Stock NH4 solution was added to 40 ml filtered stream water to raise its concentration by 0,1,5,10, 20 μg NH4-N/L. For BF standard, no stock solution was added. 10ml of OPA, the ammonium working reagent, was added to the 40 ml NH4 standard solutions. The solution was allowed to sit for 4-6 hours before a small portion was poured into a cuvette and its UV fluorescence was read by a Turner Designs Trilogy Fluorometer with UV CDOM/AM module. For BF samples, 10ml OPA solution was added to 40ml filtered stream water and its fluorescence was immediately read in the fluorometer.

 

Note: Standards are calculated by subtracting ambient (0 standard) fluorescences from all other non-BF standards. Then BF fluorescence is added to each value from the previous calculation. The standard curve is the regression of the standard value (0,1,5,10, 20, etc.) against this calculated value. 

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 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. The invertebrate community supplied about 13% of nutrient demand. Nutrient supply rate 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.