Fault-line origins have wrought a trench-like basin of remarkable
uniformity and depth, with steep rocky walls sloping to a
flat silt bed. A maximum depth of 230m (754ft) was found by
Sir John Murray's Bathymetrical Survey of 1903 which varies
little from a depth of 227m (745ft) recorded during the hydrographic
survey by the Loch Ness Project in 1991. This depth is second
only to Loch Morar (310m, 1017ft) among the British lakes.
The catchment area, of 1,775 square kilometres, is mostly
hard rock and yields few chemical nutrients to the dark peaty
water entering the loch by seven main rivers and about 100
streams.
The lack of nutrients, such as nitrates and phosphates, is
just as important as on land. These nutrients are the essential
fertilisers for plant growth (photosynthesis); feeding grass
on the land and tiny algae in water. The amount of this "primary
productivity" forms the base of the "food chain"
and normally determines the amount of animal life any habitat
can sustain. In fact lakes are sometimes divided into two
groups according to the amount of nutrients in them; eutrophic
(nutrient rich) and oligotrophic (nutrient poor) lakes like
Loch Ness. This has further consequences not only upon how
much life can exist, but which regions of the lake it can
occupy.
In any lake, the water stratifies in summer. This means that
as the upper water (the epilimnion) warms, it becomes less
dense and floats on the colder water (the hypolimnion) beneath.
They are separated by a zone of rapid temperature change called
the thermocline. Thus the "upper" lake is separated
from the "lower" lake until cooled and mixed again
during the winter gales. Photosynthesis is limited to the
upper part of the epilimnion where light can penetrate and
once the nutrients are used, they cannot be replaced from
the hypolimnion beneath until mixing occurs. The algae therefore
die. In shallow eutrophic lakes the decay of organic matter
descending into the hypolimnion uses up the oxygen there,
much to the detriment of deep water life. However in deep
oligotrophic lakes, such as Loch Ness, these profound effects
do not occur since nutrients are already low in the epilimnion
and there is little deoxygenation of the vast hypolimnion
which remains over 80% oxygen saturated. Therefore, although
Loch Ness may not be very productive it has the compensation
of stability. It is spared the seasonal booms and crashes
of more productive waters and a variety of life, including
fish, can survive in even its deepest regions.
Loch Ness is so large that the summer's warmth does not entirely
leave it until well into the next spring, in the meantime
melting the snow along its shores. For the same reason however,
the loch takes a long time to heat and no summer warms more
than the upper ten metres to more than about 15oC and only
the top few centimetres to 20oC. The loch is relatively cold
from a biological point of view and many of its inhabitants
are "relicts" from glacial times.
The illustration shows a sonar profile across the loch and
although the depth scale is exaggerated, the regularity of
trench shape is entirely real. We have added the colour to
emphasize the "Thermocline" which is the transition
where the warm upper layer of water floats on the more dense
colder water beneath.
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