138 SCIENTIFIC SURVEY OF BLACKPOOL AND DISTRICT 



stages in the post-glacial development of aquatic vegetation. This is largely 

 because the underlying rocks vary considerably in hardness, and as a result 

 some of the lakes have developed more slowly than others. These more 

 primitive lakes (e.g., Wastwater and Ennerdale) have a sparse colonising 

 vegetation in which Isoetes lacustris and Nitella opaca are most characteristic 

 of deep water, while Litorella is usually present in the shallows. Any silts are 

 coarse and poor in bases, and the silted places are likely to support Juncus 

 fluitans, Callitriche intermedia and, more rarely, Potamogeton pseudofluitans, 

 a submerged form of P. polygonifolus. As lake development progresses, 

 silting increases, and the sub-strata also become more organic in character. 

 Submerged species of Potamogeton become frequent, especially P. perfoliatus, 

 P. praelongus and P. pusillus. Lobelia Dortmanna and reeds (chiefly Scirpus 

 lacustris and Phragmites) become more frequent on the margins. 



In still later stages, when the sub-strata as a whole are organic, water-lilies, 

 Potamogeton natans, Equisetum limosum and Carices extend over the margins, 

 while submerged plants such as Sparganium minimum, P. alpinus and 

 P. obtusijolius are characteristic of greater depths of water. Many of the 

 smaller and shallower lakes have disappeared, and their position is only 

 indicated by peat mosses (e.g., Rusland and Shoulthwaite) bearing vegetation 

 of the ' lowland moor ' type. While in some cases, the reed swamps of the 

 lakes developed to moor through a well-defined woodland stage in which willows 

 and birch were prominent, in other cases the woodland stage may have been 

 less well-marked or nearly absent. The bogs at the south end of Hawes 

 Water exemplify a transitional condition of this type. In general, the wood- 

 land development seems to be associated with a greater relative abundance 

 of inorganic silts or with a more decided influence from telluric waters. 



Taxonomically, the most interesting aquatic plants are Naias flexilis and 

 Hydrilla verticillata, both confined to Esthwaite Water, and elsewhere very rare 

 in the British Isles. Potamogeton pusillus is represented mainly by the 

 sub-species lacustris, a broad-leaved form not found in other parts of England. 



The sequence of stages in lake development shown in the study of higher 

 plants is apparent also in relation to the plankton and littoral algae of the lakes. 

 The primitive or rocky lakes have a rather sparse flora in which desmids and 

 green algae are predominant. A further stage in development is marked by the 

 appearance of abundant diatoms, especially Asterionella and Tabellaria 

 fenestrata in the plankton. The latest stage is shown by an abundance of 

 blue-green algae such as Anaboena and Coelosphaerium. It is of interest 

 to note that the last transition has actually been observed in Windermere, 

 though there it is not entirely due to natural causes. 



The great influx of visitors to the district from 1910 onwards has led to an 

 increase in the volume of sewage effluents reaching the lake and hence to an 

 increase in the amount of organic matter in its waters. Hence, also, blue- 

 green algae have become a constant element in the phyto-plankton, although 

 they were practically absent in 1907-8. 



Enough has been said to indicate that the lakes offer great possibilities to 

 students of aquatic biology. The establishment of a laboratory at Wray 

 Castle, on Windermere, by the Fresh-water Biological Association may be 

 regarded as an index of the scientific and practical interest these problems 

 arouse. 



