ACTA BIOLOGICA SLOVENICA 
LJUBLJANA 2002 Vol. 45, Št. 2: 25 - 34 
Sprejeto (accepted): 2002-05-27 
Aquatic macrophytes of the mountain lake Krnsko jezero, Slovenia 
Vodni makrofiti Krnskega jezera, Slovenija 
Olga URBANC-BERČIČ, Alenka GABERŠČIK, Milijan ŠIŠKO, Anton BRANCELJ 
National Institute of Biology, Večna pot 111, I 000 Ljubljana, Slovenia; 
Tel.+386 I 4233388; E-mail : olga.urbanc@nib.si 
Abstract. The macrophyte vegetation in the mountain lake Krnsko jezero has 
been monitored since 1991. Five species of aquatic macrophytes, occupying different 
depths, were present in the lake. Changes in depth distribution were detected due to 
accelerated eutrophication caused by inputs of nutrients from the watershed. In the 
period between 1997 and 1998 repeated earthquakes additionally influenced the 
processes in the lake Krnsko jezero, by increased input of matter. The occasional 
phytoplankton blooms reduced water transparency and consequently disturbance to 
growth, development and distribution of anchored macrophytes occurred. In 1998 
macrophytes spread down to 7.0 m, reaching the maximum at 5.8 kg DW m·2 at 2 m 
depth. The total primary production in the lake was estimated at 4991 kg organic matter 
per year out of which Chara and Potamogeton species presented 95.5 % and 0.6 %, 
respectively. 
Key words: aquatic macrophytes, chlorophyll a, lake Krnsko jezero, mountain 
lake, nutrients, organic matter, primary production 
Izvleček. Makrofite v Krnskem jezeru spremljamo od 199 l leta. V jezeru je 
prisotnih pet vrst podvodnih rastlin, ki uspevajo na različnih globinah. Zaradi pospešene 
evtrofikacije, kije odraz povečanega vnosa nutrientov iz zaledja, se globina uspevanja 
spreminja. Med letoma 1997 in 1998 je bila serija potresov, zaradi česar se je vnos 
snovi iz zaledja povečal in dodatno vplival na procese v jezeru. Občasno cvetenje 
fitoplanktona je zmanj šalo prosojnost vode, kar se je odrazilo na rasti , razvoju in 
razporeditvi submerznih makrofitov. Tako so v letu 1998 makrofiti segali do globine 7 
m, njihova največja biomasa je bila 5,8 kg suhe teže m·2 na globini 2 m. Celotna 
primarna produkcija organske snovi v jezeru je bila ocenjena na 4991 kg leto-1, pri 
čemer je bil delež parožnic 95,5 %, delež dristavcev pa le 0,6 %. 
Ključne besede: vodni makrofiti , klorofil a, Krnsko jezero, gorsko jezero, 
nutrienti, organska snov, primarna produkcija 
26 Acta Biologica Slovenica, 45 (2), 2002 
Introduction 
Mountain lakes, usually located in remote natura! environment, are becoming interesting sites of 
ecological research for two reasons inter alta: they are relatively small and they are sensitive to 
different influences from the environment. The alpine lake Krnsko jezero is an eutrophic lake (GABERŠČIK 
& URBANC-BERČIČ 1996). In the last century some events had put an evident mark on this ecosystem. 
During the First World War the wider area was the centre of severe battles that had resulted in increased 
input of matter from the watershed. Latter on two species of fish were introduced in the lake; in late 
twenties Salvelinus alpinus (Linnaeus, 1758) and two decades latter Phoxinus phoxinus (Linnaeus, 
1758) (BRANCEU & al.1997). Other human activities, pasturing and mountaineering at most, presented 
an influential activity, the latter being intensified in the last decade. The influence of these activities had 
been closely studied in the nearby lake (BRANCEU & al. 2000). 
The first investigation ofbasic geographic and hydrobiologic characteristics was carried out in the 
late fifties (GAMS 1962), but no record on macrophytes was made until 1988 (BLAŽENČIČ & al. 1990). 
In 1996 the systematic research of 14 alpine lakes started as an addition to the previous 5-year 
monitoring. The study had revealed that the most abundant aquatic vegetation was in the lake Krnsko 
jezero (GABERŠČIK & URBANC-BERČIČ, 1996). Comparable studies to ours on primary production in 
mountain lakes was not found but in our case it was evident that macrophytes contributed a great deal 
to the production ofthe lake. Researches on lowland lakes, rich with aquatic vegetation showed, that 
macrophytes presen! an important factor maintaining the stability of the ecosystem (Oz1MEK & al. 1990, 
R0RSLETT 1991, RASPOPOV & al. 1996). 
The present study was carried out in order to evaluate the abundance and production of 
anchored macrophytes, related to the main trophic parameters and their contribution to the primary 
production of the lake. 
Material and methods 
Site description 
The lake Krnsko jezero is located well bellow the timberline at the altitude of 1383 m. It is of 
glacial origin. Lake is the largest and the deepest in the Triglav National Park, covering the area of 
49600 m2• The lake is 17 .6 m deep (Fig. 1 ). At the highest water leve! the to tal vol ume of 446700 
m3 of water accumulates in the lake. At heavy rains fluctuations of water leve! could be as high as 4 
m, reaching the normal leve! within two days. The lake is ice covered usually from November to 
May. During the summer it is stratified, with late spring (after ice-cover melting) and late autumn 
homothermy. The nearby watershed is diverse, formed of stones, gravel, rocks, meadows and 
pastures. The wider area is geologically unstable and some severe earthquakes with soil avalanches 
have caused strong disturbances in watershed and increased the input of matter. 
Water chemistry 
The sampling of water for chemical analyses and chlorophyll a was carried out with Van Dom 
sampler. Samples were collected on the vertical profile every 2.5 m (O m, 2.5 m, 5 m, 7.5 m, JO.O m, 
12.5 m and above bottom) in monthly intervals in the ice-free period from May 1996 to the end of 
1999. Electric conductivity was measured in laboratory at 25 °C with conductivity meter (MA 5950, 
Iskra, Slovenia) and concentration of oxygen was measured in situ with an oxygenmeter (WTW Oxi 
320, Germany). To evaluate chemical variables analytical methods according to APHA (1992) were 
applied. Ali spectrophotometric measurements were performed with spectrophotometer Perkin Elmer 
(UV NIS, Lambda 12). The temperature was measured with a thermistor (Iskra, Slovenia). Chlorophyll 
O. Urbanc-Berčič, A. Gaberščik, M. Šiško, A. Brancelj: Aquatic macrophytes of the mountain ... 27 
a concentration was determined by photometry (GOLTERMAN & al. 1978, WETZEL & LIKENs 1995). 
Biomass of phytoplankton was calculated on the basis of chlorophyll a concentrations (T ALLING 
1975). Transparency was estimated by means of Secchi disc. The physical and chemical data are 
presented as median, minimum and maximum of total measured values. 
,__ ______ \00 m 
Figure 1: Bathimetric map of the lake Krnsko jezero 
Slika 1: Batimetrična karta Krnskega jezera 
Macrophyte survey 
The survey of submersed vegetation was carried out from 1991 to 1999 along 20 permanent 
transects. The shore line was divided into the sections of different lengths, based on the uniformity 
of morphological characteristics (bathimetric map), water depth and substrate types. For graphic 
presentation the homogenous sections were additionally divided into the fina! 31 sections, with the 
length of 35±2 m (Fig. 2). Abundance of the observed macrophytes was estimated semi-
quantitatively using a 5-level scale (!- present, 2 - rare, 3 - common, 4 - abundant, 5 - predominant) 
(MELZER 1992, PALL & JANAUER 1995). The survey was made from the boat using depthmeter, view 
box and sampling rake. In September 1998, macrophyte community was surveyed by scuba-diving 
on six representative transects. The total biomass of macrophytes was collected in the area of 0.1 m2 
on the depths of 1, 2, 3, 4 and 7 m, washed thoroughly to remove periphyton, oven dried at 105 °C 
and weighted. The ratio of organic matter was determined by ignition at 550 °C following the 
method from APHA (1992). 
Results and Discussion 
As the majority oflakes world-wide the lake Krnsko jezero is subjected to accelerated eutrophication 
due to diverse reasons. The disturbances to the lake were caused by the First World War, introduction 
of fish, traditional pasturing, earthquakes and mountaineering (GABERŠČIK & al. 1997). In the period 
between 1997 and 1998 after repeated earthquakes the sedimentation rate in the lake increased. The 
analyses of the material in the sediment traps revealed that the annual amount of gathered matter was 
increasing from 0.84 g DW m 2 in 1997, 1.01 g DW m·2 in 1998 to 3.68 g DW m 2 in 1999 (MURI&. 
al. 2002). 
28 
o J;tJ 
Ei 
i 7,5 II?' ' -:<' ' ,'\ 
Cl 
s 
a 
O) 
Cl 
10 
12,5 t'Q\,'.#Ml\ 
15 JOJ 
2,5 
5 
7,5 
10 
12,5 
15 
O 100 200 300 
Tota! P mg m ·3 
o 10 20 
Water temperature °C 
Acta Biologica Slovenica, 45 (2), 2002 
7,5 
10 
12,5 
15 
o 1000 2000 3000 
Tota! N mg m·3 
12,5 <:-::"/~!/"< 
o 5 10 15 
Oxygen gm·3 
7 ,5 ~:fq:it':i~~.-:•.,-. .. , ..... , .. -.-.. ... t ... w.·.·;-'j 
10 
o 
2,5 
5 
7,5 
10 
12,5 
15 
o 6 12 18 
Chlorophylla mg m·3 
"._·ft> --------' 
··--·~x=-:::i 
O 100 200 300 400 
Conductivity µS cm·1 
Figure 2: Main physical and chemical parameters measured in the depth profile of the lake Krnsko 
jezero in the period from 1996 to 1999- Median values, minima and maxima are presented_ 
Slika 2: Glavni fizikalni in kemični parametri merjeni na globinskem profilu v Krnskem jezeru, v 
obdobju od 1996 do 1999 leta_ Pedstavljene so mediane, minimalne in maximalne vrednosti_ 
o. Urbanc-Berčič, A. Gaberščik, M. Šiško, A. Brancelj: Aquatic macrophytes of the mountain... 29 
Concentrations of nutrients in water column measured in a period from 1996 to 1999 varied to a great 
extent (Fig. 2). The median values of nutrients increased with depth, the total phosphorus ranging from 
21 to 44 mg m·3 and the total nitrogen from 915 to 1463 mg m·3. The concentrations of total phosphorus 
in the water column differed a lot, exhibiting the lowest variability in the bottom sam ples, due to releasing 
of phosphorus from the sediment under anaerobic conditions. The concentrations of total nitrogen and 
dissolved solids rneasured as electric conductivity showed less variability than total phosphorus. 
The introduction of fish influenced the food web in the lake, and consequently only one zooplankton 
species, Cyclops vicinus (Uljanin 1875) was present. In the nearby lakes zooplankton was stili diverse 
and up to 6 species could be found (BRANCEU & al. 1997). The reduction of zooplankton diversity 
resulted further in the increase of chlorophyll a concentration and in lower water transparency (Tab. 1 ). 
During the monitoring Secchi depth varied in the range from 2.5 m to 1 1 m, median value being 4 m. 
The concentration of chlorophyll a asa measure of phytoplankton production was rather constant in 
the upper layers, reaching median value about 3 mg m·3• The highest concentration was measured at the 
bottom layer (median value 13 mg m-3), likely due to alga] sedimentation, while the variability from 1 
to 17 mg m·3 was deterrnined at the depth of 7.5 m, where thermocline usually occurred. 
Table 1: Data on minimal water tranparencies and maximal depth distribution of Characeae in the lake 
Krnsko jezero from 1994 to 1999; n = 3 - 6, * BLAŽENčrč & al. (1990). 
Tabela 1: Najnižje vrednosti prosojnosti vode in največja globina uspevanja parožnic (Characeae) v 
Krnskem jezeru v obdobju od 1994 do 1999 (n=3-6). Podatek* BLAžENČIČ & al. (1990). 
Year 
Secchi depth (m) 
Maximal depth 
distribution (m) 
1988 1991 1992 1993 1994 1995 1996 1997 1998 1999 
10.0* 8.0 8.0 8.0 8.0 
4.2 
7.0 
3.5 
7.0 
3.0 
7.5 
2.5 
7.0 
2.5 
7.5 
By preventing the penetration of light, planktonic algae posed depth limits for anchored macrophytes. 
In spite of this phenomenon the littoral area of the lake Krnsko jezero is large. We defined it by the 
extension of macrophyte community (WETZEL 1990) on the basis of bathimetric map. In 1998 it 
presented about 19600 m2, that is nearly 40 % of the projected lake area. Five species of submersed 
macrophytes i.e. Chara delicatula Ag., Chara contraria f capillacea Mig., Potamogeton alpinus 
Balbis, Potamogeton pusillus L. and Batrachium trichophyllum Chaix. colonising different depths, 
were determined in the lake (Fig. 3). The community is considered as flora! rich in comparison to other 
13 Slovene alpine lakes (GABERŠČIK & URBANC-BERČIČ 1996). This coincides with the data ofnorthern 
European lakes elaborated by R0RSLETI (1991) which revealed that upland lakes exhibited proportionally 
fewer species than lowland sites. 
During the monitoring the maximal depths colonised by macrophytes oscillated (Tab. 1) showing 
trend of gradual reduction. In 1988 Chara delicatula spread down to l O m. The maximum depth of this 
species was 8 m from 1991 to 1994 and 7 - 7.5 m since then (BLAŽENČic & al. 1990, GABERŠČIK & URBANC-
BERČIČ 1996). The competition for light and nutrients with filamentous and planktonic algae was estimated 
to be the main reason for this feature. BuNoow ( 1992a,b) found out that charophytes were sensitive to the 
reduction of light more than to the increase of total phosphorus, as it was indicated in the previous studies 
(FoRSBERG 1964). LEHMANN & LACHAVANNE (1999) compared the relevance of two indices to retine 
changes in macrophyte community in Lake Geneva. They demonstrated that saprobic index based on 
organic matter inputs, was more closely related to subtle vegetation changes than index, based on 
nutrients load. Our survey indicated that increased inputs detected in sediment traps, originating from the 
disturbed watershed supposed to be a trigger for changing of submersed vegetation. Survey of 116 high 
mountain Pyrenean lakes, led by GACIA & al. ( 1994) pointed out two environmental variables, vegetation 
coverage of the catchment and altitude as the main factors that influence the distribution and composition 
30 
s o,~~----
..5 , ✓ 
& ~ Q -2 " C> 
5 
5 
Acta Biologica Slovenica, 45 (2), 2002 
Potamogeton alpinus 
~~) { o 1 D--<J ( : c~J 
10 
10 
10 
15 20 25 30 
Potamogeton pusillus 
)tli(~~ 
15 20 25 30 
Chara contraria 
15 
Chara delicatula 
Filamentous algae 
15 
Locations 
20 25 30 
20 25 30 
Figure 3: Depth distribution and abundance (rated from I to 5) of macrophytes in 31 sections (length 
35±2 m) in the littoral of the lake Krnsko jezero in September 1998. Section 1 isto the right of the lake 
outflow (counter clockwise). 
Slika 3: Globinska razporeditev in številčnost makrofitov, ocenjena od 1 do 5, na 31 odsekih litorala v 
Krnskem jezeru, september 1998. Odseki ( 1 je desno od iztoka) potekajo v obratni smeri urinega kazalca. 
o. Urbanc-Berčič, A. Gaberščik, M. Šiško, A. Brancelj: Aquatic macrophytes of the mountain... 31 
of macrophytes. These two determinants reflect potential inputs in whole while a nutrie:1t availability in 
the water as a relevant variable for the primary producers, depends on the processes in lake, as well. 
In 1998, when a detailed survey was made by scuba-di vers, the maximum biomass of macrophytes 
was determined between the depths of2 and 3 m (the estimation of abundance was 5) even lhough the 
continuous stands of stoneworts spread to the depth of7.5 m (Fig. 3). Plants were extremely elongated 
reaching the length up to 1 m at the depth of 3 m. The prevailing species in the lake were Chara 
delicatula and Chara contrariaf capillacea. In 1998 their share per lake was 2700 and 2068 kg of 
organic matter, respectively. The form er reached up to 3 kg DW m·2 on lhe depth of 3 m. The highest 
dry weight 5.8 kg m·2 of Chara contrariaf capillacea was determined on the depth of 2 m (Fig. 4). 
o 2 4 6 8 
Biomass kg DW 1112 
Figure 4: Depth distribution of macrophytes biomass in September 1998 (white column - average, 
grey column - maximum value). Figures present the values calculated from the macrophytes' samples 
obtained by scuba diving on 6 transects. 
Slika 4 : Globinska razporeditev biomase makrofitov v septembru 1998 (bel stolpec so povprečne, siv 
stolpec maximalne vrednosti). Številke predstavljajo biomaso, preračunano iz vzorcev makrofitov, 
zbranih s pomočjo potapljačev na 6 transektih. 
Both species contributed a lot to the total dry weight which was determined at 17537 kg. The value 
corresponds to 4991 kg of organic matter. Converted to energy content it means that about 108 KJ were 
integrated in the ecosystem from solar energy. With such a production the lake reaches the leve! of 
fertile ecosystems (OZIMEK & al. 1990). The predominance of stonewort species which function asa 
stabiliser of the sediment is beneficial for the system resilience conceming inputs from the watershed 
i.e. nutrients, as well as particulate matter. The other two submersed macrophytes Potamogeton alpinus 
and Potamogeton pusillus were not abundant and their biomass presented a negligible share of 21 and 
33 kg DW per lake in 1998 (Fig.S) but having the highest organic content (Fig. 6). In spite of low 
production pondweeds presen! a factor stabilising the shallow, sensitive parts of the littoral and 
promoting sedimentation of fine particles. The macrophytic biomass distribution in the lake Krnsko 
jezero reflects the characteristics of the lake, i.e. basin configuration, high availability of nutrients, 
waler leve! fluctuations and modest light conditions . Stoneworts that have lower light demands for 
photosynthesis (BLINDOW l 992a,b) express a competitive advantage over higher submersed plants. 
Occasionally, when the runoff of nutrients from the watershed was increased, the biomass of filamentous 
and planktonic algae increased. Even though planktonic algae presented a minor share in the primary 
production of the lake, they affected macrophytes (LACHAVANNE 1985, HouGH & al. 1989). Calculated 
32 Acta Biologica Slovenica, 45 (2), 2002 
to the whole lake volume their average concent~ation would be 4 mg m·3, which is equivalent to the 
production of 92 kg organic matter per lake, presenting 1.8 % of the lake primary production (Fig. 5). 
In 1998, nearly the same share of primary production was contributed from filamentous algae. They 
Cha del 
-H,4% 
Pot alp 
0,1% 
Cha con 
54,1% 
Figure 5. Partition oftotal organic matter among different primary producers in the lake Krnsko jezero 
in September 1998 when the total primary production was estimated to 4991 kg DW of organic matter. 
Periphyton biomass is excluded. 
Slika 5: Delež organske snovi med različnimi primarnimi producenti v Krnskem jezeru, september 1998. 
Celotna primarna produkcija je bila ocenjena na 4991 kg suhe organske snovi brez biomase perifitona. 
H 
~ ro s 
(.) 
·a 
ro 
e!l o 
~ 
100 
80 
60 
40 
20 
o 
'°"'•.· 0,.s 
¾ 
~ 
Figure 6: The ratio of organic matter in the biomass of different primary producers from the lake 
Krnsko jezero. 
Slika 6: Razmerje organske snovi v biomasi različnih primarnih producentov v Krnskem jezeru. 
appeared mainly in the shallow water competing with pondweeds. On the sites with steep slopes where 
the conditions for anchoring were not favourable, higher plants and stoneworts were scarce. On these 
locations filamentous algae were not in a position of direct competitors (locations from 18 to 30 on Fig. 3). 
Conclusions 
In the Jake Krnsko jezero accelerated eutrophication resulted in high primary production that 
presented very high absorbance of solar energy into the lake. In 1998, littoral presented 40 % of a 
O. Urbanc-Berčič, A. Gaberščik, M. Šiško, A. Brancelj: Aquatic macrophytes of the mountain... 33 
projected Jake area and was densely colonised with macrophytes. The majo:- share of biomass belonged 
to charophyte species (95 .5%) while planktonic and filamentous algae contributed 3.5%, only. In spite 
of this minority they presented an influential competitor to stonewort for light. Reduction of light 
penetration was a feature which have been measured in the last years. Successful competition of 
filamentous algae for nutrients was evident in the shallowest parts, with exception ofthe Jocations with 
steep slopes and unsuitable substrates for stoneworts and higher plants to anchor. The colonisation of 
littoral with anchored macrophytes increases the ecosystem resilience, by stabilising the erosion zone 
which overtakes function of a sedimentation zone. 
In spite of different influences from the watershed, the direct impact of a single disturbance was 
difficult to define on a tem pora! scale, but the consequences of integral impact resulted in reduction of 
water transparency, enhanced appearance of filamentous algae and shrinking of littoral zone. Those 
features indicated the eutrophication in lake Krnsko jezero and showed trend of accelerated succession 
in a remote mountain lake ecosystem. 
Povzetek 
Pospešena evtrofikacija Krnskega jezera se odraža v visoki primarni produkciji , kar pomeni zelo 
visoko vezavo sončne energije v jezeru . Litoralje v letu 1998 zavzemal kar40 % jezerske površine, ki 
je bila gosto poseljena z makrofiti. Večinski delež biomase je pripadel parožnicam (95,5% ), medtem ko 
je bil delež planktonskih in nitastih alg samo 3,5%. Kljub podrejenemu deležu so bile alge vpliven 
kompetitor za svetlobo, katere intenziteta se je v jezeru zmanjševala. Uspešnost nitastih alg v tekmovanju 
za hranila je bila očitna predvsem v plitvejših predelih, z izjemo odsekov na strmih brežinah, kjer je bil 
substrat neprimeren za zakoreninjenje parožnic in višjih rastlin. Poselitev litorala z makrofitskimi 
vrstami, ki se zakoreninijo, povečuje elastičnost ekosistema, saj te vrste s koreninskim sistemom 
stabilizirajo potencialne erozijske cone, ki tako postanejo sedimentacijske cone. 
Kljub očitnim vplivom iz zaledja je neposreden vpliv posamezne motnje tako časovno kot prostorsko 
težko razločiti. Vendar pa so posledice vseh motenj določljive in prepoznane kot znižanje prosojnosti 
vode, kot povečano pojavljanje nitastih alg in kot krčenje litoralne cone. Vsi ti pojavi, ki so prepoznani 
kot odraz pospešene evtrofikacije, so v Krnskem jezeru časovno zgoščeni in kot taki dober prikaz 
sukcesijskih procesov v oddaljenem gorskem jezerskem ekosistemu. 
Acknowledgement 
Project was supported by Ministry of Science and Technology Republic of Slovenia (SLO-ALPE 
project Paleoecology and ecology in mountain lakes, JI 7 414) and partly by European Commission as 
M O LAR project (Measuring and modelling the dynamic response of remote mountain lake ecosystems 
to environmental change, IC20-CT96-02 J ). 
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