The differential distribution of diatoms and dinoflagellates in the Bay of La Paz, Gulf of California, Mexico, was analyzed in summer of 2009, when a cyclonic eddy confined in the bay dominated the circulation. An uplift of the nutricline in the eddy drove high concentrations of nutrients to the euphotic layer. A differential phytoplankton distribution was observed to be associated with the eddy: there was an abundance of dinoflagellates close to the center of the cyclonic eddy, whereas diatoms were more abundant at the periphery. A significant inverse correlation (R = −0.62, p < 0.002) was found between the temperature at 25 m depth and the dinoflagellates abundance. Based on the temporal evolution of chlorophyll measured by MODIS satellite images, and a conceptual model proposed for the lifecycle of eddies, the cyclonic eddy may have been an old decaying structure. The effect of the cyclonic eddy on the phytoplankton distribution in this small semienclosed region was apparently similar to that found in larger eddies in the open ocean, but this is the first time such a differential distribution has been found associated to a confined eddy.

During 2015–2016, a strong El Niño event, nicknamed the “Godzilla El Niño,” occurred in the Pacific Ocean. Using satellite imagery, in this article we assess impacts of this event on sea-surface temperature and chlorophyll-a concentrations in the southern Gulf of California. Daily images of sea-surface temperatures and chlorophyll-a were obtained by satellite from the Moderate Resolution Imaging Spectroradiometer for the period from January 2013 to December 2017. A circular area ≈ 46.8 km in diameter in the central part of the gulf was selected to evaluate monthly variation of both parameters. Hydrographic data generated during a research cruise in November 2016 were used to evaluate water mass distributions. Results revealed strong seasonal variability, with high chlorophyll-a concentrations recorded during winter and low values during summer. Contrary to predictions, the “Godzilla El Niño” event apparently did not have as large an impact on the phytoplankton biomass, expressed as chlorophyll-a...