Náttúrufræðingurinn - 1978, Side 56

Stefánsson, U. and F. A. Richards. 1963:
Processes contributing to tlie nutrient
distribution off tlie Columbia River
and Strait of Juan de Fuca. Limnol.
Oceanog. 8, 394—410.
Strickland, J. D. H. and T. R. Parsons.
1968: A Practical Handbook of Sea-
water Analysis, Bullet. 167, Fisheries
Research Board of Canada, Ottawa,
311 pp.
Unesco. 1966: Determination of photo-
synthetic pigments in seawater.
Monographs on oceanographic me-
thodology I, Paris, 69 pp.
Unesco. 1973: Tables of oxygen solubility
in sea water. International oceano-
graphic tables, Vol. 2. National Insti-
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S U M M A R Y
Lake Miklavatn in Fljót,
North Iceland
by Dr. Unnsleinn Stefánsson
University of Iceland and
Marine Research Inslitute,
Reykjavík, Iceland
and Dr. Björn Jóhannesson
Laugarnesvegi 110, Reykjavík
Lake Miklavatn in Fljót has a surface
area of 6.6 km2, a volume of 69 X 10° m3,
and a mean deptli of 10.5 m. The month-
ly discharge into the lake varies between
4 and 25 m3/sec, with a mean value of
10.3 m3/sec.
In former times seawater coukl flow
freely into tlte lake through its estuary
at liigli tide, and as a result of tliis ex-
change of seawater between the deeper
layer of the lake and the sea outside,
various species of marine fish were com-
mon in the lake. At the present time,
however, the channel is no longer deep
enough to permit inflow of seawater.
Consequently, the saline deeper layer in-
side the lake has become stagnant and
anoxic.
Preliminary chemical observations were
carried out in August 1976. In late June
1977 more detailed observations were
made at 13 stations, and in August 1977
studies at the deepest station (St. 4, depth
~ 20 m), were repeated (Fig. 1, Table
II). The results reveal several interesting
features:
1) There is a very sharp vertical gra-
dient between fresli water on top and
salt water of about 26—27%c salinity be-
low a depth of 8—9 meters. Mixing across
tliis halocline must be very small, and
the renewal of lake water is therefore
limited to the upper fresh-water layer.
The flushing time of this 7—9 m thick
layer ranges between i/2 and 3 months.
2) Seasonal temperature variations ap-
pear to be confined to the upper fresh-
water layer, where a temperature mini-
mum of about 4° C is found in June, just
above the halocline.
3) Oxygen concentration is near or
above saturation in the fresh-water layer,
with maximum values occurring just
above the lialocline. The concentration
drops to zero across the halocline, and in
the deeper saline layer, relatively high
concentrations of sulfide are present.
4) Diatoms were found mainly in the
uppermost 5 metres and were much more
abundant in June than in August 1977,
whilst the number of flagellates was much
greater at 7.5 and 8 meter depths than
at the surface. Judging from chlorophyll
measurements, pH, oxygen saturation
values and nutrient concentrations, it is
concluded that plant production was re-
latively modest in June and August 1977,
but may have been intense in August
1976.
5) Alkalinity is relatively low in the
fresh-water layer, but increases rapidly
with deptli in the saline deeper layer,
wliere it reaches values of more tlian 6
meq/L near the bottom. Taking into
account expected conservative alkalinity
due to mixing betwetn rivcr water and
seawater, the excess alkalinity of the
deeper layer is estimated to range be-
tween 1 and 4 meq/L. The possible
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