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 as production of the N-rich toxin microcystin. Many previous studies focused on nitrate because it is the most abundant N substrate during bloom initiation. However, recent work implicated reduced N substrates like ammonium and dissolved organic N (DON) in promoting greater bloom biomass and longevity. To examine the relative importance of oxidized and reduced N substrates to phytoplankton during different bloom stages, we measured concentrations and natural abundance delta N-15 isotope values of dissolved N substrates and phytoplankton biomass throughout the entirety of the 2020 cyanoHAB in Western Lake Erie. The results provide the first data on DON dynamics and composition in Western Lake Erie, and suggest that phytoplankton, including Microcystis, likely relied on N regenerated from the DON pool in later bloom stages. In addition, the stable isotope data confirm the importance of nitrate delivered via the Maumee River to cyanobacterial growth and toxin production.