[LUM#18] Back to the Future of Marine Ecosystems
By studying the marine fauna off the coast of Peru during the last interglacial period, scientists have come to understand that the ecosystem—which is currently dominated by anchovies—could become a haven for gobies in the future. These findings overturn the projections of ocean models.
How will marine species adapt to climate change? This question is all the more crucial for ecologists because the ocean environment is particularly affected by global warming. In addition to rising water temperatures, the oceans are facing changes in ocean currents, ocean 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 coast is a highly productive oceanic zone, with cold upwellings bringing large amounts of 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 (oxygen-free) starting at a depth of just a few dozen meters below the surface. While some species are confined to the surface, others have adapted by rising to the surface at night to take in oxygen. But if the oxygen concentration in the water drops further due to rising water temperatures, some species may not be able to adapt. “Scientists have long believed that oxygen was not a key factor in the evolution of the marine environment. Yet in certain areas, such as off the coast of Peru, it is the limiting factor, ” explains Arnaud Bertrand, co-author of the study and a marine ecologist at the Marbec laboratory.
130,000 Years of Marine History
The challenge for researchers, therefore, is to predict how the ecosystem might change 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 the volume of fish decreases, on average, faster than their size,” explains Arnaud Bertrand. But two scenarios remain possible: either existing species will shrink, or new, smaller species will become established.
In the Southeast Pacific, the second hypothesis is the correct one, 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 support 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 seafloor core. Scales, vertebrae, and other remains provided insights into species diversity and abundance over the millennia.
Researchers found that the ecosystem has undergone a radical transformation in response to changing environmental conditions (particularly temperature and oxygen concentration). During the last interglacial period, 125,000 years ago, temperatures and oxygen concentrations in Peru were similar to those predicted for the end of this century. At that time, the ecosystem was dominated not by anchovies, but by small gobies. In all likelihood, the shift that occurred 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, the scientists benefited from a remarkable environment: it is rare to be able to conduct paleobiological research in a marine setting, which generally does not provide the necessary conditions for preservation. But the anoxic environment off the Peruvian coast has allowed the organic matter in the sediments to be preserved. “This sediment core has opened a window onto the abiotic conditions expected at the end of the century, ” the researcher said enthusiastically.
A less productive and less diverse ecosystem
The success of gobies can be attributed to their small size (a few centimeters, compared to about ten for anchovies) and their greater resistance to anoxic conditions. Less nutritious and lower in fat than anchovies, their dominance will alter the entire food chain, all the way up to seabirds 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.”
For the scientific community, these findings are invaluable. “It’s very difficult to test our hypotheses about the evolution of fish populations using recent observations, because we can’t tell what’s due to global warming and what’s due to fishing,” explains Arnaud Bertrand. Indeed, fishing contributes to a decrease in the size of fish populations by removing the largest specimens; it can also lead to a shift toward communities of smaller fish. For example, in Namibia, overfishing of sardines in the 1970s led to a shift in the ecosystem that benefited gobies and jellyfish (Reporterre, March 16, 2013). “This isolated case, which we used to consider an exception, may well apply to large, productive ecosystems, ” the scientist emphasizes.
*Marbec (UM, IRD, CNRS, Ifremer)
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