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

The effective quantum yield for carbon assimilation, phi (c) (mol C Einstei n(-1)), quantifies the efficiency by which phytoplankton use absorbed light energy to photochemically fix and store carbon. A five-year time series fr om the Sargasso Sea shows a high degree of variability in estimated values of phi (c). A significant seasonal cycle is found as phi (c) values are red uced in summer. However, this seasonal cycle explains only a small fraction of the total variance in phi (c), and very few environmental parameters co rrelate with phi (c). Significant correlation is observed between phi (c), and the flux of photosynthetically active radiation, Q(PAR), and between ph i (c) and the rate at which phytoplankton absorb quanta, AQ(PAR). Near-surf ace phi (c) values also are correlated with concentrations of the photoprot ectant pigment zeaxanthin, where phi (c) values are depressed when zeaxanth in concentrations are elevated. Four previously published phi (c) models ar e assessed using these data. The predictive skill for each of these phi (c) models indicates that they behave as good data interpolators, but poor pre dictors of phi (c) variability. The prognostic capability of these models d oes not improve when seasonal mean parameters are applied. The present resu lts demonstrate the difficulties in modeling primary production, as we have yet to develop a predictive understanding of the important photophysiologi cal, ecological and methodological processes regulating primary production. (C) 2001 Elsevier Science Ltd. All rights reserved.