Nutrient recycling by animals is a potentially important biogeochemical process in both terrestrial and aquatic ecosystems. Stoichiometric traits of individual species may result in some taxa playing disproportionately important roles in the recycling of nutrients relative to their biomass, acting as keystone nutrient recyclers. We examined factors controlling the relative contribution of 12 Neotropical fish species to nutrient recycling in four streams spanning a range of phosphorus (P) levels. In high‐P conditions (135 μg/L soluble reactive phosphorus, SRP), most species fed on P‐enriched diets and P excretion rates were high across species. In low‐P conditions (3 μg/L SRP), aquatic food resources were depleted in P, and species with higher body P content showed low rates of P recycling. However, fishes that were subsidized by terrestrial inputs were decoupled from aquatic P availability and therefore excreted P at disproportionately high rates. One of these species, Astyanax aeneus (Characidae), represented 12% of the total population and 18% of the total biomass of the fish assemblage in our focal low‐P study stream but had P excretion rates >10‐fold higher than other abundant fishes. As a result, we estimated that P excretion by A. aeneus accounted for 90% of the P recycled by this fish assemblage and also supplied ∼90% of the stream P demand in this P‐limited ecosystem. Nitrogen excretion rates showed little variation among species, and the contribution of a given species to ecosystem N recycling was largely dependent upon the total biomass of that species. Because of the high variability in P excretion rates among fish species, ecosystem‐level P recycling could be particularly sensitive to changes in fish community structure in P‐limited systems.

Recent discoveries have altered prevailing paradigms concerning the conditions under which nitrification takes place and the organisms responsible for nitrification in aquatic ecosystems. In Lake Superior, nitrate () concentrations have increased fivefold in the past century. Although previous evidence indicated that most is generated by nitrification within the lake, important questions remain concerning the magnitude and controls of nitrification, and which microbial groups are primarily responsible for this process. We measured water‐column nitrification rates in the western basin of Lake Superior during five research cruises from November 2009 to March 2011. Using in situ bottle incubations at 10 depths, we quantified nitrification rates using both the oxidation of ¹⁵N‐labeled ammonium () and the uptake of ¹⁴C associated with nitrification. Average rates of oxidation ranged from 18–34 nmol N L⁻¹ d⁻¹ across the five cruises, similar to values reported for the coastal ocean, and two orders of magnitude lower than values reported from other lakes. Low nitrification rates observed in the epilimnion corresponded to the absence of ammonium‐oxidizing archaea and nitrite‐oxidizing bacteria. The measured rates of nitrification are > 50‐fold greater than the long‐term rise in the lake, indicating that N is actively cycling and that long‐term change in this ecosystem is mediated by internal dynamics.