Marine heatwaves pose threat to seagrass restoration efforts

Emily Price

A marine heatwave has undermined the success of seagrass restoration efforts at a site in north Wales, new research has found.

Over 1.1 million seagrass seeds were planted at sites in two bays in north Wales between 2023 and 2024 as part of Seagrass Ocean Rescue: North Wales, the UK’s largest active seagrass restoration programme.

Despite being located less than 50km apart, the planting efforts in Bae Caergybi saw seedling establishment and overwinter survival where the planting at Bae Tremadog failed.

Research undertaken by a team of scientists from Project Seagrass, Swansea University, the Met Office, and North Wales Wildlife Trust and supported by staff from WWF and Pen Llŷn a’r Sarnau Special Area of Conservation (SAC), revealed the impact that a marine heatwave had on the restoration efforts at Bae Tremadog, alongside the implications of this for future seagrass restoration planning.

Dr Oliver Thomas, Senior Science Officer at Project Seagrass said: “As another record-breaking year for global temperatures gets underway, this research is highly relevant in showcasing the risk marine heatwaves pose to ongoing marine restoration efforts.

“Our hope is that the findings of this paper will assist other practitioners in future-proofing their own projects, facilitating greater restoration success in the long term”

Marine heatwaves are periods of extreme sea surface temperature that persist for 5 days or more and can have severe effects on our Ocean’s wildlife, including the destruction of marine habitats.

Over the last century there has been a global increase in marine heatwave frequency and duration as a result of a rapidly changing climate, including in locations which were previously considered to be low risk such as the north Atlantic.

Heatwave

Satellite and ground-level environmental data revealed that the failure of seedling establishment in Bae Tremadog coincided with a rapid-onset marine heatwave.

During the hot spell, sea surface temperatures increased by 3.5 degrees Celsius in a week and intertidal temperatures exceeded 30 degrees Celsius during low tides.

By contrast, Bae Caergybi experienced cooler and more stable conditions.

Dr Caroline Rowland, head of Ocean Cryosphere and Climate at the Met Office said: “The frequency and duration of marine heatwaves is increasing around the UK.

“As our waters warm, it becomes easier to cross the line into marine heatwave conditions.

“This week is a clear example of this, when long hours of sunshine, low winds and high levels of atmospheric moisture have triggered widespread marine heatwave conditions.”

Restoration

Ricardo Zanre, Senior Programme Manager- UK Seascapes at WWF UK said: “As the UK experiences another heatwave, this research shows that the impacts of rising temperatures don’t stop on land.

“Nature is our ally, and the lesson from this study is we need to restore nature smarter.

“By understanding which sites are most resilient to marine heatwaves and using the latest science to guide restoration efforts, we can give seagrass meadows the best possible chance to recover and support thriving marine wildlife.

“Without urgent action on climate change, temperatures will continue to rise along with the risks to both people and nature.”

Experts say the research highlights the need to incorporate regional-scale models of the likelihood of future marine heatwaves and long-term temperature trends when selecting restoration sites.

The Met Office has triggered work to provide marine heatwave forecasts so that users of marine information, such as the Seagrass Ocean Rescue partners can make informed decisions.

Prioritising sites with greater environmental stability will be essential to the success of Ocean recovery efforts.