Zooplankton of turbid and hydrologically dynamic prairie rivers

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1 Freshwater Biology (2005) 50, doi: /j x Zooplankton of turbid and hydrologically dynamic prairie 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 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 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 U.S.A. thorp@ku.edu 1474 Ó 2005 Blackwell Publishing Ltd

2 Zooplankton in prairie rivers 1475 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 organisms 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 distribution in an area constituting roughly one-third of the conterminous U.S.A. Moreover, scientific knowledge of potamoplankton in these rivers is almost nonexistent, 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 example, 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 influencing the abundance and access to food, mechanics of feeding, downstream transport versus temporary retention, direct mortality (e.g. from ultraviolet radiation), and thermal conditions. Biotic factors include competition for food, parasitism, disease, and planktivory by fish and both benthic and pelagic invertebrates. 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, previously studied rivers usually featured relatively permanent islands and slackwaters (¼shorelines, embayments, and other areas outside the main channel 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 production. 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 midwestern and north-eastern U.S.A.? In an initial step examining this question, we compared potamoplankton 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 moderately 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 characteristics, turbidity, water temperatures, or some other

3 1476 J.H. Thorp and S. Mantovani 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 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 extensively 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 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 discharge 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, Niobrara, and Platte Rivers and from multiple reaches in the Kansas River (Fig. 1a). Samples were collected in 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 These latter habitats varied substantially in current velocity (all 0.51 m s )1, with mean ¼ 0.14 m s )1 ), temperature, 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 highturbidity 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 differences 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 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 semiarid 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

4 Zooplankton in prairie rivers 1477 (a) 43 0'0"N Niobrara River Elkhorn River * * Mississippi River Platte River * Missouri River Arkansas River * * Ohio River 37 0'0N"N 102 0'0"W (b) Agricultural field * Kansas River Riparian zone 95 0'0"W Temporary sand bar island m Slack waters Fig. 1 (a) General 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.

5 1478 J.H. Thorp and S. Mantovani 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 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 Lower Saint boreal forests Lawrence Ohio Eastern deciduous forests Teays-Old Ohio Rivers in grassland watersheds Niobrara Temperate grasslands Middle Missouri Elkhorn Temperate grasslands Middle Missouri Platte Temperate grasslands Middle Missouri Kansas Temperate grasslands Middle Missouri Arkansas Temperate grasslands Southern plains arbitrary categories of discharge variability, as determined by the coefficient of variation (standard deviation/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 peasized 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 constricted-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 Lawrence 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 reservoirs on tributaries. The Kansas basin has the highest proportion of dammed tributaries. Most samples from the Kansas River were collected at sites approximately 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 navigation 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 hydroelectric dam (at Massena, NY and Cornwall, Ontario, hundreds of kilometres downstream from the Great

6 Zooplankton in prairie rivers 1479 (a) Platte Principal component analysis of environmental conditions and coniferous forests) but also contain row crop agriculture [see environmental information on most of these rivers in Benke & Cushing (2005)]. PC2 (16.5%) (b) Dimension Niobrara Kansas Elkhorn Arkansas St. Lawrence Ohio PC1 (77.9%) Platte Multi-dimensional scaling analysis of rotifer diversity and density Elkhorn Arkansas Kansas Niobrara St. Lawrence Ohio Dimension 1 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. 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 shortgrass 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 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 microcrustacean 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 S inverted microscope for rotifers and a Nikon SMZ 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 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. Microcrustaceans 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-

7 1480 J.H. Thorp and S. Mantovani Fig. 3 May to September discharge (cm) patterns in the Kansas River during the study ( ) and averaged over a period of 85 years. Sample dates during our study are shown as asterisks. ling (MDS), PCA, a community similarity index, and analysis of variance (ANOVA). Dissimilarities among rivers in their zooplankton assemblage were quantified in part using the Sorensen index (DI): DI ¼ 1 ½2a=ðb þ cþš 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. ANOVA 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 [Log 10 (x + 1)]. Results were compared between rivers using post hoc Tukey honestly significant difference (HSD) tests. We also examined relationships between zooplankton densities 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 demonstrated 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 ), indicating extremely high similarity in their taxonomic composition. In contrast, other paired comparisons showed much more dissimilar 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

8 Zooplankton in prairie rivers 1481 surprising given their substantial differences in turbidity, 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 [ANOVA d.f. ¼ 6, 51, MS (error) ¼ 0.101, F ¼ , 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 EPA samples (Fig. 4), but were highest if the larger data set from our 2004 hydrological retention study was used. There were no consistent patterns in zooplankton densities to suggest that zooplankton overall fared better or worse in prairie rivers compared to eastern rivers. Significant differences among rivers were also evident for species diversity [Shannon Index: ANOVA d.f. ¼ 6, 51, MS (error) ¼ 0.002, F ¼ , P < 0.01] and evenness [Pielou Index: MS (error) ¼ 0.001, F ¼ , 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 assemblages of Great Plains rivers. However, half the comparisons among various prairie rivers for evenness 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 zooplankton 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 Shannon Pielou Density Diversity indices Zooplankton density (no. L 1 ) 0.0 AR KA PL EL NI Prairie OH SL Rivers 0 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.

9 1482 J.H. Thorp and S. Mantovani 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 rotifers, while the St Lawrence had more nearly equal percentages of cladocera (14%) and copepods (18%). Mean densities for all zooplankton (using EPA samples 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 numbers 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 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 significant differences among rivers in taxonomic composition. 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 Diaphanosoma. 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 abundant cyclopoid copepod in both eastern rivers; but other genera, such as Mesocyclops, often were important in samples from experimental enclosure studies in the Ohio and St Lawrence Rivers (Jack & Thorp, 2000, 2002; Thorp & Casper, 2002, 2003). Significant differences (ANOVA; P < 0.01) existed among the seven rivers in densities of 10 of the 11 most common rotifer genera, which together represented >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 dominants 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 hydrological retention study (discussed below), the dominant 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 zooplankton 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 withinriver 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 R 2 -value for rotifers was moderate (0.4965) when all seven rivers were considered,

10 Zooplankton in prairie rivers 1483 but dropped precipitously when the less turbid Ohio and very clear St Lawrence Rivers were eliminated from the comparison (R 2 ¼ ). For crustaceans, the R 2 -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 R 2 -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 R 2 -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 R 2 -value (0.3362). This marginal relationship disappeared, however, when only prairie rivers were considered (R 2 ¼ ). The positive regression between crustacean abundance and mean discharge (d.f. ¼ 5, R 2 ¼ , 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 R 2 -values ( and , respectively). Effects of flow velocity in a given habitat of the Kansas River on zooplankton densities during Rotifer density (no. L 1 ) (a) Rotifer densities in 7 rivers Linear regression y = x R 2 = Crustacean density (no. L 1 ) (b) Crustancean densities in 7 Rivers Linear regression y = x R 2 = Rotifer density (no. L 1 ) (c) Rotifer densities in the Kansas river Exponential regression y = e x R 2 = Crustacean density (no. L 1 ) (d) Crustancean densities in the Kansas river Linear regression y = 0003x R 2 = Turbidity (NTU) Turbidity (NTU) 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.

11 1484 J.H. Thorp and S. Mantovani (a) Rotifer density (no. L 1 ) Rotifer densities in 7 rivers Linear regression y = 25.67x R 2 = (a) Rotifer density (no. L 1 ) Rotifer density in the Kansas river (July only) Linear regression y = x R 2 = (b) Crustacean density (no. L 1 ) Crustacean densities in 7 rivers Linear regression y = x R 2 = Discharge [log (10) cm] Fig. 6 Effects of discharge (averaged over many decades; note log scale) on densities of rotifers (a) and crustaceans (b) in seven rivers. (b) Crustacean density (no. L 1 ) Crustacean density in the Kansas river (July Sept) Linear regression Current velocity (ms 1 ) y = 0.36x R 2 = 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). summer 2004 (Fig. 7a b) often seemed influenced by an interaction of immediate and recent flow characteristics (Fig. 8). Crustacean densities were inversely related to current velocities in a given habitat throughout the summer (Fig. 7b; d.f. ¼ 23, R 2 ¼ , 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 magnitude higher in August to September when discharge was less. Effects of current velocity on rotifer numbers were only evident during the high discharge month of July (Fig. 7a) when a significant negative linear regression was present (d.f. ¼ 8, R 2 ¼ , 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 hydrological retention areas) and high turbulence, discharge

12 Zooplankton in prairie rivers Rotifers Temperature ( C) Discharge (cms) Discharge Crustaceans Rotifer density (no. L 1 ) Crustacean density (no. L 1 ) 20 0 June J n1u Jun Jun Jun JuJuly n J1 l Jul Jul Jul Temperature Aug1 Aug Aug AugSept Aug1Se Sep 0 0 Date in 2004 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. variability, and suspended sediment loads. Many of these environmentally challenging criteria characterise 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 turbulence 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 predictions 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 (Wallace & 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

13 1486 J.H. Thorp and S. Mantovani 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 comparisons of evenness (Pielou Index) revealed significant differences. Our prediction of the dominance of small zooplankton 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 microcrustaceans and rotifers. Zooplankton assemblages in the two eastern rivers differed considerably from each other, but they contained approximately times more microcrustaceans per litre than did prairie rivers. Moreover, the ratio of microcrustacean density to rotifer density was approximately 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 abundance. 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 communities in prairie rivers. Five abiotic environmental factors should be especially 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 sheltered, 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 rotifers and microcrustaceans, but we suspect that a simpler interaction is merely being altered by concomitant 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 findings (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

14 Zooplankton in prairie rivers 1487 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 environments once they disperse to them. This could account for, or contribute to, the low numbers of microcrustaceans 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 cyclopoids was linked experimentally to increased densities of rotifers in the St Lawrence River (Thorp & Casper, 2003). Hence, rotifers probably do better in turbid rivers not because this environmental condition 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, temperature, substrate type) (Thorp et al., 2005b). It is not surprising, therefore, that lotic ecologists are increasingly 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 whether hydrological retention in prairie rivers played a significant role in regulating zooplankton assemblages. Baranyi et al. (2002) found that rotifers dominated the zooplankton community of periodically 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 velocities, 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 hydrological parameter must impinge on zooplankton through current velocity, water depth, and turbulence. Crustacean densities in the Kansas River were positively related to hydrological retention throughout 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 microcrustaceans (e.g. Pace et al., 1992; Kobayashi, 1997). The relatively importance of low hydrological retention 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 suggest that a complex system of channels and slackwaters 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