1908.] 



The British Freshwater Phytoplankton. 



191 



nearly akin to the latter. In addition they have peculiarities which tend to 

 mark them off from either of those groups ; for instance, the relatively high 

 winter temperatures. Very many of these lakes never freeze, and most of 

 the others only rarely become covered with ice, and then for comparatively 

 brief periods. The summer temperatures are also comparatively low. The 

 highest temperature we have recorded in Windermere in the course of 

 twelve months' observations (including one of our warmest summers) was 

 14°-4 C. (58° F.), in Wastwater 1T'2 C. (63° F.), and in Ennerdale Water 

 15° - 5 C. (60° F.). The highest temperatures we have obtained in the Welsh 

 lakes were 17° ; 5 C. (63°-5 F.) in Llyn Ogwen and 18° C. (64 a 5 F.) in Llyn 

 Llydaw. The small sheet of water known as Llyn Elsie attained a summer 

 (August) temperature of 19° - 7 C. (67°'5 F.). The summer temperature of 

 the Irish lakes rarely exceeds 18° - 3 C. (65° F). The average range of 

 temperature in the Scottish lochs is from about 5° C. to 13° C, and the 

 highest we have ourselves measured was 16° - 6 C. (62° F.) in Loch Earn. 



In all the four British lake-areas the water is soft, with only small 

 quantities of dissolved lime, a peculiarity which accounts for the rarity of 

 snails at the lake margins. 



The phytoplankton is never of very great bulk, and it is quite exceptional 

 for it to colour the water to any appreciable extent. 



The periodicity of the phytoplankton is very variable in the different 

 lakes. In some it is conspicuous, but in others it is not very well marked. 

 Investigations at present in progress indicate that it is most conspicuous in 

 the shallower lakes, and particularly in those at considerable altitudes, but at 

 present our data are not nearly sufficient for generalisations to be made. 



The MyxophycEtE play quite a secondary part in the plankton of the 

 British lakes as compared with the Central European lakes. They are more 

 abundant in some of the Irish lakes than in any of the others, and some- 

 times the phenomenon of "water-bloom" makes its appearance. This 

 phenomenon, which is due to sudden and simultaneous maxima of a few of 

 the limnetic species of Blue-green Algae, is of very irregular occurrence in 

 the British lakes, and is practically confined to sporadic appearances in some 

 of the shallower-lakes.* 



Oscillatoria tenuis is general but never abundant, and Anabcena Zemmer- 

 manni should be specially mentioned, as its spores form deep blue-green 

 Moating clusters, which sometimes give a decided colour to the surface water. 



* The phenomenon of "water-bloom" is better seen in some of the large pools and 

 meres of the lowland parts of Britain than in any of the lake-areas. A good description 

 of the " breaking of the meres " has been given by Phillips in ' Trans. Shropshire Archseol. 

 and Nat. Hist. Soc.,' 1883, and earlier records were by Greville, Dickie, and Drummond. 



