Microplankton respiration and net community metabolism in a bay on the NW Mediterranean coast

The seasonal variation in the respiration and net community metabolism of microplanktonic communities was examined in the Bay of Blanes, an exposed NW Mediterranean bay (NE Spanish coast) which receives intermittent inputs from a torrential river, based on estimates of respiration rates of microplanktonic communities obtained over 2 yr (March 1992 to March 1994). Community respiration rates (mean ± SE = 5.232 ± 0.9 umol O₂ dm^-3 d^-1) ranged over 2 orders of magnitude, showing a distinct seasonal pattern, with the highest respiration rates in summer and late winter Differences in respiration rate were independent of ambient temperature but were closely related (r = 0.81) to the biomass of heterotrophic microplankton, which was dominated by bacteria and heterotrophic cillates. The microplankton community in the Bay of Blanes tended to be net heterotrophic on the annual time scale, with demands exceeding autochthonous production. Net daily autotrophic metabolism was only observed during periods of very low respiration rates, and heterotrophic organisms played a dominant role in the metabolism of the NW Mediterranean littoral community studied, as evidenced by a strong correlation between net community metabolism and respiration rates. There was substantial inter-annual variation in metabolic rates between years, with average respiration rates in the first year twice as high as those in the second year (6.816 ± 1.728 and 3.672 ± 0.374 μmol O₂ dm^-3 d^-1, respectively). This resulted in a more heterotrophic net daily community metabolism during the first than during the second year (-1.258 ± 0.94 and 0.978 ± 0.344 μmol O₂ dm^-3 d^-1, respectively). The net heterotrophic nature of the microplanktonic community in the Bay of Blanes indicates that carbon demands should be partially satisfied from inputs from land, consistent with the much higher rainfall in the first, compared to the second, year. Variability in rainfall and the associated differences in carbon inputs from land are, therefore, major sources of interannual differences in the microplanktonic metabolism of the NW Mediterranean littoral.