Seasonal variation of methane in the water column of Arkona and Bornholm Basin, western Baltic Sea

Abstract

Methane and the hydrographic parameters temperature, salinity and oxygen (T, Sal, O2) were surveyed at several stations and along selected transects in the Arkona and Bornholm Basin, western Baltic Sea, between 2009 and 2012. The methane distribution in the two adjacent basins show annually reoccurring as well as seasonal variations, governed by stratification of the water column during the summer period, enhanced vertical mixing during winter and the frequent inflow of oxygen-rich saline water from the North Sea. The Arkona Basin is characterized by low methane concentrations in the surface water with increasing values towards the bottom water layer. Elevated methane concentrations were found in the bottom layer during the summer period. The Bornholm Basin also shows low methane concentrations in the surface water, but high values in the bottom water layer throughout the year. For anoxic conditions, often prevailing in the Bornholm Basin in summer, a positive correlation between methane and hydrogen sulfide concentrations was observed. Strong depletion in the stable isotopic ratio of methane in the deeper waters of the Bornholm Basin reveals effective oxidation processes. The midwater region from 50 to 70m water depth in the Bornholm Basin is characterized by a methane-enriched water layer, persistent throughout the survey period. High-resolution hydrographic modelling of the physical driving forces suggest this finding to be caused by intrusion of methane-rich waters originating from the Arkona Basin into the water column of the Bornholm Basin and is shown to be a powerful tool for the interpretation of the development of the methane distribution in space and time.