REGIONAL MODELS FOR PHYTOPLANKTON ABSORPTION AS A FUNCTION OF CHLOROPHYLL-A CONCENTRATION

Empirical relationships for predicting phytoplanktonic absorption at 6 76 and 436 nm from water column chlorophyll a concentration are presen ted for distinct geographic regions defined by latitude. The forms of the predictive equations are controlled by underlying biological mecha nisms and lend insight into these mechanisms. These region-specific mo dels allow prediction of phytoplanktonic absorption from more easily m easured parameters such as chlorophyll a concentration or in situ fluo rescence and increase accuracy in modeling optical properties or prima ry production rates from specific absorption coefficients. Phytoplankt onic absorption can be predicted from chlorophyll a concentrations est imated from satellite-based ocean color measurements. Temperate and tr opical regions exhibited statistically indistinguishable relationships at low chlorophyll so these regions were combined and treated as one. Nonlinear relationships between phytoplanktonic absorption at both 43 6 and 676 nm and chlorophyll a concentration for the combined temperat e/tropical region suggested that pigment packaging effects were import ant and variable. Higher slopes between absorption and chlorophyll a a t low chlorophyll supported the concept of low pigment packaging effec ts (thus higher specific absorption) in oligotrophic, low chlorophyll a waters. Subpolar waters displayed a distinct pattern and were define d as a separate region. Near-linear and linear relationships between p hytoplanktonic absorption at 436 and 676 nm and chlorophyll a concentr ation indicated that influences of pigment packaging on phytoplankton specific absorption coefficients were relatively constant and uncouple d from water column chlorophyll a concentration in the subpolar region . Optical depth correlated inversely with specific absorption at 436 n m in the subpolar region, illustrating the role of photoadaptation in determining specific absorption and predictive relationships. Differen ces between predictive quadratic equations for particulate and phytopl anktonic absorption indicated that detritus made only a small contribu tion to the nonlinear nature of the relationships. Absorption by detri tus at 436 nm ranged from 25 to 90% of the total particulate absorptio n. The proportion of absorption by detritus did not exhibit any strong patterns as a function of chlorophyll a concentration, arguing agains t the previously invoked explanation that proportional increases in ab sorption by detritus as environments become more oligotrophic cause th e observed nonlinearity between absorption and chlorophyll a concentra tion.