Variability of the effective quantum yield for carbon assimilation in the Sargasso Sea

The effective quantum yield for carbon assimilation, φc (mol C Einstein−1), quantifies the efficiency by which phytoplankton use absorbed light energy to photochemically fix and store carbon. A five-year time series from the Sargasso Sea shows a high degree of variability in estimated values of φc. A significant seasonal cycle is found as φc values are reduced in summer. However, this seasonal cycle explains only a small fraction of the total variance in φc, and very few environmental parameters correlate with φc. Significant correlation is observed between φc and the flux of photosynthetically active radiation, QPAR, and between φc and the rate at which phytoplankton absorb quanta, AQPAR. Near-surface φc values also are correlated with concentrations of the photoprotectant pigment zeaxanthin, where φc values are depressed when zeaxanthin concentrations are elevated. Four previously published φc models are assessed using these data. The predictive skill for each of these φc models indicates that they behave as good data interpolators, but poor predictors of φc variability. The prognostic capability of these models does not improve when seasonal mean parameters are applied. The present results demonstrate the difficulties in modeling primary production, as we have yet to develop a predictive understanding of the important photophysiological, ecological and methodological processes regulating primary production.