Published online 6 August 1998 | Nature | doi:10.1038/news980806-4

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Diatoms pinpoint pollution in Lake Baikal

Lake Baikal in Siberia is the oldest lake in the world and the largest, in terms of the amount of water it holds. Stretching for some 600 kilometres and more than 1,600 metres deep in some places, it holds more than 2,500 species of plants and animals, around 75 per cent of which are found nowhere else in the world.

This priceless natural resource, which has recently been designated a World Heritage Site by UNESCO, has over the past few decades been subject to increasing pollution, both from discharge of industrial waste products and sewage into its waters and from atmospheric pollution from the factories that surround the lake.

To manage this vast resource as effectively as possible, ecologists need to know exactly how pollution has affected the lake at present and which areas are most affected. Water pollution from industrial discharges has been blamed for the sharp decline in stocks of the omul, a species of fish endemic to the lake, which is now protected, and the deaths of thousands of the endemic Lake Baikal seals in an epidemic of seal distemper in the late 1980s.

But although these disasters could be attributed to the insidious effects of pollution, some scientists believe that they could equally well be attributed to natural events or to factors other than pollution. Politicians, on the other hand, need hard evidence in order to put resources where they are most needed, and to avoid wasting resources on projects that may have no or even adverse effects in the long term.

To this end, an important new benchmark survey of the lake water quality and how it has changed over the past few hundred years is reported in the current issue of the Philosophical Transactions of the Royal Society of London by A.W. MacKay of University College London and an international team of scientists, working under the auspices of the Baikal International Centre for Ecological Research (BICER), at the Limnological Institute in Irkutsk. Despite definite evidence of pollution-driven changes in water quality in some inshore areas, they could find no evidence that the conditions in the deeper waters of the lake had deteriorated in recent years.

Pollution enters the lake from the atmosphere and from the discharge of sewage and industrial wastes from two cities at either end of the lake. The rivers that feed Lake Baikal carry more pollution in from the surrounding area. The main source of pollution at present is considered to be a pulp and cellulose processing factory at Baikalsk at the southern end of the lake. Since 1966, this has dumped some 1.5 billion cubic metres of industrial waste, including organochlorines, into the lake.

To detect changes in water quality, the BICER team used the record of diatoms in the sediment at the bottom of the lake. By taking small cores of sediment from 20 different sites, they were able to reconstruct a historical record of composition of diatom populations throughout the lake as a whole from the present day back to the mid-nineteenth century and in some cases to some 2,000 years ago. Diatoms are single-celled photosynthetic organisms with silica coats, which are preserved in sediments when the organisms die and sink to the bottom of the lake.

Their abundance and species are sensitive not only to changes in water quality due to pollution but also to the changes in climate, and a historical record that can be correlated with known climatic change is particularly valuable in showing how the diatom populations may change naturally over time. This knowledge is crucial in interpreting modern records and disentangling the changes due to pollution from those that might be expected to be occurring as part of the natural variability of all ecosystems.

By looking at the historical record for several diatom species, the research team shows that their relative abundances had indeed changed over time, but that the main change had occurred in the middle of the nineteenth century, well before any significant industrialization of the lake had occurred. They attribute this change to the warmer climate after the end of the Little Ice Age in the eighteenth century, and find no changes that indicate deterioration of the deeper offshore water quality or nutrient enrichment in more recent times.

In some inshore areas of the lake, it is a different story however. The BICER team finds recent changes in diatom populations that indicate nutrient enrichment of the water inshore near the Baikalsk paper mill and in the shallow waters of the delta of the Selenga River, one of the main tributary rivers. They also looked at evidence for recent heavy metal pollution, indicating industrial discharges, and atmospheric pollution in the form of carbonaceous particles, in their sediment cores. They found confirmation of previous records showing that atmospheric and industrial pollution is greatest in the southern area of the lake, in particular, and at the extreme northern end, around the city of Severobaikalsk.