Phytoplankton Distribution Across an Anticyclonic Eddy in the Gulf of Mexico

Abstract In the Gulf of Mexico, the Loop Current Eddies (LCEs) are highly dynamic mesoscale structures that significantly influence the physicochemical characteristics of the water column. These anticyclonic eddies modify environmental factors such as solar light penetration and nutrient availability, thereby affecting the growth, structure, and distribution of the phytoplankton community. This study was conducted in the oceanic region of the Gulf of Mexico to characterize the chemotaxonomic composition and size classes of phytoplankton across a young LCE. The analysis employed high‐performance liquid chromatography along with the CHEMTAX algorithm and complementary phytoplankton light‐absorption coefficients. These approaches revealed significant physiological adaptations in the phytoplankton assemblage related to environmental gradients within the oligotrophic LCE waters and the ecological preferences of its components. The dominance of pico‐ and nano‐sized groups, such as Prochlorococcus and haptophytes (responsible for about 70% of total chlorophyll‐ a ), highlighted the competitive advantage of these groups over micro‐sized cells (e.g., diatoms) in the warm, nutrient‐poor waters of the anticyclonic eddy. Furthermore, the vertical distribution of phytoplankton groups across different layers of the euphotic zone (from the surface to 0.1% surface irradiance) demonstrated the high level of ecological specialization of phototrophic cells, reflected in their diverse physiological and functional traits suited to thriving in this oligotrophic ecosystem. These findings offer high‐resolution observations that help improve parameterizations of phytoplankton adaptation processes in ecological models driven by environmental variability within mesoscale‐dominated ecosystems.