Dense bottom gravity currents and their impact on pelagic methanotrophy at oxic/anoxic transition zones

Abstract

We show that inflows of oxygenated waters into sulfidic layers have a strong impact on biogeochemical transformation at oxic/anoxic transition zones. Taking the pelagic methane dynamics in the Gotland Basin as an example, we performed our studies when one of the largest inflows ever recorded entered the Baltic Sea in March 2015. An inflowing gravity current transported oxic waters into the sulfidic deep layers and freshly generated a near‐bottom secondary redox interface. At the upper slope, where the inflowing water masses were vigorously turbulent and the main and secondary redox interfaces in close contact to each other, methane oxidation rates inside the transition zone were found to be higher compared to the weakly turbulent basin interior. At the main redox interface in the basin center, lateral intrusions of oxygenated waters into intermediate water depth may have stimulated the growth of the methanotrophic community and their activity.

Effects of the 2014 major Baltic inflow on methane and nitrous oxide dynamics in the water column of the central Baltic Sea

“…Already by March, oxic water and noticeable changes in salinity were detected in the deepest part of the Eastern Gotland Basin (site BY15). These initial signals were a combination of new inflowing water and water pushed out of the Bornholm Basin ahead of the inflow (Schmale et al, 2016), as also occurred during the MBI of 2003 (Feistel et al, 2003). Over the following months, a distinct mass of saline, oxic inflow water accumulated in the EGB.…”

“…This displacement of CH 4 -rich stagnant deep water by CH 4 -poor inflow water may deplete the inventory of CH 4 in the water column of the EGB. However, due to turbulent mixing at the contact between the inflow and older water masses (Schmale et al, 2016), oxidation of CH 4 may also be expected to accelerate the depletion of CH 4 . In 2015, we observed the gradual erosion of a midwater [CH 4 ] maximum between March and August, followed by a re-accumulation of CH 4 in both mid-and deep water towards the end of the year (Fig.…”

“…The inflow was the third largest MBI observed since 1880 Mohrholz et al, 2015) and caused large changes in the dissolved oxygen concentrations throughout the southern and central Baltic Sea . The inflow strongly impacted the vertical distribution of CH 4 in the Gotland Basin (Schmale et al, 2016), which is the largest sub-basin of the Baltic Sea. Both advective processes (displacement and dilution of old CH 4 -rich deep waters by the CH 4 -poor inflow water) and aerobic oxidation of CH 4 (stimulated by mixing at the contact between these water masses) contributed to a decline in [CH 4 ] during 2015.…”

“…J.-P. Myllykangas et al: Baltic inflows and dissolved gases (Reissmann et al, 2009) and new redox fronts develop between new and old water masses (Schmale et al, 2016). As such, MBIs have a strong influence on many biogeochemical processes in the Baltic Sea.…”

Propagation of Impact of the Recent Major Baltic Inflows From the Eastern Gotland Basin to the Gulf of Finland

“…The impact of December 2014 MBI on the deepwater properties and dynamics in the Gotland Basin is well investigated. Schmale et al (2016) followed the propagation of December 2014 MBI water and recorded its arrival into the Gotland Basin in the beginning of March when increasing oxygen levels in the deepest layers terminated the 10-year stagnation period. The near-bottom layer was a mixture of the inflow water, water flushed out from the near-bottom region of the neighboring Bornholm Basin, and surrounding water entrained along the pathway of the inflow.…”