15] VEGETATIONAL TYPES OF FRESH WATERS 481 



Desmidiales (Desmids) and unicellular as well as colonial Volvocales), 

 Bacillariophyceae (Diatoms), Dinophyceae (Dinoflagellates), and 

 several other groups of flagellates, etc., being commonly represented. 

 During spring or summer maxima a greenish, yellowish, or brownish 

 ' soupiness ' may be evident to the naked eye, the composition and 

 luxuriance of the community being very variable in time and space. 

 Thus from an aircraft the colours and general appearance of the 

 water of even adjacent tarns may be strikingly different, especially 

 in boreal regions. Except in the tropics where filamentous forms 

 are sometimes dominant (R. Ross in litt.), macroscopic plants are 

 commonly lacking in the real plankton, as are Rhodophyceae and 

 Phaeophyceae, though some Fungi may occur. 



Continuous investigation of a lake or pond over a period of years 

 is likely to reveal striking changes recurring seasonallv in much the 

 same combination and form each year. This variabilitv in type and 

 abundance at different seasons, or periodicity, as it is called, is shown 

 by (i) perennial species which occur in different densities at different 

 times, and (2) ephemeral types which spend the rest of the year 

 in some resistant stage usually on the shore or lake-bottom. The 

 fluctuations are due to interaction between the rates of multiplication 

 and of depletion, the former being dependent on basic biotic in- 

 fluences and the latter largely on natural mortality, predators, and 

 mechanical factors such as sedimentation. Nor do spring forms 

 commonly recur in autumn, owing to the different light and tem- 

 perature relationships ; for in other than tropical regions these tw^o 

 leading factors exhibit marked and important differences in lakes at 

 different seasons. 



The circulation of water in winter and early spring brings up 

 nutrients to the surface layers. This often leads to an early ' bloom- 

 ing ' of Diatoms. But with the onset of summer stratification, 

 accompanied by a profuse growth of Green and other Algae, the 

 stock of nutrients cannot be supplemented sufficiently to maintain 

 abundant new growth and reproduction. This is because most of 

 the nitrate and phosphate ions, particularly, have already been re- 

 moved from the upper layers and stored in the bodies of the organisms 

 that flourish there. Certain Peridinians which can manage with a 

 minimum of inorganic nutrients are then apt to appear in fair num- 

 bers. However, in late summer vertical convection gradually ex- 

 tends deeper, causing a replenishment of the upper layers from the 

 nutrient-rich water of the depths, so that population expansion can 

 again take place — cf. Fig. 165. The cycle is perennial and more or 



