Methane emission from the Pregolya River Estuary (Baltic Sea).

Measurements of dissolved methane were performed in a sequentially connected system of water bodies that form the Pregolya River Estuary: Pregolya River - Kaliningrad Sea Canal - Vistula and Curonian Lagoons - Baltic Straight - Baltic Sea. Methane concentrations were detected on a gas chromatograph with a flame ionization detector, and the fluxes were calculated using a boundary layer gas transfer model. During a study period between 2016 and 2022, the Pregolya River is characterized by the high range of methane concentrations, reaching 8838 nmol/L in the annual maximum in May. This could be result of the "cumulative effect" of the contact of fresh and brackish waters and the inhibition of river flow during the surge events, probably, in combination with episodic release of methane from the bottom sediments. The estuary water was constantly oversaturated in methane, thus, emission from water into the atmosphere reached 1018 μmol/m/day. The study area was notable source of methane to the atmosphere. In spring and in summer, subsurface maximums in methane were observed in the gas-saturated mud area on the Gdansk Deep slope, which did not seem to be related to the spreading of methane-reach water from the Baltic Straight.