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

Remotely sensed chlorophyll a (CN a) concentrations are determined by the r atio of upwelled radiances within the Soret band of Chl a (443 nm) and at 5 50 nm. Absorption at wavelengths outside this band (>460 nm) is dominated b y 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 investigato rs speculated that these accessory pigments must covary with Chl a, althoug h routine methods to measure these pigments had not yet been developed. Nea rly 7,000 (high performance liquid chromatography) pigment samples, collect ed within the euphotic zone, were measured to test the consistency of the r elationship between accessory pigments and Chl a. Despite the various sampl ing periods (1985-1998) and numerous geographic locations, consistent patte rns have emerged in the ratios of the log accessory pigments to log total C hl a (TCHLA = Chl a, Chl a allomer, Chl a epimer, and chlorophyllide a). Th ere were strong log-linear relationships within cruises for these ratios wi th an average r(2) of 0.889. An even moro impressive relationship was obser ved on a global scale when all the data were combined. individual relations hips 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 som e statistical differences between these relationships, yet on a practical s ense 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 access ory pigment concentrations. This global log-linearity largely explains the success in remotely sensed Chl a algorithms, even though phyroplankton popu lations can vary in their composition and suite of pigments.