independent of temperature; in eutrophic regions (N-NO3 >1.0 

 ug«atom/£), AN increases very slightly with temperature, while the 

 concentration of chlorophyll decreases rapidly; as a result, a drop is 

 observed in the level of photosynthesis. In this last case, the 

 temperature does not influence the rate of photosynthesis directly, but 

 rather as a factor which is negatively correlated to the concentration 

 of nutrient salts. 



Comparison of results obtained under laboratory and natural 

 condit io"ns^ In discussing the influence of light on the photosynthesis 

 of phytoplankton, we should make one general comment concerning the 

 impossibility of reproducing all of the parameters of the light field in 

 situ under laboratory conditions. This is particularly true of the top~^ 

 water layer of high irradiation. With "inhibiting" irradiation, 

 suppression of photosynthesis is usually (but far from always) 

 observed. With the same irradiation under laboratory or deck-incubator 

 conditions, the light curves of photosynthesis reach a plateau instead 

 of descending. We can name four probable causes for the decrease in 

 photosynthesis beneath the surface of the sea: inactivation of 

 chlorophyll by excess energy; inhibition of the process by the UV 

 component of the light field, not considered in measurements of PhAR; 

 incomplete utilization of the light flux, which is insufficiently 

 polarized at the surface; and underutil ization of light fluctuating as a 

 result of the focusing effect of the irregular surface of the sea. The 

 lack of inhibition of photosynthesis in highly eutrophic water speaks in 

 favor of UV radiation as the cause of the drop in photosynthesis near 

 the surface, since these waters contain a yellow substance which absorbs 

 short-wave radiation. 



Thus, the rapid displacement of the position of the light optimum 

 described above can be explained not only by adaptation of the 

 phytoplankton, but also by the fact that the energy is underutilized in 

 the near-surface layers due to certain special properties of the light 

 field or a combination of this field and the peculiarities of the 

 pigment system. Illumination conditions may be considered "optimal" at 

 the depth where this underutil ization stops. If this depth changes only 

 slightly at a given point in the ocean, different quantities of light 

 energy will reach the point as the weather changes, leading to a shift 

 of the optimum on the light curves of photosynthesis. The position of 

 the "light optimum" of photosynthesis in this case is determined not by 

 the physiologic peculiarities of the phytoplankton, but rather, by the 

 parameters of the light field, which explains the slight variation in 

 the position of the "light optimums" of photosynthesis as a function of 

 temperature, in comparison to the incident radiation (Fig. 5). 



The absolute values of the "light optimums" for photosynthesis, 

 obtained under natural conditions and in cultures, are basically 

 similar, exceptions being the data observed in cloudy weather, when the 

 light optimum of photosynthesis was found to be at much lower 

 irradiation than is usually observed under laboratory conditions. The 

 threshold value of irradiation under natural conditions was found to be 

 related not to the systematic composition of the phytoplankton (as was 

 indicated in cultures), but rather with the ecologic conditions. If we 

 consider that at the threshold value of irradiation, the light curves of 



253 



