THE FRESH-WATER LOCHS OF SCOTLAND. 139 



source of water-supply for the city of Edinburgh, objection was made to 

 the water on account of the presence of certain organisms in it, and 

 especially of the small crustacean (Daphnia) commonly called the Water- 

 flea. It was pointed out at the time by those conversant with such 

 matters that these Crustacea are usually present in all impounded waters, 

 and we shall see that the organisms found in the water of St. Mary's Loch 

 are those which are characteristic of all freshwater lakes in this country, 

 even of those which are reputed purest. Their presence does not, there- 

 fore, in itself constitute any objection to the water. 



" The water of St. Mary's Loch was examined by the Lake Survey on 

 two occasions, early in May, 1905, and in the middle of January, 190(5. 

 In May the water was fairly clear, and there were only a few species of 

 animals and some microscopic plants found in it. There were four species 

 of Crustacea : Diaptomus gracilis, Sara, Cyclops strenuus, Fischer, Daphnia 

 hyalina, Leydig, and Bosmina obtusirostris, Sars ; one rotifer, Notholca 

 longispina, Kellicott ; and three diatoms, Asterionella formosa, Hass., 

 Tabellaria flocculosa, Kiitz., and T. fenestrata, Kiitz., var. aster ionelloides, 

 Gmn. No other organism was at all abundant. 



" All the species above enumerated are among the commonest of lacus- 

 trine organisms, and it would be difficult to find a loch in Scotland in 

 which all of them are not present. 



" In January the condition of the water was very different. The quantity 

 of life was immensely greater, and rendered the water of a dull yellowish 

 colour, and so turbid that bright objects could only be seen at a depth of 

 about 3 feet. All the same species were present, but some of them were 

 more abundant than in May, and many species were present which were 

 not found in May. The Crustacea were the same, but the larvae of the 

 copepods were very abundant, and some of the Cyclops were carrying 

 eggs. Of rotifers four additional species were found : Anurcea cochlearis, 

 ConocJtilus sp., Polyarthra platyptera, and Triarthra longiseta. The con- 

 trast was greatest in the vegetable life. One greenish alga, forming little 

 clusters easily visible to the naked eye, was mainly responsible for the 

 turbidity of the water. 



" This ' flowering of the lake ' in winter is not unfamiliar in Scotland, 

 and has been seen in Loch Earn, etc. It takes place when the temperature 

 is low (in St. Mary's 3S~ Fahr.). As the increase of life is not due 

 to high temperature, occurring, in fact, when the lochs are coolest, it may 

 be supposed tbat pollution of the water by sewage may have something to 

 do with it. At any rate, both St. Mary's Loch and Loch Earn receive a 

 good deal of sewage. These facts concern the open water of the loch, 

 which alone is of much importance in relation to water-supply for towns. 

 The life of the margin of St. Mary's, and of the mud on the bottom, was 

 also studied. 



" The life of the margin, chiefly found among the mosses and other 

 aquatic plants, is much more abundant than that of the open water. It 



