478 INTRODUCTION TO PLANT GEOGRAPHY [CHAP. 



free atmosphere, which averages about 0.03 per cent, of carbon dioxide, 

 this gas, being easily soluble in water, is widely abundant in lakes. 

 Thus many contain more than 20 c.c. per litre in the depths, although 

 in oligotrophic lakes the content may be as low as i c.c. per litre. 

 Oligotrophic lakes may also show little variation in the oxygen content 

 at different depths, in marked contrast to eutrophic ones. 



The so-called ' lime ' content (particularly of calcium carbonate 

 and bicarbonate) of freshwater lakes and streams varies greatly, 

 peaty waters being especially lime-poor and ' soft ', whereas those 

 originating in calcareous districts are mostly lime-rich and ' hard '. 

 Variations also occur at different depths and times of the year : 

 thus in summer periods of relative stagnation, living organisms 

 may remove much of the ' lime ' from the upper layers, while the 

 deeper ones, which are already rich in carbon dioxide, actually be- 

 come enriched in lime. Calcium and allied carbonates and bicarbon- 

 ates have a marked ' buffering ' effect against changes in the reaction 

 or pH level of a body of water, and as the reaction (or some condition 

 associated with it) is often important in affecting the flora and conse- 

 quently the vegetation, so are such ' salts ' important. Most favour- 

 able is a weakly basic to neutral reaction, markedly acidic waters 

 being biologically unfavourable : hence the limited and peculiar flora 

 of scarcely buffered bog-waters, the acidity of which is associated 

 with marked poverty in lime. 



With regard to the impress of the environment as a whole, there 

 is insufiicient data as yet to compare the vegetational productivity 

 of different climatic zones. Thus although some tropical inland 

 waters may be more prolific as producers of plant or animal life than 

 some extra-tropical ones, others are practically barren. Many extra- 

 tropical lakes naturally rank extremely high in productivity, and for 

 many which do not so rank a great deal can be done by the addition 

 of fertilizers or by such manuring as is, for example, practised in 

 European carp-ponds. For productivity of lakes is related largely 

 to such factors as chemical content, turbidity, and light. Even in 

 the Arctic the present writer has collected samples from small lakes 

 and ponds that have shown a surprising wealth of algal forms : in 

 one instance of six samples taken in as many small vials from shallow 

 pools and peaty puddles in Baffin Island in July and August, 1936, 

 no less than 179 different species or varieties of Algae were determined. 

 Nearly all of these were microscopic, a large proportion being Des- 

 mids. Moreover a considerable number of organisms, such as the 

 familiar planktonic Dinoflagellate Ceratiiim hiriindineJla and species 



