Periphyton response to simulated nonpoint source pollution: local over regional control

We investigated functional and structural responses of periphyton communities to simulated nonpoint source (NPS) pollution over a 2-year period. Periphyton communities were examined in three unique but hydrologically connected habitats in the lower Muskegon River: river, wetland, and lake. Our study addressed several major questions: (1) what are the effects of nonpoint source pollutants on periphyton structure and function, (2) to what degree do environmental conditions, including the pollutant delivery system and hydrologic conditions, affect periphyton response, and (3) do periphyton communities show synchronous responses to nonpoint source pollution? Nutrients were added over a 21-day period in each of the four seasons over 2 years. The effect of road salt on metabolism was evaluated in the winter and spring seasons in the river habitat only. In general, the periphyton community showed complex responses to the NPS amendments, but there was little evidence of nutrient limitation in this system based on community structure, biomass accrual, metabolism, or alkaline phosphatase activity. This result was unexpected given the relatively low ambient concentrations of phosphorus. We discuss the possibilities that nutrient delivery was ineffective or that herbivory and/or irradiance might have constrained periphyton growth, but ultimately conclude that our results suggest that periphyton communities in the lower Muskegon River watershed were (1) nutrient-replete, (2) tolerant of short-term salt additions, and (3) influenced more by local conditions (habitat) than by regional (watershed) factors. In systems where nonpoint source pollutant delivery is diffuse and hydrologic residence time is short, the influence of nonpoint source pollutants on periphyton may be either modest or too difficult to detect using traditional endpoints.