Are Methods for Estimating Primary Production and the Growth Rates of Phytoplankton Approaching Agreement?

During the 1980s, estimates of primary productivity and the growth rates of phytoplankton in oligotrophic waters were controversial, in part because rates based on seasonal accumulations of oxygen in the shallow oxygen maximum were reported to be much higher than could be accounted for with measurements of photosynthesis based on incubations with C-14. Since then, much has changed: tested and standardized methods have been employed to collect comprehensive time-series observations of primary production and related oceanographic properties in oligotrophic waters of the North Pacific subtropical gyre and the Sargasso Sea; technical and theoretical advances have led to new tracer-based estimates of photosynthesis (e.g., oxygen/argon and triple isotopes of dissolved oxygen); and biogeochemical sensor systems on ocean gliders and profiling floats can describe with unprecedented resolution the dynamics of phytoplankton, oxygen and nitrate as driven by growth, loss processes including grazing, and vertical migration for nutrient acquisition. Meanwhile, the estimation of primary productivity, phytoplankton biomass and phytoplankton growth rates from remote sensing of ocean color has matured, complementing biogeochemical models that describe and predict these key properties of plankton dynamics. In a selective review focused on well-studied oligotrophic waters, I compare methods for estimating the primary productivity and growth rates of phytoplankton to see if they are converging on agreement, not only in the estimated rates, but also in the underlying assumptions, such as the ratio of gross- to net primary production — and how this relates to the measurement — and the ratio of chlorophyll to carbon in phytoplankton. Examples of agreement are encouraging, but some stark contrasts illustrate the need for improved mechanistic understanding of exactly what each method is measuring.