Abstract
Abstract. The North Atlantic spring bloom is one of the main events that lead to carbon export to the deep ocean and drive oceanic uptake of CO2 from the atmosphere. Here we use a suite of physical, bio-optical and chemical measurements made during the 2008 spring bloom to optimize and compare three different models of biological carbon export. The observations are from a Lagrangian float that operated south of Iceland from early April to late June, and were calibrated with ship-based measurements. The simplest model is representative of typical NPZD models used for the North Atlantic, while the most complex model explicitly includes diatoms and the formation of fast sinking diatom aggregates and cysts under silicate limitation. We carried out a variational optimization and error analysis for the biological parameters of all three models, and compared their ability to replicate the observations. The observations were sufficient to constrain most phytoplankton-related model parameters to accuracies of better than 15 %. However, the lack of zooplankton observations leads to large uncertainties in model parameters for grazing. The simulated vertical carbon flux at 100 m depth is similar between models and agrees well with available observations, but at 600 m the simulated flux is larger by a factor of 2.5 to 4.5 for the model with diatom aggregation. While none of the models can be formally rejected based on their misfit with the available observations, the model that includes export by diatom aggregation has a statistically significant better fit to the observations and more accurately represents the mechanisms and timing of carbon export based on observations not included in the optimization. Thus models that accurately simulate the upper 100 m do not necessarily accurately simulate export to deeper depths.
Highlights
It is estimated that about 25 % of the global oceanic CO2 uptake from the atmosphere takes place in the North Atlantic (Takahashi et al, 2009)
Data used for validation and assimilation were collected by a heavily-instrumented Lagrangian mixed-layer float (D’Asaro, 2002), which operated at the North Atlantic Bloom 2008 (NAB08) site (Fig. 1) from Year Day (YD) 95 (4 April) to YD 146 (25 May)
The only parameter measured during the NAB08 experiment was α
Summary
It is estimated that about 25 % of the global oceanic CO2 uptake from the atmosphere takes place in the North Atlantic (Takahashi et al, 2009). A major contributor to this uptake is the North Atlantic spring bloom. During the bloom phytoplankton typically grow rapidly, taking up nutrients near the surface. Export of Particulate Organic Carbon (POC) to the deep ocean results from sinking of organic particles primarily diatom aggregates and zooplankton fecal pellets. In the North Atlantic large diatoms often dominate the phytoplankton community at the beginning of the bloom (Sieracki et al, 1993) and constitute a major fraction of the sinking organic matter. Since silicate concentrations in the North Atlantic are lower than nitrate concentrations, silicate is typically the first nutrient to become depleted and to start limiting diatom growth (Allen et al, 2005). Physiological stress resulting from silicate limitation is known to increase the sinking rates of diatoms (Bienfang et al, 1982).
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