Rapid re-oxygenation of Baltic Sea sediments following a large inflow event.

The Baltic Sea is the second largest (373 000 km) brackish water system in the world and experiences long periods of anoxia and hypoxia (\2 ml l) below the halocline at approximately 80 m depth. This ‘dead zone’ could have a below-halocline extension of 70 000 km (2011), which is similar to the area of Scotland. Although the Baltic Sea has had periods of low oxygen in historical times (Zillén et al. 2008), it was not until in the middle of the last century that anthropogenetic-induced eutrophication made the situation worse. During the last decade, the expansion of Baltic Sea dead zones has been the largest in centuries. Inflow of oxygen-rich high-salinity water into the Baltic Sea, via the Danish Straits, has been reduced since the 1980s with large inflows occurring in 1993 and 2003. In December 2014 and January 2015, a large inflow of oxygen-rich water entered the Baltic Sea and raised the salinity by up to 1 unit and re-oxygenated the deepest parts. The effect of this episode on the oxidation of the reduced sulphidic sediments was examined on 2 July 2015 in the deep basin east of the island Gotland in the Baltic proper using sediment profile imagery (SPI). The oxygen-rich water reached this area in March. Images (size 16 9 24 cm) of the sediment–water interface were obtained using a digital camera (Canon EOS D70) inside a prism mounted on a tripod, which operates like an up-side-down periscope and penetrates into the sediment (Nilsson and Rosenberg 2000). For improvement of the analyses, the colours of the images were enhanced in Adobe Photoshop CS6 Extended.