Land, rivers and the sea: Mechanisms and measures to reduce hypoxia and brownification in the Baltic Sea

The oxygen level in the ocean is expected to decrease by 3-4% during the next century and the prevailing explanation for decreasing oxygen levels is eutrophication.

Despite several efforts to reduce the transport of nutrients to coastal areas, hypoxia is still increasing in for example the Baltic Sea.

One explanation might be the increasing transport of terrestrial dissolved organic matter containing organic nitrogen and phosphorous which can cause eutrophication. However, the primary component of organic matter is carbon.

The bioavailable fraction of the carbon will be utilized by bacteria in the coastal areas and, in the Baltic Sea, this utilization has been estimated to consume 1 million tons oxygen per year.

Hitherto, there is little knowledge on what controls reactivity of dissolved organic carbon (DOC) and even less is known about efficient management efforts to reduce the transport into the ocean.

The aim of this project is to 1) compare the relative importance of environmental factors for bioavailability of carbon in Baltic Sea river catchments 2) study how bioavailability is affected by increasing terrestrial DOC concentrations and if the effect differs among rivers with different catchment areas, and 3) evaluate how to most efficiently reduce DOC concentration and bioavailable carbon using constructed wetlands.

These results are crucial to determine where and how to focus measures to reduce the transport of DOC and to take urgent action in climate mitigation.