The shallow oxygen maximum layer and primary production

Abstract

The shallow oxygen maximum (SOM) is a vertical maximum in the dissolved oxygen concentration of the upper ocean. This feature develops largely from photosynthetic oxygen production, and measurement of the rate of oxygen accumulation provides the basis to make an independent estimate of primary production. Net accumulation of dissolved oxygen represents new production, which is generally presumed to require a new nutrient input consisting of some form of preformed nutrients. Here the time scales, vertical distribution or properties and new nutrient sources associated with formation of the SOM in the North Pacific ocean area considered. The SOM forms on two distinct time scales, episodic and gradual, and apparently through different processes on each scale. The episodic SOM develops from physical events in which nutricline water is transported into the mixed layer and then ventilated to the atmosphere. This process results in the local creation of preformed nutrients that stimulate a phytoplankton bloom and a vertical maximum of chlorophyll and dissolved oxygen. However, the time scale of formation of the gradual SOM is inconsistent with this mechanism because repeated mixing and ventilation events cannot result in the gradual accumulation of oxygen. An alternative new nutrient source is required. It is hypothesized that the gradual SOM develops from input of the “new” nutrients contained in organic matter transported from other depths by migrating animals and/or sinking particles. This hypothesis implies that the concept of new production must be carefully defined in context of the vertical structure of the euphotic zone. Whether or not my hypothesis is correct, these observations show that estimating primary production by measuring the increase in dissolved oxygen is very difficult. For both the episodic and gradual SOM, estimating production requires that the gas fluxes due to air-sea exchange and vertical and horizontal mixing be well known, in addition to direct measurements of the temporal change in dissolved oxygen within a water parcel.