Nutrient recycling by stream fish assemblages across a land use gradient

In many streams fishes can be an important source of recycled nutrients (N and P) to the ecosystem. We examined the role played by temperate stream fish assemblages in recycling nutrients across a gradient of agricultural land use.  We quantified nutrient excretion rates of both ammonia (NH4⁺- N) and phosphate (SRP) for fish species sampled in 12 streams in southern Ontario, Canada. Species-specific excretion rates were consistently predictable based on body size, although SRP excretion rates showed significant site effects for some fish species (ANCOVA, P < 0.005) that strongly related to periphyton N:P ratios.  To quantify reach scale demand for excreted nutrients, aerial uptake rates were measured in one forested and one agricultural watershed using short-term whole-stream amendment experiments and further compared to measurements published in the literature.  Total fish biomass and assemblage excretion increased with agricultural land use and, as such, a greater percentage of nutrient uptake demand was satisfied as agriculture increased.   Despite this increasing relative importance, it seems that fish in our study streams play a minor to moderate role in nutrient recycling as SRP excretion did not exceed 4% and NH4⁺-N ranged from 0-30% of respective stream uptake demands.