878 
LIF spectra of different phytoplankton samples excited at 308 nm are shown in Rg. 1, where re 
sults from two Blue-Green species (Nodulana and Spirulina Platensis) are compared with those from a Green 
one (Tetraselmis Suecica). Spectra appear to be clearly chacteristic of the class and not of die species. The phy- 
cocyanin band Rq (650 nm) is peculiarly overlapping the chlorophyll bands R, and R 2 (at 690 nm 740 nm re 
spectively) in cyanobacteria, so that spectral ratios R^/R, and R 2 /R, allow to distinguish between the two 
classes. However we have verified on Spirulina Platensis that, for very high algae concentration (c > 1 g/1) usu 
ally not occurring in a natural environment, there is some self-absorption of the radiation emitted by the pig 
ments, which alters the spectral ratios from the expected class values. 
In order to identify the excitation wavelength most suitable to phytoplankton detection, we meas 
ured the excitation spectra of different classes of algae throught the near UV- visible range. Significant results 
concerning the chlorophyll band at 690 nm are shown in Fig. 2 for a Blue-Green and a Green species, respec 
tively. Phytoplankton red pigments are very efficiently excited upon UV excitation, especially in the case of 
Green algae. 
300000 
- ñmmmmr Spirulina 0.162 g/1 
1 •/■./// ■ Tetraselmis 0.335 g/1 
Rg. 2 - Laser induced fluorescence of the F690 red channel for different al 
gae versus excitation wavelength. 
Detection limits of phytoplacton in saline water have been measured on solutions of the consid 
ered samples in water column of different height At sufficient water depth (h > 1 m) and diluted algae solutions 
(<50 mg/1), we have verified that the Raman water peak intensity remains constant with increasing algae con 
centration and can be used for renormalization of remote sensed data. Significant results are shown in Rg. 3, 
where the algae concentration is measured either through the phycocyanin R^ and the chlorophyll R, band in 
tensities (a) for Spirulina or through both the chlorophyll R, and R 0 band intensities (b) for Tetraselmis. 
The chlorophyll-a (Chl-a) detection limit for Spirulina Platensis as determined in laboratory 
measurements, collecting signals from a column of a maximum depth of 2 m was found close to 2 pg/1, thus 
lying in a range comparable to the expected open sea values. 
3.3 Monitoring of phytoplankton photosynthetic activity 
The recently introduced spectral ratio F690/F735, usually referred to as the red/red ratio, has been used so far as 
an indicator of photosynthetic efficiency in plants and in algae [8]. However, several laboratory and field ex 
periment seem to show that a number of different environmental and experimental factors do actually affect this 
ratio more than the bio-physiological status of the investigated sample, resulting in measurement which some 
times are ambiguous or contradictory [9]. A procedure to remove the contibution of such external factors, has