zooplankton of turbid and hydrologically dynamic prairie ... · pdf filejames h. thorp* and...

Zooplankton of turbid and hydrologically dynamicprairie rivers

JAMES H. THORP* AND SARA MANTOVANI †

*Kansas Biological Survey and Department of Ecology and Evolutionary Biology, University of Kansas, Lawrence, KS, U.S.A.†Department of Biology, University of Ferrara, Ferrara, Italy

SUMMARY

1. Compared with rivers in more humid, forested ecoregions of eastern and midwestern

U.S.A., rivers in semi-arid grassland of the U.S. Great Plains tend to be relatively shallow,

more variable in discharge, and characterised by high suspended sediment loads.

Although critical life stages of fish in prairie rivers probably depend at least partially on

zooplanktonic food, data on community and distributional patterns of potamoplankton in

these widespread ecosystems are almost entirely absent.

2. We examined summer zooplankton distribution in five prairie rivers (Arkansas,

Kansas, Platte, Elkhorn, and Niobrara Rivers) spread over six degrees of latitude during

2003–2004. We compared our results from 126 samples with previously collected data from

the Ohio and St Lawrence Rivers in forested ecoregions and correlated differences with

abiotic environmental conditions.

3. The importance of hydrological retention zones to stream biota has been recently

demonstrated for rivers with quasi-permanent islands and slackwater regions, but the

importance of slackwaters formed by ephemeral sandbar islands in prairie rivers is

unknown. We evaluated the role of hydrological retention for planktonic rotifers,

cladocera, and copepods in the Kansas River during the summer of 2004.

4. Zooplankton assemblages were extremely similar among prairie rivers (Sorensen

Dissimilarity Index: mean ¼ 0.07) but moderately disparate for comparisons of prairie

versus forested-basin rivers (mean ¼ 0.50).

5. Total zooplankton densities in prairie rivers (approximately 81 L)1) were intermediate

between the Ohio (approximately 92 L)1) and St Lawrence Rivers (approximately 43 L)1),

but relative abundances were significantly different. Rotifers represented >99% of

zooplankton individuals in grassland rivers, but only approximately 37–68% in other

rivers. Rotifer species richness was lower in prairie rivers, but relative abundances of

common genera were much less skewed compared with eastern rivers where Polyarthra

dominated rotifer assemblages (41–73%).

6. For comparisons among rivers, rotifers were significantly more abundant in turbid

rivers, while microcrustaceans were less dense. However, for comparisons within the

Kansas River over time, rotifer densities were inversely related to turbidity. We

hypothesise that rotifers indirectly benefit from river turbidity because their food

competitors (cladocera) and predators (e.g. cyclopoid copepods and visually feeding fish)

are relatively more susceptible to suspended sediments.

7. Crustacean densities were positively related to the degree of hydrological retention

(negatively to current velocities) throughout the study, but rotifer densities were

significantly depressed by current velocities only when river discharge was high, making

Correspondence: James H. Thorp, Kansas Biological Survey, Higuchi Hall, University of Kansas, 2101 Constant Ave., Lawrence,

KS 66047-3759 U.S.A. E-mail: [email protected]

Freshwater Biology (2005) 50, 1474–1491 doi:10.1111/j.1365-2427.2005.01422.x

1474 � 2005 Blackwell Publishing Ltd

slackwaters that much more valuable. Ephemeral sandbars may not provide sufficient

hydrological retention in time and space to sustain viable crustacean populations, but they

are adequate to help sustain growth of rotifer populations.

Keywords: Great Plains, hydrologic retention, Kansas River, microcrustaceans, rotifers

Introduction

Rivers in the semi-arid Great Plains of North America

are often physically rigorous and ecologically

demanding habitats for planktonic and benthic organ-

isms because of unstable sand substrates and high

suspended sediment loads. Their hydrographs are

relatively dynamic and largely controlled by highly

variable, thunderstorm-precipitation events (cf.

Dodds et al., 2004). Ecological research on North

American prairie rivers is rare, despite their distribu-

tion in an area constituting roughly one-third of the

conterminous U.S.A. Moreover, scientific knowledge

of potamoplankton in these rivers is almost non-

existent, although studies of plankton in prairie

reservoirs are not uncommon. Indeed, a computer

literature search revealed no ecological or systematics

publication on zooplankton in prairie or grassland

rivers in the last two decades.

Zooplankton are critical links in riverine food webs

between phytoplankton and fish (Jack & Thorp, 2002;

Thorp & Casper, 2003). In prairie rivers they are

probably now a primary food source for larval and

some adult fish and were undoubtedly important long

before humans began building reservoirs. For exam-

ple, adults of the large and ancient paddlefish

Polyodon spathula (Walbaum), which have attained a

body weight of at least 36.7 kg in grassland rivers of

Kansas, feed almost exclusively on zooplankton

(Cross & Collins, 1995). Consequently, determining

the factors controlling zooplankton density, diversity,

and distribution should be an important step toward

understanding the ecology of prairie rivers.

Riverine zooplankton are controlled by a poorly

understood mixture of abiotic and biotic factors

varying seasonally among and within rivers (Thorp

& Casper, 2002). Abiotic factors include those influen-

cing the abundance and access to food, mechanics of

feeding, downstream transport versus temporary

retention, direct mortality (e.g. from ultraviolet radia-

tion), and thermal conditions. Biotic factors include

competition for food, parasitism, disease, and plank-

tivory by fish and both benthic and pelagic inverte-

brates.

The relative importance of various abiotic and biotic

factors to zooplankton assemblages are likely to vary

among species, seasons, and types of rivers. For

example, the substantial role of hydrological retention

for production of riverine zooplankton has been

demonstrated in large rivers (Thorp et al., 1994;

Schiemer et al., 2001; Hein et al., 2005; J.H. Thorp &

A.F. Casper, unpublished data for the St Lawrence

River). However, those studies have focused on rivers

with relatively stable hydrographs compared to those

in rivers of semi-arid ecoregions. Moreover, pre-

viously studied rivers usually featured relatively

permanent islands and slackwaters (¼shorelines,

embayments, and other areas outside the main chan-

nel where current velocities are substantially reduced,

e.g. below 0.1 m s)1). The scientific literature does not

indicate whether the more ephemeral slackwaters

associated with relatively unstable and ephemeral

sandbar islands in prairie rivers significantly affect

potamoplankton community diversity and produc-

tion.

In our potamoplanktonic study, we asked three

primary questions. First, do assemblages of rotifers,

copepods, and cladocera differ substantially among

Great Plains rivers and between those prairie streams

and rivers with forested watersheds in the midwes-

tern and north-eastern U.S.A.? In an initial step

examining this question, we compared potamoplank-

ton among: (i) five turbid, medium to large grassland

rivers (the Elkhorn, Niobrara, Arkansas, Platte, and

Kansas Rivers) whose hydrographs range from low to

high variability; (ii) the very large, constricted Ohio

River, which carries less sediment and has a moder-

ately variable hydrograph; and (iii) the very large

St Lawrence River, which is exceptionally clear and

hydrologically stable because of its origin primarily in

the Great Lakes (Thorp, Lamberti & Casper, 2005a).

Second, are differences among rivers in zooplankton

assemblages correlated with hydrological character-

istics, turbidity, water temperatures, or some other

Zooplankton in prairie rivers 1475

� 2005 Blackwell Publishing Ltd, Freshwater Biology, 50, 1474–1491

abiotic factor? Third, what is the importance of

hydrological retention for zooplankton in prairie

rivers with ephemeral sandbars? To answer this last

question, we sampled habitats with different current

velocities in the Kansas River.

Methods

Sample sites and habitat analyses

We sampled zooplankton from five prairie rivers in the

summers of 2003 and 2004 (Fig. 1a) and compared our

results with data collected in the summers of 1992–2000

from the Ohio and St Lawrence Rivers by JHT and other

researchers (especially additional Ohio River data from

Dr Debbie Guelda at Bemidji State University, Bemidji,

MN, U.S.A.). The last two rivers were selected in part

because zooplankton have been studied more exten-

sively in these rivers than in other U.S.A. and Canadian

rivers. We also collected miscellaneous environmental

data for all prairie rivers and current velocity by habitat

for the Kansas River during 2004. Finally, we analysed

rivers for discharge and turbidity using gauging data

from the US Geological Survey (USGS). Turbidity was

measured in nephelometric units (NTUs) because

USGS data bases were more extensive for NTUs than

for suspended sediment (mg L)1). The weakness of this

approach is that suspended sediments are not the only

factor contributing to an NTU level. Some pertinent

differences among these seven rivers are shown in

Table 1, and the distribution of rivers are plotted

(Fig. 2a) using a principal component analysis (PCA;

Clarke, 1993) of turbidity, mean discharge, and dis-

charge variability. This PCA plot is useful later in the

Results for interpreting differences in zooplankton

assemblages among rivers.

Zooplankton from prairie rivers were collected

from a single reach in the Arkansas, Elkhorn, Nio-

brara, and Platte Rivers and from multiple reaches in

the Kansas River (Fig. 1a). Samples were collected

in 2003–2004 from a single habitat type (near shore in

slackwater sites) from all five prairie rivers as part of a

larger watershed-river study sponsored by the US

Environmental Protection Agency (EPA); these are

referred to as EPA data in the Results. In addition,

zooplankton were more intensively sampled from

multiple habitats in several reaches of the Kansas

River (Fig. 1b) during July to September 2004. These

latter habitats varied substantially in current velocity

(all £ 0.51 m s)1, with mean ¼ 0.14 m s)1), tempera-

ture, and other environmental parameters. To be

conservative, we used the EPA samples of the Kansas

River when comparing among all seven rivers and the

non-EPA samples from the Kansas River in 2004 for

analyses of possible effects of hydrological retention.

All five prairie rivers are relatively wide for their

discharge, shallow (often <2 m deep), and turbid

compared with a typical river in a forested ecoregion.

The Platte, Kansas, and Elkhorn are in a high-

turbidity cluster (Table 1), whereas the Niobrara and

Arkansas carry only about half as much suspended

sediment (correlated with turbidity). However, the

mean of the latter group was still twice as high as the

average for the Ohio River and nearly 60 times higher

than the average turbidity of the St Lawrence River,

which is the clearest of the top 10 large rivers of the

world (Gleick, 1993). This permitted us to analyse

turbidity effects using average values for turbidity

rather than relying on highly variable, daily values

during sample dates.

Zooplankton are directly influenced by current

velocity and turbulence and only indirectly by river

discharge, but one can gain a perspective on differ-

ences among rivers in overall hydrological conditions

by examining discharge patterns. The St Lawrence

and Ohio represent two of the four largest rivers in

North America in mean discharge, and both are at

least 15 times larger than the biggest prairie river we

studied (the Kansas). Sampling from prairie rivers in

2003 and early summer of 2004 was during a severe,

multi-year drought, but the more intensive sampling

of the Kansas River associated with habitat analysis

occurred mostly in postdrought conditions (Fig. 3).

Indeed, the summer of 2004 was the third rainiest in

the lower basin of the Kansas River since 1939.

Consequently stream discharges were much higher

in 2004 than in either 2003 or the historical averages

(Fig. 3). Habitat conditions, such as the presence and

nature of sandbars, alter as a result of changes in river

stage and current velocity.

River flows are relatively dynamic for these prairie

rivers in both drought and flood years compared with

similar-order rivers from the more humid eastern

U.S.A. This results because prairie rivers drain semi-

arid watersheds and are largely fed by surface runoff

following thunderstorms (Dodds et al., 2004) and some

groundwater, with the proportions varying among

seasons and rivers. Our seven rivers fit within four

1476 J.H. Thorp and S. Mantovani


43°0'0"N

37°0'0N"N

102°0'0"W

95°0'0"W

MississippiRiver

MissouriRiver

OhioRiver

ArkansasRiver

Kansas River

PlatteRiver

Niobrara River Elkhorn River

*

**

**

*

(a)

(b)

Slack waters0 50 100 m

Temporarysand bar island

Agricultural field Riparian zone

Fig. 1 (a) General 2003–2004 sample locations (asterisks) for the Arkansas River, Kansas, Platte, Elkhorn, and Niobrara Rivers in the

central US Great Plains. Also shown are sections of the Mississippi and Missouri Rivers and mouth of the Ohio River (data from Ohio

were taken approximately 1000 km upstream); (b) Remote sensing photograph of the Kansas River illustrating some of the many

ephemeral hydrological retention areas (slackwaters) formed by sandbar islands which appear and disappear with floods as the river

flows from left to right. The photograph is courtesy of Michael E. Houts (Kansas Applied Remote Sensing programme at the University of

Kansas). It was taken with a Duncan Tech MS 3100 multispectral camera from a fixed wing aircraft flying at approximately 3000 m altitude

originally using near infrared, red, and blue bands; the pixel size was approximately 1 m.



arbitrary categories of discharge variability, as deter-

mined by the coefficient of variation (standard devi-

ation/mean) (Table 1): high (Arkansas, Kansas, and

Elkhorn), medium (Platte and Ohio), low (Niobrara),

and exceedingly low (St Lawrence). The Niobrara has

unusually low discharge variability for a prairie river

because groundwater constitutes a significant source of

its river water (Dodds, 2002). The St Lawrence may

have one of the least variable discharge patterns (0.15;

Table 1) of the large rivers of the world because >95%

of its discharge at a point several hundred kilometres

from its origin (where most of our zooplankton data

were obtained) (Thorp et al., 2005a) is derived from the

Laurentian Great Lakes.

The abundance of hydrological retention areas –

where current velocities and sometimes turbulence are

low – varies among these seven rivers. Slackwaters in

our prairie rivers are primarily formed by ephemeral

sandbars and a few more permanent forested islands.

Semi-permanent islands with trees are relatively rare

because sandbar islands often arise and disappear

periodically with floods. Consequently, slackwater

habitats formed next to these islands are much more

ephemeral than in rivers with more stable banks and

river beds. The predominant substrate in these prairie

rivers is coarse to fine sand and silt, with some pea-

sized gravel in faster flowing habitats and more silt in

low velocity habitats. While stone substrates are rare

except for some small gravel bars, the shallow nature

of these rivers and their presence in a generally windy

climatic zone increases turbulence over the relatively

homogenous sand substrates. In contrast, the Ohio

and St Lawrence Rivers contain a few to very many

relatively permanent, wooded islands; substrates are

typically much coarser on average. The mostly con-

stricted-channel Ohio River now has few islands and

embayments and is deeper than prairie rivers. The

upper two-thirds of the river (including areas sampled

for zooplankton) has many shoreline boulders and

cobble which promote some turbulence. The St Law-

rence is replete with semi-permanent slackwater areas

formed by forested islands and embayments, and

submerged rocks are common near shore. In addition

to natural turbulence, the last two rivers are also

heavily navigated by ships (mostly St Lawrence),

barges, and pleasure craft whose wakes disturb the

water to some extent, especially near shore.

Our seven rivers vary in the number and types of

dams present on their main channels and tributaries.

High dams are absent or rare in the main channels of all

five prairie rivers, but all contain one or more reser-

voirs on tributaries. The Kansas basin has the highest

proportion of dammed tributaries. Most samples from

the Kansas River were collected at sites approximately

20–60 km below a high-dam reservoir (Perry Lake) on

the Delaware River tributary. However, we also

sampled zooplankton in the Kansas 5–10 km upstream

from its confluence with the Delaware to account for

any potentially significant reservoir effects. The main

channel of the Ohio River has many low-head naviga-

tion dams. Tributaries upstream of the sample sites on

the Ohio include some low and high dams. The St

Lawrence River was sampled in the main channel and

slackwaters between Lake Ontario and the first hydro-

electric dam (at Massena, NY and Cornwall, Ontario,

hundreds of kilometres downstream from the Great

Table 1 Selected environmental characteristics of rivers in our study. Data from the US Geological Survey, a book on North American

rivers (Benke & Cushing, 2005), and other sources. Information reflects environmental characteristics near sites where we sampled.

Discharge records span 66–102 years and represent means of daily discharge values. Turbidity records cover at least 18 years but

were less frequently taken each year.

River Biome

Freshwater

ecoregion

River order

(approximate)

Discharge

mean (cm)

Coefficient of

variation

Turbidity

(NTUs)

Rivers in forested watersheds

St Lawrence Temperate deciduous and

boreal forests

Lower Saint

Lawrence

8 7565.3 0.15 1.16

Ohio Eastern deciduous forests Teays-Old Ohio 9 3491.3 1.00 31.09

Rivers in grassland watersheds

Niobrara Temperate grasslands Middle Missouri 4 51.2 0.55 69.97

Elkhorn Temperate grasslands Middle Missouri 4 40.7 1.68 109.29

Platte Temperate grasslands Middle Missouri 5 168.4 0.95 155.25

Kansas Temperate grasslands Middle Missouri 7 221.9 1.69 125.90

Arkansas Temperate grasslands Southern plains 6 57.7 1.89 67.54



Lakes). The main channels of all seven rivers are

flowing ecosystems lacking both significant vertical

stratification and other conditions that characterise

rivers with reservoirs.

All prairie-river sample sites were in medium to

large rivers that drain tall to mostly mixed and short-

grass prairie ecoregions. The eastern portion of these

watersheds typically include extensive areas of row

crop agriculture and riparian forests, but their water is

also derived from the drier central to western regions

with cattle rangeland, row crops, and sparse riparian

forests. Direct or groundwater extraction of water for

this agriculture is a serious concern for most prairie

rivers. In contrast, the watersheds of the Ohio and St

Lawrence Rivers are naturally wooded (deciduous

and coniferous forests) but also contain row crop

agriculture [see environmental information on most of

these rivers in Benke & Cushing (2005)].

Collection and identification of zooplankton

Seventy-eight zooplankton samples from the shallow

Kansas River were collected in July to September 2004

from depths averaging approximately 32 cm using

multiple grab sampling with small buckets to obtain a

final volume of 21 L. A 1-L ‘rotifer sample’ was

immediately removed and filtered through a 20 lm

sieve, immersed in 95% ethyl alcohol (ETOH) for

approximately 30 s to kill rotifers quickly (preserving

body shape), and then preserved in 75% ETOH for

later identification. The remaining 20-L ‘microcrusta-

cean sample’ was filtered through a 106 lm sieve,

with the retained contents then preserved in 75%

ETOH. All rotifers and microcrustaceans (copepods

and cladocera only) in the 21-L sample were counted

and identified at least to genus using a Nikon TE 2000-

S inverted microscope for rotifers and a Nikon SMZ-

1500 stereomicroscope for microcrustaceans. Genera

were identified using taxonomic keys, illustrations,

and photographs in Stemberger (1979), Balcer, Korda

& Dodson (1984), and Thorp & Covich (2001).

Four to eight additional 20-L zooplankton samples

per river per year were collected in the summers of

2003–2004 from the Arkansas, Kansas, Platte, Elkhorn,

and Niobrara Rivers (total sample size ¼ 48).

Zooplankton were pumped (12-volt diaphragm

pump) from approximately 1 m depth into a 20-L

bucket and then poured through a 20 lm sieve. The

retained zooplankton were rapidly killed in 95%

ETOH before being preserved in 75% ETOH. Micro-

crustaceans were counted and identified from the

entire 20-L sample, but a subsample (approximately

1 L) was processed for rotifers.

Zooplankton were collected from the St Lawrence

Rivers using a high-speed diaphragm pump and

similar sieve sizes and from the Ohio River using

comparable sieves but either a manual (samples from

D. Guelda) or electric diaphragm pumps (samples

from JHT).

Statistical analyses

Communities of zooplankton in different rivers were

compared with techniques for multidimensional sca-

PC1 (77.9%)–2 –1 0 1 2 3 4

PC

2 (1

6.5%

)D

imen

sion

2

Dimension 1

–1.0

–0.5

0.0

0.5

1.0

1.5

Arkansas

Elkhorn

Kansas

Niobrara

Platte

Ohio

St. Lawrence

(a)

(b)

Principal component analysisof environmental conditions

–1.0 –0.5 0.0 0.5 1.0 1.5–1.0

–0.8

–0.6

–0.4

–0.2

0.0

0.2

0.4

0.6

0.8Multi-dimensional scaling analysisof rotifer diversity and density

Arkansas

Kansas

Niobrara

St. Lawrence

Elkhorn

PlatteOhio

Fig. 2 (a) Principal component analysis based on environmental

differences among seven rivers. Mean river discharge, discharge

variability (SD/mean), and turbidity contributed 0.617, )0.574,

and )0.538, respectively to the PC1 axis, and )0.133, 0.598, and

)0.791 to the PC2 axis. Ovals link prairie rivers (on left) or the

two large rivers from eastern deciduous forest regions; (b)

Multidimensional scaling analysis based on densities of rotifer

genera in the seven rivers; The very low stress value (<0.001)

indicates that the plot was highly representative of the data.



ling (MDS), PCA, a community similarity index, and

analysis of variance (ANOVAANOVA). Dissimilarities among

rivers in their zooplankton assemblage were quanti-

fied in part using the Sorensen index (DI):

DI ¼ 1 � ½2a=ðbþ cÞ�

where a and b equal the number of genera in river 1

and river 2, respectively, and c ¼ the number of genera

in all rivers (or separate groups of rivers); DI ¼ ranges

from 0 to 1 (i.e. minimum to maximum dissimilarity)

(Magurran, 1988). Community differences were also

analysed graphically with MDS techniques (Young &

Hamer, 1987) using densities of rotifer genera. ANOVAANOVA

tests were conducted on overall zooplankton densities,

species diversity/evenness (Shannon and Pielou

Indices), and abundance of the most abundant rotifer

species using transformed data [Log10 (x + 1)]. Results

were compared between rivers using post hoc Tukey

honestly significant difference (HSD) tests. We also

examined relationships between zooplankton densi-

ties and four environmental metrics (mean turbidity,

discharge, discharge variability, and current velocity)

using correlations (Pearson Index) and regressions

(examined with t-tests for significance).

Results

Zooplankton of rivers in grassland and forested

ecoregions

Principal component analysis of effects of turbidity,

mean discharge, and discharge variability demonstra-

ted that prairie rivers were in a group distinct from

Ohio and St Lawrence Rivers (Fig. 2a), and this pattern

was also apparent in their zooplankton assemblages.

An MDS plot of rotifer densities (based on the 11 most

common genera overall) clearly indicated that prairie

river zooplankton communities were different from

those in rivers of forested ecoregions (Fig. 2b). Ten

paired-river comparisons of zooplankton in the five

Great Plains rivers had an average Sorensen Index of

0.07 (range ¼ 0.03–0.11), indicating extremely high

similarity in their taxonomic composition. In contrast,

other paired comparisons showed much more dissimi-

lar communities for the Ohio versus St Lawrence (0.46),

Ohio versus prairie rivers as a group (0.51), St Lawrence

versus prairie rivers (0.49), and the two very large rivers

versus prairie rivers (0.42). These results also indicate

that zooplankton assemblages differed considerably

between the Ohio and St Lawrence Rivers, which is not

Fig. 3 May to September discharge (cm) patterns in the Kansas River during the study (2003–04) and averaged over a period of

85 years. Sample dates during our study are shown as asterisks.



surprising given their substantial differences in turbid-

ity, discharge variability, and watersheds (Table 1).

Decreasing water temperatures on a latitudinal basis

might also have caused zooplankton differences

between these two eastern rivers, but such biotic

differences were not evident along a similar gradient

for the prairie rivers.

Total zooplankton densities varied significantly

among rivers [ANOVAANOVA d.f. ¼ 6, 51, MS (error) ¼0.101, F ¼ 90 116.01, P < 0.01] (Fig. 4). Tukey HSD

tests demonstrated that densities of zooplankton in

the Ohio and St Lawrence Rivers were significantly

different (P < 0.05) from each other and from all

prairie rivers. Of 10 possible comparisons among

prairie rivers, however, only three were significantly

different, and two of these involved the more unique,

spring-fed Niobrara River (with the Kansas and Platte

Rivers). Mean zooplankton numbers for the Kansas

River were lowest among the seven rivers when using

2003–2004 EPA samples (Fig. 4), but were highest if

the larger data set from our 2004 hydrological retent-

ion study was used. There were no consistent patterns

in zooplankton densities to suggest that zooplankton

overall fared better or worse in prairie rivers com-

pared to eastern rivers.

Significant differences among rivers were also

evident for species diversity [Shannon Index: ANOVAANOVA

d.f. ¼ 6, 51, MS (error) ¼ 0.002, F ¼ 1021.51, P < 0.01]

and evenness [Pielou Index: MS (error) ¼ 0.001, F ¼262.95, P < 0.01; Fig. 4]. As with overall densities,

both species diversity and evenness indices were

significantly different (Tukey HSD tests, P < 0.05)

when comparing both the Ohio versus the St

Lawrence River and each of those rivers versus any

prairie river. In contrast, only one of 10 possible

comparisons among prairie rivers were different for

the Shannon Index (Niobrara versus Arkansas), again

demonstrating similarity among zooplankton assem-

blages of Great Plains rivers. However, half the

comparisons among various prairie rivers for even-

ness showed significant differences (two involving the

Niobrara).

The most dramatic difference between prairie rivers

and the eastern rivers concerned the dominance of

rotifers (Appendix). On average, 99% of the zooplank-

ton fauna (exclusive of protozoa) in our five prairie

rivers were rotifers, compared with 68% in the St

Lawrence River and only 36% in the Ohio River.

(Data were derived from studies using slightly

different collection techniques, but the mesh sizes

Rivers

Div

ersi

ty in

dice

s

0.0

0.5

1.0

1.5

2.0

2.5

Zoo

plan

kton

den

sity

(no

. L–1

)

0

20

40

60

80

100

120

140Shannon

PielouDensity

AR KA PL EL NI Prairie OH SL

Fig. 4 Differences among rivers in density (line), generic diversity (Shannon index, black bars), and evenness (Pielou index, gray

diagonal bars); horizontal bars are mean±1 SE. River abbreviations are AR, Arkansas; KA, Kansas; PL, Platte; EL, Elkhorn; NI,

Niobrara; Prairie, those five previous rivers; OH, Ohio; and SL, St Lawrence.



used for filtering water samples still collected roughly

the same percentage of rotifers.) The Ohio River was

much more balanced numerically among cladocera

(20%), copepods (44%, including nauplii), and roti-

fers, while the St Lawrence had more nearly equal

percentages of cladocera (14%) and copepods (18%).

Mean densities for all zooplankton (using EPA sam-

ples for all five prairie rivers) were greatest in the

Ohio River (approximately 93 animals L)1), medium

in the prairie rivers (approximately 81 L)1), and

lowest in the St Lawrence approximately 43 L)1),

despite the primary origin of the St. Lawrence in the

zooplankton-rich Great Lakes. However, such num-

bers will fluctuate over time, among habitats, and

with slightly different collection techniques. For

example, when we used samples from our 2004

hydrological retention study in the Kansas River as

a substitute for the 2003–2004 EPA samples from the

same river, the mean density for prairie rivers rose by

over 80% to 147 animals L)1. This dramatic rise was

caused primarily by the population explosion of one

genus (Proales), whose mean percentage of the total

rotifer density in Kansas River samples increased

from 9% to 51%.

More detailed taxonomic analyses also shows sig-

nificant differences among rivers in taxonomic com-

position. Crustaceans were very rare in channel and

slackwater (Fig. 1b) samples from prairie rivers but

were common in the Ohio and St. Lawrence Rivers

(Appendix). The primary cladoceran genera in the

Ohio River were Bosmina, Ceriodaphnia, and Diaphano-

soma. In the St. Lawrence, Bosmina and Daphnia were

the major cladocera throughout the international

section of the river on average; but after a hundred

river kilometres or so downstream from Lake Ontario,

Bosmina was the only common cladoceran. Calanoid

copepods were not common in any of the seven rivers,

but Eurytemora affinis – an estuarine invader – was the

overwhelming dominant calanoid in the Ohio and

St. Lawrence Rivers. Diacyclops was the most abun-

dant cyclopoid copepod in both eastern rivers; but

other genera, such as Mesocyclops, often were import-

ant in samples from experimental enclosure studies in

the Ohio and St Lawrence Rivers (Jack & Thorp, 2000,

2002; Thorp & Casper, 2002, 2003).

Significant differences (ANOVAANOVA; P < 0.01) existed

among the seven rivers in densities of 10 of the 11

most common rotifer genera, which together repre-

sented >90% of the rotifer densities. Moreover, the

11th genus, Keratella, was nearly different statistically

(P ¼ 0.062). The St Lawrence was strongly dominated

primarily by Polyarthra (approximately 73%) and

secondarily by Synchaeta (approximately 12%).

Polyarthra was also the dominant rotifer in the Ohio

River (41%), but three other genera represented at

least 9% of the rotifers (in order: Brachionus, Keratella,

and Synchaeta). In contrast, none of the rotifer dom-

inants of the Ohio and St Lawrence were important in

prairie rivers. About 78% of the zooplankton in these

Great Plains systems were composed of five genera

ranging in percentages from approximately 10–20%

(in order from most abundant: Monostyla, Notholca,

Gastropus, Tricocerca, and Proales). During our hydro-

logical retention study (discussed below), the domin-

ant rotifers in the Kansas River were Proales

(approximately 51%), Monostyla (approximately

19%), and Trichocerca (approximately 15%).

The role of water movement and turbidity among and

within rivers

We analysed relationships between zooplankton

densities and four environmental metrics for both

among- and within-river comparisons because these

potentially have different relationships to zooplank-

ton ecology. For comparisons among the seven rivers,

we evaluated effects on zooplankton from mean

turbidity, discharge, and discharge variability using

U.S.G.S. environmental data, our EPA zooplankton

data, and other data collected from the Ohio River

(data sets from Guelda, Thorp, and others) and the

St Lawrence River (by Thorp and Casper). For within-

river comparisons, however, we investigated the

relationships between zooplankton densities in the

Kansas River and both current velocity and turbidity

within specific habitats.

Relationships between zooplankton densities and

turbidity initially appeared inconsistent when making

among- and within-river comparisons, but these

differences may be explained by species adaptations

and/or biotic interactions (see Discussion). We found

a positive linear relationship between density of

rotifers and a river’s average turbidity (calculated

over many years) (Fig. 5a) and a negative regression

for crustaceans and turbidity (Fig. 5b); however, only

the first approached significance (d.f. ¼ 5, P < 0.10).

The linear regression R2-value for rotifers was mod-

erate (0.4965) when all seven rivers were considered,



but dropped precipitously when the less turbid Ohio

and very clear St Lawrence Rivers were eliminated

from the comparison (R2 ¼ 0.1632). For crustaceans,

the R2-value was lower (0.3037) for a comparison of all

seven rivers, partially because there were very few

copepods and cladocera in rivers with turbidities over

50 NTUs. Although rotifers were abundant and

crustaceans were rare in Great Plains rivers with their

high turbidities, we cannot determine whether this

was due directly to turbidity. In fact, when one

examines zooplankton densities in the Kansas River

(based on turbidity measurements at the time of

sampling), the opposite relationships seem apparent

(Fig. 5c–d). The linear regression for crustaceans

(Fig. 5d), while slightly positive and significant

(d.f. ¼ 20, P < 0.05), had a nearly zero slope and a

low R2-value (0.2193). In contrast, rotifer densities

declined exponentially with an increase in turbidity

(Fig. 5c, d.f. ¼ 20, P < 0.05), and this relationship had

a moderate R2-value (0.6205).

Rotifer densities tended to be greater in smaller

rivers (Fig. 6a), but crustaceans showed the opposite

trend (Fig. 6b). The negative regression of mean

discharge (averaged over many decades) and rotifer

density was only marginally insignificant (d.f. ¼ 5,

P < 0.10) with a moderately low R2-value (0.3362).

This marginal relationship disappeared, however,

when only prairie rivers were considered (R2 ¼0.0097). The positive regression between crustacean

abundance and mean discharge (d.f. ¼ 5, R2 ¼ 0.4980,

P < 0.05) should also be interpreted with caution

because the slope was highly influenced by the rarity

of crustaceans in prairie rivers. While rotifer densities

tended to rise and crustacean densities declined with

increasing discharge variability, both relationships

were characterised by non-significant correlations and

low regression R2-values (0.1662 and 0.0621, respect-

ively).

Effects of flow velocity in a given habitat of

the Kansas River on zooplankton densities during

0 20 40 60 80 100 120 140 160 0 20 40 60 80 100 120 140 160

Rot

ifer

den

sity

(no

. L–1

)

20

40

60

80

100

120

140

Rotifer densities in 7 rivers Linear regression

y = 0.4965x + 26.374

R2 = 0.4377

(a)

Cru

stac

ean

dens

ity

(no.

L–1

)

–10

0

10

20

30

40

50

y = –0.2219x + 28.377

R2 = 0.3037

(b)Crustancean densities in 7 Rivers

Linear regression

Turbidity (NTU)

Rot

ifer

den

sity

(no

. L–1

)

0 200 400 600 8000

200

400

600

800

1000

1200

1400

Exponential regression

y = 885.11e–0.0059x

R2 = 0.6205

(c)Rotifer densities in the Kansas river

Turbidity (NTU)

Cru

stac

ean

dens

ity

(no.

L–1

)

0 200 400 600 8000.0

0.1

0.2

0.3

0.4

0.5

0.6

y = 0003x + 0.0436 R2 = 0.2193

(d)Crustancean densities in the Kansas river

Linear regression

Fig. 5 Effects of water turbidity (NTUs) on rotifer (a) and crustacean (b) densities in seven rivers and on rotifer (c) and crustacean (d)

densities in the Kansas River. Crustaceans include copepods (nauplii, copepodids, and adults) and cladocera.



summer 2004 (Fig. 7a–b) often seemed influenced by

an interaction of immediate and recent flow charac-

teristics (Fig. 8). Crustacean densities were inversely

related to current velocities in a given habitat

throughout the summer (Fig. 7b; d.f. ¼ 23, R2 ¼0.6205, P < 0.05), and densities stayed relatively

constant and very low throughout this period. In

contrast, rotifer densities were not significantly

related to current velocities when data from the entire

summer sample period were analysed (d.f. ¼ 23,

P > 0.05), but their densities were an order of magni-

tude higher in August to September when discharge

was less. Effects of current velocity on rotifer num-

bers were only evident during the high discharge

month of July (Fig. 7a) when a significant negative

linear regression was present (d.f. ¼ 8, R2 ¼ 0.4725,

P < 0.05). Fluctuations in water temperatures during

the sample period were not correlated with changes in

zooplankton densities (Fig. 8).

Discussion

Research on freshwater zooplankton ecology has so

heavily emphasised lentic habitats that the ecology of

lotic zooplankton almost seems a footnote in a

literature review. Nonetheless, the few investigators

around the world who have studied the role of

potamoplankton have increasingly demonstrated the

importance of zooplankton not only to sustenance of

most larval fish and a few adult species (e.g. Meng &

Orsi, 1991) but also to biotic components of carbon

cycling within large riverine ecosystems in particular

(e.g. Gosselain et al., 1998; Pace, Findlay & Fischer,

1998; Gliwicz, 2002; Thorp & Delong, 2002).

Aside from natural and polluted systems with

extreme chemical or thermal conditions, the most

rigorous lotic habitats for plankton should be in rivers

with rapid downstream transport (i.e. minor hydro-

logical retention areas) and high turbulence, discharge

10 100 1000 10000

Rot

ifer

den

sity

(no

. L–1

)

0

20

40

60

80

100

120

140Rotifer densities in 7 rivers

Linear regression

(a)

y = –25.67x + 128.72

R2 = 0.3362

Discharge [log (10) cm]

10 100 1000 10000

Cru

stac

ean

dens

ity

(no.

L–1

)

0

10

20

30

40

50Crustacean densities in 7 riversLinear regression

(b)

y = 16.764x – 30.27

R2 = 0.498

Fig. 6 Effects of discharge (averaged over many decades; note

log scale) on densities of rotifers (a) and crustaceans (b) in seven

rivers.

0.0 0.1 0.2 0.3 0.4 0.5

Rot

ifer

den

sity

(no

. L–1

)

0

10

20

30

40

50Rotifer density in the Kansas river (July only) Linear regression

(a)

y = –55.016x + 34.19

R2 = 0.4725

Current velocity (ms–1)

0.0 0.1 0.2 0.3 0.4 0.5

Cru

stac

ean

dens

ity

(no.

L–1

)

0.0

0.1

0.2

0.3

0.4

0.5 Crustacean density in the Kansas river (July – Sept)

Linear regression

(b)

y = –0.36x + 0.19

R2 = 0.1778

Fig. 7 Effects of current velocity in the Kansas River on rotifers

in July (a) and crustaceans throughout the summer sampling

period of 2004 (b).



variability, and suspended sediment loads. Many of

these environmentally challenging criteria character-

ise rivers of the US Great Plains. As a whole they tend

to carry massive amounts of suspended sediment and

have highly variable discharge regimes in response to

grassland watersheds and thunderstorm precipitation

events, respectively. One might initially expect turbu-

lence to be low in prairie rivers because of the general

dearth of rocks and a typically low slope. However,

these are shallow systems in which the high winds

typical of the Great Plains contribute to the river’s

turbulence. The hydrologically related retentiveness

of organisms and nutrients in prairie rivers is less

clear. Compared with rivers of the same stream order

in more humid ecoregions, the mean discharge and

average current velocity of prairie rivers are, of

course, low. However, permanent slackwaters are

rare, although some prairie rivers are braided.

Instead, most slackwater habitats occur in association

with shorelines and ephemeral sandbar islands,

especially when the former contain substantial snags

from fallen riparian trees.

Knowing these conditions, we hypothesised that we

would find relatively few zooplankton in prairie

rivers. Of those that would likely occur, species with

small bodies and short generation times, especially

rotifers, should dominate. Because this environment

seemed rigorous, we also expected taxonomic

diversity of both microcrustaceans and rotifers to be

relatively low. As our results indicated, some predic-

tions proved correct but others were false.

Differences between zooplankton in rivers of grassland

and forested ecoregions

Compared with most other aquatic metazoa,

zooplankton genera are relatively cosmopolitan,

although apparently less so than once thought (Wal-

lace & Snell, 2001). This largely reflects the natural

phenomenon of resistant life stages dispersing great

distances on wind currents. In addition, the many

headwater lakes and artificial reservoirs throughout

the U.S.A. potentially provide a homogenising source

of lentic-selected zooplankton to rivers. Consequently,

one might expect potamoplankton assemblages in

widely separated rivers of the U.S.A. to contain

similar taxa unless local environmental factors –

rather than dispersal barriers – exert a dominant

control over species richness and density.

The zooplankton assemblages we sampled in the

US Great Plains proved to be remarkably similar

despite the fact that our five rivers are distributed

across six degrees of north temperate latitude and

two major river basins (one in the Arkansas and four

in the Missouri River basins). Our primary measure

of community uniformity (Sorensen’s Dissimilarity

Date in 2004

nJ nuJ nuJ nuJ nuJ lJ luJ luJ luJ guA guA guA guA guA eS peS

Dis

char

ge (

cms)

0

200

400

600

800

Tem

pera

ture

(°C

)

20

40

60

80

100

Cru

stac

ean

dens

ity

(no.

L–1

)

Rot

ifer

den

sity

(no

. L–1

)

0

2

4

6

8

10

12

14

0

200

400

600

800

1000

Discharge

Temperature

Rotifers

Crustaceans

June 1u July 1 Aug 1 Sept 1

Fig. 8 Variations during the summer of 2004 in river discharge (gray area), water temperature (bottom white area), rotifer density

(solid line and closed circles), and crustacean density (dashed line and open circles) in the Kansas River; note the four ordinate axes.



Index) demonstrated unusually high similarity in

species composition (mean ¼ 0.07 on a scale from 0

to 1, with low values equal to high similarity).

Moreover, few differences were evident in species

diversity (Shannon Index), although half the compar-

isons of evenness (Pielou Index) revealed significant

differences.

Our prediction of the dominance of small zooplank-

ton in these rivers was verified. Microcrustaceans

(copepods and a few cladocera) were present in all

prairie rivers, but each river was dominated very

strongly by rotifers (99% numerically).

Predictions of relatively low zooplankton density in

grassland rivers proved incorrect however. Overall

zooplankton densities were on par with those in the

Ohio and St Lawrence Rivers, exceeding them in some

cases but trailing in others. A next research step

would be to compare secondary production of

zooplankton between prairie and forested rivers.

The zooplankton production in grassland rivers is

based on many small zooplankton (rotifers) with short

generation times, while zooplankton communities of

rivers in forested ecoregions have greater numbers of

large zooplankton with concomitant longer turnover

times. Environmental differences in thermal regime,

relative food availability, and predation could also

alter production comparisons among river types.

While we found minor differences among prairie

rivers, major differences existed between grassland

rivers as a whole and the two forested-basin rivers.

The most stark differences between those seven rivers

concerned the relative abundances of microcrusta-

ceans and rotifers. Zooplankton assemblages in the

two eastern rivers differed considerably from each

other, but they contained approximately 20–80 times

more microcrustaceans per litre than did prairie

rivers. Moreover, the ratio of microcrustacean density

to rotifer density was approximately 32–64 times

higher in the Ohio and St Lawrence Rivers compared

with prairie rivers. Every prairie river we studied

contained higher absolute densities of rotifers than the

eastern rivers, and the average difference was 2.5

times greater. In fact, rotifers constituted on average

99.46% of the metazoan zooplankton in these five

rivers. While 19 rotifer genera were recorded for the

Ohio and St Lawrence Rivers combined and only 11

genera were identified from the five prairie rivers,

the taxonomic evenness of rotifers in grassland rivers

was much higher. The eastern forested rivers were

dominated by very few genera (mostly Polyarthra at

41–73%), while nearly 80% of the rotifers in prairie

rivers were in five genera of roughly equal abun-

dance. Rotifers are typically the most abundant

zooplankton in rivers (e.g. Pace, Findlay & Lints,

1992; Thorp et al., 1994; Kobayashi et al., 1996; Kim &

Joo, 2000), but the extreme dominance of rotifers, as

shown in prairie rivers, is unusual.

Environmental regulation of zooplankton in prairie

rivers

Recognising patterns of zooplankton density and

diversity is relatively easy, but identifying proximate

and ultimate causes of these patterns is a more

arduous task. While some measure of biotic control

of potamoplankton has been demonstrated in the

Ohio and St Lawrence Rivers (Jack & Thorp, 2000,

2002; Thorp & Casper, 2002, 2003), no studies of biotic

control have been published for prairie rivers. Data

from the present study suggests, however, that abiotic

factors may strongly influence zooplankton commu-

nities in prairie rivers.

Five abiotic environmental factors should be espe-

cially important to lotic zooplankton: (i) turbidity

(especially from suspended sediment); (ii) water

turbulence; (iii) hydrological retention, which is

influenced by stream discharge and access to shel-

tered, low velocity sites (slackwaters); (iv) thermal

conditions; and (v) ultraviolet radiation.

Turbidity. Our data seem initially to imply a complex

relationships between turbidity and densities of rot-

ifers and microcrustaceans, but we suspect that a

simpler interaction is merely being altered by con-

comitant changes in both competition and predation.

When we compared a river’s average turbidity over

many years with recent zooplankton sampling data,

we found rotifers fared better and microcrustaceans

did worse in turbid rivers, such as those in the US

Great Plains. This is consistent with laboratory find-

ings (e.g. Kirk & Gilbert, 1990) that suspended clay

reduces population growth rates of cladocera much

more than it affects rotifers. It is also consistent with

findings in various field studies around the world

(e.g. Shiel, 1985; Pace et al., 1992; Thorp et al., 1994). In

contrast, densities of rotifers in the turbid Kansas

River rose by an order of magnitude from July 2004,

when turbidity and discharge were high, during



August to September, when the opposite conditions

prevailed. The answer to this apparent conundrum

may relate to biotic interactions. We suggest that two

direct effects of suspended sediments are to reduce

both overall zooplankton densities and the number of

species that can successfully colonise turbid environ-

ments once they disperse to them. This could account

for, or contribute to, the low numbers of microcrus-

taceans in prairie rivers. However, two countervailing

indirect effects of suspended sediments for rotifers are

a reduction in densities of cladocera, which are often

superior food competitors (Kirk & Gilbert, 1990), and

a decrease in both visually hunting fish planktivores

(cf. McCabe & O’Brien, 1983; Cuker, 1993) and

predatory cyclopoid copepods. A decrease in cyclo-

poids was linked experimentally to increased densi-

ties of rotifers in the St Lawrence River (Thorp &

Casper, 2003). Hence, rotifers probably do better in

turbid rivers not because this environmental condi-

tion favours them but rather because pernicious

effects of competition and predation are partially

alleviated by high suspended sediment loads.

Hydrological retention. Species diversity and density

vary significantly with current velocity throughout

river networks in general and are positively correlated

with hydrological retention within the riverscape of

larger rivers, except where taxa are restricted by other

abiotic environmental conditions (e.g. oxygen, tem-

perature, substrate type) (Thorp et al., 2005b). It is not

surprising, therefore, that lotic ecologists are increas-

ingly identifying hydrological retention in slackwaters

and floodplain lakes as a major factor influencing

potamoplankton production and diversity as well as

other structural (Thorp et al., 1994; Basu & Pick, 1996;

Reckendorfer et al., 1999; Schiemer et al., 2001) and

functional characteristics of rivers (Hein et al., 2005).

Based on these observations, we examined whe-

ther hydrological retention in prairie rivers played a

significant role in regulating zooplankton assem-

blages. Baranyi et al. (2002) found that rotifers

dominated the zooplankton community of periodic-

ally isolated channels of the River Danube during

periods of low to medium water age (i.e. periods of

isolation from the main channel) but gave way to

microcrustaceans with increasing water age. Could

this be true in prairie rivers where hydrological

retention results more from ephemeral sandbar

islands (Fig. 1b) than from the relatively permanent

slackwaters formed by forested islands in most other

studies?

Perhaps the most complex task in understanding

this relationship is interpreting interactions between

zooplankton densities and current velocity because

responses are influenced by proximate current velo-

cities, average current velocities in the river, and

recent patterns of discharge in the river. Mean river

discharge by itself was not a good predictor of

zooplankton densities in our study, but this hydro-

logical parameter must impinge on zooplankton

through current velocity, water depth, and turbu-

lence. Crustacean densities in the Kansas River were

positively related to hydrological retention through-

out the summer sample period, but rotifer densities

were significantly depressed by current velocities

only during July, when mean river discharge was

high. We hypothesise that ephemeral sandbars do

not provide sufficient hydrological retention in time

and space to sustain many if any viable populations

of microcrustaceans but that they are adequate to

help sustain growth of rotifer populations. This is

consistent with conclusions of other scientists that

rotifers require shorter water retention times in rivers

for somatic and reproductive growth than do micro-

crustaceans (e.g. Pace et al., 1992; Kobayashi, 1997).

The relatively importance of low hydrological retent-

ion versus high turbidities in affecting cladocera and

copepods is not known. We also hypothesise that

hydrological retention is relatively important to

rotifers in prairie rivers only when mean current

velocities in the river are high, making slackwaters

that much more valuable.

Although rotifer densities may not be strongly and

directly influenced by hydrological retention in the

Kansas River, this contrasts with concurrent studies

on benthic invertebrates (J. Kreft, S. Moore & J. Thorp,

unpublished data) and larval fish (S. Moore &

J. Thorp, unpublished data) which reveal strong

positive effects from hydrological retention. We sug-

gest that a complex system of channels and slackwa-

ters is directly beneficial to varying degrees to most

aquatic organisms in all rivers, but individual taxa

may suffer indirectly from concomitant increases in

competition and predation in these habitats.

Other abiotic factors. The shallow nature of prairie

rivers and their minimal canopy cover can produce

high water temperatures. During July to September



habitat analysis in the Kansas River, the average water

temperature was 24.1 �C (range ¼ 16.4–31.3 �C); this

was close to a 30-year average for May to September of

24.5 �C (range ¼ 12–34 �C). It is difficult to obtain

comparable data from different rivers because sample

depth and period of record vary considerably (but see

data from USGS web sites and chapters in Benke &

Cushing, 2005). However, it appears that our five

prairie rivers have annual temperatures within about

1 �C of each other. The annual temperature of the much

deeper Ohio River seems to be no more than approxi-

mately 1–2 �C higher than our five prairie rivers; and in

shallow waters, its temperatures can occasionally

exceed 30 �C during the summer (Thorp, Alexander

& Cobbs, 2002). On the opposite extreme, the annual

temperature of the St Lawrence River is roughly 2 �Ccooler than our prairie rivers, and temperatures rarely

exceed 22 �C. The presence of microcrustaceans in the

slightly warmer Ohio River and slightly cooler St

Lawrence River and their occurrence in reservoirs

throughout the latitudinal range of the conterminous

U.S.A. suggest that water temperatures are not the

primary factor in prairie rivers contributing to low

numbers of microcrustaceans, especially cladocera.

However, warm temperatures in shallow habitats of

prairie rivers may enhance or diminish secondary

production of rotifers depending on the season.

Net primary productivity in rivers is limited in part

by the amount of time phytoplankton are swept below

the photic zone by the helical, downstream circulation

patterns of rivers and by the amount of bottom

surface area available for benthic microalgal produc-

tion. At least the former influences riverine zooplank-

ton production (Pace et al., 1992). High suspended

sediment loads decrease penetration of both photo-

synthetically active radiation (PAR) and ultraviolet

radiation (UVR), but the photons need only penetrate

a short distance to reach the bottom of prairie rivers

during much of the year. Rotifers, especially loricate

genera such as Keratella, are considered to be more

tolerant of UVR than microcrustaceans (Leech &

Williamson, 2000), but additional research is needed

to determine whether this factor significantly con-

tributes to the unusually high ratio of rotifers to

microcrustaceans in prairie rivers. Turbidity limita-

tions to total net autotrophic production and inter-

ference with feeding mechanisms in particular may

also contribute to the rarity of copepods and cladocera

in grassland rivers.

We lack sufficient information on wind-driven

turbulence in prairie rivers to begin evaluation of its

effects on zooplankton assemblages.

Biotic versus abiotic control of prairie river zooplankton

Although a widespread consensus exists that zoo-

plankton are regulated more by physical than biotic

factors (e.g. Hynes, 1970; Baranyi et al., 2002), this

conclusion is not founded on experimental evidence.

Although stochastic factors are probably more import-

ant overall and throughout the river network, signi-

ficant competitive and predator–prey interactions

involving zooplankton are likely to occur in lateral

slackwater habitats and in the main channel in areas

or times of minimal hydraulic stress (Thorp & Casper,

2003; Thorp et al., 2005b). In prairie rivers, the rarity of

competitive cladocera and predatory cyclopoid cope-

pods could contribute to the higher densities and

relative abundance of rotifers in comparison to rivers

we studied in eastern, forested ecoregions.

Acknowledgments

We appreciate the help of Stephanie Moore, Scott

Campbell, Leila Desotelle, and numerous undergrad-

uates (especially Jim Kreft, Uriah Price, and Jason

Robertson) in collecting zooplankton from five prairie

rivers in 2003–2004 and the help of Andy Casper in

collecting plankton from the St Lawrence and Ohio

Rivers. We are extremely grateful to Debbie Guelda

(with field assistance from Rick Koch and Tim Sellers)

for additional data on zooplankton from the Ohio

River, and we thank Edward Peters for environmental

data on the Platte River. Comments on an earlier draft

by Mike Delong, Jeff Jack, and two anonymous

reviewers improved the final manuscript. Collection

and processing of samples was supported both by a

S.T.A.R. grant to J. Thorp from the US Environmental

Protection Agency (RD-83059701) and by travel and

support for S. Mantovani from the Department of

Biology, University of Ferrara.

References

Balcer M.D., Korda N.L. & Dodson S.I. (1984) Zooplankton

of the Great Lakes: A guide to the identification and ecology

of the common crustacean species. University of Wiscon-

sin Press, Madison, WI, U.S.A.



Baranyi C., Hein T., Holarek C., Keckeis S. & Schiemer F.

(2002) Zooplankton biomass and community structure

in a Danube River floodplain system: effects of

hydrology. Freshwater Biology, 47, 473–482.

Basu B.K. & Pick F.R. (1996) Factors regulating phyto-

plankton and zooplankton biomass in temperate

rivers. Limnology and Oceanography, 41, 1572–1577.

Benke A.C. & Cushing C.E. (Eds) (2005) Rivers of North

America. Academic Press, San Diego, CA.

Clarke K.R. (1993) Non-parametric multivariate analysis

of changes in community structure. Australian Journal

of Ecology, 18, 117–143.

Cross F.B. & Collins J.T. (1995) Fishes in Kansas, 2nd edn.

University of Kansas Natural History Museum, Lawr-

ence, KS, U.S.A.

Cuker B.E. (1993) Suspended clays alter trophic interac-

tions in the plankton. Ecology, 74, 944–953.

Dodds W.K. (2002) Freshwater Ecology: Concepts and Envi-

ronmental Applications. Academic Press, San Diego, CA,

569 pp.

Dodds W.K., Gido K., Whiles M.R., Fritz K.M. &

Matthews M.J. (2004) Life on the edge: the ecology of

Great Plains prairie streams. BioScience, 54, 207–218.

Gleick P.H. (1993) Water in Crisis. A Guide to the World’s

FreshWaterResources. OxfordUniversity Press, New York.

Gliwicz Z.M. (2002) On the different nature of top-down

and bottom-up effects in pelagic food webs. Freshwater

Biology, 47, 2296–2312.

Gosselain V., Descy J.-P., Viroux L., Joaquim-Justo C.,

Hammer A., Metens A. & Schweitzer S. (1998) Grazing

by large river zooplankton? The case of the Meuse and

Moselle rivers in 1994 and 1995. Hydrobiologia, 369,

199–216.

Hein T., Reckendorfer W., Thorp J. & Schiemer F. (2005)

The role of slackwater areas and the hydrologic

exchange for biogeochemical processes in river corri-

dors: examples from the Austrian Danube. Archiv fur

Hydrobiologie, Suppl. 155 (Large Rivers 15), 425–442.

Hynes H.B.N. (1970) The Ecology of Running Waters.

University of Toronto Press, Toronto, 555 pp.

Jack J.D. & Thorp J.H. (2000) Effects of the benthic

suspension feeder Dreissena polymorpha on zooplank-

ton in a large river. Freshwater Biology, 44, 569–579.

Jack J.D. & Thorp J.H. (2002) Impacts of fish predation on

an Ohio River zooplankton community. Journal of

Plankton Research, 24, 119–127.

Kim H.-Y. & Joo G.-J. (2000) The longitudinal distribution

and community dynamics of zooplankton in a regu-

lated large river: a case study of the Nakdong River

(Korea). Hydrobiologia, 438, 171–184.

Kirk K.L. & Gilbert J.J. (1990) Suspended clay and the

population dynamics of planktonic rotifers and clado-

cerans. Ecology, 71, 1741–1755.

Kobayashi T. (1997) Associations between environmental

variables and zooplankton body masses in a regulated

Australian river. Marine and Freshwater Research, 48,

523–529.

Kobayashi T., Gibbs P., Dixon P.I. & Shiel R.J. (1996)

Grazing by a river zooplankton community: impor-

tance of microzooplankton. Marine and Freshwater

Research, 47, 1025–1036.

Leech D.M. & Williamson C.E. (2000) Is tolerance to UV

radiation in zooplankton related to body size, taxon, or

lake transparency?EcologicalApplications, 10, 1530–1540.

Magurran A.E. (1988) Ecological Diversity and Its Measure-

ment. Princeton University Press, New Jersey, 179 pp.

McCabe G.D. & O’Brien W.J. (1983) The effects of

suspended silt on feeding and reproduction of Daphnia

pulex. American Midland Naturalist, 110, 324–337.

Meng L. & Orsi J.J. (1991) Selective predation by larval

striped bass on native and introduced copepods.

Transactions of the American Fisheries Society, 120, 187–

192.

Pace M.L., Findlay S.E.G. & Fischer D. (1998) Effects of an

invasive bivalve on the zooplankton community of the

Hudson River. Freshwater Biology, 39, 103–116.

Pace M.L., Findlay S.E.G. & Lints D. (1992) Zooplankton

in advective environments: the Hudson River commu-

nity and a comparative analysis. Canadian Journal of

Fisheries and Aquatic Sciences, 49, 1060–1069.

Reckendorfer W., Keckeis H., Winkler G. & Schiemer F.

(1999) Zooplankton abundance in the River Danube,

Austria: the significance of inshore retention. Fresh-

water Biology, 41, 583–591.

Schiemer F., Keckeis H., Reckendorfer W. & Winkler G.

(2001) The ‘inshore retention concept’ and its signifi-

cance for large rivers. Archiv fur Hydrobiologie, Supple-

ment 135, Large Rivers, 12, 509–516.

Shiel R.J. (1985) Zooplankton of the Darling River system,

Australia. Verhandlungen. Internationale Vereinigung

Limnologie, 22, 2136–2140.

Stemberger R.S. (1979) A Guide to the Rotifers of the

Laurentian Great Lakes. EPA-600/4–79–021. US Environ-

mental Protection Agency, Cincinnati, OH, U.S.A.

Thorp J.H. & Casper A.F. (2002) Potential effects on

zooplankton from species shifts in mussel planktivory:

a field experiment in the St. Lawrence River. Freshwater

Biology, 47, 107–119.

Thorp J.H. & Casper A.F. (2003) Importance of biotic

interactions in large rivers: an experiment with

planktivorous fish, dreissenid mussels, and zooplank-

ton in the St. Lawrence. River Research and Applications,

19, 265–279.

Thorp J.H. & Covich A.P. (Eds) (2001) Ecology and

Classification of North American Freshwater Invertebrates.

2nd edn. Academic Press, San Diego, U.S.A.



Thorp J.H. & Delong M.D. (2002) Dominance of

autochthonous autotrophic carbon in food webs of

heterotrophic rivers? Oikos, 96, 543–550.

Thorp J.H., Alexander J.E. Jr & Cobbs G.A. (2002) Coping

with warmer, large rivers: a field experiment on

potential range expansion of northern quagga mussels

(Dreissena bugensis). Freshwater Biology, 47, 1779–1790.

Thorp J.H., Lamberti G.A. & Casper A.F. (2005a)

Chapter. 21. St. Lawrence River. In: Rivers of North

America (Eds A.C. Benke & C.E. Cushing), pp. 983–

1028. Academic Press, San Diego, CA.

Thorp J.H., Thoms M.C. & Delong M.D. (2005b) The

riverine ecosystem synthesis: biocomplexity in river

networks across space and time. River Research and

Applications (in press).

Thorp J.H., Black A.R., Haag K.H. & Wehr J.D. (1994)

Zooplankton assemblages in the Ohio River: sea-

sonal, tributary, and navigation dam effects.

Canadian Journal of Fisheries and Aquatic Sciences, 51,

1634–1643.

Wallace R.L. & Snell T.W. (2001) Phylum Rotifera. In:

Ecology and Classification of North American Freshwater

Invertebrates, 2nd edn (Eds J.H. Thorp & A.P.

Covich), pp. 195–254. Academic Press, San Diego,

U.S.A.

Young F.W. & Hamer R.M. (1987) Multidimensional

Scaling: History, Theory and Applications. Erlbaum,

New York.

(Manuscript accepted 16 June 2005)

Appendix 1 Mean densities (no. L)1) of zooplankton genera collected from seven rivers

Taxa/Rivers AR KA PL EL NI OH SL

Branchiopoda

Acroperus 0.00 0.00 0.00 0.00 0.00 0.00 0.28

Alona 0.00 0.00 0.00 0.00 0.00 0.00 0.02

Bosmina 0.00 0.02 0.14 0.00 0.05 7.52 2.39

Bosminopsis 0.00 0.00 0.00 0.00 0.00 0.01 0.00

Ceriodaphnia 0.00 0.00 0.00 0.00 0.00 4.83 0.38

Chydorus 0.00 0.00 0.00 0.00 0.00 0.04 0.18

Daphnia 0.00 0.00 0.00 0.00 0.00 0.89 2.10

Diaphanosoma 0.00 0.40 0.21 0.00 0.07 4.83 0.03

Eubosmina 0.00 0.00 0.00 0.00 0.00 0.00 0.13

Holopedium 0.00 0.00 0.00 0.00 0.00 0.00 0.06

Polyphemus 0.00 0.00 0.00 0.00 0.00 0.00 0.26

Sida 0.00 0.00 0.00 0.00 0.00 0.00 0.05

Total cladocera 0.00 0.42 0.35 0.00 0.12 18.10 5.88

Copepoda

Calanoids (adults + copepodids) 0.01 0.02 0.04 0.01 0.01 0.68 0.63

Eurytemora 0.00 0.00 0.00 0.00 0.00 * 0.16

Leptodiaptomus 0.01 0.08 0.03 0.00 0.01 * 0.01

Cyclopoids (adults + copepodids) 0.01 0.12 0.28 0.02 0.16 20.00 1.11

Acanthocyclops 0.00 0.00 0.00 0.00 0.00 * 0.06

Diacyclops 0.01 0.06 0.17 0.01 0.00 * 0.25

Mesocyclops 0.00 0.01 0.00 0.00 0.00 * 0.01

Harpacticoids (adults + copepodids) 0.00 0.00 0.00 0.00 0.00 * 0.22

Nauplii 0.31 0.56 0.56 0.09 0.54 19.78 6.11

Total copepods 0.35 0.70 0.88 0.13 0.72 40.46 8.07

Rotifera

Ascomorpha 0.00 0.00 0.00 0.00 0.00 0.83 0.00

Asplanchna 0.00 0.00 0.13 0.00 0.00 0.99 0.11

Brachionus 4.13 0.44 16.38 4.25 1.63 7.43 0.00

Conchiloides 0.00 0.00 0.00 0.00 0.00 0.00 0.01

Encentrum 0.00 0.00 0.00 0.00 0.00 0.00 0.11

Euchlanis 12.38 1.56 0.63 3.13 0.50 0.00 0.64

Filinia 1.75 1.56 17.25 16.88 0.13 0.27 0.00

Gastropus 8.88 6.06 6.25 3.38 3.38 0.00 0.00

Kelicottia 0.00 0.00 0.00 0.00 0.00 0.01 0.00

Keratella 1.00 2.38 1.00 0.38 1.25 4.48 1.38

Lecane 0.00 0.00 0.00 0.00 0.00 0.21 0.04



Appendix 1 (Continued)

Taxa/Rivers AR KA PL EL NI OH SL

Lepadella 0.00 0.00 0.00 0.00 0.00 0.00 0.54

Monostyla 30.50 6.75 3.88 4.00 9.88 0.00 0.89

Notholca 10.25 6.19 44.50 12.50 1.50 0.06 0.06

Platyias 0.00 0.00 0.00 0.00 0.00 0.02 0.00

Ploesoma 7.38 1.44 21.00 0.00 1.75 2.38 0.08

Polyarthra 0.00 0.00 0.00 0.00 0.00 13.73 21.43

Proales 9.75 3.13 8.13 19.63 1.50 0.00 0.00

Synchaeta 0.00 0.00 0.00 0.00 0.00 3.02 3.41

Trichocerca 6.50 4.69 9.63 12.75 20.88 0.23 0.69

Trichotria 0.00 0.00 0.00 0.00 0.00 0.00 0.07

Total rotifers 92.50 34.38 128.75 101.63 42.38 33.65 29.46

*Taxa may be present but not tabulated in this data set. Based on other data sets, Eurytemora and Diacyclops thomasi were the dominant

calanoid and cyclopoid copepods in the Ohio River.

Mesocyclops was also common in the Ohio River. Other genera present at <0.01 animals per litre in all rivers were not included here.

Rivers: AR, KA, PL, EL, NI, OH, and SL ¼ Arkansas, Kansas, Platte, Elkhorn, Niobrara, Ohio, and St Lawrence Rivers, respectively.



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