Resolving the ocean's euphotic zone

Measurements of net primary production (P) combined with calculated estimates of phytoplankton respiration (R-p) and gross primary production (G) are used to determine the depth of the ocean's euphotic zone, the autotrophic productive layer. The base of the euphotic zone, the compensation depth (where P=0 and G=R-p), 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 ocean's productive layer from satellite ocean color sensors. (C) 2013 Elsevier Ltd. All rights reserved.