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 subbasin-scale circulation features on phytoplankton community composition and primary and new productions was investigated in the eastern Mediterranean during winter. Taxonomic pigments were used as size class markers of phototroph groups (picophytoplankton, nanophytoplankton and microphytoplankton). Primary production rates were computed using a light photosynthesis model that makes use of the total chlorophyll a (Tchl a) concentration profile as an input variable. New production was estimated as the product of primary production by a pigment-based proxy of the ƒ ratio (new production/total production). 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 and picophytoplankton (determined from the pigment-derived criteria) were the dominant size classes and contributed to 60 and 27%, respectively, of Tchl a, while microphytoplankton contributed only to 13%. Subbasin and, to a certain 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 being 12.4 mg m−2) and the highest picophytoplankton contribution (40% of Tchl a). In contrast, cyclonic gyres contained the highest Tchl a concentration (40.3±15.3 mg m−2) with the highest microphytoplankton contribution (up to 26% of Tchl a). Observations conducted at a mesoscale in the Rhode gyre (cyclonic) region show that the core of the gyre is dominated by microphytoplankton (mainly diatoms), while adjacent areas are characterized by high chlorophyll concentration dominated by picophytoplankton and nanophytoplankton. 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 Levantine basin. Our results confirm the predominance of oligotrophic conditions in the eastern Mediterranean and emphasize the role of subbasin and mesoscale dynamics in driving phytoplankton biomass and composition and, finally, biogeochemical cycling in this area.

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