The role of phytoplankton dynamics in the seasonal and interannual variability of carbon in the subpolar North Atlantic-a modeling study

Abstract

We developed an ecosystem/biogeochemical model system, which includesmultiple phytoplankton functional groups and carbon cycle dynamics, andapplied it to investigate physical-biological interactions in Icelandicwaters. Satellite and in situ data were used to evaluate the model. Surfaceseasonal cycle amplitudes and biases of key parameters (DIC, TA,pCO 2, air-sea CO 2 flux, and nutrients) are significantly improvedwhen compared to surface observations by prescribing deep water values andtrends, based on available data. The seasonality of the coccolithophore and"other phytoplankton" (diatoms and dinoflagellates) blooms is in generalagreement with satellite ocean color products. Nutrient supply, biomass andcalcite concentrations are modulated by light and mixed layer depth seasonalcycles. Diatoms are the most abundant phytoplankton, with a large bloom inearly spring and a secondary bloom in fall. The diatom bloom is followed byblooms of dinoflagellates and coccolithophores. The effect of biologicalchanges on the seasonal variability of the surface ocean pCO 2 is nearlytwice the temperature effect, in agreement with previous studies. Theinclusion of multiple phytoplankton functional groups in the model played amajor role in the accurate representation of CO 2 uptake by biology. Forinstance, at the peak of the bloom, the exclusion of coccolithophores causesan increase in alkalinity of up to 4 μmol kg -1 with acorresponding increase in DIC of up to 16 μmol kg -1. During thepeak of the bloom in summer, the net effect of the absence of thecoccolithophores bloom is an increase in pCO 2 of more than 20 μatmand a reduction of atmospheric CO 2 uptake of more than6 mmol m -2 d -1. On average, the impact of coccolithophores is an increase ofair-sea CO 2 flux of about 27%. Considering the areal extent of thebloom from satellite images within the Irminger and Icelandic Basins, thisreduction translates into an annual mean of nearly 1500 tonnes C yr -1. © 2012 Author(s).