Environmental drivers of pico- and nanophytoplankton distribution in the coastal zone of Ubatuba, Brazil

Wind-driven variability strongly influences the physicochemical structure of coastal waters, yet short-term responses of size-fractionated phytoplankton biomass remain poorly resolved in the Southwest Atlantic. This study investigates how wind-driven environmental conditions regulate the vertical and cross-shore distribution of pico- and nanophytoplankton on the inner shelf off Ubatuba, southeastern Brazil. Two oceanographic campaigns were conducted in December 2018 and January 2019 using high-resolution environmental profiling and discrete water sampling at seven stations spaced approximately 1.5 km apart. Contrasting meteorological conditions generated distinct physical scenarios characterized by differences in water column stratification and nutrient availability. Enhanced vertical mixing in December resulted in weak stratification, the absence of a well-defined subsurface chlorophyll maximum (SCM), and more vertically homogeneous pico- and nanophytoplankton distributions under low nutrient concentrations. In contrast, persistent northeasterly winds in January promoted strong stratification, intensified intrusion of nutrient-rich South Atlantic Central Water (SACW), and the development of pronounced SCMs, where picoeukaryotes, nanoeukaryotes, and Synechococcus exhibited increased biomass. This study shows that wind-driven submesoscale variability governs pico- and nanophytoplankton biomass and distribution on the inner shelf by regulating water column stability and nutrient supply. Event-scale changes in stratification associated with SACW intrusions produced rapid shifts in vertical biomass structure, emphasizing the role of short-term physical forcing in shaping coastal phytoplankton communities and their contribution to carbon cycling in oligotrophic shelf systems.