Photosynthetic quotients, new production and net community production in the open ocean

Abstract

Based on balanced chemical equations for the production of proteins, polysaccharides, lipids and nucleic acids from inorganic precursors and reported estimates of the composition of microalgal biomass, the photosynthetic quotients (PQs) for new and recycled production are estimated to be 1.4 ± 0.1 and 1.1 ± 0.1, respectively. Reports that the PQs for new production is 1.8 or higher appear to result from comparisons of gross oxygen production to net CO 2 assimilation. Available experimental data suggest that net community carbon production and new production of carbon in the ocean differ by less than 10%. Evidence indicates that the ratio of carbon to nitrogen (C:N) in the organic matter exported from the photic zone exceeds the Redfield ratio of 5.7 by weight, regardless of whether the organic matter is exported in the form of dissolved organics or passively or actively transported particulates. Hence scaling nitrate uptake by the Redfield ratio to estimate net community carbon production will tend to underestimate the export of organic carbon from the photic zone, the magnitude of the probable error being roughly 15–30%.