From the chlorophyll <i>a</i> in the surface layer to its vertical profile: a Greenland Sea relationship for satellite applications

Abstract

Abstract. Current estimates of global marine primary production range over a factor of two. At high latitudes, the uncertainty is even larger than globally because here in-situ data and ocean color observations are scarce, and the phytoplankton absorption shows specific characteristics due to the low-light adaptation. The improvement of the primary production estimates requires an accurate knowledge on the chlorophyll vertical profile, which is the basis for most primary production models. To date, studies describing the typical chlorophyll profile based on the chlorophyll in the surface layer did not include the Arctic region or, if it was included, the dependence of the profile shape on surface concentration was neglected. The goal of our study was to derive and describe the typical Greenland Sea chlorophyll profiles, categorized according to the chlorophyll concentration in the surface layer and further monthly resolved. The Greenland Sea was chosen because it is known to be one of the most productive regions of the Arctic and is among the Arctic regions where most chlorophyll field data are available. Our database contained 1199 chlorophyll profiles from R/Vs Polarstern and Maria S Merian cruises combined with data of the ARCSS-PP database (Arctic primary production in-situ database) for the years 1957–2010. The profiles were categorized according to their mean concentration in the surface layer and then monthly median profiles within each category were calculated. The category with the surface layer chlorophyll exceeding 0.7 mg C m−3 showed a clear seasonal cycle with values gradually decreasing from April to August. Chlorophyll profiles maxima moved from lower depths in spring towards the surface in late summer. Profiles with smallest surface values always showed a subsurface chlorophyll maximum with its median magnitude reaching up to three times the surface concentration. While the variability in April, May and June of the Greenland Sea season is following the global non-monthly resolved relationship of the chlorophyll profile to surface chlorophyll concentrations described by the model of Morel and Berthon (1989), it deviates significantly from that in other months (July–September) where the maxima of the chlorophyll are at quite different depths. The Greenland Sea dimensionless monthly median profiles intersect roughly at one common depth within each category. Finally, by applying a Gaussian fitting with 0.1 mg C m−3 surface chlorophyll steps to the median monthly resolved chlorophyll profiles of the defined categories, mathematical approximations have been determined. These will be used as the input to the satellite-based primary production models estimating primary production in Arctic regions.