Remotely Sensed Cyanobacterial Intensity Predicts Risk for Lake Blooms and Toxins across the Contiguous U.S.

Abstract:

Cyanobacterial harmful algal blooms (CHABs) can produce toxins that impair freshwater ecosystems used for drinking water, recreation, and aquatic habitat. Field monitoring HABs is resource intensive and water managers need timely information about how identify at risk lakes. Recent advances in remote sensing of CHABs has increased the number of lakes monitored but clear linkages with in situ cyanotoxins must be developed. We tested if a remotely sensed metric of summer (Jun-Sep) bloom magnitude (cells/mL) calculated from the Cyanobacteria Assessment Network (CyAN) predicts risk for detecting the toxin microcystin in ~150 synoptic samples from the 2012 and 2007 US EPA National Lakes Assessments (NLA).

Remotely sensed summer mean bloom magnitude provided a good indicator of risk of toxin levels in NLA field data. Among the lakes with a mean summer bloom magnitude less than the World Health Organization (WHO) low risk category (≤20,000 cells/mL), microcystin was detected (>0.1 μg/L) in 32% and 8% of samples collected in the 2012 and 2007 NLA, respectively. For lakes with a bloom magnitude above the WHO high risk category (>100,000 cells/mL), microcystin was detected in 71% and 63% of samples.

We also found agreement between summer bloom magnitude and other in situ NLA measures of HABs. For lakes with a bloom magnitude below the WHO low risk category, in situ cyanobacteria cell density was consistent with this category 76% and 92% of the time in the 2012 and 2007 surveys, respectively, and 34% and 12% of time for lakes in the high-risk category. For lakes with a bloom magnitude below the WHO low risk category, chl a samples were consistent with this category (≤10 μg/L for chl a) 76% and 92% of the time in the two surveys and 34% and 12% of time for lakes in the high-risk category (>50 μg/L).

The broad agreement between remotely sensed bloom magnitude and detectable microcystin, cyanobacteria cells, and chl a illustrates the promise of such tools for prioritizing limited resources for field monitoring. Of the 2,191 lakes resolvable by CyAN, 35% have a mean summer bloom magnitude above the WHO high risk category for cyanobacterial cell density. Our work supports the use of the CyAN-derived bloom magnitude metrics to prioritize monitoring in these high-risk lakes.