OutlineThis chapter reviews the current state of knowledge of the process and measurement of microplankton respiration in marine surface waters. The principal approaches are outlined and their potentials and limitations discussed. A global database, containing 1662 observations has been compiled and analyzed for the spatial and temporal distribution of surface water respiration. The database is tiny compared to that of photosynthesis and biased with respect to season, latitude, community structure, and depth. Measurements and models show that the major portions of respiration lies in that attributable to bacteria (12-59%) and to algae A global estimate of 13.5 Pmol O 2 a −1 is derived from the mean depth-integrated rate, which significantly exceeds contemporary estimates of ocean plankton production (2.3-4.3 Pmol O 2 a −1 ). This difference is at variance with the results of mass-balance calculations, which suggest a small difference (ca. 0.18) between oceanic production and respiration. The reasons for this are discussed. IntroductionThis chapter reviews our present understanding of the process of marine planktonic respiration, and the methodologies used for its determination. We collate the global database of measured respiration and use it to assess how far we can determine seasonal, regional, and latitudinal patterns of distribution. Measurements of respiration are not yet routine within national and international marine biogeochemical programs. A significant relationship between respiration and more routinely measured parameters would substantially progress our ability to determine its spatial and temporal variability. We investigate whether such a relationship exists. Linked to this is the importance of an appreciation of the distribution of respiration within the microbial food web, and so improved food web model parameterization and verification. When considered on comparable time and space scales, the balance between plankton respiration and photosynthesis indicates the potential amount of photosynthetically fixed carbon available for export from the upper mixed layer. We assume that the epipelagic zone of the open oceans approaches this ideal, and analyze the distributions of respiration and photosynthesis in order to assess the global balance between photosynthesis and respiration. Available approaches and their constraintsRates of planktonic respiration can be derived in a number of ways: (i) from the measurement of the rate of production/consumption of a product 147

Plankton abundances, bacterial production, and the size distribution of oxygen rnetabolism 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 colnmunity production. Bacterioplankton were implied to be the major resplrers Consequently the phasing of respiration in relation to photosynthesis was strongly influenced by bacterloplankton 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 bacter~oplankton. The observations were consistent with routes via a slo\vly 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.

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