15] 



VEGETATIONAL TYPES OF FRESH WATERS 



487 



the surface in calm weather and is then absent below 4 metres (a), 

 but in windy weather is plentiful down to much greater depths (h). 

 Yet other Blue-green Algae, such as Aphanizomenon flos-aquae, can 

 achieve a vertical distribution of 6 metres or more on very calm days 

 (according to Professor G. W. Prescott in litt.). However, the really 

 lasting differences in composition occur in and below the thermo- 

 cline, where the eddy-diffusion currents are curtailed. The latter 

 may, however, extend to considerable depths during the autumn 



Fig. 166. — Diagram indicating distribution in a European lake of a cyanophycean 

 {Glosotrichia ec/iinulata), which is rendered buoyant by included gas-vacuoles : 

 (a) during calm weather, and (b) during windy weather. (After Ruttner, modified.) 



circulation, and at such times lead to a virtually uniform distribution 

 of plankton {cf. the October section of Fig. 165). Later on, under 

 the winter ice-cover of cool to frigid regions, stratification again 

 appears. Another complicating physical factor influencing stratifica- 

 tion is wave action, and yet another is water renewal, during which 

 much of the plankton-rich surface water of lakes is liable to be lost 

 by outflow and commonly replaced by inflowing river or other water 

 poor in plankton. 



The biotic factors affecting phytoplankton are far more numerous 

 and complicated than the mechanical ones, and no attempt will be 

 made to analyse them here. They are concerned with such (often 

 interrelated) processes as reproduction, photosynthesis, secretion of 



