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

Greg. M. Silsbe, Robert E. Hecky, Ralph E.H. Smith

Limnol. Oceanogr. Methods 10:736-751 (2012) | DOI: 10.4319/lom.2012.10.736

ABSTRACT: Bio-optical models predict photosynthetic electron transport rates through photosystem II (ETRPSII) from measures of irradiance (E), the absorption coefficient of pigments associated with PSII (aPSII) that are spectrally scaled to E (āPSII), and the quantum efficiency of PSII (φ'PSII). However bio-optical models currently suffer from methodological uncertainties in the quantification of āPSII, and variable stoichiometry between ETRPSII and the more ecologically-relevant carbon fixation (PC), defined here as the quantum requirement for carbon fixation (Φe,C = ETRPSII × PC-1). Here we analyze measures of PC, φ'PSII, and āPSII 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 āPSII are large. Measures of āPSII are shown to be highly influenced by methodology as different approaches significantly influence measures of ETRPSII and Φe,C. A new technique that derives āPSII from in situ spectral fluorescence measures is introduced and shown to yield ETRPSII estimates that correlate well with independent measures of PC under light limited conditions. The Φe,C inferred from this new approach agreed well with independent assessments in the lake and demonstrates that bio-optical models with well-parameterized āPSII can be usefully predictive of light-limited PC across wide biological and chemical gradients in this great lake.