Náttúrufræðingurinn - 2010, Blaðsíða 65

145
Tímarit Hins íslenska náttúrufræðifélags
vatnspest (Elodea canadensis), tjarna-
kræklu (Elodea nuttallii), linda-
hrukklu (Hydrilla verticillata), mara-
tegundina Myriophyllum aquaticum,
hrukkunykru (Potamogeton crispus)
og agnartegundina Crassula helmsii
en fjórar fyrstnefndu tegundirnar
líkjast mjög kransarfa í útliti enda af
sömu ætt. Á grundvelli upplýsinga
um lífshætti, útbreiðslumynstur og
hitaþol þessara tegunda, m.a. í þeim
heimildum sem vísað er til, má ætla
að meðal slíkra plantna kunni að
leynast tegundir sem eigi síst minni
möguleika á að ná hér fótfestu en
kransarfi.
Summary
Brazilian Waterweed in Opnur
Springs in Oelfus, Iceland
Brazilian waterweed (Egeria densa) was
recently found in the geothermal Opnur
springs (63°58.68’N, 21°10.70’W) situat-
ed in the northern fringes of the wet-
land area Oelfusforir, in Oelfus, South-
Iceland. This is the only known location
for this species in Iceland and probably
also the one farthest north. The water-
weed has become naturalized there and
is the dominant macrophyte in the
main water body of the springs, Opnur
pond (area 9,800 m2, depth mostly 0.5–
1.0 m), producing dense mats, also dur-
ing winter when other vegetation is
dormant. Patches of the waterweed oc-
curred in the first 2 km of the approxi-
mately 6 km long outlet ditch, mostly in
the downstream part. The outlet brook
runs into the shared estuary for the riv-
ers Oelfusa and Varma, the former gla-
cially fed and the latter receiving geo-
thermal water. The growth characteristics
and the general life history of the water-
weed in Opnur springs were studied
and the temperature was monitored.
For a period of 12 months the water
temperature was recorded every hour
and the distribution and the percentage
coverage of the waterweed were
mapped visually twice a month. In win-
ter the Brazilian waterweed grew in
monospecific stands but had consider-
able competition in other seasons, espe-
cially from filamentous algae. Perfoliate
pondweed (Potamogeton perfoliatus)
formed only discontinuous stands and
canopies in the summer. Before the ar-
rival of the waterweed the pondweed
was the dominant plant species in
Opnur springs. Presently it is still com-
mon there and dominates in the upper-
most part of the outlet ditch as well as
in the smaller water body of Opnur
springs called Sundlaug (area 471 m2,
depth 0.3 m). In the Opnur pond the
mats of filamentous algae were most
prominent during summer. In a small
cove in the NV part of Opnur pond,
where the Brazilian waterweed did not
form stable canopy at the surface, the
mean water temperature was 23.6°C
and the monthly means were 22.3–
25.8°C. In the SV part, at the location
where the waterweed grew most vigor-
ously, the mean temperature was 18.3°C
and the monthly means were 14.2–
22.8°C. In the pond outlet, about 870 m
downstream, the mean temperature
was 14.8°C and the monthly means
were 11.5°C–20.3°C. In the summertime
Icelandic water bodies commonly reach
surface temperatures of 10–15°C. The
establishment of the waterweed in
freshwater localities in Iceland not in-
fluenced by geothermal waters, wheth-
er it is in this watershed or elsewhere, is
therefore not excluded but is probably
not likely. Although other suitable geo-
thermally influenced freshwater bodies
most likely exist in Iceland, they are
relatively few and for the most part far
between. Therefore further coloniza-
tion of this species might proof difficult
in Iceland in the present climate.
Þakkir
Háskólasetrið í Hveragerði lagði til aðstöðu, vinnu og tæki vegna verkefnis-
ins. Heiða Gehringer annaðist hluta vettvangsvinnu. Umhverfis- og orku-
sjóður Orkuveitu Reykjavíkur styrkti verkefnið. Christopher Mitchell og
Chrystina Montuori Sorrentino aðstoðuðu við útfærslu á teiknivinnu.
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