[LUM#18] Back to the future of marine ecosystems
By studying the ocean fauna of the last interglacial period off the coast of Peru, scientists have come to understand that the ecosystem currently dominated by anchovies could become a paradise for gobies in the future. These findings overturn the projections of ocean models.
How will marine species adapt to climate change? This question is particularly crucial for ecologists, as the ocean environment is especially affected by global warming. In addition to rising water temperatures, oceans are facing changes in ocean currents, water acidification, and declining oxygen levels. A study published in Science in January 2022, in which the Marbec* laboratory participated, provides an unprecedented forecast of how the ecosystem off the coast of Peru will change by the end of the century.
Thanks to the Humboldt Current, the Peruvian coastline is a very prolific oceanic zone, with cold water upwelling bringing nutrients from the depths. But this abundance of life makes it all the more fragile. Rich in organic matter, the area is low in oxygen due to intense bacterial activity. It is even anoxic (without oxygen) a few dozen meters below the surface. While some species are stuck at the surface, others have adapted by rising to the surface at night to get oxygen. But if the oxygen concentration in the water continues to decline due to rising water temperatures, some species may not be able to adapt. "Scientists have long thought that oxygen was not a key factor in the evolution of the marine environment. However, in certain areas, such as off the coast of Peru, it is the limiting factor, " explains Arnaud Bertrand, co-author of the study and marine ecologist at the Marbec laboratory.
130,000 years of maritime history
The question for researchers is therefore to anticipate how the ecosystem might evolve as a result of rising temperatures and declining oxygen levels. Overall, scientists expect fish sizes to decrease. "For cold-blooded animals, metabolism increases with temperature. Warmer, oxygen-poor conditions therefore favor smaller individuals, which require proportionally less energy and oxygen since fish volume decreases on average faster than size, " explains Arnaud Bertrand. But two scenarios remain possible: either species will shrink, or new, smaller species will establish themselves.
In the southeastern Pacific, the second hypothesis is correct, according to the original findings published in Science. Researchers predict that the current ecosystem, characterized by an abundance of anchovies, could shift to a new state dominated by gobies by the end of the century. To reach this conclusion, they studied marine evolution on a geological timescale. Under the leadership of paleobiologist Renato Salvatteci, the international team was able to reconstruct 130,000 years of ocean life using a 5-meter-long ocean floor core sample. Scales, vertebrae, and other remains provided insight into the diversity of species and their abundance over the millennia.
Researchers have found that the ecosystem has undergone radical changes in response to changing environmental conditions (particularly temperature and oxygen concentration). During the last interglacial period, 125,000 years ago, temperatures and concentrations in Peru were similar to those predicted for the end of this century. The ecosystem was then dominated not by anchovies, but by small gobies. In all likelihood, the shift that took place then could happen again. "This is a very important discovery. None of the current models are capable of predicting that a small fish that is currently insignificant in the ecosystem could become dominant, " insists Arnaud Bertrand.
For this research, scientists benefited from a remarkable environment: it is rare to be able to conduct paleobiology in a marine environment, which generally does not offer the necessary conditions for preservation. But the anoxic environment of the Peruvian coast has preserved the organic matter in the sediments. "This core has opened a window onto the abiotic conditions expected at the end of the century, " says the researcher.
A less productive and less diverse ecosystem
The success of gobies can be explained by their small size (a few centimeters compared to about ten centimeters for anchovies) and their greater resistance to anoxic conditions. Less nutritious and less fatty than anchovies, their predominance will alter the entire food chain, right down to birds and marine mammals, explains Arnaud Bertrand: "The population of current species could therefore collapse in favor of new species, likely leading to a less productive and less diverse ecosystem."
These results are invaluable to the scientific community. "It is very difficult to test our hypotheses on the evolution of fish populations through recent observations, because we cannot know what is due to global warming and what is due to fishing," explains Arnaud Bertrand. Fishing contributes to a decrease in the size of species by removing the largest specimens; it can also lead to a shift towards smaller fish communities. For example, in Namibia, overfishing of sardines in the 1970s led to a change in the ecosystem in favor of gobies and jellyfish (Reporterre 03/16/2013). "This isolated case, which we thought was a special case, may well be valid for large productive ecosystems, " the scientist points out.
*Marbec (UM, IRD, CNRS, Ifremer)
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