Il] MAIN HABITATS, SUCCESSIONS, AND CLIMAXES 319 



distinguished quantitatively from the oHgotrophic type in bein ; 

 usually poorer in the numbers of species but richer in individuals 

 and poor in humus though commonly sihed and shallow. The 

 eutrophic type is also relatively rich in combined nitrogen, phos- 

 phorus, and often calcium, typically contains plentiful Blue-green 

 Algae, and has a broad zone of rooted Pondweeds, etc., and a sur- 

 rounding one of luxuriant reed-sw^amp {cf. Fig. 89). 



Fig. 89. — Lake of eutrophic type near Prout's Neck, Maine. It is silted and 

 shallow, with floating-leaf plants outside the broad marginal reed-swamp dominated 



bv tall Cattails. 



In addition, the reaction (or ' pH level ', whether acidic or basic) 

 of a body of water is commonly important to many organisms, 

 while lack of oxygen may be a limiting factor deep down in sheltered 

 situations. Also often limiting are seasonal and other changes of 

 level in shallow places. Any substantial tidal activities are especially 

 significant, as may be the speed and flow of currents and the presence 

 and particular powers of various living organisms. 



Even as the zoned vegetation of sea-shores indicates the existence 

 of diflrerent habitats at diflFerent levels, e.g. above and below normal 

 low-tide mark, so do zones exist at different depths around the 

 margins of deep lakes. Moreover, seas (such as the Sargasso) and 

 especiallv lakes of warm regions, may bear extensive macroscopic 



