One and done? Managing a cyanobacterial bloom in a shallow seepage lake in the North Carolina Coastal Plain

Lauritsen DD. 2025. One and done? Managing a cyanobacterial bloom in a shallow seepage lake in the North Carolina Coastal Plain. Lake Reserv Manage. 41:239–259. White Lake, a small, clearwater, historically acidic and low nutrient natural lake, has had increases in pH and total nitrogen (TN) that reflect recent changes in rainfall chemistry in the region. In the fall of 2017, a filamentous cyanobacterial bloom began, and the invasive weed hydrilla was found in >80% of the lake. An alum treatment to mitigate the bloom and reduce P levels was recommended by science advisors. A rapid increase in pH in the days prior to the treatment start was the root cause of a fish kill during the period when the lake pH was >9. By 8 weeks post treatment, filamentous cyanobacteria biovolumes were significantly reduced and total phosphorus (TP) levels dropped 66%, and these reductions have been sustained; phosphorus levels were comparable to historical levels, and the same was true of taxa dominating phytoplankton biovolumes (generally desmids). Rainfall was a diffuse source of dissolved N and P, with lake TN/TP (mass) ratios close to rainfall TN/TP ratios. Naturally occurring Al was more abundant than Fe in the muddy sediments of White Lake and nearby Lake Waccamaw, and no significant anoxic soluble reactive phosphorus (SRP) release was found in either lake. White Lake pH levels have generally remained between 6.2 and 7.2 since the treatment and little to no hydrilla has been seen, although conditions that can foster cyanobacterial blooms may again be seen in this more productive shallow lake. Sustained monitoring is critical to inform the type and timing of any future management actions.