The Botany of Iceland - 01.12.1914, Side 136

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THORODDSEN
lowing mosses are found on stones in rivulets and brooks: Fonti-
nalis antipyretica verv frequently, also F. gracilis and F. thulensis,
Ambtystegium Kneiffii, A. ochraceum, Hypnum rusciforme var. attantica,
and others1. The vegetation of lakes and pools is much richer
and differs considerably according to the depth of the water and
the nature of the bottom, etc. The plankton of the Icelandic lakes
has as yet been very little investigated; there are only a few notes
to hand from Myvatn and Thingvallavatn. In Myvatn zooplankton
only was found; Thingvallavatn contained phytoplankton in which
diatoms were dominant2. In deep lakes tliere is usually very little
or no vegetation at greater depths, only in places where it is shal-
lower does plant-life occur. In Thingvallavatn, however, there are
large areas covered with Chara and Nitelta, especially at a depth
of 13—30 metres, and they extend even down to 38 metres3. Where
the lakes are shallower various species of Potamogeton and Myrio-
phyllum and also Batrachium paucistamineum are common. Near
the margin and in smaller pools the most common, and usually
dominant, species are the following: Heleocharis palustris, Equisetum
timosum, Carex rostrata, Menyanthes trifoliata, and in the southern
lowlands Glyceria ftuitans is common; to these should be added
Hippuris vulgaris, Eriophorum angustifolium, Sparganinm minimum,
S. submuticum, Ranunculus hyperboreus aud R. reptans, Subularia
aquatica, Callitriche hamulata and C. verna, Limosella aquatica and
several others, the occurrence of which varies somewhat according
to the quantity of the water, the conditions at the bottom, elc.
Where Equisetum limosum occurs in abundance it is cut annually
and used for fodder for milch cows. In Myvatn and in other lakes
in Thingeyjarsysla NostocAumps are found in abundance, often thrown
up on the shore in very great quantities. Sometimes in warm sum-
mers large areas of Myvatn become turbid; this phenomenon is
known by the inhabitants as “leirlos” — it is said to be very injurious
to salmon-trout, their gills becoming filled with the fine parlicles —
when this occurs they retreat in great numbers to the eastern shore of
the lake, where the water is purer and clearer owing to the numer-
ous springs which here issue from the lava4.
1 H.Jónsson, 1900, p. 17, 1905, p. 7.
2 C. H. Ostenfeld and C. Wesenberg-Lund: A Regular Fortnightly Explor-
ation of the Plankton of the two Icelandic Lakes, Thingvallavatn and Myvatn.
(Proc. R. Society Edinburgh, Vol. 25, Part 12, 1906, pp. 1092—1167).
8 B. Sæmundsson in Andvari, 1904, p. 89.
4 Dr. H. Jónsson informs me that the so-called "leirlos" is probably due to
Blue-green Algæ perhaps Aphanizomenon ftos aquœ.