Regulation of nitrification in the land-ocean contact area of the Rhone River plume (NW Mediterranean)

The study of natural conditions controlling nitrification processes was undertaken in the estuarine area of the Rhone River (NW Mediterranean Sea) over an annual cycle. Nitrification rates (N-oxidation rates, CO2 fixation rates, specific counts of ammonium and nitrite oxidisers) as well as environmental parameters (temperature, salinity, suspended matter, inorganic nitrogen) were measured monthly over a 1 yr period. Depth profiles were obtained in low salinity plume water, at the edge of the plume (intermediate salinity) and in the sea, to study the evolution of nitrification processes from the mouth of the river to the sea of both the brackish surface layer and the bottom nepheloid layer. Nitrification rates in the surface plume generally declined with distance from the river and with increasing salinity, whereas nitrification rates in the benthic nepheloid layer exhibited little spatial variation. This was illustrated by high variability of activities in the plume (50 +/- 37 and 33 +/- 21 nM N oxidised h(-1) for ammonium and nitrite oxidation rates respectively) versus lower variability in the benthic nepheloid layer (30 +/- 8 and 22 +/- 6 nM N oxidised h(-1) for ammonium and nitrite oxidation rates respectively). Nitrification rates in both the plume and benthic nepheloid layer were higher in the summer (142 to 175 nM N oxidised h(-1) for ammonium oxidation and 64 to 96 nM N oxidised h(-1) for nitrite oxidation) than during the winter months (5 to 30 nM N oxidised h(-1) for both activities). Highest N-oxidation rates were related to low salinity in the surface layer, corresponding to the highest concentrations of ammonium from the river water. Ammonium oxidation (AO) is regulated by in situ ammonium concentration (r(2) = 0.741, p = 0.0001, n = 132) and nitrite oxidation (NO) by the ammonium oxidation rate (r(2) = 0.850, p = 0.0001, n = 132). AO rate = 14.5 + 10 [NH4+] and NO rate = 4.7 + 0.56 AO rate. Q(10) values were calculated for ammonium oxidation (2.72) and nitrite oxidation (3.08) rates. The counts of nitrifiers reached 10(6) cell dm(-3), representing less than 2 % of the total bacterial counts. Nitrogen N-oxidation rates were correlated to nitrifying cell abundances (r = 0.89 to 0.99), and the activities per cell ranged from 1.2 to 1.9 x 10(-15) mol N oxidised cell(-1) d(-1). The biomass production of the ammonium oxidisers was demonstrated by the decrease of N oxidised/C fixed ratio from 15.8 in winter to 10.8 in summer, in spite of the increase of AO rate in summer. A seasonal fluctuation of N oxidised/C fixed ratio for nitrite oxidisers was not demonstrated; however, the ratio increased from 21.8 +/- 4.4 in winter to 29.3 +/- 2.9 in summer. We estimated that 10% (winter) and 20% (summer) of ammonium was nitrified at the mouth of the river. During summer, nitrification is supported by the flux of ammonium from in situ mineralisation and contributes to nitrate excess in the plume. The contribution of nitrification to the N requirement of the primary producers was estimated to be 14 % at the edge of the plume (intermediate salinity) and 66% at the sea station.