Improved estimation of carbon fixation rates from active flourometry using spectral fluorescence in light-limited environments

Abstract

Bio-optical models predict photosynthetic electron transport rates through photosystem II (ETR PSII) from measures of irradiance (E), the absorption coefficient of pigments associated with PSII (a PSII) that are spectrally scaled to E (ā PSII), and the quantum efficiency of PSII (φ' PSII). However bio-optical models currently suffer from methodological uncertainties in the quantification of ā PSII, and variable stoichiometry between ETR PSII and the more ecologically-relevant carbon fixation (P C), defined here as the quantum requirement for carbon fixation (Φ e,C = ETR PSII ×?P C-1). Here we analyze measures of P C, φ' PSII, and ā PSII across optical, thermal, nutrient and phytoplankton composition gradients in Lake Erie. We show that φ' PSII in the light-limited portion of the water column is relatively constant despite the wide range of biological and environmental conditions, but that variations in ā PSII are large. Measures of ā PSII are shown to be highly influenced by methodology as different approaches significantly influence measures of ETR PSII and Φ e,C. A new technique that derives ā PSII from in situ spectral fluorescence measures is introduced and shown to yield ETR PSII estimates that correlate well with independent measures of P C under light limited conditions. The Φ e,C inferred from this new approach agreed well with independent assessments in the lake and demonstrates that bio-optical models with well-parameterized ā PSII can be usefully predictive of light-limited P C across wide biological and chemical gradients in this great lake. © 2012, by the American Society of Limnology and Oceanography, Inc.