ZoBell — 14 — Marine Microbiology 



mixed by the action of wind and waves, is fairly homogeneous. Below 

 this homogeneous layer, which is lo to loo meters deep, the temperature 

 decreases rapidly with depth to about 5° C. or less at a depth of 200 to 

 1000 meters, depending upon the latitude. The zone of maximum temper- 

 ature gradient is called the thermocline. Below the thermocline the tem- 

 perature continues uniformly low to the bottom of the sea. Minor irregu- 

 larities in the temperature curve may be caused by the intrusion of layers 

 or tongues of colder or warmer water. 



The temperature range of water exceeding 1000 meters in depth is 5° 

 to — 1,5° C. Thus, about 90 per cent of the marine environment is per- 

 petually colder than 5° C. However, Smith (1940) points out that more 

 than half of the ocean's surface ranges from 15° to 30° C, and only 27 

 per cent of the surface water of the ocean has a mean annual temperature 

 below 10° C. 



The effects of water temperatures on the activities and distribution of 

 marine organisms have been summarized by Harvey (1928), Ekman 

 (1935), SvERDRUP et al. (1942), and others. The temperature of the water 

 also influences the density and movement of water masses. 



The movements of sea water : — Although the marine environment 

 is fairly stable in most respects, in many places the water is in a state of 

 continuous movement. In virtue of the mobility of its waters, the sea 

 responds readily to all forces acting upon it. The principal movements are 

 waves, tides, tidal streams, and currents. 



A wave is a ridge or swell of water normally having a forward motion. 

 Waves may be propagated for long distances. Upon reaching shallow 

 water, the wave steepness increases causing the waves to curl over and 

 form breakers. In deep water the water particles within progressive as 

 well as standing waves move in circles (Sverdrup, et al., 1942). Over 

 deep water, surface waves sometimes attain a maximum height of nearly 

 100 feet, the height of a wave being the vertical distance from the bottom 

 of the trough to the top of the crest. Besides tending to mix water, wind 

 waves produce considerable agitation of bodies in the water, particularly 

 at the surface and sometimes on the sea bottom. The action of waves 

 and breakers along the coast and in shallow water is an important ecolog- 

 ical factor. 



The attraction of the moon and sun causes tides, the alternate rising 

 and falling of the surface of the ocean and of gulfs, bays, and other bodies 

 of water connected with the ocean. The biological effects of tides are most 

 noticeable along the coast, especially in the intertidal zone. The latter is 

 alternately inundated and then left exposed to the air. In different parts 

 of the world the vertical distance between the highest spring tide and the 

 lowest neap tide ranges from less than one foot to 42 feet or more. Ac- 

 cording to Marmer (1932), the largest observed tidal ranges occur in the 

 Bay of Fundy near Nova Scotia, Port Gallegos at the southern end of Ar- 

 gentina, and Frobisher Bay on BafSn Island where the maximum tidal 

 ranges are 42, 36, and 35 feet respectively. Depending upon the bottom 

 gradient, the intertidal zone may extend from a few feet to several miles 

 from land. 



For further information on tides, tidal streams, ocean currents, and 

 other movements of sea water, the reader is referred to Johnstone (1928), 

 Marmer (1932), Patton and Marmer (1932), and Sverdrup e/ a/. (1942). 



The movements of sea water affect marine organisms in several ways 



