The cause is not the widely published issues of plastic pollution and overfishing, but a less visible phenomena – ocean deoxygenation.
“Our oceans are losing oxygen,” says physical oceanographer Patricia Handmann. “Since the 1950s, they have lost already 2% and they are predicted to lose 7% in the future, in the next 100 years – which is, of course, a challenge for all [animals and plants] living in the ocean.”
In 1960, there were 45 coastal areas with dangerously low oxygen levels, known as hypoxia. Almost 60 years later, a 2018 research paper found there were more than 700 worldwide.
The main cause, as with many other environmental issues, is climate change. As the surface of the ocean warms, it holds less oxygen. It is also more buoyant, so it distributes less oxygen to lower levels, which naturally contain less of it.
Other culprits include sewage and fertiliser run-off, which cause nutrient pollution known as eutrophication. This leads to algal blooms, which further deplete oxygen levels, killing fish and seagrass. Bacteria that decomposes the algae also uses oxygen before mobilising other substances in the sediment, such as phosphorus, which feeds new algae.
It is a “vicious cycle”, Handmann says, which can continue until environments are unable to sustain underwater life. That is bad news not just for plants and animals, but also the humans that rely on them for food and livelihoods.
“There are large parts of the population that live within coastal areas and they are directly affected by this,” says Handmann, oxygen adviser for French green hydrogen firm Lhyfe. “The sad news is that, until now, there were no real mitigation measures for that, apart from limiting nutrient import and also limiting, of course, greenhouse gas emissions.” These are both slow processes, however.
This led Lhyfe to consider the opportunities presented by offshore hydrogen production, which it has successfully trialled off the coast of Western France.
Electrolysis of water generates not just hydrogen but oxygen, which is normally seen as a ‘waste’ byproduct. Lhyfe hopes it could instead be used to combat some of the worst effects of climate change in marine environments.
Breathing new life into coastal areas
Aiming to replace ‘grey’ hydrogen produced from fossil fuels, the Nantes-headquartered company plans to use wind turbines to generate green hydrogen offshore. It will further test offshore production technology in the upcoming Hope project in Belgium, which could eventually produce up to four tonnes of hydrogen per day.
Until now, electrolyser engineering has focused on maximising hydrogen production, with oxygen usually vented into the atmosphere. Lhyfe’s upcoming BoxIn project, which it is seeking funding for, aims to find out what would happen if it was instead captured and pumped underwater to reoxygenate depleted areas.
The Sealhyfe green hydrogen platform (right) and the floating offshore wind turbine used to power it
The pilot experiment will test injection of oxygen at one of three potential Swedish sites, identified in a report with regional hydrogen project developer Flexens and Stockholm University: Byfjorden in Uddevalla, or Säbyviken or Skarpösundet in Stockholm.
Liquid oxygen storage will be connected to a vapouriser, which will transmit gaseous oxygen through a tube placed into the water column and anchored to the seafloor. The flow rate will be regulated from shore by changing the size of bubbles allowed through the porous tube.
“Salinity and temperature play a crucial role for all living things in the water, and also to the [water] dynamics, of course, and that's something that we don't want to interrupt, which means that the bubble size has to be adapted to a very small size that can be absorbed by the surrounding water,” Handmann says.
The pilot project is expected to cost €5-6m and last around six years, during which time the habitat will be monitored to ensure it is recovering. If successful, the longer-term objective of the project will be integrating oxygen injection and electrolysis.
Finding the right electrolysis process and equipment will be one of the main challenges, Handmann says. The oxygen will need to be pressurised, which is possible with alkaline electrolysers, but they are not well-adapted to harsh offshore conditions. Proton exchange membrane (PEM) electrolysis could be used instead, but that would require additional equipment to pressurise the oxygen.
Alongside environmental considerations, other issues in full-scale deployment will include technical challenges around distributing oxygen in deeper waters.
“The technique is thought to be very nicely scalable,” Handmann says. “It has been shown also in fresh water environments.” A larger-scale system to tackle the issue around the Baltic Sea could require 10 to 20 platforms, she adds.
‘We have to be proactive’
Capturing and using oxygen from electrolysis could offer a neat way of avoiding hazards caused by venting, says Dr James Carton, a sustainable energy expert at Dublin City University who is not involved in the project. “Oxygen is really highly corrosive, very reactive, when it's in high purity coming off an electrolysis system,” he says. “There's challenges of electrostatic discharge build-up, etcetera.”
The pipes used in a reoxygenation system will need to cover a “huge” area, according to Carton, to make sure the oxygen is distributed away from the platform and throughout the water. If a floating platform is used, they will also need to be flexible to prevent damage as it shifts on the waves.
Seawater electrolysis also often involves desalinating the water first, which will create brine that will need to be managed. Additional power will be needed for that process, Carton says, and to pump the oxygen underwater.
Although several challenges remain to be solved, the project is a “positive thing” according to Carton: “We have to be proactive in solving the problems we've caused, such as eutrophication and deoxygenation of our seawater.”
He adds: “If we're producing hydrogen through electrolysis, there's oxygen. We can do stuff with that. And if this is one of the really valuable things to do with it, let's do it.”
Local solutions, global problems
For some people reckoning with the impact of human expansion on our natural environments, more meddling might feel like the last thing we need. Those viewpoints are being considered, Handmann says, as well as the views of coastal communities.
“In the end, it's a political decision to implement things like this, and for that, of course, scientifically led guidelines have to be put in place now. We are also working with Unesco, for example.” The previous BoxHy project, which assessed the feasibility of the process, was also endorsed by the UN Decade of the Ocean.
She continues: “This is really important to us. It's not something that has to be done just for the image of the company, or ‘just for doing it’. It has to be done to really help mitigate the problem that's there, and it only has to be done if it's actually good for the environment.”
A similar process has been floated for other areas around the world, including the Gulf of St Lawrence in Canada, which has lost oxygen “faster than almost anywhere else” according to the University of Washington.
While it can make a real difference, Handmann stresses that it is a local restoration method. “It's not a solution to a global problem. The only solution to this global problem is to limit greenhouse gas emissions and our nutrient inputs to the coastal environment.”
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