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

Jens C. Sorensen¹, David A. Siegel^1,2

¹Institute for Computational Earth System Science and Department of Geography, University of California at Santa Barbara, Santa Barbara, CA 93106-3060, USA

²Donald Bren School of Environmental Science and Management, University of California at Santa Barbara, Santa Barbara, CA 93106-3060, USA

Abstract

The effective quantum yield for carbon assimilation, 0_c (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 0_c. A significant seasonal cycle is found as 0_c values are reduced in summer. However, this seasonal cycle explains only a small fraction of the total variance in 0_c, and very few environmental parameters correlate with 0_c. Significant correlation is observed between 0_c and the flux of photosynthetically active radiation, Q_PAR and between 0_c and the rate at which phytoplankton absorb quanta, AQ_PAR. Near-surface 0_c values also are correlated with concentrations of the photoprotectant pigment zeaxanthin, where 0_c values are depressed when zeaxanthin concentrations are elevated. Four previously published 0_c models are assessed using these data. The predictive skill for each of these 0_c models indicates that they behave as good data interpolators, but poor predictors of 0_c 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.