Biological response of phytoplankton community structure after warm-water anomalies conditions and its implications on Mexican Pacific coastal waters.

Different hydrographic events impacted the pelagic ecosystem of the California Current during 2013 to 2016, drastically reducing the phytoplankton biomass available for higher trophic levels. In the fall of 2013 a large warm temperature anomaly (WWA) developed in the upper ocean along the axis of the North Pacific Current, by the end of 2015, this WWA stretched from Alaska to Baja California, Mexico. In the early summer of 2015, a weak-to-moderate El Niño led to an above-average Sea Surface Temperature (SST) across the Equatorial Pacific. SST anomalies were the warmest ones recorded in the last 13 years in the California Current. The aim at this work was to compare 2017 and 2019 phytoplankton community structure during with previously recorded data during WWA conditions. Data from four campaigns at two seasons (spring and autum) in Todos Santos Bay, Baja California, were used. Niskin bottles sampling at different depths and Utermöhl counting method for identified cells were used. Hydrographic condition was documented using a CTD RBR maestro 6Hz equipped with a fluorescence sensor (Turner) and light sensor (PAR). We observed Chlla concentration from 6 to 12 µg l-1 (ten time higher than 2014-2015). Phytoplankton community during the WWA were dominated by small cells (> 5 µm), surface waters in this study were dominated by dinoflagellates (70%) (Ceratium sp, Scrippsiella trochoidea, Gyrodinium sp., Prorocentrum sp. highlighting P. micans and P. gracile), diatoms (30% relative abundance) Chaetoceros sp., Corethron hystrix, Haslea wawrikae and Pseudo-nitzschia sp. The domain of the dinoflagellates is more evident at depth, having a percentage between 90% and 70% in most of the stations. In this study we have seen how the population has changed, with the larger cells having dominance among others that were already common before these warm phenomena showing. Our results suggest an apparently recovery of the community where harmful phytoplankton has benefited.