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

Abstract

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...

Spatial variability of nitrous oxide in the Minho and Lima estuaries (Portugal)

“…Though no information on benthic AOA and AOB communities along the Minho and Lima estuaries is available, NH 4 + concentration in the upper Minho estuary (maximum 4.4 μmol L -1 ) seems more suitable for the occurrence of nitrification than in the Lima estuary (maximum 1.8 μmol L -1 ). Nitrification reactions typically happen within a DO range of 15.6-78.0 μmol L -1 , and the Minho and Lima surface waters were well (Bianchi et al 1999, Teixeira et al 2013. Our results from the upper Minho are in accordance with these findings, as the potential nitrification occurred at low salinity (between ~2 and ~10).…”

Compositional changes in free‐living bacterial communities along a salinity gradient in two temperate estuaries

“…The ␤-proteobacteria include bacteria that are capable of using reduced gases, such as H 2 and methane, and also nitrifying bacteria (Zavarzin et al 1990). Whereas the first two nutritional processes remain trivial in terms of total fluxes in the water column, nitrification appears as a general and essential process in the freshwater part of estuaries (Bianchi et al 1999;de Wilde and de Bie 2000). Nitrification has been observed in the upper Choptank (Ward and Twilley 1986) and the Potomac River estuaries (Elkins et al 1982).…”

Functioning of the planktonic ecosystem on the Gulf of Lions shelf (NW Mediterranean) during spring and its impact on the carbon deposition: a field data and 3-D modelling combined approach

“…To avoid this bias, a set of parameters specially dedicated to the Rhone River plume environment has been designed considering the plume-specific diversity of plankton communities (e.g. bold parameters in Table A5) as well as the high but unbalanced nutrient inputs from the river (Moutin et al, 1998;Bianchi et al, 1999;Joux et al, 2005;Ludwig et al, 2009).…”