Seasonal Study of Methane and Nitrous Oxide in the Coastal Waters of the Southern Baltic Sea

Abstract Atmospheric and dissolved methane (CH 4 ) and nitrous oxide (N 2 O) were measured in the unique coastal ecosystem of the Bodden waters, including the western Oder estuary, (southern Baltic Sea) during five campaigns between 1994 and 1997. The CH 4 saturations, ranging from 105–15 500%, showed great spatial and temporal variability with maximum values in September and minimum values in December. The N 2 O saturations were in the range of 91–312% with a maximum in March. Enhanced concentrations of both gases were observed only in the western Oder estuary near the mouth of the Peene River. Thus, we conclude that the distributions of CH 4 and N 2 O in the investigated Bodden waters are, directly or indirectly, linked to the Peene River runoff and not to the Oder River. Our estimate of the annual CH 4 emissions from the Bodden waters to the atmosphere indicates a significant contribution ( c . 17%) to the overall CH 4 emissions from the Baltic Sea. In contrast, the Bodden waters represent only a small source for atmospheric N 2 O. CH 4 production rates estimated from sediment slurry experiments revealed a significant spatial variability and indicated that methanogenic activity was related to acetate consumption in the surface sediment layer. Sedimentary CH 4 production might depend on different amounts of accumulation of organic material.