Vertical variation of primary production and phytoplankton biomass were assessed by short time series studies at an anchor station, at the 45m isobath, in São Sebastião coastal region, Southeast Brazil. “In situ” measurements and samplings were conducted during summer of 1994 (72 h series) and during spring of 1997 (78 h series). Meteorological, hydrological, dissolved nutrients and phytoplankton biomass (Chl-a, Chl-b, Chl-c, carotens, “in vivo” fluorescence, cell densities, DNA and RNA) were monitored. Primary productivity rates were evaluated through in situ incubations by 14C technique, covering the morning and afternoon periods. Phytoplankton physiological status was evaluated by the rates active Chl-a / total Chl-a, 480/665, RNA/DNA and C assimilation. Picophytoplankton contribution to the bulk biomass and for the primary production rates was estimated by size fractionation filtration. Most variables were monitored at three-hour intervals, in nine depths. In summer a subtle intrusion of South Atlantic Central Water (SACW), occurred under 30 m depth. In the springtime, SACW had an intense intrusion reaching the 25 m in only three hours. It displayed the same advection pattern 36 h later, leaving the sampling point under intense eastward currents action. Despite SACW advection, water column displayed a high tendency to static stability, revealed by Brunt-Väisälä frequency. In the spring high tendency to dynamic stability was also revealed by computing the Richardson Number. Isolated instability points occurred linked to lateral advection of saltier waters. High nitrogen levels in the surface layers ( > 1 micro-M) in both periods, were related to the rainy period before the sampling. Higher phosphate and silicates were detected in the less saline waters suggesting continental runoff contribution to the place. Evidences of nitrification processes were detected in the water column during summer. SACW influence was also denoted by the higher nutrient values linked to it. Temperature, nitrate and silicate profiles showed some influence of the tide signal close to the bottom. Phytoplankton biomass ranged from 15.91 to 18.39 (summer) and 40.41 to 46.22 mg Chl m^-2 (spring) picophytoplankton comprised 44% of total Chl and represented 60% of the total cell number. Primary production rates ranged from 379.98 to 618.9 mg C m^-2 d^-1 (summer) and from 191.97 to 413.53 mg C m^-2 d^-1 (spring). Mean picophytoplankton photosynthetic efficiency were in the range of 0.61 a 0.93 mg C m^2 /mgChl E (summer) and 0.51 a 0.74 mg C m^2/ mgChl E (spring). Such values were higher than the ones computed for the entire biomass. On the account of the high static stability observed, photoaclimation processes could be evidenced in short time periods (h), mainly related to carotenoids concentrations. Phytoflagellates dominated the whole water column. DNA, Chl-a measurements and “in vivo” fluorescence displayed same distribution patterns. Surface DNA and Chl-a were positively correlated which is not the case in bottom waters. DNA concentrations in the spring ranged 203.3 to 224.3 mg m^-2. RNA ranged from 137.7 to 196.9 mg m^-2. Temporal profiles of studied variables and PCA analysis information indicated phytoplankton was phosphate limited during summer and, during spring, limitation was related to light.