(Table 1) Concentration of standard and pure chlorophyll a in water layers with various illumination levels in the Black Sea in February-April 1991

In February and in the first half of March surface chlorophyll a concentration (C_s) varied from 0.65-0.90 mg/m3 in the southeastern Black Sea to 1.59-2.67 mg/m3 in regions of the western and eastern cyclonic gyres, and to 2.47 mg/m3 over the northwestern shelf. The average concentration of this pigment in the photosynthetic layer (C'_ph ) varied from 0.69 to 2.39 mg/m3; this value reached its maximum in the central parts of both main cyclonic gyres. In March and April C_s remained high (>1 mg/m3) over the whole studied area, but was lower than in February. C'_ph value varied from 0.56 to 2.53 mg/m3. The subsurface chlorophyll maximum (SCM) was not pronounced at the end of February and in the first half of March; it occurred above the upper boundary of the main pycnocline. At the end of March and at the beginning of April SCM was more pronounced; this situation is typical for the period of transformation from the winter-spring state of phytocoenosis to its late spring state. On the basis of data collected during a long period (1976-1991) maps of C_s distribution in February-March and in April-May were constructed. Graphs of C_s variations during the winter-spring period in different regions of the eastern and western parts of the Black Sea from December to May are presented. Data of field observations are compared with results of remote sensing monitoring.