Diatom associations in shelf and oceanic waters off Santos Basin in relation to environmental drivers, southern Brazil (22 ^o S – 28 ^o S)

A mesoscale study on diatoms from the Santos Basin (23°S to 28°S), southern Brazil, was carried out during early spring 2019, based on 60 oceanographic stations stretching from coastal to open-ocean waters. Multivariate analysis based on counting diatom valves on permanent slides was performed in relation to the main environmental parameters (temperature, salinity, nutrients, and depth). Four diatom associations were discriminated, affected by the water masses and environmental drivers operating in the Santos Basin. The inner- to mid-shelf association was influenced by nutrient discharge from rivers and estuaries, as well as the Subtropical Shelf Front; Actinocyclus octonarius, Chaetoceros didymus, Fragilariopsis doliolus, and Pseudo-nitzschia pungens were the highest relative valve contributors. Another association composed of uncommon diatoms occurred as isolated peaks along the continental slope and was affected by meanders and eddies of the Brazil Current, as well as deep waters brought up by local coastal upwelling. Delphineis surirella, Navicula pennata, Pleurosigma diversestriatum, Pseudo-nitzschia cf. delicatissima, Pseudo-nitzschia calliantha, and Skeletonema tropicum were its main components. A typical subtropical/tropical association from oligotrophic warm waters was delimited in outer-shelf and oceanic waters such as Azpeitia spp., Asteromphalus spp., and Roperia tesselata. The fourth association comprised diatoms widely distributed across the Santos Basin, overlapping with the tropical association in certain areas. In this association, Nitzschia bicapitata, Thalassionema nitzschioides, Chaetoceros spp. and Bacteriastrum hyalinum were the most important taxa. Multivariate analysis identified for the first time diatom associations related to water masses and their properties in the Santos Basin on a regional scale. These patterns of mesoscale distribution provide a more detailed understanding of diatom biogeography than previously documented in broader-scale investigations.