Phytoplankton community and the fluorescence-derived photo-physiological parameters in the South Atlantic Ocean

Abstract Assessing the composition and photo-physiology of phytoplankton communities are fundamental pieces of information to understand their role in global biogeochemical cycles mediated by primary production rates in the oceans. Photo-physiological parameters, namely the maximal photochemical efficiency (Fv/Fm) and the effective absorption cross-section of Photosystem II (σPSII), are currently used to estimate primary production rates in the laboratory and nature, and although variable fluorescence techniques substantially expand opportunities for new measurements in both time and space, their results reflect synergies among phytoplankton composition, cell size, limiting nutrients, and light history that need to be accounted for. The current work compared the variability of Fv/Fm and σPSII, as well as phytoplankton groups derived by pigment-CHEMTAX analyses, at the surface and deep chlorophyll maximum (DCM) of two sections of oceanographic stations over in the productive South Atlantic Subtropical Front (STF) at 35°S of latitude, and crossing the adjacent oligotrophic waters of the South Atlantic Gyre. High chlorophyll-a concentrations were found in the STF, where the presence of cyclonic eddies displaced the pycnocline upwards, and Fv/Fm varied according to specific photo-protective mechanisms of the phytoplankton groups in the surface. In the DCM, higher Fv/Fm were related to the Chl-a synthesis, suggesting a more favorable growing condition at this layer. In the Gyre transect, Fv/Fm showed significant linear relationships with the taxonomical composition, with larger values related to the presence of Prochlorococcus and Chl b-containing green flagellates. Overall, σPSII varied independently from Fv/Fm but were related to the presence of diatoms and the relative proportion between Synechococcus and Prochlorococcus in the STF and Gyre, respectively. Our results encourage the development of more complex models for primary production that should include instantaneous and high-resolution fluorescence-derived parameters as proxies for phytoplankton community and physiological acclimation state.