Back to the future of marine ecosystems

By studying the oceanic fauna of the last interglacial period off Peru, scientists understand that the ecosystem dominated today by anchovies could in the future become a paradise for gobies. Results that reverse the projections of ocean models.

How will marine species adapt to climate change? This question is all the more crucial for ecologists as the ocean environment is particularly affected by warming. In addition to the increase in water temperature, the oceans are facing changes in marine currents, acidification of the water and a decrease in oxygen content. A study published in Science in January 2022, in which the Marbec laboratory* participated, provides an unprecedented prediction on the transformation of the ecosystem off Peru by the end of the century.

Thanks to the Humboldt current, the Peruvian coasts are a very prolific oceanic zone, the upwelling of cold water bringing quantities of nutrients from the depths. But this profusion of life makes it all the more fragile. Rich in organic matter, the zone is poor in oxygen because of the intense activity of bacteria. It is even anoxic (without oxygen) from a few dozen meters below the surface. If some species are trapped at the surface, others have adapted by coming to the surface at night to oxygenate themselves. But if the concentration of oxygen in the water drops further because of rising water temperatures, some species may not adapt. "Scientists have long thought that oxygen was not a key factor in the evolution of the marine environment. However, in some areas such as off 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 marine history

The question for researchers is therefore to anticipate how, under the effect of the increase in temperature and the decrease in oxygen, the ecosystem could change. Overall, scientists expect the size of the fish to decrease. "For cold-blooded animals, metabolism increases with temperature. Warmer and oxygen-poor conditions therefore favor smaller individuals, which need proportionally less energy and oxygen since the volume of fish decreases on average faster than their size," explains Arnaud Bertrand. But two hypotheses remain possible: either the species will shrink, or new, smaller species will establish themselves.

In the southeast Pacific, the second hypothesis is the correct one according to original results published in Science. The researchers predict that the current ecosystem characterized by the abundance of anchovies could switch to a new state dominated by gobies by the end of the century. To convince themselves of this, they studied marine evolution on a geological time scale. Under the leadership of paleobiologist Renato Salvatteci, the international team was able to reconstruct 130,000 years of ocean life using a 5-meter core of the ocean floor. Scales, vertebrae... have thus allowed to know the diversity of species and their abundance over the millennia.

The researchers found that the ecosystem was radically transformed, depending on the evolution of environmental conditions (temperature and oxygen concentration in particular). During the last interglacial period, 125,000 years ago, temperatures and concentrations in Peru were those predicted for the end of our century. The ecosystem was then dominated not by anchovies, but by small gobies. In all likelihood, the tipping that took place then could thus be repeated. "This is a very important discovery. None of the current models are capable of predicting that a small fish that is currently negligible in the ecosystem can become dominant," insists Arnaud Bertrand.

For this research, the scientists benefited from a remarkable environment: it is rare to be able to make paleobiology in marine environment, which in general does not offer the necessary conditions of conservation. But the anoxic environment of the Peruvian coast has allowed to preserve the organic matter of the sediments. " This core has opened a window on the abiotic conditions expected at the end of the century," says the researcher.

A less productive and less diverse ecosystem

The success of the gobies is explained by their small size (a few centimeters compared to about ten for anchovies) and their better resistance to anoxic conditions. Less nutritious and less fatty than anchovies, their predominance will modify the entire food chain, up to birds and marine mammals, explains Arnaud Bertrand: "the population of current species could therefore collapse in favor of new species, probably towards a less productive and less diverse ecosystem.

For the scientific community, these results are valuable. " 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 or to fishing," explains Arnaud Bertrand. Indeed, fishing contributes to reducing the size of species by removing the largest specimens; it can also lead to a shift towards smaller fish communities. For example, in Namibia, the overfishing of sardines in the 1970s led to a modification of the ecosystem in favor of gobies and jellyfish (Reporterre 16/03/2013). "This isolated case that we took for a special case could well be valid for large productive ecosystems," emphasizes the scientist.

*Marbec (UM, IRD, CNRS, Ifremer)