Cyanobacterial harmful algal blooms (cyanoHABs) are the most common form of harmful algal blooms in freshwater systems. Cyanobacterial blooms can negatively affect human health, the environment, and the economy. Satellite data has proven to be an effective monitoring tool for cyanobacteria in inland lakes. A recent study analyzed how often cyanobacterial blooms occur in over 2,000 lakes across the United States. National results confirmed previously observed patterns with occurrence peaking in late-summer and early-fall. Southeastern regional results, however, differed from the national response. Cyanobacterial occurrence in the southeast peaked over the winter months contradicting our previous understanding of bloom dynamics. This study explores this anomaly by first categorizing over 100 Florida lakes by their bloom seasonality before assessing the primary drivers of cyanobacterial blooms across each category. Monthly satellite estimates of cyanobacterial occurrence were obtained from the European Space Agency Sentinel-3A Ocean and Land Colour Imager (OLCI) for 2016 through 2019 and summarized as the median cyanobacterial occurrence for each lake. A temporal clustering algorithm was used to classify each lake based on meteorological seasons. A random forest was generated for each temporal cluster and variable importance was assessed using environmental parameters as drivers.
Results/Conclusions
115 Florida lakes were grouped into six distinct temporal seasonalities, two of which exhibit winter bloom phenology. The majority of lakes peaking in the wintertime were located within the Central Florida Ridgelands and Uplands EPA Level IV Ecoregion. A random forest algorithm considered the following environmental parameters, using a combination of remotely sensed and field observations: meteorological conditions (air temperature, incoming solar radiation, precipitation), hydrologic parameters (lake depth, water temperature), and agricultural characteristics. Lake depth was the primary driver of cyanobacterial occurrence for four of the six temporal clusters. The temporal cluster that characterized the highest number of lakes and exhibited strong winter bloom tendencies, was driven primarily by incoming solar radiation and air temperature. While this study focused on Florida lakes, satellite observations suggest that several inland lakes across the southeastern United States, spanning from North Carolina to Alabama, are characterized by cyanobacterial peaks throughout the winter months. Our previous understanding of cyanobacterial bloom dynamics has assumed that blooms are not present, or are reduced in biomass, during the winter months. However, results presented here suggest otherwise, providing a more accurate representation of cyanobacterial bloom dynamics in the humid, subtropical climate of the southeastern United States.