Náttúrufræðingurinn - 1985, Side 43

SUMMARY
The biology of the blackfly
Simulium vittatum Zett.
(Diptera: Simuliidae)
in the River Laxá, northern
Iceland.
by
Gísli Már Gíslason,
Institute of Biology, University of Iceland,
Grensásvegur 12, 108 Reykjavík, Iceland
and
Vigfús Jóhannsson,
Department of Zoology, University of
Newcastle upon Tyne, NEl 7RU, England.
River Laxá, 40 m3/s, is the outlet of the
productive Lake Mývatn and is one of the
most productive rivers in Iceland (Figs 2-
3). The lake owns its productivity to nutri-
ent rich springs (Ólafsson 1979b) that feed
it and solar radiation of 77,000 kcal/cm2/
year (Einarsson 1979). The conditions of
the lake control the productivity of the
river.
The study focused on the the population
dynamics of the blackfly Simulium vitta-
tum, and which factors controlled its
density and production. The study area
was the upper part of the river, above the
Laxárvirkjun hydroelectric power station.
Three sampling sites were chosen as repre-
sentative for the river, Midkvísl, in the
outlet, Helluvad, 4 km from the lake, and
Thverá, 22 km downstream (Fig. 2).
S. vittatum dominates the fauna of R.
Laxá (Fig. 5) (Gíslason 1985). It belongs
to 2 sibling species, IIIL-1 and IS—7
(Klaus Rothfels in lit.). It is about 70—
85% of the secondary production in the
river. It is partly a univoltine species. In
the upper reaches of the river (close to the
outlet) each generation was clearly divided
into 2 cohorts (Figs 1 & 6-7). One cohort
grew very rapidly over the summer, with a
life span of about 1-2 months. The other
cohort grew more slowly, with a life span
of one year. In the lower reaches of the
upper part of the river, only the slow—
growing cohort existed.
The larvae fed on fine particulate orga-
nic matter (fPOM) drifting from L.
Mývatn. There was no difference in the
composition of their gut contents and the
seston of the river (Figs 9 & 11). Usually
detritus was not less than 50% of the gut
contents (Fig. 9). Diatoms accounted for
10—70% of the gut contents in 1977 and
25—45% in 1978. The most frequently
ingested species was Fragilaria spp.
(mainly F. construens) (Fig. 11), which is
abundant in the bottom sediment of L.
Mývatn. Other frequently ingested
diatoms were Stephanodiscus hantzschii,
Nitzschia spp., Synedra spp. and Rhoicos-
phenia curvata. In August 1977 the blue-
green alga Anabaena flos-aquae was the
most frequntly ingested alga and it was
abundant in the gut contents in July (Fig.
9). In 1978 this alga was rarely found.
In 1978 to 1982, the quantity of fPOM
was reduced 30-60% (Fig. 8). This was
due to reduction of the primary production
in the phytoplankton in L. Mývatn (Aðal-
steinsson 1984), where it fell by 80-90%
in June to September. Before 1978 and
from 1983 A. flos-aquae formed an exten-
sive water bloom in the lake (Fig. 10), but
in the period from 1978 to 1982, it failed to
do so.
This led to changes in the life history of
S. vittatum. The relative proportion of the
fast-growing larvae in the summer
population fell from ca. 40% (1977) to
about 20% (1978—82) in the outlet and
nearly disappeared at Helluvad (Figs 6—
7). In 1983 drifting fPOM increased and in
1984 the quantity had reached the previous
level, with increasing proportion of the
blackfly larvae growing rapidly in the
summer.
Similar changes occurred in the produc-
tion of S. vittatum. In Midkvísl (outlet) the
generations 1976-7 and 1977-8, had a
production from 867 to 1231 g wet-weight/
m2/year but in 1978-82 the annual produc-
tion fell to 312—437 g ww/m2/year. There
was an increase in the production of the
1983—84 generation to 1504 g ww/m2/year
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