The Impact of Legacy Nutrient Loading from Lake Sediments on Cyanobacteria Bloom Severity

Nutrient pollution and cyanobacteria harmful algal blooms (cyanoHABs) are critical challenges shared among surface waters, largely driven by nutrient releases from nonpoint sources. Tools that inform the selection of nutrient source control and/or timing of implementation would further efforts to reduce nutrient pollution and public health impacts. We provide a modeling framework that uses both mechanistic and statistical models for quantifying the relative importance of external and internal phosphorus (P) loads on cyanoHAB severity and identifying subwatersheds with potential upstream legacy stores. We demonstrated the framework using data from a freshwater lake and found that recently added P from the internal load was significant in explaining cyanoHAB severity (24%), more so than recently added P from external sources (1.1%). Using counterfactual scenarios, we found that a 90% reduction in the recently added internal P load would significantly reduce cyanobacteria cell densities, leading to less severe blooms. Notably, we found that the relative importance of the internal and external P loads varied among years, which can infer when nutrient control strategies may be more/less successful. As such, this framework can help identify the most significant source of P across time and space to better inform nutrient source control.