64 



The layer in which the sudden change occurs is known as the 

 " Sprungschicht " or " thermocHne." Its position varies, getting 

 deeper and deeper as the summer advances, until it is obliterated 

 by the complete turn-over of the water in late autumn. Professor 

 Birge has shown that the presence of the " Sprungschicht " in a lake 

 may be of extreme importance to the organisms living therein, and 

 that it is therefore necessary for the biologist to pay special attention 

 to this matter. 



Concerning the movements of the water of a lake, there are, in 

 addition to the slow circulation due to temperature just described, 

 various currents and oscillations of level brought about by the action 

 of the wind and other causes, including, perhaps, sudden variations 

 in atmospheric pressure. To the latter cause must almost certainly 

 be ascribed the very peculiar periodic alterations of level known as 

 " seiches," which represent a rhythmic swinging of the whole mass 

 of the water. They may vary from movements of a yard or more to 

 scarcely perceptible differences of level, but in all cases they occur 

 in series with a gradually diminishing amplitude. The time of swing 

 may vary from a few minutes to an hour or more.* 



The biological investigation of lakes now remains to be con- 

 sidered, but the subject is so large that it will be quite impossible to 

 deal with it adequately in this paper. A typical lake may, from a 

 biological standpoint, be divided into three regions, each characterised 

 by peculiar conditions of existence and the occurrence of special 

 animals and plants. These regions are — (i) the littoral, (2) the 

 abyssal, and (3) the pelagic, and they necessarily require different 

 methods for their examination. 



In the littoral region of lakes the conditions of existence are 

 rather severe, owing to the action of the waves, the great and sudden 

 variations in temperature, light, etc. The fauna and flora are not 

 usually very rich, unless there happen to be small sheltered bays 

 and shallow creeks here and there round the lake. The littoral 

 plants, when present, are arranged in more or less definite zones 

 corresponding with the gradually increasing depth of water as we 

 proceed from the strand out on to the top, and down the face of the 

 " lake-terrace." Thus we get first the zone of Fhragmites, Scirpus, 

 etc. ; then the water-lily zone ; then the Potamogeton zone, with such 

 species as P. crispus and P. lucens ; then the zone of Chara, Nitella, 

 and Foniinalis ; and lastly a zone consisting only of ground algae, such 

 as Cladophora and Khizocloiiiuin. This marks the lowest point at 

 which green plants can thrive, and probably coincides with the limit 

 of the penetration of light in an appreciably active form. The 

 fauna of the littoral, considered in its entirety, is very varied, but the 

 species are mostly similar to those of ponds and other shallower 

 waters. For collection purposes a net fastened to a stick and 



* One of the results of the Lake Survey under Sir John Murray has been 

 the demonstration of the occurrence of a seiche in one of the Scotch lochs 

 (see "Nature" for 12th June, 1902, p. 162). 



