Pigment chemotaxonomic distributions of phytoplankton during summer in the western Mediterranean

Abstract

Pigment distributions were investigated in the western Mediterranean basin during July 1993 to document the trophic status of the summer phytoplankton community. The characteristic deep chlorophyll maximum (DCM) was observed at all oceanic stations, and chlorophyll a concentrations of up to 1700 ng 1−1 were measured in the DCM in the northern regions. High chlorophyll a levels (2000–3000 ng 1−1) were determined in the lower reaches of the Rhone River, accompanied by high fucoxanthin levels. Fucoxanthin was also the dominant accessory pigment at the inshore stations influenced by the Rhone, while hexanoyloxyfucoxanthin was the major carotenoid at all other northern sites. Divinyl chlorophyll a concentrations were very low in the north (<30 ng 1−1) and only accounted for a maximum of 8% of the total chlorophyll a. Chlorophyll a levels were much lower in the southwestern Mediterranean; we estimate that divinyl chlorophyll a contributed 11–40% to the total chlorophyll a. Fucoxanthin was the prominent accessory pigment at Gibraltar, but hexanoyloxyfucoxanthin, chlorophyll b, zeaxanthin and divinyl chlorophyll a were more important at the other southern stations. The pigment data were used to estimate the contributions of prokaryotes (cyanobacteria and prochlorophytes) and eukaryotes to the total chlorophyll a at the surface and in the DCM. Overall, we determined that eukaryotes accounted for most of the chlorophyll a biomass, contributing 53–98%, and the prokaryote proportion was 2–47%. The pigment pattern revealed that the phytoplankton assemblage was not homogeneous and trophic conditions ranged from eutrophic in coastal and frontal regions where fucoxanthin containing diatoms dominated, to oligotrophic throughout most of the basin. Higher chlorophyll a biomass and dominant hexanoyloxyfucoxanthin containing prymnesiophytes were observed in the northern sector, while an increased prominence of prokaryotes in the south suggested that the southern sector was more oligotrophic.