Reconciling Between Optical and Biological Determinants of the Euphotic Zone Depth

The conventional use of optically determined 1% of surface photosynthetically available radiation (PAR) depth (Z(1%)(PAR), lambda = 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 lambda = 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.

Automated summary

In this study, discrepancies between optically determined surface photosynthetically available radiation depth (Z(1%)(PAR)) and biologically determined compensation depth (Z(c)) were explored using a large dataset of downwelling irradiance and primary production profiles. The results suggest potential alternatives, such as Z(0.5%)(PAR), Z(0.9%)(USR), or Z(1.5%)(490), to bridge the gap between optical and biological determinants of euphotic zone depths in open ocean waters at low to middle latitudes.