The deep waters of a Swedish fjord are hospitable to life after a team of scientists installed pumps to reverse oxygen deprivation.
Advanced marine species need oxygen in their water to survive. However, a number of processes can deplete oxygen, leading to dead zones, and human activities are making many of these more frequent and extreme. The most common example involves algal blooms, where excess nutrients produce a sudden burst of cyanobacterial activity, leaving an oxygen-deprived aquatic desert behind.
Feedback mechanisms that keep ecosystems healthy turn backwards, leading to a self-preserving state that can be very hard to fix. Thousands of natural resource managers have probably thought of pumping oxygen to where it might be useful; after all, it's what we do in millions of aquariums. However, actually trying it out on an ecosystem-wide scale is a different matter.
A team from the University of Southern Denmark and the University Gothenburg were worried about the spread of dead zones in the almost landlocked Baltic Sea. According to USD's Dr. Michael Forth, their "Swedish colleagues got the idea to use a pump to mix oxygen-rich surface water into the deeper parts of the water column in the fjord which was lacking oxygen.”
To test their idea, the team conducted their study in Byfjord, Sweden, a sill fjord that is 4 kilometers long (2.5 miles) and reaches 51 meters (167 ft) deep, but has a shallow entrance that prevents large amounts of oxygen-rich water from the open ocean reaching the main basin. The shape of the fjord means the low salinity surface water mixes particularly poorly with the denser water beneath, preventing oxygen from penetrating the depths.
Organic material from the surface sinks into the fjord's basin where it rots and consumes what little oxygen there is, sometimes triggering the production of hydrogen sulfide (rotten egg gas) which is toxic to fish.
The scientists responded by pumping surface water to the depths and comparing samples from the water column both with the situation in Byfjord before the pumping began and with two nearby control fjords. In the International Society for Microbial Ecology Journal, they report that the once dominant anaerobic bacteria declined and aerobic species appeared at previously unseen depths.
“Overall, the bacterial community in the formerly anoxic bottom waters changed to a community structure similar to those found in oxic waters, showing that an engineered oxygenation of a large body of anoxic marine water is possible and emulates that of a natural oxygenation event,” the authors report.
Most significantly, oxygen levels stayed high even after pumping stopped, raising hopes the idea might be applied to the whole Baltic Sea.
Why care which bacterial clade dominates an environment that humans barely visit? The SUP05 bacteria driven out by the oxygen contributes to the production of greenhouse gasses. Their replacements, on the other hand, can be food for larger creatures. "In the later phase of the experiment the entire water column began to look healthy," says Forth. "Many of the oxygen-needing bacterial species had returned and new bacterial communities similar to those in natural oxic fjords formed."