Bacterioplankton abundance and biomass stimulated by water masses intrusions over the Southern Brazilian Shelf (between 25°57'S and 29°24'S)

In order to understand the distribution of bacterioplankton along the continental shelf off the Santa Catarina coast (25°57'S and 29°24'S) and whether water masses were affecting this distribution, we measured abundance, biovolume, and biomass of bacterioplankton, and the physico-chemical characteristics of the water masses in three transects in two distinct seasons: summer and winter of 2010. During the austral summer, high continental runoffs from the Itajaí River and Babitonga Bay mix over the shelf with Tropical Water (TW) and originate the summer Subtropical Shelf Water (STSW). Bacterial biomass reached an average of 2.5-5.0 μgC L−1 in summer, with values observed in the southernmost transect (Araranguá) being one order of magnitude higher when compared to the others, where nutrient-rich South Atlantic Central Water (SACW) intrusions are often near the coast. On the contrary, in the austral winter, the Plata Plume Water (PPW) spread on the first 40 m of water positively influenced bacterial abundance and biomass, which were on average ten times higher than those observed in the summer (abundance of up to 2.5 × 109 cells L−1, and biomass up to 60 μgC L−1). As a conclusion, PPW, indicated by the colder waters, high silicate and Kd values, and high concentrations of particulate organic carbon have a positive effect on the abundance and biomass of bacterioplankton, whereas the intrusions of SACW, and local freshwater runoffs have secondary effects on bacterial parameters. This is the first description of the influence of PPW intrusions over the dynamics of bacterioplankton, considering two contrasting environmental periods, and, consequently, to carbon budget studies in the Southern Brazilian Shelf (SBS). Taking into account both abundance and biomass of bacterioplankton and their association with water masses in the SBS, our results provide fundamental information for further biogeochemical studies.