VARIABILITY IN SPECTRAL AND NONSPECTRAL MEASUREMENTS OF PHOTOSYNTHETIC LIGHT UTILIZATION EFFICIENCIES

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 bio-optical models of in situ primary production (P). Field samples were collected at 4 coastal stations during a 1 d transect (July 23, 1988) across the Southern California Counter Current (SCCC) at a time when phytoplankton communities were dominated largely by picoplankton (0.4 to 5-mu-m). Concurrent determinations were made of downwelling spectral irradiance [Q(d)(lambda,z)], photosynthelically available radiation (Q(par)), spectral attenuation coefficients (K(d)(lambda,z)], detrilal-corrected phytoplankton absorption [a(ph)(lambda,z)], white light photosynthesis-irradiance parameters (P(max), alpha, I(k)) and carbon action spectra alpha(DELTA-lambda(h)Z). From these parameters, spectral estimates of in situ phytoplankton absorbed radiation [AQ(ph)(DELTA-lambda(h)Z)], maximum quantum yield [phi(max)(DELTA-lambda(jr)z)], operational quantum yield [phi(DELTA-lambda(jr)z)], radiation utilization efficiency [f(DELTA-lambda(jr)Z)] and productivity P(DELTA-lambda(jr)Z) were derived. 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 phi(DELTA-lambda(ir)Z) and P(DELTA-lambda(jr)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 their associated impact upon absorption-based production parameters, increased with light depth. One consequence was a close correspondence between AQ(ph) and P(DELTA-lambda(jr)Z) at depth which was not evident in surface waters. A second consequence was that while spectrally weighted and white light estimates of quantum yield were occasionally similar 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 et al. 1989) and provide a conceptual base on which to improve existing and future spectral models of in situ photosynthetic quantum yield.