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

Authors: Taylor, Jason; RODMAN, ASHELY - Us National Park Service; SCOTT, JEFFERSON - Baylor University

Submitted to: Freshwater Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/10/2019
Publication Date: 12/27/2019
Citation: Taylor, J.M., Rodman, A.R., Scott, J.T. 2019. Stream algal biomass response to experimental phosphorus and nitrogen gradients: a case for dual nutrient management in agricultural watersheds. Freshwater Science. 49:140–151. https://doi.org/10.1002/jeq2.20039.
DOI: https://doi.org/10.1002/jeq2.20039

Interpretive Summary: Excess nutrients from wastewater and agricultural runoff globally threaten freshwater and marine ecosystems. There is widespread recognition that phosphorus enrichment can impact freshwater resources critical to natural ecosystems and human society primarily due to nuisance algae growth. However, as understanding of nutrient impacts on aquatic resources has matured, there is growing evidence that the combined effects of phosphorus and nitrogen pollution can be more detrimental to freshwater ecosystems than phosphorus alone. In this study, researchers from USDA-ARS, the National Park Service, and Baylor University experimentally compared the effects of phosphorus pollution to the combined effects of nitrogen and phosphorus pollution on stream algae growth in the Buffalo River, a federally-protected river and a state-designated Extraordinary Resource Water and Natural and Scenic Waterway. The study found that algae growth responses to the combined influence of excess nitrogen and phosphorus was 3 times greater than growth responses to phosphorus alone. These results support the need for management of both nitrogen and phosphorus runoff in watersheds experiencing agricultural intensification and can be used to inform best management practices for nutrients within the Buffalo River

Technical Abstract: We experimentally tested the role of phosphorus (P) and nitrogen (N) in limiting stream periphyton biomass by conducting a 31-day streamside mesocosm experiment within the Buffalo National River Watershed in Arkansas. Cobbles transplanted from two different tributary streams within the watershed were exposed to a range of P treatments (0, 0.012, 0.025, 0.05, 0.1, and 0.2 mg/L P) and N (0, 0.12, 0.25. 0.5, 1.0, and 2.0 mg/L N) during P-only and N + P enrichment periods to assess responses of benthic chlorophyll-a (chl. a) and ash-free dry mass (AFDM). Mean AFDM was significantly higher on day 31 of the N + P enrichment when compared to day 17 of the P-only enrichment (p < 0.001). Significant differences in AFDM, but not chl. a, were observed between cobbles from different stream sites. Phosphorus enrichment stimulated benthic chl. a biomass, but enrichment effects were significantly greater when streams were enriched with N + P (p < 0.001). Chlorophyll-a ranged from 4.4 to 57.9 mg/m2 and increased with increasing P concentrations (p < 0.001) after 17 days of P enrichment. However, mean chl. a biomass was three-fold greater after 14 additional days of N + P enrichment, and all excess N was immobilized, resulting in chl. a biomass that ranged from 13.3 to 171.1 mg/m2 across a gradient of increasing N + P. Results support the need for controlling both N and P in freshwater systems to avoid excessive algal biomass accrual and provide insight into how potential increases in nutrient loading associated with changing landuse patterns may influence accumulation of nuisance algae within the Buffalo National River Watershed.