Algal pigment distribution and primary production in the eastern Mediterranean as derived from coastal zone color scanner observations

Abstract

About 300 coastal zone color scanner (CZCS) scenes, gathered over the eastern Mediterranean basin mostly during the years 1979–1981, have been processed from level 1 by using improved pixel‐by‐pixel procedures for the atmospheric correction and pigment retrieval. The seasonal evolution of the upper ocean pigment concentration is described and analyzed within the whole basin and its subbasins. From the chlorophyll concentration in the top layer, and by using statistical relationships, the depth‐integrated pigment content is estimated and used in conjunction with a light‐photosynthesis model to estimate the carbon fixation. The model relies on a set of physiological parameters, selected after the validation of the light‐photosynthesis model and not on locally measured parameters. Additional information needed in the modeling are the photosynthetically available radiation (computed from astronomic and atmospheric parameters, combined with a cloud climatology), sea temperature and mixed‐layer depth (taken from Levitus (1982)). Actually, the model is used to generate look‐up tables in such a way that all possible situations (concerning available radiation, chlorophyll concentration, and temperature) are covered. The appropriate situation associated with any pixel is selected from these tables to generate primary production maps. Despite a relatively good spatial coverage, studying the interannual variability of the pigment distribution and primary production appeared to be impossible. Therefore 12 “climatological” monthly chlorophyll maps have been produced by merging the data corresponding to several years. The carbon fixation rates in each of the subbasins have been computed on a monthly basis, and annual mean values derived thereafter. The primary production values are compared with sparse field determinations. They are also compared with those previously derived for the Western basin, also by using CZCS data (Morel and André, 1991). When put together, these companion works provide a kind of record of the trophic status of the entire Mediterranean Sea in the early 1980s. Ocean color sensors to be launched next, like SeaWIFS, will allow the seasonal and interannual variabilities in the late 1990s to be addressed.