a b s t r a c t The combined influence of freshwater inputs and wind-driven upwelling may generate contrasting environmental conditions over small spatial scales in the coastal ocean. Over two consecutive years (mid-2006 through to mid-2008), we compared the springtime and wintertime composition, biomass, and primary production of the main phytoplankton groups at two coastal stations (RV and UW) near the Itata River mouth in the upwelling area off central Chile. Hydrographic and nutrient profiles showed distinct seasonal features: a relatively weak surface thermocline and weak river influence at both stations in spring, and in winter a stronger surface halocline at station RV, located closer to the river mouth. At both stations, primary production (24–8000 mg C m À2 d À1) and chlorophyll-a concentrations (5–20 mg Chl-a m À3) were highest in the spring, with a dominance of microphytoplankton (Chaetoceros spp., Thalassiosira spp.). Total primary production and chlorophyll-a in the winter corresponded mostly to smaller size fractions (pico-and nanoplankton), which dominated the phytoplankton community (>50%) in terms of carbon biomass at station RV. At this river-influenced station, small autotrophic and heterotrophic groups (<20 lm), including picophytoeukaryotes, photo-and heterotrophic nanoflagel-lates, and ciliates, were two to four times more abundant than at station UW. We conclude that most of the integrated carbon biomass and production rates during winter months are accounted for by small cells in the microbial food web. This component of the phytoplankton community may be enhanced in response to the additional surface input of nutrients by river discharges into the nearshore environment.