MEPS 281:13-26 (2004)  -  doi:10.3354/meps281013

ABSTRACT: Pigment and ocean colour data were used to characterise the phytoplankton at the surface along an eastern boundary transect of the Atlantic Ocean in May and June 1998. Five major regions were delineated, including the Benguela and Canary upwelling systems off SW and NW Africa, the temperate NE Atlantic, the northern oligotrophic gyre and a southern oligotrophic zone. Diagnostic indices were derived to assess the chemotaxonomic composition of the phytoplankton community, while photo-pigment indices were derived to investigate pigment adaptations across the regions. A large range in chlorophyll a concentrations was observed along the transect, varying from 0.03 to 30.6 mg m^-3. The levels were high in the high productivity regions, with mean concentrations being 3-fold greater in the Benguela region compared to the Canary and temperate NE Atlantic. Chlorophyll a levels were low in the oligotrophic regions, but concentrations in the southern oligotrophic region were double those in the northern. Diagnostic indices revealed that microplankton dominated the community in the Benguela region, nanoplankton dominated the temperate NE Atlantic and a mixed community of micro- and nanoplankton were present in the Canary region. Picoplankton comprised the majority of the communities in the 2 oligotrophic regions. Photo-pigment indices indicated that variation in the chlorophyll a proportion of total pigments was small, averaging 45% across the transect, but the accessory pigment pool varied considerably. Chlorophyll c and photosynthetic carotenoids were prominent in the high productivity regions, resulting in photosynthetic pigments (chlorophylls plus carotenoids) accounting for 87 to 93% of total pigments. In contrast, the level of photoprotective carotenoids was high in oligotrophic regions, resulting in a photosynthetic pigment content of only 68%.

KEY WORDS: Pigment indices · Ocean colour · Atlantic Ocean · Eastern transect