ACTA BIOLOGICA SLOVENICA 
 LJUBLJANA 2009             Vol. 52, [t. 2: 49–59
Changes in physico-chemical characteristics and the succession of 
phytoplankton in the lake Velenjsko jezero following its restoration
Zdenka Mazej, Gabrijela Triglav Brežnik, Rudi ramšak 
ERICo Environmental Research and Industrial Co-operation d.o.o., Velenje, Slovenia
Correspondence: Zdenka Mazej, ERICo Environmental Research and Industrial Co-operation Ltd., 
Koroška 58, 3320 Velenje, Slovenia; E-mail: zdenka.mazej@erico.si
Abstract: 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 develop-
ment 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 oligo-
trophic 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, SO42–. 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.
Keywords: pH, lake water, nutrients, phytoplankton
Izvleček: Vrstna sestava in abundanca fitoplanktona se v Velenjskem jezeru določa že 
od leta 1994. Vrednost pH jezerske vode je bila 12 vse do leta 1994, ko so v Termoelektrarni 
Šoštanj uvedli zaprti krog transportne vode (oktober 1994). Ob visokem pH, so bile v jezeru v 
velikem številu prisotne le filamentozne cianobakterije Oscillatoria ssp., z redko prisotnostjo 
taksonov Synedra sp. in Ceratium sp.. V letu 1995, ko je pH zgornjih plasti jezera narastel do 9, 
je število taksonov zraslo na število 7 (Coelosphaeria sp., Gomphosphaeria sp., Scenedesmus 
sp., Pediastrum sp., Asterionella sp., Synedra sp. in Ceratium sp.) in v letu 1996, ko je pH padel 
na 8, smo lahko v jezeru določili že 13 taksonov. Jezero nudi zelo ugodne razmere za razvoj 
alg, saj je bogato s hranili. Od leta 1996 sicer ostajajo koncentracije hranil v epilimniju bolj ali 
manj enake, v spodnjih plasteh pa se z leti povečujejo koncentracije amonija (NH4+) in kisikova 
krivulja je postala klinogradna. Medtem ko je bilo jezero v letu 1996 še prezračeno do dna (45 
m), je že v letu 2000 kisik skoraj popolnoma izginil pod globino 20 metrov. Velenjsko jezero 
lahko glede na OECD klasifikacijo uvrstimo glede na količino celotnega fosforja (120 µg L–1) 
in celotnega dušika (1500 µg L–1) med hiperevtrofna jezera, na osnovi povprečne prosojnosti 
(5.38 m) med mezoevtrofna jezera, in na osnovu povprečne koncentracije klorofila a (1,03 µg 
L–1) med oligotrofna jezera. Kljub veliki koncentraciji hranilnih snovi v jezeru, pa primarna pro-
dukcija ni tako velika kot je v drugih podobnih jezerskih ekosistemih. Vzrok za to lahko iščemo 
50 Acta Biologica Slovenica, 52 (2), 2009
v drugačnem kemizmu, saj se zaradi bližine industrije jezero polni z ioni Ca2+, K+, Mg2+, Na+, 
Cl–, še posebej pa  z SO42–. Prevladajoči algi v letu 2007 sta bili cyanobacteria Pseudanabaena 
cf. catenata in Planktothrix rubescens. Cvet prve vrste se je pojavil v juniju, druga vrsta pa je 
cvetela od novembra do januarja. Pogosti vrsti v letu 2007 sta bili tudi dynophyta Ceratium 
hirundinella in Peridinium cinctum. 
Ključne besede: pH, jezero, hranila, fitoplankton
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 orga-
nisms 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 characteris-
tics, 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 sea-
sonal 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 com-
munity 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 eutrophica-
tion processes, is one of the biological elements 
considered within the Water Framework Directive 
(WFD). Phytoplankton biomass, composition 
and abundance, together with frequency and in-
tensity 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.
Materials and Methods
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 m2 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. 
51Z. Mazej, G. Triglav Brežnik, R. Ramšak: Changes in physico­chemical characteristics …
Physical and chemical parameters
The water samples from the different depths 
at deepest part of Velenjsko jezero were taken 
and analysed four times a year (spring, summer, 
autumn, winter) in the years 1996, 1998, 2000, 
2002, 2004 and 2007. Transparency was measured 
by Secchi disk. Temperature, pH and oxygen 
profiles were obtained using a portable oxygen 
meter WTW multiline P4. Water samples for 
laboratory analysis were obtained from different 
depths using a depth (Van­Dorn) sampler. Before 
2000, parameters were determined by following 
standard methods: total phosphorus (SIST ISO 
6878:1996), ammonium nitrogen (SIST ISO 5664: 
1996), nitrate nitrogen and sulphate (SIST ISO 
10304­2: 1996), SEP (DIN 38404), magnesium 
and calcium (SIST EN ISO 7980). From 2000 
onwards, parameters were determined by the 
following standard methods: total phosphorus 
(SIST ISO 6878: 1996), ammonium nitrogen 
(SIST ISO 5664: 1996), nitrate nitrogen, chloride 
and sulphate (SIST ISO 10304: 1998), SEP (SIST 
EN 27888:1998), magnesium and calcium (SIST 
EN ISO 7980). 
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.
The plankton samples were taken and analysed 
four times a year (spring, summer, autumn, winter) 
in the years 1994, 1995, 1996, 2000, 2002, 2004 
and 2007. Qualitative 20 µm mesh plankton net 
samples were taken as a vertical profile, preserved 
in 3% formaldehyde and analysed for phytoplank-
ton species community composition. The species 
were identified using a light microscope accord-
ing to Hindak (1978), eTTl & gärTner (1988), 
ettl & al. (1999), krammer & lange-BerTaloT 
(1991, 1997, 2000a, 2000b, 2004), komarek & 
al. (2005), starMach (1985), popovski & pfiesTer 
(1990), sTreBle & krauTer (2002), vrHovšek & 
al. (2006). Their abundance was rated into three 
categories: present (1), subdominant (3) and 
dominant (5). Unicellular Cyanobacteria were 
counted like trichomes.
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 abun-
dance, 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 de-
creased 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 eutrophica-
tion, due to non-point sources of nutrients from 
drainage areas, nevertheless occurred between 
Table 1: OECD recommendations for classification of lakes into trophic categories (OECD 1982) based on total 
phosphorus, total nitrogen and chlorophyll contents and transparency of the water. The values for Ve-
lenjsko jezero in 2007 are shaded.
Tabela 1: Priporočila OECD (1992) za uvrstitev jezer v trofične kategorije na podlagi povprečnih letnih koncentracij 
celotnega dušika in fosforja, koncentracij klorofila a ter prosojnosti vode.  Kategorije, v katere lahko 
uvrstimo Velenjsko jezero so osenčene.
OECD value Total phosphorus
(µg L–1)
Total nitrogen
(µg L–1)
Transparency
(m)
Chlorophyll
(µg L–1)
Ultraoligotrophic <4 <200 >12 <1
Oligotrophic <10 200–400 >6 <2.5
Mesoeutrophic 10–35 300–650 6–3 2.5–8
Eutrophic 35–100 500–1500 3–1,5 8–25
Hyper eutrophic >100 <1500 <1.5 >25
52 Acta Biologica Slovenica, 52 (2), 2009
1996 and 2007. 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). While 
in 1996 the lake water was fully aerated to the 
bottom (45 m), oxygen was almost completely 
exhausted below a depth of 20 m in August and 
in November 2000 and November 2007. As a 
consequence the concentration of NH4+ started 
to arise, and the concentration of NO3– decreased 
below 10 metres.
Average values measured at different depths 
and the volume of each stratum was used to calcu-
late 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) deter-
mined 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 pres-
ence 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 chlo-
rophyll a, while Bacillariophyceae, Dinophyceae 
and Chrysophyceae are of lesser importance 
(kasprzak & al. 2008). 
A small but general increase of all taxons 
was observed in the period from 1996 to 2007, 
but the increase of taxons of Bacillariophyceae, 
especially Chlorophyta was notable. Of the 66 
species recognized, only a few contributed at 
least once during the year to the major percentage 
of total density (Cyanophyceae: Pseudanabaena 
cf. catenata, Planktothrix rubescens and Phor-
midium sp., Dynophyta: Ceratium hirundinella 
and Peridinium cinctum and Bacillariophyceae: 
Cyclotella meneghiniana and Stephanodiscus 
sp.) (Table 3). The phytoplankton assemblage, in 
which a number of rarer species were found among 
the dominant ones, shows the eutrophic status of 
the Lake. From 1996 onwards, a very significant 
part of the phytoplankton biomass consisted of 
dinoflagellates from genus Ceratium and Perid-
inium. They had been prevailing in the biomass 
till 2004, when the predominance of filamentous 
Table 2: Mean physico-chemical characteristics of water of 30 cm depth in Velenjsko jezero in the years of sam-
pling; n=4, average value ± SD
Tabela 2: Povprečni rezultati fizikalnih meritev in kemijskih analiz vode iz globine 30 cm v Velenjskem jezeru v 
letih vzorčevanja; n=4, povprečna vrednost ± SD
1996 1998 2000 2002 2004 2007
Transparency (m) 7.75±1.77 4.83±1.08 4.35±0.92 4.50±1.74 6.78±2.81 5.38±1.73
pH 8.75±0.25 8.75±0.26 8.40±0.20 7.80±0.10 8.20±0.20 8.80±0.40
SEP (µs cm–1) – 661±126 905±71.9 924±68.2 1085±81.0 1398±29.9
NH4+ (mg L–1) – 0.05±0.005 0.33±0.14 0.36±0.07 0.30±0.08 0.26±0.26
NO3– (mg L–1) – 4.36±0.51 2.90±0.42 3.38±0.52 4.70±0.63 3.74±0.75
P – total (mg L–1) – 0.05±0.011 0.05±0.03 0.08±0.05 0.09±0.06 0.06±0.03
Chlorophyll a (µg L–1) – – 1.40±0.51 2.10±0.98 1.70±0.10 1.03±0.38
SO42– (mg L–1) – 159±65.5 369±38.7 403±43.1 595±32.5 623±31.2
Ca2+ (mg L–1) – 70.8±8.10 81.3±22.1 119±14.2 175±15.3 192±18.6
K+ (mg L–1) – 42.4±4.24 44.1±9.5 40.0±6.01 48.0±3.52 43.1±2.25
Mg2+ (mg L–1) – 11.8±0.87 13.1±1.27 14.6±2.20 14.2±1.06 16.2±0.49
Na+ (mg L–1) – 41.6±0.85 46.2±5.40 48.2±3.42 46.7±2.70 63.7±8.60
Cl–+ (mg L–1) – – – – 21.1±0.22 34.2±4.57
53Z. Mazej, G. Triglav Brežnik, R. Ramšak: Changes in physico­chemical characteristics …
Table 3: Relative abundance (1 – present, 3 – subdominant and 5 – dominant) of phytoplankton taxons in Velenjsko 
jezero was assessed throughout the years 1996, 2002, 2004 and 2007.
Tabela 3: Relativna abundanca (1 – redko prisotna vrsta, 3 – zmerno prisotna ali pogosta vrsta; 5 – prevladujoča 
ali množična vrsta) fitoplanktonskih taksonov v Velenjskem jezeru v letih 1996, 2002, 2004 in 2007.
Taxa 
Spring Summer Autumn Winter
M
ay
 1
99
6
20
. 0
4.
 2
00
0
06
. 0
5.
 2
00
2
11
. 0
5.
 2
00
4
17
. 0
4.
 2
00
7
Av
g.
 1
99
6
17
. 0
8.
 2
00
0
18
. 0
7.
 2
00
2
27
. 0
7.
 2
00
4
08
. 0
8.
 2
00
7
Se
pt
. 1
99
6
09
. 1
1.
 2
00
0
23
. 1
0.
 2
00
2
25
. 1
0.
 2
00
4
08
. 1
1.
 2
00
7
D
ec
. 1
99
6
19
. 1
2.
 2
00
0
16
. 1
2.
 2
00
2
07
. 1
2.
 2
00
4
20
. 1
2.
 2
00
7
ph: Cyanobacteria 
(Modrozelene alge)
    Anabaena constricta 1
    Pseudanabaena cf. catenata. 3 3 3 1 1 1
    Aphanizomenon flos- aquae 1 1 1 1 1
    Aphanizomenon gracile 1 1
    Aphanizomenon sp. 1
    Dactylococcopsis sp. 
    (D. fascicularis)
1
    Coelosphaeria sp. 1
    Gomphosphaeria sp. 1
    Oscillatoria agardhii 1
    Oscillatoria brevis 1 1 1
    Oscillatoria formosa 1 1 1
    Oscillatoria limosa 1
    Oscillatoria limnetica 1
    Oscillatoria sp. 1 1 1 1 1 1 1 1 1 1 1
    Oscillatoria tenuis 1 1
    Oscillatoria rubescens 
   (Planktothrix rubescens)
5 5 5 5 5 5 5 5
    Merismopedia sp. 1
    Phormidium sp. 1 1 1 3 1 5
    Phormidium retzii 1
    Spirulina sp. 1
ph: Pyrrhophyceae
(Dinoflagellatae – ognjene alge)
cl: Gymnodinales
    Ceratium hirundinella 1 3 1 2 5 3 5 3 3 1 5 5 3 1 1 3
    Gymnodinium paradoxum 1
    Gymnodinium sp. 1
    Peridinium sp. 1
    Peridinium cinctum 
   (Peridinopsis penardii)
1 1 1 3 1 1 5 1 1 1
    Peridinium tabullatum 1
    Peridinium umbonatum
54 Acta Biologica Slovenica, 52 (2), 2009
Taxa
Spring Summer Autumn Winter
M
ay
 1
99
6
20
. 0
4.
 2
00
0
06
. 0
5.
 2
00
2
11
. 0
5.
 2
00
4
17
. 0
4.
 2
00
7
A
ug
. 1
99
6
17
. 0
8.
 2
00
0
18
. 0
7.
 2
00
2
27
. 0
7.
 2
00
4
08
. 0
8.
 2
00
7
Se
pt
. 1
99
6
09
. 1
1.
 2
00
0
23
. 1
0.
 2
00
2
25
. 1
0.
 2
00
4
08
. 1
1.
 2
00
7
D
ec
. 1
99
6
19
. 1
2.
 2
00
0
16
. 1
2.
 2
00
2
07
. 1
2.
 2
00
4
20
. 1
2.
 2
00
7
ph: Chlorophyta 
    Ankistrodesmus acicularis 
    (Monoraphidium griffithii)
1 1 1 1
    Ankistrodesmus falcatus 
    (Monoraphidium contortum)
1
    Chlorococcum infusionum 1 1 1 1 1
    Coelastrum microporum 1 1 1
    Coelastrum reticulatum 1 1 1 1 1 1
    Coelastrum sphaericum 1
    Cosmarium bloculatum 1
    Monoraphidium convulatum
    Microspora sp. 1
    Pandorina morum 1 1 1
    Pediastrum clathratum 1 1 1 1 1
    Pediastrum boryanum 1 1 1 1 1
    Pediastrum gracillinum 1
    Pediastrum simplex 1
    Pediastrum sp. 3 2 1
    Planktosphaeria gelatinosa 1
    Scenedesmus obliquus 1 1
    Scenedesmus platydiscus 1
    Scenedesmus sp. 1 1 1 1 1
    Sphaerocystis sp. 1
    Staurastrum gracile 1 1
    Staurastrum paradoxum 1
    Volvox sp. 1
ph: Euglenophyta
Euglena sp. 1 1
ph: Heterokontophyta 
cl: Chrysophyceae 
    Dinobryon divergens 1
    Dinobryon sp. 1 1
cl: Bacillariophyceae (Diatomeae)
Achnantes minutissima 1
    Amphora veneta 1 1
    Amphora ovalis 1
    Amphora sp. 1 1 1
    Asterionella formosa 1 1 1 1 1 1 1 1 1
    Caloneis amphisbaena 1 1 1
    Cocconeis sp. 1
55Z. Mazej, G. Triglav Brežnik, R. Ramšak: Changes in physico­chemical characteristics …
Taxa
Spring Summer Autumn Winter
M
ay
 1
99
6
20
. 0
4.
 2
00
0
06
. 0
5.
 2
00
2
11
. 0
5.
 2
00
4
17
. 0
4.
 2
00
7
A
ug
. 1
99
6
17
. 0
8.
 2
00
0
18
. 0
7.
 2
00
2
27
. 0
7.
 2
00
4
08
. 0
8.
 2
00
7
Se
pt
. 1
99
6
09
. 1
1.
 2
00
0
23
. 1
0.
 2
00
2
25
. 1
0.
 2
00
4
08
. 1
1.
 2
00
7
D
ec
. 1
99
6
19
. 1
2.
 2
00
0
16
. 1
2.
 2
00
2
07
. 1
2.
 2
00
4
20
. 1
2.
 2
00
7
    Cyclotella meneghiniana
    (C. kuetzingiana,
    C.melosiroides)
1 1 1 3 1
    Cyclotella sp. 1 1 1 1
    Cymatopleura solea
    (C. librilis)
1
    Cymbella lanceolata 1
    Diatoma elongatum (D. tenue) 1 1
    Diatoma vulgare (D. vulgaris) 1
    Fragilaria crotonensis 1 1 1
    Hantzschia amphioxys 1
    Gyrosigma attenuatum
   (G. acuminatum)
1
    Melosira varians 1
    Navicula cryptocephala 1
    Navicula cuspidata
    Navicula radiosa 1
    Navicula sp. 1 1 2 1 1
    Nitzschia sigmoidea 1
    Nitzschia sp. 2
    Stephanodiscus sp.
    (S.hantzschii)
3 5
    Surirella sp. 1 1
    Synedra acus 3 1 3 1 1 3 1 1 2 1
Cyanobacteria was observed. Nutrient enrichment 
of lakes is usually accompanied by characteristic 
shifts within the phytoplankton community. During 
eutrophication, small flagellated taxa are replaced 
by increasing proportions of green algae, with 
cyanobacteria finally predominating (mcQueen 
& al. 1986). The distribution of dinoflagellates 
as a function of major chemical and physical 
factors shows that most dinoflagellate species 
have restricted ranges with respect to calcium, 
pH, dissolved organic matter, and temperature 
(Taylor & pollingHer 1987). Some are however 
highly tolerant and widespread, especially species 
of Ceratium and Peridinium, which were present 
in high abundance in Velenjsko jezero. Many 
other (micro) algae species, especially greens (22 
species), occurred infrequently during warmer 
periods of the year. 
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*106 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 
56 Acta Biologica Slovenica, 52 (2), 2009
Fig. 1: Vertical distribution of oxygen, nitrate, ammonium and chlorophyll a concentrations in Velenjsko jezero 
in summer and autumn of 1996, 2000 and 2007.
Slika 1: Vertikalna razporeditev koncentracij kisika, nitrata, amonija in klorofila a poleti in jeseni v letih 1996, 
2000 in 2007.
57Z. Mazej, G. Triglav Brežnik, R. Ramšak: Changes in physico­chemical characteristics …
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 ac-
tive (Micheletti & al. 1998). It usually grows at a 
depth where the penetrating PAR is around 1–5% 
of the surface values (cHorus & BarTram 1999). In 
favourable meteorological and climatic conditions, 
it migrates to the surface forming a surface bloom 
frequently covering almost the entire lake surface. 
Such blooms can persist on the surface even in 
January and can grow under ice cover (sedmak 
& kosi 2001). In such cases microcystin-YR can 
normally be detected in bloom samples (sedmak 
& kosi 2001). Not all cyanobacteria blooms are 
toxic, and even blooms caused by known toxin 
producers may not actually produce toxins, or may 
only do so at undetectable levels. The triggers of 
toxin production are not known well. This type 
of toxin has been shown to persist in water for a 
week or more after the bloom has disappeared. 
No human deaths have been directly associated 
with these cyanotoxins, however they may cause 
skin irritations or nausea (carMichael 1997). The 
presence of microcystins can also influence the 
growth of other phytoplanktonts in the bloom. 
High densities of Planktothrix can inhibit the 
growth of other phytoplankton species and thus 
reduce the number of alternative food particles 
for zooplankton. The diversity in the blooms is 
thus low. It has been suggested that there this is 
due to the combined effect of light limitation and 
microcystin influence on susceptible phytoplankton 
species (sedmak & kosi 2002).
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). Wash-
ing 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 SO42– L–1. 
SO42– has no influence on the trophic status of 
the water. Velenjsko jezero contains very high 
concentrations of divalent cations, especially Ca2+, 
providing good conditions for the development 
of green algae, which have high requirements for 
Ca2+. 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 nutri-
ent 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 con-
ductivity (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 normalisa-
tion of pH, Cyanobacteria have replaced 
Dinophyta as the predominant species. The 
predominant algae in the lake ten years later 
were cyanobacteria Pseudanabaena cf. cat-
enata and Planktothrix rubescens as well as 
dynophyta Ceratium hirundinella and Perid-
inium cinctum. The first bloom of the former 
usually occurs in June and the second from 
November to January.
58 Acta Biologica Slovenica, 52 (2), 2009
Literature
carMichael W. W. 1997: The Cynotoxins. In: calloW J. A. (ed.): Advances in Botanical Research, 
Incorporating advances in Plant pathology 27: 211–256, Academic Press, London, San Diego.
cHorus i. & j. BarTram 1999: Toxic Cyanobacteria in water. A guide to their public health consequence, 
monitoring and management. E&FN SPON, London: 416 pp.
Directive 2000/60/EC of the European Parlament and of the Council of 23 October 2000 establishing 
a framework for Community action in field of water policy, http://europa.eu.int/smartapi/cgi/sga_d
oc?smartapi!celexapi!prod!CELEXnumdoc&lg=en&numdoc=32000L0060&model=guichett
eTTl H. & g. gärTner 1988: Süßwasserflora von Mitteleuropa, Chlorophyta II, Tetrasporales, Chlo-
rococcales, Gloeodendrales, Band 10, Gustav Fischer, Verlag Stuttgart, 436 pp.
komárek j. & k. anagnosTidis 1999: Cyanoprokaryota, Band 19/1, Teil: Chroococcales. in: eTTl 
H., g. gärTner, H. Heynig, d. mollenHauer (Eds.): Süßwasserflora von Mitteleuropa, Gustav 
Fischer Verlag Stuttgart, 548 pp.
Harmful Algae, 2004, Harmful Algae Blooms in Maryland (Anabaena, Aphanizomenon) http://www.
dnr.state.md.us/bay/hab/anabaena.html 28.10.2004.
Hindak f., p. marvan, j. komarek, k. rosa, j. popovsky & o. lHoTsky 1978: Sladkovodne riasy, 
Slovenske pedagogicke nakladatelstvo, Bratislava, 724 pp.
kasprzak p., j. padisák, r. kosHel, l. krieniTz, f. gervais 2008: Chlorophyll a concentration across 
a trophic gradient of Lakes: An estimator of phytoplankton biomass? Limnologica 38: 327–338.
komárek, j., k. anagnosTidis, B. Büdel, g. gärTner, l. krieniTz & m. scHagerl 2005. Cyano-
prokaryota II, 2.Teil/Part2: Oscillatoriales, Band/Volume 19/2, Spektum AkademischerVerlag, 
Elsevier GmbH.
krammer k. & H. lange-BerTaloT 1991, 2004:  S., Geb. Süßwasserflora von Mitteleuropa, Bd. 02/4: 
Bacillariophyceae, Teil 4: Achnanthaceae, Kritische Ergänzungen zu Achnanthes s.l., Navicula 
s.str., Gomphonema, Gesamtliteraturverzeichnis Teil 1–4, Ergänzter Nachdruck. 468 pp. 
krammer k. & H. lange-BerTaloT 1997: Süßwasserflora von Mitteleuropa, Bd. 02/2: Bacillariophy-
ceae, Teil 2: Bacillariaceae, Epithemiaceae, Surirellaceae, Band 2/2, XIV, 612 pp., Softcover. 
krammer k. & H. lange-BerTaloT 2000a: Süßwasserflora von Mitteleuropa, Bd. 02/5: Bacillario-
phyceae, Vol. 2/5, 2000, XIV, 311 pp.
krammer k. & H. lange-BerTaloT 2000b: Bacillariophyceae, Teil 3: Centrales, Fragilariaceae, Euno-
tiaceae, Süßwasserflora von Mitteleuropa , Band 2/3 , XIV, 600 pp.
mazej z. & m. epšek 2005: The macrophytes of lake Velenjsko Jezero, Slovenia – the succession of 
macrophytes after restoration of the lake. Acta biol. slov. 48: 21–31.
Mazej z. & M. GerM 2008: Seasonal changes in the contents of nutrients in five macrophyte species 
from the lake Velenjsko jezero (Slovenia). Acta biol. slov. 51 (1): 3–11.
mcQueen d., j. r. posT & e. l. mills 1986: Trophic relationships in freshwater pelagic ecosystems. 
Can. J. Fish. Aquat. Sci. 43: 1571–1581.
micHeleTTi s, f. scHanz & a. e. WalsBy 1998: The daily integral of photosynthesis by Planktothrix 
rubescens during summer stratification and autumnal mixing in Lake Zurich. New Phytol. 193: 
233–246.
OECD 1982: Eutrophication of waters, monitoring, assessment and management. Paris.
Official Gazette of the Republic of Slovenia No. 11/2002. Rules on chemical status of surface wa-
ters.
popovsky j. & l. a. pfiesTer 1990: Süßwasserflora von Mitteleuropa, Dinophyceae (Dinoflagellida), 
Band 6, Gustav Fischer Verlag Stuttgart, 272 pp.
ramšak r. & m. rejic 1995: Raziskave in spremljanje kakovosti jezer v Šaleški dolini : poročilo za 
leto 1994. ERICo Velenje, 70 pp. 
ramšak r. 1996: Opravljene meritve temperature, kisika in pH na Velenjskem jezeru in mikrobiološke 
analize na turističnem jezeru 19.7.1996. ERICo, Velenje, 3 pp. 
59Z. Mazej, G. Triglav Brežnik, R. Ramšak: Changes in physico­chemical characteristics …
remec rekar Š. 2008: Kakovost jezer v letu 2007 – Quality of the lakes in 2007. Ministry for Envi-
ronment and Spatial Planning, Environmental Agency of The Republic of Slovenia, Ljubljana. 
(http://www.arso.gov.si/vode/jezera/Porocilo_jezera_2007.pdf).
revilla m., j. franco, j. Bald, á. Borja, a. laza, s. seoane & v. valencia 2009: Assessment of the 
phytoplankton ecological status in the Basque coast (northern Spain) according to the European 
Water Framework Directive. J. Sea Res. 61 (1–2): 60–67.
reynolds c. s. 1984: Phytoplankton assemblages and their periodicity in stratifying lake systems. 
Holarctic Ecology, 3: 141–159.
reynolds c. s. & a. c. peTersen 2000: The distribution of planktonic Cyanobacteria in Irish lakes 
in relation to their trophic states. Hydrobiologia 424: 91–99.
round f. e. 1981: The Ecology of Algae. Cambridge University Press, Cambridge.
sedmak B., & g. kosi 1997: Cvetenje cianobakterij v ribnikih Republike Slovenije in njihova toksičnost 
= Cyanobacterial blooms in fish ponds of Slovenia and their toxicity. Ichthyos (Ljublj.), 14 (1): 
9–21.
sedmak B. & g. kosi 2002: Harmful cyanobacterial blooms in Slovenia – bloom types and micro-
cystin producers = Škodljiva cianobakterijska cvetenja v Sloveniji – tipi cvetenj in proizvajalci 
mikrocistinov. Acta biol. slov. 45 (1): 17–30.
sTarmacH k. 1985: Süßwasserflora von Mitteleuropa, Chrysophyceae und Haptophyceae, Band 1, 
Gustav Fischer Verlag Stuttgart, 515 pp (Dinobryonaceae).
sTreBle H. & d. krauTer 2002: Das Leben im Wasertropfen, Mikroflora und Mikrofauna des Süß-
wassers Ein Bestimmungsbuch, Franckh-Kosmos Verlags-GmbH & Co, 428 pp.
Taylor f. j. r. & u. pollingHer 1987: Ecology of dinoflagellates. In: Taylor f. j. r. (ed.): The 
Biology of Flagellates. Blackwell Scientific Publs., Oxford. pp: 398–529.
Wetzel r. 2001: Limnology, 3rd edition. Academic Press, New York, USA.
Willén e. 2001: Four decades of research on the Swedish large lakes Málaren, Hjälmaren, Vättern 
and Vännern: the significance of monitoring and remedial measures for a sustainable society. 
Ambio 30: 458–466.
vrHovšek d., g. kosi, a. krivograd klemenčič & n. smolar-žvanuT 2006: Monografija sladkovodnih 
in kopenskih alg v Sloveniji, Založba ZRC, ZRC SAZU, 172 pp.