The role of internal nitrogen loading in supporting non-N-fixing harmful cyanobacterial blooms in the water column of a large eutrophic lake

Western Lake Erie cyanobacterial harmful algal blooms (cyanoHABs) occur every summer as a result of anthropogenic nutrient loading. Although the physiological importance of nitrogen (N) in supporting bloom biomass and toxin production is established, the role of internal N recycling in the water column to support bloom maintenance is not as well understood. Over three field seasons (2015–2017), we collected water from western Lake Erie and employed bottle incubations with 15N-ammonium ((Formula presented.)) enrichments to determine (Formula presented.) regeneration and potential uptake rates in the water column. Potential (Formula presented.) uptake rates followed spatial and seasonal patterns, with greatest rates measured nearest the Maumee River inflow and during peak bloom months (August and September). Regeneration followed a similar spatial pattern but was greatest in early summer (June and July) and supported ~ 20–60% of potential (Formula presented.) demand during the height of the bloom. Basin-wide internal (Formula presented.) regeneration during the April–October period could supply (Formula presented.) at 60–200% of annual external N loading to the western basin. These results help explain how non-N-fixing cyanoHABs in Lake Erie and other large, eutrophic lakes continue producing biomass and N-rich toxins long after spring nutrient loads are exhausted or transported to other areas. Internal N loads are ultimately driven by external N loads; in low precipitation years, external nutrient loads result in smaller blooms, producing less substrate for subsequent internal N loads. Overall, these findings, along with others, confirm that both internal and external N loading must be considered when evaluating cyanoHAB management strategies