Methane dynamics in a shallow non-tidal estuary (Randers Fjord, Denmark)

Methane concentrations, oxidation rates, exchanges at the sediment-water interface and emissions to the atmosphere were studied between February and December 2000 along an estuarine gradient in Randers Fjord (Denmark), Methane concentrations measured at 3 stations, 1 in freshwater, 1 in brackish water (salinity 3 to 7) and I in saltwater (salinity 17 to 23), showed high supersaturation with respect to atmospheric equilibrium, with concentrations ranging from 186-420, 70-290 and 28-124 nM and median concentrations of 347, 125 and 41 nM respectively. Calculated median fluxes to the atmosphere were 355, 126 and 40 mumol m(-2) d(-1) at the 3 stations respectively. The contribution of water-column methane oxidation to the total methane sinks (oxidation and emission) was 22 to 42% in the river (depth 8 m), but fell to less than 3% at the brackish station, owing to lower rates and shallow depth (1.7 m). No oxidation could be detected in the saltwater. Methane fluxes through the sediment-water interface were directed downwards at the brackish station (from -19 to -353 mumol m(-2) d(-1) in December and July respectively) and upwards at the saltwater station (from 3 to 400 mumol m(-2) d(-1) in March and July respectively). At the brackish station, methane uptake by the sediment accounted for 16 to 55% of the total methane sink. Potential aerobic methane oxidation in surface sediments revealed the presence of a population of methanotrophs active at ambient methane concentrations at the brackish station, but not at the saltwater station. During summer, methane production at the saltwater station appeared to occur in the first 1 cm of the sediment and was 40 times higher than at the brackish station. The turnover time of methane relative to all sinks was 4 to 7 d in the freshwater, 0.5 to 1.8 d in the brackish water and 0.6 to 4.7 d in the saltwater. Our results confirm the important role of the estuarine zone in recycling methane, Most of the methane carried by the river is oxidised and released to the atmosphere in the upper estuary, and new production of methane occurs in the lower estuary, where, in addition, oxidation is inefficient.