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Despite the recent increased interest in large river ecosystems, our knowledge of the zooplankton of these habitats remains fragmentary. With some notable exceptions [e.g. (Kofoid, 1903; Reinhard,1931)] comparatively little early research was conducted on the phytoplankton and zooplankton of large rivers compared to the numerous studies on fish and freshwater mussels. This lack of research may have resulted from the impression that rivers were not suitable environments for zooplankton. Stream flow (velocity and turbulence) can have negative effects on zooplankton by transporting them into unfavourable environments, physically damaging them, diluting their food resources, and hindering reproduction (Rzsoka, 1978;Pace et al., 1992).The relationship between zooplankton biomass and water residence time has been recently examined in rivers of various sizes. In a comparison of 31 north temperate rivers, Basu and Pick (Basu and Pick, 1996) found that zooplankton biomass was directly proportional to water residence time (r 2 = 0.33) although a small positive relationship was also present with chlorophyll a (Chl a). In many other rivers that have been studied, zooplankton densities or species richness are generally lowest during high flow periods (

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.

Laboratory experiments compared the susceptibilities of six ciliates and the rotiferKeratella cochlearis to predation and interference from Daphnia pulex and Bosmina longirostris. 2. Susceptibilities of the ciliates to D. pulex were similar to or less than that of the rotifer, and decreased with increasing ciliate size. Most ciliates were just as susceptible to B. longirostris as to the much larger D. pulex. The jumping response of the oligotrich Strobilidium gyrans appeared to be an effective defence against 6. longirostris. 3. Clearance rates of B. longirostris and D. pulex on different ciliate species at a density of 1.3 ciliates ml~^ ranged from 1-30 to 5-24ml ind."^ day"', respectively. In natural plankton communities, cladocerans could impose high mortality rates on ciliates and shift the size structure of ciliate assemblages towards larger and less susceptible species.

Summary 1. 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.