GLOBAL warming will cause the bottom layers of Scotland’s deepest freshwater lochs to stagnate and rise in temperature, threatening their sensitive ecology, The Scotsman has learned.
In particular the survival of the Arctic char, one of our least known and most endangered fish species, could be put in jeopardy.
Freshwater lochs such as Loch Lomond, Loch Doon and Loch Morar have a layered structure where the surface and deep waters differ in temperature and density. To keep the bottom layers fresh, the layers have to mix, and for this to happen they have to reach a similar temperature and density for a time. This normally happens in winter when the surface waters cool.
But biologists at Glasgow University have found that if the climate warms as predicted, the surface waters will not cool sufficiently for the layers to mix.
The findings are corroborated by studies in France where research revealed that a mere 1C change in air temperature would be enough to upset the thermal mixture of famous continental lakes including Germany’s Lake Ammersee and Lake Annecy in France.
The fear now is that the diversity of Arctic char in Scotland, with lochs holding an estimated 200 distinct populations that have adapted to different environmental niches, could be reduced.
Such a loss would constitute an environmental tragedy, as the variety of Arctic char is the best example in Scotland of Darwinian evolution.
So unique is each of the 200 loch variants of char - a cousin of the salmon and trout - that some may be classed as separate species. In some instances the species in a single loch also show genetic variations.
Dr Colin Adams, an expert on Arctic char at Glasgow University’s Rowardennan Field Station by Loch Lomond, said: "This process of heat layering in lochs is known as thermal stratification. If the period in the year in which this stratification occurs changes, as it may well do with global warming, then Arctic char are likely to be threatened by the overall increase in temperature."
Arctic char will be put at risk because the warming of the deep water prevents their eggs from developing. They require temperatures below 7C to develop through to the hatching phase.
Dr Adams added: "We are in real danger of losing the genetic diversity of our species of Arctic char. It is the most southerly populations at low, sea-level altitudes in Scotland that are at greatest risk - such as Loch Doon in Ayrshire.
"The most important thing is to try to retain the diversity of the species by creating a refuge population of those that are most at risk.
"Obviously this would need to be done for the Loch Doon char, but there are a whole host of other populations which are relatively valuable because they seem to be very discrete from other populations."
Lack of oxygen caused by stagnation could also lead to many deep-dwelling crustaceans dying from suffocation.
Although Arctic char are not unique to Scotland, they have enjoyed a less threatened existence than elsewhere. In Scandinavia, Canada and the United States, the char have been put under pressure from angling, with waters having to be stocked afresh and fish transported from one lake to another. In Ireland, which has experienced significant problems from agricultural pollution, there have so far been at least six char extinctions.
Dr Adams said: "The physical way that lochs operate is set to change, and we need to take steps to prepare for that to preserve the diversity of our species."
Meanwhile, conservationists say rising temperatures in the North Sea have caused the deaths of hundreds of seabirds. Up to 1,000 shags, also known as green cormorants, have been washed up along the Scottish east coast.
Studies show cold currents that normally pour into the North Sea in winter - keeping the temperature down - did not occur this year, causing the temperature to increase.
The warmer water has affected the plankton that forms the base of the food chain and feeds sand eels, on which the shags survive.
Mark Grantham, recovery officer for the British Trust for Ornithology, says studies of the seabirds’ bodies showed they were suffering from malnutrition.