BIOLOGY OF THE SCOTTISH LOCHS
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the shallow Continental lakes. Even so far north as Denmark the
annual range seems to be about 10°'0 Fahr. greater; in the great
Balatonsee of Hungary it is much higher—nearly 50°-0 Fahr. The
greater range of temperature will favour seasonal variation. The
higher summer temperature will be likely to encourage a richer fauna
and flora; the low winter temperature will be unfavourable to life.
An exceptionally severe winter may cause some of the smaller lochs
of the first class to be for a time transferred to the second.
We are not in a position to trace the actual relation of the lifechanges throughout the year to the changes of temperature, except
in the case of Loch Ness, which may be taken as a fair type of the
first class. That loch was examined regularly for more than a
complete year. Loch Morar has been examined at all seasons, but
irregularly and at long intervals. Several lochs which freeze in winter
have been examined in midsummer and in midwinter.
In the great lakes which never freeze there is no very marked decrease
in the quantity of organisms in winter. Many of the species persist
all the year round ; but as those which are absent in winter are the
most conspicuous of the Crustacea, the difference between the winter
and the summer plankton appears rather striking.
Holopedium
Polyphemus, Bythotrephes and Leptodora are all absent during a part
of the year.
Cosmopolitan Element in the Plankton.—For information as to
the general plankton of Europe and other regions I am mainly
indebted to various papers by Dr Wesenberg-Lund, who has made
wider comparative studies of plankton.
Only those species which are generally distributed over Scotland
can be taken into account in comparing the plankton of Scotland
with that of Europe generally. The animals which are dominant or
common both in Scotland and the rest of Europe are: Diaptomux
gracilis, Daphnia hyalma, Diaphanosoma brachyurum, Leptodora
kindtii, Conochilus unicornis, Asplanchna priodonta, Polywthra
platyptera, Anurtva cochlearis, Notholca longispina Cerathnn hirundinella, and Asterionella
All of these, according to Dr Wesenberg-Lund, belong to the
general plankton association of the great European plain, or are
even cosmopolitan.
PECULIARITIES OF THE SCOTTISH PLANKTOH
The Scottish plankton differs from the plankton of the Central
European plain and from the cosmopolitan fresh-water plankton
in several respects. The most striking peculiarity is the extraordinary
richness of the phytoplankton in species of Desmids, shared only, in