Satellites predict lakes at risk from cyanobacteria and microcystin toxins.
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Cyanobacterial harmful algal blooms (HABs) can produce toxins that impair freshwater ecosystems used for drinking water, recreation, and habitat for aquatic biota. With HABs increasing in extent and intensity globally, water managers need information about how to prioritize lake monitoring. Remote sensing is a promising monitoring tool, but a challenge is connecting satellite data with field bloom conditions across a broad spatial extent. We combined Cyanobacteria Assessment Network satellite data of lakes between 2008 and 2011 with HAB field data from the USEPA’s National Lakes Assessments conducted in 2007 and 2012. There were 210 lakes with both satellite data and were surveyed for microcystin toxin, cyanobacteria, and chlorophyll a. We calculated the mean summer bloom magnitude for each satellite-resolved lake and used logistic regression to estimate the likelihood of each lake exceeding the human health risk thresholds of 1.0 µg/L microcystin, 100,000 cells/mL cyanobacteria, or 50 µg/L chlorophyll a. In all cases, a unit increase in the lake bloom magnitude was associated with a 15-40% likelihood increase in exceeding HAB thresholds. We used the modeled relationships to predict the likelihood of threshold exceedance for all 2,192 lakes monitored by satellite. Among these lakes, 37-69 were highly likely (>75% likelihood) to exceed the thresholds. This approach leverages existing national scale data to identify lakes that are at risk for exceeding bloom thresholds that is both geographically comprehensive and grounded in field data. Such analyses are critical for connecting satellite information with field conditions to aid setting monitoring priorities for lake management.