14 University of California Publications in Zoology [Vol. 19 



CONDITIONS OP LIFE IN THE SEA 



It is advisable to point out something of what is known about the 

 physical conditions of the sea in the region where the work of the 

 Scripps Institution has been carried on, since the experiments here 

 considered relate especially to the behavior of animals found under 

 those conditions. The recent paper of McEwen (1916) summarizes 

 the knowledge of the conditions in the San Diego region, and the state- 

 ments that follow are based on that paper. The points that I desire 

 especially to emphasize are the variations in temperature, salinity, and 

 light intensity. 



It is stated (McEwen, 1916, p. 267) that the annual range of sur- 

 face temperatures is between 6° and 7°C, the upper and lower limits 

 varying somewhat in different areas. The lower limit is from 14° to 

 15° and the upper from 20° to 22° ; the coldest months are February, 

 March, and April ; the warmest season is the month of August. The 

 water contains less salt in the spring than during the summer. The 

 salinity varies from 33.50 °/ 00 in February to 33.75 °/ 00 in July or 

 August in one area ; in another section the minimum is 33.55 "/„„ in 

 January, the maximum 33.85 °/ 00 in July or August. The annual range 

 of temperature "decreases from 6?2 C at the surface to 1?0 C at two 

 hundred meters, while it can scarcely be detected below four hundred 

 meters" (p. 268). Different months differ as to the monthly average 

 of temperature and salinity, as do different days of the same month ; 

 and "each year is distinct from the others" (p. 269). Furthermore, 

 there are significant differences in temperature at positions as little as 

 sixty feet apart (p. 269). The bearing of such facts on the field study 

 of plankton organisms is obvious and is noted by McEwen on page 270. 



The diurnal variation in environmental elements is of more im- 

 portance than the seasonal so far as the daily vertical migration is 

 concerned. It is stated (p. 271) that the diurnal range in surface 

 temperature in a region well offshore is about 1?0 C ; the lower limit of 

 the range occurs between 1 a.m. and 2 a.m., and the upper between 

 5 p.m. and 7 p.m. This slight diurnal variation decreases as the depth 

 increases, and it does not appear at depths exceeding 25 meters. 



The diurnal variation of salinity at the surface is given as about 



0.05 %o (P- 271). 



It will probably be granted without argument that daily variations 

 in surface temperature and salinity are too small to have a marked 



