ALBERT

Notes: 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 = 0.07) but moderately disparate for comparisons of prairie versus forested-basin rivers (mean = 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 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.

Notes: SUMMARY 1. Zebra mussels (Dreissena polymorpha) have established a much greater range in North America and Europe than quagga mussels (D. bugensis), which occupy a very similar niche.2. We hypothesised that quaggas are physiologically capable of sustaining populations in warmer rivers currently occupied only by zebra mussels and that unidentified, non-physiological factors account for their more limited distribution.3. Growth and survival of individually tagged mussels (976 D. bugensis from Lake Erie; 2625 D. polymorpha from Lake Erie and the Ohio River) were recorded monthly for up to 15 months in an outdoor stream mesocosm receiving unfiltered water from the Ohio River.4. Extreme temperatures stressed both species; but in contrast to several previous laboratory studies, quaggas survived high temperatures better than zebra mussels. We suspect this was the result of species-specific differences in their ability to obtain, assimilate and/or catabolise food at high, sublethal temperatures.5. A unimodal growth pattern was observed in both species, with the highest growth rates from late spring to early autumn.6. Our survival and growth data suggest that quaggas are not physiologically limited from expanding southward.7. While lacking definitive proof that dreissenid populations in rivers are ecologically sustainable without upstream lentic ecosystems and/or unintended human intervention, we suggest that complex river currents and upstream retentive and highly productive slackwater habitats in rivers may help sustain downstream populations of these meroplanktonic, dreissenid mussels.

Notes: 1. Suspension feeding by bivalves exceeds that by other planktivores in many North American rivers, and food webs may be altered substantially by differences in feeding patterns between native unionid mussels and invading dreissenid mussels.2. We conducted an experiment comparing zooplanktivory by one unionid and two dreissenid species that addressed several primary questions. Is benthic planktivory important in this river? Has this linkage been altered substantially by dreissenids? Do the two dreissenid species differ in planktivory, and is this ecologically important if quagga mussels extend their geographical range?3. Our 12-day experiment consisted of controls (no mussels) and treatments with unionid (Elliptio complanata), quagga (Dreissena bugensis) or zebra (D. polymorpha) mussels in 3500-L, 80-μm mesh enclosures placed in a slackwater area of the St Lawrence River.4. The density of the most abundant calanoid copepod Eurytemora affinis increased in the presence of dreissenids, probably as an indirect food web response. By day 12, a cumulative effect was shown by the most overwhelmingly abundant rotifer, Polyarthra, whose density declined dramatically in dreissenid enclosures compared with control and unionid enclosures. Rotifer densities in unionid enclosures were not different from controls, nor were dreissenid treatments different from each other. The effects on rotifers were probably from predation, as Chl-a did not vary among treatments.5. We conclude that benthic-pelagic coupling via planktivory is important in slackwater areas. Dreissenids have strengthened this linkage, but range extension of quaggas should not appreciably alter effects produced by a similar biomass of zebra mussels.

Notes: 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.

Notes: 1. In situ exclosure experiments in the Mississippi and Ohio Rivers determined the importance of fish predation in regulating zebra mussels (Dreissena polymorpha), an increasingly important constituent of the benthic invertebrate assemblages in both rivers.2. We evaluated the effects of predatory fish on the density, biomass and size distribution of zebra mussels in a floodplain reach of the upper Mississippi River and in a naturally constrained reach of the Ohio River. Fifty, six-sided, predator-exclusion cages and fifty ‘partial’ cages (mesh at the upstream end only) were deployed, with half the cages containing willow snags and half clay tiles suspended 12–16 cm above the bottom. A single snag or tile sample unit was removed from each cage at approximately monthly intervals from July to October 1994. Types and relative abundances of molluscivorous fish were evaluated by electrofishing near the cages in both rivers. Actual and potential recruitment of young zebra mussels on to the substrata were measured using benthic samples in both rivers and estimated (Ohio River only) from counts of planktonic veligers.3. Zebra mussels were consumed by at least three fish species in the upper Mississippi River (mostly carp, Cyprinus carpio, and redhorse suckers, Moxostoma sp.) and five species in the Ohio River (primarily smallmouth buffalo, Ictiobus bubalus, and channel catfish, Ictalurus punctatus), but potential recruitment seemed adequate to replace consumed mussels, at least in the Ohio River. The number of juvenile benthic mussels showed no apparent link with the density of veligers soon after initiation of reproduction. Recruitment of juveniles on snags and tiles was not affected by cage type (thus eliminating a potentially confounding ‘cage effect’).4. Fish significantly influenced mussel populations, but the impact was often greatest among low density populations in the upper Mississippi. Density and biomass differed in both rivers for cage type (higher inside cages), substratum (greater on tiles), and date (increased over time). Presumed size-selective predation was present in the Mississippi (greater on larger size classes) but was not evident in the Ohio. We hypothesize that fish in the Mississippi can more easily select larger prey from the low density populations; whereas size-selective predation on tightly packed zebra mussels in the Ohio would be difficult.5. Although fish can reduce numbers of Dreissena polymorpha in the two rivers, current levels of fish predation seem insufficient to regulate zebra mussel densities because of its great reproductive capacity. The recent invasion of zebra mussels, however, could lead to larger fish populations while promoting greater carbon retention and overall ecosystem secondary production.

Notes: STUDIES AND REPORTS

Notes: Summary I examined the relationship between temperature and agonistic behavior in the crayfish Cambarus latimanus LeConte a species which is reproductively active in winter. Crayfish were acclimated for 2 wks at 9.5, 14, 22 and 30°C in summer and at 9.5 and 22°C in winter. Agonistic behavior of pairs of the same sex was recorded for 1 h following acclimation. The entire acclimation, testing, and temperature-readjustment procedure was repeated until all crayfish pairs had been tested at each acclimation temperature. Published information on lobsters and catfish indicate that agonistic behavior is directly related to temperature. In contrast, for C. latimanus in the present study, duration of total agonistic behavior, maximum duration of a single agonistic interaction, and average length of a agonistic encounter were all inversely related to acclimation temperature. I hypothesized that for ectothermic species whose agonistic behavior is closely associated with reproductive processes (including competition for mates) the level of agonism will vary either directly or inversely depending upon whether the reproductive period is cued by increasing or decreasing seasonal temperatures, respectively.

Notes: Summary 1) Functional response curves were constructed for Celithemis fasciata larvae feeding on 6 different densities of midge larvae at 10, 15, 20, and 25°C. Values for attack rate and handling time were estimated with Rogers's random predator equation. 2) Polynomial regression revealed that the functional response curves were linear although a tendency toward decreasing consumption rates at higher densities was shown. 3) The mean number of prey eaten increased with temperature; however, temperature did not alter the fundamental shape of the functional response curve. 4) The attack rate and handling time were linearly related to temperature in general, but changed relatively little between 15 and 20° C.

Notes: Summary Males of three sunfish species (Centrarchidae) construct nests for spawning and often share them sequentially in the littoral zone of a 4-hectare lake in New York State. To determine spatial and temporal effects of this reproductive behavior on zoobenthos in “patches,” I sampled bottom assemblages from inside nests and from adjacent (〈1 m from nest perimeter), relatively undisturbed sites during the reproductive season of 1986 and immediately prior to nest-building in 1987. The reproductive behaviors of largemouth bass (Micropterus salmoides), redbreast sunfish (Lepomis auritus), and pumpkinseeds (L. gibbosus) altered relative abundances and significantly decreased benthic invertebrate diversity and density. These effects were extremely pronounced during the reproductive season and were partially detectable the following year. Community changes were probably the result of both bioturbation (modification of sediment size and organic content) and predation. The ecosystem-wide effects of nest-building are evaluated in terms of the number, distribution, and longevity of patches.