Predicting Toxic Algal Bloom Risk in 124,500 lakes across the Conterminous United States using Lake and Landscape Drivers of Water Quality
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The presence of cyanobacterial harmful algal blooms (cyanoHABs) in freshwater lakes across the United States poses a serious threat to ecosystems and human health. National-scale datasets allow for engagement with broad spatial patterns such as watershed and waterbody specific characteristics that can increase or diminish the risk of cyanoHABs. With spatially-explicit regression models, we use lake morphology, watershed nutrient input, land cover, and geographic and meteorological data to predict cyanobacteria abundance and the probability of the cyanotoxin microcystin in approximately 124,500 US lakes. Approximately 65% of lakes (81,535) were predicted to exceed a high threshold of 100,000 cyanobacterial cells/mL at least once during the summer, and 15% (18,779) had a high probability of detectable microcystin during this same period. Many of these predictions were expected based on anthropogenic nutrient inputs. Inconsistent with our expectations, 36% of the high cyanobacteria predictions and 7% of likely microcystin detections occurred in watersheds with low nutrient inputs. Within these subsets of lakes, we investigated other drivers influencing cyanoHABs beyond anthropogenic nutrient inputs and the model covariates. Lakes with low nutrient inputs and high cyanoHAB risk were observed in watersheds with higher drainage ratios (watershed:lake area) than other lakes. Lakes with higher cyanobacteria risk, regardless of nutrient inputs, also had higher lake area:depth ratios than low cyanobacteria lakes, providing additional insight the increased risk associated with shallower lakes. This research can help determine how watershed and lake characteristics, in addition to nutrient supply, may inform cyanoHAB bloom mitigation efforts.