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 15 N-ammonium (NH4+) enrichments to determine NH4+ regeneration and potential uptake rates in the water column. Potential NH4+ 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 similar to 20-60% of potential NH4+ demand during the height of the bloom. Basinwide internal NH4+ regeneration during the April-October period could supply NH4+ 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.