Dynamics of Two Greenhouse Gases, Methane and Nitrous Oxide, Along the Rhone River Plume (Gulf of Lions, Northwestern Mediterranean Sea)

The concentrations of two greenhouse gases, nitrous oxide (N2O) and methane (CH4), and the bacterial processes involved in their production (nitrification and denitrification for N2O, and methanogenesis for CH4) were determined in a water column, along the Rhone River plume, in the Gulf of Lions, northwestern Mediterranean Sea. High concentrations of dissolved CH4 and N2O were recorded in the surface waters, up to 1,300 nM for CH4 and 40 nM for N2O, versus, 2–3 nM, and 5 nM in the open ocean surface waters. Along the Rhone River plume, the highest concentrations of CH4 were detected near the river mouth, whereas N2O remained present throughout the plume, up to the marine station. Bacterial production rates of N2O and CH4 indicated that the Rhone River plume constituted an area favorable for the development of nitrifiers, denitrifiers, and methanogens, promoting their activities in suspended particles. Nonetheless, no direct relationship could be found between the concentration and the production of the biogases, suggesting that the dissolved gases were not necessarily produced in situ, in surface seawater, but could be produced in deep water or bottom sediment, and then transported to the surface waters. The presence of extremely high concentrations of CH4 and N2O, in superficial seawater implies they can escape directly to the atmosphere; consequently, these near-shore waters enriched in greenhouse gases may play an important role in the atmospheric increase of both CH4 and N2O concentration, and thus in the global climate change.