Íslenskar landbúnaðarrannsóknir - 01.03.1972, Side 122

120 ÍSLENZKAR LANDBÚNAÐARRANNSÓKNIR
S U M M A R Y
LIMNOLOGICAL STUDIES AT MÝVATN
Geir Gigja
Agricultural Research Institute, Reykjavik, Iceland
Limnological investigations were carried out at Lake Mývatn during the years
1942 to 1956 and 1960 to 1964, mainly in the vicinity of Grímsstadir at the
northern end (Fig. 1).
Of the insect life at Mývatn, the Chironomidae has the greatest number of
species. Fifteen species of Chironomidae have been found there, and the abund-
ance is tremendous. Because of this abundance, the Chironomidae is of great
importance as nutrition for the bird and fish population of the lake. The
Chironomidae spends its winter life in the lake in the larva stage. The pupa,
and then the adult stages take over with the coming of spring.
The Chironomidae at Mývatn usually first appears in the latter half of May.
However, there have been occasions when it has not been observed before
June. The Chironomidae disappears in September and October (Fig. 2.). The
swarms may last for one day at a time, or up to periods of a whole consecutive
week. These may be so numerous and dense that they give the impression of
clouds obscuring the sun.
During summer Lake Mývatn becomes opaque in various places, largely due
to the presence of Anabaena flos aquae.
Although Simulidae are often mentioned in connexion with Mývatn, they
are not native to the Lake, but come from the running waters in the vicinity,
such as the River Laxá, where they breed.
Lake Mývatn usually becomes covered with ice in September—October and
ice-free in May. However, on 25th May 1949 there was ice one metre thick in
places, and in that year the ice did not leave the lake until 21st June.