Coastal ecosystems being ‘burned’ by double whammy of rising temperatures and ocean acidification

Shellfish wiped out and complex habitats reduced to slime-covered barren grounds

High carbon dioxide levels turn seabeds into a mat of slimy algae that smothers life (Nicolas Floc’h)

Coastal ecosystems are being “burned” by rising sea temperatures and ocean acidification, stripping them of life including shellfish and kelp habitats for dolphins, scientists have found.

The deadly combination is stripping complex coastal habitats of their biodiversity and reducing them slime-covered barren grounds, they are warning.

A changing climate is heating the seas, causing ocean “tropicalisation”, while rising levels of carbon dioxide in the water increase acidity.

The experts say their findings highlight the urgent need to control greenhouse gas emissions.

In two studies, the scientists looked at temperate waters – those that do not normally experience temperature extremes – in Japan, finding widespread evidence of “burnt seashores”, where kelp forests and shellfish fisheries were wiped out.

Jason Hall-Spencer, a University of Plymouth professor and one author of the papers, told The Independent: “We had hoped rising temperatures would simply mean that tropical fish would replace the temperate species, but unfortunately we found the whole ecosystem crashed.

“Most fish simply could not survive when warming and acidification combined because the acidified waters literally ate away at the habitats the fish need to hide, feed and breed, leaving a really simplified habitat dominated by small seaweeds.

“It was like replacing an ancient forest with a patch of nettles.

“Other casualties included mussels, oysters, snails, crabs and lobsters, all of which support jobs and food security.”

Temperate waters are also found off the UK, and the Atlantic coasts of Norway, Germany, Belgium, France and Spain, as well as Canada and the US, Chile, Argentina, Peru, New Zealand, South Africa and Russia.

“At present, temperate waters are changing, becoming ‘tropicalised’ and dominated by warm-water species of corals, fish and seaweeds,” said Prof Hall-Spencer.

The diversity of coral species declined as the CO2 increased, reducing the ecosystem “services” they provide (Paul Naylor)

“But another effect of increasing greenhouse gas emissions – ocean acidification – complicates matters.

“Acidification reduces carbonate in the ocean, which is needed by reef-building corals to create their structure.”

Kelp forests, which are habitats for lobsters, fish and dolphins, are already being lost globally as a result of warmer sea surface temperatures and heatwaves.

And warm-water corals are shifting northward into temperate reefs and could replace cold-water species, the experts say.

In their studies, published in Global Change Biology, the teams from the University of Plymouth, with scientists from Japan, Italy, France and Spain, reported how they used three locations, one representing the present; one representing a future with ocean warming, and the third a future with ocean warming plus acidification.

The team found that in both the ocean warming and acidification scenarios, kelp forest disappeared but there was no increase in tropical species. Instead, the seabed was degraded into a “simple turf-algal habitat”.

The seabed became a mat of slimy algae that smothered and killed marine life.

Ben Harvey, of the Marine Research Centre at Tsukuba, Japan, said: “Warmer waters facilitate the growth and colonisation of reef-building corals, but ocean acidification appears to negate these benefits.

“Kelp transplants did not survive in warmer waters, largely because they were eaten by warm water fishes.”

The diversity of coral species declined as the CO2 increased, reducing the ecosystem “services” they provide. “It’s like pulling the rug from under the whole ecosystem,” said Prof Hall-Spencer, who called for global action.

“Temperate coastal waters are facing major degradation of seascapes due to the unseen hand of rising CO2 levels,” he added.