l6] VEGETATIONAL TYPES OF SEAS 509 



which they live, and also to the general stability of the physical 

 characteristics of that medium, slight changes in the environment 

 are apt to be reflected with particular promptness in the plant and 

 animal population, whose components commonly lack the protection 

 their terrestrial counterparts have developed. Moreover the 

 organisms themselves may modify the chemical nature of their 

 environment by withdrawing or adding substances, so that, for 

 example, the surface layers of sea- water often become so impoverished 

 in the essential combined nitrogen and phosphorus as greatly to limit 

 growth and otherwise change the character of the plankton. Such 

 impoverishment is due to absorption by organisms whose dead 

 bodies sink to the depths where they gradually decay, so that 

 replenishment takes place chiefly by vertical convection currents in 

 the cooling period of autumn. This replenishment occurs most 

 actively in temperate and colder seas, and explains the often richer 

 plankton of these regions than of tropical seas where the lastingly 

 warm and light surface waters shut off the deep ones and little 

 vertical convection takes place. Indeed, while tropical seas are 

 estimated to contain on the average 5 planktonic organisms per 

 cubic centimetre, the figure for arctic seas varies from 100 to 500. 

 Nevertheless there is some compensation provided by the greater 

 depth to which the photosynthetic zone extends in the lower latitudes, 

 and still more in the short duration of activity each year in the high 

 latitudes, so that some authorities have recently claimed that there 

 is little or no significant difference in total productivity between 

 the seas of high and low latitudes. But for one reason or another, 

 in spite of the relative uniformit}^ of marine habitats, and the similarity 

 that prevails over vast areas especially in the open ocean, great 

 differences do in fact occur in different regions, which are reflected 

 in their plant as well as animal life. 



Among other chemical items it may be presumed that the content 

 of dissolved oxygen and carbon dioxide, and the variable reaction 

 (pH), are all important. Where vertical currents are active in seas, 

 the gas-content and composition of the water will be kept similar 

 in all layers reached by such currents ; but where no such aeration 

 etc. takes place, extreme conditions may occur. Thus in the Black 

 Sea, oxygen is found only to a depth of 183 metres ; below this, 

 normally respiring plants and animals cannot exist, and much the 

 same appears to be true in some tropical lakes. Although the situa- 

 tion with carbon dioxide is substantially reversed through photo- 

 synthesis, during which it is absorbed and replaced by oxygen. 



