Volume 61, Issue 18

This study evaluates runoff and different methods for the estimation of water balance and runoff genesis in four small alpine catchments, which lie outside the standard network of hydrological and climate networks. These test catchments, whose size ranges between 2.3 and 110 ha, are located above the timberline at elevations between 1,784 and 2,380 m. Their land surfaces consist of lakes, rock formations, debris deposits, and alpine meadows. Hydrological data were collected for the water year 2001. The catchments were instrumented by three automatic weather stations recording global and net solar radiation, air temperature, humidity, precipitation, and soil temperature. Lake water levels were registered with staff gauges and runoff evaluated from water storage calculations. Runoff genesis was investigated by means of chemical tracers (Rhodamine WT and Lithium chloride). Hydrological process estimations were made using standard methods including: the input of precipitation and snowmelt, both potential and actual evaporation, which was estimated by the approaches of Hamon, Priestley-Taylor, Penman and Grindley, and runoff calculated from the lake storage, were compared with results of the conceptual hydrological Brook90 model. The empirical results show that hydrological processes are governed by the temperature-dependent regime of high mountain snowmelt. However, the major differences in water yield and runoff genesis between watersheds were due to differences in the morphologies of the lakes and their basins, the soilvegetation complex, and runoff routing. Evaluating approaches to estimation, Penman’s combination of both aerodynamic and energy balance method provides the best fit to observed data with observed evapotranspiration being 78 to 99% of the potential calculated. The deterministic Brook90 model is effective for precipitation-runoff genesis studies in small headwater catchments. In the L’adové pleso basin predicted and observed water yield show close correlation. The annual sum of actual evapotranspiration calculated by BROOK90 (352 mm) corresponds closely to that estimated by the approach of Penman (386 mm).

In the framework of the international project EMERGE, species composition of epilithic diatoms of 34 selected high mountain lakes of Slovak part of the Tatra Mountains were investigated. In all, 127 taxa of diatoms belonging to 26 genera were recorded. Comparison of the epilithic assemblages of the investigated lakes showed differences both in relative abundance and taxa present in the individual lakes. On the basis of species composition and relative abundance of epilithic diatoms using hierarchical cluster analysis, two main groups of lakes were created. The first group includes 4 lakes which can be classified as strongly acidified and 9 lakes which are acidified or threatened by acidification. In this group, acidophilous species such as Achnanthes helvetica, A. marginulata, Aulacoseira distans and Tabellaria flocculosa prevailed. The second group includes 19 non-acidified lakes. The most common diatoms in these lakes were Achnanthes minutissima, A. helvetica, Cymbella minuta and Denticula tenuis which occur practically in all these lakes but with different abundances. The division of lakes due to their diatom species composition mostly confirmed the status of these lakes as judged from their water chemistry.

In our study, we focused on littoral Cladocera living and feeding in shallow shore parts of 46 mountain lakes in the Tatra Mountains (Slovakia and Poland). The studied lakes underwent a major acidification event in the 1980s and are now in the process of recovery. Lakes were divided into three categories based on their sensitivity to acidification: 5 extremely sensitive (ES), 11 acid sensitive (AS), and 30 non-sensitive (NS) lakes. In our study, we included historical data from the literature, and data from sediment core and littoral samples, which together represent the evolution of the littoral communities from a pre-industrial period up to the present. In total, 11 littoral species were found belonging to three cladoceran families. Most of the species were members of the family Chydoridae: Alona affinis, A. quadrangularis, A. rectangula, A. guttata, Acroperus harpae, Alonella excisa, A. nana, Chydorus sphaericus, and Eurycercus lamellatus. One species belonged to each family Daphniidae (Ceriodaphnia quadrangula) and Polyphemidae (Polyphemus pediculus). The most numerous littoral taxa were Alona affinis, Acroperus harpae, and Chydorus sphaericus. All species reacted to decreased pH levels during peak acidification in the 1980s by disappearing from most of the lakes of all categories; the only persisting species was Chydorus sphaericus. Most species returned to the lakes when pH started to increase in the 1990s, although their return was noticeably slower in AS lakes. Alona quadrangularis decreased its distribution range over the studied period; Polyphemus pediculus was mostly detected in the 1910s only. The number of species was highest in all lake categories when dwarf pine was present in the lake catchment. On the whole, the littoral community was richest in NS lakes.

Data from two surveys of the Tatra Mountain lakes (Slovakia and Poland) performed in the autumns of 1984 (53 lakes) and 1993 or 1994 (92 lakes) were used to estimate spatial variability in water chemistry in this lake district during the period of maximum European acid deposition. The ionic content of the lakes was generally low, with conductivity (at 20°C) ranging from 1.1 to 4.7 mS m−1 and 23% of the lakes had a depleted carbonate buffering system. Major factors governing differences in lake-water chemistry were bedrock composition and amount of soil and vegetation in their catchment areas. Compared to lakes in the predominantly granitic central part of the Tatra Mountains, lakes in the West Tatra Mountains had higher concentrations of base cations and alkalinity due to the presence of metamorphic rocks in the bedrock. Concentrations of phosphorus, organic carbon, organic nitrogen, and chlorophyll-a were highest in forest lakes and decreased with decreasing density of vegetation and soil cover in the catchment areas. Concentrations of nitrate showed an opposite trend. Several exceptions to these general patterns in chemical and biological composition were due to exceptional geology or hydrology of the lake catchments.

Species composition of planktonic Crustacea in 102 lakes in the West and High Tatra Mountains, studied during the peak of anthropogenic acidification (1978–1996), is presented in this work. Zooplankton of the Tatra lakes have been studied since the middle of the 19th century, which later enabled the recognition of lake acidification and the assessment of its effect on the plankton community of lake ecosystems. In the pre-acidification period, the distribution of zooplankton was determined namely by the lake altitude and orientation (north vs. south) and by the catchment character. Crustacean zooplankton in larger lakes consisted of a limited number of species, with Acanthodiaptomus denticornis and Daphnia longispina dominating lakes in the forest zone, and Arctodiaptomus alpinus, Cyclops abyssorum, Daphnia longispina, Daphnia pulicaria, and Holopedium gibberum dominating lakes in the alpine zone. Ceriodaphnia quadrangula, Daphnia obtusa, Daphnia pulex, and Mixodiaptomus tatricus occurred in lakes with high concentrations of dissolved organic matter and in strongly acidified waters. Anthropogenic acidification has caused drastic changes in both the chemistry and biology of the Tatra lakes. Based on their status during the acidification peak, lakes were divided into three categories: non-acidified (with no change in the species composition of crustacean zooplankton due to the acidification), acidified (planktonic Crustacea disappeared in lakes with meadow-rocky catchments), and strongly acidified lakes where original Crustacea in meadow-rocky catchment lakes disappeared and were replaced by populations of the acid-tolerant littoral species Acanthocyclops vernalis, Chydorus sphaericus, and Eucyclops serrulatus. The acidification-induced processes of oligotrophication and toxicity of aluminium played a key role in the extinction of species. Despite the first signs of biological recovery observed in the early 2000s, acidification remains the most important factor governing the structure of plankton in the Tatra lakes.

All species of Daphnia (Cladocera) produce, at some stage in their life cycle, diapausing eggs, which can remain viable for decades or centuries forming a “seed bank” in lake sediments. Because of their often good preservation in lake sediment, they are useful in paleolimnology and microevolutionary studies. The focus of this study was the analysis of cladoceran resting eggs stored in the sediment in order to examine the ephippial eggs bank of Daphnia pulicaria Forbes in six mountain lakes in the High Tatra Mountains, the Western Carpathians (northern Slovakia and southern Poland). Firstly, we analyzed distribution, abundance and physical condition of resting eggs in the sediment for their later used in historical reconstruction of Daphnia populations by genetic methods. To assess changes in the genetic composition of the population through time, we used two microsatellite markers. Although DNA from resting eggs preserved in the High Tatra Mountain lake sediments was extracted by various protocols modified for small amounts of ancient DNA, DNA from eggs was not of sufficient quality for microsatellite analyses. Distribution curves of resting eggs from sediment cores correspond to the environmental changes that have occurred in the High Tatra Mountains area during last two centuries (atmospheric acid deposition, fish introduction) and demonstrate their influence on natural populations. Evaluation of ephippia physical condition (the most common category was empty ephippial covers) suggests that the majority of resting eggs hatched to produce a new generation of Daphnia or may be due to failed deposition of resting eggs by Daphnia to the chitinous case. In conclusion, age, low quantity and poor physical condition of resting eggs from these Tatra lake sediments proved to be unsuitable not just for use in genetic analyses, but also the possibilities of autogenous restoration of Daphnia populations from the resting egg banks in the Tatra sediments are negligible.

Littoral benthic macroinvertebrates of 45 mountain lakes in the Tatra Mountains were sampled using a semi-quantitative method in September 2000. A total of 32,852 specimens were identified to 93 taxa belonging to 14 higher taxonomic groups. Multivariate statistics (CCA, RDA) and nine biotic metrics (AQEM/STAR) were used to explain relationships between macroinvertebrate assemblages and environmental variables. Up to 57% of the ecological position of littoral macroinvertebrate assemblages were explained by variance of environmental variables divided into chemical, trophic, physical, catchment and location. Five types of Tatra lakes were recognized using CCA: A — strongly acidified lakes (small catchment, low pH, high concentration of TP, DOC, highest amount of POM in littoral); B — alpine acidified lakes (low amount of POM, low values of biotic metrics); C — alpine non-acidified lakes (high value of diversity index, predominance of Diptera); D — subalpine acidified lakes (high values of biotic metrics: number of families, proportion of crenal and rhithral taxa/total taxa); E — subalpine non-acidified lakes (high values of biotic metrics: number of families, number of genera, BMWP score, number of taxa and abundance of EPT taxa). RDA was used to design five levels of macroinvertebrate taxa acidification tolerance. The Tatra Acidification Index (TAI) was established to assess the acidification status of the lakes in the Tatra Mts.

The composition of invertebrates of inlets and outlets of selected West Tatra and High Tatra lakes (Slovakia, 49°10′ N, 20°00′ E) was analyzed. Twenty-seven sampling sites representing 11 inlets and 16 outlets of 19 mountain lakes were investigated. In total, 10,692 aquatic invertebrates from 27 samples were sorted, 7,314 from inlets and 3,378 from outlets. Altogether, 110 zoobenthic taxa (81 in inlets and 85 in outlets) belonging to 12 higher taxonomic groups were identified. The most constant benthic groups found both in lets and outlets, at more than half of all sites, were Oligochaeta (93% of all sites), Chironomidae (78%), Trichoptera (63%), Plecoptera (59%) and Turbellaria (56%). The most abundant groups were Chironomidae (4,697 ind.), Oligochaeta (2,473 ind.) and Turbellaria (2,236 ind.). No statistically significant differences were found between the benthic communities of inlets and outlets. On the contrary, significant differences (P < 0.05) were found in some parameters of benthic communities of inlets situated in different altitudes (< 1800 m a.s.l., 1800–2000 m a.s.l., > 2000 m a.s.l), outlets of different altitudes, and finally, outlets of lakes with different areas (< 5 ha and ≥ 5 ha). Cluster analysis relatively clearly separated outlets of subalpine lakes from the alpine lakes. Also, outlets of lakes with the lowest pH were separated from the other sites. In the case of inlets, West Tatra sites were separated from the High Tatra sites.

The structure of macrozoobenthos of two different high-mountain Tatra lakes, namely Starolesnianske pleso (strongly acidified) and Nižné Terianske pleso (reference site) is described. The latter is characterised by a large catchment and total area, and is relatively deep with strong inlet and outlet. It is not affected by acidification. Univoltine species of mayflies, stoneflies and caddisflies dominate in the littoral, mainly scrapers. Predators are represented by stoneflies. Shredders dominate in the lake outlet. From the zoogeographic point of view, Carpathian endemites and Central European species prevail. Forty percent of species are sensitive to acidification. The former has a smaller catchment and total area, and is relatively shallow, without inlet. It is strongly affected by acidification. Semivoltine species, mainly predators (beetles) and shredders prevail. From the zoogeographic point of view Palaearctic species dominate. Species sensitive to acidification are missing. Emergence of water insects has two peaks during spring and fall circulation.

The occurrence of preimaginal stages of Diptera (excluding Chironomidae and Simuliidae) in the Tatra Mountain lakes, their inlets and outlets, and the influence of acidification on the presence of these taxa were assessed. At 21 sites, 18 Diptera taxa were recorded in 2000 and 2001. The most frequent were Dicranota sp. (73%), Pedicia (C.)rivosa (50%) and Wiedemannia sp. (14%). Diptera were ordinated by RDA; relationships between taxa and environmental variables were significantly determined by temperature and habitat characteristics (altitude, habitat, alkalinity, Ca2+, pH, temperature, conductivity). The RDA diagram confirmed the existence of four Diptera assemblage groups corresponding with four habitat types. Group 1 — alpine and subalpine oligotrophic lakes at higher altitudes threatened by acidification, with the acid tolerant taxa Tipula rufina, T. alpium, Dicranota sp. and Rhypholophus sp. Group 2 — subalpine oligotrophic lakes not threatened by acidification with the acid sensitive taxa Wiedemannia sp., Hemerodromia sp., Tipula (S.) benesignata, Orimarga sp., Chionea sp., Pedicia (C.) rivosa and Bazarella subneglecta which prefer higher temperature. Group 3 — subalpine oligotrophic lakes, with the acid sensitive species Berdeniella illiesi, which prefers cold water. Group 4 — outlets of submontane oligotrophic lakes with characteristic taxa Tricyphona sp. and Tipula goriziensis. Acidified alpine and subalpine Tatra lakes are situated mostly at higher altitudes (1705–2145 m. a.s.l.) and have specific taxonomic composition.

Chironomid assemblages in thirty-three mountain lakes situated above tree line in the Slovakian part of the Tatra Mountains were studied during 2000–2002. Chironomid species/taxa, collected as pupal exuviae, were correlated with physical, chemical, and lake morphometry variables of 22 lakes. Two-way indicator species analysis (TWINSPAN) was used to classify the lakes into four distinct groups: higher situated alpine lakes, lower situated alpine lakes, subalpine lakes and acidified lakes. Presence/absence of eight taxa was identified as indicative for this classification. In discriminant function analysis, pH, dissolved organic carbon, altitude and lake area were the most significant variables reflecting differences among groups of lakes. This model of four variables allowed 77% success in the prediction of group membership. A multiple regression model with lake area, concentration of magnesium and total phosphorus accounted for 37% of the variance in taxa richness. Lakes with greater area contained more chironomid taxa than smaller ones. Lakes with higher alkalinity and higher trophic status tend to support more taxa. Canonical correspondence analysis (CCA) indicated that most variation in the composition of chironomid assemblages was related to pH and to altitude. The results can be used as reference data for long-term monitoring of the Tatra lakes, especially in connection with a recovery from acidification and global climatic change.

The profundal community of L’adové pleso (an oligotrophic high mountain seepage lake at an altitude of 2,057 m with a max. depth of 18 m and an ice-cover period from October–July) was studied from December 2000–October 2001. Chironomidae, the most significant part of the studied community, are represented by four taxa and dominated by Micropsectra radialis Goetghebuer, 1939 and Pseudodiamesa nivosa (Goethgebuer, 1928). These two species showed a 1-year life cycle. The total densities of chironomids varied from 0 to 5,927 ind. m−2; no chironomids, or very low densities, were found during the winter/spring period, probably due to low oxygen concentrations in the medial part of the lake. These low oxygen concentrations probably caused the relocation of larvae from the medial part of the sedimentary area at the same time.

Ninety-one lakes distributed along the Tatra Mountains (most of lakes > 1 ha and 65% of lakes > 0.01 ha) were sampled and analysed for ionic and nutrient composition in September 2004 (15 years after reduction in acid deposition). Eighty-one lakes were in alpine zone and ten lakes in Norway spruce forest. The results were compared to similar lake surveys from 1994 (the beginning of water recovery from acidification) and 1984 (maximum acidification). Atmospheric deposition of SO42− and inorganic N decreased 57% and 35%, respectively, in this region from the late 1980s to 2000. Lake water concentrations of SO42− and NO3− have decreased both by ∼50% on average (to 23 and 19 μmol L−1, respectively, in 2004) since 1984. While the decrease in SO42− concentrations was stable throughout 1984–2004, most of the NO3− decrease occurred from 1994 to 2004. The declines in SO42− and NO3− concentrations depended on catchment coverage with vegetation, being most rapid for SO42− in forest lakes and for NO3− in rocky lakes. Concentrations of the sum of base cations (dominated by Ca2+) significantly decreased between 1984 and 2004, with the highest change in rocky lakes. Most of this decline occurred between 1994 and 2004. Acid neutralising capacity (ANC) did not change in the 1984–1994 period, but increased on average by 29 μmol L−1 between 1994 and 2004, with the highest change in rocky lakes. Over the last decade, the proportion of lakes with ANC > 150 μmol L−1 increased from 15% to 21% and that of ANC < 20 μmol L−1 decreased from 37% to 20%. The highest decline in H+ and Al concentrations occurred in the most acid lakes. On a regional basis, no significant change was observed for total phosphorus, total organic nitrogen, and dissolved organic carbon (DOC) in the 1994–2004 period. However, these parameters increased in forest lakes, which exhibited an increasing trend in DOC concentrations, inversely related (P < 0.001) to their decreasing ionic strength (30% on average in 1994–2004).

Three lakes were studied in the High Tatra Mountains at altitudes from 2000 to 2157 m a.s.l., which represent three categories of acidity status recognised in the Tatra lakes in the 1980s: non-acidified, acidified, and strongly acidified. Subfossil chironomid remains from dated sediment cores covering ca. throughout 200 years were analysed. The chironomid thanatocoenoses of all cores layers reflected ultra-oligotrophic non-acidified conditions in L’adové pleso. Nevertheless, the finding of the acid-tolerant species Zalutschia tatrica in littoral samples in 1980s indicates that the littoral zone was more influenced by acidification than deeper areas of the lake. Three stages of lake developmental history can be distinguished in the acidified lake Vyšné Wahlenbergovo pleso based on the chironomid subfossil record: the pre-acidification stage before the 1920s, the anthropogenic acidification stage between the 1920–1980s, and the recovery of the lake from the end of the 1980s. The period of acidification was accompanied by a decline in chironomid numbers, while chironomid fauna composition was unchanged during the whole investigated history of the lake. None of the chironomid taxa present prior to the acidification period disappeared during the peak of acidification. The most apparent change in the sediment record taken from the strongly acidified lake Starolesnianske pleso was recognised in the layers corresponding to the period 1960–1980. It is characterised by the disappearance of the dominant acid-sensitive Tanytarsus lugens group and the dominance of acidtolerant Tanytarsus gregarius group. The most recent chironomid assemblage probably benefits from the amount of food resources as a result of increased lake productivity induced by acidification.

The basic physical, chemical, and biochemical properties of mountain soils were determined in alpine-zone meadow and moraine areas of the Tatra Mountains (Slovakia, Poland) in 2000–2001. The amount of soil (dry weight soil < 2 mm) varied from 38 to 255 kg m−2 (average of 121 kg m−2) in alpine meadows and averaged 13 kg m−2 in moraine areas. Concentration of organic C was the parameter that most strongly and positively correlated with N, P, S, effective cation exchange capacity (CEC), exchangeable base cations, exchangeable acidity, and all biochemical parameters (C, N, and P in microbial biomass and C mineralisation rates). The relationship between C and P was less straightforward due to inorganic P forms associated with Fe and Al oxides. The average pools of C, N, P, and S, were respectively 696, 41, 2.9, and 1.9 mol m−2 (i.e., 84, 5.7, 0.91 and 0.61 t ha−1) in meadow soils, and 38, 2.1, 0.45 and 0.12 mol m−2 (i.e., 4.5, 0.30, 0.14 and 0.04 t ha−1) in moraine areas. Soil pH was generally low, with the lowest pHH 2 O values (3.8–4.9) in the A-horizons. Average pools of CEC were 12 and 0.7 eq m−2 in meadows and moraine areas, respectively. The base saturation (BS) was 4–45% (12% on average) of CEC, and was primarily based on Ca2+ and K+ (∼40% and ∼22% of BS, respectively). C:N molar ratios (14–20) were only slightly lower than those observed in the alpine Tatra Mountain zone ∼40 years ago. Concentrations of C, N, and P in soil microbial biomass were high (on average 1.6, 3.4, and 25% of total C, N, and P concentrations), suggesting high microbial activity in alpine soils.

Sediment cores from nine different lakes in the Tatra Mountains, collected as part of the EU funded AL:PE, MOLAR and EMERGE projects investigating natural environmental records stored in remote mountain lake sediment sequences, were dated radiometrically by 210Pb and 137Cs. At five sites, Długi Staw Gąsienicowy and Zielony Staw Gąsienicowy on the Polish side of the Tatra Mountains and Starolesnianske pleso, Nižné Terianske pleso, and L’adové pleso on the Slovak side of the Tatra Mountains, the cores were sectioned at close intervals and analysed in detail to produce a high resolution chronology. For the remaining four sites, Zmarzly Staw Gąsienicowy (Poland), and Vel’ké Hincovo pleso, Vyšné Temnosmrečinské pleso, Vyšné Wahlenbergovo pleso (Slovakia), it was sufficient to establish a low resolution sketch chronology and only a few samples were analysed from each core. At L’adové pleso, multiple cores were collected in order to establish spatial distribution of sediments over the bed of the lake. Cores from all sites had good records of the fallout radionuclides from which it was possible to construct reliable chronologies of the recent sediments.

Concentrations of major nutrients (C, N, P) and acid soluble metals (Ca, Mg, K, Al, Fe, Mn, Pb, and Zn) were determined in modern (0–1 cm) and pre-acidification (5–10 cm) sediment layers collected from 37 alpine and 3 forest lakes in the Tatra Mountains (Slovakia, Poland) in 1996–1998. Sediment composition reflected catchment characteristics and productivity of lakes. In the sediments of alpine lakes, C and N concentrations decreased and Mg increased with a decreasing proportion of vegetation and soil in the catchment. Decreasing Ca:Mg ratios in sediments along the vegetation gradient was inverse to that in water, and could be associated with different ratios of cations in water leachate from catchments and in solids which enter the lake due to soil erosion. Phosphorus concentrations increased with the proportion of moraine areas, with till soils rich in P. Concentrations of C, N, P, and Ca in sediments positively correlated to their concentrations in water. Sediment concentrations of Al and Al:Ca ratios increased with decreasing sediment and water pH. A negative correlation between water pH and concentrations of organic C in water and sediments indicated the important impact of organic acids on the acid status of the lakes exposed to higher terrestrial export of organic matter. Compared to the pre-acidification period, the modern sediments had significantly higher Fe, Mn, Zn, Pb, and K, but lower Mg concentrations. The Zn and Pb enrichment was more evident in oligotrophic alpine lakes than in more productive forest lakes and was independent of lake water or sediment pH. Fe and Mn concentrations in the modern sediments were higher than in ambient soils and bedrock, while those in pre-acidification sediments were similar to contemporary soils and to the rock layer. The enrichment of the modern sediments with Fe and Mn thus probably resulted from both their redox recycling and ecosystem acidification.

In 2000 and 2001, miniature thermistors with integrated data loggers were employed to measure lake surface water temperatures (LSWTs) and temperature profiles in high-altitude mountain lakes lying between 1580 and 2145 m a.s.l. on both the Slovak and Polish sides of the Tatra Mountains. This allowed the annual cycle of water temperatures and ice cover in these lakes to be described quantitatively, and their dependence on lake altitude above sea level to be investigated. LSWTs in the Tatra Mountains are found to decrease approximately linearly with increasing altitude from late spring to autumn. LSWT in summer can be modelled well in terms of exponentially smoothed ambient air temperature. Although the timing of ice-off is dependent on altitude, the timing of ice-on is not; the dependence of the duration of ice cover on altitude is therefore wholly due to the altitudinal dependence of the timing of ice-off. The temperature profile measurements allow quantitative characterization of summer and winter stagnation, and spring and autumn turnover.

A one-year study of the phytoplankton of remote mountain lake L’adové pleso (2057 m. a.s.l.), situated in the High Tatra Mountains, was carried out in 2000–2001. Seasonal development of the species structure and depth distribution of biomass, as well as chlorophyll-a concentrations, and chemical characteristics of the lake water were examined. The phytoplankton was made up of a low number of nanoplanktonic unicellular species, mainly flagellates from Cryptophyceae and Chrysophyceae. Plagioselmis lacustris, Cryptomonas cf. erosa and Ochromonas spp. were the most important with regards to both abundance and biomass. The phytoplankton showed a marked seasonal development. During most of the ice-covered period, abundances of phytoplankton remained very low. In regard to the vertical distribution, both species composition and total biomass were clearly stratified during most sampling dates. The amount of chlorophyll-a per unit biovolume varied considerably, and significantly higher values were found on sampling dates with complete or partial ice cover. Our data on pH and phytoplankton were compared with a previous study in 1990–1991 in order to evaluate possible changes associated with the decrease of acid deposition. The extent of episodic acidification of the lake water has diminished both in time and space, and lake water pH has slightly increased. We observed a shift in species composition, and an important increase of total phytoplankton abundance. This could be considered as a first sign of biological response to changing chemical conditions.