Phytoplankton Communities in the Eastern Tropical Pacific Ocean off Mexico and the Southern Gulf of California During the Strong El Niño of 2023/24

This paper analyzes phytoplankton communities in the Eastern Tropical Pacific Ocean off Mexico (ETPOM) and the Southern Gulf of California (SGC) during the strong El Niño event of 2023/24. A multidisciplinary research cruise was conducted in the winter of 2024, during which high-resolution hydrographic data and water samples for phytoplankton cell determinations were collected at 33 sites. Additionally, satellite data were obtained to evaluate sea surface temperature and chlorophyll-<i>a</i> levels. A total of 269 phytoplankton species were identified, comprising one hundred and fifty diatoms, one hundred and twelve dinoflagellates, five silicoflagellates, one ciliate and one cyanobacteria. The dominant species included the diatom <i>Pseudo-nitzschia pseudodelicatissima</i>, the dinoflagellate <i>Gyrodinium fusiforme</i>, the silicoflagellate <i>Octactis octonaria</i>, and the ciliate <i>Mesodinium rubrum</i>. The cyanobacterium <i>Trichodesmium hildebrandtii</i> was also identified. In terms of total abundances, diatoms were the most prevalent, with 224,900 cells L^-1, followed by dinoflagellates at 104,520 cells L^-1, ciliates at 20,980 cells L^-1, cyanobacteria at 1760 cells L^-1, and silicoflagellates at 1500 cells L^-1. Notably, interesting differences emerged in species richness and abundance when comparing both regions. These results enhance our understanding of phytoplankton dynamics associated with strong El Niño events. The ETPOM remains a region that requires further monitoring through in situ observations.