Plankton abundances, bacterial production, and the size distribution of oxygen metabolism and chlorophyll a concentration were determined through 3 seasonal cycles in the Menai Strait (North Wales, UK). Spring blooms were comprised of a diatom to Phaeocystis succession. Meso- and microphytoplankton dominated phytoplankton production and biomass during diatom blooms, and nanophytoplankton predominated during summer, when activity and biomass were low. Correlation analysis showed temperature to be the best predictor for chlorophyll a-specific gross community production. Bacterioplankton were implied to be the major respirers. Consequently the phasing of respiration in relation to photosynthesis was strongly influenced by bacterioplankton metabolism and abundance changes. The respiration maximum occurred 1 to 2 wk after the Phaeocystis abundance maximum. An explanation for this temporal lag was sought by considering the time scales of flow of organic material between the phytoplankton and the bacterioplankton. The observations were consistent with routes via a slowly cycling pool, such as polymeric organic material. This pool would function as a reservoir and result in microheterotrophic respiration persisting after the decline of photosynthesis, and causing a positive to negative temporal sequence in net community production.