Plankton metabolism and dissolved organic carbon use in the Bay of Palma, NW Mediterranean Sea

A study was conducted to assess the annual variability in planktonic metabolism and dissolved organic carbon (DOC) utilization in an oligotrophic Mediterranean Bay (Bay of Palma, Spain) and to test the role of elevated DOC concentrations in driving planktonic metabolism off balance. We examined, at monthly intervals over 17 mo, gross primary production (GPP), community respiration (R), net community production (NCP), DOC concentration, total chlorophyll a (chl a) concentration, and, for a smaller subset of 11 to 14 mo, net DOC fluxes, bacterial abundance (BA) and bacterial respiration (BR). The community was net heterotrophic in autumn, winter and the first summer studied, and shifted to net autotrophic towards the end of the study period. This period of sustained autotrophy was an anomalous period characterized by frequent storms that stimulated autotrophic processes in the bay, leading to the development of a bloom of the cyanobacteria Synechoccocus. Use of DOC was consistent with the trophic state of the system, as DOC consumption was observed during periods when the system was net heterotrophic and there was a net DOC production when the system shifted to autotrophic. Bacterial respiration accounted for, on average, 51.76% of R and increased as the percent of cells with high DNA content increased. The planktonic community was net heterotrophic on an annual basis, suggesting that the system imports DOC. In particular, the organic carbon import may derive from the excess production of the underlying Posidonia oceanica meadow.