The Development of Vegetation in the English Lakes. 



279 



and that this is due primarily to the increased number of Diatoms (and 

 Myxophyceas). In this connection the results of Apstein,* taken with those 

 of Brandtjf are of considerable interest. They show that in Holstein, lakes 

 rich in nitrates contain the greatest bulk of plankton (chiefly Diatoms and 

 Myxophyceae), whilst scarcity of nitrates is accompanied by paucity of 

 plankton. The water analyses show that this conclusion holds also for the 

 English lakes, and it is therefore probable that Diatoms and Myxophyceae are 

 favoured by the presence of nitrates, provided their other habitat require- 

 ments are present. 



Since it has already been shown that the f ^^^^l ^ ratio, silica, nitrate 



CaO + MgO 



and organic contents of the lake waters are dependent upon the physical 

 condition of the lake basin and drainage system, there are strong grounds for 

 the conclusion that the development of the phytoplankton, both in quality 

 and in bulk, in any of these lakes, is directly due to the degree of develop- 

 ment of the lake basin. W. and G. S. West regard contamination of 

 the drainage system by cultivation and sewage as an important cause of 

 plankton and Diatom abundance. While this recognises the effect of the 

 probable increase of nitrates caused by habitations and cultivations, it is not 

 in itself a fundamental distinction. As weathering and erosion change the 

 topography of the lake basin, there is a constant increase in nitrification and 

 change in the characters of the surface waters. Farms and cultivation follo'W 

 the development of soil and gentle slopes. They perhaps accelerate or 

 increase the developmental reaction ; they do not inaugurate it, and, therefore, 

 the stage of evolution of the lake basin must he regarded as being the fundamental 

 factor affecting the distribution of the phytoplankton, since it is upon this factor 

 that the characters of the waters depend. 



Finally, according to the views laid down previously, we may regard these 

 lakes as illustrating a post-glacial development of the phytoplankton. The 

 succcession would then become : — 



Desmids dfc. Diatoms dt. Diatoms dt. Diatoms 



Peridinium {Tahellaria spp.) (Asterionella) {AstetHonella) 



Willei ah. >- Desmids ab. JEudorina ah. >- and 



SpJissroci/stis' Ceratium ah. Myxophyoese dfc. 



ScTirateri ab. 



Rhizosolenia 



morsa fr. 



Fish — Economic Significance of Results. 



The economic importance of all limnological studies is largely bound up 

 with their bearing on the problems of fresh-water fisheries. The following 



* 'Das Siisserwasserplankton,' Kiel und Leipzig, 1896. 

 t ' Wiss. Meeresunt.,' Kiel Komm., voL4, p. 215 (1899). 



