Another look at the nitrite and chlorophyll maxima in the central North Pacific

Abstract

A field study of the deep chlorophyll maximum and associated nitrite maximuum of the central North Pacific compared the standing crop, growth rates, and physiological characteristics of phytoplankton from the mixed layer, the chlorophyll maximum, and a depth well below the chlorohyll maximum. (1) The chlorophyll maximum is primarily due to increased cellular chlorophyll rather than an accumulation of cells at depth, and (2) the cells within and below the chlorophyll maximum are liiving but limited by radiant energy.A simple box model was constructed to cover the depth internal of the primary nitrite maximum. Solution of the model, which assumed constant vertical mixing, indicated sufficient uptake of upwardly diffusing nitrate to account for the appearance of nitrate. It is suggested that nitrous acid diffuses out of phytoplankton, and that there is a steady-state concentration of extracellular nitrite in which the rate of uptake by phytoplankton equals its rate of passive diffusion out of the cells.Culture studies of the marine diatom,Thalassiosira pseudonana, indicate that the cells produce nitrite at rates sufficient to account for the primary nitrite maximum, and that there is a steady-state concentration of nitrite that does not exceed 4 μM.