140 Temperature 



mocline leads to the deeper water which is still quite cold. With the 

 onset of autumn solar radiation has become reduced and the winds 

 have been stronger with the result that the surface waters are stirred 

 downward, and the thermocline is shifted deeper and eventually de- 

 stroyed. By November the whole water column has become mixed 

 at an intermediate temperature and remains uniform as it cools to its 

 minimum temperature during the winter. 



Temperature,°C 

 2 4 6 8 10 12 14 16 18 20 22 24 



Fig. 5.6. Seasonal changes in the vertical distribution of temperature in coastal 

 water off New York, showing the positions of the thermocHne ( T ) ( Modified from 



Clarke, 1940.) 



The radiation received from the sun reaches a maximum in June in 

 the North Temperate Zone, but owing to the lag in the heating effect 

 the highest temperatures on land ordinarily occur in July. At the 

 surface of water the maximum heat of the season is not experienced 

 until August, and in subsurface layers the thermal peak is even more 

 delayed. If you imagine yourself a worm living in the bottom mud 

 in the situation represented by Fig. 5.6, "summer" for you would not 

 begin until November! In similar fashion the seasons are often com- 

 pletely reversed in deep lakes. 



This review of the thermal conditions has shown that in the ocean 

 and in lakes there is no temperature too high and no temperature too 

 low for active life of some kind. Temperature changes are much 

 less and much slower than in the terrestrial environment. Further- 

 more, organisms can usually get out of excessively high or low tem- 

 peratures by a short journey into deeper water. In these respects life 

 in the water is much easier than life on land. The results of this 



