Il] MAIN HABITATS, SUCCESSIONS, AND CLIMAXES 317 



be reduced to chains of pools in dry weather, and lakes can have 

 considerable convection and wind-engendered currents as well as 

 run-off streams, so that here again any distinction is not wholly 

 valid. Major variations in salinity found in different seas, estuaries, 

 salt-lakes, etc., and the light, temperature, tranquillity or shelter 

 from disturbance, size and depth of the body of water, possibility 

 of attachment, and content of dissolved substances, are all im- 

 portant factors leading to the existence of whole series of different 

 aquatic habitats. Further variable factors may be the ' reaction ' 

 (acidity, neutrality, or basicity), aeration, and seasonal or tidal or 

 other changes in the level of the surface. 



Let us briefly consider examples of these and some other factors 

 as they may affect aquatic habitats : further details are given in 

 Chapters XV and XVI. Light, being essential for photosynthesis, 

 severely limits to those depths to which a sufficiency penetrates the 

 possibilities for normal plant development, while towards the lower 

 limit at which photosynthesis is possible, this vital function is in- 

 sufficiently active to sustain life unless it be of specially adapted 

 organisms. Some Red Algae seem to be so adapted, for they can 

 grow at depths of nearly 200 metres in exceptionally clear seas, while 

 some phytoplanktonic organisms have been dredged from, and can 

 apparently live at, fully 200 metres. The larger Brown Algae, on 

 the other hand, do not seem to be able to grow at any such depths, 

 while vascular plants in fresh water usually extend no deeper than 

 10 metres even in the clearest lakes, and in shallower water form 

 zones correlated with their light requirements. However, in the 

 Mediterranean Sea one flowering plant, Posidonia, is reported to 

 extend down to depths of 80 or even 100 metres. 



Of the other factors, temperature differences frequently have much 

 the same effect in aquatic as in aerial habitats, although major bodies 

 of water will act as reservoirs militating against rapid changes in 

 temperature. Consequently, conditions in water tend to remain 

 more ' even ' than in the air, with the result that aquatic organisms 

 and communities are often surprisingly widespread. Shelter from 

 wave or ice action, and tranquillity from currents and tides, is another 

 important factor profoundly affecting the habitat and attendant 

 vegetation, macroscopic {i.e. visible to the naked eye, as opposed 

 to microscopic) plants often being limited to sheltered bays, etc. 

 This is often bound up with the size and depth of the body of water, 

 on which convection and wind-engendered or other currents 

 frequently depend, as does the degree (if any) of freezing. But 



