Bodden waters (southern Baltic Sea) as a source of methane and nitrous oxide

Abstract A first assessment of N 2 O and CH 4 emissions from Bodden waters (southern Baltic Sea) was carried out with the aim of investigating the seasonal and spatial variability of the emissions as well as characterizing the corresponding biological processes (i.e. denitrification, nitrification, and methanogenesis) along pronounced gradients in trophic status and salinity. Saturation values of CH 4 varied from 102 to 16,300%. Supply of organic carbon to the sediments, temperature, and input of riverine CH 4 can be considered as important controls on methane concentration. The influence of salinity seems to be insignicant in this respect. On average, 86% of sedimentary methane is oxidized within the sediment. In contrast, methane oxidation is restricted to the water column when stagnation of water bodies occurs. In addition to the seasonal pattern, a diurnal rhythm of methane concentrations was detected. The N 2 O saturation values of the Bodden waters were low (90 to 309%), andshowed a significant correlation with nitrate concentrations (r=0.83). The Peene River was identified as a strong source for allochthonous N 2 O. From the present data it is hypothesized that, in the inner Bodden waters, high riverine loads of nitrate (which fuels denitrification) and allochthonous N 2 O might cause enhanced concentrations of N 2 O. In the outer Bodden, waters, nitrification may be the prevailing source of nitrous oxide, especially in summer. A preliminary estimate gives an annual emission of 58 tonnes N 2 O, and 960 tonnes CH 4 for the whole Bodden area. Thus, Bodden waters constitute an important source of methane, and a modest source of nitrous oxide in the Baltic region.