Nutrient cycling by animals in aquatic ecosystems: from individuals to ecosystems

Dr. Michael J. Vanni

Animals can affect the fluxes of nutrients such as nitrogen (N) and phosphorus (P) in many ways and in many biomes. For example, all animals release N and P as waste products, and some animals can move transport great distances via their movements. In addition, animals can have strong indirect effects of nutrient cycling by regulating populations of plants and microbes, and by modifying physical habitats. While the importance of animals has been demonstrated in many situations, there has not been a synthesis of the role of animals in nutrient cycling. This talk will attempt to synthesize information on the roles of animals in mediating biogeochemical cycles, from individuals to biomes, and is structured in three parts. First, data from freshwater and marine taxa were compiled to assess the extent to which body size, temperature and animal body nutrient content can predict excretion rates of animals in the field. This synthesis of over 8000 observations shows that body size alone is an excellent predictor of both N and P excretion rates, but also that temperature and body nutrients are significantly related to excretion rates and the N:P ratio excreted. Metabolic scaling coefficients relating body size and excretion rates show negative allometry, but usually are different from 0.75 (the coefficient predicted by metabolic ecology theory). Temperature has a greater effect on N excretion than on P excretion, and excretion N:P is negatively related to body N:P, as predicted by ecological stoichiometry theory. The second part of the talk presents case studies on the role of animals (mostly fish) in nutrient cycling in lakes. These studies show that nutrient excretion by fish can exceed nutrient supply from other sources known to be important (e.g., watersheds) and can support a significant fraction of ecosystem nutrient demand. However, the importance of fish varies considerably both spatially and temporally, even within a single ecosystem. The third part of the talk provides predictions regarding the roles of animals in nutrient cycling across aquatic and terrestrial biomes. Based on energy flow and other ecosystem properties, direct fluxes of nutrients through animals are predicted to be most important in pelagic ecosystems, least important in forests and of intermediate importance in grasslands. In contrast, the importance of indirect effects of animals in mediating nutrient cycling (via trophic cascades, selective feeding, ecosystem engineering) relative to direct effects (direct flux through excretion) is predicted to be greatest in forests and least in pelagic systems. General conclusions are that animals are under-appreciated in terms of their importance in nutrient cycling and have not been adequately integrated into conceptual models of biogeochemical cycling. A major challenge is predicting and testing for variation across ecosystems and over various scales.


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