The soundscape of an unusual dinoflagellate bloom in the Gulf of Maine

In April–September 2023, an unusual dinoflagellate Tripos Muelleri bloom persisted in nearshore and offshore waters of the Gulf of Maine. Cell counts exceeded normal baselines by a factor of 10–50, and the bloom persisted for 5 months instead of the typical 4 weeks in the month of September. Tripos muelleri does not produce harmful toxins and naturally occurs in the Gulf of Maine, but high biomass blooms like this one have the potential to cause low oxygen conditions when they decay leading to negative ecosystem impacts. Dense phytoplankton blooms that sink to the seafloor are consumed by bacteria that can create hypoxic conditions affecting fish, shellfish, lobster, and higher trophic level predators; thus a change in the vocal presence and feeding behavior of local predators related to the bloom could potentially alter the soundscape. This work assessed the potential of passive acoustic soundscape metrics as indicators of change in ecosystem dynamics in association with the unusual dinoflagellate bloom. Metrics of sound amplitude, impulsiveness, and uniformity were explored in conjunction with temperature, salinity, dissolved oxygen, carbon dioxide, and volume backscatter. Results are interpreted in relation to the broader impacts on ecosystem function, grazers, and marine mammals.