Different optical processes in water in different parts of the 
measured spectrum supply us with the key of interpreting re- 
sults. Absorption by yellow substance and phytoplankton pigments 
cause changes mainly at shorter wavelengths, whereas scattering 
on phytoplankton and suspended matter is the most important 
phenomen for longer wavelengths. To study the contribution of 
various optical processes to the spectrum of on the upwelling 
light, calculations on upwelling radiance just above the sea sur- 
face (RASS) on 16 wavelengths (400 ¢ N € 700 nm) were carried out. 
Three kinds of optically active matter (OAM) were taken into 
account - suspended matter, yellow substance and chlorophyll. 
RASS was calculated for OAM maxima on depths 2, 5, IO, I5 meters 
as well as for homogeneous vertical distribution. The RASS spec- 
tra are presented in normalized and unnormalized curves. Normal- 
izing makes the RASS spectral curves less sensitive to nonselec- 
tive components. The absolute values of the unnormalized RASS 
spectral curves are sensitive to scattering and backward  scat- 
tering coefficients. It shows that if we use Scattering for de- 
coding, it is better to use the absolute values of the spectral 
curve, but if absorption is used, the normalized spectral curves 
must be preferred. 
Studyng the normalized RASS spectral curves three cases 
should bepointed out: I) optically pure water - RASS values are 
decreasing if A is increasing; 2) medium water transmittance 
and all three kinds of optically active matter - minimum in 
RASS values on A = 440 nm can be observed, which enables to 
detect phytoplankton from medium heights (helicopter, airplane) ; 
3) turbid water - minima A = 400 nm is smoothed - high concen- 
trations of other optically active matters conceal chlorophyll 
concentration variations (also in case of high phytoplankton - 
for example during plankton blooms) . Using optical data for the 
Baltic Sea water and optically active matter we can see that 
the normalized RASS spectra for the Baltic depend only on the 
surface concentrations of the phytoplankton, whereas it is ver- 
tical distribution in the water column (as concentrations of the 
other optically active components) is practically unimportant. 
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