Long-term monitoring of zooplankton diversity in the Mediterranean Sea in a context of climate change over 55 years

Climate change affects the oceans directly (e.g.,  increase in salinity and temperature of surface waters) or indirectly (e.g., changes in precipitations and winds for example). Those modifications affect living organisms, especially plankton that, by definition, can not move when facing unfavorable conditions. For this reason, plankton is considered as a good sentinel of the global state of open sea ecosystems. In addition, plankton plays a key role in the functioning of marine ecosystems, as a main actor of the carbon cycle or as the base of food webs. Monitoring and understanding how planktonic communities respond to global changes is therefore a major priority at global scale, as evidenced by the recent "Plankton Manifesto" edited by the UN or plankton's status as an Essential Ocean Variable according to the Global Ocean Observing System.

In this context, long time series of plankton biodiversity observations are a key tool. In the Mediterranean Sea, considered as a global warming hotspot, the historical marine station of Villefranche-Sur-Mer continues to carry out one of the oldest zooplanktonic time series in the world. Starting in 1967, a 330 µm mesh sized net has been deployed bimonthly, all year round, between 75 m and the surface. Over 900 formalin-preserved samples were recently re-processed with a quantitative imagery device (the ZooScan) and the resulting 2M individual images processed through an artificial intelligence pipeline to provide individual measurements and taxonomic classifications.

From this data, we describe long term trends,  seasonal changes, and regime shifts in the mesozooplanktonic community, in relation to environmental conditions (temperature, oxygen , chlorophyll a). And, since the images provide access to individual measurements, we can also explore changes in ecologically-relevant traits such as size, transparency, etc. Among other things, we show, together with significant warming and decrease of chlorophyll a concentration; changes in zooplankton concentrations and shifts in community composition. These changes also translate into significant changes in individual-level traits. Causal inference approaches indicate significant top-down control of zooplankton on phytoplankton, which seems to be increasing with time. Such extraordinarily long observations give us access to these long term, climate-scale processes and are therefore precious in our globally changing world.