Bacterial nitrification and denitrification rates in the Rhone River plume (northwestern Mediterranean Sea)

Abstract

Water samples were collected in May 1992 from turbid plume water along several transects of increasing salinity from the RhBne River mouth to the sea. Nitrogen salt concentrations (NH,', NO?. NO,), nitrification, denitrification and nitrate reduction were determined. NH,' values, measured in the plume water, were lower than those corresponding to a conservative dilution, demonstrating a loss of 2 pm01 1 ' of NH,' In situ concentrations of N O 1 differed by 10 to 30 pm01 I-' from theoretical values (conservat~ve 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 pm01 I-' d' respectively. These 2 rates correlated well with in situ NH,' concentrations. 15 ':c, of the allochtonous NH,' was nitrified. Dissimilative nitrate and nitrite reduction rates displayed similar values, decreasing from 380 to 7 umol I' d' Denitrification ranged from 0 to 7 pm01 I-' d . ' , independently of the salinity value. 3.5:1 of the allochthonous NO; was denitrified. In the plume, denitrification rates were 30 to 100 times lower than nltrite reduction, whlle 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 theoretlcal and In sltu N O 3 concentrations. The deficit in nitrate in the plume could a r ~ s e from the nitrate reduction process.