Trends of simulated oxygen dynamics and hypoxia in the Baltic Sea

Over the last century hypoxia is increasing in large parts of the Baltic Sea. Inflowing salt water brings new supplies of oxygen to the bottom water however is at the same time enhancing stratification and thereby creating favourable conditions for hypoxia. Moreover, it is the increased flux of organic material to the bottom water and sediments due to nutrient enrichment, which has disrupted the balance between oxygen supply through physical processes and oxygen consumption from decomposition of organic material. The aim of the present work is to explore the decadal changes of oxygen availability in the Baltic Sea on the base of a 3D numerical model. The used 3D hydrodynamic model is the General Estuarine Transport Model code (http://getm.eu), which is implemented for the whole Baltic Sea including the Kattegat and is forced with measured water level data at the open boundary. The simulated time period covers the 5 decades from 1960 until the end of 2010. Oxygen concentration in the surface layer is mainly controlled by the air-sea gas exchange depending on wind speed, temperature and salinity. Oxygen consumption in the bulk and oxygen demand at the bottom are both parameterised simple as exponentially increasing functions of temperature and salinity. Despite this simplistic model approach, modelled oxygen concentrations agree well with independent observational data. The statistical analysis of the model data with regard to trends and structural breakpoints in oxygen concentration and anoxic bottom area reveal overall increasing trends. The detailed picture is however more complicated as shown by decreasing trends in the 60ties until the 70ties followed by a trend reversal and accelerated increasing trends after that time. The time series are characterised by a statistical significant breakpoint around the end of the 70ties.