Thin layers are vertically compressed, horizontally extensive, highly concentrated features comprised of plankton and/or particles. They are critical components of the marine ecosystem, likely playing a key role in the life histories and evolutionary trajectories of species found in, or, interacting with them. These structures have been reported in diverse marine environments around the globe. However, the mechanisms of thin layer formation/dissipation in the southwestern Atlantic Ocean were unknown until this contribution. To assess the temporal evolution of thin phytoplankton layers on the inner shelf off Ubatuba, Brazil, we conducted two oceanographic fixed station cruises, including optics, acoustics, and imaging techniques. Over a period of 2 days, three thin layers were observed: within the pycnocline close to the maximum stratification, and below the pycnocline where phytoplankton were affected by enhanced nutrient supply provided by the South Atlantic Central Water (SACW). Changes in regional wind patterns influenced the presence of SACW, which directly affected shear and stratification: the primary physical mechanisms we attribute to thin layer formation in this region. The associated biological mechanisms contributing to thin layer formation were biomass accumulation (in situ growth) and likely the mobility of dinoflagellates. The dominant organisms in the thin layer depths and surroundings, by our in situ imaging system, were cyanobacteria, diatoms, dinoflagellates, and crustaceans. Thin Layers likely have crucial importance for meso‐oligotrophic environments, representing important feeding resources for higher trophic levels.

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The interaction between biological and short-term physical processes affects plankton distribution and composition variability, with consequences for the entire ecosystem. Aspects of the plankton distribution at smaller scales have been understudied, mainly in coastal regions where highly variable conditions promote variations in the biophysical environment. This work aimed to investigate the short-term variations in the distribution of plankton along the inner shelf off Ubatuba, Brazil. We quantified the abundance and diversity of the main planktonic taxa (>100 μm) and their spatiotemporal distribution in winter 2018 and summer 2019 using high-resolution sensors and a shadowgraphic imaging system at cross-shore and fixed stations (12 and 24 h). Summer–winter environmental contrast was evident in the water column stability and plankton abundance. In the summer, higher values of plankton abundance and diversity were obtained. Our results suggested that spatiotemporal variations related to wind-induced mixing and water mass intrusions lead to variabilities in plankton populations on a time scale of a few hours, vertically between depth layers and at spatial scales ˂2 km. This study contributes to the knowledge of the mechanisms of biophysical interactions and their spatiotemporal variation, which is crucial for assessing the effects of climate variability on plankton-mediated ecosystem services.