by Tatyana MOISEENKO, RAS Corresponding Member, RAS Institute of Water Management
Since arctic and high-mountain lakes (tarns) have been spared direct anthropogenic, man-caused effects, we can learn a good deal about global changes of the environment by studying lacustrine bottomset beds (deposits). The pollution of the upper layers of the atmosphere and the cross-border transfer of air masses from industrial centers have touched off substantial ecological changes in these what looks like vestal bodies of water.
Paleoecological studies of lakes figure prominently in major international programs: PEP (Pole-Equator-Pole) for Europe and Africa, CAPE (Circumpolar Ambience in Past Epochs), MOLAR (Mountain Lakes Research), LIMPACTs (Limnetic impact of human activity), among others. This approach has proved highly productive. The available fresh data show up the aftereffects of the booming industries in Europe, namely the pollution of high-mountain lakes in the Alps, Tatra Mountains and Pyrenees with oxides, heavy metals and radionuclides.
WATER "MEMORY LAYERS"
Now why in particular high-mountain and arctic lakes? How do they help diagnose global environmental changes? The thing is that their water catchment area
contains no immediate, direct pollutants (say, contributed by industrial or agricultural sources). That is why we can speak of the decisive impact of the cross-border transport of substances on the chemical composition of water, one that is formed by atmospheric precipitation often contaminated by heavy metals, acidiferous toxic organic substances and radionuclides. Such lakes have little, if any, soil and plants capable of retaining or digesting contaminants. Thus actually all polluting agents get into water.
The low temperatures as well as the ultrafresh and oligotrophic nature of these bodies of water diminish their self-purification capacity, and hence impurities get accumulated in bottomset beds (deposits). According to MOLAR data, the rate of sedimentation in tarns and arctic lakes is not above 1 mm a year. Also, ecosystems are quite vulnerable in a cold climate, a trend resulting in a rapid response of biological communities to a change of abiotic factors, be it the fluctuations of temperatures and of acid alkali balance or the action of toxic agents. Dying, aquatic organisms are deposited on the floor in conformity with their habitation and trophic status. Therefore analysis of diatom flora valves (diatoms-microscopic one-celled plants) and of remains of invertebrates, if combined with geochemical composition and age-related data, makes it possible to reconstitute the living conditions of these organisms over many centuries. That is to say, by obtaining information on the state of lacustrine and ground systems, we can track down the climate- and man-caused changes there.
In fact, diatoms (diatomic algae) are part and parcel of many freshwater (limnetic) communities. In ecological elasticity and biological productivity such algae are superior to other organisms and actually have no analogs. Since their qualitative and quantitative spectrum is bound up with the chemical composition of water and temperature conditions, diatoms are often used as biological indicators of the extent and character of anthropogenic loads (pressure). Thereby we learn about the rate and intensity of eutrophication (works by Dr. Natalia Davidova of the RAS Institute of Lacustrine Studies, and Dr. Lyudmila Kagan of the Institute for the Problems of Industrial Ecology of the North, the RAS Kola Research Center) and about water acidification (works by the British and Scandinavian diatomologists Richard Batterby of London University and Ingmar Renberg of Lulio University); we also learn about the presence of toxic impurities (with Dr. Lev Razumovsky and other biologists of our Institute involved with the subject).
We should know a remarkable lot if we want to interpret and evaluate the information concealed within lacustrine floors. We should command specific knowledge about the dietary (trophic) ways of different diatom species, about their resistance to toxic components (toxiresistance) and also about their ecological valence relative to ambient conditions (temperature and acidity above all) most favorable for their growth. Limnologists have compiled corresponding databases and keep updating them. Lately organisms other than diatoms have been taken up for paleoecological studies, namely the cladocera of zooplankton, and the chironomids of zoobenthos. Studying their horny remains, we can identify their species and rank, and consequently, habitation conditions. These methods, however, are not widely used yet.
In 1998, working within the MOLAR framework, experts employed at the Institute for the Problems of the Industrial Ecology of the North (RAS Kola Research Center) carried out research that made it possible to track down the dynamics of anthropogenic pressure in the Far North of European Russia. These studies were supervised by the author of the present article. We chose the Kola Peninsula as an area of the transborder transfer of air masses from Europe to the Arctic. Our data show that upper layers of the atmosphere are implicated in this process which takes place during long arctic winters; how-
Stratigraphic profiles of the relative number (percent) of diatomic reference species, diatomic algae grouped according to pH optimum (acb - acidobionts, acp - acidophiles, circ - circumneutrals, alkp - alkalophiles), and theoretical pH values in bottomset beds.
ever, in summer this transfer changes direction to the opposite, and polluting agents descend on the peninsula.
This is a region of major copper-and-nickel industries founded back in the 1940s. But the heavy, badly polluted air does no rise high into the atmosphere, and so the level of pollution in high-mountain lakes is not above the overall regional background. The quality of their water (these lakes are located in granite-gneiss blocks) largely depends on atmospheric precipitation, a factor that makes it highly sensitive even to weak anthropogenic loads.
ON THE EUROPE-ARCTIC BORDER
Well, we chose a mountain lake in the Chuna tundras (mountainous locality in the western part of the Kola Peninsula) at 475.3 m above sea level and 18 m deep. We took core samples of bottomset beds as deep as we could go (i.e. in the accumulation zone of deposits) and sliced the cores into 1 cm - thick layers. The chemical composition of the samples was analyzed by the method of atom-absorption spectrophotometry. The age of benthic deposits was determined by radiometry at Liverpool University from 210Pb (isotope of lead) chronology.
In its hydrochemical characteristics this oligotrophic superfresh-water lake is typical of northern and mountain tundras (treeless plains). The average concentration of heavy metals in water was found as follows: nickel - 1.1; copper - 1.3; cadmium - 0.13; and lead - 0.5 µg/l, which is characteristic of the putative background values in outlying districts of Kola's north. As to pH values, they were in a 5.8 to 6.4 range, that is the acidification of the lake was but weakly pronounced. We collated our data with the characteristics of lakes with pellucid water and pH below 5, where technogenic sulfates predominated in the ionic composition. Accordingly, we calculated a critical load for the lake, i.e. the topmost permissible level of acid-forming substances in the water catchment area; it was 0.3 g of sulfur per m2 a year. Even if this margin is exceeded by 0.02 g of sulfur annually, the pH value will keep going down. Yet another thing was of much interest to us-the current rates of the ongoing acidification processes.
Dr. Lyudmila Kagan made a diatom analysis by standard methods and in keeping with updated ecological characteristics of species described in the literature. Theoretical pH values for each layer of the samples were calculated from Ingmar Renberg's equation allowing to do that with good reliability for the percentage of groups of species having an identical pH optimum: acidobionts (preferring an acid medium at pH<5.5); acidophiles growing in weak acid medium at pH>5.5 below pH=7); circumneutrals (living in a neutral medium at pH=7) and alkalophiles (existing in an alkaline medium, i.e. at pH>7).
Studying the composition and dynamics of diatom communities in the bottomset beds of Lake Chuna Tundr, our team reconstructed its ecological state and external effects on the historical time scale. All in all we detected 90 species and varieties, of which 70 percent belong to acidophiles, that is growing predominantly in an acid medium. Most numerous were Brachysira brebissonii - as much as 50 percent, and Frustulia rhomboids v. saxonica - up to 18 percent. The diversity of diatoms was the highest deeper in the bottomset deposits, with the genera Eutonia (26) and Pennularia (14) in the lead, a fact indicative of the lake's weak natural acidity. The pH value in the pre-industrial period (layers 5 to 10 cm thick) was found to be close to 6.5 (i.e. almost neutral), which is typical of lakes in Kola's north.
But moving upwards to 5 - 6 cm layers deposited at the close of the 19th century, we detect structural changes in the community: namely, the population of circumneutrals
Concentration of metals (µg/q dry weight) in bottomset deposits in Lake Chuna Tundr (in retrospect).
and alkalophiles (P. viridis, E. arcus, Nitzschia palea) is down, while that of acidobionts (E. exigua, E. monodon, E. serra, P. biceps) goes up. Which means that the water pH started moving into a more acidic region. Diatoms respond to all dynamic processes in the chemical composition of the lake in the course of the vegetative (growing) season - in particular, to the utmost drop of pH in high-water periods, a phenomenon widespread now due to the acidification of meltwater by sulfate masses of technogenic origin.
Late in the 19th century the Kola Peninsula was inhabited largely by aborigines, the Saami, and there were none of the local sources of industrial pollution. Meanwhile, though the industrialization of Europe was proceeding apace, one was not awake yet to the need of waste disposal and purification of effluents. The combustion of coal and fuel oil as well as metal smelting polluted the atmosphere with acid-forming substances. Drawn into air flows in the upper layers of the atmosphere, they traveled far and wide and fell in water catchment areas thus contributing to water acidification. In Scandinavia experts say that by the 1940s and 1950s many lakes there had turned lifeless owing to the transfer and precipitation of technogenic sulfates of European origin.
Evaluating our data, we saw that the negative trends become more pronounced from 3 - 4 cm layers upwards, i.e. in deposits formed in the mid - 20th century. Compared with the natural conditions, the number of circumneutrals and alkalophiles marked a further downturn (~ 30 percent and ~ 60 percent respectively), while the percentage of acidobionts soared 10 fold. Remarkably, such rare diatom forms proper to acidic, dystrophic lakes as Stenopterobia intermediata showed up, along with degenerative, deformed circumneutrals like E. praerupta and E. arcus; P. viridis valves showed lesions at the suture. This period is noted for lowest pH values in the acid region inferred from the correlation of diatom species.
NORTHERN CHEMOSPHERE IN THE MIRROR OF VESTAL LAKES
A retrospective study of the geochemical composition of bottomset beds deposited throughout the 20th century showed up accumulation of certain metals, though their presence in the lake water was rather low. Thus, the accumulation of nickel, copper and cobalt tracked down to the 1940s is definitely connected with the industrial development of the Kola Peninsula and copper-nickel ore dressing. But what interested us here in particular was whether the global pollution of the northern chemosphere with heavy metals was mirrored in Kola, and if it was, how. Lately the world scientific community has become worried about the enhanced presence of cadmium (Cd) and lead (Pb) in the global environment.
Our research shows that the lacustrine ecosystems that we have studied have been accumulating lead since the end of the 19th century. At that time the Kola north was not an industrial area, and that is why the initial stage of accumulation of this heavy metal (lead) could be put down to the transfer of polluted air masses from Europe. In fact, our data agree with arctic studies carried out by American scientists who point to the global contamination of the northern chemosphere with lead.
The cadmium-related dynamics is more complicated, though: the Cd concentration marked a downturn towards the close of the 19th century, which was followed by a rise towards the 1930s and 1940s, then by another drop about the mid - 1970s and by another rise that is still on.
Considering the acidification trends in the Kola lake studied by us, we can explain some similarity in the behavior of such labile elements as cadmium (Cd) and
Distribution of lead (Pb) and cadmium (Cd) in lacustrine bottomset beds in European Russia.
zinc (Zn). The fact is that they are capable of active diffusion from bottomset deposits in acidified water persisting in the low (acidic) region of pH; this might have been the cause of the decline in Cd accumulation in the late 19th century. The subsequent industrial development of the Kola area spurred Cd accumulation trends. The associated falls in pH and in Cd concentration by the mid - 1970s might be due to the fact that the Kola copper-and-nickel industries began to use the ore mined at Norilsk, and its processing resulted in a dramatic increase in sulfur dioxide (SO2) discharges* as recorded in the "memory" of bottomset beds.
So: the first signs of Cd accumulation in a remote virgin lake of the Chuna tundras concur with the onset of industrialization in Europe and the ensuing global pollution of the atmosphere of the Northern Hemisphere. The anthropogenic acidification uptrend, as seen in the makeup of diatom communities in lacustrine deposits, is likewise related to the transport of acid-forming substances from Europe at the end of the 19th century. But is this true of other regions as well?
Accordingly, in 2003 and 2004 we made a study of core samples of deposits from other Russian lakes - namely Kentenyavr (in the Kola tundra), Glubokoe (in central European Russia) and Khmelevskoe (Caucasian mountains). In so doing we adhered to the same approach - the lakes should be as far as possible from direct pollution sources. Common trends were traced in all these three lakes. Located ca. 2000 m above sea level, Lake Khmelevskoe is devoid of run-off (no rivers flowing in and out), and it is getting water through atmospheric precipitation. The geological structure of its water catchment area is predominantly of granite, and this makes it vulnerable to acid aggression. Although no industries are found in the locality, the lake is acidified: pH - 5.5, and Pb concentration in water is 2 µg/l.
Now, the concentration of lead in Lake Chuna Tundr is 0.5 µ.g/l, or much lower, and pH is above 6, or in the weak acidity region. Does it mean that acidification is proceeding more vigorously in the Caucasus? The first data on the chemical composition of bottomset beds revealed a buildup of deleterious Cd and Zn in the past. So the anxiety about the global contamination of the environment with these and other heavy metals is by no means groundless. Such trends have been traced in three natural-climatic zones-from the treeless plains (tundras) of the north to the subtropics down south. Obviously, Caucasian tarns, which are a good way from Russia's industrial centers, are accumulating lead and cadmium more readily compared with other regions because of the pollution of the upper layers of the atmosphere.
Further in-depth investigations are needed in the group of Caucasian high-mountain lakes for more information on current trends and developments in the global environment. The ongoing processes out there in the Caucasus are well consistent with what is happening in the remote districts of the Alps. That is why tarns are good object of study for assessing the level of atmospheric pollution and of anthropogenic pressure. This research, however, is quite laborious and costly, be it sample taking (when mountain lakes are hard of access and helicopters have to be hired), or measurements and datings.
Diatom flora studies may be helpful for an insight into climatic variations of the past. We have done this job for the Kola north in the Chuna tarn. We collected a 3-meter core of bottomset deposits by using a deep-freeze sampler; a diatom analysis of the flora and reconstruction of temperatures were made by our research scientist, Dr. Nadezhda Solovyeva, at London University. The pattern of temperature dynamics thus established for the last few millennia does not indicate any distinct tendencies towards climatic warming in the Kola North. Additional research is evidently needed.
Be that as it may, the work we are doing is of vital significance. As to our paleoecological method, it is highly informative for tracking global changes of the environment and climate.
Illustrations supplied by the author
* See: L. Leontyev, "Ecological Problem of Norilsk: Ways of Solution", Science in Russia, No. 5, 2006. -Ed.
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