2005-2019 CTD (48M, Sea & Sun Technology GmbH, Trappenkamp, Germany) measurements and water samples at the surface (0 m depth), at 7 m and 18 m depth were taken biweekly between 2005 and 2019 at the “Wittlingskuhle” a bit off the GEOMAR pier in the Inner Kiel Fjord (N 54°19.69, E 10°09.06). The oxygen concentration of these water samples was measured by the Winkler- iodometric titration method (Winkler 1888) in mg/L and are converted to oxygen-saturation values by correcting for temperature, salinity and pressure.