Light absorption by phytoplankton in the North Pacific Subtropical Gyre

To constrain the energy fueling photosynthesis in the North Pacific Subtropical Gyre (NPSG) we characterize the variability of phytoplankton absorption spectra in conjunction with that of the light field at Station ALOHA (22°45′N, 158°00′W). Furthermore, we decompose the phytoplankton absorption into photosynthetic and photoprotective components based on high-performance liquid chromatography pigment analysis. Between January 2006 and December 2012 the variability in chlorophyll-specific absorption (a*_f) above the deep chlorophyll maximum (DCM) layer was driven by changes in photoprotective carotenoid concentrations while the chlorophyll-specific absorption of photosynthetic pigments (a*_f(PSP)) remained nearly constant with a mean (± SD) value of 0.008 ± 0.001 m² (mg chl a)⁻¹. In contrast, below the DCM layer changes in a*_f resulted from increases in the relative contribution of photosynthetic pigments with depth, suggesting that the constancy in a*_f(PSP) above the DCM layer is controlled by nutrient limitation. While the daily photon fluxes absorbed by photosynthetic pigments in the upper 45 m did not vary at a seasonal scale, averaging 0.45 ± 0.12 mol quanta m⁻² d⁻¹ in winter and 0.46 ± 0.10 mol quanta m⁻² d⁻¹ in summer, when integrated over the upper 200 m these fluxes ranged from 0.64 ± 0.16 to 0.79 ± 0.19 mol quanta m⁻² d⁻¹ in winter and summer, respectively. Based on the rate of photons trapped by the photosynthetic pigments and on the seasonal euphotic zone depth integrated gross O₂ evolution rates derived from H¹⁸₂ O in situ incubations we estimate a mean photosynthetic yield of ∼0.1 mol O₂ evolved per mol quanta absorbed by photosynthetic pigments.