LIMNOLOGICAL PROBLEMS 
385 
greenish-blue, but often green ; the abundant supply of huniic 
substances washed out of the ground by the rivers generally causes 
the colour to be of a more or less brownish tint, but hardly ever 
so intense as that of the Scottish lakes (see especially Ule, 1898, 
pp. 69-7^2). This brown colour is, so far as I understand, more con- 
spicuous in North German and South Swedish than in Danish lakes, 
of which the larger, in my opinion, cannot be said to be brown but 
much rather green, perhaps owing to the great quantity of lime in 
the water. The brown tones of colour are most distinct in small 
lakes and in summer ; the green colour in spring, soon after the ice 
has broken up. The influence of the plankton on the original lake- 
colour will be referred to later. 
As to the chemical composition of the lake-water I need only refer 
to the greatly varying abundance of lime from lake to lake. The 
chalky nature of the surrounding country, of course, exercises a very 
great influence. The waters of lakes situated in moraine clay rich 
in lime have higher percentages of lime than those in territories with 
sandy ground. This zone, in contrast to the foregoing, may no doubt 
on the whole be said to contain many lakes having a high percentage 
of lime. Those which are remarkable for their greater transparency, 
purer water, and colours of a more greenish tinge have generally a 
richer organic life than waters with a small percentage of lime and 
coloured more or less brown by the humic acids. In describing the 
conditions in the Baltic lakes, and not least the chemical composition 
of the lake- water, it ought to be remembered that many of these 
lakes have formerly been in much more intimate touch with the sea 
than now. In the so-called beach-lakes the degree of salinity varies 
very much ; even in the true inland lakes, where, of course, it is slight, 
it may sometimes exceed 10 in 100,000 parts (Halbfass, 1901, p. 90). 
Concerning the quantity of dissolved organic material little is as yet 
known ; it is most probably on the whole pretty great — greater in 
shallow than in deep lakes, greatest in autumn and least in winter 
(Halbfass, 1901, p. 94). 
The broad littoral zone is covered with vegetation, everywhere 
arranged in very uniform belts, first a belt of Scirpus and Phragmites^ 
and then a belt comprising species of Potamogeton with P. lucens and 
P. perfoliata as main forms, and outside that again a belt of CharacCce 
mainly formed by species of Chara which hardly descends deeper than 
about 5-7 m. ; a Nitella belt is absent or weakly developed, but 
amongst the Characese and also outside the latter we find a peat- 
forming community of sterile submerged mosses, in our lakes at 
any rate going out as far as 9 m., in South Swedish lakes as far as 
7 m. (Carlsson, 1902, p. 27). The main forms are Amblystegkmi 
scorpioides and Fontiiialis antipyretica. I have known this moss belt 
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