| Summary: | International audience The composition of marine ecosystems is determined by spatial and temporal patterns of global biogeochemical cycles. Shifts in marine ecosystem composition driven by changes in climate can in turn affect biogeochemical cycles, especially through their impact on air-sea fluxes of CO2 and trace gas concentrations in the atmosphere. However, the response of marine ecosystems to climate is difficult to assess at global scale due to the scarcity of large-scale in-situ biological monitoring programmes. Here, we combine and analyse remote-sensing observations, in-situ observations, and a global ocean biogeochemistry model to gain insight into interactions between marine ecosystem composition and climate variability over the last decade, in the North Atlantic and in the Southern Ocean. Our results show large-scale shifts in the dominance of diatoms, with mean anomalies 63.3% higher during extreme positive phases of North Atlantic Oscillation index compared to its extreme negative phases and with mean anomalies 134% higher during extreme positive phases of Southern Annular Mode in the Southern Ocean over the period 1998-2008. Significant changes in the frequency of diatom dominance are detected concurrently in the three data sources studied. Model outputs indicate that this increase is driven by nutrient supply from deep waters during windier and more turbulent climate conditions.
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