Abstract
To investigate how pico- and nanoplankton respond to oceanographic conditions in the Southwestern Atlantic Ocean, we assessed the influence of a summer intrusion of the South Atlantic Central Water (SACW) on the spatial and vertical dynamics of planktonic abundance and carbon biomass across environmental gradients. Seawater samples were collected from six depths within the euphotic zone at nine oceanographic stations in a transect on the Brazilian continental shelf in January 2013. The abundance of pico- and nanoplankton populations was determined by flow cytometry, and carbon biomass was calculated based on conversion factors from the literature. The autotrophic Synechococcus spp., picoeukaryotes and nanoeukaryotes were more abundant in the surface layers of the innermost stations influenced by Coastal Water (maximum of 1.19x105, 1.5x104, and 8.61x103 cell·mL−1, respectively), whereas Prochlorococcus spp. dominated (max. of 6.57x104 cell·mL−1) at the outermost stations influenced by Tropical Water and in the uplifting layers of the SACW around a depth of 100 m. Numerically, heterotrophic bacterial populations were predominant, with maximum concentrations (2.11x106 cell·mL−1) recorded in the surface layers of the inner and mid shelves in Coastal Water and the upper limits of the SACW. Nutrient-rich (high silicate and phosphate) and relatively less saline waters enhanced the picoeukaryotic biomass, while Synechococcus and heterotrophic bacteria were linked to higher temperatures, lower salinities, and higher inputs of ammonia and dissolved organic carbon. The relative importance of each group to carbon biomass partitioning under upwelling conditions is led by heterotrophic bacteria, followed by picoeukaryotes, Synechococcus and Prochlorococcus, and when the SACW is not as influential, the relative contribution of each phytoplanktonic group is more evenly distributed. In addition to habitat preferences, the physical structure of oligotrophic waters has a large impact on the vertical and spatial distribution patterns of picoplankton, reflecting the strong effect of the SACW intrusion.
Highlights
Pico-sized plankton make a significant contribution to phytoplankton primary production and carbon cycling in the ocean (Partensky et al, 1999; Agawin et al, 2000; Bell and Kalff, 2001 and references therein)
Despite increasing efforts to study microbial communities on the Brazilian continental shelf (Alves et al, 2014; Gérikas-Ribeiro et al, 2016a), the relative contribution of photoautotrophs and heterotrophic bacteria to the carbon cycle is largely unknown in terms of abundance and carbon biomass, their distribution within distinct water masses in the South Atlantic
We examined the influence of the South Atlantic Central Water (SACW) summer intrusion on the spatial dynamics of abundance and carbon biomass partitioning of pico- and nano-plankton communities across environmental gradients
Summary
Pico-sized plankton make a significant contribution to phytoplankton primary production and carbon cycling in the ocean (Partensky et al, 1999; Agawin et al, 2000; Bell and Kalff, 2001 and references therein). During spring and summer, the predominant northeasterneastern winds favor the onshore intrusion of the oceanic South Atlantic Central Water (SACW, sensu Sverdrup, 1954; see review in Castro et al, 2006) and the stratification of the water column (Castro, 2014; Cerda and Castro, 2014) This relatively cold (5–20◦C) and nutrient-rich water mass fertilizes the base of the euphotic zone, forms deep chlorophyll maximum (DCM) layer, and is relevant to the dynamics of planktonic communities (Brandini et al, 2014). Despite increasing efforts to study microbial communities on the Brazilian continental shelf (Alves et al, 2014; Gérikas-Ribeiro et al, 2016a), the relative contribution of photoautotrophs and heterotrophic bacteria to the carbon cycle is largely unknown in terms of abundance and carbon biomass, their distribution within distinct water masses in the South Atlantic. We examined the influence of the SACW summer intrusion on the spatial dynamics of abundance and carbon biomass partitioning of pico- and nano-plankton communities across environmental gradients
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