BG Biogeosciences BG 1726-4189 Copernicus GmbH Göttingen, Germany 10.5194/bg-8-1291-2011 Optimizing models of the North Atlantic spring bloom using physical, chemical and bio-optical observations from a Lagrangian float Bagniewski W. 1 2 Fennel K. 2 Perry M. J. 1 D'Asaro E. A. 3 University of Maine, School of Marine Sciences, Orono, ME, USA Dalhousie University, Department of Oceanography, Halifax, NS, Canada University of Washington, Applied Physics Laboratory, Seattle, WA, USA 25 05 2011 8 5 1291 1307 This is an open-access article ditributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. This article is available from http://www.biogeosciences.net/8/1291/2011/bg-8-1291-2011.html The full text article is available as a PDF file from http://www.biogeosciences.net/8/1291/2011/bg-8-1291-2011.pdf

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 CO<sub>2</sub> 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.

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