Resolving the oceanʹs euphotic zone

Measurements of net primary production (P) combined with calculated estimates of phytoplankton respiration (Rp) and gross primary production (G) are used to determine the depth of the oceanʹs euphotic zone, the autotrophic productive layer. The base of the euphotic zone, the compensation depth (where P=0 and G=Rp), is found to be consistently deeper than the traditionally assumed ‘1% light depth’. It is found to occur, however, at a depth that encompasses the depth range of all, or nearly all, autotrophic biomass. The estimated compensation depth also occurs near the depth of 1% of surface blue light (490 nm), supporting the determination of the oceanʹs productive layer from satellite ocean color sensors.