ABSTRACT: Planktonic gross primary production (GPP), community respiration (CR) and net community production (NCP) were evaluated for the first time in the central part of the Southern Brazilian Bight (SBB), in the São Sebastião Channel (SSC); and their response to the South Atlantic Coastal Water (SACW) intrusion was investigated. Sampling experiments took place during late spring 2015 and summer 2016 at a fixed station in the southern portion of the SSC. Although both chlorophyll a and nutrient concentrations were characteristic of oligotrophic and low productive ecosystems, all volumetric planktonic metabolism experiments showed high net and gross primary productions rates. Depth-integrated NCP and GPP values had a global mean ±SE of 31 ± 10.5 and 106.7 ± 11.5 mmol O₂ m^-2 d^-1, respectively. The communities tended to be net heterotrophic (i.e. GPP/CR < 1) at volumetric GPP values less than 7.10 mmol O₂ m^-3 d^-1. The intrusion of nutrient-rich and -mixed water (partly SACW) observed during the summer experiments had a strong influence on autotrophic planktonic communities, enhancing their NCP rates about 3-fold. Nutrient supply by terrestrial inputs and by upwelled SACW intrusion in the SSC enables high primary production to be sustained with a dominance of autotrophic communities.