Oxygenated deep bottoms beneath a thick hypoxic layer lack potential of benthic colonization

After many years with mainly anoxic conditions, the deepwater in the East Gotland Basin (EGB) in the Baltic became oxygenated when a huge inflow of new deepwater from the Kattegat in December 2014 reached the EGB (e.g. Mohrholz et al. 2015). During such inflows, the rather saline and dense new deepwater intrudes along the seabed. Some of the residing old deepwater was mixed into the new deepwater, which successively reduced both the salinity and the oxygen concentration of the intruding new deepwater. The unmixed portion of the old deepwater was lifted over the new deepwater. In March 2015, about 3 months after the inflow from the Kattegat, the new deepwater reached station BY15 in the EGB as can be seen from the strong increase of salinity (Fig. 1a) and the appearance of oxygen (Fig. 1b). BY15 is located ca 10 km east of Station 5 (Fig. 2). On 2 July 2015, about 4 months after the oxygenation of the deep bottoms in the EGB, Rosenberg et al. (2016) took sediment profile images (SPI) of the upper part of the sediment at Stations 1 to 5, located at 60, 140, 175, 200 and 210 m depth, respectively. Station 1 is a reference station located at the top of the halocline above the deepwater. The images showed thin oxidized layers at the sediment surface at all stations except Station 2 (Rosenberg et al. 2016). The chemical conditions in the top sediments in 2015 are described in that paper in relation to the redox conditions. The sediment images showed signs of macroscopic life only at shallow Station 1 where signs of bioturbation by sediment-dwelling animals (infauna) were observed. The lack of signs of animals on the deepest stations, 3 to 5, was believed to be due to a too brief time of exposure to oxygenated water above the sediment. In an earlier engineering experiment, with forced oxygenation of the usually anoxic deepwater in the By Fjord on the Swedish west coast, fauna was established about one year after oxygenation (Stigebrandt et al. 2015). To follow the progress of possible colonization and the progress of chemical oxygenation and reduction processes of the bottoms overlain by hypoxic water in the deeper EGB, a SPI survey similar to the July 2015 survey of Rosenberg et al. (2016) was undertaken on 10 April 2016. The results of these observations are shown and discussed below. A long-term historic development of oxygen records of the Baltic and the Gotland Basin is given by Conley et al. (2009).