PHOTOSYNTHETIC PIGMENTS AS BIOMARKERS OF PHYTOPLANKTON POPULATIONS AND PROCESSES INVOLVED IN THE TRANSFORMATION OF PARTICULATE ORGANIC-MATTER AT THE BIOTRANS SITE (47-DEGREES-N, 20-DEGREES-W)
Résumé
Particulate samples were collected throughout the water column (0-4200 m) in June/July 1988 at the Biotrans site and their carotenoids and chlorophylls analysed by HPLC. These photosynthetic pigments were used as biomarkers to characterize the autotrophic populations, their utilization by heterotrophs and sedimentation of particles out of the euphotic zone. In the upper 50 m the pico- and nanophytoplankton accounted for 85% of the chlorophyll a biomass. The major pigment of the nanophytoplankton fraction was 19'-hexanoyloxyfucoxanthin (prymnesiophytes), whereas the main pigment in the microphytoplankton was peridinin (dinoflagellates). The peaks in the distributions of phaeophorbide a and nanophytoplankton pigments (19'-hexanoyloxyfucoxanthin, 19'-butanoyloxyfucoxanthin, chlorophyll b, lutein and/or zeaxanthin) coincided between 75 and 100 m, which pointed to an active grazing of nanophytoplankton by zooplankton. These pigments were detected in particles > 20-mu-m from the Double Longhurst Hardy Plankton Recorder down to 1000 m, probably as a consequence of their incorporation into sedimenting faecal material. In contrast, the vertical distributions of phaeophorbide a and peridinin (microphytoplankton pigment) did not coincide, and this carotenoid was not detected below 400 m in particles > 20-mu-m. A vertical profile (0-4200 m) shows, at 2300 m, the presence of nanophytoplanktonic material similar in its pigment pattern and composition to that of surface populations, suggesting fast sedimentation of Prymnesiophyte floc.