The effect of catchment urbanization on nutrient uptake and biofilm enzyme activity in Lake Superior (USA) tributary streams

Abstract

We compared landscape, habitat, and chemistry variables with nutrient spiraling and biofilm extracellular enzyme activity (EEA), to assess the response of nine Lake Superior tributaries to the level of urbanization in their catchments. We found no significant correlation between uptake metrics for NH4+ and PO43− and the level of catchment urbanization. NH4+ and PO43– uptake velocities (Vf) were, however, positively correlated with biofilm EEA and with biofilm respiration (DHA). In general, biofilm EEA was negatively correlated with indicators of increased urbanization (e.g., % impervious surface cover and Cl− concentration) and positively correlated with % forest cover. Biofilm respiration measured as dehydrogenase activity (DHA) decreased with indicators of increased urbanization (e.g., % ISC, storm sewer length, % of the stream channel shaded by the riparian canopy, and Cl−) and increased with non-urban indicators (e.g., % forest, % wetland, and stream width and depth). Regression of Vf and uptake rate (U) versus ambient nutrient concentrations indicated phosphorus limitation in the study streams, a result supported by regression biofilm peptidase versus phosphatase activity. There was no evidence of NH4+ saturation or limitation. This is the first study to demonstrate correlations between nutrient uptake and biofilm EEA in streams, with linkages to catchment-scale disturbances.