Retrieval of near-surface bio-optical properties of the Arabian Sea from remotely sensed ocean colour data

Abstract

The use of CZCS-type band-ratio algorithms to estimate the diffuse attenuation coefficient, percentage light depths, and near-surface optically weighted phytoplankton pigment concentrations from remotely sensed ocean colour data was investigated on two cruises in the Arabian Sea and Gulf of Oman during autumn and winter 1994. The variations of upwelling radiance and downwelling irradiance with depth were measured along with phytoplankton pigment concentrations by HPLC. A spectroradiometer was used on the second cruise to investigate the feasibility of measuring water-leaving radiance from above the sea surface. Retrieval of the diffuse downwelling attenuation coefficient at 490 nm was accurate to within 22% of the actual value across both cruises. There was also a robust relationship between the diffuse attenuation coefficient and the 10, 1 and 0.1% light depths. Above-surface estimates of water-leaving radiance agreed with SeaWiFS-standard estimates to within 10% between 443 and 555 nm. The global 443 : 555 band-ratio algorithms of Aiken et al. [NASA Tech. Memo 104566, Vol. 29, SeaWiFS Technical Report Series, 34 pp] estimated near-surface chlorophyll- a and fluorometric pigment concentrations with mean absolute errors of less than 35% of the actual values (which were all less than 2.0 mg m -3). The performance of algorithms based on the 490 : 555 ratio was poorer. The estimates given by the algorithms were generally higher than the measured pigment concentrations and the variance of the accuracy of the estimates was high. There appears to be no significant change in the performance of the algorithms between cruises (approximately 2 1 2 months apart in time). There is no evidence that the Gulf of Oman should be treated as a separate bio-optical province to the Arabian Sea/Omani shelf area for the purpose of the retrieval of near-surface pigment concentrations from ocean colour observations.