1. Introduction

      Lake Baikal is situated in East Siberia. It is the most ancient (25 myr.), the deepest (1637 m.) and the greatest (23 thousands of cubic kilometers) freshwater body on earth. Geographic information on the lake and its environment can be found in a recently published Atlas (Atlas Baikala, 1993), and we shall not repeat these data. The volume of water stored in Lake Baikal is 20% of the fresh water contained in all rivers and lakes of our planet (The Water Encyclopedia, 1990). If water of Lake Baikal was spread. All over the surface of continents, a layer which is 20 cm thick would be obtained. Lake Baikal receives 60 cubic kilometers per year from its riverine tributaries. Approximately the same volume flows out of the lake through Angara River. The time during which waters of Lake Baikal are substituted by waters of tributaries is 330 years (Afanasyev, 1960).

     Considering the problem of pollution of Lake Baikal and the possible changes of its ecological system under anthropogenic impacts, one has to remember about its enormous volume.

      Table 1.1 indicates the amounts of pollutants which would have to be discharged into Lake Baikal in order that their concentrations could reach different levels.

Table 1.1. Amounts of pollutants which would have to be discharged into Lake Baikal in order that their concentrations could reach different levels.

      Some papers on Lake Baikal present data on its pollution which seem to be unrealistic for the simple reason that sources of the large amounts of pollutants which are needed to raise their concentrations to the published values are unknown. For example, the concentration of sulphate anion in Lake Baikal is ca. 5.5 mg/1. Some papers claim that this concentration increased during the recent years to 6.5 mg/1 due to anthropogenic impact. Such an increase could happen after discharge of 23,000,000 tons of sulphate (see Table 1.1.). To bring this amount, it would be necessary to load 8000 rail-road trains. It is highly unbelievable that an operation of such scale would remain unnoticed, not to speak that it is hard to imagine that any institution could finance it. A much simpler explanation of this "observation" on the increased concentration of sulphate is an error of analytical determination. The turbidimetric method of determination of sulphate which is routinely used for its monitoring in Lake Baikal gives a statistic error of ± 30 %. This example makes it evident that the chemical analysis methods employed on Lake Baikal should have a very high accuracy.

     The problem of the purity of the water of Lake Baikal and of the state of different elements of its ecological systems attract a high attention of the Russian and international environmentalists.   The fact that the ecosystem of Lake Baikal is unique and has a globalimportance has been legally confirmed when the lake and its environment were included in 1996 into the List of World Heritage of UNESCO. A Law of the Russian Federation on the Protection of Lake Baikal was accepted in 1999.

      Everybody understands that Lake Baikal has to be protected. However, actions which have to be done in order to achieve this goal are very expensive. Decision makers have to be sure that these expenses are not in vain. Unfortunately, some scientists and mass media, wishing Baikal all the best, tended to exaggerate the extent of its pollution. The examples of such exaggeration are calculations made by G. I. Galazy (1988). This author cites evidence according to which purified waste water of the Baikalsk Pulp and Paper Plant diluted 10,000 times changes the behavior offish in special experiments. Knowing the volume of waste waters discharged into the lake by this plant during the time of its existence (ca.2 km³), he claims that already two-thirds of the waters of Lake Baikal (2 x 10000 = 20000 km³) are already changed compared to the native state. Evidently, this "calculation" is nothing else than a polemic figure. Neither the chemical nature, nor the concentrations of the compounds which reportedly change the behavior offish are known. Any compounds changing the behavior offish should disappear from the ecological system at a certain rate due to evaporation, sedimentation, chemical decomposition, as well by consumption and metabolism by aquatic organisms. "Calculations" of this kind cannot be a basis for sound economical or political decisions. In order to come to decisions, it is necessary to understand the nature of pollutants, to know their concentrations at different depths and locations, to study their faith.

      Even a more complicated task is evaluation of the state of natural populations of phyto - and zooplankton, fish and seals, because their abundances are subject to strong natural oscillations in time and in space. For example, it is known that the concentration of one of the leading planktonic diatom algae of Lake Baikal Aulacosira baicalensis changes in different years from analytical zero to more than 400,000 cells per litre (Fig. 1.1.).

      Information on Lake Baikal, if it is used as a basis for practical decisions, has to be obtained by accurate and precise methods, and by highly qualified specialists. Beginning from 1987, Lake Baikal has become, due to an initiative of the Limnological Institute, an object of large-scale international scientific co-operation. Siberian Branch of the Russian Academy of Sciences together with The Royal Belgium Institute of Natural Sciences, The Royal Society of London, The University of South Carolina, The Japanese Association for Baikal International Research Programs, The Swiss Federal Institute for Technology have founded in 1990 the Baikal International Center for Ecological Research (BICER). More than 200 international expeditions on Lake Baikal took place under the auspices of BICER since that time. Highly-qualified Russian and international experts took part in these expeditions. Sophisticated modern scientific instruments were used to analyze the samples obtained. The results were published in peer-reviewed international scientific journals. This gives a certain guarantee of quality. The present review considers these new data, compared to historical evidence, in order to obtain a reliable estimate of the present state of the ecological system of Lake Baikal according to international standards (Grachev, 1994)).

      In order to make perception of this evidence easier, we compare some of the characteristics of the ecological system of Lake Baikal with those of two other thoroughly investigated large lakes, in particular, of the Great Lakes of North America, and of Bodensee (Lake Constance). The choice of these two ecosystems was to some extent occasional. One of the arguments was that they have been thoroughly studied for many decades, and .are natural properties of a special value for the countries where they belong.

Fig 1.1. Dynamics of the maximum abundance of the planktonic diatom alga Aulacoseira baicalensis in Southern Baikal. Before 1981 - maximum abundances found with samples taken every 10 days at the pelagic station near Bolshie Koty. After 1981 - maximum abundances with samples taken during spring expeditions of the Limnological Institute.

      The system of the Great Lakes of North America has almost the same volume as Lake Baikal ( 30,000 km³). It includes Lakes Superior, Huron, Erie, Michigan and Ontario. The area of these lakes is much greater than that of Lake Baikal, but they are much younger and shallower. Anthropogenic impact upon different Great Lakes broadly varies. For example, many great industries of USA and its Chicago megapolis are situated on the shores of Lake Michigan. The catchment basin of this lake is one of the greatest centers of agriculture of the nation. On the contrary, only a small number of plants is found on the shores of Lake Superior, and there is practically no intense agriculture; the density of human population in the catchment is similar to that found around Lake Baikal (1-10 per sq. km, Atlas of the World, 1981, p. 57). Therefore, Lake Superior is the least polluted of the five Great Lakes.

      The other reference fresh-water ecosystem - Bodensee - is situated in Central Europe at the junction of the borders of Switzerland, Germany and Austria. The greatest river of Germany - Rhein -flows of this lake. The volume of Bodensee is 50 km³ (Rossknecht, 1996), water residence time in it is 6 years. The lake attracts attention as a valuable natural property, and is also one of the greatest German sources of potable water.