Patterns in sources and forms of nitrogen in a large eutrophic lake during a cyanobacterial harmful algal bloom

Western Lake Erie experiences an annual, toxic cyanobacterial harmful algal bloom (cyanoHAB), primarily caused by excess anthropogenic inputs of nitrogen (N) and phosphorous (P). Because the non-N fixing cyanobacteria species Microcystis dominates these blooms, N availability is hypothesized to play a central role in cyanoHAB progression, as well production N-rich toxin microcystin. Many previous studies focused on nitrate because it most abundant substrate during initiation. However, recent work implicated reduced substrates like ammonium dissolved organic (DON) promoting greater biomass longevity. To examine relative importance oxidized phytoplankton different stages, we measured concentrations natural abundance δ15N isotope values throughout entirety 2020 Erie. The results provide first data DON dynamics composition Erie, suggest that phytoplankton, including Microcystis, likely relied regenerated from pool later stages. In addition, stable confirm delivered via Maumee River growth production.