Phytoplankton pigment variations during the transition from spring bloom to oligotrophy in the northwestern Mediterranean sea

Phytoplankton taxonomic pigments and primary production were measured at the JGOFSFrance time-series station DYFAMED in the northwestern Mediterranean Sea during May 1995 to investigate changes in phytoplankton composition and the biogeochemical implications (DYNAPROC experiment). The study period covered the transitional situation from late spring bloom to pre-oligotrophic. The late spring bloom situation, occurring at the beginning of the study, revealed high chlorophyll a concentrations (maximum 3 mg m~3 at 30 m) and high primary production (maximum 497 mg C m~2 14 h~1). At the end of the experiment, the trophic regime shifted towards pre-oligotrophic and was characterized by lower chlorophyll a concentrations (( 1m g m~3), although primary production still remained high (659 mg C m~2 14 h~1). At termination of the spring bloom, the phytoplankton community was composed of chromophyte nano#agellates (38$4%), diatoms (29$2%), cryptophytes (12$1%) and cyanobacteria (8$1%). During the transition to the pre-oligotrophic period, the contribution of small cells increased (e.g. cyanobacteria 18$2%, green #agellates 5$1%). Vertical pro"les of pigments revealed a partition of the phytoplankton groups: cyanobacteria were most abundant in the surface layer, nano#agellates containing 19@-HF#19@BF at the depth of chlorophyll maximum, whereas diatoms were located below the chlorophyll maximum. At termination of the spring bloom, a wind event induced vertical transport of nutrients into the euphotic layer. Phytoplankton groups responded di!erently to the event: initially, diatom concentrations increased (for 24 h) then rapidly decreased. In contrast, all others groups

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