We studied the importance of dissolved primary production in a coastal, productive ecosystem in relation to phytoplankton biomass, community structure and productivity. The photosynthetic production of dissolved organic carbon (DOCp) and particulate organic carbon was determined in mesocosm experiments during four contrasting oceanographic periods in the Ria de Vigo (NW Iberian Peninsula). We also determined the size-fractionated chlorophyll a concentration and primary production, phytoplankton taxonomic composition and bacterial production. Phytoplankton biomass was dominated by the > 20 mu m size fraction (mostly diatoms), except in winter, when the 2-20 and < 2 mu m size fractions (flagellates and picophytoplankton) increased in importance. The percentage of extracellular release (PER) had an average value of 19% and was independent of oceanographic period, phytoplankton biomass and production, taxonomic composition and size structure. During phytoplankton blooms, PER increased significantly from 14% in the exponential growth phase to 23% in the senescent phase. Bacterial carbon demand and DOCp were uncoupled, suggesting that other processes in addition to photosynthate exudation contribute most of the labile carbon to fuel bacterial metabolism. Dissolved primary production remains an important process in coastal phytoplankton assemblages throughout the year, irrespective of size-structure and community composition, but attaining higher significance during the decaying phase of blooms.