Thin Phytoplankton Layer Observation in the Coastal Zone of Ubatuba, Brazil.

The vertical distribution of phytoplankton is influenced by an interplay of physical, chemical and biological factors, which may lead to the formation of the so-called thin layers. Thin phytoplankton layers are common features in coastal and open oceans ecosystems and have important implications for trophic dynamics and the carbon cycle. This work presents the results of a field campaign carried out in January 2019 off Ubatuba, Brazil, which investigated the spatial occurrence of plankton thin layers and their relationship with accompanying environmental conditions, including nutrient concentration, dissolved oxygen, temperature, turbidity and fluorescence of chlorophyll-a. High resolution environmental sensors were used coupled to a plankton imaging system. A chlorophyll-a fluorescence peak occurred in association to temperatures lower than 18°C, indicating South Atlantic Central Water (SACW) intrusion onto the inner shelf. The presence of SACW was also evidenced by low dissolved oxygen and high nutrient (especially silicate) concentrations in the bottom layers. Data from the imaging system revealed a thin layer (~ 0.80 m thick) of diatoms associated with the chlorophyll-a fluorescence peak. The layers occurred below the pycnocline, in a region with low buoyancy frequency [<0.0005 (rads/s)²]. In situ observations indicated that phytoplankton may have been affected by locally enhanced nutrient supply provided by the SACW advection in a convergence zone. This is the first observation of fine scale biological structures in a pelagic ecosystem off the Brazilian coast.