Abstract
Abstract. In late 2014, a large, oxygen-rich salt water inflow entered the Baltic Sea and caused considerable changes in deep water oxygen concentrations. We studied the effects of the inflow on the concentration patterns of two greenhouse gases, methane and nitrous oxide, during the following year (2015) in the water column of the Gotland Basin. In the eastern basin, methane which had previously accumulated in the deep waters was largely removed during the year. Here, volume-weighted mean concentration below 70 m decreased from 108 nM in March to 16.3 nM over a period of 141 days (0.65 nM d−1), predominantly due to oxidation (up to 79 %) following turbulent mixing with the oxygen-rich inflow. In contrast nitrous oxide, which was previously absent from deep waters, accumulated in deep waters due to enhanced nitrification following the inflow. Volume-weighted mean concentration of nitrous oxide below 70 m increased from 11.8 nM in March to 24.4 nM in 141 days (0.09 nM d−1). A transient extreme accumulation of nitrous oxide (877 nM) was observed in the deep waters of the Eastern Gotland Basin towards the end of 2015, when deep waters turned anoxic again, sedimentary denitrification was induced and methane was reintroduced to the bottom waters. The Western Gotland Basin gas biogeochemistry was not affected by the inflow.
Highlights
The Baltic Sea is a shallow, semi-enclosed brackish water body
In this study we focus on two gases that are strongly influenced by the spatial distribution of hypoxia and anoxia in the Baltic Sea: methane (CH4) (Schmale et al, 2012) and nitrous oxide (N2O) (Hietanen et al, 2012)
The earliest major impact of the inflow on deep water oxygen was observed at the southernmost station BY10, where oxygen was detected already in March
Summary
The Baltic Sea is a shallow, semi-enclosed brackish water body It receives large fresh water inputs from the rivers along its coast, and exchanges saline water with the North Sea through the narrow Danish straits, principally via the Darss Sill and the Drogden Sill (Fig. 1). Intermediate-depth water masses in the southern areas of the Baltic Sea are ventilated frequently, deep waters of the central Baltic are renewed only during major Baltic inflow (MBI) events, during which large amounts of saline oxygenrich water enter the Baltic through the Danish straits over a short period of time (Schinke and Matthäus, 1998). When oxygen is introduced by MBIs, large quantities of the previously accumulated reduced compounds in the deep waters are subsequently oxidized
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