Stoichiometry of Excreta and Excretion Rates of a Stream-dwelling Plethodontid Salamander

Nutrient recycling by fish and amphibians can contribute significantly to ecosystem processes in freshwater ecosystems. Variation in the components that comprise excreta and the factors that control excretion rates may result in differences in the rates and ratios of nutrients excreted by these taxa. These factors can subsequently influence the ecological role of fish and amphibians in freshwater systems. We assessed the composition of excreta of larval Eurycea cirrigera in six headwater streams, and investigated the influence of body size, organismal stoichiometry, and stream nutrient concentrations on mass-specific excretion rates. Excretion components and excretion rates of E. cirrigera were compared to fish in stream and river systems. Excretion of urea comprised the majority of excreted waste in E. cirrigera. This is considerably different from freshwater fish, where the majority of excreted nitrogenous waste is in the form of NH4. Excretion of NH4, soluble reactive phosphorus (SRP), and urea by E. cirrigera differed across streams. Body size was the most important predictor of excretion rates of NH4 and the ratio of N:P in the larvae of E. cirrigera, and no other variables tested influenced the rates of excretion. These estimates reveal larval stream-dwelling plethodontids' excretion rates of inorganic nutrients (NH4 and SRP) are lower compared to fish in similar systems. However, the contributions of urea (organic forms of nitrogen) are higher. Considering many headwater streams are fishless, stream-dwelling plethodontids, such as E. cirrigera, could represent a primary method of nutrient recycling of limiting nutrients in headwater streams, particularly more labile forms of organic nitrogen.