Transient, tidal time-scale Nitrogen transformations in an estuarine turbidity maximum-fluid mud system (The Gironde, France)

The maximum turbidity zone (MTZ) of the Gironde Estuary is a site of important mineralization of particulate organic nitrogen. Moreover, this MTZ is characterized by intense cycles of settling and resuspension of anoxic fluid mud at both tidal and neap-spring time-scales. In the upper layer of the fluid mud, which was deposited at tidal slack and eroded during each ebb and flood, a denitrification rate of up to 65 μmol 1-1 h-1 was measured during the transient period of settling. High concentrations of NO2- (up to 13 μM), and N2O (up to 1500 nM) were observed in this layer at tidal slack, either due to an incomplete nitrification at low oxygen concentrations or to high denitrification activity. This suggests that deposition and erosion of the upper fluid mud at the tidal time-scale could be an important source of N2O for the estuarine overlying waters. The deepest layer of the fluid mud was more turbid, NO3- depleted and remained stable throughout tidal cycles, during several days. In this layer, ammonification was responsible for an increase of NH4/+. Most probable number (MPN) counts of ammonium oxidizing bacteria in turbid water and fluid mud revealed that they were closely associated with particles, whereas potential nitrification rates were rather constant (10-14 μmol 1-1 h-1) irrespective of the suspended matter concentrations. In the MTZ of the Gironde, nitrogen behaves like in activated sludge reactors, with oxic/anoxic oscillations occurring at the tidal time scale. Nitrification takes place in the oxic water column and denitrification in the anoxic fluid mud. However at the whole estuarine scale the loss of particulate nitrogen and the gain of total dissolved inorganic nitrogen are in the same order of magnitude, which suggests that the net loss of nitrogen from the system is rather limited. This seems due to the fact that high denitrification rates are limited both in time (tidal slacks) and space (upper layers of fluid mud). However, our study emphasises the importance of settling/erosion transition periods on the nitrogen dynamics in estuarine maximum turbidity zones.