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.
- Publication:
-
Journal of Geophysical Research (Oceans)
- Pub Date:
- May 2018
- DOI:
- 10.1029/2017JC013686
- Bibcode:
- 2018JGRC..123.3227M
- Keywords:
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- hypoxia;
- coastal seas;
- Baltic Sea;
- environmental monitoring;
- numerical modeling;
- oxygen consumption