Interactions between zooplankton and phytoplankton profiles in the eastern tropical Pacific Ocean

Data from 86 detailed Zooplankton profiles taken during the EASTROPAC cruises of 1967 to 1968 have enabled a first-order description to be made of Zooplankton distribution in the upper 1000 m of the water column in relation to density, light, oxygen, and phytoplankton. A layer of abundant epiplankton contains a subsurface maximum that tends to coincide with the bottom of the mixed layer and with the depth of maximum carbon fixation, but lies above the chlorophyll-a maximum. Zooplankton abundance declines sharply downwards across the pycnocline, forming a discontinuity between the epiplankton and the low-biomass plankton below. Coincident with the deep sonic scattering layers are diurnal layers of migrant interzonal species which rise at night into the epiplankton. Major regional differences are caused by the shoaling of the pycnocline at the equatorial divergence, and the very deep pycnocline of the southern gyral region. An hypothesis, based upon the EASTROPAC data, is stated: that the form of phytoplankton profiles, in periods or regions of stable production, may be primarily determined by a depth-differential in herbivore grazing pressure, rather than by differential cell-sinking rates as is suggested in classical production models.