Macroinvertebrate excretion rates and their contribution to nutrient cycling in a rocky mountain stream

Abstract

Nutrient supply into an ecosystem is an important factor that can influence its productivity. While some nutrients enter the ecosystem from external sources, cycling of nutrients internally can also be an important process in determining nutrient supply. In this study I examined the role of benthic stream macroinvertebrates in supplying nutrients back into the water column. I measured excretion rates of NH4-N for all readily identifiable taxa in the upper East River in Gothic, CO and examined how these rates varied by body size and across taxa. I also measured nutrient uptake rates in order to determine what percent of ecosystem demand is met by the macroinvertebrate community. I found that invertebrate N excretion rates were positively correlated with individual body mass. Additionally both mean N excretion rate per individual and per unit of biomass varied by nearly three orders of magnitude across invertebrate taxa sampled. The invertebrate community supplied about 13% of nutrient demand. Nutrient supply by macroinvertebrates was dominated by the mayfly Baetis bicaudatus, Chironimidae, and the caddisfly Arctopsyche grandis. I also found that taxa with high mass-specific excretion rates tend to be most tolerant to organic pollution. This result could indicate the presence of a positive feedback chain where an initial disturbance leads to continued change in the invertebrate community. I determined that macroinvertebrates do play an important role in nutrient cycling in this Rocky Mountain stream and that changes in the macroinvertebrate community could have potentially large effects on stream nutrient cycling.

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