Methane dynamics in the Baltic Sea: investigating concentration, flux and isotopic composition patterns using the coupled physical-biogeochemical model BALTSEM-CH₄ v1.0

Methane (CH4) cycling in the Baltic Sea is studied through model simulations that incorporate the stable isotopes of CH4 (12C-CH4 and 13C-CH4) in a physical-biogeochemical model. A preliminary CH4 budget identifies benthic release as the dominant CH4 source, which is largely balanced by oxidation in the water column and to a smaller degree by outgassing. The contributions from land loads and net export to the North Sea are of marginal importance. Simulated total CH4 emissions from the Baltic Sea correspond to an average 0.04 g CH4 m−2 y−1, which can be compared to the calibrated sediment source of 0.3 g CH4 m−2 y−1. A major uncertainty is that spatial and temporal variations of the sediment source are not well known. Further, the coarse spatial resolution prevents the model to resolve shallow-water near-shore areas for which measurements indicate occurrences of considerably higher CH4 concentrations and emissions compared to the open Baltic Sea. Modeling of stable CH4 isotopes can help to constrain process rates; to our knowledge this is the first time that CH4 isotopes have been included in a physical-biogeochemical model. A large-scale approach is used in this study, but the parametrizations and parameters presented here could also be implemented in models of near-shore areas where CH4 concentrations and fluxes are typically substantially larger and more variable. Currently, it is not known how important local shallow-water CH4 hotspots are compared to the open water outgassing in the Baltic Sea.