Until recently, denitrification was thought to be the only significant pathway for N 2 formation and, in turn, the removal of nitrogen in aquatic sediments. The discovery of anaerobic ammonium oxidation in the laboratory suggested that alternative metabolisms might be present in the environment. By using a combination of 15 ؊ . We observed a shift in the significance of anaerobic ammonium oxidation to N 2 formation relative to denitrification, from 8% near the head of the estuary to less than 1% at the coast. The relative importance of anaerobic ammonium oxidation was positively correlated (P < 0.05) with sediment organic content. This report of anaerobic ammonium oxidation in organically enriched estuarine sediments, though in contrast to a recent report on continental shelf sediments, confirms the presence of this novel metabolism in another aquatic sediment system.Since the 1970s, substantial research has focused on the ability of estuarine sediments to attenuate riverine nitrogen (N) loads before they affect coastal seas (4,18,19,31,33). Estuarine sediments are essentially anaerobic below a few surface millimeters, and the mineralization of organic matter proceeds via alternate electron acceptors such as NO 3 Ϫ and SO 4 2Ϫ (20,27). In turn, the reduction of NO 3 Ϫ removes NO 3 Ϫ from the overlying waters. Until recently, it was largely thought that NO 3 Ϫ could be either reduced to N 2 gas via denitrification (a facultative metabolism mediated by a variety of bacteria) and lost from the system or reduced to ammonium (NH 4 ϩ ) by fermentative metabolisms and hence conserved within the sediments (dissimilatory nitrate reduction to ammonium [DNRA]) (8,23). It had been demonstrated that changes in sediment organic loadings and estuarine NO 3 Ϫ concentrations may affect the partitioning between these two end products of NO 3 Ϫ reduction (11, 12). The discovery within the laboratory (17) of anaerobic ammonium oxidation revealed a novel metabolism that could short circuit the N cycle, bypassing what was previously thought to be a critical aerobic nitrification phase and potentially providing an alternative pathway for N 2 gas formation in the environment (Fig. 1).Originally, it was thought (17) that anaerobic ammonium oxidation coupled the oxidation of NH 4 ϩ to the reduction of NO 3 Ϫ :Further work, however, showed that the oxidation of ammonium was actually coupled to the reduction of nitrite rather than nitrate (34, 35):The application of this process to the treatment of nitrogenous waste has received a great deal of attention (9, 10), and more recently, the organism responsible has been classified as a new autotrophic planctomycete (28). Anaerobic ammonium oxidation was recently reported to account for as much as 24 and 60% of N 2 formation in continental shelf sediments in relatively deep water (380 and 695 m, respectively) but less than 2% of N 2 formation in eutrophic shallow coastal bay sediments (30). The drop in the significance of anaerobic ammonium oxidation for N 2 formation relative to denitrificatio...

Abstract. River deltas are particularly important in the marine carbon cycle as they represent the transition between terrestrial and marine carbon: linked to major burial zones, they are reprocessing zones where large carbon fluxes can be mineralized. In order to estimate this mineralization, sediment oxygen uptake rates were measured in continental shelf sediments and river prodelta over different seasons near the outlet of the Rhône River in the Mediterranean Sea. On a selected set of 10 stations in the river prodelta and nearby continental shelf, in situ diffusive oxygen uptake (DOU) and laboratory total oxygen uptake (TOU) measurements were performed in early spring and summer 2007 and late spring and winter 2008. In and ex situ DOU did not show any significant differences except for shallowest organic rich stations. Sediment DOU rates show highest values concentrated close to the river mouth (approx. 20 mmol O 2 m −2 d −1 ) and decrease offshore to values around 4.5 mmol O 2 m −2 d −1 with lowest gradients in a south west direction linked to the preferential transport of the finest riverine material. Core incubation TOU showed the same spatial pattern with an averaged TOU/DOU ratio of 1.2±0.4. Temporal variations of sediment DOU over different sampling periods, spring summer and late fall, were limited and benthic mineralization rates presented a stable spatial pattern.Correspondence to: C. Cathalot (cecile.cathalot@lsce.ipsl.fr) A flood of the Rhône River occurred in June 2008 and delivered up to 30 cm of new soft muddy deposit. Immediately after this flood, sediment DOU rates close to the river mouth dropped from around 15-20 mmol O 2 m −2 d −1 to values close to 10 mmol O 2 m −2 d −1 , in response to the deposition near the river outlet of low reactivity organic matter associated to fine material. Six months later, the oxygen distribution had relaxed back to its initial stage: the initial spatial distribution was found again underlining the active microbial degradation rates involved and the role of further deposits. These results highlight the immediate response of the sediment oxygen system to flood deposit and the rapid relaxation of this system towards its initial state (6 months or less) potentially linked to further deposits of reactive material.

International audienceEditorial handling by M. Kersten a b s t r a c t The early diagenesis of the major carrier phases (Fe and Mn minerals), trace elements (As, Co, Cr, Hg, MeHg, Ni) and nutrients (RNO 3 , NH þ 4 , RPO 4) and their exchange at the sediment water/interface were studied in the Berre Lagoon, a Mediterranean lagoon in France, at one site under two contrasting oxygen-ation conditions (strictly anoxic and slightly oxic) and at an adjacent site with perennially well-oxygen-ated water. From the concentration profiles of the primary biogeochemical constituents and trace elements of the pore and bottom waters, as well as the total and reactive particulate phases, it was pos-sible to locate and identify the diagenetic reactions controlling the mobility of trace elements in the sed-iments and quantify their rates by coupling one-dimensional steady-state transport-reaction modelling and thermodynamic speciation calculations. Under oxic conditions and in the absence of benthic organisms, the main redox reactions were well identified vertically in the surface sediments and followed the theoretical sequence of oxidant consump-tion: O 2 > RNO 3 =MnO 2 > FeðOHÞ 3 > SO 2À 4 . However, under anoxic conditions, only MnO 2 , Fe(OH) 3 and SO 2À 4 reduction were present, and they all occurred at the interface. The main biogeochemical controls on the mobility of As, Cr, Hg, MeHg and Ni in the surface sediments were identified as the adsorption/ desorption on and/or coprecipitation/codissolution with Fe oxy-hydroxides. In contrast, Co mobility was primarily controlled by its reactivity towards Mn oxy-hydroxides. In sulphidic sediments, As, Hg and MeHg were sequestered along with Fe sulphides, whereas Co and Ni precipitated directly as metallic sulphides and Cr mobility was enhanced by complexation with dissolved organic ligands. The fluxes of trace elements at the sediment–water interface are essentially dependent on the localisation of their remobilisation and immobilisation reactions under the interface, which in turn is governed by the ben-thic water oxygenation conditions and kinetic competition among those reaction and diffusion processes. Under oxic conditions, the precipitation of Fe or Mn oxy-hydroxides in the surface sediments constitutes the most efficient mechanism to sequester most of the trace elements studied, thus preventing their dif-fusion to the water column. Under anoxic conditions the export of trace elements to the water column is dependent on the kinetic competition during the reductive dissolution of Fe and/or Mn oxy-hydroxides, diffusion and immobilisation with sulphides. It is also shown that benthic organisms in the perennially oxygenated site have a clear impact on this general pattern. Based on the extensive dataset and geochem-ical modelling, it is predicted that the planned re-oxygenation of the entire lagoon basin, if complete, will most likely limit or reduce the export of the trace elements from the sediments to the water column and therefore, limit the impact of the contaminated sediment