Bio-optical properties during the summer season in the Sea of Okhotsk

The Sea of Okhotsk is one of the most productive ocean regions in the world. However, the in situ bio-optical properties, which are crucial for satellite ocean-color of the productivity, remain uncertain in this region because little data have been available. We conducted an in situ observation and evaluated the bio-optical properties in terms of chlorophyll a (Chl a) concentration, spectral remote sensing reflectance (Rrs), and the light absorption coefficients of phytoplankton (aφ), non-algal particles (aNAP), and chromophoric dissolved organic matter (aCDOM) in the summer of 2006. The data covered a wide range of Chl a levels in surface waters from 0.3 to 8.5 mg m−3. At 443 nm, aCDOM dominated (64% on average) the total non-water absorption (at-w) in this study area. Based on the in situ Rrs data, surface Chl a concentrations that were estimated using the sea-viewing wide field-of-view sensor (SeaWiFS) OC4v6 and the moderate-resolution imaging spectroradiometer (MODIS) OC3M algorithms were significantly higher than the in situ data by more than 160% and 260%, respectively. In particular, the largest overestimation occurred in the region where aCDOM at 443 nm accounted for more than 80% of at-w near the mouth of the Amur River. However, except the CDOM-rich stations, the performance of the OC4v6 and OC3M algorithms became better (i.e., their mean normalized biases were reduced to 50% and 66%, respectively). We conclude that the operational global algorithms were applicable to the summer season in the Sea of Okhotsk except the CDOM-rich region, in which new approaches for ocean-color algorithms (i.e., local algorithms) would be required.