The effect of net-spinning caddisflies on nitrate uptake in stream mesocosms

Abstract

Streambeds are thought to play an important role in reducing downstream nitrate export since they provide habitat for microorganisms that use nitrate to build biomass and generate energy. Hydropsychid (net-spinning caddisfly) larvae inhabit the streambed, where they construct silk net and retreat structures. Previous work suggests that the ecosystem engineering activity of net-spinning caddisfly larvae can support higher levels of streambed sediment biomass and create habitat for denitrifying microorganisms, yet it remains unclear whether these effects lead to changes in whole-stream nitrate uptake. We hypothesized that net-spinning caddisflies increase the capacity of the streambed microbial community to take up nitrate. To test this hypothesis, we constructed 15 stream mesocosms and added caddisflies to each according to one of five density treatments ranging from 0 to 2,500 larvae m -2. We measured whole- mesocosm nitrate uptake and characterized hydrologic exchange three weeks after releasing the caddisflies by adding potassium nitrate and sodium chloride to each mesocosm and measuring nitrate + nitrite concentrations over the next 10 hours. To characterize rates of nitrate uptake in the mesocosm streambed, a proxy for the capacity of the streambed microbial community, we used a model that accounts for hydrologic differences among the mesocosms using specific conductance measurements. We found some evidence that higher net-spinning caddisfly densities resulted in higher rates of whole-mesocosm nitrate uptake, or greater nitrate removal. However, we found little evidence that net spinning caddisflies had a substantial effect on nitrate uptake rates in the streambed. We outline several limitations of our study and provide recommendations for improving the experimental set-up and modeling approach in future work. Overall, this research demonstrates some potential for net-spinning caddisflies to serve as ecosystem engineers by altering nitrate removal in streams, though research is needed to lend additional evidence to our findings and to better understand the mechanism by which caddisflies impact nitrate uptake.