Effects on land use on trophic structure in agricultural U.S. watersheds

There is much interest among landscape and watershed ecologists in understanding the link between landscape pattern and stream trophic processes.  We used geographic, nutrient, biological and landscape data from 15 ecoregions across the U.S. to study effects of nutrient enrichment on stream ecosystems.  The data are from a 4-year study conducted by a U.S.G.S. National Water Quality Assessment program that focuses on stream ecosystems in eight agricultural study units across the U.S representing a broad spectrum of environmental settings (climate, geology, topography, and soils).

Nutrients, algae and benthic invertebrates varied within and between study units. Nutrients and algae were strongly positively correlated with agricultural land use in both the basin and stream buffer (0.2 to 0.6) while primary invertebrate consumers were more strongly correlated with local habitat variables (macrophyte cover, substrate). We constructed a set of Structural Equation Models to examine direct and indirect causal effects between land use, natural conditions, nutrients, algal biomass, and invertebrate grazers and predators.  Initial results suggest that % agriculture in the stream buffer and % basin forested cover strongly effect stream nutrients and that the effect of nutrients on benthic algae was relatively stronger then the effect of grazers.