Continuity in the photosynthetic production of dissolved organic carbon from eutrophic to oligotrophic waters

We studied the photosynthetic production of dissolved organic carbon (DOCp) during summer oligotrophic conditions in the Celtic Sea. The production rate of size-fractionated, particulate organic carbon (POCp) was also determined, as well as the relationship between irradiance and DOCp and the kinetics of DOCp during a 24 h light-dark cycle. Mean (1 SE), euphotic layer-integrated POCp was 510 +/- 61 mg C m(-2) d(-1), and cells in the < 5 mu m size fraction accounted for 58 +/- 3 % and 63 +/- 4 % of total integrated chlorophyll a concentration and POCp, respectively. The mean, integrated percentage of extracellular release [PER, calculated as DOCp / (DOCp + POCp)] was 22 +/- 2 %. (DOC)-C-14 accumulation stopped during the dark. In photosynthesis-irradiance experiments, DOCp remained constant across a wide irradiance range, despite the strong light dependence of POCp. These observations suggest that direct release from intact cells is the main pathway for DOCp. Significant DOCp took place throughout the euphotic layer and across a wide irradiance range, suggesting an important role for passive diffusion in the release of recent photosynthate. When comparing our measurements of DOCp in the Celtic Sea with previous observations in a coastal, eutrophic ecosystem, a remarkable continuity is observed, both in the relative significance and likely mechanisms of DOCp. Across a productivity range from < 100 to > 15 000 mg C m(-2) d(-1), PER was relatively constant at similar to 20%, and integrated DOCp could be predicted from integrated POCp with the equation: log (DOCp) = 0.96 x log (POCp) - 0.51 (r(2) = 0.90, n = 35, p < 0.001).