Náttúrufræðingurinn - 2020, Blaðsíða 93

Tímarit Hins íslenska náttúrufræðifélags
93
Samkvæmt athugunum árið 2017
höfðu framangreindar þörungategundir
þá náð sér aftur nokkuð vel á strik.59
Vegna óvissu um framhaldið er mik-
ilvægt að fylgjast áfram grannt með
þróun þörungaflórunnar í vatninu, rétt
eins og á við um vöktun Þingvallavatns
í heild. Ástand vistkerfisins nú kann að
vera viðkvæmara og sveiflukenndara en
áður vegna hlýnunar og annarra sam-
verkandi þátta. Til þessa benda einnig
niðurstöður vöktunar á svifdýrafánu
Þingvallavatns sem greint er frá annars
staðar í þessu hefti.60
ENGLISH SUMMARY
WARMING OF LAKE ÞINGVALLAVATN
AND THERMAL PROCESSES IN THE LAKE
In this study we account for the ther-
mal evolution of Lake Þingvallavatn,
SW-Iceland, on a yearly basis over a 55
year period, during 1962–2017. We also
look on thermal stratification processes
in the lake, with monitoring data availa-
ble for the period 2011–2016.
Lake Þingvallavatn is the second larg-
est lake in Iceland, 83 km2, 2.9 km3 in vol-
ume, with a mean depth of 34 m and max
depth of 114 m. The lake is fed mainly by
spring-water inlets, amounting to ca. 90%
of the total inlet, 100 m3/s. The catchment
is 1,300 km2, composed largely of lava
bedrock from Holocene, and reaching
north on to the glacier Langjökull in the
highlands. Presently, the glacial compo-
nent of the lake´s inlet is estimated to be
15–20 m3/s, and the water is estimated to
take a decade or so to reach the lake. Most
of the spring-water entering the lake is
2.7–4.0°C all year round. The retention
time is estimated to be around 330 days.
Measurements of water tempera-
ture at the lake outlet were obtained at
two stations from the National Power
Company of Iceland, with a minimum
of 1–4 records per day every year over
the period 1962–1994 and 2000–2017
(Fig. 1), covering 47 years. During
2000–2017, measurements were done
24–48 times per day. Analyses of lake
temperature were done on daily means,
usually calculated from 3–48 recordings
per day, and daily means were used to
calculate monthly and annual means.
In all, 304.678 temperature recordings,
giving 17.153 daily means, were used for
comparison of annual means over the
47-year period with recordings.
To verify the reliability of the lake
outlet temperature as an indicator of
the water temperature within the lake
itself, we performed linear regression
analyses on temperature recordings
performed simultaneously at the outlet
(station LV2) and at the pelagic station
(NK2) in middle of the lake (Figs. 1 and
2, Table 1). At the pelagic station, meas-
urements were done by data loggers at 1
hour interval, at 4, 8, 16, 24, 32 and 40 m
depth during 5.6.–24.10. 2012, 6.6.–14.10.
2013, 9.5.–24.10. 2014 and 23.5.–25.10.
2015. This data, in all 596 measure-
ments, defined the data set for the linear
regression. We also used measurements
from the data loggers to analyse thermal
stratification development in the lake in
the period 2011–2016.
Temperature of Lake Þingvallavatn
has increased significantly for the
past 30 years, from the end of the cold
wave that lasted between 1965–1986
and onwards, congruent with a rise in
air temperature in the catchment area
(Figs. 3 and 4). Annual mean lake tem-
perature has risen on average by ca.
0.15°C per decade, similar to warming
observed in other large, deep lakes in
the northern heimisphere.13,43
Temperature has risen in all months,
except February-May, with the most
profound warming occurring in sum-
mer (June-August), with an increase of
1.3–1.6°C per month on average during
1962–2016, closely followed by autumn
and winter (September-January), with
a rise of 0.7–1.1°C on average per month
(Table 2). Because of warming, freez-
ing of Þingvallavatn occurs less fre-
quently than before (Fig. 6). If the lake
freezes, it does so nowadays later in the
winter and ice breaks up sooner in the
spring than 30 years or so ago. Warm-
ing of the lake may also have resulted
in stronger thermal stratification of
the lake, as observed by greater tem-
perature difference between epi- and
hypolimnion during 2010–2016 com-
pared to 1974–1981.
The ecological consequences of
warming of Lake Þingvallavatn are dis-
cussed, some of which may already have
been verified, e.g. by increased primary
production in the lake as measured by
chlorophyll-a concentrations.16,26,29 Also,
in relation to warming and response
of primary producers, nitrogen loads
to Lake Þingvallavatn appear to have
increased, measuring 29–42 μg/l NO3
in inlet springs of the lake in 1975,
compared to 27–68 μg/l NO3 in the
years 2007–2016.39 The aforementioned
changes are in line with those observed
in freshwater ecosystems elsewhere at
northerly latitudes.15,46,53–55
Recent changes, without precedent,
have also taken place in species com-
position of phytoplankton in Þingvalla-
vatn. Populations of large diatomes of
the genus Aulacoseira (A. islandica and A.
italica), along with Asterionella formosa,
the principal species in primary pro-
duction in the lake for decades, suddenly
crashed in 2016 and the species were
not found in monitoring samples.26,59
Similar changes in lake diatom commu-
nities in northern latitudes have been
ascribed to concurrent effects of warm-
ing and eutrophication.57,58