THE FRESH- WATER LOCHS OF SCOTLAND. 31 



amount of the organic life in the fresh-water lochs are subject to great 

 variation in the different lochs when compared with each other, and in 

 the same loch at different seasons of the year. Large numbers of 

 observations are being collected, and we may look for interesting results 

 when these are in a state for discussion. Generally speaking, the 

 pelagic fauna and flora are much more abundant in the warm summer 

 months than at other times of the year, and are also more abundant in 

 the shallow lochs than in the deep ones. In the spring months there is 

 a great development of diatoms and other Phytoplankton, which render 

 the water less transparent than at other times of the year. 



Mr. Thomas Scott has lately been comparing the fauna in several of 

 the Scottish lochs at different seasons of the year ; some of his results 

 for the lochs now under consideration may be noted. 



In Loch Katrine the Entomostraca and other invertebrates were 

 scarcer than in the other lochs examined. Fourteen species are recorded, 

 Bosmina longispina being the only species present in all the gatherings ; 

 Leptodora was entirely absent from the gatherings collected during the 

 colder months. Cyclops strenuus and Polyphemus appeared to be more 

 frequent in the upper part of the loch, and Bosmina and Leptodora in 

 the lower part. The sides of Loch Katrine do not generally present 

 conditions very favourable to shore-dwellers, and an examination of the 

 shore about Stronachlachar yielded scarcely anything that differed from 

 the tow-net captures, while at the lower end the shore between the 

 Trossachs pier and Ellen's isle yielded much better results. Here forty 

 species of Crustacea were obtained, as well as one or two species of 

 Mollusca, but they were all individually scarce. The Cladocera were 

 more numerous in species in the warmer than in the colder months, 

 while with the Copepoda the reverse was observed, though the difference 

 was not so great.* 



In Loch Arklet, Holopedium gibberum, one of the most remarkable 

 species of the Cladocera in Britain, was moderately common in the 

 tow-net gatherings collected in September and November, 1897, and in 

 June, 1898, it was abundant all through the water, but when the loch 

 was visited in March, 1898, not a trace of Holopedium could be seen. 

 In June, when Holopedium was so abundant, other species previously 

 observed were either very scarce or absent, as if they had been more 

 or less crowded out by this particular cladoceran. Eleven crustacean 

 species are recorded, Daplinia being the only form obtained in all the 

 gatherings ; Bythotreplies was observed in September and June, but not 

 in November and March, and Leptodora occurred only in September. 

 Infusoria (Ceratium, &c.) and micro-algae were much less frequent in 

 June than in the other gatherings. Forty-two species of Entomostraca 

 were obtained by hand-net round the shores of Loch Arklet, including 



* Scott, Seventeenth Annual Report of the Fishery Board Jor Scotland, pt iii. pp. 148-151, 1899. 



