River Thames was pumped full of oxygen in 2022 to prevent fish deaths

The river Thames in London was so starved of oxygen in August 2022, due to sewage and hot weather, that it was effectively on life support, New Scientist can reveal. A specialised boat was used to directly inject the gas into the water for 11 days that month to prevent plants and fish dying. As temperatures rise due to climate change, this may have to be done more often, despite recent efforts to clean up the river and improve oxygen levels.

Maintaining healthy oxygen levels in rivers is vital for plant life and fish, says John Sullivan at Thames Water, the company that oversees the river. But sewage spills and hot weather can lead to a dramatic drop. For example, on 19 June this year, thousands of fish were found dead in the river Soar in Leicester, UK, due to low oxygen levels.

Hot weather is a factor because water holds less oxygen at higher temperatures, says Sullivan, while sewage encourages the growth of bacteria, plants and algae that consume the gas.

To address this, Thames Water operates two boats, Bubbler and Vitality, that can each inject 30 tonnes of oxygen into the river a day.

Last year, Vitality pumped this gas into the Thames for 11 days across August, according to a request made by New Scientist under environmental information regulations, which are designed to ensure such details become public. “[This was] due to [low] oxygen levels in the water, resulting from the summer and pollution build-up in the water,” says a Thames Water spokesperson.

Charles Tyler at the University of Exeter, UK, says this injecting technique is a “last resort”. “If you have to go into a river and pump it full of oxygen to save the lives of fish or invertebrates – it’s crisis management,” he says. “I’m not saying it’s a bad thing, but I just think that it’s tackling the problem a little bit late in the game.”

Both vessels have been in use for decades, but the need for them has been greatly reduced since 2015, when the development of several sewage overflow and treatment facilities cut the discharge of raw sewage into the Thames by close to 50 per cent, says Sullivan. The boats were used for 330 hours in 2015, but only a handful of hours in the intervening years, before logging 215 hours in 2022.

Sullivan suggests that a new London super sewer due to be operational in 2025 should stop up to 95 per cent of the overflow of raw sewage into the Thames, which may make the boats obsolete.

But climate change could mean that Thames Water will have to keep injecting oxygen into the river, says Sullivan. “Warmer temperatures and less rainfall don’t add up to a good concept for oxygen levels in the Thames,” says Sullivan.