Highly variable bottom water oxygen concentration in the shallow Arkona Basin (Baltic Sea)

Shallow western Baltic Sea waters show seasonal low oxygen concentrations in bottom waters from summer into early autumn. A record of hourly oxygen measured from 2012 to 2017 at a platform in the central Arkona Basin ascertained important developments of distinct short-term variability in bottom water oxygen concentration. Analysis of these years indicated that severe hypoxia of 0–2 mg L −1 occurred in July with a duration between 0 and 10 h, in August between 0 and 61 h and in September again between 0 and 10 h. Using 4 mg L −1 as the threshold, the ranking of the respective years from longest to shortest duration in hypoxic events showed 2012 (982 h) as the worst, followed by 2016 (854 h), 2013 (769 h), 2014 (659 h) and 2015 (351 h). The clear improvement in 2015 was caused by a Major Baltic Inflow that ventilated the system. Interpreting the drivers of short term hypoxia from the in situ measurements alone was difficult, as adequately resolved biogeochemical parameters were not measured. However, coincident three dimensional high resolution coupled hydrographic-biogeochemical modelling, reproduced the oxygen concentrations at the platform, and additionally revealed considerable spatial variability in the oxygen distribution in the area and its causes. Microbial degradation of organic matter in bottom waters was found to be the major oxygen consumption process with nitrification also contributing in some years. Recovery from seasonal periods of hypoxia is linked to the break-up of the seasonal thermocline or is caused by lateral transport of differently preformed oxygenated waters. The study demonstrates the utility of combining high resolution in situ observations with coincident biogeochemical modelling, as a powerful, integrated tool for monitoring and understanding drivers of short-term seasonal hypoxia. • Bottom waters of the shallow Baltic Sea show seasonally low oxygen concentrations. • High resolution measurements with biogeochemical modelling uncover oxygen behavior. • Drivers of hypoxia were determined with 3D hydrographic-biogeochemical modelling. • Saltwater inflows that bring less oxygen in warming oceans need observation.