Nitrous oxide and methane in European coastal waters

Coastal areas such as continental shelves, estuaries, deltas, fjords and lagoons can release high amounts of nitrous oxide (N2O) and methane (CH4) to the atmosphere. However, estimates of trace gas emissions are often biased by incomplete spatial and temporal coverages. Based on a compilation of literature data, the distributions of N2O and CH4 in European coastal areas (i.e. Arctic Ocean, Baltic Sea, North Sea, northeastern Atlantic Ocean, Mediterranean Sea, Black Sea) were reviewed and their emissions to the atmosphere reassessed. Maximum N2O saturations were found in estuarine systems, whereas the shelf waters, which are not influenced by freshwater plumes, are close to equilibrium with the atmosphere. This implies that N2O is mainly formed in estuarine systems. European coastal waters are a net source of N2O to the atmosphere (0.33e0.67 Tg N year 1) with the major contribution coming from estuarine/river systems and not from open shelf areas. European shelf areas contribute significantly (up to 26%) to the global oceanic N2O emissions. CH4 saturations show a high temporal and spatial variability with maximum values in estuarine/fjord systems. European coastal areas are a source of atmospheric CH4 (0.35e0.75 Tg C year 1) and contribute significantly to the overall global CH4 oceanic emissions. However, this estimate still seems to be a severe underestimation since CH4 fluxes from estuaries and shallow seeps are not adequately represented. Future N2O and CH4 emissions from coastal areas strongly depend on the degree of eutrophication of coastal waters and might increase in the future.