The Develoivnent of Vegetation in the English Lakes. 281 



abundant phyto- and zoo-plankton in these lakes, and lience the fundarmntal 

 factor in the abundance of fish hecomes in this case the stage of development 

 shovm ly any particidar laJce. Apart from the increase in the bulk of 

 plankton, this factor operates in another way, for, as a lake develops, silts 

 become richer and more abundant, and rooted vegetation more luxuriant. 

 Hence there are increased areas on which epiphytic algs can grow, and 

 increased shelter and food for the smaller aquatic animals. The submerged 

 vegetation of Esthwaite, as an extreme example, is literally teeming with 

 life, and offers a marked contrast in this respect with vegetation from a 

 primitive lake like Wastwater. Pond,* on different and theoretical grounds, 

 also concludes that abundant rooted vegetation is favourable to fish pro- 

 duction. He shows that rooted submerged vegetation draws its mineral 

 nutriment chiefly from the substratum. Hence he argues that the seasonal 

 decay of this vegetation enriches the water in salts at the expense of the 

 soil, in this way favouring an increase in the bulk of the plankton. It 

 must be remarked that Pond's assumption can only hold where organic 

 decay is rapid and complete, and it is therefore more likely to operate in 

 relatively evolved lakes where silts are abundant and rich than in waters 

 where they are scarce, and the rate of organic decay is consequently slow. 

 In the latter type of water, organic matter accumulates in an undeeayed 

 condition. 



Kofoid,f however, concludes that the bulk of plankton (and hence of fish) 

 is inversely proportional to that of the larger submerged plants. In the 

 lakes to which he refers, the larger plants are almost entirely Ceratophyllum, 

 a free floating form which must draw the whole of its mineral nutriment 

 from the lake waters, and hence directly compete with the plankton. It is 

 clear, from Kofoid's account of Flag Lake, that he recognises that an 

 abundant rooted, vegetation probably, on decaying, favours the production of 

 a rich plankton. Hence his \'iews are consistent with those advanced above, 

 since the larger free-floating plants are absent from this lake area. 



It would thus appear that the increase in rooted vegetation, in the bulk of 

 the plankton, and in the abundance of fish, are all changes to be correlated 

 one with another, and ultimately depend upon the topographic development 

 of the lake. 



Xo satisfactory evidence is as yet available to account for the decrease in 

 numbers of trout in the evolved lakes. The indications are that trout food 

 (moUusca, flies, larvae and fresh-water shrimps) is most abundant in evolved 

 lakes. It can only be suggested that the decline of this fish may be due to 



* 'Eep. U.S. Comm. Fish.,' 1905, p. 435. 



t ' Bull. ILL State Lab. Nat. Hist.,' vol. 6, p. 95 (1899). 



