Phytoplankton pigment distribution in relation to upper thermocline circulation in the eastern Mediterranean Sea during winter

Using a sampling grid of 67 stations, the influence of basin-wide and subba sin-scale circulation features on phytoplankton community composition and p rimary and new productions was investigated in the eastern Mediterranean du ring winter. Taxonomic pigments were used as size class markers of phototro ph groups (picophytoplankton, nanophytoplankton and microphytoplankton). Pr imary production rates were computed using a light photosynthesis model tha t makes use of the total chlorophyll a (Tchl a) concentration profile as an input variable. New production was estimated as the product of primary pro duction by a pigment-based proxy of the f ratio (new production/total produ ction), For the whole eastern Mediterranean, Tchl a concentration was 20.4 mg m(-2), and estimated primary and new production were 0.27 and 0.04 g C m (-2) d(-1), respectively, when integrated between the surface and the depth of the productive zone (1.5 times the euphotic layer). Nanophytoplankton a nd picophytoplankton (determined from the pigment-derived criteria) were th e dominant size classes and contributed to 60 and 27%, respectively, of Tch l a, while microphytoplankton contributed only to 13%. Subbasin and, to a c ertain extent, mesoscale structures (cyclonic and anticyclonic gyres) were exceptions to this general trend. Anticyclonic gyres were characterized by low Tchl a concentrations (18.8 +/- 4.2 mg m(-2), with the lowest value bei ng 12.4 mg m(-2)) and the highest picophytoplankton contribution (40% of Tc hl a). In contrast, cyclonic gyres contained the highest Tchl a concentrati on (40.3 +/- 15.3 mg in with the highest microphytoplankton contribution (u p to 26% of Tchl a). Observations conducted at a mesoscale in the Rhode gyr e (cyclonic) region show that the core of the gyre is dominated by microphy toplankton (mainly diatoms), while adjacent areas are characterized by high chlorophyll concentration dominated by picophytoplankton and nanophytoplan kton. We estimate that the Rhodes gyre is a zone of enhanced new production , which is 9 times higher than that in adjacent oligotrophic areas of the L evantine basin. Our results confirm the predominance of oligotrophic condit ions in the eastern Mediterranean and emphasize the role of subbasin and me soscale dynamics in driving phytoplankton biomass and composition and, fina lly, biogeochemical cycling in this area.