The most significant biologic zonation is the division into 

 the euphotic or photic zone, in which there is sufficient solar 

 radiation for active photosynthesis, and the deeper aphotic region. 

 The photic zone may extend to about 180 m in clear oceanic water at 

 low latitudes, or may be as shallow as a few meters in the more turbid 

 coastal regions of temperate latitudes (fig. 3-2). 



There is considerable seasonal change in surface solar 

 radiation in the temperate zone; therefore, thickness of the photic 

 zone varies throughout the year. In the spring, when radiation levels 

 increase, plant life grows rapidly, using nutrients supplied to surface 

 waters from depth by winter mixing. Surface heating in spring imposes 

 a stability on the water column that reduces vertical mixing and 

 increases the residence time of phytoplankton in the layer above the 

 developing thermocline. Retention of phytoplankton near the surface, 

 increased radiation, and available nutrients trigger the spring bloom. 

 In the tropics, low nutrient supply prevents high productivity of 

 organisms; in temperate regions, both light and nutrients may limit 

 production; and in polar regions, light alone controls productivity. 

 Nutrients are supplied to the photic zone by: (1) rivers which supply 

 land material, and (2) upwelling of water from depths below the photic 

 zone where nutrients have accumulated due to decomposition. In the 

 sea, as on land, photosynthetic plant life must exist in order for 

 animal life to survive. 



b. Organisms and the Composition of Seawater 



Seawater is more than a solution of salts in a certain proportion, 

 for marine life is an inherent part of the sea, and marine organisms 

 play an important role in determining the chemical and physical 

 properties at any particular location and time, e.g., they have an effect 

 on oxygen and carbon dioxide gaseous contents. Beyond this, certain 

 elements occurring in small concentrations in seawater are removed in 

 relatively large amounts by marine life for skeletal, structural, and 

 other uses. Inorganic compounds of nitrogen, phosphorus, iron, and 

 silicon — the plant nutrients — fall in this class, and their 

 distribution is affected appreciably by biological activity. 



Life cycles of the various organisms in the sea are closely 

 interwoven. Plants of all sizes furnish the basic food for all life 

 and grow in the upper illuminated layer. These plants absorb carbon 

 dioxide from solution and release oxygen. They also absorb phosphates 

 and nitrogen salts and convert them into organic matter. However, the 

 inorganic substances that they use for growth must be made available 

 to them near the surface, in spite of downward movement of particulate 

 matter in the sea under the influence of gravity. Availability of 

 these materials at the surface results from circulation of ocean waters, 

 convective mixing, and upwelling from the bottom. The inflow of 

 minerals from land is a small factor in supplying nutrients for sea 

 plants. Whenever plant nutrients are abundant near the surface, large 



