Authors: Israel Medina-Gómez, Miguel Cahuich-López, Ana Aguilar-Trujillo, Giuliana Cruz-Trejo, Miryam Juárez, Ismael Mariño-Tapia, Jorge Herrera-Silveira, Cecilia Enriquez
Chlorophyll-a concentrations were recorded in three oceanographic surveys (GOMEX-4, GOMEX-5, GOMEX-6) during 2015–2018 throughout the Yucatan shelf, southeast Gulf of Mexico (GoM). Such campaigns encompassed the warm-phase, cold-phase, and neutral-phase of El Niño-Southern Oscillation (ENSO), respectively, as well as a progressively increasing upwelling intensification scenario. We undertook the analysis of the Chl-a concentrations in the Yucatan shelf through the multiyear GOMEX series to understand how the interplay between the coastal upwelling, precipitation regime, and wind patterns influence the distribution, magnitude and timing of the phytoplankton biomass. These results suggest that substantial rainfall late summer during ENSO’s warm-phase in GOMEX-4 survey improved the coastward Chl-a (1.69 ± 0.07 mg m−3). This pattern could be associated to the increased cold-front activity usually modulated by the wintertime warm-phase of ENSO. The wind-driven mixing during these cold fronts led to water column mix, which contributed to enhance the distinguishing between coastline and marine Chl-a levels. The coastal upwelling pulses depicted a wide-shelf influence over the Chl-a concentration (0.47 ± 0.01 mg m−3 and 0.53 ± 0.03 mg m−3 average concentrations in the marine and coastal areas, respectively). An onshore excursion of cold water masses intermingled with seasonal precipitation traits sustained high nearshore Chl-a in zones already altered by subaqueous freshwater input in the karstified Yucatan seascape (varying within the interval of 1.50–2.76 mg m−3). These results suggest that, similarly to riverine-influenced areas in the northern GoM, warm-phase ENSO episodes have the potential to alter the phytoplankton biomass also in karstified shelves, as the Yucatan platform. This large-scale pattern overlaps processes occurring over a broad spatial and temporal scale, like seasonal rainfall and upwelling, establishing recurrent environmental gradients driving biogeochemical fronts along the seascape. The relationships found allow a better grasp of sound ecologically processes as phytoplankton blooms along the Yucatan shelf, which might be also sensitive to human-driven disturbances.