||Microbial plankton carbon fluxes were comprehensively assessed at 4 contrasting sites in Waquoit Bay estuary (Massachusetts, USA) in 2008, focussing on the role of phytoplankton as dissolved organic matter suppliers for heterotrophic bacterioplankton. Strong seasonal patterns in environmental and microbial variables were observed, with concurrent summer maxima and minima in December for bacterial biomass (17.7-164.7 mu g C l(-1)) and production (4.8-121.5 mu g Cl-1 d(-1)). Phytoplankton biomass (0.7-32.6 mu g chlorophyll l(-1)) and primary production (PP, 164-3570 mu g C l(-1) d(-1)) varied similarly to bacterial variables, with dissolved PP (DPP), i.e. released extracel-lularly, ranging from 3 to 36% of the total PP. The flux of DPP represented between 0.4 and 20% of bulk dissolved organic carbon (DOC, 92-346 mu mol C l(-1)) daily turnover. In addition to temperature, moderate bottom-up control of bacterial assemblages was suggested by biomass-production regressions. Bacteria apparently depended more on phytoplankton-derived DOC than on its total concentration. The difference became larger when the total flux of carbon or bacterial carbon demand (BCD, 19-468 mu g C l(-1) d(-1)) was considered, with DPP explaining 53% of its variance compared with 37% from bulk DOC. On average, DPP could meet 50% of BCD, indicating a larger than expected trophic dependence of bacteria on phytoplankton in the whole estuarine ecosystem.