Evaluation and regional optimization of bio-optical algorithms for central Lake Superior

Bio-optical properties of the ocean have been broadly investigated using Coastal Zone Color Scanner (CZCS) and Sea-viewing Wide Field-of-view Sensor imagery (SeaWiFS), from which numerous globally and regionally optimized bio-optical algorithms have emerged. As a first step in evaluating bio-optical algorithms for the Laurentian Great Lakes, we tested ten published marine bio-optical algorithms (9 empirical and 1 semi-analytical), in central Lake Superior using optical data and discrete water samples collected in 1998-2000. Although the semi-analytical algorithm provided a better result than the empirical algorithms, all of the algorithms overestimate chlorophyll a concentration. We derived optimized empirical parameters using two of the best algorithms (Ocean Color-Version 4 (OC4) and semi-analytical algorithms), but the models do not simulate chlorophyll a concentrations well. Compared to other coastal waters, the chlorophyll a concentration in Lake Superior is low (0.2-1.5 μg L-1) and the fraction of total absorbance attributable to chromophoric dissolved organic matter (CDOM) is high. The ratio of absorbance due to chlorophyll a to that of CDOM ranges from 0.001 to 0.262 between 443 and 555 nm in Lake Superior, which is 2-3 times lower than in case 1 waters, and indicates that semi-analytical, as opposed to strictly empirical, bio-optical algorithms may provide better estimates of chlorophyll a concentration in Lake Superior