Phytoplankton pigment, absorption and ocean colour characteristics in the southern Benguela ecosystem

Pigment and remotely sensed data were used to investigate the distribution of phytoplankton and associated absorption characteristics in the southern Benguela ecosystem in June 1999. Chlorophyll a concentrations ranged from 0 .3 to 18.5 mg m(-3) at the surface, with fucoxanthin, the biomarker for dia toms, being most abundant in the inshore environment. Pigment signatures fo r nanoflagellates and cyanobacteria tended to be more important than fucoxa nthin at offshore localities. A north-south divide of the chemotaxonomic pi gments suggested that a mixed population of diatoms and small flagellates w as prevalent on the shelf in South African waters, while diatoms dominated the Namibian shelf zone. Satellite ocean colour images complemented the pig ment data by confirming the division of the phytoplankton into northern and southern components, and highlighted the complexity of the distribution pa ttern at the meso-scale. A plot of SeaWiFs chlorophyll a as a function of m easured chlorophyll a gave a poor relationship, with a slope of 0.56 and on ly 55% of the variance being explained. The regression line indicated that SeaWiFs overestimated chlorophyll a for concentrations <2 mg m(-3) and unde restimated concentrations that were >2 mg m(-3). Estimates of phytoplankton and pigment absorption, computed from pigment concentrations and absorptio n coefficients from the literature, revealed that the major absorption of l ight in diatom-nanoflagellate communities was by photosynthetic chlorophyll s and carotenoids. In contrast, there was significantly greater absorption by the photoprotective carotenoids in the upper layers of the water column in nanoflagellate-cyanobacteria dominated populations.