The influence of basin-wide and sub-basin scale circulation features on phytoplankton community composition, primary and new productions, has been investigated in the Eastern Mediterranean during winter, using a sampling grid of 67 stations. Taxonomic pigments were used as size class markers of phototroph groups (pico-, nano- and microphytoplankton). Primary production rates were computed using a light-photosynthesis model which 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 f-ratio (new production/ total production). On average and for the whole eastern Mediterranean, Tchl a concentration was 20.4 mg m-2, primary and new production were 0.27 and 0.04 g C m-2 d-1, respectively. Nano- and pico-phytoplankton were the dominant size classes and respectively contributed to 60 and 27% of Tchl a, while microphytoplankton contributed for only 13%. Sub-basin and, with some extent, mesoscale structures (cyclonic and anticyclonic gyres), were an exception to this general trend. Anticyclonic gyres were characterized by the lowest Tch la concentration (18.8 ± 4.2 mg×m-2) and the highest picoplankton contribution (40 % of Tchl a), while cyclonic gyres presented the highest T chl a concentration (40.3 ± 15.3 mg×m-2) with an elevated microphytoplankton contribution (up to 26% of Tchl a). Observations conducted at mesoscale in the area of the Rhode gyre (cyclonic) show that the core of the gyre is dominated by microphytoplankton (mainly diatoms), while adjacent areas are characterized by high chlorophyll concentration dominated by pico-and nano-phytoplankton. We estimate that the Rhodes gyre is an enhancement zone of new production, by a factor of 9 compared to the adjacent oligotrophic areas of the Levantine basin. Our results confirm the predominance of oligotrophic conditions in the Eastern Mediterranean and emphasize the role of mesoscale dynamics in driving phytoplankton biomass and composition, and finally biogeochemical cycling in this area.
Phytoplankton pigment distribution in relation to upper thermocline circulation in the Eastern Mediterranean Sea during winter.
Luchetta A;
2001
Abstract
The influence of basin-wide and sub-basin scale circulation features on phytoplankton community composition, primary and new productions, has been investigated in the Eastern Mediterranean during winter, using a sampling grid of 67 stations. Taxonomic pigments were used as size class markers of phototroph groups (pico-, nano- and microphytoplankton). Primary production rates were computed using a light-photosynthesis model which 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 f-ratio (new production/ total production). On average and for the whole eastern Mediterranean, Tchl a concentration was 20.4 mg m-2, primary and new production were 0.27 and 0.04 g C m-2 d-1, respectively. Nano- and pico-phytoplankton were the dominant size classes and respectively contributed to 60 and 27% of Tchl a, while microphytoplankton contributed for only 13%. Sub-basin and, with some extent, mesoscale structures (cyclonic and anticyclonic gyres), were an exception to this general trend. Anticyclonic gyres were characterized by the lowest Tch la concentration (18.8 ± 4.2 mg×m-2) and the highest picoplankton contribution (40 % of Tchl a), while cyclonic gyres presented the highest T chl a concentration (40.3 ± 15.3 mg×m-2) with an elevated microphytoplankton contribution (up to 26% of Tchl a). Observations conducted at mesoscale in the area of the Rhode gyre (cyclonic) show that the core of the gyre is dominated by microphytoplankton (mainly diatoms), while adjacent areas are characterized by high chlorophyll concentration dominated by pico-and nano-phytoplankton. We estimate that the Rhodes gyre is an enhancement zone of new production, by a factor of 9 compared to the adjacent oligotrophic areas of the Levantine basin. Our results confirm the predominance of oligotrophic conditions in the Eastern Mediterranean and emphasize the role of mesoscale dynamics in driving phytoplankton biomass and composition, and finally biogeochemical cycling in this area.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
