MARINE ECOLOGY PROGRESS SERIES Mar Ecol Prog Ser Vol. 230: 171–181, 2002 Published April 5 INTRODUCTION Methane is an atmospheric trace gas that contributes significantly to the greenhouse effect. The methane concentration in the atmosphere has been increasing over the last century, indicating that sources are larger than sinks. Marine ecosystems are presently estimated to play only a modest role in the global methane bud- get, their contribution representing between 1 and 10% of the total emissions (Cicerone & Oremland 1988, Bange et al. 1994). Although shelf and estuaries represent only 15 and 0.4% of the global ocean area, they contribute about 68 and 7 to 10% of the total oceanic emissions respectively (Bange et al. 1994, Upstill-Goddard et al. 2000, Middelburg et al. 2001). Methane concentrations in estuaries are generally orders of magnitude higher than the atmospheric equi- © Inter-Research 2002 · www.int-res.com * *Present address: Université Bordeaux 1, Département de Géologie et Océanographie, CNRS-UMR EPOC 5805, Ave- nue des Facultés, 33405 Talence, France **Corresponding author. E-mail: i5ni@civil.auc.dk Methane dynamics in a shallow non-tidal estuary (Randers Fjord, Denmark) Gwenaël Abril * , Niels Iversen ** Aalborg University, Department of Life Science, Environmental Engineering Laboratory, Sohngaardsholmsvej 57, 9000 Aalborg, Denmark ABSTRACT: Methane concentrations, oxidation rates, exchanges at the sediment-water interface and emissions to the atmosphere were studied between February and December 2000 along an estu- arine gradient in Randers Fjord (Denmark). Methane concentrations measured at 3 stations, 1 in freshwater, 1 in brackish water (salinity 3 to 7) and 1 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 μmol 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 μmol m –2 d –1 in December and July respectively) and upwards at the saltwater station (from 3 to 400 μmol m –2 d –1 in March and July respectively). At the brackish station, methane uptake by the sed- iment accounted for 16 to 55% of the total methane sink. Potential aerobic methane oxidation in sur- face 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 pro- duction 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. KEY WORDS: Riverine methane · Estuary · Sediment · Aerobic methane oxidation · Methane production Resale or republication not permitted without written consent of the publisher