Le programme de recherche PhytoDev

(2016-2017/19, PI E. Martinez)

Phytoplankton, the microalgae that populate the upper lit layers of the ocean, fuel the oceanic food web and affect the concentration of oceanic and atmospheric CO₂ through photosynthetic carbon fixation. While seasonal and inter-annual cycles of phytoplankton are rather well characterized, large uncertainties remain about their decadal cycles. Nevertheless decadal cycles of phytoplankton (in terms of chlorophyll biomass, community composition and carbon fluxes) have to be well characterized for three major reasons: (1) They can accentuate, weaken or even mask the secular (climate-related) trends (the recent debate about the observed cooling in the context of climate change illustrates the crucial need for better understanding decadal variability); (2) The observed changes in phytoplankton during warm phases of decadal cycles may provide insight into how future climate warming-induced changes will alter carbon cycle and marine food web; (3) Predicting future global changes in oceanic phytoplankton with some confidence requires first a comprehensive understanding of how climate changes influenced them in the past.

Seasonal and inter-annual cycles of phytoplankton biomass and production are relatively well characterized in the Global Ocean, especially thanks to the large amount of studies based on remote-sensing data collected over the last 19 years. This knowledge has allowed the development of coupled physical-biogeochemical models. Due to the unavailability of observations over a continuous time series longer than two decades and at basin scale, phytoplankton decadal variability at global scale can only be investigated through physical-biogeochemical modeling. However, there are large differences between simulations of ocean dynamics obtained from different models, which leads to large uncertainties in the predicted biological response at decadal and longer time scales (Henson et al., 2009 vs. Patara et al., 2011, Séférian et al. 2013).

The PhytoDeV project aims to investigate the phytoplankton time variability at decadal and longer time scales and the underpinning physical processes in the Global Ocean. Due to the limited availability of appropriate in situ observations combined with uncertainties in bio-physical modeling for the timescales considered, we propose an original statistical method that couples ocean color and physical satellite observations with ocean physical models to infer decadal changes in Chlorophyll-a concentration (Chl, a proxy of phytoplankton biomass) at global scale. The changes in phytoplankton community structure and associated primary production will then be inferred from the Chl values. The most significant expected results of PhytoDeVarean 1) understanding of the past decadal variability of phytoplankton biomass, assemblages and associated production as well as their underpinning physical mechanisms in the global ocean; 2) give an original insight into carbon cycle forecast and IPCC scenarios, and provide a measure of the decadal-like signal in the climate trends from the IPCC models.

Financement : CNES

Contact : Elodie MARTINEZ