Pigment distribution and primary production in the western Mediterranean as derived and modeled from coastal zone color scanner observations

Abstract

A set of 114 coastal zone color scanner (CZCS) images of the western Mediterranean (mainly in the year 1981) have been processed and analyzed to describe the algal biomass evolution and estimate its potential carbon fixation. For that, the pigment concentration in the top layer, Csat, is used through empirical relationships to infer the depth‐integrated pigment content of the productive column, 〈C〉tot. A spectral light‐photosynthesis model driven by 〈C〉tot is operated with additional information, namely, about sea temperature and photosynthetically available radiation (computed from astronomical and atmospherical parameters then combined with a cloud climatology). This model also includes a standard set of physiological parameters which account for the light capture by algae and for the use of this radiant energy in photosynthesis. This model allows a climatology of ψ* the cross section for photosynthesis per unit of areal chlorophyll, to be produced and then convoluted with the biomass maps after they have been averaged and composited. On average and for the whole western Mediterranean, the pigment concentration in the upper layer is about 0.25 mg Chl m−3, leading to an areal mean concentration of 21 mg Chl m−2. The maximum (bloom) occurs in early May in all zones. Seasonal variations in algal biomass are much more marked in the northern part than in the southern part (apart from Alboran Sea); the south Tyrrhenian basin and the central part of the Algerian basin are steadily oligotrophic. The mean annual carbon fixation rate for the whole basin is about 94 g C m−2 yr−1, or 106 and 87, for the northern and southern basins when separately considered. The seasonality is expressed by a six‐fold change in the production rate (between February and May) within the northern zone, whereas only a two‐fold change occurs in the southern zone between the same months. These estimates, which compare well with previous episodic field data, considerably extend our knowledge of the temporal progression of productivity within the entire western Mediterranean and its various provinces.