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Estimating New Primary Production in Upwelling Areas from Space: A Case Study for the Benguela System using ENVISAT Satellite Data and a Model Dependent on the Phytoplankton Community Size Structure.

Ana Silio-Calzada(1), Annick Bricaud(1), Julia Uitz(2) and Bernard Gentili(1)

(1) Laboratoire d’Océanographie de Villefranche, CNRS, Quai de la Dase B.P. 8, 06238 - Villefranche sur Mer CEDEX, France
(2) Scripps Institute of Oceanography, 9500 Gilman Drive, 92093-0238, La Jolla, United States

Abstract

The estimate of new primary production, the fraction of primary production supported by nitrogen imported from outside the euphotic zone (Dugdale and Goering 1967), is considered to be an indicator of the flux of organic matter transported to the deep ocean by sinking particles. Therefore the spatial and temporal variations of the new primary production within the marine ecosystems have a crucial importance for the study of biogeochemical fluxes in the ocean; however, they have been poorly documented. In this study, we propose a revision of the “shift-up” new production model developed for upwelling areas by Dugdale et al. (1989), and later adapted by Kudela and Dugdale (1996), using ocean color and sea surface temperature satellite data as inputs. The two major improvements of the model are: 1) a new approach for the estimation of nitrate surface concentrations, specifically developed for areas influenced by upwelling processes, that relates the nitrate concentration to an indicator of the time elapsed since upwelling, in the form of the difference between SST and the estimated temperature of the upwelled water (variable with latitude and season), T, and 2) the estimation of nitrogen uptake rates taking into account the phytoplankton community size structure, estimated for each pixel using the method of Uitz et al. (2006). Appropriate physiological parameters are then used for each size class. This revised model has been applied to MERIS and AATSR data acquired over the year 2003, using the Benguela upwelling area as a test site. The combination of the new production model with a total production model allows the seasonal variations of new production (including the respective contributions of the three size classes), and of the new-to-total production ratio (f-ratio), to be estimated. Even though microphytoplankton are generally considered to be responsible for new production, our results show that the contribution of small cells should no be overlooked in the upwelling-affected areas. The values observed for the f-ratio (generally < 0.5) are lower than usually assumed for upwelling areas, although in agreement with published in situ data from the Benguela system.

 

Workshop presentation

Keywords: ESA European Space Agency - Agence spatiale europeenne, observation de la terre, earth observation, satellite remote sensing, teledetection, geophysique, altimetrie, radar, chimique atmospherique, geophysics, altimetry, radar, atmospheric chemistry