The once common kelp forests and abalone fisheries of the Shikine Island in Japan have now vanished. Scientists from Japan identified that these temperate coastal marine ecosystems are transforming into much “simpler” ones, deprived of their biodiversity aesthetic values and complexity.
Scientists from the University of Tsukuba along with their international collaborators investigated the combined effects of ocean warming and acidification on the temperate coastal marine ecosystems.
Coral reefs are synonymous with the tropical coastal seas. When the ocean temperatures cool in the direction of the poles, corals yield to kelp as the main habitat-forming species. This shift from coral to kelp can be seen evidently on the 2000 km coastline of Japan, where modifications to these ecosystems are ongoing.
Kelp forests are being lost globally as a result of warmer sea surface temperatures and heatwaves. 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.
Dr. Sylvain Agostini, Study Lead Author, Shimoda Marine Research Center, University of Tsukuba
Three likely scenarios are possible when coastal species shift. Temperate reefs can become more tropicalized and predominated by warm-water fishes, corals and other species. On the other hand, reefs may be dominated by turf algae or tropical seaweeds.
However, another impact of growing greenhouse gas emissions — ocean acidification — further complicates matters. Acidification decreases the carbonate amount in the ocean, which is required by reef-building corals to build their structure. Reduced carbonate ion concentrations restrict the colonization of new areas by rapid-growing coral species.
To identify the potential changes along Japan’s coast, the researchers identified three locations at a similar latitude representing three distinct scenarios (present, ocean warming and ocean warming plus acidification). They investigated the prevailing communities and later transplanted coral and kelp species, thus estimating their survival and growth at the selected sites.
The researchers discovered that with both acidification and ocean warming, coastal ecosystems possibly lose kelp forests but may not gain reef-building corals. The outcome is a simplified turf-dominated habitat.
According to Dr. Ben Harvey, “Warmer waters facilitate the growth and colonization of reef-building corals. 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 consequences of these changes is that warm temperate coastal waters are facing major simplification which is clearly seen in the degradation of the seascape.
Nicolas Floc'h, Study Co-Author, Professor and Artist, Ecole Européenne Supérieure d'Art de Bretagne
Lost kelp forests could be replaced by simpler turf-dominated communities that supply a fraction of the ecosystem services of more biodiverse tropical reefs. On the whole, the results emphasize the pressing need for carbon emissions control and restricting the factors of ocean change.
Harvey, B. P., et al. (2021) Ocean acidification locks algal communities in a species-poor early successional stage. Global Change Biology. doi.org/10.1111/gcb.15455.