Pigments and species composition of natural phytoplankton populations: effect on the absorption spectra

Data from three cruises (Arabesque 1 and 2 cruises in the Arabian Sea and the Vancouver Island cruise) were examined to assess the importance of species composition and accessory pigments in modifying specific absorption coefficients. The three cruises differed widely in their phytoplankton assemblages with small cells dominating the Arabesque 2 cruise and large diatoms the Vancouver Island cruise. Absorption spectra from each cruise were decomposed into 13 Gaussian bands representing absorption by the major chlorophylls and accessory pigments. The maximum specific peak height (p*(m)) for each Gaussian band was obtained by regressing Gaussian peak heights against the concentration of the pigment responsible for the absorption band. This relationship was generally non-linear and was fitted with a rectangular hyperbolic function. Changes in the maximum specific peak heights between cruises reflect changes in the packaging effect, which were most apparent at Gaussian bands of high absorption (400-490 nm), but were close to zero at the Gaussian band centered around 623 mn, associated with chlorophyll a (Chi a). The maximum specific peak height of this Gaussian band, p*(m)(623), may be used to obtain reliable estimates of Chi a for any phytoplankton assemblage, unaffected by variations caused by the package effect. Comparing the Arabesque 2 peak heights with the Vancouver data, an apparent flattening effect of 0.42 at 440 nm and 0.62 at 676 nm was found for the Vancouver data relative to the Arabesque 2 data (assuming a zero flattening effect for the latter). Multiple linear regression analysis suggested that 29-42% of the variability in the specific absorption coefficient of phytoplankton at 440 nm was due to changes in pigment composition, while the remaining 58-71% was due to changes in the package effect. An inverse relationship was found between the proportion of non-photosynthetic pigments (NPC) and ambient Chi a concentration, suggesting that small cells (generally found in oligotrophic waters) had higher proportions of photoprotective pigments.