UTILIZATION OF SOLAR ENEROY RY AQUATIC ORGANISMS 



35 



continues to hold for the shorter, and more 

 important. Avaveleng'ths or whether there 

 is an abi'upt npward turn. A similar dis- 

 parity is to be fonnd anion;::- the few mea- 

 surements which have been made of natural 

 waters {cf. Clarke and James 1939) dne 

 largely to tlie lack of a sensitive instrument 

 suitable for field use. The exact deter- 

 mination of the rate of penetration of this 

 important type of radiation is tlnis a matter 

 Avhicli urgently needs attention. 



Turning now to the visible component 

 of the spectrum, we find that many observa- 

 tions exist on the degree of activity of fish 

 and of plankton animals in relation to its 

 intensity. The information obtained, how- 

 ever, has not yet been adequately applied 

 to the ecological relationships of the spe- 

 cies concerned. A prominent case in point 

 is the diurnal migration of ]^lanktoii. Al- 

 though research in this field has shown con- 

 clusively that the daily change in illumina- 

 tion is primarily responsible for the migra- 

 tion, the internal physiological reactions 

 which control direction and speed of swim- 

 ming are poorly understood (Welsh 1938). 

 Even less clear are the reasons why the 

 vertical migration of one species differs 

 from that of another and why the behavior 

 of the same species varies profoundly from 

 time to time and from place to place (Clarke 

 1934, Kikuchi 1938. and Johnson 1938). 

 Nevertheless the stratification of 200 plank- 

 ton, which results from these reactions to 

 light, is an aid to the investigation of cer- 

 tain important problems such as the feeding 

 of these animals and their role as food for 

 fish. 



According to a recent calculation (Clarke 

 1936b) sufficient illumination for vision in 

 at least one species of fish exists at the 

 bottom in those lakes and coastal oceanic 

 areas whose transparencies have been 

 measured. The minimum illumination re- 

 quired by other species should be similarly 

 tested and the information applied to all 

 types of natural Avaters. Since the spectral 

 distribution of the light at only moderate 

 depths differs so materially from that at the 

 surface, careful testing of the spectral sen- 

 sitivity of the various species is impera- 



tive. Tlie liglit receptors of some organisms 

 must be exceedingly sensitive since Water- 

 man, Nuimemacher, and Clarke (1939) have 

 recently demonsti-ated a distinct diurnal 

 migration of plankton at depths as great as 

 800 meters in the ocean. New information 

 on the possibilities of vision for fish will 

 have valuable applications to fisheries prob- 

 lems. It should be feasible to determine 

 generally the depth and time of day when 

 feeding requiring vision is possible, on the 

 one hand, and when fish nets might be per- 

 ceived and avoided on the other. The dis- 

 tance could be ascertained at which a sub- 

 merged light is visible and its effect in at- 

 tracting or repelling various types of fish. 



The intensity of illumination controls 

 photosynthesis and hence limits the depth 

 of growth of both ])ottom-living and pelagic 

 plants. For example, the lower limits for 

 fixed plants in the following oceanic areas 

 are:* in Baltic, 20 meters; off Iceland, 50 

 meters; and in Mediterranean, 130-1(30 

 meters. In lakes the lower limits are:t 

 Trout Lake, 10-12 meters; Crystal Lake, 

 15-20 meters ; and in Crater Lake, 12 meters. 

 The maximum abundance of phytoplankton 

 has been reported at the following depths :* 

 off' Syracu.se, 50 meters; off' California, 25- 

 55 meters; and in north European waters, 

 10-30 meters. At these depths most of the 

 radiant energy present is in the green or 

 blue, but red light is most effective for 

 ]ih()tosynthesis (Manning 1938). Diatoms 

 and other algae appear, however, to be able 

 to use other parts of the visible spectrum 

 effectively (Stanbury 1931). The mechan- 

 ism which makes this possible requires 

 elucidation. 



The relative rates of photosynthesis at 

 various levels beneath the surface have been 

 investigated by measuring the oxygen 

 evolved by phytoplankton placed in bottles 

 and suspended at diff'erent depths. Near 



*From Marshall and Orr (1928). 



t From Jiiday (private cominuiiieation). See 

 also Jiulay (1934) and Hasler (1938). In lakes 

 liotli fixed and planktonic plants have frequently 

 been reported at depths far too great for photo- 

 syntliesis. These plants must either be in a dor- 

 mant or dying condition or else are living sapro- 

 ])liyti(-al]y (cf. Kozmiiiski 1938). 



