WINDERMERE FRESHWATER BIOLOGICAL STATION 297 



FRESHWATER BIOLOGICAL STATION, WINDERMERE. 



Report of Committee appointed to aid competent investigators selected by 

 the Committee to carry out definite pieces of work at the Freshwater 

 Biological Station, Wray Castle, Windermere (Prof. F. E. Fritsch, 

 F.R.S., Chairman ; Prof. P. A. Buxton, Secretary ; Miss P. M. 

 Jenkin, Dr. C. H. O'Donoghue (from Section D) ; Dr. W. H, 

 Pearsall [from Section K). 



During the current year the British Association's table at the laboratory 

 has been occupied by Mr. R. Misra, working under the general direction of 

 Dr. W. H. Pearsall of the University of Leeds. Mr. Misra has carried out 

 his work in close collaboration with members of the Association's staff. A 

 full account of his work has been prepared for publication as a scientific 

 paper. The following short account of the work has been drawn up by 

 Mr. Misra. 



Lake Muds and their Plant Successions. 



Under the guidance of Dr. W. H. Pearsall this subject has been studied 

 in the Lake District during a period of two years. The investigations were 

 carried out in the Freshwater Biological Laboratory for the two summers 

 1935-36, and the rest of the period has been occupied in chemical analysis 

 of field collections at the University of Leeds. 



It has been shown during my work that the plants attached to lake bottoms 

 are greatly influenced by the nature of the substratum ; their successions 

 have been traced to chemical and physical changes taking place in the mud. 

 In shallow water Littorella and Lobelia give way to Phragmites as the bottom 

 gets older and more organic, and in deeper water Isoetes gives place first to 

 pond weeds, then to water-lilies, and finally to sedges as similar changes take 

 place in the mud. These changes in the lake floor are therefore very im- 

 portant in deciding the quality and quantity of vegetation in a lake. 



During the development of a plant succession, the lake muds do not 

 become very acid as do terrestrial soils undergoing similar changes in the 

 same district ; for instance, in young or newly-colonised muds the p¥L 

 range is very wide. But the pW range for organic muds is very narrow, 

 the average figure being 5-8-60. Since the organic muds become ex- 

 tremely acid upon exposing them to air for some time, it is believed that the 

 acidity under water is kept down by special types of anaerobic decomposition. 



The ammonium thiocyanate test for detecting soil sourness was developed 

 by Comber, and is frequently used by agriculturists for that purpose ; but 

 it would not work with lake muds, for it depends upon the presence of ferric 

 iron, and in lake muds the iron is present in a reduced condition. This 

 method has now been modified to apply to lake muds. This is done by 

 oxidising the iron by hydrogen peroxide. A large number of samples 

 studied by the modified technique show that the muds become deficient 

 in lime as they accumulate organic matter. Plants like Littorella and 

 Isoetes cannot grow upon these lime-deficient muds, although the pond weeds 

 do well there. 



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