Extra oxygen breathes life back into a deep fjord

By pumping oxygen-rich surface water into the oxygen-starved water at the bottom of the sea, a Danish/Swedish research team has helped resuscitate a dying fjord.

A report on their efforts has been reported in the multidisciplinary journal of microbial ecology, ISME. The pilot project, in which pumping took place intermittently between 2010 and 2013, saw the return of oxygen-loving organisms on the bottom of the fjord.

Low levels of oxygen in bottom waters, also known as hypoxia, is a particular problem in the Baltic Sea. It is caused by nutrients from farming and other human activities flowing into the sea and causing algal blooms. The blooms sink to the bottom consuming more oxygen as they decompose than is added by natural processes. Anoxia, or lack of oxygen, kills organisms that require oxygen and creates dead zones in the sea.

Scientists from the University of Southern Denmark and the University of Gothenburg in Sweden wanted to test if pumps could enhance a natural venting process that takes oxygen-rich water into the deeper parts of the sea. The experiment took place in the 51-metre deep-Byfjord in Sweden. Surface water was pumped to a depth of 35 metres, and was replaced with new water flowing in from the Kattegat Ocean. Overall, 50% of fjord’s bottom water was replenished in this way.

Postdoctoral researcher Michael Forth and Professor Alexander Treusch from the Nordic Centre for Earth Evolution at the University of Southern Denmark said:

“In the later phase of the experiment the entire water column began to look healthy. Many of the oxygen-needing bacterial species had returned and new bacterial communities similar to those in natural oxic fjords formed. This showed us that the idea worked.”

Scientists do not yet know if the oxygen levels will fall again now the pumps have stopped. They believe the experiment could be replicated in the Baltic Sea where conditions are similar.

Large Oxygen Minimum Zones exist off the coast of Chile and in the Arabian Sea and are growing. According to climate models this growth may lead to more organic nitrogen being lost and may affect the global carbon cycle.

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