Variability in spectral and nonspectral measurements of photosynthetic light utilization efficiencies

Abstract

The aim of this study was to quantify the variability in and differences between spectral and nonspectral measurements: of light utilization efficiencies for natural phytoplankton communities, in order to evaluate possible consequences for blo-opt~cal models of in s j t ~l p]-1ma1-y production (P). Field samples were collected at 4 coastal stations durlng a 1 d transect (July 23, 1988) across the Southern California Counter ('urrent (SCCC) at a time when phytoplankton communities were dominated largely by picoplankton (0 4 to 5 pm). Concurrent determlnations were made of d o w n ~v e l l ~n g spectral ~rradiance [QCl(A,z)], photosynthetically available radiation (Qpclr), spectral attenuation coefficients [&(A,z)], detrital-corrected phytoplankton absorption [a,,,,(A,z)], white light photosynthesis-irradiance parameters (P,,,,. a, I*) and carbon action spectra a(AA,,z). From these parameters, spectral estimates of in situ phytoplankton absorbed radiation [AQ,,h(AA,.z)l, maximum quantum yield [d,,,.(AA,.z)], operational quantum yield [&(AA,,z)l, radiation utilization efficiency [c(AA Significant spatial variability in all bio-optical parameters was noted for communities in the surface waters and the chlorophyll maximum. For surface waters, there was significant variability in the 525 to 600 nm region of the spectral signatures of dl[dA,,z) and P(AA,,z) which was attributable to phycobilins not resolved In absorption spectra. The importance of light absorption by photosynthetic pigments other than chlorophyll a (chl), and thcir assoc~ated Impact upon absorption-based production parameters, increased w ~t h light depth. One consequence was a close correspondence between AQ,, and P(AA,,z) at depth w h ~c h was not evident in surface waters A second consequence was that while spectrally weighted and whlte l ~g h t estimates of quantum yield were occasionally sim~lar In surface waters, spectral estimates for all chlorophyll maximum communities were 4-to 6-fold higher than white light measurements. Results confirm previous observations that white light measures of quantum yield can significantly underestimate quantum yield for subsurface communities of phytoplankton (Prezelin e t al. 1989) a n d provide a conceptual base on which to improve existing and future spectral models of in situ photosynthetic quantum yield.