Recently Accelerated Oxygen Consumption Rates Amplify Deoxygenation in the Baltic Sea

Abstract

Many coastal seas suffer from expanding bottom‐water deoxygenation and hypoxia primarily because of excessive nutrient loads from land. The Baltic Sea in northern Europe has one of the largest anthropogenically induced oxygen‐deficient bottom zones in the world. Despite the decrease of nutrient supply after the 1980s, recently observed oxygen consumption rates are higher than ever observed, limiting the impact of natural ventilation by oxygen‐enriched saltwater intrusions. We have estimated oxygen consumption rates after saltwater inflows during subsequent stagnation periods from monitoring observations and model results for 1850–2015. In recent years, ventilating water that originates mainly from the surface layer has contained higher concentrations of organic matter, zooplankton, and higher trophic levels. As a result, oxygen consumption in the water column has increased relatively more than oxygen consumption in the sediment, primarily due to respiration of zooplankton and higher trophic levels. Subsequently, natural ventilation has become less effective in alleviating hypoxia, instead amplifying deoxygenation of the deep water. We propose that such a detrimental, positive feedback may also affect other coastal seas with nutrient excess and with an intense, internal recirculation. Other drivers of oxygen consumption, like warming, were found to be less important under contemporary conditions.

A multi-year observation of nitrous oxide at the Boknis Eck Time Series Station in the Eckernförde Bay (southwestern Baltic Sea)

“…In December 2014, a strong saline water inflow from the North Sea was observed, which was the third strongest ever recorded (Mohrholz et al, 2015). Although the salinity in December 2014 was comparable to other years, a remarkable increase in salinity was observed in the following several months.…”

Summer hydrographic changes in the Baltic Sea, Kattegat and Skagerrak projected in an ensemble of climate scenarios downscaled with a coupled regional ocean–sea ice–atmosphere model

“…The Baltic Sea is a brackish sea and has very limited water exchange with marine water originating from the North Sea and therefore has long flushing times in the order of 2-3 decades (e.g. Meier 2007;Meier et al 2018b). The average depth is about 50 m. It is permanently stratified in most areas due to a strong halocline at 60-80 m depths (e.g.…”

“…Together with the Danish straits this area constitutes the pathway for the more or less sporadic severe inflows of salt-rich water masses from the North Sea to the Baltic Sea (Major Baltic inflows, MBIs) which can be monitored far into the deep Baltic Proper and which play an important role in controlling oxygen conditions in the deep Baltic (e.g. Matthäus and Franck 1992;Meier et al 2018b).…”

A multi-year observation of nitrous oxide at the Boknis Eck Time-Series Station in the Eckernförde Bay (southwestern Baltic Sea)

“…A significant nutrient decline has been observed at the BE Time-Series Station since the mid-1980s, however, Lennartz et al (2014) found that bottom O 2 concentrations were still decreasing over the past 60 years. The ongoing oxygen decline was attributed to the temperature-enhanced O 2 consumption in the bottom water (Meier et al, 2018) and a prolongation of the stratification period at the BE Time-Series Station (Lennartz et al, 2014). Please note that the trends in nutrients and O 2 concentrations were detected based on the data collection which lasted for approximately 30 and 60 years, respectively, while the N 2 O observations at BE Time-Series…”