Influence of mesoscale processes in the phytoplankton community on the chlorophyll maximum in the southern Gulf of California

We hypothesized that mesoscale physical processes, specifically anticyclonic eddies and coastal upwelling, generate distinct vertical phytoplankton community structures at the chlorophyll maximum in the southern Gulf of California. From November 5th to 21st, 2020, an oceanographic cruise was carried out aboard the RV "Dr. Jorge Carranza Fraser" of the Instituto Mexicano de Investigación en Pesca y Acuacultura Sustentable, in the southern Gulf of California. Temperature, salinity, dissolved oxygen, and chlorophyll variables were measured at 54 stations using a conductivity, temperature and depth instrument. We identified the depth of the chlorophyll maximum and obtained an aliquot of water was obtained for phytoplankton identification. We processed satellite images of sea surface temperature and chlorophyll, obtained with the MODIS Aqua sensor (L3 level) with a five-day resolution and 1 km per pixel. Images of geostrophic flux and absolute sea level height were obtained to identify the mesoscale physical processes present in the study area that influence the structure of the phytoplankton community. We analyzed phytoplankton diversity and abundance using standard ecological indices, evaluating the influence of mesoscale features on community structure. We observed surface chlorophyll maxima (less than 10 m) and high abundances of Thalassiosira , Thalassionema , and Chaetoceros at upwelling-influenced coastal stations. Meanwhile, we observed deeper maxima (more than 40 m) and the dominance of Thalassionema and Chaetoceros in oceanic anticyclonic eddies. These niche differentiations of the dominant phytoplankton species provide essential fundamental for applying predictive models of fishery productivity under mesoscale regimes in changing ocean environments due to global warming.