ABSTRACT:

Biogeochemical rates within streams vary with ecosystem properties including the distribution of fishes. While many studies investigate the singular effect of fishes on ecosystem components, there is a limited understanding of how fish presence interacts with other ecosystem properties to affect ecosystem structure and function. Here, we used path analyses to elucidate direct and indirect effects of fish presence, and other ecosystem properties on ecosystem respiration (ER), gross primary production (GPP), and ammonium uptake. Experimental responses of fish removal on patch scale (300 cm2) benthic rates of ER, GPP, and ammonium uptake were measured at two sites in a prairie stream. Biogeochemical rates associated with benthic substrata were quantified by monitoring fluxes of dissolved oxygen and ammonium inside sealed acrylic chambers with internal circulation systems. The effect of fish was determined by comparing substrata from field exclosures with fish absent to substrata exposed to fish. Total path model-explained variance was greatest for ER (R2=0.55) and least for ammonium uptake (R2=0.36) and GPP (R2=0.34). Fish decreased algal biomass and directly increased all biogeochemical rates. The relative importance of the different abiotic ecosystem properties varied by process; however, FBOM and substrata size were important for most rates. This study provides evidence for primarily direct effects of fish on both stream structure and function in a prairie stream. Our results emphasize that interactions between biotic and abiotic factors should be considered when determining drivers of biogeochemical activity .

ABSTRACT:

We are quantifying the benefits of watershed-scale agricultural conservation practices that may reduce nutrient runoff to downstream freshwaters as a part of the USDA Regional Conservation Partnership Program (RCPP) and the Indiana Watershed Initiative (IWI). We have monitored Shatto Ditch Watershed (SDW; Kosciusko Co, IN) since 2012 to assess the effects of watershed-scale planting of winter cover crops on 62-68% of croppable acres and the construction of 7 km of two-stage ditch. In Spring 2015, we added a second watershed, the Kirkpatrick Ditch Watershed (KDW; Jasper Co, IN), where we increased winter cover crop acreage to 13-32% of the croppable acres and monitored an existing 1 km of two-stage ditch. We collected water samples every 14 days from representative tile drains (n=6-25) and the base of the watershed at both SDW and KDW to quantify the effects of cover crops on nitrogen and phosphorus export. We collected soil samples from a subset of fields with and without cover crops in each watershed seasonally after cash crop harvest in the fall and before cover crop termination in the spring. Soil samples were analyzed for water extractable phosphorus, nitrate, ammonium, and soil organic matter.