Stable isotopes of carbon, nitrogen, and sulfur are used as ecological tracers for a variety of applications, such as studies of animal migrations, energy sources, and food web pathways. Yet uncertainty relating to the time period integrated by isotopic measurement of animal tissues can confound the interpretation of isotopic data. There have been a large number of experimental isotopic diet shift studies aimed at quantifying animal tissue isotopic turnover rate λ (%·day-1, often expressed as isotopic half-life, ln(2)/λ, days). Yet no studies have evaluated or summarized the many individual half-life estimates in an effort to both seek broad-scale patterns and characterize the degree of variability. Here, we collect previously published half-life estimates, examine how half-life is related to body size, and test for tissue- and taxa-varying allometric relationships. Half-life generally increases with animal body mass, and is longer in muscle and blood compared to plasma and internal organs. Half-life was longest in ecotherms, followed by mammals, and finally birds. For ectotherms, different taxa-tissue combinations had similar allometric slopes that generally matched predictions of metabolic theory. Half-life for ectotherms can be approximated as: ln (half-life) = 0.22*ln (body mass) + group-specific intercept; n = 261, p<0.0001, r2 = 0.63. For endothermic groups, relationships with body mass were weak and model slopes and intercepts were heterogeneous. While isotopic half-life can be approximated using simple allometric relationships for some taxa and tissue types, there is also a high degree of unexplained variation in our models. Our study highlights several strong and general patterns, though accurate prediction of isotopic half-life from readily available variables such as animal body mass remains elusive.

Functional variation among consumer communities can alter ecosystem nutrient cycling. These impacts on ecosystem function can be specifically driven by interspecific variation in stoichiometric traits; thus, functional trait‐based approaches can be used to explain the processes controlling ecosystem stoichiometry. However, eutrophication may reduce the functional importance of consumers in ecosystems by eliminating heterogeneity in nutrient recycling among taxa. To test whether zooplankton functional diversity, i.e. aspects of the stoichiometric trait space occupied by zooplankton communities, varies over gradients in trophic state and nutrient stoichiometry, we examined functional and taxonomic variation in the zooplankton communities of 130 lakes in the agriculturally dominated state of Iowa (U.S.A.) over 7 years. Stoichiometric functional dispersion decreased with trophic state index, supporting the trait abundance shift hypothesis that hypereutrophic lakes are characterised by different combinations of functional traits than their less eutrophic counterparts. Zooplankton communities became increasingly N‐rich relative to P as TSI increased. Specifically, P‐poor Bosmina, Chydorus, and cyclopoid copepods increased in abundance with eutrophication. Stoichiometric trait distributions of zooplankton shift with eutrophication, which implies that the unique functioning of hypereutrophic lakes could be due in part to the consumers inhabiting them. As zooplankton N:P increased with trophic state while lake total nitrogen to total phosphorus ratio decreased with trophic state, P‐poor zooplankton taxa may exacerbate excess P availability in these hypereutrophic systems by differentially recycling P at higher rates.

3. Consumption rate of N, but not P, was significantly negatively affected by diet N:P. 25Effect sizes of diet elemental composition on consumption-specific excretion N, P and 26 N:P in laboratory studies were all significantly different from 0, but effect size for raw 27 excretion N:P was not significantly different from zero in laboratory or field surveys.

Consumer-driven nutrient recycling can have substantial effects on primary production and patterns of nutrient limitation in aquatic ecosystems by altering the rates as well as the relative supplies of the key nutrients nitrogen (N) and phosphorus (P). While variation in nutrient recycling stoichiometry has been well-studied among species, the mechanisms that explain intraspecific variation in recycling N:P are not well-understood. We examined the relative importance of potential drivers of variation in nutrient recycling by the fish Gambusia marshi among aquatic habitats in the Cuatro Ciénegas basin of Coahuila, Mexico. There, G. marshi inhabits warm thermal springs with high predation pressure as well as cooler, surface runoff-fed systems with low predation pressure. We hypothesized that variation in food consumption among these habitats would drive intraspecific differences in excretion rates and N:P ratios. Stoichiometric models predicted that temperature alone should not cause substantial variation in excretion N:P, but that further reducing consumption rates should substantially increase excretion N:P. We performed temperature and diet ration manipulation experiments in the laboratory and found strong support for model predictions. We then tested these predictions in the field by measuring nutrient recycling rates and ratios as well as body stoichiometry of fish from nine sites that vary in temperature and predation pressure. Fish from warm, high-predation sites excreted nutrients at a lower N:P ratio than fish from cool, low-predation sites, consistent with the hypothesis that reduced consumption under reduced predation pressure had stronger consequences for P retention and excretion among populations than did variation in body stoichiometry. These results highlight the utility of stoichiometric models for predicting variation in consumer-driven nutrient recycling within a phenotypically variable species.

Calcium carbonate (CaCO 3 ) deposition is common in aquatic ecosystems and may reduce phosphorus availability via coprecipitation of phosphate, an impact with broad implications for ecosystem processes. To determine if CaCO 3 deposition in streams increases phosphorus (P) retention in minerals while reducing P availability to organisms, we studied paired streams (with and without active CaCO 3 deposition) subjected to experimental shading and monitored changes in ecosystem attributes (e.g., periphyton biomass content, nutrient spiraling, periphyton nutrient limitation, and leaf litter decomposition). Shading reduced rates of CaCO 3 deposition by over 50%, suggesting that a substantial proportion of CaCO 3 deposition is supported by photosynthetically induced changes in alkalinity. Shading-induced reductions in CaCO 3 deposition led to increases in epilithon biomass P content (P < 0.05) and periphyton growth (F 2,12 = 5.79, P < 0.05). Reductions in CaCO 3 deposition also relieved P limitation of periphyton growth (F 3,16 = 59.32, P < 0.001), extended P uptake lengths at least an order of magnitude, and reduced both P mass transfer velocity and areal uptake rates by over 80% (F 2,3 = 22.62, P < 0.05 and F 2,3 = 13.19, P < 0.05, respectively). Finally, while shading caused reductions in leaf litter decomposition in the non-CaCO 3 depositing stream (F 5,7 = 22.45, P < 0.001), shading had no effect on leaf litter decomposition in the stream with active CaCO 3 deposition. These results indicate that CaCO 3 deposition can regulate P bioavailability and retention in streams and may drive streams to be P limited, as has been suggested in lake and wetland ecosystems.

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Abstract. Altered thermal regimes under climate change may influence host-parasite interactions and invasive species, both potentially impacting valuable ecosystem services. There is considerable interest in how parasite life cycle rates, growth, and impacts on hosts will change under altered environmental temperatures. Likewise, transformed thermal regimes may reduce natural resistance and barriers preventing establishment of invasive species or alter the range and impacts of established exotic species. The Laurentian Great Lakes are some of the most invaded ecosystems and have been profoundly shaped by exotic species. Invasion by the parasitic sea lamprey (Petromyzon marinus) contributed to major declines in many Great Lakes fish populations. In Lake Superior, substantial progress has been made towards controlling invasive sea lamprey and rehabilitating native fish populations. Surface water temperatures in Lake Superior have been increasing rapidly since 1980 presenting a new challenge for management. Here we test how thermal changes in Lake Superior have impacted the feeding and growth of the parasitic sea lamprey. Sea lamprey have increased in size corresponding with longer durations of thermal habitat (i.e., longer growing seasons) for their preferred hosts. To compare regional differences in sea lamprey feeding and growth rates, we used a bioenergetics model with temperature estimates from a lake-wide hydrodynamic model hindcast from 1979-2006. Spatial differences in patterns of warming across the lake result in regionally different predictions for increases in sea lamprey feeding rates and size. These predictions were matched by data from adult sea lamprey spawning in streams draining into these different thermal regions. Larger sea lampreys will be more fecund and have increased feeding rates, thus increasing mortality among host fishes. Resource management should consider these climate driven regional impacts when allocating resources to sea lamprey control efforts. Under new and evolving thermal regimes, successful management systems may need to be restructured for changing phenology, growth, and shifts in host-parasite systems towards greater impacts on host populations.