Phytoplankton assemblage in a cyclonic eddy confined to a coastal region of the Gulf of California

This study characterizes the phytoplankton assemblage within a cyclonic eddy confined in the Bay of La Paz, Gulf of California, integrating high-resolution hydrographic data and water samples collected in August 2017 complemented by satellite observations. The eddy featured a distinct cold core with elevated chlorophyll- a concentrations and supported a diverse phytoplankton community of 151 species, dominated by dinoflagellates (37,780 cells L -1 ) and diatoms (35,780 cells L -1 ). Notable taxa included Planktoniella sol , Pseudo-nitzschia subfraudulenta , Coscinodiscus asteromphalus , Protoperidinium punctulatum , Gyrodinium fusiforme , Protoperidinium robustum , Dictyocha fibula, Mesodinium rubrum , and Trichodesmium hildebrandtii . Horizontal distribution patterns revealed significant spatial heterogeneity, with species composition diverging between the eddy core and periphery in response to temperature and density gradients. Multivariate analysis identified temperature as the primary driver of community abundance. Furthermore, our findings suggest that a previously undocumented predator-prey dynamic between autotrophic and heterotrophic species may exert an additional influence on these patterns. These findings clarify how cyclonic eddies structure phytoplankton communities and provide critical insight into regional primary production, biogeography, and the distribution of species, some of which capable of forming harmful algal blooms. • This study identified a total of 151 phytoplankton species. • The circulation pattern was characterized by a well-defined cyclonic eddy. • Some species were found to be more abundant near the center of the eddy. • Other species were more abundant at the periphery of the eddy.