Relationship between microcystin and nutrients during harmful algal blooms in South Florida waterways

Despite their unwelcome presence, harmful algal blooms (HABs) have become increasingly familiar to the marine and freshwater communities of South Florida and around the world. Understanding the conditions which promote HABs is a pressing topic for the health of both human and aquatic life. During a HAB, the water becomes contaminated with toxins that vary depending on the algal species and have a range of toxicities and environmental fates. One well-studied biotoxin that has heavily impacted South Florida and the Great Lakes in recent years is the hepatotoxin class microcystin, produced by Microcystis and other freshwater cyanobacteria. A known factor influencing algal bloom occurrence and density is nutrient concentrations, both phosphorus (P) and nitrogen (N); the relationship between nutrients and bloom toxicity is less well-defined, but is critical for the health of communities impacted by HABs. To study the relationship between microcystin concentrations and nutrient concentrations, field samples were acquired from the nutrient-rich waters of Florida’s Lake Okeechobee, down the Caloosahatchee river, and into the estuarine environment of Cape Coral. Samples were collected at five to nine sites on three different days across the intense HAB events that occurred across South Florida in 2018. Water was filtered to collect particulate material for toxin analysis, and the filtrate was analyzed for nutrients. Microcystin was quantified via an extraction in methanol and analysis by liquid chromatography triple quadrupole mass spectrometry, analyzing for eight different microcystin congeners. Nutrient analyses included phosphate (molybdenum blue spectrophotometric analysis), dissolved organic phosphorus (by persulfate digestion followed by phosphate analysis), and nitrate (by NOx box). By analyzing the relationship between nutrients, nutrient ratios and toxin concentrations, we can better understand HAB toxicity dynamics to inform public health concerns about HABs, and potentially inform policies limiting nutrient inputs. Ultimately, this research allows us to better understand the mechanisms that lead to HABs so that we can protect aquatic and human lives in the future.