ACTA BIOLOGICA SLOVENICA 
 LJUBLJANA 2008           Vol. 51, [t. 1: 13–20
Sprejeto (accepted): 27.10.2008
Competitive advantages of Najas marina L. in a process of littoral 
colonization in the lake Velenjsko jezero (Slovenija)
Tekmovalne prednosti vrste Najas marina L. pri kolonizaciji litorala 
Velenjskega jezera (Slovenia)
 Zdenka MAZEJ1*, Mateja GERM2
1 ERICo d.o.o., Environmental Research and Industrial Co-operation, Koroška 58, 3320 Velenje, 
Slovenia; 
E-mail address: zdenka.mazej@erico.si (*corresponding author)
2 Biotechnical Faculty, Department of Biology, Večna pot 111, 1000 Ljubljana, Slovenia
Abstract. Najas marina is the dominant macrophyte species in Velenjsko jezero. It appeared 
in the lake in 1997 and soon prevailed over the species Myriophyllum spicatum L. and Pota-
mogeton crispus L., which used to be the two most abundant species in the lake in the past. The 
physico-chemical and geomorphological characteristics of the lake are discussed in relation to the 
attributes of Najas marina, presenting competitive advantages in this environment. Conditions in 
the lake such as warm water and unstable sediment enabled successful growth and life strategy 
of Najas marina, which is a summer-annual plant with short life cycle, quick propagation from 
seeds and a very extensive root system.
Key words: artificial lake, physico-chemical and geomorphological characteristics of the 
lake, plant invasion, aquatic macrophytes, Najas marina L.
Izvleček. Najas marina je prevladujoča makrofitska vrsta v Velenjskem jezeru. V jezeru se 
je pojavila leta 1997 in kmalu prevladala nad vrstama Myriophyllum spicatum L. in Potamogeton 
crispus L., ki sta do takrat prevladovali v jezeru. V članku so izpostavljene fizikalno kemijske 
in geomorfološke značilnosti jezera, ki so ene izmed pomembnih dejavnikov, ki vplivajo na 
kompeticijske lastnosti določene vrste. Razmere v jezeru, kot so topla voda in nestabilen 
sediment, omogočajo uspešen razvoj in rast vrste Najas marina, ki je toploljubna enoletnica s 
kratkim življenjskim ciklom, hitrim načinom razmnoževanja iz semen in z zelo ekstenzivnim 
koreninskim sistemom.
Ključne besede: umetno jezero, fizikalno kemijske in geomorfološke značilnosti jezera, 
invazivnost rastlin, vodni makrofiti, Najas marina L.
Introduction
The ability of a plant species to invade a region depends not only on the attributes of the plant, 
but also on the physico-chemical characteristics of the lake habitat invaded (Ali & SoltAn 2006). 
Besides penetration of radiation and temperature, the key factor determining the composition and 
vertical distribution of submersed macrophyte communities is substrate composition (average grain 
size, sorting level, silt fraction, organic matter). Sediments influence vegetation in two ways, i.e. by 
serving as an anchor for roots and rhizoids (HAndley & dAvy 2002) and as potential nutrient reservoir 
14 Acta Biologica Slovenica, 51 (1), 2008
(PerAltA & al. 2003). ScultHorPe (1967) pointed out that the principal influence of the substrate on 
the distribution of rooted aquatic plants is due to its physical texture rather than chemical composition. 
The sediment texture is a very important characteristic, which has very great influence on the physical, 
chemical and, indirectly, on the biological properties of the sediment. The mechanical properties and 
instability of the substrate in aquatic habitats might prevent seedling establishment and increase the 
chances of dislodgement (tituS & Hoover 1991) in many species. In contrast seedlings of Najas marina 
L. can not colonize on firm sediments (HAndley & dAvy 2002). Authors who have investigated the 
interaction between vegetation and sediment (lindner 1978, Selig & al. 2007) clearly distinguished 
between plant communities’ characteristic for mineral sediments with low nutrient content and plant 
communities’ characteristic for muddy sediments with high nutrient content. 
Najas marina is a rare species in Slovenia, registered in only six locations (Vreš & Kaligarič 1999, 
germ & al. 2008) beside Velenjsko jezero, where Najas marina is the dominant species. It appeared 
in the lake in 1997 (mAzej 1998) and outcompeted both Myriophyllum spicatum L. and Potamogeton 
crispus L., which used to be abundant in previous years. However, they had never occurred in the 
lake to such an extent as Najas marina. Myriophyllum spicatum and Potamogeton crispus are known 
as very competitive species, which establish large monospecific weed beds in many lakes (nicHolS 
& SHow 1986; BolduAn & al. 1994), including some Slovenian meso-eutrophic lakes (mAzej 1998; 
Mazej & gaberščiK 1999). 
We hypothesize that physico-chemical and geomorphological characteristics of the lake are the 
main factors, which allow the broad expansion of Najas marina. Species composition was related 
to environmental characteristics in order to point out the reasons for the successful growth of Najas 
marina, which prevailed over other macrophytes. Some of the attributes of Najas marina were also 
stressed.
Materials and methods
Study Area 
Velenjsko jezero is situated in the Šalek valley, in the Sub-alpine part of Slovenia near the Aus-
trian border at an altitude of 366 m, with a surface area of 135000 m2 and a maximum depth of 54 m. 
Huge lignite-coal reserves, which are dug in Velenje Colliery are the crucial factor of human caused 
changes and pollution of the Šalek valley. The most remarkable consequences of coal mining are three 
subsidence lakes, Škalsko, Velenjsko and Družmirsko jezero. Velenjsko jezero came into existence 
after the World War II. At the beginning of its existence it was used as a reservoir for ash transport 
water from the Šoštanj thermal plant. The pH of transport water is around 12. The pH of lake was the 
same so any sorts of organism could not survive in such an alkaline environment. Up to early eighties 
ash slurry had run into lake, but afterwards the building of the ash landfill was begun. Ash reminded 
on the landfill and only transport water ran to lake. The pH remained 12 because the only reason for 
high alkinity was transport water. The closed loop system for the ash was built in 1994 and this has 
an impact on the lake quality. In only three years the lake pH has almost been normalized and biota 
appeared in the lake again (ŠterBenk, 1999). It was recolonized by phyto- and zooplankton, fish, 
macrophytes and other organisms. The pH-value of the lake is now around 8. 
Sampling
Three sampling locations (L1, L2, L3) with highest species richness and similar morphometric 
characteristics of littoral on the south-eastern part of the lake, were chosen. Samples of water and sedi-
ment were taken adjacent to vegetation from three sampling locations monthly from June to September 
2004. Samples of water were taken at 0.5 m depth in plastic bottles, while sediment was sampled by a 
15Z. Mazej, M. Germ: Competitive advantages of Najas marina L. in a process of littoral colonization …
handled plastic scoop. Water transparency was measured in the middle of the lake with a Secchi disk. 
Temperature at 30 cm and pH at all three locations were measured with a MultiLine P4. 
Presence and abundance of macrophytes
At the same time the distribution of macrophyte species at the three sampling locations was 
assessed using a boat, a depth meter, a viewing box and a sampling rake. Species abundance was 
evaluated according to koHler (1978) on a five level descriptor scale (1 – very rare, 2 – infrequent, 
3 – common, 4 – frequent, 5 – abundant, predominant). 
Water and sediment analysis
The samples of water and sediment were brought to the laboratory and stored at 4 oC. The contents 
of total nitrogen (TN), total phosphorus (TP) and soluble reactive phosphate (SRP) in sediment were 
analysed by the standard methods: ISO 11261:1995, ISO 11263:1995 and ÖNORM L 1088:2005, 
while TN and TP in the water were determined according to the standard methods ISO 10304-2:1995 
and ISO 6878:2004. 
Soil texture, sorting level and the content of organic matter were also determined in the sedi-
ment. Soil texture was determined by mechanical analysis – the sedimentation stactometer method 
with American classification according to Hodnik (1988). The percentages of four fractions were 
determined: sand (grain size > 0.2 mm), coarse silt (grain size 0.05<x<0.2 mm), fine silt (grain size 
0.02<x<0.05 mm) and loam (grain size < 0.02 mm). The sorting level was calculated according to 
nAuScH & ScHlungBAum (1984). The organic matter was determined according to the method described 
in international standard ISO 14235.
Results
The littoral of Velenjsko jezero is relatively steep (Table 1). The geomorphology of the chosen 
transects was quite homogenous, as the areas of the littoral at different depth zones were similar.
Table 1: Average distance and area of the littoral from the shore to a defined depth of the lake basin measured at 
three sections each 200±2 m in length.
Tabela 1: Povprečna razdalja od obale do določene globine jezera in površina enometerskih globinskih pasov v 
treh izbranih transektih (širina enega 200±2 m).
Depth of lake 
basin (m)
Distance from the 
shore (m)
Depth zone 
(m)
Area of the littoral of the three sections (m2)
         L1                    L2                      L3
1  6.1±2.30 0 – 1  710 1396 1572
2 11.6±2.09 1 – 2  644 1177 1473
3 16.9±1.40 2 – 3  784 1047 1357
4 22.2±1.43 3 – 4  808  948 1368
5 27.4±1.15 4 – 5  874  959 1307
6 32.7±1.00 5 – 6  971  933 1298
7 38.1±1.00 6 – 7 1013  921 1308
We detected large seasonal changes in the presence and abundance of macrophyte species in the 
three chosen locations of the lake (Table 2). Potamogeton pectinatus L. prevailed in June and July, 
while in August Najas marina became the most abundant and colonized almost all littoral. Potamoge-
ton crispus was abundantly present mainly in June, while its abundance drastically decreased during 
16 Acta Biologica Slovenica, 51 (1), 2008
July. Chara sp., Potamogeton lucens L., Najas minor All. and Myriophyllum spicatum were present 
in low abundance over the whole season.
Table 2: Average relative abundance of different macrophyte species in Velenjsko jezero in the year 2004 at the 
three chosen transects 200±2 m in length. Abundance: a five level descriptor scale (0 – absent, 1 – very 
rare, 2 – infrequent, 3 – common, 4 frequent, 5 – abundant, predominant) (koHler 1978).
Tabela 2: Prisotnost in pogostost makrofitov v Velenjskem jezeru v treh izbranih transektih od junija do septembra 
2004. Abundanca temelji na petstopenjski skali: 0 – odsotna, 1 – zelo redka, 2 – posamična, 3 – pogosta, 
4 – množična, 5 – prevladujoča) (koHler 1978).
L1 L2 L3
Jun Jul Aug Sep Jun Jul Aug Sep Jun Jul Aug Sep
Chara sp. 1 1 0 0 1 0 0 0 0 0 0 0
Myriophyllum spicatum 0 1 1 2 1 1 0 0 1 1 0 0
Najas marina 0 1 5 4 0 2 5 3 0 2 5 3
Potamogeton crispus 0 2 0 0 2 1 0 0 2 1 0 0
Potamogeton pectinatus 3 3 3 2 0 0 0 0 3 3 2 2
Potamogeton lucens 0 0 0 0 1 2 2 1 1 2 2 1
Najas minor 0 0 0 0 0 0 2 2 0 1 2 2
The mean values of physical characteristics of water observed in Velenjsko jezero throughout the 
season 2004 are shown in Table 3. The temperature of the water was the highest in August (23.9 oC), 
when the transparency of the lake was the lowest (3.1 m). In September lake water was still relatively 
warm (22.5 oC), but the transparency had increased (8 m). pH value was stable and the water was well 
aerated throughout the whole season. 
Table 3: Physical characteristics of water over the period from June to September 2004 at three chosen locations 
(average±SD; n=3). 
Tabela 3: Rezultati fizikalnih meritev in kemijskih analiz vode opravljenih enkrat mesečno od junija do septembra 
2004 na izbranih transektih (povprečna vrednost ±SD; n=3).
Date
2004
Temperature
(oC)
Oxygen
(mg/L)
Oxygen saturation 
(%)
pH Transparency
(m)
08.06. 19.0±0.5 10.0±0.3 111.0±1.1 8.2±0.1 4.5
14.07. 21.0±0.1 11.7±0.5 127.4±0.9 8.1±0.1 3.7
18.08. 23.9±0.2  7.8±0.2 100.0±1.2 8.1±0.1 3.1
08.09. 22.5±0.2  7.5±0.3  93.9±0.9 8.2±0.1 8.0
The contents of TN and TP in water and sediment and SRP in sediment varied throughout the 
season (Table 4), at a rather high level.
Sediment parameters were variable, when comparing different sampling sites, but on average 
sand (83.6%) was the prevailing fraction. The sorting level (S=1.6) reflects the fact that one-grain 
size fractions clearly dominated (more than 50%), which is often the case in exposed areas. The level 
of organic matter varied from 2.5 to 13 % (Table 5). 
17Z. Mazej, M. Germ: Competitive advantages of Najas marina L. in a process of littoral colonization …
Table 4: The contents of nutrients in sediment and water measured every month from June to September at three 
chosen locations (average±SD; n=3).
Table 4: Povprečna vsebnost hranil v sedimentu in vodi, merjena enkrat mesečno od junija do septembra (povprečna 
vrednost ±SD; n=3).
                               SEDIMENT WATER
Date Total N
(%DW)
Total P
(mg/kg)
SRP
(mg/kg)
Total N
(mg/l)
Total P
(mg/l)
08.06. 0.067±0.05 302±169 7.14±1.51 1.23±0.01 0.06±0.02
14.07. 0.102±0.04 456±140 6.40±2.32 1.20±0.00 0.14±0.05
18.08. 0.098±0.01 456±93.2 6.75±1.89 1.83±0.58 0.17±0.02
08.09. 0.098±0.008 505±54.6 6.51±1.31 1.10±0.00 0.08±0.01
Table 5: Results of the analysis of sediment texture, sorting level and organic matter content at three chosen 
locations (povprečna vrednost ±SD; n=3).
Tabela 5: Tekstura in sortirna stopnja sedimenta ter vsebnost organskih snovi v sedimentu iz treh izbranih transektov 
(average±SD; n=3).
 
loam
(%)
fine silt
(%) coarse silt (%)
sand
(%) sorting level organic matter (%)
5.42±5.80 7.06±5.42 3.88±1.58 83.6±11.5 1.62±0.29 5.8±4.7
Discussion
Many authors reported Myriophyllum spicatum as outcompeting Najas marina in different lake 
ecosystems (AgAmi & wAiSel 1986, SimonS & al. 1994, BootSmA & al. 1999, Ali & SoltAn 2006). In 
Velenjsko jezero the situation is the opposite (Tab. 2). Prior to 1997, when Najas marina appeared in 
the lake, Myriophyllum spicatum and Potamogeton crispus were the most abundant species (mAzej 
1998, Mazej & gaberščiK 1999, Mazej & epšeK 2005). After 1997 Najas marina had spread rapidly 
within a few years, forming weed beds that covered larger areas. Abundance of Myriophyllum spicatum 
and Potamogeton crispus declined even in spring (mAzej & ePŠek 2005), when Najas marina is not 
competitive, since it is a summer-annual plant, starting its life cycle in July. Potamogeton crispus and 
Myriophyllum spicatum show different life cycles in comparison to Najas marina. They overwinter, 
and then grow rapidly in the spring. By early summer the plants undergo senescence processes and 
then remain dormant until autumn (BolduAn & al. 1994). The biomass production of Potamogeton 
crispus often reaches its maximum in early summer, avoiding competition with other species (e.g. 
with Najas marina) in the same habitat, which begins their growth later in the season (toBieSSen & 
Snow 1984). In contrast to the other species in the lake, Najas marina propagates from seeds (AgAmi 
& weiSel 1986) that enable quick colonization of new habitats. 
Optimum germination temperatures of Najas marina are between 20 and 25 oC (Proctor 1967). 
That might be a reason why the species is rare throughout Europe (HAndley & al. 2002). It begins its 
growth in July, when nutrients in lakes are usually impoverished and the littoral is fully overgrown 
with other macrophytes. This is not the case in Velenjsko jezero. In July soft sediment littoral is still 
not colonized and the amounts of nutrients in the water and sediment do not decrease before Sep-
tember (Tabs. 2, 4). Velenjsko jezero could be classified as eutrophic according to the level of total 
phosphorus (0.11 mg/L) and total nitrogen (1.34 mg/L) in the water (oecd 1982). Najas marina is 
reported to thrive in oligotrophic and mesotrophic shallow freshwater habitats (TP in sediment <50 
µg g-1 dw) and was considered to be nearly extinct due to hypereutrophication (Selig & al. 2007). For 
18 Acta Biologica Slovenica, 51 (1), 2008
example, SimonS & al. (1994) beside the decline of charophytes, described the decline of Najas marina 
stands in a shallow peat lake (Botshol, The Netherlands) due to nutrient input. The same effect was 
observed in the research of BootSmA & al. (1999) in the shallow lakes of Naardemeer nature reserve 
(The Netherlands). Najas marina disappeared from the lakes when the concentration of total P in the 
water was 0.1 mg/l and it reappeared when the concentration of total phosphorus had dropped to 0.05 
mg/l. Our results cannot confirm these findings as the sediment (mean TP in sediment is 329 µg g-1 dw) 
and water (mean TP in water is 0.11 mg/l) of Velenjsko jezero are very rich in phosphorus. However, 
Velenjsko jezero, in contrast to Botshol and the lakes in Naardemeer, is a deep lake (max depth is 
54.6 m) with steep slopes along greater parts of the lakeshore (Tab. 1). Its transparency (mean Secchi 
depth was 4.8 m) is much better than the transparency in the lakes of Naardmeer (from 0.3 to 1.05 m). 
Stelzer & al. (2005) claim, that Najas marina is a taxon with a high preference for good ecological 
status in a case of German lakes. Selig & al. (2007) found Najas marina stands in eutrophic waters, 
even though TP was comparatively low.
The littoral of Velenjsko jezero is decisive for low sediment stability. Therefore species, which 
develop extensive root systems, were expected to be more successful. Unstable sediment that pre-
vails in Velenjsko jezero prevented spreading of Myriophyllum spicatum and Potamogeton crispus. 
Myriophyllum spicatum is reported to have difficulty in becoming rooted in a soft and moving bottom 
(nicHolS & SHAw 1986). Najas marina with a branching growth form (HAndley & dAvy 2000) forms 
dense beds and develops very extensive root systems (HAndley & dAvy 2002). Roots can account for 
more than 30% of plant biomass compared to the values of 10% or less for the majority of submersed 
aquatic plants (AgAmi &weiSel 1986). The sorting level and sediment fraction type reflected other 
abiotic parameters such as wave exposure and water current. Selig & al. (2007) found that Najas 
marina stands prefer sediment with a higher sorting level in the range similar to the sediment of 
Velenjsko jezero (S =1.6) (Tab. 5). This level describes open sandy areas with little sedimentation. 
Myriophyllum spicatum and Potamogeton pectinatus can be found in sediments with a wide range 
of sorting level but with the lower median (1.3) (Selig & al. 2007). It is known from literature that 
Myriophyllum spicatum (and also Potamogeton crispus) grow best on fine sediment with a relatively 
high organic matter concentration, e.g. 10-25% (nicHolS & SHAw 1986) and 9-13% (vAn wijck & al. 
1994), which is higher than the concentration in the sediment of Velenjsko jezero.
Conclusion
This investigation revealed that species composition in a particular lake depends on plant strategy 
and on the physico-chemical and geomorphological characteristics of the lake. The same species cannot 
be equally successful in habitats with differing physico-chemical and geomorphological characteristics. 
Favourable circumstances in the lake Velenjsko jezero like warm water, unstable sediment and life 
strategy of Najas marina as a summer-annual plant with short life cycle, rapid propagation by seed 
and a very extensive root system, enable this species to grow successfully and dominate over other 
macrophytes in the lake.
Acknowledgement
The authors are grateful to Prof. Dr. Alenka Gaberščik and Prof. Roger Pain for critical reading 
of the manuscript. This research was a part of the project financed by the Šoštanj Thermal Power 
Plant: “The role of macrophytes in maintaining ecological balance of a lake ecosystem in the case of 
Velenje Lake”, and of the project financed by the Slovenian Research Agency (ARRS), Ljubljana: 
“Determination of heavy metal pollution in the lake ecosystems by using bioindicators on the case 
of the Šalek lakes No. L1-7144-1007. Additional support was given by the research programme 
19Z. Mazej, M. Germ: Competitive advantages of Najas marina L. in a process of littoral colonization …
“Communities, relations and communications in ecosystems” (P1-0255), financed by the Slovenian 
Research Agency (ARRS), Ljubljana. Parts of analyses were performed in the laboratory at National 
institute of Biology.
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