Functional relationships and bio-optical properties derived from phytoplankton pigments, optical and photosynthetic parameters; a case study of the Benguela ecosystem

The relationships between phytoplankton pigments, optical properties and photosynthetic parameters for different phytoplankton functional types (derived by diagnostic pigment indices, DPI) were determined from data acquired in the Benguela ecosystem and the offshore region in October 2002. We observed robust inter-pigment relationships: total chlorophyll-a (TChla) was highly correlated with total pigment (TP) and accessory pigment (AP). However, the regression equations for stations dominated by flagellates differed from the equations for stations dominated by diatoms and dinoflagellates. The pigment ratio TChla/TP and the optical ratio a676/a440 were not constant but increased non-linearly with increasing TChla or TP: complimentarily the AP/TP and a490/a676 ratios decreased. There were significant non linear relationships between the photosynthetic parameters F-v/F-m or sigma(PSH) measured by Fast Repetition Rate Fluorometry and TChla or TP. Pigment ratios, optical ratios, F-v/F-m and sigma(PSH) were all inter-correlated with high significance. We determined the distinctive bio-optical properties associated with dominant phytoplankton functional types (derived by DPI) that conformed to the classical partitioning: flagellates (nano-plankton, comprising several taxa) had low biomass, low TChla/TP fraction and low F-v/F-m and high sigma(PSH); diatoms and dinoflagellates (micro-plankton) had high biomass, pigment ratios, F-v/F-m and low sigma(PSH).