Gas bubble release areas as new potential hot spots for water column enrichment with nutrients in eutrophicated sea

The study examined how physicochemical and hydrochemical conditions in the water column above methane seeps differ from those at reference stations. Seawater samples were collected over gaseous sediments with active gas outflow and nearby sites without gas leaks. 2-year observations of a water column above active pockmarks in the south-eastern Baltic Sea (Gulf of Gdańsk) were complemented by a study of three hydrologically different regions of the southern Baltic (Arkona Basin, Mecklenburg Bay and Eckernförde Bay). Higher nutrient concentrations (in particular phosphate and silicate) were observed above gaseous sediments, and the difference between seep and reference sites was most pronounced when a strong halocline was present in the water column. The results suggest that the release of gas from the seabed can intensify the exchange of substances at the sediment-water interface and contribute to an increased concentration of nutrients in the water. Strong enrichment of the water below the halocline can be expected especially in autumn, when a large amount of organic matter is decomposed after the growing season, and the intensity of gas outflow from the bottom also increases. Future studies are needed to assess the impact of this process on the marine environment. Nutrients released from the sediments can be transported to the euphotic layer through upwelling and increased mixing in autumn and winter and support phytoplankton production in subsequent seasons.