Algal biomass and sea surface temperature in the Mediterranean Basin - Intercomparison of data from various satellite sensors, and implications for primary production estimates

The Mediterranean Basin, subject both to climate changes and to increasing anthropogenic inputs, is an appropriate test site for observing the evolution of algal biomass and primary production on a long-term basis. With this aim, it is first necessary to study the consistency of the various sets of satellite data as provided by the space agencies, and to compare them to in situ available data. Pixel-by-pixel comparisons of the Level 3 chlorophyll products derived from the ocean color and temperature scanner (OCTS; Version 4, August 1999), polarization and directionality of earth reflectances (POLDER; reprocessing no. 2, July 2000), and the sea-viewing wide field-of-view sensor (SeaWiFS; reprocessing no. 3, May 2000) reveal a reasonably good agreement. Discrepancies, however, appear particularly in oligotrophic areas: weekly (or 10-day) means for OCTS and POLDER (which were operating simultaneously) differ in these areas by 30-70% on average, and OCTS and SeaWiFS weekly means, at 1-year distance, reveal differences by up to a factor of 2. Comparisons with measurements at sea, performed during various cruises, show that all these sensors tend to overestimate chlorophyll concentrations in oligotrophic waters. A "regional algorithm" is proposed to reduce this bias. The impact of using the various datasets for chlorophyll concentration, and for seawater temperature (Reynolds sea surface temperature [SST] analyses, Levitus climatological profiles) for primary production computations is shown. Because they are simultaneous to ocean color data, Reynolds analyses appear to be the most appropriate inputs to such computations. They have, however, to be combined with climatological vertical profiles of seawater temperature, so as to provide representative values for the productive layer. (C) 2002 Elsevier Science Inc. All rights reserved.