Organic carbon and bacterial heterotrophic activity in the maximum turbidity zone of the Seine estuary (France)

The aim of this study is to understand the sources, biodegradability and fate of organic carbon entering the maximum turbidity zone (MTZ) of the Seine estuary. Dissolved (DOC) and particulate organic matter (POC), and their biodegradable fractions (BDOC and BPOC) were investigated immediately upstream and within the MTZ in various hydrological conditions together with bacterial production of free-living and particle-associated cells. At the entrance of the MTZ, average DOC was 3.6 mg C l−1 with a biodegradable fraction of 23%. The DOC and BDOC/DOC ratio decreased linearly with increasing salinity, indicating a dilution of river freshwater with a higher DOC and higher BDOC/DOC ratio than marine waters. The organic content of suspended matter (SM) at the entrance of the MTZ was high (POC/SM ratios up to 10.6%), especially during phytoplankton blooms when high BPOC/POC ratios were also found. Within the MTZ, the carbon content of SM was lower and less variable; the biodegradable fraction of POC in the MTZ was lower than upstream, illustrating the progressive bacterial degradation of BPOC within the MTZ. Total bacterial production in the MTZ fluctuated between 0.2 and 5.1 μg C l−1 h−1; average production of free-living bacteria was 0.77 μg C l−1 h−1 while that of attached bacteria was 1.47 μg C g SM−1 h−1. Growth rates of attached bacteria were, on average, three times higher than those of free-living ones. Bacterial activity can explain the decrease in the POC/SM ratio observed between summer and fall in the MTZ.