New research from an international team of scientists has shown that when ocean warming and acidification combine, complex coastal habitats can be stripped of their variety and reduced to slime-covered barren grounds.
Across Japan, the team has found widespread evidence of 'isoyake' - or 'burnt seashore' - with the disappearance of kelp forests and abalone fisheries as temperate coastal marine ecosystems lose biodiversity and become more 'simple'.
In two studies published in Global Change Biology today, the team from the University, with counterparts in Japan, Italy, France and Spain, used the volcanic islands of Shikine-jima, approximately 160 kilometres south of Tōkyō, and Vulcano Island, off the coast of Italy, as living laboratories to analyse the impact of rising pH levels and temperatures.
"Kelp forests are being lost globally as a result of warmer sea surface temperatures and heatwaves,"
said first author, Dr Sylvain Agostini, from the Shimoda Marine Research Center, at the University of Tsukuba.
"In Japan, this 'isoyake', or 'burnt seashore', is widespread. As ocean temperatures continue to increase, warm water corals are shifting northward into temperate reefs and could replace cold-water species."
"At present, temperate waters are changing, becoming 'tropicalized' and dominated by warm water species of corals, fish and seaweeds,"
added corresponding author Professor Jason Hall-Spencer, of the School of Biological and Marine Sciences at Plymouth.
"But another effect of increasing greenhouse gas emissions - ocean acidification - complicates matters. Acidification reduces the amount of carbonate in the ocean, which is needed by reef-building corals to create their structure. Decreases in carbonate ion concentrations limits the poleward spread of corals."
For the studies, the team used three locations at a similar latitude, each representing a different scenario: the present; a future with ocean warming, and a future with ocean warming plus acidification. At each site, they examined the existing communities, and also transplanted kelp and coral species and measured their growth and survival at the different sites.
The team found that with both ocean warming and acidification scenarios, there was a loss of kelp forest but no gain in tropical species. Instead, the seabed was degraded into a simple turf-algal habitat.
"Warmer waters facilitate the growth and colonization of reef-building corals,"
said Dr Ben Harvey, also of the Marine Research Center at Tsukuba.
"But ocean acidification appears to negate these benefits. And kelp transplants did not survive in warmer waters, largely because they were eaten by warm water fishes."
The second study focused specifically upon coralline algae as these act as foundation species and their loss has negative effects on coastal ecosystem services. This study applied molecular tools to assess the biodiversity of coralline algae and to detect which groups are most sensitive to ocean acidification. In both study areas, the diversity of coralline species declined as the CO2 increased.
"We show a decrease in the diversity of coralline algae with increasing acidification with more than half of the species lost in high carbon dioxide conditions,"
said first author, Dr Viviana Peña, of the BioCost Research Group, in the Universidade da Coruña, Spain.
"We found a substantial diversity of coralline algae and many of these disappear with ocean acidification."
Professor Hall-Spencer said that global action is required to address the risk:
"Temperate coastal waters are facing major degradation of seascapes due to the unseen hand of rising CO2 levels. Overall, our results highlight the urgent need for control of carbon emissions and a limit to the drivers of ocean change,"
he concluded.
