Dinoflagellate Blooms and Spinning Fish Coincide with Sargassum Inundation, Nitrogen Enrichment and Ocean Warming in the Florida Keys, USA

The Florida Keys archipelago in southernmost Florida is reknown as the third longest barrier coral reef in the world and its popularity as a sport fishing destination. Beginning in the 1980s, phytoplankton and macroalgae blooms became evident in nearshore waters, and increased dramatically in the 1990s when water managers increased freshwater flows from Lake Okeechobee south to Florida Bay and the Florida Keys. Increasing nutrients from human waste, along with non-point source nutrients (mostly nitrogen) from the Everglades, contributed to HABs, die-off of seagrasses and hard corals, and the designation of the Florida Keys as a Dead Zone in 2003. In fall of 2023 and continuing into spring 2024, observations of unprecedented “spinning fish” behavior and mortality events were reported for the Lower Keys on social media and local news outlets. By July 31, 2024, at least 54 critically endangered small-tooth sawfish had died. A monitoring response by local anglers, the Florida Fish and Wildlife Conservation Commission and various research groups indicated over 80 species were impacted, including commercially-important fishes and crustaceans such as spiny lobster (Panulirus argus) and stone crab (Menippe mercenaria). The research revealed unprecedented blooms of benthic dinoflagellates, including Gambierdiscus, Prorocentrum, Ostreopsis, and Coolia, as well as a complex “cocktail” of toxins that included ciguatoxins, gambierones, and okadaic acid. Additional monitoring showed that the dinoflagellate bloom coincided with extremely high total dissolved nitrogen (TDN), TDN:TDP ratios, warm winter temperatures, and recurring Sargassum inundation events that could have seeded the Lower Keys with increasing densities of these dinoflagellates.