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

Abstract

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 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 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 mg/m3), 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 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 z1% is calculated via Rrs‐derived [Chl]. Further, the importance of deriving euphotic zone depth from satellite ocean‐color remote sensing is discussed.