similar to the examples presented above of light adaptation in the 

 Pacific and Atlantic Oceans. This phenomenon has not been observed for 

 marine waters, but has been observed in Lake Erie (Verduin, 1956). The 

 maximum of photosynthesis of phytoplankton in this lake is observed 

 during the various seasons on sunny days at higher levels of irradiation 

 than is the case on cloudy days. 



Thus, many reported facts indicate that the position of the light 

 optimum for photosynthesis is related to the conditions of illumination 

 at the moment of measurement. In contradiction to this, some authors 

 (Finenko, in print; Aruga, 1965) believe that the light optimum of 

 photosynthesis of phytoplankton depends primarily on temperature. To 

 check the correctness of these hypotheses using a single mehod (Yerlov, 

 1970), data were processed from 77 stations from both domestic and 

 foreign expeditions, in which the measurements of photosynthesis in situ 

 were accompanied by optical observations. The values of the light 

 optima were taken from the light curves of AN obtained at these 

 stations. They were related to the incident radiation on the date of 

 measurement (Fig. 5a) and to the temperature at the depth of the optimum 

 (Fig. 5b). We can see that the position of the optima depends to a 

 greater extent on incident radiation than on temperature. Thus, the 

 assumption of rapid alteration of the light optimum for photosynthesis 

 as a function of weather conditions is confirmed. It is possible that 

 this alteration is related not so much to adaptation of the 

 phytoplankton to the level of irradiation present as to the different 

 depths of penetration of the near ultraviolet, deactivating pigments, 

 and other peculiarities of the light field. This would explain why, in 

 highly eutrophic waters, with significant quantities of impurities, the 

 light optimum for photosynthesis is found at the surface under 

 conditions of much more intensive illumination than is the case in less 

 productive waters. It is also possible that light inhibition of 

 phytoplankton is to some extent an artifact, since during exposure of 

 bottles, the algae are exposed to inhibiting radiation longer than is 

 the case in nature, where they can move vertically. It has been found 

 that in bottles which are moved vertically by ten meters each half hour, 

 light inhibition of photosynthesis is weaker than in nonmoving 

 bottles. Still, however, this effect apparently does occur under 

 natural conditions: we know for instance, (Koblentz-Mishke, 1971) that 

 the maximum ratio of chlorophylls a:c is present at the depth of the 

 light optimum of photosynthesis. 



The data mentioned earlier, from measurements in situ , were used to 

 plot generalized light curves of the photosynthesis of phytoplankton 

 living under various ecologic conditions--for various trophic zones of 

 the tropical area. The curves are the envelopes of the fields of points 

 on the graphs relating primary production to level of illumination at 

 the depth of measurements (Fig. 6a) and standardized curves for the 

 maximum of photosynthesis (Fig. 6b). The optimal irradiation in the 

 tropical area is usually between 30 and 100 cal/cm^-day, averaging 70 

 cal/cm^«day, which amounts to 15-50% of the mean incident PhAR. Quite 

 different curves are produced for highly eutrophic inshore zones: here 

 the maximum photosynthesis is observed at the surface of the sea at the 

 highest values of underwater irradiation observed anywhere under these 

 ecologic conditions. 



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