Microplankton productivity in the oligotrophic ocean

Abstract

Uncertainty about the absolute levels of biological productivity over vast tracts of the world's oceans is a fundamental limitation to our understanding of the marine ecosystem1,2. Various workers have sought clarification through a comparison of the fluxes of oxygen and carbon in the photic layer3–5. A re-examination of data4 used to compare short-term (<1 day) in vitro carbon assimilation with long-term (≈100 day) in situ oxygen accumulation at a station in the north central Pacific has shown that the molar fluxes of oxygen and carbon are, in fact, consistent within the resolution of the comparison6. This test, however, is lacking in discrimination7. A recent comparison of short-term in vitro carbon and oxygen fluxes off Hawaii has been interpreted5 as indicating that short-term (≈12h) carbon uptake in open ocean regions provides an unbiased measure of gross primary production, Pg. We show here that comparisons of short-term oxygen- and carbon-flux measurements do not provide a powerful test of the accuracy of either of the measurements, both of which can be biased by trophic interactions in the microplankton. Consideration of trophic interactions, implicit in the reported data5, leads to the important new conclusion that the active biomass of microheterotrophic organisms was large compared with that of autotrophs, a situation which may be generally applicable to the pelagic zone of the open ocean.