Nutrient recycling by insect and fish communities in high-elevation tropical streams

Abstract

High- to mid-elevation streams are often oligotrophic, but harbor diverse groups of aquatic animals that can satisfy a substantial proportion of nutrient demand. Therefore, we tested the proportion of nutrient demand met by two dominant guilds of animal consumers in the Andes to ask: (1) Do excretion rates vary between insects and fish in montane tropical stream ecosystems? (2) What consumer guild dominates areal nutrient regeneration? (3) What is the nutrient demand and what proportion are consumer taxa regenerating? We combined aquatic insect and fish biomass estimates with measured excretion rates of two fish species (one native, one introduced) and six aquatic insects and estimated nutrient demand in streams by conducting nutrient uptake measurements. Insect taxa had higher per-capita excretion rates than fish and had higher excretion N:P. Aquatic insect biomass tended to be higher than fish biomass and consequently total areal excretion rates by insects were higher. Collectively, communities contributed up to 15–24% of NH4–N demand and 1–19% of SRP demand. The additive effect of these groups on nutrient availability is likely an important function in low-nutrient tropical streams. Further work needs to be conducted to examine the interactions within entire communities and consequential impacts on nutrient cycling.