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The data collected in the southern Black Sea during the period of July 1997 to September 1998 showed that light penetrated into the upper 20-35 m, with a downward attenuation coefficient varying between 0.1 and 0.25 m -1 . The chlorophyll-a concentration for the euphotic zone ranged from <0.5 to 1.5 µg l -1 . Coherent sub-surface chlorophylla maxima formed near the base of the euphotic zone at all study points and a secondary one was observed at very low level of light (<0.1% of the surface light) in the nearshore regions. Production rates were estimated in between 62 and 785 mg C m -2 d -1 in this period. Bioassay experiments (using extra nitrate, ammonia, phosphate, silicon and iron) showed that under optimum light conditions the phytoplankton population is nitrate limited in open waters. Phosphate seems to control the growth in the nearshore regions of the southern Black Sea. Silicon concentration also influenced the phytoplankton growth since the majority of the population was determined to be diatoms.

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