COS 105-6 - Nutrient excretion of vertebrates and invertebrates across trophic positions and a stream gradient of dissolved organic matter: is “a change gonna come”?

Background/Question/Methods

Nutrient recycling in aquatic ecosystems is a key biogeochemical process that can be mediated by aquatic animals. The importance and the characteristics of animal-mediated nutrient recycling may vary relative to the taxonomic classification (vertebrates vs. invertebrates), trophic position and abiotic factors such as ambient nutrient conditions and dissolved organic matter (DOM) quality and quantity, measured as units of carbon (DOC). In this study, we examined the effects of DOC and DOM composition on the nitrogen and phosphorus release of mayflies and fish in eleven streams that varied in ambient DOC and DOM composition. First, we used Generalized Additive Mixed Models (GAMM) to analyze mass-specific N, P, and N:P excretion of vertebrates and invertebrates relative to DOC and DOM. Second, we grouped DOC and DOM into two broad clusters using a hierarchical cluster analysis: 1) low DOC and microbial-like DOM, and 2) high DOC and humic-like DOM. We then compared population N, P, and N:P excretion rates of four trophic levels between the two clusters.

Results/Conclusions

For both individual and population nutrient excretion rates, invertebrate nutrient excretion rates were the most sensitive to DOC and DOM. At the individual level, mass-specific invertebrate N excretion rates increased with DOC and DOM, while mass-specific invertebrate and vertebrate P excretion rates responded nonlinearly following a bimodal trend. At the population level, all three trophic levels except for carnivores showed an increase in N and P excretion rates from low DOC and microbial-like DOM to high DOC and humic-like DOM and a hierarchical response across trophic levels. Conversely, carnivores showed a decrease in N and P excretion rates between the two DOM clusters. These results suggest that 1) DOC and DOM have a stronger effect on mass-specific rather than population excretion, 2) invertebrates may be the most sensitive to changes in DOM, and 3) trophic position determines animal relative contribution to nutrient recycling. Our findings demonstrate that as our waters become browner, animal-mediated nutrient recycling will change and that the magnitude and direction of this change will be specific to the animal taxonomic classification, its trophic position, and the scale at which we are observing it (individual vs. population level).