Advancing interpretations of 14C-uptake measurements in the context of phytoplankton physiology and ecology

The C-14-uptake method is the most common approach employed for estimating primary production in the ocean. Normalizing C-14-uptake to chlorophyll a and time yields a value termed the assimilation number, which is thought to reflect phytoplankton physiology. It is often assumed that the measured rate of C-14-uptake is between net and gross primary production, depending on the time scale of the incubation. Recent studies employing multiple oxygen and carbon isotopic methods to measure photosynthesis of phytoplankton grown over a range of steady-state division rates have provided mechanistic insights on the relationship between C-14-uptake and gross-to-net primary production. Results from these studies show that short-term (< 12 h) "photosynthesis-irradiance" measurements are not a reliable means of estimating net production, gross production or nutrient limitation, but can provide important information on the photoacclimation state of the phytoplankton. Long-term (24 h) incubations yield assimilation numbers that are in good agreement with net production rates, but are independent of nutrient-limited division rates. Despite complications in interpreting C-14-uptake data, we suggest that these measurements are important for understanding phytoplankton physiology and carbon cycles while, at the same time, efforts are needed to establish new incubation-free methods for measuring phytoplankton division rate and biomass.