Phytoplankton Responses to Mesoscale and Submesoscale Processes in a Tropical Meander

Abstract

The mesoscale activity of western boundary currents can induce changes in the oligotrophic conditions of the oceanic subtropical gyres. Off SE Brazil, recurrent meanders in the Brazil Current can modulate the pelagic ecosystem through eddy-induced upwelling, as well as drive coastal upwelling and cross-shelf exchanges. Despite their relevance, direct measurements of such meanders are scarce. At the end of the Spring of 2019, we acquired measurements of both physical and biogeochemical properties along a transect through one of these features, with 5–10 km spacing between stations. Displacements of the isopycnals at the meander edges arose from submesoscale filaments along the meander's rim, associated with the signature of shelf water masses. Chlorophyll distributions along the transect indicated phytoplankton responses to the different dynamic regimes in the meander: eddy stirring in the surface mixed layers versus eddy pumping in the subsurface waters, and mesoscale dynamics in the meander center versus submesoscale dynamics in its periphery. Subsurface phytoplankton biomass was enhanced in the meander center, likely due to eddy pumping raising the nutricline into the euphotic zone. Eddy pumping also appears to have contributed to a shift in the phytoplankton community composition to larger organisms. Our results indicate that the physical dynamics create different habitats across the meander, leading to spatial patchiness in the chlorophyll distributions, with different types of organisms in distinct parts of the meander.