Nitrification rates were measured along a salinity gradient in the Rhône River estuary, using specific inhibitors (allylthiourea and chlorate) coupled with the measurement of change in nitrite concentration and inorganic carbon uptake by nitrifiers. Rates of ammonium and nitrite oxidation were similar up to 15 practical salinity units (from 1 to 2 μmol N oxidized liter(-1) day(-1)). For higher salinities, nitrite and ammonium oxidation rates were 0.14 and 0.23 μmol N oxidized liter(-1) day(-1), respectively. Ammonium oxidizers assimilated 19-150 × 10-3 μmol C liter(-1) day(-1), while nitrite oxidizers fixed 4.8-72.6 × 10-3 μmol C liter(-1) day(-1). The amounts of nitrogen oxidized and C incorporated demonstrated a linear correlation (r (2) > 0.99). The ratio of N oxidized to C incorporated ranged between 14.3 to 12.3 for ammonium oxidizers, and between 31.6 and 29 for nitrite oxidizers, the lower values being measured in seawater.
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.
ABSTRACT. The study of natural conditions controhng nitrification processes was undertaken in the estuarine area of the RhBne River (NW Mediterranean Sea) over an annual cycle. Nitrification rates (Noxidation 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-' for ammonium and nitrite oxidation rates respectively) versus lower variability in the benthlc nepheloid layer (30 * 8 and 22 -c 6 nM N oxidised h-' 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-' for ammonium oxidation and 64 to 96 nM N oxidised h-' for nitrite oxidation) than during the winter months (5 to 30 nM N oxidised h-' 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 (r2 = 0.741, p = 0.0001, n = 132) and nitrite oxidation (NO) by the ammonium oxidation rate (r2 = 0.850, p = 0.0001, n = 132). A 0 rate = 14.5 + 10 [NH,'] and NO rate = 4.7 + 0.56 A 0 rate. Qlo values were calculated for ammonium oxidation (2.72) and nitrite oxidation (3.08) rates. The counts of nitrifiers reached 106 cell dm-3, representing less than 2 % of the total bacterial counts. Nitrogen 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-" m01 N oxidised cell-' d-'. 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 A 0 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, nitnfication is supported by the flux of ammoni.um 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 salinit...
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