Microphotometric characterization of phytoplankton and detrital absorption properties in the Sargasso Sea

Microphotometric techniques were used to characterize the spectral absorption properties of individual phytoplankton cells and detrital particles from two sites in the Sargasso Sea during spring 1985. Direct determinations of the absorption efficiency factor, Qa(λ), geometric cross-sectional area, G, and particle type were made for about 50 individual particles for each water sample analyzed. Large intrasample and intersample variations of Qa(λ) spectra for both phytoplankton and detrital particles were observed. For the mesotrophic northern station, mean values of Qa(λ) and G for phytoplankton cells showed insignificant vertical variations. For detrital particles, however, the mean geometric cross-sectional area decreased with depth while their mean Qa(λ) values increased. For the oligotrophic southern station these same relationships were observed, although more small-scale variability was apparent. The statistical comparison of the mean Qa(λ) values with the total particulate absorption coefficient spectrum, ap(λ), enabled quantification of the contributions phytoplankton and detrital materials made to ap(λ). Results showed that phytoplankton abundances accounted for ∼50–70% of the particulate absorption at 440 nm, although instances were observed where phytoplankton cells accounted for >90% of ap(440 nm). The reconstructed particulate absorption spectrum, âp(λ), compared favorably with the total ap(λ) indicating that nearly all of the optically relevant absorbing particulate material was sampled with the microphotometric technique.