Náttúrufræðingurinn - 2002, Side 39

Tímarit Hins íslenska náttúrufræðifélags
Skilaboðin geta verið með ýmsu
móti. Til dæmis má með honum
vekja athygli á ýmsu er varðar líf og
lífsskilyrði í vötnum eða á jörðinni í
heild. Kúluskíturinn á það nefnilega
sameiginlegt með jörðinni að vera
grænn og hnattlaga og þurfa að snú-
ast til að njóta birtu allan hringinn.
SUMMARY
Lake balls, or Marimo, in Lake
Mývatn
Lake balls in Lake Mývatn were first
observed by scientists in 1977. This
growth form of the green alga Aega-
gropila linnaei forms densely packed
colonies in well defined patches on the
lake floor at depths ranging between 2
and 2.5 m. To our knowledge, such
colonies only exist in one other lake,
Lake Akan on Hokkaido, Japan, where
they are strictly protected as a "special
natural treasure" and are a tourist
attraction.
Other growth forms also exist in
Lake Mývatn. They include "free-float-
ing filaments", i.e. small, loosely
packed tufts that form an unattached
mat on the muddy lake bottom in
association with Cladophom glomerata.
Another growth form, recently found
in the lake, is growing on rocks in the
littoral zone. This form is also known
from Lake Þingvallavatn in Iceland.
The lake balls grow to the size of 10-
12 cm diameter in Lake Mývatn and
more than double that size in Lake
Akan. They can form two, three, or
even more layers on the lake bottom.
Large lake balls are also known from
Lake Öisu in Estonia. Small lake balls
(about 5 cm in diameter) occur in two
other lakes in Iceland (Lakes Kringlu-
vatn and Snjóölduvatn).
The lake ball patches of Lake Akan
and Lake Mývatn certainly rank among
the strangest plant communities world-
wide. Their existence depends not only
on very special adaptation but also on
dynamic interaction between currents,
light and sediment accumulation. The
spherical form has an unfavourable
surface:volume ratio which means that
the plants need good light conditions to
grow and can "only" reach a theoretical
maximum width of 30 cm. Their round
form reduces the risk that the balls get
covered in sediment, not least because
they tend to roll back and forth by
wave action in stormy weather. The
balls are green all way round which gu-
arantees that they can photosynthesize
no matter which side is turned
upwards. Inside, the ball is also green
and packed with chloroplasts which
become active in matter of hours in
case the ball breaks apart.
The regeneration of lake balls in
Lake Mývatn is poorly known. They
might grow from tufts detached from
the littoral zone or from fragments of
broken balls. The rapidly declining
population of lake balls in Mývatn is of
special concern, but for unknown rea-
son some of the main patches have all
but vanished in recent years. At Lake
Akan a great effort is spent on the con-
servation of the lake balls. This inclu-
des an annual three-day marimo festi-
val in which the Ainu people, the indig-
eneous people of Hokkaido, play an
important part. Because of their app-
ealing appearance the lake balls also
serve as a medium for environmental
education. They bear a certain likeness
to the Earth in being green and round
and in their need to rotate in order to
receive light on all sides.
Heimildir
Arnþór Garðarsson, Árni Einarsson, Gísli Már Gíslason, Guðmundur V.
Helgason & Jón S. Ólafsson 1987. Yfirlitskönnun á botnlífi Mývatns.
Rannsóknastöð við Mývatn, skýrsla 3. Náttúruverndarráð, fjölrit 18.
57 bls.
Arnþór Garðarsson & Árni Einarsson 1991. Lífið á botni Mývatns. Bls.
190-217 í: Náttúra Mývatns. Ritstj. Arnþór Garðarsson & Árni Einars-
son. Hið íslenska náttúrufræðifélag, Reykjavík.
Árni Einarsson, Hlynur Óskarsson & Hafliði Hafliðason 1993.
Stratigraphy of fossil pigments and Cladophora and its relationship
with tephra deposition in Lake Mývatn, Iceland. Journal of Paleolim-
nology 8. 15-26.
Árni Einarsson, Jón S. Ólafsson, Arnþór Garðarsson & Gerður Stefáns-
dóttir 1994. Cladophora í Syðriflóa Mývatns. Umhverfisráðuneytið,
fjölrit. 30 bls.
Gunnar Steinn Jónsson 1992. Photosynthesis and production of epilithic
algal communities in Thingvallavatn. Oikos 64. 222-240.
Hanyuda, T. & K. Ueda 2002. New views of phylogenetic relationship
between Marimo (Aegagropila linnaei) and some species of Cladoph-
orales. Algae 2002. 26th annual and 50th anniversary congress of Japa-
nese Society of Phycology and 3rd Asian Pacific Phycological Forum,
Joint Conference Ábstracts: 55.
Helgi Hallgrímsson 2002. Vatnaskúfur, vatnadúnn og vatnabolti Cladoph-
ora aegagropila. Náttúrufræðingurinn 70.179-184.
Hunding, C. 1979. The oxygen balance of Lake Mývatn, Iceland. Oikos 32.
139-150.
Nagasawa, Sv I. Wakana & M. Nagao 1994. Mathematical characterization
of photosynthetic and respiratory property regarding the size of
Marimo's aggregation. Marimo Research 3.16-25.
Pétur M. Jónasson, ritstj. 1979. Ecology of eutrophic, subarctic Lake Mý-
vatn and the River Laxá. Oikos 32.
Vokohama, Y., M. Nagao, I. Wakana & T. Yoshida 1994. Photosynthetic
and respiratory activity in the inner part of spherical aggregation of
"Marimo". Marimo Research 3. 7-11.
Yoshida, T., M. Nagao, I. Wakana & Y. Yokohama 1994. Photosynthetic
and respiratory property in the large size spherical aggregations of
"Marimo". Marimo Research 3.1-6.
Yoshida, Tv T. Horiguchi, M. Nagaov I. Wakana & Y. Yokohama 1998.
Ultrastructural study of chloroplasts of inner layer cells of a spherical
aggregation of "Marimo" (Chlorophyta) and structural changes seen in
organelles after exposing to light. Marimo Research 7. 1-13.
PÓSTFANG HÖFUNDA/AUTHORS' ADDRESSES
Ámi Einarsson
Marianne Jensdóttir
myvatn@hi.is
Náttúrurannsóknastöð við Mývatn
660 Mývatn
UM HÖFUNDANA
Árni Einarsson (f. 1954) lauk BS-prófi í líffræði frá Há-
skóla íslands 1976 og doktorsprófi í vistfræði frá há-
skólanum í Aberdeen í Skotlandi 1986. Hann hefur
starfað við Mývatnsrannsóknir frá árinu 1975 og er nú
forstöðumaður Náttúrurannsóknastöðvarinnar við
Mývatn.
Marianne Jensdóttir (f. 1977) lauk BS-prófi í líffræði frá
Háskóla Islands 2001. Marianne leggur nú stund á
meistaranám við sama skóla og er samband kúluskíts
og botndýralífs aðalviðfangsefni hennar.
39