The effects of water, plankton algae, and other suspended and dissolved substances on light attenuation for the total euphotic layer of the ocean were investigated. All three of these factors play different roles depending on the depth of the euphotic zone. When the euphotic zone is shallow (about 10 m) the dominant factor is plankton algae. In the deepest euphotic zones found, most of the light attenuation can be attributed to water itself. If the euphotic zone is of intermediate thickness (30–40 m), more than one-half of the light attenuation must be attributed to other paniculate and dissolved substances. The maximum amount of chlorophyll (as a measure of phytoplankton biomass) that can be expected to be found under a m2 of sea surface is 333 mg.