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.. University of California Press and American Institute of BiologicalSciences are collaborating with JSTOR to digitize, preserve and extend access to BioScience. Limnologists have been studying patterns in lake primary productivity for more than 60 years (Elster 1974). More recently, concern about eutrophication has focused attention on nutrient supply as a regulator of lake productivity. However, nutrient supply cannot explain all the variation in the primary productivity of the world's lakes. Schindler (1978) analyzed a sample of 66 lakes that were likely to be limited in productivity by phosphorus because their nitrogen/phosphorus ratios exceeded five. Phosphorus supply, corrected for hydrologic residence time, explained only 48% of the variance in primary production, and lakes with similar phosphorus supply rates differed nearly a thousandfold in productivity. Phosphorus loading explains 79-95% of the variance in chlorophyll a concentration (Dillon and Rigler 1974, Oglesby 1977, Schindler 1978), but chlorophyll a concentration is a poor predictor of primary production (Brylinsky and Mann 1973, Oglesby 1977).

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A simple energy budget equation is developed to yield a bioenergetics model designed to simulate fish growth. Parameters for the model are estimated from the literature for application to yellow perch (Perca flavescens) and walleye (Stizostedion vitreum vitreum). Simulations are presented that demonstrate model output as functions of body size, activity level, ration level, food quality, and environmental temperature. Sensitivity analyses identify the importance of food consumption, activity, and excretion as biological processes represented in the parameters. On the basis of temperature conditions in selected lakes and specified feeding levels, simulations are presented to quantify the importance of year-to-year variation of temperature in determining growth. In heterothermal systems, temperature selection by percids can have a significant effect on growth. For walleye on fixed rations, annual growth can vary from zero to twofold increments due entirely to differences in summer temperatures. Variations in food quality have lesser effects. Key words: Perca, Stizostedion, bioenergetics model, growth, sensitivity, simulations

Catastrophic ecological regime shifts may be announced in advance by statistical early-warning signals such as slowing return rates from perturbation and rising variance. The theoretical background for these indicators is rich but real-world tests are rare, especially for whole ecosystems. We tested the hypothesis that these statistics would be early-warning signals for an experimentallyinduced regime shift in an aquatic food web. We gradually added top predators to a lake over three years to destabilize its food web. An adjacent lake was monitored simultaneously as a reference ecosystem. Warning signals of a regime shift were evident in the manipulated lake during reorganization of the food web more than a year before the food web transition was complete, corroborating theory for leading indicators of ecological regime shifts.

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.