Cutting Edge

February 25, 2000

Scientists have solved the mystery of Loch Ness, but their discovery has more to do with what Nessie eats than whether she is alive Driving along the shore of Loch Ness you are sure to spot some watchers maintaining a vigil in the hope of catching a glimpse of Nessie. Nothing will shake their conviction that some strange creature lurks unseen. While they wait, they might reflect that the brooding waters of Loch Ness actually conceal a different mystery, and one that is yielding to patient scientific study.

For almost a decade, researchers from Lancaster University's department of biological sciences have been studying the loch. We have been interested in the extent to which organic matter produced by land plants and washed into a lake from its surrounding catchment can contribute to the open water food web, even in large lakes such as Loch Ness. We know that the chemistry of lake water depends on the surrounding geology and land use. The biological productivity of lakes is determined by the supply of nutrients from the catchment. Even atmospheric deposition of chemicals, such as acid rain, transported from far away can profoundly influence lake ecology. So the idea that organic matter from the catchment can help to fuel a lake's food web is an extension of an established principle.

Loch Ness is notoriously deep and murky, so conditions are unfavourable for growth of the tiny photosynthetic plants that live suspended in the water and are usually considered to form the base of the food web in lakes and oceans. Our early studies in Loch Ness showed that the small animals that feed on these plants were too numerous to be supported by the limited plant life alone, in which case they must utilise another source of organic matter and energy. In fact, scarcely any of the organic carbon in the loch is in living organisms. The bulk is in detritus, mostly the peaty material that colours the loch brown. This detritus must have been produced on land. Could this be the missing supplement for the aquatic food web?

Stable isotope analysis appeared to offer a solution to this question. Modern instruments can measure the relative proportions of naturally occurring stable isotopes (for example carbon-13 to carbon-12) giving an "isotopic signature" for a sample. The isotopic signature of an animal reflects its diet. If an animal is feeding on two alternative food sources that have distinctive isotopic signatures, a knowledge of the signatures of the animal and its food sources lets us measure how much each food contributes to the growth of the animal.

We established that the detritus in Loch Ness has a carbon signature corresponding exactly to land plants from the catchment. By contrast, the plants growing within the loch show a lower proportion of carbon-13. Since the two major sources of organic carbon for the food web in Loch Ness do indeed have different isotopic signatures, we have been able to determine that during 1998, organic carbon from the land contributed about 40 per cent of the total carbon in the bodies of small animals in the open-

water region of Loch Ness.

As well as solving one mystery in Loch Ness, our results are contributing to a more general reappraisal of how lakes are influenced by the organic matter they receive from their catchment and hence release more carbon dioxide into the atmosphere than they take up. If climate change alters the transport of organic carbon from land to lakes, we must be able to predict the impact on lake ecology if we are to manage our water resources and fisheries wisely in future.

Roger Jones is senior lecturer in biological sciences at the University of Lancaster.

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