200 



THE PHYTOPLANKTON OF THE ENGLISH 

 LAKE DISTRICT. 



Wm. west, F.L.S., 



AND 



G. S. WEST, M. A., D.Sc, F.L.S. 



{Contiiuied from page igj). 



After carefully considering the occurrence of Asterionella 

 in the British lakes, we are compelled to agree with Wesenberg- 

 Lund that Whipple's explanation is insufficient to explain the 

 great maxima which occur with a considerable degree of regu- 

 larity in so many of these lakes. Whipple's observations were 

 carried out in reservoirs and in the laboratory, and not under 

 conditions such as obtain in large lakes of considerable depth. 

 In the first place, it is unlikety that any living individuals 

 would exist at the bottom of a deep lake ; and assuming they 

 did, it would be quite impossible for them to be raised up from 

 the bottom, either by storms or convection currents, in suffi- 

 cient quantities to cause an enormous maximum in the plankton. 

 Moreover,, although Asterionella attains its maxima in both 

 spring and autumn, many other plankton-diatoms have only 

 one maximum, and in some species this is attained in the winter 

 and in others in the summer. 



In stormy times, large numbers of individuals are probabl}^ 

 carried into the plankton from the littoral region, and this 

 doubtless accounts for the sudden maxima of certain plankton 

 Diatoms a fevv' days after a storm, such as in those cases 

 recorded both by Whipple and Wesenberg-Lund. 



We think, however, that the supply of plankton-recruits 

 from the littoral region would be totally insufficient to cause 

 the enormous maxima which occur regularly in certain plank- 

 ton-species unless the other determining factors were of the 

 most favourable nature. These determining factors would 

 most probably be temperature, food-supply, and aeration of 

 the water. 



It would appear that temperature is a factor of importance, 

 as the vernal and autumnal maxima occur at approximately the 

 same water-temperature. This temperature (about 7° — 8°C.), 

 is probably the optimum for Astevionella gracillima. In the 

 spring the food-supply would be at its greatest because of the 

 large quantity of decomposed organic matter accumulated in 



Naturalist, 



