Could climate change and biodiversity of marine plankton in North Atlantic affect carbon cycle?

These results have been obtained by a researcher from the Laboratoire d'Océanologie et de Géosciences (CNRS/Université Lille 1/Université du Littoral-Cote d'Opale, Wimereux) in collaboration with the Sir Alister Hardy Foundation for Ocean Science (Plymouth) and the Laboratoire d'Océanologie de Villefranche (CNRS/Université Pierre et Marie Curie). Researchers demonstrate that this structural modification of biological systems could bring about an alteration to the carbon sink in the North Atlantic and a reduction in the presence of subarctic fish such as cod.

The work has recently been published in the Proceedings of the National Academy of Sciences.

Observations show that 84% of global warming occurs in oceans. Numerous results already show that marine organisms respond to this rise in temperature. However, few studies have been carried out on the consequences of global climate change on the evolution of marine biodiversity on a large spatial scale.

The Continuous Plankton Recorder program based in Plymouth in the United Kingdom has been monitoring, every month since 1946, the presence and the abundance of nearly 450 species of plankton in the North Atlantic Ocean. The team, headed by Grégory Beaugrand of the Laboratoire d'Océanologie et de Géosciences (CNRS/Université Lille 1/Université du Littoral-Côte d'Opale, Wimereux), has analyzed the 97 million items of data stemming from this program. The researchers focused on the taxonomic diversity of key groups of phytoplankton, such as dinoflagellates and diatoms, and zooplankton, particularly copepods that ensure the transfer between primary producers (phytoplankton) and upper trophic levels.

Their analysis has evidenced for the first time that the rise in temperatures has been accompanied by an increase in the biodiversity of these plankton groups in the North Atlantic Ocean and by a 25 to 33% reduction in the average size of copepods, of which one hundred or so species live in this part of the ocean. The size of these organisms has in fact decreased from 3-4 mm to 2-3 mm on average in a number of regions situated between the temperate and polar systems.

The researchers then focused on the consequences of this surprising evolution. They demonstrated that the decrease in the mean size of copepods, which ensure the transfer of atmospheric carbon dioxide from the surface to the bottom of the oceans through the food chain, could lead to a reduction, not yet quantifiable, in the amount of atmospheric carbon trapped by the North Atlantic Ocean, which accounts for one quarter of the total atmospheric carbon trapped by the world's oceans.

Such weakening of the carbon sink in the North Atlantic Ocean, added to that predicted by biogeochemical models, namely that the rise in temperatures will increase thermal stratification of the water column, will make it more difficult for nutritive salts to reach the surface from the deeper layers, eventually causing marine productivity to decline. Researchers have also highlighted a faster circulation of biogenic carbon from organism to organism within the trophic network, reflecting an increase in the ecosystem metabolism, which is entirely consistent with the fact that the smaller an organism, the faster it develops and dies.

Finally, using data from models designed to assess the probability of cod presence based on the characteristics of their environment, researchers have found that the presence of cod was inversely proportional to the taxonomic diversity of zooplankton. Consequently, the increase in the diversity of zooplankton and its decline in size could result in reduced cod presence in the North Atlantic, a phenomenon that would amplify the effect of over-exploitation by fishing of this subarctic species.

This study reveals that an increase in taxonomic biodiversity, often considered positive as far as the ecosystem is concerned, could, if it occurred in all oceans, temporarily alter important functions for humans, such as the regulation of carbon dioxide and the exploitation of marine resources. This increase, which had never been observed on such a large scale, is a sign of profound structural upheaval within the biological systems of the North Atlantic in response to global warming.