Is simultaneous arrival of pelagic Sargassum and Physalia physalis a new threat to the Atlantic coasts?

Abstract

The massive influxes of pelagic Sargassum and Physalia physalis have become an increasingly recurrent phenomenon on the Atlantic coasts, affecting the economy and the structure of coastal ecosystems. For the first time, a study assesses the simultaneous arrival of these pelagic organisms. This study was conducted from June/2019 through June/2021 on the littoral of La Habana, one of the circulation points of the currents that form the North Atlantic Subtropical Gyre (NASG) run. Transects of 40 m were located parallel to the shoreline, the biomass of pelagic Sargassum was weighed, and the number of colonies of P. physalis was counted at the intertidal zone. The biomass of pelagic Sargassum was estimated as dry biomass. The simultaneous arrival of pelagic Sargassum and P. physalis was reported. Simultaneous arrivals of these pelagic species were recorded in the winter seasons, with the occurrence of cold fronts, low mean temperatures (22–27 °C), and strong northerly winds. Most months with the arrival of these pelagic species coincided with a negative average magnitude of the Arctic Oscillation Index, which favors the occurrence of cold fronts and northerly winds. The mean landing dry biomass of Sargassum during the peak months was low (0.73 ± 0.54 kg/m2) compared to the Mexican Caribbean. 145 P. physalis colonies over 100 m of coast length per year were reported during the study period. The higher visual occurrence of Sargassum natans I and the higher percentage of left-handed P. physalis colonies (56.16 ± 3.37) may indicate that the NASG area, which encloses the Sargasso Sea, could be the primary source of arrivals to La Habana littoral. As reported, the distribution of sightings of pelagic Sargassum and P. physalis coincided in several regions in the Atlantic Ocean and represents an urgent call for coordinated monitoring and development of predictive forecasting of beach landings. This work suggests that there are Atlantic coastal sites such as La Habana littoral that could host the dangerous simultaneous arrivals of pelagic Sargassum and P. physalis. Finally, the use of remote sensing techniques with in situ observations is considered important for future work, since using remote sensing techniques alone seems to miss important events such as those documented in this study.