FRESH-WATER PLANKTON 
337 
of Ceratium hirundi?iella, and finally a specially rich development of 
Chlorophycea^. 
For North Uist and Benbecula, an abundance of Dhiptomus 
ivierzejshii and apparently absence of Diaptomus laciniatus ; otherwise 
much like Lewis. The plankton material from Mull and the small 
island of Lismore is too small to justify an exact comparison with the 
above-mentioned areas, but I shall refer to it later on. 
Li the four areas which we have just distinguished the lochs diff'er 
somewhat in physical features: — (1) The Shetland lochs are mostly 
shallow, some few are relatively deep, and thev are elevated 20 to 300 
feet above sea-level ; (2) the Orkney lochs are all broad, shallow, and 
flat-bottomed, and they are elevated about 50 feet ; (3) of the lochs 
surveyed in Lewis, the greater part are relatively deep, some few are 
shallow, and they are elevated 100 to 400 feet above sea-level ; (4) 
the North Uist and Benbecula lochs are nearly all shallow, a few are 
of moderate depth, and they are elevated only a few feet above sea- 
level. As regards the temperature of the waters in these same areas, 
there appears to be a small but decided difference, the Shetland lochs 
being colder than those of the other areas. In the following table I 
have given the surface temperatures taken in all the lochs of the islands 
which were visited by the Survey. As will be seen, the Lewis and 
Shetland lochs were surveyed contemporaneously, and at that time the 
Shetland lochs were superficially several degrees colder than those of 
Lewis ; and I am supposing also that in general a like difference in 
thermal conditions would obtain for all the upper strata where the 
plankton organisms live. In North Uist and Benbecula the survey 
commenced in the second week of May, when the surface temperature 
was about 49° Fahr. After that a rapid warming of the waters took 
place, and by the fourth week in May the lochs had a surface tempera- 
ture of over 60° Fahr., and early in June a maximum reading of 68° 
Fahr. was recorded. It should be noted, however, that the difference 
in the whole thermal conditions of these lochs is often not so great as 
would seem to be the case from a consideration of surface temperature 
alone, for in the heated lochs of early June there was actually found to 
be a very rapid fall of temperature in passing from the surface to the 
lower layers of water, in several cases a range of 16^ Fahr. occurring 
in a vertical distance of only 20 or 25 feet. On the other hand, the 
colder lochs of early May had a more or less uniform temperature 
from top to bottom ; further, it will be seen that the deeper lochs 
remained cooler than the shallower lochs of the same neighbourhood. 
So in all the areas under consideration, especially in North Uist, the 
tow-nettings were taken from lochs differing quite appreciably in their 
thermal conditions, and later on we shall see to what extent the 
plankton varies accordingly. 
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