Changes in physico-chemical characteristics and the succession of phytoplankton in the lake Velenjsko jezero following its restoration

The species composition of phytoplankton in the artificial lake Velenjsko jezero has been monitored since 1994 while physico-chemical characteristics of the lake water since 1998. Before the year of lake remediation, 1994, the pH of lake water was around 12. In 1994, only filamentous cyanobacteria Oscillatoria ssp. were present in high abundance, with the rare appearance of Synedra sp. and Ceratium sp.. In 1995, the pH in the upper water layers decreased to 9, as a consequence of the construction of a fly ash system with a closed loop water cycle in October 1994. The number of algae taxons increased to 7 (Coelosphaeria sp., Gomphosphaeria sp., Scenedesmus sp., Pediastrum sp., Asterionella sp., Synedra sp. and Ceratium sp.). In 1996, when the pH fell to 8, it increased to 13. The lake provided good conditions for algal development since it was rich in nutrients. Since 1996 the level of nutrients in the upper layers of the water column has remained more or less the same, but in the deeper layers the reduced form of nitrogen (NH4) has increased and the oxygen curve has become clinograd. Velenjsko jezero can be classified according to OECD, as hypereutrophic on the basis of the level of total phosphorus (120 μg L–1) and total nitrogen (1500 μg L–1), the average transparency of 5.38 m corresponds to mesoeutrophic status, and the average concentration of chlorophyll a at 1.03 μg L–1 to oligotrophic status. Despite the high availability of nutrients the primary production was not as high as in a similar natural lake ecosystem, which could be ascribed to the high concentration of ions Ca2+, K+, Mg2+, Na+, Cl– and particularly, SO4. The predominant algae in the lake in 2007 were cyanobacteria Pseudanabaena cf. catenata, Planktothrix rubescens, from which the first bloom occured in June and the second from November to January, and dynophyta Ceratium hirundinella and Peridinium cinctum.


Introduction
The lake Velenjsko jezero can be used as a model of colonization of an empty habitat, since the high pH prevented the existence of most organisms before 1994. Colonization of macrophytes has been already described (mazej & epšek 2005, Mazej & GerM 2008. The massive development of submersed macrophytes indicated that Velenjsko jezero is very rich in nutrients. Coexistence of a number of phytoplankton species is a conspicuous feature of fresh waters. Although a few species commonly dominate a phytoplankton assemblage, a number of rarer algae coexist with the dominant species. Many differences in algal physiological characteristics, requirements, and tolerances, together with seasonal and spatial variations in environmental parameters, permit an apparently multispecific equilibrium to exist for short periods. Algae have defined temperature optima and tolerance ranges that interact with other parameters to cause seasonal succession. For example, many diatoms can photosynthesize successfully at cooler water temperatures, whereas the temperature optima of many green algae and cyanobacteria are higher (Wetzel 2001).
Attributes considered to be symptoms of negative impacts of nutrient enrichment in many ecosystems include blooms of toxic algae, increased growth of epiphytic algae, the growth of macroalgae, the loss of submerged vegetation due to shading, the development of hypoxic (and anoxic) conditions due to the decomposition of accumulated biomass, and the changes in the community structure of benthic animals due to oxygen deficiency or the presence of toxic phytoplankton species (revilla & al. 2009). The phytoplankton, because of its relationship with the eutrophication processes, is one of the biological elements considered within the Water Framework Directive (WFD). Phytoplankton biomass, composition and abundance, together with frequency and intensity of blooms, are the metrics to be assessed according to the WFD. Among the advantages of using phytoplankton to assess water quality are the rapid response of this group of organisms to the changes in the environment, their primary role in the food web and their influence on other organisms (Willén 2001).
In this paper we evaluated the trophic status of Velenjsko jezero between 1996 and 2007, considering some physico-chemical and biological parameters and analysed the succession of algae species composition and abundance following completion of the restoration measures in 1995.

Study area
Velenjsko jezero is located in central Slovenia, in the Šalek Valley, at an altitude of 366 m, It has a surface area of 135,000 m 2 and a maximal depth of 54 m. It is an artificial lake resulting from mining activity. Whole settlements, meadows and fields were submerged and flooded as a result of subsi dence. Until 1983, fly ash slurry from the Šoštanj Thermal Power Plant was transported by pipeline and emptied into Velenjsko jezero. This brought ash and calcium hydroxide to the lake, raising the pH of the water to 12. Since 1983 the ash has been used to build embankments, but effluent with a pH around 12 remained the predominant polluter of the lake until 1994. After construction of a fly ash system with a closed loop water cycle in October 1994, biota appeared in the lake. It was colonized by phyto and zooplankton, fish, macrophytes (mazej & epšek 2005) and other organisms. The pH of the lake is now around 8 and the lake is dimictic.

Biological parameters
Samples for Chl-a were obtained using a VanDorn sampler. After filtration through glass microfibre Watman GF/C filter they were analyzed by the standard method ISO 10260.

Results and Discussion
After construction of a fly ash system with a closed loop water cycle in October 1994, pH of water started to decrease (ramšak & rejic 1995, ramšak 1996). Only filamentous cyanobacteria (Oscillatoria sp.) were present in higher abundance, and rare appearance of Synedra sp., and Ceratium sp. was observed in 1994, when the pH was still above 11. In 1995, when water quality improved, the pH in the upper water layers decreased to 9, the number of algae taxons increased to 7 (Coelosphaeria sp., Gomphosphaeria sp., Scenedesmus sp., Pediastrum sp., Asterionella sp., Synedra sp. and Ceratium sp.) and to 13 in 1996, when the pH fell to 8. The lake was rich in nutrients, providing good conditions for algae development (Table 1, Table 2). Velenjsko jezero is relatively deep lake, but accelerated eutrophication, due to non-point sources of nutrients from drainage areas, nevertheless occurred between Average values measured at different depths and the volume of each stratum was used to calculate average annual concentrations of parameters. On the basis of the levels of total phosphorus (120 µg L -1 ) and total nitrogen (1500 µg L -1 ) determined in 2007, Velenjsko jezero was classified as hyper eutrophic, while the average transparency of 5.38 m corresponded to meso eutrophic status and the average concentration of chlorophyll a 1.82 µg L -1 to oligotrophic status (oecd 1982). It was expected that primary production would be higher due to the relatively high concentration of nutrients, but it appeared that other factors limited development of phytoplankton. Concentration of chlorophyll a in Velenjsko jezero is smaller in comparison with the lakes with the same trophic status (remec rekar 2008). The concentration of chlorophyll a is directly connected with the presence of phytoplankton and cyanobacteria, which are holders of primary production in lake water. Chlorophyceae, Cryptophyceae and cyanobacteria have a high impact on the concentration of chlorophyll a, while Bacillariophyceae, Dinophyceae and Chrysophyceae are of lesser importance (kasprzak & al. 2008).
Populations of filamentous cyanobacteria is increased in hypereutrophic lakes (Wetzel 2001). Although the number of cyanobacteria taxons was only 9 in 2007, their biomass was greater than that of other taxons almost all the year (reMec rekar 2008). Only in April 2007 the diatoms prevailed, in June the density of cyanobacteria Pseudanabaena sp. was very high especially in the metalimnium (>24.42*10 6 cells/L) While in August Chlorophyta and Dinophyta constitutes 50% and Cyanobacteria 50% of the phytoplankton biovolume, in November a bloom of Planktothrix rubescens occurred prevailing over other taxons (remec rekar 2008). It is generally recognized that cyanobacterial blooms are the direct consequence of eutrophication (reynolds & peTersen 2000). In Velenjsko jezero massive, long-lasting blooms of Planktothrix rubescens were observed (from November 2007 to February 2008). Planktothrix rubescens is a cold-water stenotherm species distributed mainly in middle European (reynolds 1984) and Southern subalpine lakes. During the summer stratification it is usually located within the metalimnium (cHorus & BarTram 1999, sedmak & kosi 1997, where it is photosynthetically active (Micheletti & al. 1998 Compared with other lakes (remec rekar 2008), very high average annual concentrations of sulphate (>590 mg L -1 ), chloride (>40.0 mg L -1 ), sodium (>60 mg/L -1 ) and potassium (>50 mg L -1 ) were detected in Velenjsko jezero (Table 2). Washing out of the ash disposal site is the most probable ion's source. The concentration of sulphate was almost four times higher than the maximum level in rivers provided for by Slovenian legislation (OGRS No. 11/2002). The usual concentration in lakes is in the range about of 5 to 30 mg L -1 , with an average value of about 11 mg SO 4 2-L -1 . SO 4 2has no influence on the trophic status of the water. Velenjsko jezero contains very high concentrations of divalent cations, especially Ca 2+ , providing good conditions for the development of green algae, which have high requirements for Ca 2+ . Sodium can influence the development of large populations of cyanobacteria and maximal growth of several cyanobacteria species has been found at 40 mg L -1 (Wetzel 2001), but values in Velenjsko jezero were even higher (63.7 mg/L -1 ) in 2007. Diatoms were also the dominant species in the lake in early spring, since they dominate in very hard water lakes, like Velenjsko jezero, with ratios: monovalent cations:divalent cations much less than 1.5 (round 1981). Distribution of most species of desmids of the Conjugales is limited to water with low concentrations of calcium and magnesium.

Conclusions:
1. The transparency of the lake and the nutrient concentration, and the concentration of chlorophyll a in the epilimnium remained at the same levels from 1996 to 2007, while the concentrations of ions were increasing regularly, especially sulphate and calcium, and consequently the specific electrical conductivity (SEP). Changes were detected in the phytoplankton community structure, blooms of toxic algae and the development of hypoxic (and anoxic) conditions in the hypolimnium occurred in the last years. 2. Concentration of chlorophyll a in Velenjsko jezero was smaller in comparison with the lakes with the same trophic status. Very high concentrations of sulphate, chloride, sodium and potassium can be one of the reasons for that phenomenon. 3. Filamentous cyanobacteria Oscillatoria sp., diatom Synedra sp., and dynophyta Ceratium sp. grew in lake even at pH 11. In recent years, following the first year after the normalisation of pH, Cyanobacteria have replaced Dinophyta as the predominant species. The predominant algae in the lake ten years later were cyanobacteria Pseudanabaena cf. catenata and Planktothrix rubescens as well as dynophyta Ceratium hirundinella and Peridinium cinctum. The first bloom of the former usually occurs in June and the second from November to January.