Action spectrum and maximum quantum yield of carbon fixation in natural phytoplankton populations: implications for primary production estimates in the ocean

Abstract

Spectral and non-spectral measurements of the maximum quantum yield of carbon fixation for natural phytoplankton assemblages were compared in order to evaluate their effect on bio-optical models of primary production. Field samples were collected from two different coastal regions of NW Spain in spring, summer and autumn and in a polar environment (Gerlache Strait, Antarctica) during the austral summer. Concurrent determinations were made of spectral phytoplankton absorption coefficient [ a ph( λ)], white-light-limited slope of the photosynthesis–irradiance relationships ( α B), carbon uptake action spectra [ α B( λ)], broad-band maximum quantum yields ( φ m), and spectral maximum quantum yields [ φ m( λ)]. Carbon uptake action spectra roughly followed the shape of the corresponding phytoplankton absorption spectra but with a slight displacement in the blue–green region that could be attributed to imbalance between the two photosystems PS I and PS II. Results also confirmed previous observations of wavelength dependency of maximum quantum yield. The broad-band maximum quantum yield ( φ m) calculated considering the measured spectral phytoplankton absorption coefficient and the spectrum of the light source of the incubators was not significantly different form the averaged spectral maximum quantum yield [ φ max (λ) ] ( t-test for paired samples, P=0.34). These results suggest that maximum quantum yield can be estimated with enough accuracy from white-light P– E curves and measured phytoplankton absorption spectra. Primary production at light limiting regimes was compared using four different models with a varying degree of spectral complexity. No significant differences ( t-test for paired samples, P=0.91) were found between a spectral model based on the carbon uptake action spectra [ α B( λ) — model a] and a model which uses the broad-band φ m and measured a ph( λ) (model b). In addition, primary production derived from constructed action spectra [ a c B( λ) from a ph( λ) and α B (model c) was also not significantly different from that derived from total spectral model a ( t-test for paired samples, P=0.60). It was found, however, that primary production at low light regimes can be strongly overestimated (44%) when a ph( λ) is derived from chlorophyll concentrations. A white-light model based on broad-band α B (model d), which does not consider phytoplankton light absorption, yields values 17% lower than those of model a. It is concluded that primary production at light-limited conditions can be computed accurately from broad-band maximum quantum yield estimates or from constructed action spectra provided that a ph( λ) is measured. However, given that phytoplankton absorption coefficients are necessary for both approaches and as computations based on φ m showed less variability, we suggest that the maximum quantum yield proxy should be used.