Bio-optical models can increase our ability to resolve the temporal/spatial variability in phytoplankton production. The bio-optical models are based on the linkages between watercolumn optical properties, penetration of photosynthetically available radiation, and phytoplankton production. These models require a term for the quantum yield of carbon fixation which is the efficiency with which radiation absorbed by phytoplankton is converted to photosynthate. The quantum yield is difficult to measure and has been assumed to be constant or have a predictable relationship related the in situ light field. The quantum yield of carbon fixation has also been assumed to be independent of wavelength. The variability in the magnitude and wavelength- dependency of the quantum yield was assessed. The results are used to comment on the bio-optical productivity models.

The variability in the quantum yield for field populations was measured. The magnitude of the quantum yield was dynamic and empirically unpredictable in the coastal waters of California using traditional oceanographic measurements. Proxy measurements for the quantum yield of carbon fixation will need to be developed to improve the accuracy of the bio-optical models. The quantum yield was wavelength-dependent for field populations and sensitive to community composition of the phytoplankton.

The wavelength-dependency of the quantum yield for carbon fixation was also measured on laboratory cultures of diatoms, prymnesiophytes, and dinoflagellates. In all cases the quantum yield was wavelength-dependent. Both the magnitude and wavelength-dependency of the quantum yield was sensitive to light history of the culture. The wavelength-dependency was determined by presence of photosynthetically incompetent pigments, Emerson enhancement effects, and processes downstream of the photochemical events at the reaction centers. The wavelength-dependency of the bio-optical properties in phytoplankton suggests that the error in photosynthetic rates measured using artificial light sources is large; however the systematic error in using an artificial light source could be assessed if information on the absorption properties of the phytoplankton was available.