by Nikolai FILATOV, Dr. Sc. (Geogr.), Institute for Northern Water Studies, Karelian Research Center, RAS, and Dmitry POZDNYAKOV, Dr. Sc. (Phys. & Math.), Nansen International Center for Remote Study Methods and the Environment, St. Petersburg

The last decade has been marked by visible global climatic changes and their effect on water systems, and, happily, an easing of man's pressure on them. As things are today, the health of large lakes around the world has shown an improvement from what was a relatively short while ago.

The ecosystems of Ladoga and Onega have been deteriorating for some time now, especially since the 1970s. A threat looms over the life and health of millions in St. Petersburg and Leningrad Region, and in the nearby Republic of Karelia. Lake Ladoga, for one, is not only the principal source of water for domestic and industrial needs, but also a major influence on the biosphere of a territory that spreads over Russia's northwestern fringes and Finland, the Neva River, and, to an extent, the Gulf of Finland and the Baltic Sea.

EUTROPHY THREATENS LAKES

The "phosphorus burden" on Ladoga has been growing since the early 1960s, turning the previously oligotrophic(*) lake into a mesotrophic(*) one in the mid-1980s, and threatening to make it eutrophic(**) by the end of the 1980s. Similar processes were developing in two of the North American Great Lakes, Erie and Ontario (the remaining three having been spared). They have changed significantly the principal functional relationships in these lakes, wrested the elements of their ecosystems out of balance that had been building up for millennia. True enough, experts tended to believe that Ladoga can escape the fate of the two American lakes because of its cold water, lying as it is in relatively high latitudes, and the rock type of its basin. Their hopes have not been fulfilled, however. Despite the phosphorus burden stabilization at 6,100 tons a year by the early 1990s, and even its decrease in more recent years, the threat of pollution and eutrophication continues to hang over the lake.

Regrettably, though, the Academy's Limnology Institute has only occasionally monitored Ladoga's aquatic environment through the 1990s, so the vegetative (summer) period cannot be described in full measure. Systematic studies were, however, conducted by employees of the Institute for Northern Water Studies, RAS Karelian Science Center, in the northern part of Ladoga. Their findings allow, to a certain extent only, the lake's eutrophication problems to be assessed.

An analysis of the situation in other large lakes of the world shows that their ecosystems were changing rapidly in the mid-20th century, entering a relatively stable phase in the late 1980s and early 1990s. Efforts to restore the oligotrophic regime completely have, in effect, failed in all the great lakes of Europe and America. Ladoga, for one, is

*An oligotrophic body of water is one which has little organic matter, so organisms develop slowly in it.-Ed.

* A mesotrophic body of water has a moderate content of nutrients and clear and translucent water.- Ed.

** An eutrophic body of water is shallow and well-warmed, with abundant nutrients and, therefore, highly productive organisms; its water is unclean and impure.-Ed.

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today going through a period of secondary pollution, which threatens to have far more extensive and harmful consequences than the original effect of effluent discharges.

Experts say that the current condition and evolution of any inland body of water are exposed, in a varying degree, to zonal parameters (such as geochemistry of the landscape, climate, and weather), and depend on the specifics of the lake itself (basin shape, lithology, runoff hydrology, water exchange with the surroundings) and on the type and intensity of human activities. Not surprising, therefore, lakes of identical origin and nearly similar trophic conditions are at times nowhere alike in evolution because of either the changeable local (regional) climate or specific local impact of human activities. It is interesting, in his connection, to look at the modem-age changes in the ecosystems of large lakes in North America and Europe, which used in the past to have many close hydrophysical, hydrochemical, and hydrobiological parameters.

SIMILARITY AND DIFFERENCE

The American Great Lakes have been exploited much more heavily than Europe's largest lakes in the 19th and 20th centuries. Today, the areas around the five American lakes have a much larger population than their European counterparts, as in the United States and Canada, several megalopolises and giant industrial centers sit on the lake shores, compared to less than 300,000 people living in Petrozavodsk, the biggest city on the shores of Lake Onega.

In addition, the North American lakes have a surface area at least ten times as large as all of Europe's biggest lakes and nearly 20 times as much water, and maintain a much more vigorous water exchange with the surrounding areas than do the lakes in Europe. In thermohydrodynamic conditions, though, the lakes are very much alike: they have been dominated by cyclone-type water circulation over the 30-year climate average. The latest studies show it to be more intense in wintertime in the Great American Lakes, which (except Erie) do not freeze over. By contrast, European lakes freeze over completely or partially, so water intermixing in them is slow, facilitating eutrophication and pollution.

The cumulative erosive effect of the glaciers and, much more earlier, tectonic faults of the Tertiary were the principal landscaping process that formed the basins of these lakes lying between 40 N and 60 N.

Lakes Onega and Ladoga lie at the junction of the ancient Baltic crystalline shield and the Russian Platform. Their beds originated as early as the Proterozoic and have undergone complex transformations as part of either the continent or the sea. Subsequently, active denudation

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(*) and glacial exaration(**) led to terrain differentiation and formation of deep bays of modern-age bodies of water in areas where sedimentary rocks proved the weakest. The last glacier (10,000 to 12,000 years ago) deepened the ancient depressions and covered the terrain with an overburden of morainic, fluvioglacial, and limnoglacial sediments. In North America, similar forces produced the basins of Lakes Superior and Huron, while Michigan and Erie found themselves, together with the southern provinces of Huron and Ontario, in the zone of morainic sediments. This took place a short 10,000 years ago.

The Great Lakes of North America and large lakes in Europe share relatively low water mineralization with pH of about 7. Normally, the content of silicon and nitrogen here is sufficient to promote the development of plankton, but that of phosphorus is low (less than 10 mkg/ liter), restricting the growth ofphytoplankton.

ECOSYSTEMS OF LADOGA AND ONEGA

Lake Ladoga maintained its oligotrophic status until the second half of the 1960s, when the content of total phosphorus in the lake was about 10 mkg/liter. The shift toward mesotrophy that set in afterward was caused by humans. Man's influence came in at least five waves. Between 1976 and 1983, as manufacturing and farming burgeoned, with biological compounds used extensively for a long time, phosphorus concentration in the lake shot up rapidly (to 26 mkg/liter on average) and organic compounds started to accumulate. The average phosphorus concentration then, from 1983 to 1986, fell to 22 mkg/liter, against the background of stabilization (first time since the late 1960s) in the lake's primary productivity and growing bacterial destruction. Phytoplankton typical of oligotrophic lakes was taking over again. In 1986 to 1989, the average phosphorus concentration in the lake held at 21 mkg/liter. More organic matter was building up in bottom sediments. From 1991 to this day, phosphorus concentration declined to 17 mkg/liter. For the low average level of heavy metals in lake water (in comparison with the American lakes), insignificant amounts of cadmium, chromium, mercury, vanadium, lead, and cerium carried by effluents into Lake Ladoga were killing offphytoplankton.

What happened to cause improvement in the ecological condition of the lake?

* Denudation is a process, in which rock products of wind erosion are blown away from or washed down uplands and accumulate in terrain depressions.-Ed.

** Exaration is destruction of the glacial bed by rock fragments frozen into the ice.- Ed.

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A series of measures undertaken in the 1980s through 1990s included conversion of the Priozersk pulp and paper plant, improvements in the manufacturing processes at the Volkhov aluminum plant, paper plants at Pitkaranta and Laskela, and the tonnage of fertilizers applied on the farmlands was reduced by between 80 percent and 90 percent because of the economic crisis in Russia.

Because of the vast bulk of lake water and the inertia of its pollution and self-cleaning processes, a trend toward reduction in the population of valuable fish species, white- fish in particular, is clearly in evidence today. Halocarbons and DDT have been observed to accumulate in the tissues of the lake's fauna, including seals and various fish species, whose internal organs are severely damaged by pathological processes.

Lake Onega has to this day managed to maintain its oligotrophic status. One reason is that its underlying basin is composed by highly resistant Cambrian rocks which keep mineralization of stream emptying into it and the water of the lake itself at a very low level, some 37 mg/ liter, which is from 30 percent to 60 percent below the level in Ladoga and 20 percent to 25 percent of the level common for the American Great Lakes.

Bioproductivity of all trophic elements of Lake Onega is low because of the lake's cold water. Phyto- and zooplankton has been observed growing in the lake's open zones over the past two decades, a result of man-made pollutants flowing chiefly from the bays of Kondopoga and Petrozavodsk. In the content of original products, these bays closely resemble mesotrophic ecosystems, the northern part of the Kondopoga bay being, in effect, an eutrophic ecosystem.

The average phosphorus concentration in the lake today is within 8 to 10 mkg/liter. If more phosphorus flows into the lake, man-induced eutrophication may pick up at a much faster rate than it develops in Lake Ladoga. This is a real prospect because Onega has an average depth of 30 percent below that of Lake Ladoga, so it gets warmer much faster and has more favorable conditions for phytoplankton growth. Since total phosphorus concentration in Lake Onega must remain below 15 mkg/liter for the lake to maintain its oligotrophic status, it is today, all things considered, at a stage of ecosystem destabilization and early eutrophication.

FIVE LAKES-FIVE FATES

Geography and peculiar runoff largely contributed to slow natural aging of the Great Lakes in North

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America. In quality and quantity, the water here is originally oligotrophic. The plowing up of the adjacent lands, industrialization, population growth, increasing shipping and recreational activities, and hydroelectric power projects added up to intensify the eutrophication and toxic contamination of this giant lake system in the 1960s to 1980s. Erie, Michigan, and Ontario were worst affected.

Actually, eutrophication of Ontario began in the early decades of the 20th century, continuing right up to 1974, when the inflow of man-made phosphorus compounds decreased sharply, with the enforcement of the US-Canadian Great Lakes Water Quality Agreement of 1972. This agreement, and a later one signed in 1978, helped to lower phosphorus concentrations in the lakes from 25 to 10 mkg/liter. The photochemical processes have been turned around, and the lakes are today returning to their previous, meso-oligotrophic status. Between 1975 and 1985, the average chlorophyll concentration in the lakes dropped from 4 to 2.5 mkg/liter.

Lake Huron is oligotrophic, except Saginaw Bay and the southern part of Georgian Bay, where overall conditions stimulate eutrophication. Engineering and administrative efforts, backed with legislation, initiated two decades ago, have helped to maintain phosphorus inflow at virtually the same level (except Saginaw Bay, the average concentration in the lake does not exceed 5 mkg/liter). The lake's fisheries are, however, seriously threatened with overfishing, contracting spawning areas, toxic contamination of lake water and bottom sediments. Chlorophyll concentrations here vary within the range of 0.5 to 1.5 mkg/liter.

Among the five lakes, Erie is worst polluted by man. Beginning in the late 1960s, much of the lake (its bottom-most layers, above all) has been experiencing an acute shortage of dissolved oxygen, giving the media a reason to call Erie a "dead lake". Actually, its limnological characteristics are different in three of its principal parts- west, center, and east. The first, shallow part is heavily affected by the Detroit and Maumee rivers. It receives much biological matter to support large fish populations. The second part of the lake, with depths of 20 m at most, is moderately eutrophied and occasionally short of oxygen. Sediments are rapidly deposited here from the debris of the eroding shoreline. The deepest and coldest part has been eutrophied least of all. Because of xenobiotics discharged into the lake, considerable concentrations were detected here in the tissues of endemic whitefish as early as 1970, and whitefish fishing was banned promptly. Over the successive decades, the measures taken to

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ease man's influence on Lake Erie have made it much more healthy. They have been greatly facilitated by the high flow-through rate of the lake's water, which is fully renewed within a short 2.6 years.

Lake Michigan is the second most eutrophied among the Great Lakes. This overall condition notwithstanding, most of its water off its southern and eastern shores is oligotrophic and mesotrophic. Measures undertaken here are gradually reducing the levels of dichlorodiphenyl-trichloroethane (DDT) and polychlorinated biphenyls (PCBs) in the tissues of local fish species (trout, for example), even though they are still above permissible levels. Levels of PCBs, cadmium, lead, and mercury remain high in the Manistik and Fox rivers and in some bays of Lake Michigan.

Lake Superior has the world's second largest surface area, after the Caspian Sea, and is second only to Lake Baikal in Siberia in water volume. Large depths, a relatively small catchment area, erosion-resistant shores, and limited human activities permit it to retain its oligotrophic status. Despite its privileged status changes have lately been observed in the trends of its phytochemical processes, which are, unlike those in the other lakes of this lake system, attributed, above all, to the invasion of predators and, in that order, to the inflow of biological elements. Sustained environmental measures launched to protect this lake are almost a certain guarantee against its eutrophication or toxic contamination.

LAKES AND CLIMATIC CHANGES

The changes in the ecological conditions of lakes in Europe and North America discussed above have been occurring against the background of significant climatic instability in separate regions and the planet in general. An analysis of air temperatures on the ground surface in the last 100 years shows that intensive warming that continued until the 1940s was superseded by a short spell of cooling, which ended in the early 1940s. In the mid-1960s and 1990s, the climate was growing cooler again.

This instability has been caused by reorientation of the global climate, probably toward overall warming. The short spells of cooling were induced by natural changeability of the climate, and invigoration of volcanic activity in some periods, and, as a result, a considerable decrease in solar radiation reaching the Earth through the atmosphere of reduced transparency. Despite this general trend, regional climatic changes in the areas of the lakes described here differ significantly within the century- long time frame.

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In particular, more precipitation has been falling in Russia's northwestern region since the late 1970s, the water table has been rising in lakes, the duration of snow cover in the catchment area decreased, and the Baltic and Barents seas remained under ice for shorter periods. On a global scale (from an analysis of averaged air temperatures for 1990 to 1998), a trend has been observed for air temperatures to rise toward the late 1990s, particularly in 1997. In Fennoscandia (the area of Europe's largest lakes), however, air temperatures at ground level have been normal, regardless of the overall trend toward higher air temperature.

In the western part of the Northern Hemisphere, at the latitude of the American Great Lakes, global climate warming over the above period was manifested in air temperatures rising in all seasons of the year and evaporation increasing from the lake surface and the catchment area in general.

A conclusion to be made about the consequences of climatic variations and human influence over the past 40 years is that man's economic activities have been a major factor affecting the dynamics of the two large ecosystems-northern lakes in America and Europe. Natural changeability of climate has, however, been the dominating force. This means that as human pressure continues to relax in the future, the role of climatic factors in shaping the dynamics of these ecosystems may become overwhelming. As the air temperature rises in response to growing concentration of carbon dioxide in the atmosphere (greenhouse effect), the current trends in the lake regions of America and Europe will reverse.

Estimates have been obtained with the help of global climate models to illustrate the kind of changes in the American Great Lakes. As the carbon dioxide contents doubles, precipitation and runoff will decrease in territories between 42 N and 47 N, while higher temperatures in the lowermost atmosphere will lead to greater evaporation. As a result, minimum changes are expected in Lake Superior, and the water table will fall to its lowest mark 1 m to 2.5 m below the present level in lakes Huron, Michigan, and Erie. Meanwhile, since the local economy depends heavily on water resources (using lakes for hydroelectric power generation, water supply, shipping, recovery of mineral resources, and as a sink for effluents, including heated waste water discharged by nuclear power

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plants), it will be severely mauled by falling natural water surface level.

According to model simulations, atmospheric precipitation in Europe's northwest will increase by 5 percent to 15 percent over the same period, with the amount of carbon dioxide doubling, which will cause water tables to rise slightly in the lakes here. Fluctuations in precipitation will cause changes in runoffs, the time and intensity of spring floods, and, of course, will affect the ecosystem as a whole. The marine climate in the Leningrad Region, Karelia, and Finland will be even more pronounced. The lakes will, on average, remain under ice cover a few weeks less. In turn, this will result in rising water temperatures, more intense water mass dynamics, and thinner bottom sediments. Eutrophication of the lakes will further cut into dissolved oxygen supplies in them. As more mineral salts are drained into the lakes, the topsoil layer will thin out in the catchment area, and larger quantities of organic remains and biological matter washed off the catchment area will speed up formation of bacterial products and phytoplankton. Zooplankton will proliferate in the lakes and its diversity will diminish, with green algae growing in greater quantities. These factors will promote replacement of existing fish species.

Opposite trends will, therefore, win over in precipitation, runoff, and water table levels in the two lake provinces surveyed here. This will be the principal difference between the largest lakes in Europe and North America in the future.

To sum up, despite some similarity in basin origins and original oligotrophic status between the large lakes in northwestern Europe and North America, their ecosystems, however, followed their own ways, particularly in the 20th century.

Moreover, eutrophication and toxic contamination processes may differ greatly within a single lake system. The differences may result from specific lake origins, morphometry, thermodynamic processes, developments within the catchment area, legislation enforced to manage and protect individual lakes, investments in protective measures, and economic activity in the lakes and their catchment areas. Moreover, significant differences exist between the limnological processes in their ecosystems because of their dissimilar response to climatic factors.

Curiously, with thousands of miles between them, the largest European and North American lakes are exposed in the same way to industrial influences and worldwide climatic variations.

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