Accessory pigments versus chlorophyll a concentrations within the euphotic zone:: A ubiquitous relationship

Remotely sensed chlorophyll a (CN a) concentrations are determined by the ratio of upwelled radiances within the Soret band of Chl a (443 nm) and at 550 nm. Absorption at wavelengths outside this band (>460 nm) is dominated by accessory pigments and for the successful measurement of Chl a (e.g., 490:550 nm and 520:550 nm ratios) early Coastal Zone Color Scanner investigators speculated that these accessory pigments must covary with Chl a, although routine methods to measure these pigments had not yet been developed. Nearly 7,000 (high performance liquid chromatography) pigment samples, collected within the euphotic zone, were measured to test the consistency of the relationship between accessory pigments and Chl a. Despite the various sampling periods (1985-1998) and numerous geographic locations, consistent patterns have emerged in the ratios of the log accessory pigments to log total Chl a (TCHLA = Chl a, Chl a allomer, Chl a epimer, and chlorophyllide a). There were strong log-linear relationships within cruises for these ratios with an average r(2) of 0.889. An even moro impressive relationship was observed on a global scale when all the data were combined. individual relationships were also calculated for case I and case II waters, as well as for the first optical depth (K-1), termed the remote sensing depth. There were some statistical differences between these relationships, yet on a practical sense many could be combined. Despite a wide range of environments sampled, the overall slope of the log accessory pigments:log TCHLA was found to be 0.934 with a relative-difference root-mean-square error of 28% in log accessory pigment concentrations. This global log-linearity largely explains the success in remotely sensed Chl a algorithms, even though phyroplankton populations can vary in their composition and suite of pigments.