Reconciling Between Optical and Biological Determinants of the Euphotic Zone Depth

Abstract The conventional use of optically determined 1% of surface photosynthetically available radiation (PAR) depth ( Z 1% PAR , λ = 400–700 nm) as a metric for the euphotic zone depth ( Z eu ) has been a matter of debate for several decades because of frequent inconsistencies with the base of euphotic zone determined biologically, that is, the compensation depth ( Z c ). In this study, we attempt to reconcile between optical and biological determinants of the euphotic zone through the use of a large data set of coincidental profiles of downwelling irradiance and primary production. These measurements cover open ocean waters in the tropics, subtropics, temperate regions, and from two time‐series stations, the Hawaii Ocean Time‐series (HOT) and the Bermuda Atlantic Time‐series Study (BATS). We report that, at least for these measurements, Z 1% PAR is too shallow (by 14.1%) compared to Z c , while Z 0.1% PAR is too deep (by 32.7%). Further, the irradiance at Z c (i.e., the compensation irradiance, I c ) varies by a factor of more than five, but its ratio to surface irradiance is relatively stable. In general, I c corresponds to 0.48 ± 0.23% of surface PAR, or 0.87 ± 0.40% of surface usable solar radiation (USR, for λ = 400–560 nm), or 1.50 ± 0.67% of surface downwelling irradiance at 490 nm. These results suggest that Z 0.5% PAR , or Z 0.9% USR , or Z 1.5% 490 could be promising alternatives to bridge the optical and biological determinants of the euphotic zone depths for waters in the open ocean at low to middle latitudes.