920 
scale of horizontal variability of phytoplankton photosynthetic activity. In 
some cases changes of 17 up to 4 times were observed at the distance of 5 km 
(Fig.4). This and other relevant results (e.g. Kolber et al., 1990) on spatial 
variability of photosynthesis efficiency clearly indicate that actual range of 
variations even at local scale can be much higher than it is usually supposed 
relying on low-resolution conventional measurements (e.g. Platt et al., 1991). 
6.2. Lidar mapping in the North-Western Atlantic 
During first field applications of pump-and-probe lidar technique in the 
North-Western Atlantic (1990) we obtained a valuable information about both 
character and scales of horizontal variability of phytoplankton photosynthesis 
efficiency and Chl-a (Chekalyuk and Gorbunov, 1992; Gorbunov and Chekalyuk, 
1993) distributions. The pattern of horizontal distribution of photosynthesis 
efficiency rj in the examined area of synoptic scale (see Fig.5a) was 
reconstructed relying on night measurements to avoid quenching influence of 
solar illumination on variable Chl-a fluorescence. It was found, that due to 
complex hydrobiological situation in the area the values of A4>/$m indicated 
variations in a wide range: from 0.2 (low activity, 30% reaction centers of PS 
II were active) to 0.65 (high activity, about 95% of RCs were active). 
Figure 5. Distributions of the Efficiency of Photosynthesis 
17 = M>/4>m (a), and Chlorophyll-a Concentration (b). 
The North-Western Atlantic, May 1990. 
The positions of many mesoscale features of rj distribution (Fig.5a) 
indicated high spatial correlation with locations of hydrological structures 
in the area (Chekalyuk and Gorbunov, 1992). General explanation of the 
observed phenomena can be the enhancing phytoplankton photosynthetic activity 
due to pumping nutrients into euphotic zone by vertical water streams 
(Falkowski et al., 1991). Relying on 17 data, the correction of Chl-a 
fluorescence measurements was performed (Gorbunov and Chekalyuk, 1993) taking 
into account the regulation of Chl-a fluorescence by solar illumination in 
nearsurface water layer. This approach allows to improve the accuracy of Chl-a 
concentration estimates up to 2-3 times. The resulting distribution of Chl-a 
concentration is presented in Fig.5b.