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.

We evaluated a bioenergetics model for largemouth bass, Micropterus salmoides, using independent field data on temperature, feeding, and growth of bass in Lake Rebecca, Minnesota. Model predictions of body mass based on observed temperature and daily ration estimates fell within 2 SE of observed mean body masses on seven of nine sampling dates over a 4-mo period. We employed three statistical methods to evaluate the fit between predicted and observed growth. A lack-of-fit test detected no significant lack of fit between simulated and observed body masses; partitioning mean squared error showed that 76% of the variance was due to random variation rather than to systematic errors, and a reliability index indicated agreement between predicted and observed masses within a factor of 1.12. Growth simulations were relatively robust with respect to simulated errors in the input variables temperature and swimming speed, but were sensitive to errors in initial body mass.The model used observed growth to predict cumulative food consumption over the sampling season; the prediction was within 8.5% of an extrapolation from field data. Estimation of consumption rate as a constant proportion of maximum ration for the whole season was shown to be inappropriate; however, three intervals that used different constant proportions of maximum ration fit the data and corresponded to observed changes in bass diet.

The rivers and streams of west‐central Mexico are becoming increasingly degraded and warrant expanded conservation efforts. We have developed an index of biotic integrity based on fish assemblage characteristics to aid in the preservation of the biological integrity and biodiversity of these waters. Our version is an adaptation of previous versions of the index of biotic integrity, which have proven effective in environmental assessment and monitoring in the United States and elsewhere. It consists of 10 fish assemblage attributes, termed metrics, that are related to environmental quality, ecosystem integrity, and biodiversity in west‐central Mexican streams and rivers: number of native species, percentage of benthic species, number of water column species, number of sensitive species, percentage of tolerant species, percentage of exotic species, percentage of omnivores, percentage of native livebearing species, relative abundance, and percentage of diseased or deformed. Data on fish communities from 27 sites on streams and small rivers in the vicinity of the Sierra de Manantlán, southwestern Jalisco, and in the upper Río Duero basin, northern Michoacán, were used to develop expectations and scoring criteria for each metric. The index value for a site was the sum of the scores for the 10 metrics. Index values and associated ratings of biotic integrity for the 27 sites corresponded closely with independent ratings based on habitat and water quality. However, additional tests of the index with independent data and refinement of metrics and scoring criteria are clearly warranted. The index shows promise for identifying key watersheds for the protection of intact native biotic communities and individual endangered and threatened species and for the long‐term monitoring and evaluation of biodiversity and ecosystem integrity in the streams and rivers of west‐central Mexico. Our version could serve as a model for the development of similar indices for other regions and other types of aquatic ecosystems in Mesoamerica.

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