Euphotic Zone Depth: Its Derivation and Implication to Ocean-Color Remote Sensing

Euphotic zone depth, Z1, reflects the depth where photosynthetic available radiation PAR is 1 of its surface value. The value Of Z1 is a measure of water clarity, which is an important parameter regarding ecosystems. Based on the Case-1 water assumption, Z1 can be estimated empirically from the remotely derived concentration of chlorophyll-a Chl, commonly retrieved by employing band ratios of remote sensing reflectance Rrs. Recently, a model based on waters inherent optical properties IOPs has been developed to describe the vertical attenuation of visible solar radiation. Since lOPs can be near-analytically calculated from Rrs, so too can Z1. In this study, for measurements made over three different regions and at different seasons Z1 were in a range of 4.3-82.0 m with Chl ranging from 0.07 to 49.4 mgm3, Z1 calculated from Rrs was compared with Z1 from in situ measured PAR profiles. It is found that the Z1 values calculated via Rrs-derived lOPs are, on average, within 14 of the measured values, and similar results were obtained for depths of 10 and 50 of sufface PAR. in comparison, however, the error was 33 when Z1 is calculated via Rrs-derived ChI. Further, the importance of deriving euphotic zone depth from satellite ocean-color remote sensing is discussed.