lnp logoLNP LOGO
Home Archive
Loch Ness
Loch Ness
Loch Ness Explorers
& Links
Loch Ness

Loch Ness An Underwater World

Click the buttons for pictures and a description of life and events beneath the surface
Back to Explore Loch Ness

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.

Back to Explore Loch Ness