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−1 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