Determination of Absorption and Fluorescence Excitation Spectra for Phytoplankton

Investigations of photosynthetic processes in both aquatic ecosys tems and in the laboratory have sought to couple measurements of the rates of production with quantitative assessment of the ambient light field and the absorption capacity of the cells. Kiefer etal. (1979) and Bricaud etal. (1983) have described techniques for determination of ab sorption coefficients for cultures. For field studies, however, where the cell concentration is very dilute, direct measurement of absorption is not possible. Most field studies have assumed a constant absorption per chlorophyll, and have estimated the amount of absorbed light from measurements of the chlorophyll concentration and the available light for photosynthesis (Rhode,1965; Tyler,1975; Dubinsky and Berman,1976; Morel, 1978). We present here a technique for measuring the absorption coeffi cient (ap) for cultures or field samples. Yentsch (1957) first applied the technique of direct measurement of absorption on filters for algal cultures, and for qualitative analysis of suspended marine particulates (1962). Faust and Norris (1982) have used derivative absorption spectros copy of cultured phytoplankton on glass fiber filters in order to assess pigment concentration.