Abstract
Abstract. We developed an ecosystem/biogeochemical model system, which includes multiple phytoplankton functional groups and carbon cycle dynamics, and applied it to investigate physical-biological interactions in Icelandic waters. Satellite and in situ data were used to evaluate the model. Surface seasonal cycle amplitudes and biases of key parameters (DIC, TA, pCO2, air-sea CO2 flux, and nutrients) are significantly improved when compared to surface observations by prescribing deep water values and trends, based on available data. The seasonality of the coccolithophore and "other phytoplankton" (diatoms and dinoflagellates) blooms is in general agreement with satellite ocean color products. Nutrient supply, biomass and calcite concentrations are modulated by light and mixed layer depth seasonal cycles. Diatoms are the most abundant phytoplankton, with a large bloom in early spring and a secondary bloom in fall. The diatom bloom is followed by blooms of dinoflagellates and coccolithophores. The effect of biological changes on the seasonal variability of the surface ocean pCO2 is nearly twice the temperature effect, in agreement with previous studies. The inclusion of multiple phytoplankton functional groups in the model played a major role in the accurate representation of CO2 uptake by biology. For instance, at the peak of the bloom, the exclusion of coccolithophores causes an increase in alkalinity of up to 4 μmol kg−1 with a corresponding increase in DIC of up to 16 μmol kg−1. During the peak of the bloom in summer, the net effect of the absence of the coccolithophores bloom is an increase in pCO2 of more than 20 μatm and a reduction of atmospheric CO2 uptake of more than 6 mmol m−2 d−1. On average, the impact of coccolithophores is an increase of air-sea CO2 flux of about 27%. Considering the areal extent of the bloom from satellite images within the Irminger and Icelandic Basins, this reduction translates into an annual mean of nearly 1500 tonnes C yr−1.
Highlights
Coupled biogeochemical-physical numerical models together with observations are an essential tool to understand the interaction between physical and biological processes that create the observed temporal variability, on time scalesPublished by Copernicus Publications on behalf of the European Geosciences Union.S
Coccolithophores occupy a central role in the carbon cycle by the conversion of dissolved inorganic carbon (DIC) to both particulate organic carbon (POC) and particulate inorganic carbon (PIC) forms, albeit uncertainties exist over whether coccolithophore blooms are net sinks or sources of CO2 to the atmosphere (Boyd and Trull, 2007)
The coherent model prediction of growth of coccolithophores and the variations in PIC concentration are consistent with the interannual changes in PIC observed by the satellite, which confirms the relevance of the Zeu/mixed layer depth (MLD) ratio
Summary
Coupled biogeochemical-physical numerical models together with observations are an essential tool to understand the interaction between physical and biological processes that create the observed temporal variability, on time scales. We developed a coupled biogeochemical-physical model system, which includes multiple phytoplankton functional groups and carbon cycle dynamics, and applied it to assess the role of phytoplankton dynamics in the seasonal and interannual variability of carbon in the subpolar North Atlantic. South of the AF waters are saltier (∼35 psu) and warmer (8 to 12 ◦C), as a result of Atlantic water intrusions These hydrographic characteristics have an impact on the seasonal vertical mixing, as shown in model simulations. There are indications from Argos floats that deep convective chimneys occasionally occur in the central Irminger Sea (as in early 2008) down to nearly 1000 m (Vage et al, 2009) These large seasonal changes in stratification and vertical mixing play an important role in the euphotic zone nutrient renewal and on the onset and duration of the phytoplankton spring bloom (Henson et al, 2006). The blue circles (1994) and red crosses (2004) indicate the location of the two repeat CARINA transects
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