Bacterial abundances and production, and the size distribution of oxygen metabolism and chlorophyll a concentration were followed through two seasonal cycles in the Menai Strait (North Wales, U. K. ) and during austral summer in the Southern Ocean. In the Menai Strait, spring blooms were characterised by a diatom to Phaeocystis succession. In both the Menai Strait and the Southern Ocean, meso- and microphytoplankton dominated phytoplankton production and biomass during diatom blooms. Nanophytoplankton predominated when production and biomass were low, i. e. during the summer in the Menai Strait, in waters near the Polar Front, and in some samples from the Weddell Sea. In both ecosystems substantial respiration resided in the bacterial (< 0.8 gm) size-fraction. Consequently during the Menai Strait temporal study, phasing of respiration in relation to photosynthesis was strongly influenced by bacterial metabolism and abundance changes. The respiration maximum occurred 1-2 weeks 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 bacteria. 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, causing a positive to negative temporal sequence in net community production. There was no evidence for differences in any measure of microbial biomass between the Southern Ocean and the Menai Strait. General relationships could be derived for both ecosystems: (a) the biomass quotient (< 20 µm phytoplankton / unfractionated phytoplankton) generally increased sharply as unfractionated phytoplankton biomass decreased, (b) bacterial biomass generally increased as phytoplankton biomass increased, (c) the biomass quotient of bacteria to unfractionated phytoplankton increased sharply as unfractionated phytoplankton biomass decreased. Different relationships were derived for the oxygen fluxes in terms of phytoplankton biomass for the Southern Ocean and Menai Strait observations. In these relationships, the oxygen fluxes were generally relatively (relative to the explanatory variable: phytoplankton biomass) higher in the Menai Strait. In contrast, a single relationship for DCR in terms of GCP was fitted for both data sets. This difference is consistent with a temperature effect on the oxygen fluxes, with GCP and DCR similarly suppressed at lower temperatures.