Evaluating sediments as an ecosystem service in western Lake Erie via quantification of nutrient cycling pathways and selected gene abundances

Abstract Lake Erie experiences annual summer cyanobacterial harmful algal blooms (HABs), comprised mostly of non-nitrogen-fixing Microcystis, due to excess nitrogen (N) and phosphorus (P) inputs (eutrophication). Lake Erie's watershed is mostly agricultural, and fertilizers, manure, and drainage practices contribute to high nutrient loads. This study aimed to clarify the role of western Lake Erie sediments in either exacerbating or mitigating conditions that fuel HABs via recycling and/or removal, respectively, of excess N and reactive P. Sediment-water interface N and orthophosphate (ortho-P) dynamics and functional gene analyses of key N transformations were evaluated during a dry, low HAB year (2016) and a wet, high HAB year (2017). On average, western basin sediments were a net N sink and thus perform a valuable ecosystem service via N removal. However, sediments were a source of ortho-P and chemically reduced N. Western basin sediments can theoretically remove 29% of average annual watershed total N loading. Denitrification rates were lower during the high (2017) versus low bloom year (2016), suggesting that high external N loading and large HABs inhibit the capacity of sediments to perform that ecosystem service. Despite being a net N sink on average, western basin sediments released ammonium and urea, chemically reduced N forms that are energetically conducive to non-N-fixing, toxin-producing cyanobacterial HABs, especially during the critical period of low external loading and high biomass. These results support other recent work highlighting the urgent need to include N cycling and internal load dynamics in ecosystem models and mitigation efforts for eutrophic systems.