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

Abstract

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 < .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−2) 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−2 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.

Role of Lakes, Flood, and Low Flow Events in Modifying Catchment‐Scale DOC:TN:TP Stoichiometry and Export

“…A relative nutrient‐to‐C limitation decreases C‐use efficiency, influencing inorganic C fixation or organic C excretion by autotrophs, while heterotrophs respire more C (Hessen et al., 2004). The carbon:nitrate ratio, for instance, impacts nitrate accumulation in aquatic systems by regulating microbial processes that couple organic carbon and nitrate cycling (Taylor et al., 2020). Phosphorus is often limiting, and can be removed by biological uptake (Elser et al., 2009) or burial in lakes (Maranger et al., 2018), consequently reducing downstream P flux.…”