l6] VEGETATIONAL TYPES OF SEAS 51I 



periodicity in which hght plays such an important part, it is clear 

 that temperature may be decisive in favouring the development, 

 for example, of northern species very much farther south than usual 

 in the earlier part of the year, and of southern species northwards 

 when the water warms up in summer. But although the flora tends 

 to be more diverse in warm than cold seas, in the matter of total 

 productivity the reverse may hold — at least of phytoplankton in the 

 upper layers {see p. 509). 



Light is often the dominant factor in determining the local 

 distribution and extent of aquatic vegetation, being rapidly reduced 

 with increasing depth. The degree to which water allows light to 

 penetrate its depths naturally varies greatly with turbidity and other 

 conditions, but in general the total radiation is reduced to little 

 more than half its surface intensity at a depth of 10 centimetres, 

 and to a little over one-seventieth at 100 metres. Nevertheless in 

 some seas there may be a noticeable effect on a photographic plate 

 as deep down as 1,000 metres. Although photosynthesis doubtless 

 ceases far above such limits, and probably often in the uppermost 

 100 metres, it still commonly continues to much more considerable 

 depths in seas than in fresh waters, owing to the greater transparency 

 of the former. Thus the euphotic zone in the ocean normally 

 extends down to 80 or more metres, and the dysphotic zone, of dim 

 light and consequently very limited plant production, from the base 

 of the euphotic zone to 200 or more metres. Moreover, because of 

 the greater thickness of the turbulent layer affected by vertical 

 diffusion currents, as well as of this deeper photosynthesis allowed 

 by deeper light-penetration, the habitat of the ' surface plankton ' 

 commonly goes far deeper in oceans than in lakes. 



The depth to which photosynthesis extends will naturally vary 

 greatly in different circumstances, being limited markedly by dis- 

 persion of light due to suspended particles both living and inert. 

 Moreover, because of the lowering angle of incidence of the sun 

 and consequently reduced penetration of its rays, this depth usually 

 gets shallower and shallower at increasingly high latitudes. It is 

 commonly taken by oceanographers as the dividing-line between the 

 bottom of the infralittoral (also called by some the sublittoral) and 

 the top of the deep-sea system, being often placed at around 200 

 metres as in the accompanying diagram (Fig. 170). This maximum 

 depth to which photosynthesis extends is also the approximate depth 

 of water at the outer edge of the continental shelf, and separates 

 the neritic (shallow water) province from the oceanic province of 



