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fluorescent efficiency of different phytoplankton populations and 
by changes in water absorption that reduces the light available 
for fluorescence (e.g., Falkowski and Kiefer, 1985). 
Nevertheless, experience gained from several observations 
indicates that for a limited period and area these variables 
change within a limited range thus allowing the successful 
application of the method in ocean and coastal waters (Gower, 
1986). Although this technique seems to be more useful for Chi 
detection, generalizations and comparisons based on previous 
studies are still very difficult to make, especially for 
inland waters with highly variable biooptical properties. 
The fluorescence signal is hardly useful in productive 
inland waters. For high Chi content the processes of 
absorption by pure water and by Chi govern spectral 
behavior of reflectance. Recently it was found that the 
position and magnitude of the peak of reflectance near 700 nm 
strongly depend on Chi content (Gitelson and Nikanorov, 1986; Vos 
et al., 1986; Gitelson, 1992, 1993; Dekker, 1993). This 
phenomenon was the basis of algorithms for Chi assessment 
(e.g., Gitelson et al., 1993; Dekker, 1993). The parameters of 
the developed models depend on inherent optical properties of 
phytoplankton. Therefore, they should be adjusted for those 
phytoplankton species prevailing in the investigated water 
body at different periods. 
The objective of this study was to devise optical models of 
the lake, relating the reflectance obtained just above the water 
surface to Chi concentration in surface waters. 
METHODS 
Two experiments were carried out in a period when the 
phytoplankton density was relatively low; however, the background 
concentration of non-organic suspended matter (SM) was high and 
variable. Two experiments were executed during a period of high 
phytoplankton density ( Peridinium period, Berman et al., 1992). 
Therefore, the four experiments covered the typical situation of 
suspendoids concentrations in the lake. 
At every sampling station, the upwelling radiance, 1^, above 
the water surface and (indirectly) the downwelling radiance, L d , 
(with the aid of a standard reflectance white plate) were 
measured above the water surface with a portable LI-1800 
spectrometer in the region 400 to 850 nm. The measurements were 
taken using a telescope with a field of view of 15°. Each 
observed radiance spectrum of water was divided by the 
appropriate downwelling radiance spectrum to give a reflectance 
of R=L U /L d . 
Vertical profiles of underwater irradiance were measured 
with a LI-185A radiometer; absorption and scattering coefficients 
of the water were calculated. 
Water was sampled with a 5 L Aberg-Rodhe sampler 0.0 to 0.5 
m below the surface. Two 100 ml subsamples were immediately 
filtered and stored on ice in 5 ml 9 0% acetone. Upon return to 
the laboratory, 5 ml of 9 0% acetone was added and each sample 
was sonicated for 1 min, and left in the dark at 4 b C, 
overnight. The extract was cleared by centrifugation and 
measured fluorometrically.