Biological Oceanography

(^) Most such satellite estimators of chlorophyll in the sea surface in a pixel are functions of ratios of Rrs
at several wavelengths. For example, one formulation used for SeaWiFS (termed Oc4v4) first
determines R, the greatest of three possible ratios divided by Rrs555:
(2)
(^) which is basically a ratio of blue light to green light coming up from the ocean (the numerical values
refer to wavelengths in nanometers). Notice that smaller R corresponds to more chlorophyll (more
green light relative to blue). Finally, chlorophyll (mg m−3 = μg liter−1) is calculated from the fitted
function:
(3)
On the other hand, the NOMAD (or any sea-surface-”truth” data-set) can be
dissected into regional subsets, and those often show substantial offsets from the
matching surface data, particularly in coastal regions. For example, a plot of surface
chlorophyll concentration from NOMAD vs. SeaWiFS Ca estimates from Eqn. 3 (see
Box 2.6) from coastal sites in the Atlantic Ocean (Fig. 2.17) shows substantial
overestimates from the satellite algorithm for both moderately rich North American
waters (near 1 mg Chl m−3) and for the very oligotrophic Mediterranean. The match is
better, at least apparently unbiased, for estimates from near the southwest coast of
Africa. In contrast, Ca is generally underestimated for waters around the Palmer
Peninsula in Antarctica (not shown).
Fig. 2.17 Comparison of OC4v4 SeaWiFS chlorophyll estimates to NOMAD
estimates from just Atlantic coastal sites. Lines represent the 1 : 1 relation and ½- to
two-fold variation. Open squares are from along the North American coast; filled
diamonds from off SW Africa; and Xs from the Mediterranean.
(^) (Analysis and graph generously provided by Janet W. Campbell, University of New Hampshire.)