Chapter XVI 



VEGETATIONAL TYPES OF SEAS 



We have now dealt at least cursorily with each of the main vegeta- 

 tional types of the chief plant habitats, excepting only those of the 

 oceans and salt-seas which, however, occupy between them slightly 

 over 70 per cent, of the surface of the globe. Nevertheless they 

 are relatively uniform over very wide areas and so can be treated 

 fairly briefly. This is especially true of the open ocean and the 

 planktonic communities developing in it. On the other hand, on 

 shallow marine bottoms and particularly around sea-shores, condi- 

 tions and the attendant plant communities tend to be far more 

 variable, so that altogether we get a set of categories which are 

 largely comparable — for example in their planktonic or, alterna- 

 tively, benthic nature — with those of fresh waters. But before we 

 deal with each of these in turn, we must consider the sea in general 

 as a habitat for plants. 



Some Features of the Marine Environment 



That the marine environment is widely different from the fresh- 

 water one is evidenced by the fact that salt-water plants placed in 

 fresh water, or vice versa, almost invariably perish. The light and 

 other physical conditions can be, and often are, much the same in 

 the two media, and temperature fluctuations of the air tend to be 

 similarly ' damped down ', the differences in sahnity being commonly 

 the decisive factor so far as inhabiting plants are concerned. 



The salt-content of the free oceans is around 35 per cent., but 

 in bays and inland seas it may deviate widely from this figure owing 

 to concentration by evaporation or, alternatively, dilution by fresh- 

 water streams. Thus preponderance of evaporation causes the Red 

 Sea to have a salinity of well over 4 per cent, at the surface, while 

 abundant inflow of fresh water makes the salinity of the Baltic Sea 

 in many places less than i per cent., and in some only o- 1-0-2 per 

 cent. Such reductions in any particular region -apparently regard- 

 less of climatic or other changes — tend to be accompanied by very 



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