The spectral light absorption characteristics (400 to 700 nm) of 10 main classes, covering 31 species, of bloom-forming phytoplankton (diatoms, dinoflagellates, prymnesiophytes, euglenophytes, prasinophytes, raphidophytes, cryptophytes, chlorophytes, chrysophytes and cyanobacteria) have been examined. The survey is based on in vivo chlorophyll (chl) a-specific light absorption spectra ["a&), 400 to 700 nm] of low-and high-light adapted monocultures grown in the laboratory. Pigments were isolated by means of high-performance liquid chromatography (HPLC) to obtain visible spectra of isolated pigments to identify peaks and shoulders of the in vivo absorption spectra. A total of 217 "a&) spectra were log-transformed and normalized at 675 nm [a,,,(h)] to minimize photoadaptational effects on the spectral characteristics due to differences in pigment composition and the package effect. These alog(h) spectra were analyzed by stepwise discriminant analysis to determine sets of optimum wavelengths for classification. Discrimination and classification were most effective when lowand high-light adapted phytoplankton were grouped separately. A set of only 3 wavelengths (481,535, 649 nm) chosen on the basis of discriminant analysis classified, according to the jackknife technique, 93% of the a,,,(h) spectra. By using combinations of 4 (481, 535, 586, 649 nm) or 5 (481, 535, 586, 628, 649 nm) chosen wavelengths, 97 to 99% of the spectra were classified correctly. For pooled data (lowand high-light adapted cells), 60 to 86% of the spectra were correctly identified using a combination of 3 to 5 selected wavelengths, indicating that variations due to photoadaptation were not entirely removed by log-transforming and scaling of the spectra at 675 nm. By using the above combination of 3 wavelengths, 4 main groups of phytoplankton were clearly separated, depending mainly on their accessory chlorophylls, i.e. chl b (prasinophytes, euglenophytes, chlorophytes), chl c , and/or c;?(diatoms, dinoflagellates, prymnesiophytes, chrysophytes, raphidophytes, cryptophytes), chl c3 (toxic prymnesiophytes and dinoflagellates) and no accessory chlorophylls (cyanobacteria). The wavelengths employed here correspond to the peaks and shoulders of the in vivo absorption spectra. We conclude that different phytoplankton classes may be identified during blooms on the basis of in situ bio-optical measurements at 3 to 5 appropriately chosen wavelengths.
