Biological trait responses of river macroinvertebrate assemblages to a phosphorus gradient.

Phosphorus is the most important nutrient driving anthropogenic eutrophication of inland fresh waters. Several river basins in the Ozark Highlands and Boston Mountains ecoregions of central North America have elevated concentrations of total phosphorus, due to both point-source discharges and nonpoint source runoff in their catchments. Benthic macroinvertebrate responses, expressed as density and biomass of biological trait groups (functional feeding group, voltinism), were modeled across a steep phosphorus gradient spanning 35 river locations. Biomass and density increased across the gradient, and communities shifted from diverse, insect-dominated communities to communities dominated by small, multivoltine taxa (such as Chironomidae) and benthic algal grazers, particularly pleurocerid snails. These shifts are likely related to increased benthic primary production and supply of phosphorus to small-bodied consumers with high phosphorus demand (under the growth rate hypothesis). These results imply that phosphorus enrichment can have significant effects at multiple trophic levels in river ecosystems.