Náttúrufræðingurinn - 2020, Side 76

Náttúrufræðingurinn
76
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vallavatn. Undraheimur í mótun (ritstj. Pétur M. Jónasson & Páll Hersteinsson).
Mál og menning, Reykjavík.
2. Árný E. Sveinbjörnsdóttir & Sigfús J. Johnsen 1992. Stable isotope study of the
Thingvallavatn area. Groundwater origin, age and evaporation models. Oikos,
64. 136–150.
3. Jón Ólafsson 1992. Chemical characteristics and trace elements of Thingvalla-
vatn, Iceland. Oikos 64. 151–161.
4. Hákon Aðalsteinsson, Pétur M. Jónasson & Sigurjón Rist 1992. Physical charact-
eristics of Thingvallavatn, Iceland. Oikos 64. 121–135.
5. Freysteinn Sigurðsson & Guttormur Sigbjarnason 2002. Grunnvatnið til Þing-
vallavatns. Bls. 120–135 í: Þingvallavatn. Undraheimur í mótun (ritstj. Pétur M.
Jónasson & Páll Hersteinsson). Mál og menning, Reykjavík.
HEIMILDIRÞAKKIR
Umhverfisstofnun, Þjóðgarðurinn á Þingvöllum, Orkuveita Reykjavíkur og
Landsvirkjun kostuðu þetta verk. Við þökkum fulltrúum þessara stofnana og
fyrirtækja fyrir þann áhuga og stuðning sem þeir hafa sýnt rannsókninni. Einnig
þökkum við starfsmönnum Veðurstofunnar, sem staðið hafa að rannsókninni
með okkur, fyrir samstarfið í gegnum tíðina og má þar sérstaklega nefna þær
stöllur Svövu Björk Þorláksdóttur og Jórunni Harðardóttur. Héðinn Valdimars-
son sviðstjóri á umhverfissviði Hafrannsóknastofnunar fær sérstakar þakkir
fyrir að benda okkur á möguleg áhrif sólblettavirkni á frumframleiðni í Þing-
vallavatni. Kærar þakkir fá þeir Halldór Ármannsson og Hilmar J. Malmquist
fyrir yfirlestur á handriti greinarinnar og góðar ábendingar við vinnslu hennar.
SUMMARY
CHEMICAL CHARACTERISTICS
OF LAKE ÞINGVALLAVATN
Monitoring of the concentration
of dissolved elements and particulate
organic carbon and nitrogen has been
conducted in Lake Þingvallavatn since
2007. Samples were collected from two
springs in the northern part of the lake,
Silfra and Vellankatla, and from the outlet
of Lake Þingvallavatn at Steingrímsstöð
(the outlet). Here we present and discuss
the results from 2007 to 2014.
The concentration of total dissolved
solids in Silfra and the outlet was sim-
ilar, and lower in Vellankatla, which
indicates that most of the inflow to the
lake is from Silfra or other springs with
a similar chemical composition as Silfra.
The pH in the spring water was
between 9 and 9.5, which is typical for
spring water in basaltic bedrock due to
water-rock interaction in the bedrock in
the absence of atmospheric-, soil- or mag-
matic-derived CO2. The pH of the spring
water decreases in few minutes after it
comes in contact with the atmosphere.
The concentration of many metals
is related to dissolved oxygen concen-
tration and pH of the water. The oxy-
gen concentration affects the solubility
of metals like Fe and Mn. In reducing
conditions, the dissolved concentration
of these metals is relatively high, but
decreases when the water comes in con-
tact with the atmosphere. This is likely
to influence the metal concentration in
Lake Þingvallavatn but the concentration
of these metals is higher in the spring
water inflow than in the outlet water.
The concentration of the dissolved
nutrients silica, nitrogen and phospho-
rus was higher in the spring water inflow
than in the outlet of the lake due to
uptake by primary producers in the lake.
Silica diatoms use silica as the building
material for their shells, but nitrogen and
phosphorus are essential for photosyn-
thesis along with many other micro nutri-
ents. Nitrogen is a major nutrient and it is
the limiting nutrient in Lake Þingvallav-
atn since dissolved phosphorus is there
in a relatively high concentration in the
spring water, draining the soluble, glassy
basalt of the Icelandic volcanic zone.
Comparison between data collected in
1975 and the present study indicates a sul-
phur concentration decrease in the inflow
to Lake Þingvallavatn due to North-Amer-
ica and Europe’s restrictions on sulphur
emissions since the late 1970’s. The com-
parison also suggests a fixed nitrogen con-
centration increase in the spring-fed inflow,
resulting in a rising N/P ratio in the water,
which further demonstrates the increase
in the fixed dissolved nitrogen concen-
tration, the limiting nutrient in the lake.
Despite evidence of an increased
NO3 concentration in the spring water
since 1975 it is not possible to determine
changes in NO3 concentration at the out-
let of Lake Þingvallavatn since most of
the NO3 available is consumed by pho-
tosynthesis during the water's residence
time. However, a marked decline with
time was detected in the concentration of
the nutrients silica and phosphorus at the
outlet during 2007–2014, demonstrating
increased photosynthesis with time. The
silica concentration decrease in the lake
water is correlated to the annual number
of sunspots, which seems to be affecting
the diatom productivity. The increased
uptake of silica and phosphorus strongly
suggests increased primary production of
diatoms, either due to photosynthesis at
deeper levels in the lake, increased sup-
ply of fixed nitrogen in the lake due to
anthropogenic effects on the catchment
and/or increased production of N-fixing
bacteria in the lake. Primary production
of N-fixing bacteria is independent of the
fixed nitrogen concentration in the lake
since they can fix N2 from the atmosphere.
They use molybdenum, iron or vanadium
to produce enzymes to break the chem-
ical bonds of N2. The dataset collected
from 2007 to 2014 indicates a decrease
in vanadium concentration, possibly
because of an increased uptake of N-fix-
ing bacteria, which has been identified in
the lake. Recent studies show that there
is a symbiosis between N-fixing bacte-
ria and a few species of silica diatoms in
Lake Þingvallavatn, increasing the sup-
ply of nitrogen to the silica diatoms.
Increased influx of nitrogen into
Lake Þingvallavatn due to anthropo-
genic activities will cause increased pri-
mary productivity in the lake since there
is an excess of phosphorus in the water
with respect to the Redfield ratio, which
describes the nutrient ratio needed by
primary producers. Increased primary
production can cause deceased trans-
parency in the water which reduces
the penetration of light into the lake
and thus the primary production by the
benthic flora, which is important for
biological acitivity in the lake. Thus, the
influx of nitrogen has to be restricted
by reducing the direct influx from agri-
culture and sewage. A large part of the
nitrogen influx to the lake, however,
originates from long distance air masses
which bring transboundary pollution.
Increased nitrogen concentration in
the atmosphere causes increased influx
of nitrogen into freshwater systems
and can cause euthrophication in lakes
where nitrate is the limiting nutrient
for primary productivity. International
co-operation is needed to control atmos-
pheric nitrogen concentration, similar
to what was done regarding sulphur
emission restrictions in North-America
and Europe in the 1980's and carbon
emissions during the last decades.