Euphotic zone depth: Its derivation and implication to ocean‐color remote sensing

Abstract

Euphotic zone depth, z 1% , reflects the depth where photosynthetic available radiation (PAR) is 1% of its surface value. The value of z 1% is a measure of water clarity, which is an important parameter regarding ecosystems. Based on the Case‐1 water assumption, z 1% can be estimated empirically from the remotely derived concentration of chlorophyll‐a ([Chl]), commonly retrieved by employing band ratios of remote sensing reflectance ( R rs ). Recently, a model based on water's inherent optical properties (IOPs) has been developed to describe the vertical attenuation of visible solar radiation. Since IOPs can be near‐analytically calculated from R rs , so too can z 1% . In this study, for measurements made over three different regions and at different seasons ( z 1% were in a range of 4.3–82.0 m with [Chl] ranging from 0.07 to 49.4 mg/m 3 ), z 1% calculated from R rs was compared with z 1% from in situ measured PAR profiles. It is found that the z 1% values calculated via R rs ‐derived IOPs are, on average, within ∼14% of the measured values, and similar results were obtained for depths of 10% and 50% of surface PAR. In comparison, however, the error was ∼33% when z 1% is calculated via R rs ‐derived [Chl]. Further, the importance of deriving euphotic zone depth from satellite ocean‐color remote sensing is discussed.