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Climate: marine ecosystems experience abrupt changes

As we know, between industrial fishing and climate change, marine biodiversity is undergoing many upheavals. But the oceans are difficult to access and what happens there is poorly known. A study published Monday in Nature Climate Change reveals that marine ecosystems are regularly subject to large-scale, rapid and localized changes. These events, termed "climate surprises", are caused by natural atmospheric phenomena and can have important social consequences (collapse of fishing areas). Especially since there is an increase in their frequency and strength, possibly related to the disruption of climate caused by man.

Virtual fish

Oceans cover more than two-thirds of the earth's surface, but the biodiversity therein remains poorly understood. According to the oceanographer Grégory Beaugrand, research director at the CNRS, who led the study, we currently know only 250,000 – most of them rather badly – of the 2 million species that are found in the seas . And the field observations, slow and expensive, focus on the coast.

To overcome these shortcomings, researchers have modeled marine biodiversity. They created millions of "virtual species" that they turned into a model, in the absence of an aquarium. A sort of gigantic mathematical simulation in which fictitious species – each with their own climatic preferences – are grouped into communities and evolve together in response to changes in temperature. The majority of species – including plankton at the base of the food chain – can not regulate their own temperature, making them more susceptible to abrupt changes.

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These models make it possible "Restore the biodiversity and the communities present in the oceans", explains Gregory Beaugrand, and observe the occurrence of "Climate surprises" in the past. Rapid changes in ecosystems, marked by the appearance or disappearance of species from certain regions. By comparing these theoretical results with concrete observations, the authors were able to confirm the adequacy of their model. Before extending it to all oceans.

Acceleration

The study shows that over the period 1960-2015, on average 2.8% of the ocean surface was affected each year by phenomena of abrupt reorganizations. Contrary to what one might think, climate change is not the main cause of this phenomenon. Except in the Arctic, where melting ice causes a major reorganization of ecosystems, natural atmospheric variability largely explains these upheavals.

The French researcher nevertheless notes that"There is an increase in the extent and strength of these climate surprises". In particular, between 2012 and 2015, almost 14% of ocean ecosystems have undergone abrupt changes (2013, 2014 and 2015 are in the top 10 of the hottest years ever recorded). A shift from 10 to 50 million square kilometers affected, may be due to climate change, which is known to intensify some extreme weather events like El Niño (climate phenomenon that produces warmer waters along South America which influences the global climate). "In the future, climate surprises may increase in the context of climate change", adds Gregory Beaugrand.

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These upheavals are reflected locally by appearances or disappearances of species sometimes central in the economies. Using this model allows early detection and prediction of abrupt falls in some species – for example, the current decrease in plankton and Northeast Pacific salmon, or the re-emergence of bluefin tuna in the English Channel – and attracts attention to the areas at risk with one year ahead of the problems. Once the temperatures are known, the predictions go faster than the actual biological changes, which go to the rhythm of fish reproduction. What help researchers, industrialists and fishermen to avoid unpleasant surprises.

Nathan Mann

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