i^] Esterly: Reactions of Various Plankton Animals 15 



effect in causing diurnal migration. But temperature and salinity 

 vary with the depth, as is well known, so that the migration of the 

 animals carries them through a greater range of temperature and 

 salinity. McEwen has given in plates 34 and 35 the relations of the 

 mean annual temperature and salinity to depth. 



The maximum of the mean annual temperature (pi. 34) is at the 

 surface of course, and is 17° C. At 25 meters it is between 14° and 

 15° ; at 50 meters between 12° and 13° ; at 100 meters between 10° 

 and 11°; at 150 meters it is 10° C; while at 200 meters it is about 

 9?5 C. From that level it gradually decreases to 7?5 C at 400 meters. 

 The temperature curve is much steeper between the surface and 100 

 meters than anywhere else. 



The salinity increases with the depth (pi. 35), except that from 

 the surface to about 25 meters there is a slight decrease. The mean 

 annual salinity at the surface is 33.65 %„ ; at 25 meters it is between 

 33.50 %o and 33.55 %„ ; at 50 meters it is about 33.57 % I at 75 meters 

 about 33.68 %„ ■ aiul at 10 ° meters it is 33.80 % . Prom that level to 

 200 meters the increase is at about the same rate for each 25 meters, 

 the value of the salinity at the 200-meter level being 34.15 %„. The 

 increase is less rapid from there down, and at 400 meters the per- 

 centage is a little in excess of 34.39 %„. 



The light intensity at different depths has not been measured 

 directly in the San Diego region, but McEwen has given approxima- 

 tions of the average distribution of radiant energy with respect to 

 depth and time of day (pi. 38). The diagram is based on a formula 

 that expresses the relation between intensity of radiation and depth 

 and an "empirical one representing approximately the results of ob- 

 servations on the diurnal variation in the intensity of solar radiation 

 on a horizontal surface at sea-level, between 20° N and 40° N" (p. 279). 

 While McEwen 's diagram gives theoretical results he states that 

 the values agree well with photographic measurements made in the 

 Atlantic. 



According to the figures given in plate 38, if the intensity at a 

 depth of one meter at noon is called 1000, it is relatively about 600 at 

 nine meters, 300 at 20 meters, 200 at 25 meters, 100 at 35 meters, 50 

 at about 48 meters, 25 at 58 meters, 10 at a little over 70 meters, and 

 5 at 82 meters. It is doubtless understood that the light intensity at 

 any depth increases until noon and decreases after noon, so thai timt 

 of day is a rough expression of relative intensity. 



