Fróðskaparrit - 01.07.2004, Síða 98

96
MERCURY IN THE FAROE ISLANDS — A REVIEW OF AVAILABLE DATA
The mercury concentration in brown
trout is similar to the common level in Arc-
tic Canada (Lockhart et al., 2001). It is still
elevated above the level in Finland (AMAP
1998) where brown trout in Finnish Lapp-
land was less than 0,1 mg/kg.
Several studies have been undertaken to
elucidate the factors influencing the uptake
of mercury from a watershed/lake. It ap-
pears that apart from the magnitude of the
áctual atmospheric mercury deposition, a
wet and boggy upland with high organic
matter content is a driving force for elevat-
ed fish mercury concentration in a lake
(Fjeld, 1993), and than the mobilization is
increased by disturbances not only from
flooding in connection with electricity pro-
duction, but also from other erosion-creat-
ing activity like agriculture and the use of
heavy terrain-going machinery (Gustin,
2002; St.Louis et al., 2004).
This indication that what would be
termed moderate watershed erosion in oth-
er circumstances could render the water
less suitable in monitoring the long range
transported (also called “background lev-
el”) pollutants is bad news for monitoring
in the Faroe Islands where there is no fish-
containing water where the watershed is
not being used for some sort of erosion-
creating activity like sheep grazing and
growing of crops. The natural erosion how-
ever, i.e. that driven by wind, temperature
and wet deposition, is significant and may
overshadow the anthropogenic initiated
erosion.
Sedimcnts from freshwater lakcs
During the summer-period of 2000, sedi-
ments were collected from different Far-
oese fresh water lakes as part of the Nor-
Lake project (Landkildehus et al., 2002).
Some cores were made available for retro-
spective mercury analyses where the sedi-
ments are cut in sections representing a cer-
tain time period as defíned by radiological
analyses, and where the mercury concen-
tration is measured in each slice (Table 4).
Sediment cores were taken from Sørvágs-
vatni and Leynavatni and each core was
sliced in 5 cm thick segments. Knowledge
about mercury concentration and the age of
the various sediment slices allows the
drawing a profile of the mercury pollution
from the time of the sedimentation of the
oldest slices in the core to the present time.
The uppermost slices of the sediment from
Leynavatni could not be dated because of
extensive bioturbation in these sections, so
the estimated sedimentation rate applies
only to the two slices at the bottom of the
core. Fig. 3 shows the mercury concentra-
tion in the sediment profile sampled in
Sørvágsvatn. To eliminate variability stem-
ming from the tendency of mercury to bind
to the organic fraction, the mercury con-
centration in the figure has been corrected
for the organic content of the actual seg-
ment by dividing with the loss on ignition
(in g/kg dw).
Comparing the mercury concentration in
the segments of two cores that corresponds
to sediments deposited approx. 100 years
ago, shows that the concentration in lake
Sørvágsvatn sediments was approx. twice
as high as in the Leynavatn sediments. Cor-
recting, however, both mercury concentra-
tions for the organic material content, ex-