LIMNOLOGICAL PROBLEMS 
389 
Klunzinger, 1901, p. 321, and 1902, p. 338; also Zacharias, 1902, 
p. 700, 1903, p. 296). As these plankton organisms are different in 
the different lakes and vary during the seasons in each lake, the 
colour of the lake undergoes much greater variation than in other 
zones. We may say that as a rule the brownish -yellow Diatom-colour 
characterises our lakes in spring and autumn, the blue-green Cyano- 
phyceae-colour in summer ; in cold and deep lakes the brownish- 
yellow colour is also preserved in summer, but this is due to Ceratium 
hirund'mella. Sometimes other tinges of colour break in and replace 
the former quite suddenly and for a^ short time. Thus the Oscillatorioe 
gave Fureso a whitish tint in May 1903, and the Lynghya a cherry - 
red tint in September and October 1902. Botryococcus Braunii 
sometimes gives a reddish colour to several of our lakes. In mild 
winters the lakes keep the Diatom -colour. It is in early spring that 
the plankton has least influence on the colour, but even then the 
true colour of the lake is not apparent, as just at that time the huge 
masses of detritus give it a brownish or greyish tint. The true 
colour of the lake-water is most apparent in May, when the Diatom 
maxima are almost over and the large Cyanophyce^e maxima have 
not yet begun and the detritus has gone to the bottom (see Wesenberg- 
Lund, Prometheus^ 1906, p. 785). An impression of the enormously 
large quantities of plankton which are developed, especially during the 
Cyanophyceae maxima, is obtained by filtering the water, which then 
shows a faint milky colour, most probably caused by the Phycocyan 
set free in the processes of putrefaction. How far these observations 
from the Danish lakes also apply to the lakes in the remaining part 
of the zone is at present unknown. In North Germany, Ule and 
Halbfass have remarked upon the great importance of the plankton 
in determining the natural colour of the lake. Compared with the 
alpine lakes we may say that the Baltic lakes almost always have 
in reality " water-bloom.'*' 
The organic life, especially the enormous quantities of plankton, 
reduces also the transparency. The great yearly variations in trans- 
parency may always with certainty be traced back to corresponding 
variations in the amount of plankton. Whereas in the high-alpine 
lakes the quantities of detritus reduce the transparency, this is only 
to a slight extent the case at least in the greater part of the Danish 
lakes, except immediately after the ice breaks up and the drifting ice 
has scratched up the bottom. The quantity of plankton (water-bloom) 
may be so great that it acts as a wave-subduer. More than once I 
have seen a gale blowing the greater part of the water-bloom down into 
a corner of a lake, and in the centre raising high waves with spray ; 
nevertheless on the windward side the surface was almost smooth, with 
but a lono; swell throuo;h the thick water filled with water-bloom. 
