Diet strategy and body stoichiometry create keystone nutrient recyclers in low-nutrient streams

Nutrient recycling by consumers is an important biogeochemical process in aquatic ecosystems.  Functional traits of individual species can result in some species playing disproportionately large roles in the recycling of phosphorus (P) relative to their biomass, acting as keystone nutrient recyclers.  Here we examine factors controlling the relative contribution of twelve Neotropical fish species to nutrient recycling in streams spanning a range of P-levels.  In high-P conditions (135 µg L^-1 soluble reactive phosphorus, SRP), many species showed similarly high mass-specific P-excretion rates.  In low-P conditions (3 µg L^-1 SRP), most, but not all, species showed steep declines in P-excretion rates.  One species, Astyanax aeneus (Characidae), maintained high P-excretion rates and therefore contributed a disproportionate amount (82%) to total P-recycling by this fish assemblage.  This finding indicates that the importance of species identity is context-dependent, as A. aeneus was only a major contributor to P-recycling in low-P conditions.  Species that have relatively high body N:P ratios and feed on high-P diets are most likely to maintain high P-excretion rates in low-P environments, and thus play a disproportionately important role in ecosystem P-recycling.  The loss of a keystone nutrient recycler in a low-P stream has the potential to dramatically alter ecosystem functioning.