosphorus Tolerance 

 Dissolved Sus- Ammonia Ammonia Ammonia, Nitrite, Nitrate Total Dissolved Ortho, value  Variable pended plus plus dissolved dissolved plus dissolved from  organic, organic, nitrite, literature  dissolved total dissolved 

Invertebrate taxa–Continued 

Paranyctiophylax – – – positive – – – – – – 5Labrundinia – – – – – – – – – – 6Ancyronyx variegata – – – – – – – – – – 6Atrichopogon – – – – – – – – – – 6.8

Cryptochironomus – – – – – – – – – – 8

Helichus lithophilus – – – – positive – – – – positive 5

Gomphidae – – – – – – – positive positive positive 5 

Nectopsyche diarina – – – – – – – – positive positive 3Acroneuria – – – – – – – – – – 0Pteronarcys – – – – – – – – – – 0

Bryozoa – – – – – – – – – – N/A 

Pycnopsyche – – – – – – – – – – 4 

Invertebrate assemblage metrics 

Taxa richness – negative negative – – – – – positive – N/A EPT taxa richness – negative – – – – – – – – N/A Shredders/total abundance – – – – – – – – – – N/A Scrapers/collector-filterers – – – – – – – – – – N/A Dominant taxa (%) – positive positive – – – – – – – N/A EPT abundance (%) – – – – – – – – negative – N/A Chironomidae abundance (%) – – – – – – – – – – N/A 

Invertebrate functional feeding groups (% total abundance) 

Shredder – – – – – – – – – – N/A Scraper – – – – – – – – – – N/A Collector-filterer – positive positive – – – – – negative – N/A Collector-gatherer – negative negative – – – – – – – N/A Predator positive – – positive positive – – – – – N/A 
28 Benthic Invertebrate Assemblages and Their Relation to Physical and Chemical Characteristics of Streams in the Eastern Iowa 

Basins, 1996–98

simulans; and the Chironomidae Polypedilum, 

Stenochironomus, and Thienemannimyia group.

Six pairs of sites sampled in 1996 and 1997 

(fig. 3) illustrate the results of a multivariate ordination 

of sites by taxa for 31 locations in the EIWA study 

unit. Sites in figure 3 were assigned to one of four 

TWINSPAN groups on the basis of taxonomic composition and the correspondence of sites with attributes 

of the benthic invertebrate assemblages (metrics). 

One-half of the site pairs were assigned to the same 

group. These were samples from sites 21, 22, and 24. 

There was more temporal variation observed for sites 

7, 16, and 19 between 1996 and 1997, with the sites 

assigned to different groups. Sites 7, 16, and 19 

sampled in 1996 (suffix “A” in fig. 3) are in group 3, 

one characterized by the numerical importance of the 

Chironomidae. Assemblages collected at these same 

sites in 1997 are present in group 1, which is characterized by the numerical importance of EPT taxa.

Sampling time was relatively consistent among 

years, but precedent hydrologic conditions varied 

somewhat. Discharge within the EIWA study unit was 

lower in 1996 than in subsequent years (fig. 2). 

Johnson and others (1993) observed that merely 

changing the time of sampling from one year to the 

next, such as from late summer to early autumn, 

altered the TWINSPAN classification of lakes. In 

addition, considerable natural spatial and temporal 

variations of benthic invertebrates have been documented for life-history aspects, such as emergence, 

feeding and growth, and movements and migration 

(Resh and Rosenberg, 1989). The relations between 

EPT taxa and Chironomidae may relate more to 

natural factors such as climate or hydrology than land 

use, which remained constant during the study.

Differences in Benthic Invertebrates 

Among Site Groups

An ordination that used benthic invertebrate 

data and derived metrics for 37 samples at 31 sites 

(fig. 3) identified relations among sites and confirmed 

that the patterns in community structure were consistent with site groups. Assignment to a group was 

consistent between the TWINSPAN and DCA analyses with one exception (site 28 in fig. 3). This consistency in assignment to a group is not unusual because 

TWINSPAN and DCA are both based on reciprocal 

averaging (Hill, 1979; ter Braak and Smilauer, 1998). 

Site scores from the first two DCA axes were correlated with the invertebrate assemblage and metric data 

to identify which taxa or metrics contributed significantly to the relations observed among the sites. The 

first two axes of the DCA explained 22.3 percent of 

the variance.

The DCA axis 1 identified two groups of sites 

based on the importance of Chironomidae (positively 

correlated with DCA axis 1) and EPT taxa (negatively 

correlated with DCA axis 1). Chironomidae became 

increasingly dominant with increasing basin area. 

Groups 1 and 2, where the relative abundance of EPT 

taxa was higher and that of Chironomidae lower (table 

5), represented sites with smaller basin areas (mean 

556.6 km2

). Groups 3 and 4, where the relative abundance of Chironomidae was higher and that of EPT 

taxa lower, contained the sites with larger basin areas 

(group 3, mean 773.7 km2

; group 4, mean 15,089.0 

km2). The relative abundance of EPT taxa did not 

correlate significantly with basin area, discharge, or 

discharge per unit area using data from the 37 samples 

at 31 sites in 1996–97. The relative abundance of 

Chironomidae was significantly correlated with basin 

area and discharge. The regression plot in figure 4 

shows that the frequently more tolerant Chironomidae 

generally increase significantly in abundance from 

group 1 to 4, while the more environmentally sensitive 

EPT taxa decrease in abundance.

Other benthic invertebrate taxa and derived 

metrics that positively correlated with site scores on 

DCA axis 1 included the net-spinning caddisfly 

Hydropsyche orris and the chironomid Polypedilum. 

The net-spinning caddisflies, including H. bidens and 

Potamyia flava, are among the most common filterfeeding insects in streams and rivers; they use salivary 

secretions to spin nets on any solid surface to filter 

coarse particles (Lamberti and Moore, 1984). One 

reason for their abundance in eastern Iowa streams 

could be their ability to exploit a wider food base than 

other filter-feeding organisms by feeding on algae, 

detritus, associated microflora, and any animal material that they harvest from their nets (Lamberti and 

Moore, 1984). The relative abundance of Polypedilum, 

the second most common and widely distributed 

aquatic insect observed among the sites, increased 

from about 6 to nearly 18 percent from group 1 to 4 

(table 4). Rheotanytarsus, also widely distributed 

among all groups, was more abundant in group 4. 

Whereas Polypedilum is able to exploit sandy 

substrates, Rheotanytarsus is a sedentary species that 
DISTRIBUTION OF BENTHIC INVERTEBRATES 29

uses snags and other woody debris to colonize 

(Minshall, 1984). Simpson and Bode (1980) observed 

that Polypedilum was usually associated with Rheotanytarsus, Cheumatopsyche, and Nais in New York 

streams and considered these taxa as community indicators of abundant sources of suspended food.

The percentage of total abundance represented 

by the shredder functional feeding group was also 

positively correlated with DCA axis 1. Shredders were 

significantly more abundant in groups 3 and 4, 

reflecting the importance of the chironomid genera 

Polypedilum and Glyptotendipes (tables 4 and 5). 

Simpson and Bode (1980) characterized Glyptotendipes as a burrower favoring soft substrates in slowmoving streams and rivers. Fewer taxa were classified 

as shredders than any other functional feeding group 

of benthic invertebrates. Other numerically important 

taxa classified as shredders include the Chironomidae 

Cricotopus/Orthocladius, Cricotopus bicinctus, and 

Brillia and caddisflies in the genus Nectopsyche. 

The relative abundances of the chironomid Nilotanypus and the microcaddisfly genus Hydroptila were 

negatively correlated with site scores on DCA axis 1. 

Nilotanypus was widely distributed at sites in groups 1 

and 2, but its presence declined sharply in group 3, and 

it was not collected in group 4. Hydroptila exhibited a 

similar pattern. Available ecological information for 

species of Nilotanypus suggests that the larvae are 

found in uncontaminated, small- to medium-sized, 

cool streams and rivers. Nilotanypus lives in sand and 

gravel substrates, occasionally on rocks, in shallow, 

flowing water (Roback, 1986). These conditions were 

not characteristic of the sites in groups 3 and 4. 

Hydroptila is an herbivorous genus of microcaddisfly 

whose larval instars pierce filamentous algal cells 

and consume the contents; it might also feed on 

periphyton (Wiggins, 1996). It is possible that Hydroptila has ecological requirements similar to those of 

Nilotanypus because its food source probably is not 

limited in groups 3 and 4.

Taxa and associated metrics positively correlated significantly with site scores on DCA axis 2 

included the relative abundance of two detritivores that 

filter fine particles from the water column for food 

(net-spinning caddisflies such as Potamyia flava and 

the chironomid Rheotanytarsus) and the relative abundance of the collector-filterer functional feeding 

group. These variables effectively separated sites in 

group 4 from those in groups 1, 2 and 3 (fig. 3). Fifty 

percent of the organisms present at sites in group 4 

were classified as collector-filterers, capable of 

exploiting the significantly higher concentrations of 

suspended organic carbon (table 9). The percentage of 

collector-filterers was significantly higher than in any 

other group because of the abundances of Hydropsyche bidens, Polypedilum, and Rheotanytarsus.

Site scores on DCA axis 2 were negatively 

correlated significantly with the percent total abundance of the collector-gatherer functional feeding 

group, the ratio of scrapers to collector-filterers, and 

the relative abundance and distribution of five genera 

of Chironomidae (Ablabesmyia, Labrundinia, Thienemannimyia, Phaenopsectra, Brillia). The collectorgatherer functional feeding group predominated 

among all samples combined, with an overall relative 

abundance of 42.7 percent (table 5). It predominated in 

site groups 1, 2, and 3, groups with much smaller 

watersheds (mean basin area 634.7 km2

, range 308 to 

1,487 km2) than group 4 (mean basin area 15,089.0 

km2, range 6,050 to 32,365 km2

). The collector-filterer 

functional feeding group dominated at the large-river 

sites of group 4. 

Taxa classified as collector-gatherers are more 

generalized or opportunistic feeders that collect finely 

divided organic material that has settled on the sediments in depositional zones, such as pools or stream 

margins (Lamberti and Moore, 1984). Predominant 

collector-gatherers included the aquatic oligochaete 

worms in the families Naididae and Tubificidae; the 

mayflies Tricorythodes, Baetis intercalaris, Fallceon 

quilleri, Acentrella, Caenis, Centroptilum/Procloeon, 

Pseudocloeon, Stenacron, and Hexagenia; the riffle 

beetles Macronychus glabratus and Ancyronyx variegata; and a number of Chironomidae (for example, 

Dicrotendipes, Parakiefferiella, Chironomus, Rheocricotopus, Thienemanniella, Saetheria, Nanocladius, 

and Corynoneura).

Taxa that feed by grazing on the algae and 

detritus on the surface of rocks and other solid objects 

are classified as scrapers. Although these organisms 

were most abundant in groups 1 and 2, decreased in 

abundance in group 3, and even more in group 4, their 

abundance was low compared to that of the collector 

functional feeding groups in the study area. Among 

abundant taxa classified as scrapers were mayflies in 

the genus Stenonema and Heptagenia diabasia, the 

caddisfly Mayatrichia ayama, the beetles Stenelmis 

grossa and Helichus lithophilus, and the chironomid 

Phaenopsectra. 
30 Benthic Invertebrate Assemblages and Their Relation to Physical and Chemical Characteristics of Streams in the Eastern Iowa 

Basins, 1996–98

Because scrapers are herbivores, their distribution and abundance are influenced by the patterns of 

primary productivity in streams (Gregory, 1983) and 

linked to suitable hard substrates for the development 

of their algal food source. Other investigators have 

suggested that the relative abundance of scrapers 

would be greatest in the intermediate reaches, such as 

those in site groups 1, 2, and 3, that are found along a 

longitudinal gradient from first-order, headwater 

streams to large rivers (Vannote and others, 1980). At 

the large-river sites in group 4, hard substrates were 

limited to snags and woody debris whose horizontal 

surfaces were thickly covered by sand and fine sediments at the time of sampling, which may have limited 

the growth of periphytic algae. The low values associated with the ratio of scrapers to collector-filterers is 

another indication of the low frequency of scraper taxa 

and the importance of collector-filterers at sites in 

group 4 (table 5). 

The five genera of Chironomidae (Ablabesmyia, 

Labrundinia, Thienemannimyia, Phaenopsectra, 

Brillia) were either abundant in group 3 or collected 

infrequently or not at all at the large-river sites of 

group 4. The chironomid taxa expressing this pattern 

of distribution crossed functional feeding designations, including several predators (Ablabesmyia, 

Labrundinia, and Thienemannimyia), a shredder 

(Brillia), and a scraper (Phaenopsectra).

Group 4 was the most biologically distinct site 

group, with six sites having high scores on DCA axes 

1 and 2 (fig. 3). These were river sites with the largest 

basin areas (greater than 6,000 km2

); high productivity 

(mean 0.36 g O2/m3

/hr); channels that were significantly wider (mean width 122.8 m), deeper (mean 

depth 1.0 m), and faster (mean velocity 0.44 m/s); 

and high concentrations of nutrients and suspended 

organic carbon (table 9). Biologically in group 4, the 

relative abundance of Chironomidae was significantly 

higher, a few taxa predominated the invertebrate populations to a higher degree (55.9 percent) than in the 

other site groups, and collector-filterers were significantly more abundant (50.5 percent) (tables 4 and 5). 

The organisms that filter particles from the water 

column probably responded to the higher concentrations of suspended organic matter available at these 

large-river sites (table 9).

Taxa richness, the number of distinct taxa in a 

collection or at a site, is a component and estimate of 

community structure (Resh and Jackson, 1993). Taxa 

richness was significantly lower among group 4 sites 

(table 5). Richness often increases with improved 

water quality, habitat diversity, or habitat suitability 

(Weber, 1973; Resh and Grodhaus, 1983; Plafkin and 

others, 1989) but might also reflect mild organic 

enrichment in some situations. Lower taxa richness 

and the predominance of a few taxa might represent 

invertebrate assemblages under stress (Plafkin and 

others, 1989) or merely the natural changes in populations attributable to longitudinal succession (Fisher, 

1983). 

Commenting on the predictability of changes in 

stream systems from source to mouth, Stanford and 

Ward (1983) stated that, whereas constancy prevailed 

in the headwaters and higher stream orders (above 

stream order 7; none among the 31 sites sampled in the 

EIWA Basins), environmental conditions such as 

temperature and discharge in the middle reaches 

(stream orders 5 to 7) were highly variable. Stanford 

and Ward (1983) agreed with Vannote and others 

(1980) that species diversity should be greatest within 

the middle reaches of the stream continuum because 

the range of annual change in temperature and 

discharge across stream orders is greatest for stream 

orders 5 to 7. Within the EIWA study unit, taxa richness was highest in groups 2 and 3, EPT taxa richness 

increased from group 1 to 2, and the abundance of the 

shredder and collector-gatherer functional feeding 

groups did not decline sharply until downstream at 

group 4 sites. Many taxa displayed this general pattern 

of abundance within groups 2 and 3, including the 

small riffle beetle Dubiraphia; the caddisfly Nectopsyche candida; and the Chironomidae Brillia, Cricotopus bicinctus group, Paratanytarsus, Saetheria, and 

Tanytarsus (table 4).

INFLUENCE OF PHYSICAL AND 

CHEMICAL CHARACTERISTICS OF 

STREAMS ON BENTHIC INVERTEBRATE 

ASSEMBLAGES

Environmental variables monitored at the 

12 basic fixed sites were used to assess their potential 

influence on the benthic invertebrate assemblages in 

an ordination with a data set of 33 taxa and 12 sites 

(fig. 5). The first two axes of the DCA accounted for 

41.3 percent of the variance observed in the invertebrate populations. Site scores from the first two DCA 

axes were correlated with the environmental variables 

to identify those that were important in describing 

relations among the invertebrate assemblages (table 8). 
INFLUENCE OF PHYSICAL AND CHEMICAL CHARACTERISTICS OF STREAMS ON BENTHIC INVERTEBRATE ASSEMBLAGES 31

Identification of Important 

Environmental Variables

Environmental variables with significant positive correlations with site scores on DCA axis 1 

included agricultural land use and land cover (row 

crops, other agriculture), stream-segment gradient, 

stream-habitat features (bank vegetation stability, 

canopy angle), forms of phosphorus (dissolved phosphorus, dissolved orthophosphate), and some other 

water-quality variables (pH; concentrations of manganese, dissolved silica, calcium, alkalinity, bicarbonate) 

(fig. 5; table 8). Sites with low scores on DCA axis 1 

are associated with nonagricultural land cover and 

land use, lower stream-segment gradient, and less 

bank stability and shading of the stream by riparian 

vegetation (table 9). 

Environmental variables with significant negative correlations with site scores on DCA axis 1 

included basin area, nonagricultural features of land 

use and land cover (barren, urban, wooded), variables 

associated with population or possible human influences on water quality (population density; concentrations of sodium, chloride, sulfate, total ammonia plus 

organic nitrogen, suspended organic carbon, total 

phosphorus), and features of stream habitat (stream 

width, depth, velocity) (fig. 5; table 8). Sites with low 

scores on DCA axis 1 are associated with deeper and 

wider stream channels, higher mean velocities, larger 

basin areas, more land in nonagricultural use, greater 

population densities, probable human influences 

on water quality (for example, higher concentrations 

of sodium, chloride, sulfate, total ammonia plus 

organic nitrogen, suspended organic carbon, and total 

phosphorus). Basin area is a principal factor in the 

distribution of sites along DCA axis 1. Basin areas of 

the six sites in group 4 range from 6,050 to more than 

32,300 km2

. The basin areas of the remaining six sites 

in groups 1, 2, and 3 have smaller watersheds that 

range from 389 to 1,083 km2. Sites with high scores 

on DCA axis 1 were located in smaller watersheds in 

headwaters, other upstream areas, or tributaries.

The environmental variables correlated significantly with site scores on DCA axis 2 include geological features and contribute additional evidence of the 

importance of point and nonpoint sources of organic 

and nutrient enrichment from urban development and 

agriculture that affect the structure and composition of 

benthic invertebrate assemblages (fig. 5). Environmental variables positively correlated with site scores 

on DCA axis 2 included the percentage of the basin 

area on the Iowan Surface landform, the percentage of 

the basin area occupied by wetlands, bank width, and 

dissolved iron. Nutrients (dissolved nitrite plus 

nitrate), herbicides (particularly the degradation products deisopropylatrazine and acetochlor ethanesulfonic 

acid), and some other water-quality variables (magnesium and specific conductance) were all negatively 

correlated with site scores on DCA axis 2.

Sites are arranged along DCA axis 2 by the 

percentage of the basin area that includes each of four 

geological landforms, especially the Iowan Surface. 

The basin areas of sites in group 4 are large enough to 

encompass two to four of the recognized landforms in 

the EIWA study unit (fig. 1). The watersheds of the 

remaining six sites are each sufficiently small to be 

contained within only one geological landform. The 

sites assigned to group 1 (sites 21 and 22) are entirely 

within the Des Moines Lobe landform. The watershed 

of site 24, the only representative of group 2 among 

the basic fixed sites, is entirely within the Southern 

Iowa Drift Plain. Group 3 includes sites 7, 16, and 19, 

all of which are within the Iowan Surface or the Iowan 

Karst, a subgroup of the Iowan Surface. Other investigators have observed distinguishable regional differences in the distribution patterns of aquatic biota, 

especially fishes, in Iowa (Griffith and others, 1994). 

Paragamian (1990) noted that the composition and 

habitat of assemblages of fishes varied among five 

landform regions in Iowa. Menzel (1987) concluded 

that features such as geology, topography, soils, vegetation, drainage features, and land use within each 

landform influenced the regional distribution of fishes 

in Iowa.

The remaining area that is classified as wetlands 

might be considered as a surrogate for riparian vegetation because 87 to 96 percent of the basin area has 

been converted to some form of agriculture. Bottomland forest, a wetland type, is extensive in the upper 

Wapsipinicon River Basin (site 7). Preliminary data 

reviewed by Schnoebelen and others (1999) suggested 

that the Wapsipinicon River might have a more extensive riparian zone than other large rivers in the EIWA 

study unit. They speculated that an extensive riparian 

zone could reduce the transport of nutrients and pesticides to the river. Streams might be less turbid and 

periphytic algae of more significance with increased 

stream gradient, bank vegetation stability, and canopy 

shading (variables positively correlated along DCA 

axis 1) (M.A. Harris and S.D. Porter, U.S. Geological 
32 Benthic Invertebrate Assemblages and Their Relation to Physical and Chemical Characteristics of Streams in the Eastern Iowa 

Basins, 1996–98

Survey, written commun., 2000) and lower concentrations of the variables with negative correlations on 

DCA axis 2. The negatively correlated variables that 

reflect increased herbicide and nutrient concentrations 

and poorer water quality are evident at sites farthest 

downstream (for example, sites 4 to 6) and in areas 

with concentrations of confined, high-density hogfeeding operations (for example, sites 21 and 22 in the 

upper Iowa River watershed). Fewer of these operations are in the watersheds of sites 7, 16, and 19. 

Schnoebelen and others (1999) commented on the 

number of hog-production operations that began in the 

1990's in the study unit. They noted that the upper 

parts of the Iowa and Skunk River Basins had more 

than twice the number of permits issued from 1993 to 

1996 compared to 1987–93 and speculated generally 

on the possibility of negative effects on surface-water 

quality.

Distinctions Among Site Groups

The invertebrate assemblages in the four site 

groups reflect the cumulative effects of land use (agricultural and urban), point and nonpoint sources of 

organic and nutrient enrichment, landform, and longitudinal stream succession (Fisher, 1983). These 

factors, especially the natural changes from upstream 

to downstream, were influential in describing the 

benthic invertebrate assemblages defining the site 

groups. 

The invertebrate assemblage associated with the 

sites in group 4, those with the largest basin areas, was 

clearly defined. This group was distinct, although it is 

difficult to distinguish among the contributions of 

large basin area, concentrations of nutrients and pesticides, and decreasing instream habitat diversity. Of 74 

taxa included in the ordination of 31 sites (fig. 3, table 

4), 25 taxa were not represented in group 4, another 15 

were present at the lowest relative abundance observed 

among all samples, and 11 were present at the highest 

abundances recorded. Of these 11, significant differences in relative abundance among site groups were 

found for the net-spinning caddisflies Hydropsyche 

bidens, H. orris, and Potamyia flava; the Chironomidae Polypedilum, Rheotanytarsus, Parachironomus, and Corynoneura; and the dance fly 

Hemerodromia. Among the invertebrate metrics, the 

percent abundance of the three dominant taxa, 

Chironomidae, and the collector-filterer functional 

feeding group were highest in group 4. Taxa richness, 

EPT taxa richness, the ratio of scrapers to collectorfilterers, and the percent abundance of the functional 

feeding groups of scrapers, collector-gatherers, and 

predators were lowest in group 4. 

Sites in groups 1, 2, and 3 reflect conditions 

found more commonly in smaller streams, such as 

higher stream-segment gradients and the potential for 

more diverse or extensive riparian habitat. 

Groups 1 and 2 were distinct from groups 3 and 

4 in many respects. This result seems to be the most 

definitive distinction among the benthic invertebrate 

assemblages identified in eastern Iowa streams. The 

two pairs of groups were distinguished principally by 

the relative abundance of the EPT taxa (higher in 

groups 1 and 2) and Chironomidae (lower in groups 1 

and 2) (fig. 4, table 5). Of 74 taxa included in the ordination of 31 sites (fig. 3, table 4), 10 taxa were not 

represented in groups 1 and 2, 8 were present at the 

lowest relative abundance observed among all 

samples, and 27 were present at the highest abundances recorded.

Although most of the 27 taxa were still present 

at low abundances, mayflies, stoneflies, and caddisflies predominated. Cumulatively, they contributed to 

the principal factor that distinguished these two pairs 

of site groups—the difference in relative abundance of 

the EPT taxa and Chironomidae. Using data from the 

37 samples at 31 sites during 1996–97, the relative 

abundance of EPT taxa was not significantly related to 

increasing basin area or discharge whereas the relative 

abundance of Chironomidae was. In general, the 

majority of EPT taxa are considered to be environmentally sensitive groups (Lenat, 1988; Plafkin and others, 

1989). Their abundance is assumed to reflect less 

disturbed or enriched conditions, whereas chironomids 

tend to increase in dominance in terms of relative 

abundance or taxonomic composition along a gradient 

of increasing organic and nutrient enrichment and 

basin area (Merritt and others, 1984).

In their assessment of 69 small- to moderatesized Illinois, Iowa, and Minnesota streams, Harris and 

Porter (M.A. Harris and S.D. Porter, written commun., 

2000) concluded that stream velocity influenced 

whether invertebrate assemblages were dominated by 

EPT taxa (sites with faster velocity) or Chironomidae 

(sites with slower velocity). Although neither the relative abundance of EPT taxa nor Chironomidae correlated significantly with mean velocity in the present 

study, velocity was one of several significant variables 
INFLUENCE OF PHYSICAL AND CHEMICAL CHARACTERISTICS OF STREAMS ON BENTHIC INVERTEBRATE ASSEMBLAGES 33

that contributed to the relations among the invertebrate 

populations at the 12 basic fixed sites (fig. 5). As basin 

area, stream depth, width, and velocity increased, the 

relative abundance of Chironomidae increased (tables 

1, 5, and 9). None of the basin areas of streams studied 

by Harris and Porter (M.A. Harris and S.D. Porter, 

written commun., 2000) exceeded 2,771 km2

; six sites 

in the present study exceeded 6,000 km2

. Velocity 

contributes to the explanation of the composition of 

benthic invertebrate assemblages in the streams in 

eastern Iowa, but may be less important locally than in 

the regional context of Harris and Porter.

Hydrology influenced the composition of the 

invertebrate assemblages, particularly the abundances 

of EPT taxa and Chironomidae collected at some sites 

in 1996 and 1997. Because these two groups of taxa 

were influential in assigning sites to a group, differences in the composition of assemblages at three sites 

between years caused temporal shifts in site membership between groups 1 (abundance of EPT taxa) and 3 

(abundance of Chironomidae) (fig. 3). The shift in 

group membership was attributed to differences 

observed in rainfall-runoff and streamflow conditions. 

Discharge within the study unit was lower in 1996 

than in 1997 (fig. 2). Although time of sampling was 

relatively consistent between years, precedent hydrologic conditions varied. With lower flow, the composition of the invertebrate populations shifted to favor 

Chironomidae. Harris and Porter (M.A. Harris and 

S.D. Porter, written commun., 2000) also concluded 

that spatial and temporal (for example, from year to 

year) differences in regional rainfall conditions were 

likely to influence the structure of invertebrate 

communities and the interpretation of water-quality 

conditions. They recommended that invertebrate 

communities in streams and rivers probably should be 

evaluated during contrasting (wet and dry) years 

before assigning water-quality classifications or 

criteria to specific water bodies.

Ground water from the limestone bedrock aquifers contributes to the flow of rivers such as the Cedar 

and Wapsipinicon on the Iowan Surface landform 

during low-flow periods (Prior, 1991). In a study that 

included 69 stream reaches in Illinois, Iowa, and 

Minnesota, Harris and Porter (M.A. Harris and S.D. 

Porter, written commun., 2000) found that streams in 

eastern Iowa receiving proportionally more groundwater sources, such as those in the upper parts of the 

Cedar and Wapsipinicon Basins, had greater total taxa 

and EPT taxa richness. They speculated that differences between physicochemical conditions in groundwater discharges compared to surface-water sources 

and increased microhabitat complexity in streams with 

substantial ground-water contributions could provide 

additional niches for exploitation by invertebrate taxa, 

leading to higher taxa and EPT taxa richness. Newbury 

(1984) commented that the variety of sources of 

streamflow contributed to the variability of the water 

quality of insect habitats. He reported that concentrations of major ions were abruptly diluted as surface 

flows and shallow, temporary ground-water flows 

entered streams during snowmelt or rainstorms. 

Responses of Benthic Invertebrates to 

Nutrients and Organic Enrichment

The responses of benthic invertebrates to environmental variables, especially nutrients and indicators of organic enrichment, or specific contaminants, 

are used in biotic indices to evaluate water or habitat 

quality. States, including Iowa, developed biocriteria 

and associated indices to perform rapid biological 

assessments for wadeable streams and rivers (T.F. 

Wilton, Iowa Department of Natural Resources, 

written commun., 1998). As the basis for these 

indices, a taxon is assigned a rating from 0 (most 

sensitive) to 10 (least sensitive) or a category (for 

example, tolerant, intolerant or sensitive, facultative) 

based on the objective derivation of tolerance values 

(see Lenat, 1993, for discussion) or “expert opinions.” 

Organisms classified as facultative are able to survive 

under a range of conditions, are more “intermediate” 

in their preferences—not exactly fitting either the 

tolerant or intolerant categories (Olive and Dambach, 

1973). 

Following a procedure used by Cuffney and 

others (1997) to describe the tolerance of algal taxa to 

nutrients, the relative abundances of 74 taxa from 31 

sites in the EIWA study unit were correlated with 

concentrations of forms of organic carbon, nitrogen, 

and phosphorus. The purpose was to identify relations 

with benthic invertebrate taxa and to refine tolerance 

values used for streams in eastern Iowa. Although the 

responses of individual taxa to organic carbon and 

nutrient concentrations varied, nearly 50 percent were 

found to be facultative, showing no significant correlation to any of the variables (table 10). Many of the 

predominant taxa in table 4 could be classified as 

facultative, such as the mayflies Tricorythodes, Baetis
34 Benthic Invertebrate Assemblages and Their Relation to Physical and Chemical Characteristics of Streams in the Eastern Iowa 

Basins, 1996–98

intercalaris, Fallceon quilleri, Stenonema, and Caenis; 

the net-spinning caddisflies Hydropsyche simulans and 

Ceratopsyche; the riffle beetle Stenelmis grossa; and 

the Chironomidae Rheotanytarsus, Glyptotendipes, 

Tanytarsus, Stenochironomus, and Cladotanytarsus.

Most of the remaining taxa showed significant 

correlations with one to three of the 10 constituents 

examined, reflecting either tolerance (significant positive correlation) or sensitivity (significant negative 

correlation) to a particular form of organic carbon, 

nitrogen, or phosphorus (table 10). Only six of the 

remaining taxa (mayflies Acentrella and Centroptilum/Procloeon, caddisfly Nectopsyche candida, 

elmid beetle Dubiraphia, and Chironomidae Dicrotendipes and Thienemanniella) had more than three 

significant correlations with the 10 variables examined. 

Tolerance values, compiled by the Iowa Department of Natural Resources from its own data and 

published sources (particularly Hilsenhoff, 1988; 

Lenat, 1993) (T.F. Wilton, Iowa Department of Natural 

Resources, written commun., 1998), were included in 

table 10 to determine if site-specific data for the invertebrates were comparable to existing values or could 

be used to refine values for streams in eastern Iowa. 

Facultative taxa have values near the midrange 

between 0 and 10; values between 4.5 to 6.5 roughly 

correspond to those described by Lenat (1993). Values 

less than 4.5 are considered as sensitive or intolerant; 

values higher than about 6.5, tolerant. 

The tolerance values of most taxa in the EIWA 

study unit that might be classified as facultative (that 

is, no significant correlation to any variable examined) 

ranged from 4.5 to 6.5 (table10). Facultative taxa 

whose tolerance values probably should be lower for 

streams in eastern Iowa included the mayfly Caenis; 

net-spinning caddisfly Hydropsyche simulans; and the 

Chironomidae Glyptotendipes, Cladotanytarsus, 

Chironomus, Phaenopsectra, Parachironomus, Ablabesmyia, and Cryptochironomus. Facultative taxa 

whose tolerance values probably should be higher for 

streams in eastern Iowa included the mayflies Tricorythodes, Stenonema, and Heptagenia flavescens; the 

stoneflies Acroneuria and Pteronarcys; the caddisfly 

Pycnopsyche; the true fly Atherix variegata; and the 

chironomid Nanocladius. 

Taxa with significant correlations to one or more 

forms of organic carbon, nitrogen, and phosphorus and 

whose tolerance values probably should be higher for 

streams in eastern Iowa include the mayflies Pseudocloeon and Isonychia; and the caddisflies Hydropsyche 

bidens, Brachycentrus numerosus, Nectopsyche 

candida, and N. diarina. Taxa with significant correlations to one or more of the variables and whose tolerance values probably should be lower for streams in 

eastern Iowa include the Chironomidae Cricotopus/

Orthocladius and Thienemanniella, and the mayfly 

Acentrella (table 10).

SUMMARY 

Over 250 benthic invertebrate taxa were identified from snags and woody debris in streams and rivers 

of the Wapsipinicon, Cedar, Iowa, and Skunk River 

Basins in the Eastern Iowa Basins (EIWA) study unit 

of the U.S. Geological Survey National Water-Quality 

Assessment Program. The composition, distribution, 

and abundance of 74 predominant taxa were related to 

environmental conditions in the study unit, using 

habitat, hydrologic, and water-quality data. Four 

groups of sites were defined, based on the distribution 

and relative abundance of taxa. Detrended correspondence analysis was used to identify relations in the 

structure of the invertebrate assemblages, and the 

correspondence of taxa and sites in the groups was 

related to habitat, hydrologic, and water-quality information. Responses of invertebrate assemblages were 

explained by natural factors, such as surficial geology 

or physical habitat conditions, as well as human influences, such as agriculture or high-density hog-feeding 

operations.

Mayflies, caddisflies, and true flies were well 

represented in streams and rivers of the EIWA study 

unit. The mayflies Tricorythodes and Baetis intercalaris, the net-spinning caddisflies Hydropsyche bidens

and H. simulans, and the Chironomidae Glyptotendipes, Polypedilum, and Rheotanytarsus predominated. Spatial variation in benthic invertebrate 

assemblages within a site was less than that observed 

among sites. Assemblages from 3 years of sampling 

generally grouped by site, with exceptions related to 

differences in discharge among years.

The benthic invertebrate assemblages associated 

with the four groups of sites reflected the cumulative 

effects of agricultural and urban land use, sources of 

nutrient and organic enrichment, and longitudinal 

stream succession—the natural sequence of communities in streams from headwaters to large rivers. These 

factors, especially the natural changes from upstream 
REFERENCES CITED 35

to downstream, were influential in characterizing the 

benthic invertebrate assemblages of the site groups. 

Stream size, a reflection of basin area, was a 

principal influence in categorizing the benthic invertebrate assemblages, with sites that have the largest 

basin areas forming a separate group. Although it is 

difficult to distinguish among the contributions of 

large basin area, increased concentrations of nutrients 

and pesticides, and decreasing instream habitat diversity, the resulting invertebrate assemblage described 

was distinct. Of 74 taxa considered in this study, 33 

percent was not present at these larger sites, another 20 

percent was found at the lowest relative abundance 

observed, and 15 percent reached the highest abundances recorded. Numerically important taxa at these 

larger stream sites included the mayfly Amercaenis 

ridens; the net-spinning caddisflies Hydropsyche 

bidens, H. orris, and Potamyia flava; and the Chironomidae Polypedilum and Rheotanytarsus. Other 

attributes of the invertebrate assemblage, such as 

differences in relative abundances by functional 

feeding group or taxa richness, also were distinct for 

this group of larger sites and reflected natural changes 

in population with longitudinal stream succession.

The remaining sites were headwater or tributary 

streams. They reflected conditions found more 

commonly in smaller streams, such as higher gradients 

and the potential for more diverse or extensive riparian 

habitat, but were distinguished by landform. Following 

basin area in importance, landform contributed to the 

differences observed among the benthic invertebrate 

communities at the remaining sites. Sites separated by 

the percentage of the basin area included each of four 

geological landforms, primarily the Iowan Surface. 

Sites with the larger basin areas encompassed two to 

four of the recognized landforms in the EIWA study 

unit whereas the remaining sites typically had basin 

areas occupying only one landform.

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Eastern Iowa Basins—Hydrologic and biologic data, 

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the lower Susquehanna River Basin, 1993–95: U.S. 

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Canton, S.P., and Chadwick, J.W., 1988, Variability in 

benthic invertebrate density estimates from stream 

samples: Journal of Freshwater Ecology, v. 4, p. 291–

297.

Capel, P.D., and Larson, S.J., 1996, Evaluation of selected 

information on splitting devices for water samples: 

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Cuffney, T.F., Gurtz, M.E., and Meador, M.R., 1993, 

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SUPPLEMENTAL DATA
S

UPPLEMENTALDATA41

Table 11. Percentages of land use/land cover and landform at 31 stream-site basins in the Eastern Iowa Basins, Iowa and Minnesota [%, percent; human population data, see Sorenson and others, 1999; no./km2, number per square kilometer; –, not available]  Site U.S. Geological Human Land use/land cover (% of total basin area) Landform (% of total basin area)  number Survey station population Agri- Barren Grass- Row Urban Wetland Wooded Des Moines Iowan Iowan Southern  (fig. 1) identification density, 1990 culture land crop Lobe Surface Karst Iowa Drift  (no./km2) Plain  1 05464020 24.9 91 0.1 5.9 85 3 2 4 28.2 32.1 39.7 0  2 05453100 13.3 91 .1 9.9 81 3 2 4 44.5 18.4 0 37.2  3 05422000 13.2 87 0 10 78 2 3 7 0 84.3 0 15.7  4 05465000 28.2 89 .1 8.9 82 3 2 4 19.0 43.9 26.7 10.4  5 05465500 24.6 89 .1 10.9 79 3 2 5 21.8 32.4 16.6 29.2  6 05474000 18.5 87 0 15 72 3 2 8 25.3 0 0 74.7  7 05420680 9.5 89 0 8 81 2 4 5 0 100 0 0 

 8 05420720 9.0 90 0 7 84 1 3 5 – – – –  9 05420900 12.8 89 .1 5.9 84 2 1 7 – – – –  10 05421700 10.6 90 0 10 80 1 2 7 – – – –  11 05421870 16.5 95 0 9 86 2 0 3 – – – –

 12 05456510 11.0 91 0 6 87 2 2 3 – – – –

 13 05457950 6.1 93 0 6 88 1 2 3 – – – –  14 05458870 5.2 95 0 5 90 1 1 3 – – – –  15 05459300 11.5 91 0 10 82 2 3 3 – – – – 

 16 05461390 6.9 95 0 3 92 1 1 3 0 0 100 0 

 17 05462770 7.3 95 0 5 91 1 1 2 – – – –  18 05463510 13.6 95 0 4 92 2 1 1 – – – – 

 19 05464220 9.2 96 0 5 90 2 1 2 0 100 0 0 

 20 05449200 11.7 95 .1 1.9 93 2 1 2 – – – – 

 21 05449500 10.7 95 .1 3.9 90 2 2 2 100 0 0 0  22 05451210 4.0 95 0 4 91 1 1 3 100 0 0 0 

 23 05452020 7.4 93 0 15 78 2 1 4 – – – – 

 24 05455100 12.9 92 0 23 69 3 1 4 0 0 0 100 

 25 05455500 7.6 91 0 23 68 3 1 5 – – – –  26 05465310 12.2 91 0 12 79 2 1 6 – – – –  27 05469980 11.3 94 0 4 90 3 1 2 – – – –  28 05471120 11.1 94 0 5 89 2 1 3 – – – –  29 05473060 12.8 90 0 11 80 2 1 6 – – – –  30 05473400 14.1 85 0 17 69 2 2 10 – – – –  31 05473550 28.8 87 0 15 73 3 1 8 – – – – 
42 Benthic Invertebrate Assemblages and Their Relation to Physical and Chemical Characteristics of Streams in the Eastern Iowa 

Basins, 1996–98

Table 12. Pesticide and fertilizer applications in stream-site basins, and physical variables at 12 basic fixed sites in the Eastern Iowa Basins, Iowa and Minnesota [x 106, multiply by one million; oC, degrees Celsius; kg, kilograms; μS/cm, microsiemens per centimeter at 25 degrees Celsius; mg/L, milligrams per liter; –, not available]  Site U.S. Geological Reach Chemicals applied in basin (kg x 106) Nutrients from manure Nitrogen pH Specific Dissolved Water 

 number Survey station Pesticides Nitrogen Phosphorus applied in basin (kg x 106

) from conduc- oxygen tem- 

 (fig. 1) identification Nitrogen Phosphorus atmosphere tance (mg/L) perature in basin (μS/cm) (oC) 

 (kg x 106) 

1996 

 1 05464020 main 2.05 87.6 11.8 21.9 9.66 7.17 8.47 416 13.1 25.1  2 05453100 main 1.02 43.4 5.85 16.7 7.86 4.33 8.24 450 10.1 23.0  3 05422000 main .859 37.5 4.99 12.8 5.27 3.69 8.48 334 10.9 24.3  4 05465000 main 2.93 125 16.8 32.6 14.2 11.2 9.12 423 12.6 24.8  5 05465500 main 4.48 191 25.8 59.6 26.5 18.4 8.44 455 7.4 25.0  6 05474000 main 1.29 54.9 7.42 21.8 9.78 6.82 7.22 577 11.7 24.8  7 05420680 main .128 5.48 .737 1.94 .798 .499 7.60 422 6.7 21.0  16 05461390 main .048 2.04 .276 .426 .194 .169 7.49 446 8.4 17.0  19 05464220 main .105 4.49 .603 1.11 .458 .445 8.10 575 8.9 24.0  21 05449500 main .173 7.32 .991 2.13 1.04 .599 7.81 720 8.2 22.6  22 05451210 main .094 4.02 .542 2.38 1.2 .34 8.08 675 9.0 26.0  24 05455100 main .056 2.43 .324 .959 .393 .315 8.03 504 8.1 21.4 

1997 Multiple reach 

 5 05465500 upper – 169 .607 72.5 31.7 17.9 8.36 548 10.6 27.9  5 05465500 main – 169 .607 72.5 31.7 17.9 8.35 565 8.6 29.3  5 05465500 lower – 169 .607 72.5 31.7 17.9 8.26 557 10.3 26.5  19 05464220 upper – 3.94 26.0 1.53 .637 .417 7.74 640 8.2 21.5  19 05464220 main – 3.94 26.0 1.53 .637 .417 7.97 598 8.2 25.4  19 05464220 lower – 3.94 26.0 1.53 .637 .417 8.02 604 8.4 22.3  21 05449500 upper – 6.68 1.02 2.03 1.06 .554 8.02 576 10.4 22.3  21 05449500 main – 6.68 1.02 2.03 1.06 .554 8.21 583 12.9 25.2  21 05449500 lower – 6.68 1.02 2.03 1.06 .554 7.74 575 8.6 19.8 

1998 

 5 05465500 main – – – – – – 7.87 539 7.7 25.5  19 05464220 main – – – – – – 7.78 599 8.2 21.5  21 05449500 main – – – – – – 7.71 709 7.8 20.9 
SUPPLEMENTAL DATA 43

Table 13. Streambank characteristics and canopy shading at 12 basic fixed sites in the Eastern Iowa Basins, Iowa and Minnesota [m, meters; %, percent; mean, average of measurements taken along a transect; max., maximum]  Site U.S. Geological Reach Bank width (m) Bank height (m) Bank vegetation Canopy shading (%) number Survey station mean max. mean max. stability mean max. 

 (fig. 1) identification mean max. 

1996 

 1 05464020 main 6.4 13.7 4.0 6.0 1.7 3 14.5 24.4  2 05453100 main 3.0 7.7 2.8 4.2 2.1 4 33.3 46.1  3 05422000 main 5.9 37.3 1.1 1.5 1.4 3 33.4 46.1  4 05465000 main 3.7 5.3 2.3 2.7 1.6 3 12.7 18.9  5 05465500 main 5.6 11.4 3.7 5.5 1.5 3 9.3 11.1  6 05474000 main 9.3 12.2 5.1 5.4 2.3 4 26.0 34.4  7 05420680 main 2.6 4.9 1.2 1.4 2.0 4 73.4 100  16 05461390 main 5.1 16.3 1.6 2.8 2.9 4 85.7 100  19 05464220 main 4.2 7.4 1.9 2.7 2.8 4 25.6 38.3  21 05449500 main 2.7 5.5 1.2 2.0 1.2 2 65.5 77.2  22 05451210 main 4.0 10.5 1.6 2.3 2.6 4 48.1 62.8  24 05455100 main 6.6 9.9 6.2 7.5 2.8 4 65.2 77.8 

1997 Multiple reach 

 5 05465500 upper 3.3 8.1 2.5 3.3 1.3 2 9.7 12.8  5 05465500 main 31.9 125 2.3 3.3 1.7 3 9.1 12.8  5 05465500 lower 15.1 52.4 2.2 3.1 1.2 2 12.8 18.9  19 05464220 upper 7.9 15.0 2.0 2.7 2.5 4 19.5 33.3  19 05464220 main 2.6 5.5 1.7 2.4 3.4 4 19.4 34.4  19 05464220 lower 4.4 7.6 2.2 3.3 3.3 4 71.3 100  21 05449500 upper 2.8 5.4 1.4 2.0 1.4 2 67.2 100  21 05449500 main 3.3 5.1 1.6 2.4 1.1 2 56.1 73.3  21 05449500 lower 5.9 25.4 1.4 2.0 3.7 4 15.1 19.4 

1998 

 5 05465500 main 3.0 11.0 1.4 2.3 1.0 1 9.6 12.8  19 05464220 main 2.3 9.1 1.8 2.6 2.8 4 25.9 30.6  21 05449500 main 3.4 11.6 1.4 3.4 1.0 1 63.2 78.9 
44 Benthic Invertebrate Assemblages and Their Relation to Physical and Chemical Characteristics of Streams in the Eastern Iowa 

Basins, 1996–98 Table 14. Streambed materials and stream characteristics at 12 basic fixed sites in the Eastern Iowa Basins, Iowa and Minnesota [%, percent; m/s, meters per second; m/km, meters per kilometer; max., maximum; –, not available] 

 Site U.S. Geological Reach Bottom material, 1995 (%) Stream width, Stream depth (m) Stream velocity (m/s) Sinu- Stream

 number Survey station clay silt sand wetted channel (m) mean max. mean max. osity gradient

 (fig. 1) identification mean max. (m/km)

1996 

 1 05464020 main 18 23 59 104 114 .94 1.82 0.46 0.57 1.06 0.36 2 05453100 main 21 42 37 101 134 1.39 2.37 .53 .60 1.41 .30 3 05422000 main 28 57 15 97.5 125 .95 2.40 .44 .63 1.61 .47 4 05465000 main 9 20 71 133 200 .56 1.39 .50 .84 1.25 .32 5 05465500 main 21 74 5 214 262 1.43 3.23 .34 .61 1.23 .27 6 05474000 main 16 34 50 88.4 99.4 .74 1.39 .36 .46 1.14 .15 7 05420680 main 21 16 63 15.5 19.2 .40 1.05 .28 .42 1.26 .35 16 05461390 main 11 14 75 8.3 9.1 .15 .36 .23 .30 1.08 .78 19 05464220 main 16 45 39 29.5 34.2 .30 .65 .28 .50 1.31 .59 21 05449500 main 30 45 25 30 42.7 .59 .86 .24 .42 1.25 .16 22 05451210 main 2 7 91 14.7 21 .37 .74 .34 .60 1.21 1.2  24 05455100 main 14 41 45 16.3 19.8 .21 .59 .34 .49 1.26 .57

1997 Multiple reach 

 5 05465500 upper – – – 229 232 4.66 3.66 .74 1.07 1.23 .27 5 05465500 main 21 74 5 214 262 1.78 3.63 .59 1.40 1.23 .27 5 05465500 lower – – – 129 179 2.29 5.61 .63 1.11 1.23 .27 19 05464220 upper – – – 25.1 31.0 .35 .73 .35 .64 1.31 .59 19 05464220 main 16 45 39 30.4 33.0 .33 .64 .35 .48 1.31 .59 19 05464220 lower – – – 19.2 27.5 .46 .76 .36 .63 1.31 .59 21 05449500 upper – – – 26.9 36.6 .47 .82 .18 .23 1.25 .16 21 05449500 main 30 45 25 26.8 33.0 .50 .75 .19 .24 1.25 .16 21 05449500 lower – – – 26.8 32.0 .38 .88 .22 1.30 1.25 .16

1998 

 5 05465500 main 21 74 5 225 265 2.23 3.30 .81 1.16 1.23 .27 19 05464220 main 16 45 39 29.7 35.0 .40 .70 .19 .25 1.31 .59 21 05449500 main 30 45 25 28.7 34.7 .72 1.04 .30 .47 1.25 .16