Abstract
The carbon fluxes between phytoplankton and heterotrophic bacterioplankton were studied in two coastal oligotrophic sites in the NW Mediterranean. Phytoplankton and bacterial production rates were measured under natural conditions using different methods. In the Bay of Villefranche, the temporal variability revealed net heterotrophy in July-October and net autotrophy in December-March. The spatial variability was studied in the Bay of Palma, showing net autotrophic areas in the west and heterotrophic areas in the east. On average bacterial respiration, represented 62% of the total community respiration. Bacterial growth efficiency (BGE) values were significantly higher in autotrophic conditions than in heterotrophic ones. During autotrophic periods, dissolved primary production (DPP) was enough to sustained bacterial metabolism, although it showed a positive correlation with organic carbon stock (DOC). Under heterotrophic conditions, DPP did not sustain bacterial metabolism but bacterial respiration correlated with DPP and bacterial production with DOC. Temperature affected positively, DOC, BGE, bacterial respiration and production when the trophic status was autotrophic. To summarize, the response of bacterial metabolism to temperature and carbon sources depends on the trophic status within these oligotrophic coastal systems.
Highlights
In oligotrophic Mediterranean coastal waters, it has been shown that large supply of dissolved organic carbon (DOC) from land can sustain elevated community respiration, resulting in net heterotrophy[9,10,11]
Bacterial carbon demand includes all the organic carbon assimilated by heterotrophic bacteria, which can be allocated into anabolic and catabolic processes
Bacterial carbon demand is governed by bacterial growth efficiency (BGE = bacterial production/bacterial carbon demand), there is a large range of variation in Bacterial growth efficiency (BGE) (5–60%23;)
Summary
In oligotrophic Mediterranean coastal waters, it has been shown that large supply of dissolved organic carbon (DOC) from land can sustain elevated community respiration, resulting in net heterotrophy[9,10,11]. To improve our knowledge of the trophic status in oligotrophic bays, it is necessary to focus the studies on temporal and spatial variability, despite the given idea that these environments as steady state systems. To reach this goal, we performed simultaneous measurements of net community production, particulate and dissolved primary production, bacterial production and respiration, allowing direct BGE, bacterial carbon demand and conversion factor values. The results will improve further understanding of the link between carbon fluxes and the metabolic trophic status, of these marine coastal ecosystems
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