BACTERIAL NITRIFICATION AND DENITRIFICATION RATES IN THE RHONE RIVER PLUME (NORTHWESTERN MEDITERRANEAN-SEA)

Water samples were collected in May 1992 from turbid plume water along several transects of increasing salinity from the Rhone River mouth to the sea. Nitrogen salt concentrations (NH4+ NO2-; NO3-), nitrification, denitrification and nitrate reduction were determined. NH4+ values, measured in the plume water, were lower than those corresponding to a conservative dilution, demonstrating a loss of 2 mu mol l(-1) of NH4+. In situ concentrations of NO3- differed by 10 to 30 mu mol l(-1) from theoretical values (conservative dilution), showing a net consumption of this compound. NO2- concentrations stayed closed to the conservative dilution curve plots. Along the salinity gradient, ammonium and nitrite oxidation rates decreased from 2 to 0.2 and 1 to 0.1 mu mol l(-1) d(-1) respectively. These 2 rates correlated well with in situ NH4+ concentrations. 15 % of the allochtonous NH4+ was nitrified. Dissimilative nitrate and nitrite reduction rates displayed similar values, decreasing from 380 to 7 mu mol l(-1) d(-1) Denitrification ranged from 0 to 7 mu mol l(-1) d(-1), independently of the salinity value. 3.5 % of the allochthonous NOB was denitrified. In the plume, denitrification rates were 30 to 100 times lower than nitrite reduction, while at a salinity > 20 psu, these 2 processes occurred at similar rates. A significant correlation was demonstrated between the nitrate reduction rate and the difference between theoretical and in situ NO3- concentrations. The deficit in nitrate in the plume could arise from the nitrate reduction process.