132 Temperature 



only 30 cm the highest temperatiire of the day occurred near midnight 

 and the lowest temperature occurred near noon! 



In the tidal zone changes in temperature are the result of a com- 

 bination of the differences in the amount of heat delivered from the 

 sun and of differences in the temperature of the air and of the water. 

 All organisms living between tide marks are subject to rapidly chang- 

 ing temperatures that may test their tolerance to the limit. At one 

 locality on the coast of Maine the temperature of the mud exposed at 

 low tide was observed to rise to 38°C under the midday summer sun. 

 A short time later plants and animals living in this area were flooded 

 by the incoming tide with a temperature of 10°C. In the winter 

 when the tide is out these organisms might be exposed to tempera- 

 tures as low as — 25°C. The occurrence of these extreme and rapid 

 fluctuations in temperature adds to the rigorous conditions of the tidal 

 zone. 



Plants and animals are also concerned with seasonal cycles of tem- 

 perature. In both tropical and polar seas the intensity of heat does 

 not vary by more than about 5°C throughout the year. In the tem- 

 perate seas, however, changes of 10 to 15°C from summer to winter 

 are common, and sometimes seasonal differences of 23°C or more are 

 observed. At increasing depths in the water these differences become 

 less. Measurements off Portsmouth, New Hampshire, where there 

 was a 15-degree change in the surface temperature showed no meas- 

 urable variation from summer to winter below a depth of 140 m. 

 Probably more than 95 per cent of the oceanic environment exhibits 

 no seasonal change in temperature that is of significance to living or- 

 ganisms. In lakes and ponds seasonal fluctuations are generally 

 greater than those found in the sea. Springs, however, form a very 

 special fresh-water habitat in which the temperature may vary by only 

 a degree or two the year around. A typical example of the thermal 

 cycle in a lake showing the reduced amplitude in the deeper layers is 

 presented in Fig. 5.2. 



On land the seasonal changes in temperature are familiar to every- 

 one and are almost always of ecological significance. The greatest 

 variations are found in continental areas in the temperate zones with 

 lesser seasonal ranges in the tropics and near the coasts. At St. Paul, 

 Minnesota, the average difference between summer and winter is 

 33.7°C. Possible extremes in range are even greater. From Tibet 

 comes a report of temperatures varying by 77°C from — 37°C in the 

 winter to -|-40°C in the summer. In contrast to the foregoing are 

 certain localities where temperatures vary only slightly throughout 



