Recent regime of persistent hypoxia in the Baltic Sea

Abstract

Oxygen deficiency, in the form of hypoxia and anoxia, is a direct consequence of the eutrophication of the Baltic Sea. There is ongoing debate concerning the increasing extent of hypoxia. We analyse the integral metrics of hypoxia and anoxia: (1) temporal evolution of the hypoxic and anoxic area and volume, (2) the spatial distribution of the probabilities of hypoxia and anoxia occurrence in the Baltic Sea. The results are based on the state-of-the-art coupled physical and biogeochemical numerical model reanalysis data from Copernicus Marine Environment Monitoring Service for the period of 1993–2017. Statistical analysis showed that the variability of hypoxic and anoxic areas since the year 2000 represents stationary processes around their respective mean levels. From 2000 to 2017, the hypoxic area varies between 50000 and 80000 km2 and the anoxic area varies between 10000 and 50000 km2. Different methods and data sources indicate that the uncertainties of the estimates account for about 10000 km2. We suggest that the loss of stationarity of the time series of the hypoxic area would be an indication of the regime change of hypoxia development in the Baltic Sea. Probability distribution maps of hypoxia and anoxia provide detailed information about the persistency of hypoxia and anoxia in different parts of the Baltic Sea. The probability of hypoxia exceeds 0.9 in the eastern and western Gotland basins and in the deep area of the Bornholm basin. The Gulf of Finland and the shallower areas that connect different deep basins of the Baltic Sea exhibit seasonal and episodic hypoxia. The 80 m and 120 m isobaths are the approximate bathymetry limits of hypoxia and anoxia occurrence, respectively. Our study supports previous knowledge that hypoxia development is controlled to a large degree by the depth of the permanent halocline.