Stream algal biomass response to experimental phosphorus and nitrogen gradients: A case for dual nutrient management in agricultural watersheds.

Watershed managers generally focus on P reduction strategies to combat freshwater eutrophication despite evidence that N co-limits primary production. Our objective was to test the role of P in limiting stream periphyton biomass within the Buffalo River watershed in Arkansas by conducting a 31-d streamside mesocosm experiment. To represent potentially different starting states, cobbles were transplanted from two different tributary streams and initially exposed to a range of P (0, 0.012, 0.025, 0.05, 0.1, and 0.2 mg L P) to assess benthic ash-free dry mass (AFDM) and chlorophyll-a (chl a) and responses during a P only enrichment period. Later, the experiment was continued under a N/P (10:1 molar ratio) enrichment gradient to examine co-limitation. Mean AFDM was higher on Day 31 of the N+P enrichment compared with Day 17 of the P-only enrichment (p < .001). Overall differences in AFDM and chl a were observed between cobbles from different stream sites. Phosphorus enrichment stimulated benthic chl a biomass, but enrichment effects were greater when streams were enriched with N+P (p < .001). Chlorophyll-a increased (4.4-57.9 mg m ) with increasing P concentrations (p < .001) after P enrichment but was threefold greater after N+P enrichment, increasing from 13.3 to 171.1 mg m across the enrichment gradient. Results support the need to consider both N and P limitation in freshwater systems and demonstrate that potential increases in nutrient concentrations may influence accumulation of algae on cobble substrates from the Buffalo River watershed.