3. Marine Biology 



a. The Sea as a Biological Environment 



The sea has several distinct advantages over the land for the 

 development of life. Water, necessary to all life, is immediately 

 available. Beyond this, seawater contains in solution the required 

 gases and other materials for growth, and is very similar in composition 

 and solute concentration to the body fluids of all animals. The result- 

 ing near equality of the osmotic pressure of seawater and body fluids 

 permits the direct intake of nutrients and excretion of waste in some 

 organisms. Buoyancy provided by the high density of seawater reduces 

 the need for skeletal structure, whereas the high specific heat of 

 seawater reduces the possibility of rapid temperature change. The 

 range of pressures experienced in the ocean as a whole is very great, 

 and many organisms are consequently restricted to specific layers, but 

 all life is not excluded from the deep abyssal regions. Bathyscaphe 

 dives, in particular, have confirmed the existence of marine life in the 

 deepest parts of the ocean. 



Overall, a considerable variety of conditions exists in terms 

 of salinity, temperature, illumination, and pressure in the ocean. 

 Great variability with time is largely confined to coastal waters and 

 shallower layers. Here, marine forms must be adaptable to changes or 

 capable of adjusting themselves to the optimum condition. Light in 

 the surface layer is particularly important for photosynthesis by 

 floating microscopic plants. Notably, there is an adjustment to 

 lighting conditions by some marine life, resulting in a diurnal migra- 

 tion toward and from the sea surface. Large populations of this marine 

 life can form sound scattering layers. As distinct from regions where 

 marked variability exists, large volumes of deep water have remarkably 

 constant conditions, and specific forms appropriate to the particular 

 environment can develop. Circulation processes in the sea are important 

 to life in many ways. These processes maintain oxygen content, 

 replenish the supply of nutrients, and aid in dispersal of waste material 

 and young life in the form of spores, eggs, larvae and young adults. 

 Clearly, however, unexpected and unseasonable disturbances of the 

 circulation pattern can, by moving life into an unacceptable environment, 

 result in considerable destruction. 



Formally, the marine biological environment is divided into the , 

 water region, called pelagic, and sea floor region, referred to as 

 benthic. The water region is subdivided horizontally into the neritic 

 zone, which lies over continental shelves (low tide to 200-m depth) and 

 the oceanic zone, which lies seaward of the shelves. The oceanic zone 

 is further divided into depth regions: epipelagic, from the sea surface 

 to about 200-ra depth; mesopelagic, 200 m to 1,000 m; bathypelagic, 1,000 

 to 4,000 m; and abyssopelagic (hadal) , the deepest parts of the ocean 

 (fig. 3-1). 



