Warming and tropicalization of temperate zones

© Nicolas Floch / Fondation Tara Expéditions

A grain of sand in the gears and the whole machine seizes up. Climate change is causing major disturbances in ecosystems. The third chapter of Tara’s scientific mission in Japan will focus on one of these phenomena — the displacement of species. Information about the presence of new predators with fins:

Because of its length and geographical position, the Japanese archipelago has significant temperature variations from north to south. This is what scientists call the “temperature gradient”. Underwater landscapes differ according to latitude: In the north, macro-algae such as Laminaria are predominant, whereas in the Ryukyus — Japan’s southernmost chain of islands — coral reefs form the underwater topography. But global warming knows no boundaries and impacts every ecosystem.


Plongee_Shikine_jour_1_credit_Elodie_Bernollin-2180964First dive in Shikine to study the effects of acidification © Elodie Bernollin / Tara Expéditions Foundation


Corals are not alone in suffering from the rising temperatures. Algae present in temperate waters are being harmed by the warming, and also by the increasing number of predators arriving because of warmer temperatures.

Over the past 20 years, scientists have observed a shift of tropical or subtropical fish species from south to north, correlated with rising ocean temperatures: some fish are appearing in areas they had never colonized before. “When it’s warmer, there is also a physiological phenomenon in these fish: they eat more”, says Sylvain Agostini, Tara’s scientific coordinator in Japan. More numerous and more hungry, these herbivores with scales control the amount of algae present in the ecosystem. Faced with increasing pressure, Laminaria are losing ground.

“In the future, these fish will be able to reach even higher latitudes and perhaps create space for corals by eliminating the Laminaria. But watch out for the diabolical companion of climate change: the acidification of the oceans! One can imagine that the increase in temperatures would favor the displacement of corals to the northernmost regions of Japan. Except that all the parameters do not work in favor of this. The increase in temperatures effectively stimulates growth of corals, but acidification of the water limits the development of their skeletons. These two antagonistic parameters prevent us from foreseeing what will happen in the future”, explains Sylvain Agostini.


DCIM101MEDIADJI_0029.JPGTara’s last day in Yokunami © François Aurat / Tara Expéditions Foundation


The phenomenon of tropicalization of temperate ecosystems will be studied by the Taranauts this month, along the Japanese coasts and especially in Kochi which is used as an example by scientists all over the world. “The bay where we’re going to dive is extremely interesting. On one side of the bay, the original ecosystem dominated by algae is still present. On the other side, we observe a radical tropicalization and the Laminaria have been replaced by corals. Why? This is what we will try to understand through the data collected aboard Tara. “


Noëlie Pansiot


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