Seasonality of Phytoplankton in two satellite products and novel CMIP6 climate models

Marine phytoplankton photosynthesis is a critical part of the biological carbon pump, which transfers to depth part of the resulting organic carbon, leading to carbon ingassing at the surface and oceanic carbon sequestration.

Here we compare phytoplankton phenology in two satellite products and the new generation of CMIP6 climate models. SeaWiFS 9km data on reflectances and absorption was used here for the period 2003-2007. The reflectance-based satellite algorithm of Roy et al. (2013, 2017) is interpreted as total living phytoplankton biomass (organic carbon) at the ocean surface. In contrast, the absorption-based algorithm of Kostadinov et al. (2016) is sensitive to all backscattering particles and is interpreted as total particulate carbon in the ocean, including phytoplankton, zooplankton, detritus.

Here we analyze annual averages and seasonality characteristics (including bloom timing and relative variability) for living carbon (phytoplankton) and total carbon biomass concentrations globally and also for three subpolar regions: a subpolar North Pacific location (near station OSP: 45N 50N; 150W 140W), subpolar Atlantic (near station NABE: 45N 50N; 35W 25W), and the subpolar Southern Ocean.

The satellite-derived biomass concentrations and relative fractions for three size groups [picoplankton (0.5-2 um), nanoplankton (2-20 um), and microplankton (20-50 um)] are analyzed jointly with the output of the latest generation (CMIP6) climate models.

Global maps reveal comparable patterns of total and living particulate carbon distribution globally . However, large differences appear in regional variability, such as timing of maximum and minimum concentrations in the OSP, NABE and S Ocean areas. Comparison of satellite-derived data to CMIP6 model outputs shows comparable spatial distribution of principal features, such as lower biomass in oligotrophic gyres and increased seasonality and biomass concentrations at the boundaries of the subtropical gyres, yet disagree in detail at subpolar locations. We explore a variety of physical indices to understand how seasonal changes in mixing affect ocean nutrients and light to try to illuminate the differences among the various products in the subpolar regimes.