Abstract. Animals can be important in modulating ecosystem-level nutrient cycling, although their importance varies greatly among species and ecosystems. Nutrient cycling rates of individual animals represent valuable data for testing the predictions of important frameworks such as the Metabolic Theory of Ecology (MTE) and ecological stoichiometry (ES). They also represent an important set of functional traits that may reflect both environmental and phylogenetic influences. Over the past two decades, studies of animal-mediated nutrient cycling have increased dramatically, especially in aquatic ecosystems. Here we present a global compilation of aquatic animal nutrient excretion rates. The dataset includes 10,534 observations from freshwater and marine animals of N and/or P excretion rates. These observations represent 491 species, including most aquatic phyla. Coverage varies greatly among phyla and other taxonomic levels. The dataset includes information on animal body size, ambient temperature, taxonomic affiliations, and animal body N:P. This data set was used to test predictions of MTE and ES, as described in Vanni and McIntyre (2016; Ecology

The burial of organic matter within lake sediments can be a significant component of landscape carbon cycling. Whether organic matter deposited in lake sediments is sequestered or mineralized depends on factors limiting the decomposition rate of organic matter, such as temperature and the availability of oxygen. In stratified lakes, the distribution of temperature and oxygen is determined by the depth of the thermocline, and therefore sediment organic matter burial should be sensitive to changes in thermocline depth. Using a survey of more than 30 lakes over 3 years in the Alaskan Arctic, we found that thermocline depth during the summer was positively correlated with water transparency. Furthermore, using sediment incubations from 3 lakes, we found that variation in sediment oxygen demand is primarily affected by variation in temperature and the availability of oxygen with limited effect of the source of the sediments. Because variation in temperature and oxygen concentration in stratified lakes is mainly determined by the depth of thermocline, these results indicate that changes in transparency can have indirect effects on the rate of organic matter mineralization in lakes. A reduction in thermocline depth that results from decreased lake transparency may decrease the breakdown of sediment organic matter and increase the storage of organic carbon in lake sediments.

The importance of fish nutrient recycling for lake primary production increases with lake productivity. However, fish in low-productivity lakes may have substantial indirect effects on nutrient recycling from lower trophic levels. We measured nutrient excretion rates from fish and zooplankton in oligotrophic Arctic lakes and investigated direct and indirect fish effects on consumer nutrient recycling. Fish nutrient excretion rates were small relative to phytoplankton nutrient demand. Zooplankton excretion, however, supplied 19%–130% and 37%–200% of phytoplankton nitrogen and phosphorus demand, respectively. Fish had a significant effect on zooplankton biomass; in lakes with fish, this was approximately 80% lower than in lakes without fish. The difference in zooplankton biomass was due primarily to a decrease in zooplankton density; no significant difference in average zooplankton size was observed between fish and fishless lakes. Fish also impacted zooplankton community composition; communities in lakes with fish were dominated by copepods compared with cladoceran dominance in lakes without fish. Because of lower zooplankton biomass, lakes with fish showed lower rates of zooplankton nitrogen and phosphorus excretion relative to lakes without fish. Thus, our results support the hypothesis that fish have major indirect effects on lake nutrient cycles, even when direct excretion from fish is minimal.