934 
Figure 5. Diurnal variations in phytoplankton Chl-a fluorescence 
yield measured by shipboard lidar. Dashed line - solar irradiance. 
Diurnal rhythm of fluorescence yield leads to appropriate modulation of 
fluorescent monitoring data when measurements are performed for several days. 
This should be taken into account for correct interpreting the remotely sensed 
data. 
This problem is strongly complicated by critical dépendance of diurnal 
variation parameters on the state of photosynthetic apparatus, and, as a 
result, by high spatial variability of its parameters (Gorbunov and Chekalyuk, 
1993). 
High variability of diurnal rhythm characteristics called for development 
of an express algorithm for determination of its parameters and correction of 
remotely sensed fluorescent data. 
Field and laboratory studies of the mechanisms and features of diurnal 
variations in chlorophyll-a fluorescence and phytoplankton photosynthetic 
activity have been carried out (Gorbunov and Chekalyuk, 1992). On this 
background the express algorithm for correction of remotely sensed fluorescent 
data taking into account diurnal rhythm of phytoplankton fluorescence yield is 
being under development at present time. The algorithm relies on measurements 
of phytoplankton photosynthetic activity and in principle allows to 2-3 times 
increase the accuracy of mapping chlorophyll distribution while measurements 
are performed for several days. 
7. - CONCLUSION 
The data presented in this communication show that in spite of attractive 
possibilities of lidar techniques relying on Chl-a LIF measurements, there are 
a number of problems complicating the interpretation of LIF data. One of the 
key problems is high variability (more than 3-5 times) of in vivo Chl-a 
fluorescence yield, caused by several factors. The first group of factors is 
connected with natural variations of in vivo Chl-a fluorescence yield due to 
variations in environmental conditions (light, nutrient availability, presense