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SUMMARY
The bottom of Lake Mývatn:
Past, present and future.
by
Arni Einarsson
Institute of Biology
Grensásvegur 12,
108 Reykjavík.
The history and function of the bottom of
Mývatn in the lake’s ecosystem is
described. The present lake had a precurs-
or similar in area but deeper. The present
lake came into being about 2300 years ago
when the precursor lake was overrun by a
lavaflow from the Threngslaborgir/
Lúdentsborgir crater row. Sediment from
the precursor lake can be found underlying
the sediment formed in the present lake,
as well as mixed with scoria in pseudo-
craters to the south of Lake Mývatn.
Palaeolimnological investigations have
shown that the green alga Cladophora
aegagropila increased considerably in the
early 17th century. The most probable
cause is a gradual decrease in water depth
due to sedimentation. C. aegagropila is a
dominant macrophyte on the bottom of
the Syðriflói basin and serves as a subst-
rate for invertebrates, i.a. Eurycercus
lamellatus, which is important as food for
diving ducks (Anatidae) and arctic char
(Salvelinus alpinus).
The importance of the benthos in
maintaining the exceptionally high
diversity of waterfowl is discussed in the
light of spatial and temporal heterogeneity
of the benthos. The various species of
waterfowl concentrate in different areas of
the lake according to their food prefer-
ences, stage in their own breeding cycle
and stages in the life cycles of their prey
species. The role of long-term temporal
variation in maintaining diversity in the
ecosystem (the “intermediate-disturbance
hypothesis") is discussed.
The bottom sediments of the lake are
currently being mined for the production
of diatomite. About 25% of the bottom
area of the Ytriflói basin has been stripped
of sediments. The possible effects of this
mining are poorly known. The lake is
shallow and the entire bottom is therefore
accessible for waterfowl feeding. The most
direct effect of mining is a deepening of
the lake, thus making extensive feeding
grounds inaccessible to several waterfowl
species.
Other effects concern Cladophora.
Because Cladophora is not attached to the
bottom this important macrophyte is prob-
ably vulnerable to changes in bottom
topography caused by the mining activity.
Certain bottom areas are clearly very
important for wildlife conservation. These
include (1) spawning sites of arctic charr
and trout (Salmo trutta)-, (2) areas that are
icefree in winter and hence important to
the Barrow‘s goldeneye (Bucephala islan-
dica)\ (3) major brood-rearing areas; (4)
areas where large flocks of ducks occur
regularly; (5) focal areas of common scoter
(Melanitta nigra), (6) moulting sites of
whooper swans (Cygnus cygnus) and (7)
gadwall (Anas strepera).
More research is urgently needed in
order to determine how much of the sedi-
ments can be mined without harmful
effects on this famous ecosystem.
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