The light curves of AN are usually approximated by the formula of 

 Steele (1962); the position of the optimums on these curves differs 

 somewhat depending on the method of measurement used (Kabanova et al . , 

 1964). The most representative are curves of the assimilation number 

 obtained in situ , the least representative are the results of 

 measurement in incubators with neutral filters, since the spectral 

 composition of the light which penetrates these filters differs 

 significantly from the spectral composition of the radiation which 

 penetrates the sea. The optimal illumination (in the photosynthetically 

 active radiation [PhAR] range) for photosynthesis of phytoplankton in 

 the upper layers of the euphotic zone in most cases is 0.03-0.15 

 cal/cm"-10^ lux (Mandelli et al . , 1970; Burkholder et al., 1967). The 

 absence of light depression under these conditions can be explained to a 

 significant extent by the good conditions of mineral nutrition and (or) 

 the dominance of dinoflagellates, which are more resistant to strong 

 irradiation than are the diatoms, as well as the presence of large 

 quantities of yellow organic matter, absorbing ultraviolet radiation. 

 In addition to populations which are resistant to light, populations 

 have also been found, the photosynthesis of which reaches a maximum at 

 very low values of irradiation: 0.003-0.015 cal/cm^«min, or 2-10 

 cal/cm^«day (Koblentz-Mishke et al., 1970; Vedernikov, Solov'yeva, 

 1972). These value of saturating intensity of light were obtained on 

 foggy or cloudy days. 



The phenomenon of light adaption has been extensively studied in 

 cultures of algae. As in these cultures, the cells of planktonic algae, 

 adapted to intensive light and placed to the conditions of saturating 

 light, manifest higher values of AN than do "shade" cells. It is well 

 known that the light curves of photosynthesis of phytoplankton living at 

 various depths differ in terms of the position of the optimums: for 

 surface phytoplankton, they correspond to higher levels of irradiation 

 than for deep-water phytoplankton. The difference depends on the degree 

 of stratification of the water, i.e., the time of adaptation of algae to 

 specific conditions of illumination. 



In a number of regions of the World Ocean, a direct relationship 

 has been noted between the incident irradiation and irradiation for 

 which the maximum values of AN are observed. This relationship was 

 detected in July-August of 1966 over the Kuril -Kamchatka trench in the 

 Pacific Ocean (Koblentz-Mishke et al . , 1970). It has been shown that 

 the higher the irradiation at a given station, the higher the value of 

 irradiation which corresponds to the optimum on the light curve of AN, 

 determined by experiments in situ . Analogous results were obtained on 

 expeditions of the Institute of Oceanography, USSr Academy of Sciences, 

 in the southwestern Atlantic and southeastern Pacific (Vedernikov, 

 Starodubtsev, 1971; Kabanova et al . , 1974a, b). In the littoral waters 

 of the Barents Sea in the summer of 1967 a parallel was observed between 

 incident radiation and the position of the light optimum of the AN 

 (Vedernikov, Solov'yeva, 1972). The variation found in the Barents Sea 

 manifests seasonal adaptation, appearing as a sharp drop in the light 

 optimum in the fall, and a less clearly expressed rapidly occurring 

 adaptation for cultures and natural populations of marine phytoplankton 

 (Steemann Nielsen, Hansen, 1959; Menzel , Ryther, 1961b; Ichimura, Aruga, 

 1964; Krupatkina, 1970). The latter type of adaptation is probably 



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