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 intermi ttent inputs from a torrential river, based on estimates of respiratio n rates of microplanktonic communities obtained over 2 yr (March 1992 to March 1994). Community respiration rates (mean +/- SE = 5.232 +/- 0 .9 mu mol O-2 dm(-3) d(-1)) ranged over 2 orders of magnitude, showing a distinct seasonal pattern, with the highest respiration rates in su mmer and late winter. Differences in respiration rate were independent of ambient temperature but were closely related (r = 0.81) to the bio mass of heterotrophic microplankton, which was dominated by bacteria a nd heterotrophic ciliates. The microplankton community in the Bay of B lanes tended to be net heterotrophic on the annual time scale, with de mands exceeding autochthonous production. Net daily autotrophic metabo lism was only observed during periods of very low respiration rates, a nd heterotrophic organisms played a dominant role in the metabolism of the NW Mediterranean littoral community studied, as evidenced by a st rong correlation between net community metabolism and respiration rate s. There was substantial interannual variation in metabolic rates betw een years, with average respiration rates in the first year twice as h igh as those in the second year (6.816 +/- 1.728 and 3.672 +/- 0.374 m u mol O-2 dm(-3) d(-1), respectively). This resulted in a more heterot rophic net daily community metabolism during the first than during the second year (-1.258 +/- 0.94 and 0.978 +/- 0.344 mu mol O-2 dm(-3) d( -1), respectively). The net heterotrophic nature of the microplanktoni c 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. Variabili ty in rainfall and the associated differences in carbon inputs from la nd are, therefore, major sources of interannual differences in the mic roplanktonic metabolism of the NW Mediterranean littoral.