480 INTRODUCTION TO PLANT GEOGRAPHY [CHAP. 



Static. Rather is it the turbulence of the water caused by the eddy- 

 diffusion currents that maintains a state of continuous mixing par- 

 ticularly in the epilimnion, and keeps in this upper layer a sufficiency 

 at least for survival of the non-motile phytoplankton which chiefly 

 flourishes here. 



Whereas in the sea the diflterentiation of open-ocean (pelagic) and 

 near-shore (neritic) planktonic organisms is often marked, so that 

 the two communities may be very different, in inland lakes the dis- 

 tinction is relatively poor. Indeed, close relatives of nearly all the 

 species of phytoplankton here occur also in the littoral, whence the 

 open waters of the pelagial were evidently colonized. Moreover, 

 the number of truly planktonic species in lakes is relatively small, a 

 large proportion of the types found being really ' tychoplankton ' 

 {see above), which have scarcely more claim to membership of the 

 community than have the particles of inorganic and dead organic 

 suspended matter (' tripton ') present. Still, the numbers of actual 

 ' plankters ' (planktonic individuals) may be great, especially when 

 they exist in highly profuse ' blooms '. 



Freshwater plankton communities are composed of (a) producers 

 and (h) consumers of organic matter, the producers being almost 

 entirely chlorophyll-containing autotrophic plants. The consumers 

 or non-producers, including normal Bacteria and Fungi which make 

 up most of the so-called ' saproplankton ', are dependent upon the 

 carbohydrates and fats and proteins synthesized by the chlorophyll- 

 bearing plankters. In general the phytoplankton is composed not 

 only of the relatively large and obvious types which dominate the 

 so-called ' net-plankton ' and include most of the Diatoms and Dino- 

 flagellates, but also of extremely minute ' nannoplanktonic ' (micro- 

 planktonic) species which pass through even very fine nets (of No. 25 

 silk bolting-cloth) and are usually obtained by centrifuging. In 

 lakes, from one to a very few species commonly predominate and 

 make up the vast bulk of the phytoplankton at any one time, and 

 these may include minute nannoplanktonic types particularly of 

 Peridinians or other flagellates (e.g. Rhodomonas lacustris). Bacteria 

 normally contribute only a small proportion of the total ' biomass ', 

 and so may even the large species that dominate the net-phyto- 

 plankton. In the zooplankton, on the other hand, such large types 

 as Daphnia tend to be most prominent. 



In general the most plentiful organisms in freshwater phyto- 

 plankton are unicells or small colonies — whether flagellated or 

 non-motile — the Bacteria, Cyanophyceae, Chlorophyceae (including 



