Seasonal cycles of plankton ecology and CO2: subpolar lessons from satellite data and coupled climate models

Marine phytoplankton photosynthesis is a critical part of the biological carbon pump (BCP), which transfers to depth part of the resulting organic carbon (C), leading to C in-gassing at the surface and oceanic C sequestration. Seasonal, interannual and multi-decadal environmental changes alter ocean nutrients, light, and temperature, affecting in turn ocean plankton and BCP.

Our project goal is to contrast seasonal behavior of ocean plankton ecology and links with air-sea CO2 fluxes comparatively in the subarctic North East Pacific (NP) and subpolar North Atlantic regions (NA). This study aims to provide background mechanistic understanding for the NASA-sponsored EXPORTS project (oceanexports.org) for these regions.

We analyze seasonal variability in phytoplankton biomass (PB) in multiple satellite color products and contrast this analysis with CMIP5 and preliminary CMIP6 model results. We also discuss coupling/decoupling between biomass (in different size groups), zooplankton grazing, productivity, export, and air-sea C fluxes across models.

We find that the PB spring bloom is much weaker in the NP compared to the NA in some satellite observations and in the most ecologically-complex CMIP5 climate models. We hypothesize and verify across datasets and models that this is due to less iron and a tighter zooplankton control on phytoplankton in the NP. We also analyze the climate-induced changes in plankton seasonality and spring bloom dynamics across a set of climate models for the 21st century. We attempt to verify if the different seasonal dynamics at NP and in the NA holds over the 21st century, using the high emission scenario in a few CMIP5 and CMIP6 models.

In subpolar regimes such as NP and NA, total pCO2 represents a close competition between the temperature-driven and non-temperature components. We analyze the connection between PB and surface pCO2, in observations and across CMIP5 and CMIP6 models. We try to elucidate whether the role of phytoplankton biology in setting surface pCO2 is different at the two locations of interest and across the 21st century.