Summary1. Despite non‐point‐source (NPS) pollution being perhaps the most ubiquitous stressor affecting urban streams, there is a lack of research assessing how urban NPS pollution affects stream ecosystems. We used a natural experimental design approach to assess how stream macroinvertebrate community structure, secondary production and trophic structure are influenced by urban NPS pollution in six streams.2. Differences in macroinvertebrate community structure and secondary production among sites were highly correlated with stream‐water specific conductivity and dissolved inorganic phosphorus (DIP) concentrations. Macroinvertebrate richness, the Shannon diversity index and the Shannon evenness index were all negatively correlated with specific conductivity. These patterns were driven by differences in the richness and production of EPT and other intolerant taxa. Production of the five most productive taxa, tolerant taxa, non‐insect taxa and primary consumers were all positively correlated with stream‐water DIP.3. Despite the positive correlation between primary consumer production and DIP, there was no correlation between macroinvertebrate predator production and either total or primary consumer macroinvertebrate production. This was observed because DIP was positively correlated with the production of non‐insect macroinvertebrate taxa assumed to be relatively unavailable for macroinvertebrate predator consumption. After removing production of these taxa, we observed a strong positive correlation between macroinvertebrate predator production and production of available prey.4. Our results suggest that urban NPS pollution not only affects macroinvertebrate community structure, but also alters secondary production and trophic‐level dynamics. Differences in taxon production in our study indicate the potential for altered energy flow through stream food webs and potential effects on subsidies of aquatic insect prey to riparian food webs.

Seasonal changes in macroinvertebrate taxon abundances that are related to life history introduce temporal variation into macroinvertebrate community structure that can potentially confound bioassessments. In this study, macroinvertebrates from three rural and three urban streams were sampled monthly to assess temporal variability in community structure, determine how taxon life-cycle seasonality affects this variability, and determine if this temporal variability confounds biotic index interpretations. Temporal variation in taxon abundances was higher in the rural streams than in the urban streams, and 17 of 23 indicator taxa from rural streams exhibited, or are known to exhibit, either fast- or slow-seasonal life cycles. Conversely, five taxa were identified as indicators for urban streams, but only two of these taxa exhibited seasonal life cycles. Further, the life-cycle seasonality of many rural stream indicator taxa appeared to be linked to the seasonality of environmental conditions such as hydrologic permanence and benthic leaf detritus availability.Macroinvertebrate bioassessment index (MBI) scores were higher in the rural streams than in the urban streams during all months except June, July, and October when no difference was detected. We attribute the reduced discriminatory power of the MBI in the summer and early autumn to the life-cycle of a single taxon (Stenelmis sp.), and its effects on % Clingers - a component metric of the MBI. Stenelmis sp. exhibited a non-seasonal life cycle with a larval development time of approximately 12 months with considerable overlapping of cohorts present during the summer and early autumn when an extended first instar recruitment period was evident. During this overlap period, urban stream MBI values increased resulting in no statistical difference in the MBI between urban and rural streams in June, July, and October. Analyses of temporal variability in taxon abundances and life-history characteristics, as completed in this study, can greatly contribute to the creation and/or validation of bioassessment protocols. (C) 2011 Elsevier Ltd. All rights reserved.

Daphnia lumholtzi has been very successful in colonizing North America since its appearance in Texas in 1990. Although previous studies have sought to link its success as an invasive species with various aspects of its population biology, there is little experimental data linking the invasion success of D. lumholtzi with its autecology, specifically its reproduction strategy. In this study we sought to link food quality and quantity to diapause in D. lumholtzi through a variation in phosphorus (P) content of algae, food quantity, and light level. We also assessed the effect of Daphnia peak population densities on reproductive rates and production of resting eggs. We found that when food is abundant, per capita ephippia production may be limited by P, but under food limitation conditions, there is no significant effect of food quality on ephippia production. Our results suggest that a combination of food quality/quantity and population density may work together to induce the production of resting eggs in this invasive species.

Trihalomethanes (THMs) are contaminants of drinking water produced by the reaction between chlorine and natural organic matter. Determination of THM formation potential (THMFP) is a means of quantifying precursor abundance in waters from diverse sources. THMFP in river water entering and leaving Taylorsville Lake (Ky.) was measured to assess internal and external sources of THM precursors. THMFP in Taylorsville Lake was largely determined by watershed inputs. External inputs from tributary streams accounted for 80% of reservoir THMFP, and internal processes resulted in a net generation of 20%. Chlorophyll concentrations in the main tributary (Salt River) were comparable to those measured in Taylorsville Lake, suggesting that algal production in source waters may be important in regulating precursor supply to the reservoir. The highest THMFP was found in hypolimnetic samples, and peak export from the reservoir occurred during fall turnover, suggesting that decomposition of sedimenting organic matter both delayed and enhanced precursor release.

Summary1. The spatial distribution and movement patterns of zooplankton in large rivers are little known compared with those in lake environments. We conducted a series of studies in the Ohio River (U.S.A.) during the low flow period to assess diel vertical (DVM), longitudinal and lateral movement of crustacean zooplankton.2. The dominant large zooplankter, the copepod Eurytemora affinis, showed a consistent vertical migration pattern of daytime ascent and night‐time descent during all sampling periods – the reverse of the most common migratory pattern of zooplankton in lakes. The cladoceran Bosmina migrated in a similar way in two of the three sampling periods. Surveys taken longitudinally in the river showed similar trends for both taxa.3. During the lateral surveys, E. affinis was significantly more abundant in the shallow littoral zone during the night than in the daytime. The combination of vertical and lateral movement patterns along with the diel distribution of zooplanktivorous fish suggest that these movements are a predator‐avoidance mechanism.4. Sampling programmes in large rivers should consider that larger zooplankton such as E. affinis may not be randomly distributed in the river channel and behaviours such as diel vertical migration may be just as evident in river habitats as in lakes.

ABSTRACT: The processing of waste from confined animal feeding operations (CAFOs) presents a major environmental challenge. Treatment of waste and subsequent land application is a common best management practice (BMP) for these operations in Kentucky, USA, but there are few data assessing the effect of runoff from such operations on aquatic communities. The authors sampled a stream bordering a CAFO with a land application program to determine if runoff from the fertilized fields was adversely affecting stream communities. Water chemistry, periphyton, and macroinvertebrate samples from riffle habitats downstream of the CAFO were compared to samples collected from an upstream site and a control stream in 1999 and 2000. Riffle communities downstream of the fertilized fields had higher chlorophyll a levels than other sites, but there were no significant differences in macroinvertebrate numbers or in biometrics such as taxa richness among the sites. The BMP in place at this site may be effective in reducing this CAFO's impact on the stream; however, similar assessments at other CAFO sites should be done to assess their impacts. Functional measures such as nutrient retention and litter decomposition of streams impacted by CAFOs should also be investigated to ensure that these operations are not adversely affecting stream communities.

Daphnia lumholtzi, a cladoceran native to Australia, South Africa and Asia, has been spreading through the rivers and reservoirs of the Southern and Midwestern US since its first detection in 1989 in Lake Texoma. Although several studies have documented D. lumholtzi dispersal in the US, there is little data linking its life history characteristics with its colonization success. In this study we investigated D. lumholtzi's body stoichiometry, growth and fecundity responses on natural seston vs. uni-algal cultures of Scenedesmus acutus (high and low quality and quantity). We also assessed resting egg production via a series of growth and population experiments to see if these life history parameters are linked with its invasion success. The first experiment examined the effect of diet quality and quantity on growth rates and fecundity of D. lumholtzi. The second experiment examined the growth performance of D. lumholtzi on ambient and lower concentrations of natural seston vs. uni-algae (S. acutus) treatments. In the third experiment, the relationship of D. lumholtzi population density and resting egg production was compared with two other widely distributed (Northern Hemisphere) species (D. pulicaria and D. magna). Growth rate, fecundity and body % P (dry mass) data from the quality-quantity experiment showed that D. lumholtzi performed best under P-rich, high food conditions and worst under P-deficient, low food conditions, exhibiting effects of both food quality and quantity. None of the life history characteristics we examined were significantly different from those of the tested native species of Daphnia. However, %RNA (dry mass) of D. lumholtzi was significantly higher than the tested native species (D. lumholtzi approximate to 10 %; D. pulicaria, D. magna < 8 %). The algae-seston experiment also showed that D. lumholtzi growth performance did not differ from that of the tested native species, but the population and resting egg production experiment showed that at similar food and environmental conditions D. lumholtzi produced significantly more resting eggs than either D. magna and D. pulicaria. The higher RNA levels in D. lumholtzi may facilitate quicker resting egg production, consistent with the Growth Rate Hypothesis. Higher resting egg production may be an important component in invasion success of D. lumholtzi in North America.

Feeding experiments were performed with riverine Bosmina to investigate how their somatic growth rate responded to variation in food resources arising from changing hydrodynamic conditions. Experimental manipulations of food quality and quantity were achieved by diluting riverine suspended particulate matter (seston) to obtain a range of food concentrations and by amending natural seston with laboratory‐grown Scenedesmus acutus . Bosmina experienced food limitation during periods of elevated discharge when the P content of seston was low (C : P > 600). Growth rates covaried with body RNA and P contents ( R 2 = 0.96 and 0.86, respectively; content expressed as percent of dry mass), which is consistent with the hypothesis that high growth rates require increased allocation to P‐rich ribosomal RNA and that high seston C : P induces P limitation in riverine Bosmina . P limitation arises in riverine consumers of riverine seston when hydrologic conditions favor terrestrial inputs, sediment resuspension, and low algal productivity, thereby resulting in seston fractions dominated by P‐poor materials. During low discharge, riverine seston was P‐rich relative to Bosmina requirements and growth rates were decoupled from body P content. Bosmina RNA content was strongly and linearly related to growth over a broad range of resource conditions, suggesting that it may be a useful surrogate to assess dietary sufficiency of food resources in natural settings.

Food limitation effects on life history traits of lake zooplankton have been well documented but few studies have examined linkages between population growth rates and food resources in riverine environments. In rivers, allochthonous inputs of particulate organic matter may mitigate food limitation effects allowing density-independent mechanisms associated with washout (discharge) and feeding interference (turbidity) to assume greater importance. We experimentally manipulated densities of commonly occurring riverine zooplankton (Bosmina longirostris and cyclopoid copepods) within 20001 mesocosms containing ambient or algal-enriched food resources. The experiment was repeated through time (July, August, September) to represent the range of zooplankton densities and food resource levels observed in the Ohio River during warm-water, low-flow conditions. High growth rates and low sensitivity to density-dependent effects were observed during July when particulate organic carbon (POC) and chlorophyll concentrations were highest. Lower growth rates and stronger response to density-dependent effects were observed during August and September experiments when POC and chlorophyll concentrations were lower. Direct manipulations of algal abundance resulted in higher growth rates when gains in chlorophyll were accompanied by increases in the edible size fraction (September experiment). Algal C concentrations were found to be a significant predictor of variation in population growth rates for Bosmina but not cyclopoids. Algal C concentrations in the Ohio River rarely fell below experimentally derived minimum food thresholds but were often below saturation thresholds suggesting that population growth rates were constrained by autochthonous food resources despite the prevalence of allochthonous carbon. Copyright (c) 2005 John Wiley & Sons, Ltd.

Summary1. We compared growth, reproduction and life history characteristics of Bosmina raised on Ohio River seston versus a unialgal culture (Scenedesmus acutus), to assess potential nutritional constraints experienced by riverine populations.2. Bosmina grew well in both treatments during their juvenile stage. Analysis of variance showed that Bosmina growth prior to the start of reproduction did not differ significantly between the treatments. After the onset of reproduction Bosmina fed on Scenedesmus grew faster and exhibited higher fecundity than their counterparts fed river seston.3. Significant quantities of triacylglycerol (visible lipid droplets) were gradually accumulated in Bosmina fed on Scenedesmus. Visual lipid ovary indices were positively correlated with adult growth rate and fecundity and negatively correlated with longevity.4. Biochemical analysis showed that Scenedesmus had significantly greater total ω3, ω6 and polyunsaturated fatty acids (PUFA) whereas river seston had more eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA). Thus, in contrast to Daphnia, EPA and DHA do not appear to influence Bosmina growth or reproductive performance, but linolenic acid or total PUFA may be important.5. Comparatively weak performance by Bosmina feeding on river seston suggest that nutritional constraints may be important despite high particulate organic carbon and low C : N and C : P of river seston.

Litter decomposition in temporary aquatic environments has not been experimentally studied as much as it has in perennial systems. However, litter is likely a critical resource for organisms inhabiting ephemeral aquatic habitats. In this study, we used litterbags under different conditions of submergence and water physical and chemical properties/characteristics to study mass and nutrient losses of terrestrial materials in an ephemeral karst lake in south-central Kentucky (USA). In the first experiment, which was designed to compare decomposition rates in submerged and dry sites, total mass and carbon declined more rapidly in the litter at fully submerged sites than in dry sites. In the second experiment, which was designed to compare decomposition rates in two different submerged environments, total mass and carbon showed similar decomposition trends between the two submerged areas with different seasonal temperature patterns. Nitrogen patterns were variable but in general nitrogen levels increased in the litter in both experiments over a period of several months. These results are similar to those found in some perennially inundated systems and indicate that litter decomposition dynamics in this temporary lake can be greatly affected by lake hydrology. Year-to-year variations in hydrology may thus have strong impacts on nutrient and energy release within this system, which may affect the organisms within this karst lake and in other areas of the karst ecosystem that are ecologically connected to it.

Compared to lentic systems, much less is known about the factors that structure zooplankton communities in large liver environments. In this study, we used an in situ mesocosm system, the potamocorrals, to assess the impact of larval fish on the zooplankton community in the Ohio River (USA). The responses of zooplankton to increasing biomasses of fish were taxon-dependent. The population growth rates of the most common zooplankton, Diacylops thomasi, varied inversely with fish biomass, while other crustacean zooplankton showed no significant responses to the fish treatment. The reverse pattern was seen for the rotifer Poyarthra sp. whose population growth rates increased with increasing fish biomass. This is the first in situ evaluation of fish predation on zooplankton in a large liver system and demonstrates that predation as well as physical factors may influence riverine zooplankton densities and community structure.

1. We conducted a series of in situ enclosure experiments to assess the impact of zebra mussels (Dreissena polymorpha) on the plankton of the Ohio River. Adult mussels were suspended in pelagic enclosures (‘potamocorrals’) at three densities (0, 1000, 2500 mussels per corral) and incubated for 6 days with daily plankton and physiochemical sampling. 2. The presence of adult zebra mussels was correlated with a shift in composition of the phytoplankton community and a severe reduction in some rotifers. The effects of zebra mussels on the larger zooplankton were taxon‐dependent, but bacterial density showed no trend among treatments. 3. Zebra mussels may have significant negative impacts on zooplankton, which may in turn alter riverine food webs.

Chaney Lake State Nature Preserve is a 68 ha ephemeral karst lake in southern Warren County, Kentucky (USA). Chaney typically fills with water in the winter as groundwater levels rise and then slowly dries via evaporation and drainage back into the subsurface over the summer months. The lake contains several habitat types, identifiable by the dominant plant communities, which may have different physiochemical characteristics. Since these differences may result in an increase in habitat heterogeneity and niches available for colonization by resident zooplankton, a suite of physiochemical and nutrient parameters was monitored in these habitats from January 1997–August 1998. Zooplankton densities and community structure were assessed from January 1997 through May 1998. There were few significant differences among the habitats but there were significantly higher concentrations of NOx and higher specific conductivities associated with some of the groundwater inlet areas in the lake. Zooplankton were abundant, but there were no differences in zooplankton densities among the areas sampled. These results suggest that the physiochemical properties in the lake are heavily influenced by the groundwater inputs and that land use patterns around Chaney Lake may be having an impact on the preserve.

ABSTRACT. Ciliates are often important members of aquatic communities in terms of their biomass, productivity, trophic roles, or numerical abundance. The interaction of metazoan predators with ciliates will be mediated by a number of biotic factors, including the potential of ciliate populations for growth, the relative size of ciliates and metazooplankton, the species structure of the metazooplankton, and the defenses of ciliates. This paper reviews some of the recent laboratory an field data pertaining to these particular factor. Studies have generally shown that metazoans can reduce ciliate population growth rates, but this impact varies greatly with the ciliate and metazoans involved. Smaller ciliates are generally more vulnerable to metazoan predators than larger species, although this relationship will be affected by the defenses a ciliate may possess. The structure of the metazooplankton community itself will also affect ciliatemetazoan interactions. The suppression of ciliate populations by metazoans has important ecological consequences, and more study is needed to understand the interaction of these groups in aquatic systems.

Intact phytoplankton and microzooplankton communities from eutrophic Star Lake were incubated for 4 days with and without Daphnia pulex, Daphnia galeata mendotae, or a natural assemblage of Daphnia species. They were sampled at the onset and termination of the experiment for bacterial, phytoplankton, ciliate, rotifer, copepod and cladoceran densities. The cladocerans had varied effects on the rotifers, ranging from significant suppression of most rotifer species (Keratella cochlearis, Polyarthra remata, Keratella crassa) in the D.pulex jars, to the suppression of one (K. crassa) or no species in the D.galeata mendotae and Star Lake Daphnia assemblage jars, respectively. Small ciliates (<30 mum, longest dimension), such as Strobilidium sp. and Pseudocyclidium sp., were adversely affected by most of the cladoceran treatments, while several larger ciliates (> 81 mum) were unaffected in all such treatments. Ciliates were not consistently more vulnerable to cladoceran suppression than similarly size rotifers. The suppression of ciliates and rotifers was attributable to both direct effects (predation, interference, or both) and indirect effects (e.g. resource competition) of the cladocerans.

Clearance rates of Synchaeta pectinata, Brachionus calyciflorus and Asplanchna girodi on Tetrahymena pyriformis (46 μm in length) at a density of 10 cells ml−1, in the presence of algal food, were 2.5 to 6.1 ml rot.−1 day−1. Clearance rates of these rotifers were, respectively, about 2, 3, and 13 times lower on Strobilidium gyrans (58 μm in length) than on T. pyriformis, indicating that the saltations of S. gyrans are an effective escape response. Clearance rates of S. pectinata were considerably lower on Colpidium striatum (81 μm) than on S. gyrans, suggesting that S. pectinata may not be able to ingest ciliates of this size. S. pectinata had a clearance rate of 19 ml rot.−1 day−1 on S. gyrans at a density of 1.2 cells ml−1, in the absence of edible algal food. Rotifers may prey extensively on ciliates in natural plankton communities, ingesting 25 to 50 individuals in the 45–60 μm size range day−1.