Recent large-scale seagrass declines have prompted experimental investigations of potential mechanisms. Although many studies have implicated eutrophication or reductions of epi-phyte grazers in these declines, few experiments have simultaneously manipulated both factors to assess their relative effects. This study used meta-analyses of 35 published seagrass studies to compare the relative strength of 'top-down' grazer effects and 'bottom-up' nutrient effects on epiphyte biomass and seagrass above-ground growth rate, above-ground biomass, below-ground biomass, and shoot density. A surprising result was that seagrass growth and biomass were limited in situ by sediment nutrients; light limitation has been emphasized in the literature to date. Water column enrichments, which were correlated with increased epiphyte biomass, had strong negative effects on seagrass biomass. Grazers overall had a positive effect on shoot density, but negligible effects on sea-grass biomass and growth rate. However, analyzing epiphyte grazers separately from other grazers revealed positive effects of grazing on seagrass response variables and corresponding negative impacts on epiphyte biomass. The positive effects of epiphyte grazers were comparable in magnitude to the negative impacts of water column nutrient enrichment, suggesting that the 2 factors should not be considered in isolation of each other. Until the determinants of epiphyte grazer populations are empirically examined, it will be difficult to address the contribution that overfishing and cascading trophic effects have had on seagrass decline. Because increases in water column nutrients are documented in many regions, efforts to reduce coastal eutrophication are an appropriate and necessary focus for the management and conservation of seagrass ecosystems. KEY WORDS: Seagrasses · Meta-analysis · Epiphyte · Nutrients · Grazers · Management · Eutrophication · Top-down/bottom-up Resale or republication not permitted without written consent of the publisher herbivory, and substratum turnover. Limnol Oceanogr 32: 986-992 Brooks JL, Dodson SI (1965) Predation, body size and composition of plankton. Science 150:28-35 Camp DK, Cobb SP, Van Breedfield JV (1973) Overgrazing of seagrasses by a regular urchin, Lytechinus variegatus.

Predation can have both direct and indirect effects on the population dynamics and community structure of freshwater plankton communities, but its effects on the bacterial component of aquatic systems are less well known. We used a series of laboratory reconstructions of the detritus-based food web in the leaves of the northern pitcher plant Sarracenia purpurea to test the hypothesis that interactions at higher trophic levels could control bacterial densities and community structure. The typical pitcher community is composed of a basal-level bacterial assemblage, bacterivorous protozoa and rotifers, and larvae of the pitcher plant mosquito Wyeomyia smithii.Using organisms isolated from natural pitchers, we constructed food webs comprising 1-4 consumer species (all possible combinations of the presence and absence of Colpoda, Cyclidium, Bodo, and Wyeomyia larvae) along with a constant bacterial species pool in a factorial design experiment. Bacterial community structure was modified by the direct effects of grazing by protozoa and mosquito larvae, by the indirect effects of competitive interactions among the three protozoans, and by the cascading effects of predation by mosquito larvae. Each combination of grazers/predators produced a different, species-specific pattern of bacterial species relative abundances. Changes in nutrient supplies and other abiotic characteristics of the microcosm environment resulting from the feeding activities of Wyeomyia and the protozoa also had indirect effects on bacterial species profiles. We found an apparent trophic cascade that was mediated by the species composition and relative abundances of the intermediate-level protozoan grazers.Our data support the hypothesis that Wyeomyia smithii serves as a keystone predator in the pitcher community. Mosquito larvae were responsible for the overall architecture of the pitcher food web and the subsequent interactions among grazers and the basal-level bacterial community. At low densities, mosquito larvae controlled patterns of coexistence among protozoan species by modifying competitive interactions, while at normal field densities, Wyeomyia rapidly drove the two ciliates to extinction but permitted continued existence of heterotrophic microflagellates. Wyeomyia larvae also played a dominant role in structuring the bacterial assemblage. Bacterial species profiles in food webs containing Wyeomyia were more similar to each other than to the patterns observed in webs without mosquito larvae.Our results suggest that the interactions among members of microbial communities are just as complex as those observed in plant and animal communities and require study at the species level. Because interactions among higher trophic levels can cascade down to the microbial level, it is therefore appropriate to consider the microbes as integral parts of the entire ecosystem, not merely as decomposers or food resources, but as fully interacting members of the community.

JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org.Wiley is collaborating with JSTOR to digitize, preserve and extend access to Ecology This content downloaded from 198.91.37.2 on Mon, Abstract. In sampling 22 lakes in the Upper Peninsula of Michigan, I found that 2 species of Chaoborus, C. punctipennis and C. americanus, never co-occurred (von Ende 1975). Chaoborus punctipennis was found only in lakes with fish whereas C. americanus occurred alone in stained, bog lakes without fish. Third and 4th instar C. americanus lack extensive diel, vertical migratory behavior and are found near the surface waters in stained bog lakes. Adults of this species emerge in the middle of May. Third and 4th instar C. punctipennis exhibit diel, vertical migration. They are benthic during the day. This species emerges at the end of June.In situ rearing experiments indicate that Chaoborus americanus larvae can survive in lakes with fish, when isolated from the fish. Fish (Umbra limi) added to a lake with C. americanus eliminiated this Chaoboruis species from the lake. It is concluded that the absence of C. americanus from lakes with C. punctipennis is due to fish predation on the older larvae of C. americanus.Chaoborus punctipennis adults are able to disperse to lakes from which their larvae are absent. In situ experiments indicate C. punctipennis can survive on the zooplankton in a stained, fishless bog lake, but is subject to heavy predation by 3rd and 4th instar C. americanus larvae. It is concluded that because of early recruitment by C. americanus, as well as its lack of extensive vertical migration, this species excludes C. punctipennis from stained, fishless bog lakes.

Chaoborus populations were studied in two lakes in which fish predation was experimentally reduced, and in a reference lake. In Tuesday Lake, major reduction of fish predation led to substantial increases in density of Chaoborus punctipennis. Analysis of crop contents and estimates of consumption rates suggested that C. punctipennis caused declines of rotifer and copepod populations following the manipulations. In Peter Lake, lesser changes in fish predation caused no major change in density of Chaoborus flavicans, perhaps because food limitation compensated for effects of reduced predation. In both Peter Lake and the reference lake, Paul Lake, C. flavicans preyed heavily and selectively on Daphnia less than about 1.4 mm in total length. Bioenergetic calculations indicated that up to 46% of the daphnids were consumed daily. At some times of the year, certain zooplankton populations (especially noncolonial rotifers, small copepods, and small cladocerans) were strongly influenced by Chaoborus predation.

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