Náttúrufræðingurinn - 1978, Page 57

sources o£ tliis excess alkalinity are con-
sidered. It is concluded that it is princip-
ally due to sulfide formed by reduction
of sulphate in the deeper layer.
6) In tlie fresh-water layer nitrate, ni-
trite and ammonia concentrations were
found to be very small. In the anoxic
lower layer nitrate was undetectable, but
nitrite was present in significant con-
centrations. Ammonia concentrations also
increased markedly at the interface be-
tween the two layers and remained rela-
tively high in tlie anoxic layer. Tliis in-
crease in ammonia is attributed to deni-
trification, which could also lead to
formation of free nitrogen. To assess this
nitrogen of biogenic origin, it is proposed
tliat the nitrogen/argon ratio in the an-
oxic water be determined.
7) Reactive silicate increases fom less
than 200 yug-at/L in the surface layer to
more than 500 /xg-at/1 in the deepest part
of the lake. Using measured value of the
silicate concentrations of the river water
(~ 164 fxg-at/L), assigning probable value
to the originally inflowing seawater (~ 7
/ug-at/L), and assuming the mixing be-
tween fresh water and seawater to be
linear in the absence of biological pro-
cesses, the silicate of biogenic origin was
computed. The mean ratio between the
excess alkalinity and the silicate of bio-
genic origin AA/ASi, was found to be
122/15, in good agreement with that ex-
pected from the ntodel of Richards (1959,
1965).
8) Of particular interest are the re-
markably high concentrations of phos-
phate (up to 80 /rg-at/L) found in the
anoxic layer. Phosphate seemed to be a
linear function of alkalinity and the ratio
of change, AP/AA, was found to be about
twice as large (1 /54) as that predicted
from tlie models of Richards (1965) and
Dyrssen and Gundersen (Skirrow 1975).
The possible sources o£ tlie anomalously
liigh phosphate concentrations are dis-
cussed.
9) On the basis of the available data,
it is impossible to determine liow long
the conditions liave remained anoxic in
Lake Miklavatn. It is known, however,
that during violent storms and heavy seas
in winter in 1936, the outlet to the sea
was obstructed and a new channel formed
farther to the east. l'he hypothesis is pre-
sented that the communication between
the deep water of the lake and the sea
has been blocked from this time.
10) It is proposed that the outlet be-
tween the lake and the sea be deepened
by removing a layer of loose gravel and
sand from the estuary near its head. In
this way seawater could enter the lake
and renew its anoxic deep water, thereby
greatly improving biological conditions in
the deeper part of the lake.
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