Bacterial dynamics during the transition from spring bloom to oligotrophy in the northwestern Mediterranean Sea: relationships with particulate detritus and dissolved organic matter

The variability in microbial communities (abundance and biomass), bacterial production and ectoaminopeptidase activity, particulate and dissolved orga nic carbon (POC, DOG), and particulate and dissolved lipids was examined in spring 1995 in the northwestern Mediterranean, where a transition from the end of a bloom to pre-oligotrophic conditions was observed. Four time seri es of 36 h each and 4 h sampling intervals were performed at 5 m and at the chlorophyll maximum (30 m) between 11 and 31 May. Simultaneous measurement s of pigments, abundance of hetero- and autotrophic flagellates, bacteria a nd POC enabled the estimation of living POC (defined as autotrophic-C plus heterotrophic-C biomass), and thus the detrital organic carbon. During the first 2 time series (11 to 15 May), the bacterial-C biomass was higher than the autotrophic-C biomass at 5 m (ratio 1.4 and 1.7), whereas the opposite trend was observed in the chlorophyll peak (ratio 0.7 for the first cycle) . However, at the end of May, autotrophic-C biomass was equivalent to bacte rial-C biomass at both depths studied. The detrital pool remained a more or less constant fraction of the POC (52, 53 and 47% on 11-12 May, 14-15 May and 30-31 May) at the chlorophyll peak, whereas it decreased significantly with time (62 to 53%) at 5 m. Relationships between bacterial activities an d evolution of available resources were not systematically evidenced from o ur 36 h diel cycle data. Nevertheless, at the monthly scale, comparison of bacterial carbon demand (BCD) to potential carbon resources (detrital POC a nd DOG) showed that bacteria fed differently on the various pools. From ect oaminopeptidase turnover rates and detrital POC, the potential hydrolysis r ate of detritus was calculated. Depending on the choice of conversion facto rs for bacterial production and estimates of hydrolysis turnover rates, it was shown that bacterial hydrolysis of detritus could be one of the DOC acc umulation sources. We observed that the percentage of BCD supplied by detri tal POC hydrolysis increased in the surface and decreased in the chlorophyl l peak. An index of lipid degradation in POC, the lipolysis index, increase d during the month at 5 m, also indicating a higher hydrolysis of POC. The opposite trend was observed in the chlorophyll maximum layer. The selective decrease in dissolved Lipids in DOC in the chlorophyll maximum layer, part icularly free fatty acids, also suggests that bacteria utilized increased f ractions of carbon sources from the DOG. We concluded that partitioning bet ween DOC and detritus as resources for bacteria can change during the rapid transition period from mesotrophy to oligotrophy in the northwestern Medit erranean.