The relationship between phytoplankton species dominance and environmental variables in a shallow lake (Lake Vrana, Croatia)
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SHALLOW LAKES
The relationship between phytoplankton speciesdominance and environmental variables in a shallow lake(Lake Vrana, Croatia)
Marija Gligora Æ Andelka Plenkovic-Moraj ÆKoraljka Kralj Æ Istvan Grigorszky Æ Danijela Peros-Pucar
� Springer Science+Business Media B.V. 2007
Abstract The shallow Lake Vrana was studied
over a 1-year period, special attention being paid
to the phytoplankton. Phytoplankton was inves-
tigated monthly with respect to temporal vari-
ability of selected environmental factors. The
regular annual development observed was in
species contribution to total biomass rather than
in seasonal changes in species composition. The
assemblage was dominated by Cosmarium tenue
Arch. and Synedra sp. In winter and in spring the
phytoplankton assemblage was dominated by
Cosmarium tenue and high contribution of Syne-
dra sp. was observed during the summer and
autumn. Results suggest that concentrations of
inorganic nitrogen and phosphorus were critical
in regulating phytoplankton biomass and species
dominance.
Keywords Phytoplankton � Shallow lake �Species dominance
Introduction
One of the two states in shallow lakes is a clear
state rich in submerged vegetation (Scheffer,
1998). Macrophytes provide refuge for pelagic
grazers (Stephen et al., 1998), support a diverse
fish population, prevent sediment resuspension
and are also one of several mechanisms that can
suppress phytoplankton growth by reducing nutri-
ent concentration in the lake water column
(Jeppesen et al., 1997; Søndergaard et al., 2003;
Fernandez-Alaez et al., 2004). The rates at which
organisms consume resources depend on the
availability of such resources. Phytoplankton
utilizes nutrients and becomes a component of
the pelagic food web (Lampert & Sommer, 1997;
Gliwicz, 2002; Stephen et al., 2004; Auer et al.,
2004). Interannual variability in nutrient
resources can play an important role in the
determination of phytoplankton distribution and
abundance (Sommer, 1987; Reynolds, 1997;
Naselli-Flores, 2000). Phytoplankton is affected
by external factors but is also influenced by the
outcome of the competition itself (Kilham &
Guest editors: R. D. Gulati, E. Lammens, N. De Pauw &E. Van DonkShallow lakes in a changing world
M. Gligora (&) � A. Plenkovic-Moraj �K. KraljDivision of Biology, Department of Botany, Facultyof Science, University of Zagreb, Rooseveltov trg 6,HR-10000 Zagreb, Croatiae-mail: mgligora@zg.biol.pmf.hr
I. GrigorszkyBotanical Department, Debrecen University,P.O. Box 14, Debrecen 4010, Hungary
D. Peros-PucarPublic Health Institute Zadar, Kolovare 2, Zadar,Croatia
123
Hydrobiologia (2007) 584:337–346
DOI 10.1007/s10750-007-0590-0
Tilman, 1979; Tilman et al., 1982; Huisman &
Weissing, 1999). There were two working hypoth-
eses during this investigation. First, that environ-
mental variables influence phytoplankton species
composition and biomass. Second, that phyto-
plankton seasonality and changes in species
dominance are correlated with nutrient availabil-
ity in a shallow vegetated lake.
Study site
Lake Vrana (Vransko jezero) is situated on the
East Adriatic coast. It is a freshwater (salinity
range 0.7–1.2&), karstic cryptodepression con-
nected at the south west to the Adriatic Sea by a
narrow artificial channel. Lake Vrana is polymic-
tic, shallow waterbody with a mean depth of 2 m.
Catchments are characterized mainly by karst.
Because of the karst morphology and the agri-
cultural nature of the surrounding area, in con-
junction with the presence of a bird sanctuary in
the north west part, the lake receives considerable
external nutrient loads. The lake is dominated by
macrophytes, especially during the warm summer
months. Macrophyte coverage, as visually esti-
mated, exceeded 50% of the surface area during
summer (dominants: Najas marina L., Potamog-
eton pectinatus L. and Potamogeton perfoliatus
L.). The macrophyte association also includes
Phragmites australis (Cav.) Trin. ex Steud., Scir-
pus triqueter L. Tavlen, Myriophyllum spicatum
L., Utricularia australis R. Br, Chara sp., Typha
angustifolia L. and Potamogeton lucens L., as
accompanying species (M. Mrakovcic, pers.
comm.).
Materials and methods
Phytoplankton was sampled monthly at four
selected sites, in the period from January to
December 2004. Samples for phytoplankton anal-
ysis were collected at 0.5 m depth, preserved in
2% formaldehyde (final concentration) and
stored at 4�C. After 24 h sedimentation, 10 ml
subsamples were analyzed. Cell counts were
obtained with the inverted microscope following
Uthermohl’s method (1958). A minimum of 400
settling units were counted, nanophytoplankton
(<20 lm) at 1000· magnification in 15 randomly
selected fields and microphytoplankton (>20 lm)
cells in a transect at 400· magnification, providing
a counting error of <10% (Lund et al., 1958).
Measurements of the 20 randomly chosen cells
were obtained by AxioVision software and bio-
mass was calculated according to Rott (1981).
Transparency was measured with a Secchi disc.
Samples for water chemistry were taken simulta-
neously with phytoplankton samples. They were
stored in refrigerated boxes and analyzed in the
laboratory for nitrate, nitrite, ammonium, soluble
reactive phosphorus, total phosphorus, chlorides,
and pH, alkalinity, conductivity, oxygen and its
saturation (APHA, 1995). Chlorophyll a concen-
trations were analyzed fluorometrically according
to Method 445.0 after filtration onto Whitman
GF/F glass filters and acetone extraction (Arar &
Collins, 1997).
For statistical evaluation PRIMER 5 software
package was employed (Clarke & Warwick,
2001). Principal-component analysis (PCA) of
all physical and chemical variables (Table 1,
excepted chl a) was used to identify the main
environmental variables in the dataset. Distances
between samples on the ordination attempt to
match the corresponding dissimilarities in the
environmental data. The correlation between the
individual and combined environmental parame-
ters and the phytoplankton biomass was analyzed
using PRIMER 5 submodule BIO-ENV. A stan-
dard Spearman’s rank correlation was used in this
procedure. Only taxa contributing more than 5%
to the total phytoplankton biomass were included
in the analysis. All physical and chemical data as
well as phytoplankton biomass were normalized
for PCA and BIO-ENV. To reduce the influence
of absolute biomass, the data were standardized
to a 0–1 range (Jackson, 1993). The same trans-
formation was used for all physical and chemical
parameters since data were not normally distrib-
uted (except conductivity and chloride values).
The draftsman plot of all pairwise combinations
suggested that linearity was satisfied under this
transformation, which is appropriate for these
variables. The analysis of differences between
samples and groups was restricted to non-para-
metric tests, Kruskal–Wallis and Mann–Whitney
338 Hydrobiologia (2007) 584:337–346
123
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Hydrobiologia (2007) 584:337–346 339
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U tests. Pearson’s correlation coefficients, as well
as Kruskal–Wallis and Mann–Whitney U tests,
were calculated using Statistica 6.0 software.
Results
In the course of the year water temperature in
Lake Vrana ranged between 1.6�C and 26.2�C
and was greatly influenced by the Mediterra-
nean climatic conditions. It was typically higher
during the summer (Table 1, group III) and
lower in the winter period (Table 1, group I).
The water was alkaline (7.7–9.4), with very high
conductivity (1638–3960 lS cm–1). Nitrate con-
centrations varied between 0.5 mg l–1 and
2.34 mg l–1 reaching, a maximum in March and
decreasing towards summer. The lowest nitrate
concentration was recorded in September.
Ammonium ranged from 0.004 mg l–1 to
0.3 mg l–1 with a decreasing trend from winter
to spring-summer. Mean concentration of
ammonium during summer (from July to Sep-
tember, Group III) was 0.018 mg l–1. Following
lower summer values, an increase in ammonium
concentration was recorded. Nitrite concentra-
tions ranged from 0.0013 mg l–1 to 0.023 mg l–1.
Nitrite concentrations were higher (mean
0.018 mg l–1) during the spring period (from
April to June, Group II) and decreased in the
lake during the summer and autumn. Monthly
variation of total phosphorus concentrations
changed between 0.01 mg l–1 and 0.059 mg l–1
except in January, when a mean value of
0.25 mg l–1 was recorded. Soluble phosphorus
ranged from 0.001 mg l–1 to 0.025 mg l–1 during
the study period except for a peak in January
(January mean value 0.04 mg l–1). Dissolved
inorganic N (as NH4–N + NO2–N + NO3–N) to
P (PO4–P) ratio was higher than 30:1 during
whole research period.
Among the 55 species identified during 2004,
only 15 species contributed more than 5% to the
total phytoplankton biomass. The assemblage was
dominated by Cosmarium tenue Arch. and
Syneda sp. The winter and spring assemblages
were dominated by Cosmarium tenue. The assem-
blage was determined by a high contribution of
Synedra sp. during summer and autumn (Fig. 1).
The high species diversity in January was caused
by the highest species number (24). Diversity
significantly decreased from January to June and
it was lowest in June, due to the great predom-
inance of Cosmarium tenue.
The two axes in PCA analyses accounted for
60.3% of the cumulative variance in physico-
chemical data set with eigenvalues of 5.29 and
2.55, respectively (Table 2). Axes 3, 4 and 5
accounted for 29.5% and are not discussed
further. PCA 1 was presented by temperature,
in the positive direction (r = 0.395) and ammo-
nium in the negative (r = –0.382), explaining
40.7% of the variance. PCA axis 2 was positively
influenced by NO2–N (r = 0.490) and NO3–N
(r = 0.575) (Fig. 2).
The BIO-ENV procedure presented phyto-
plankton assemblages and biomass in correlation
with concentrations of inorganic nitrogen com-
pounds, temperature and chlorides (qw = 0.389).
According to the analyses, winter samples
(January/March) were joined in Group I and
correlated with high ammonium concentrations
(Table 1) but also with high oxygen, chlorides and
alkalinity values (Fig. 2). The dominant species
during this period was Cosmarium tenue, accom-
panied by Gonatozygon sp. (Table 3). There was
significant difference in phosphorus concentration
between January and March (p < 0.05). Group II
joined the spring samples (April/May/Jun) with
the clear dominance of Cosmarium tenue (Ta-
ble 3) and high concentrations of nitrites and
nitrates, but lower ammonium values (Table 1).
The temporal distribution of Synedra sp. mostly
affected groups III and IV. According to the
statistical analysis, Group III represents summer
(July/August/September) assemblages dominated
by Synedra sp. associated with low nitrogen
concentrations, oxygen and chlorides (Fig. 2). In
terms of species dominance two subgroups of
Group III can be recognized. The first subgroup
consists of July samples where Synedra sp. was a
codominant species due to biomass, and
Gomphosphaeria sp. was the dominant species
accompanied by Planktolyngbya contorta (Lem-
mermann) Anagnostidis & Komarek and
Cosmarium tenue. The second subgroup was
represented by August and September assem-
blages with the clear dominance of Synedra sp.,
340 Hydrobiologia (2007) 584:337–346
123
and Gomphospaheria sp. as accompanying
species (Table 3). There was a statistically signif-
icant difference in levels of nitrites and nitrates
between subgroups within Group III (p < 0.01).
During autumn, nitrogen compounds again
reached higher values (Table 1) determining the
assemblage in Group IV with the dominance of
Synedra sp. and codominance of Cosmarium
tenue.
Other species, Pseudanabaena catenata Lauter-
born, Chroococcus sp., Crucigenia tetrapedia
(Kirchner) W. & G. S. West, Monoraphidium
minutum (Nageli) Komarkova-Legenerova,
Ankistrodesmus densus Korshikov, Koliella tenuis
(Nygaard) Hindak and Navicula sp. also typified
the assemblages in Lake Vrana but contributed
less to the total phytoplankton biomass during
most of the investigation period, with no distinct
seasonal differences in the assemblages.
Kruskal–Wallis and Mann–Whitney U tests
performed on the four statistically identified
groups confirmed significant differences in the
nitrogen concentrations and also temperature,
alkalinity, oxygen, concentrations of chlorides
and chlorophyll a (Table 1).
Concentrations of chlorophyll a ranged from
0.05 lg l–1 to 9.54 lg l–1 and clearly correlated
with concentration of nitrites (r = 0.66, p <
0.001). Biomass ranged from 0.94 mg l–1 to
51.60 mg l–1 and reached a higher concentration
during winter period due to the high contribution
of large benthic diatoms to the phytoplankton
biomass. Total phytoplankton biomass showed a
Fig. 1 Changes inShannon–WienerDiversity Index (H)through the investigationperiod (mean values andstandard deviation onfour sampling points)indicating groupsidentified by statisticalanalyses and dominantspecies
Table 2 Summary statistics for the first five axes of PCAperformed on the environmental data set during theresearch period
PCA axis 1 2 3 4 5
Eigen values 5.29 2.55 1.76 1.27 0.80% Variation 40.7 19.6 13.5 9.8 6.2Cumulative % variation 40.7 60.3 73.5 83.5 89.7
Hydrobiologia (2007) 584:337–346 341
123
great positive correlation with phosphates
(r = 0.54, p < 0.001) but also a very high correla-
tion with all nitrogen compounds, especially with
nitrites (r = 0.33, p < 0.05).
Discussion
The present study indicates that nutrients are
significant variables influencing phytoplankton
biomass. The regular annual development ob-
served was in species contribution to total bio-
mass rather than in seasonal changes in species
composition. According to the statistical analysis,
it was the inorganic nitrogen compounds in the
water column that mostly affected species dom-
inance. PCA showed seasonality of nitrogen
compounds, which was followed by changes in
species dominance. High nitrogen concentrations
during winter and especially during spring period
coincided with the dominance and high biomass
of Cosmarium tenue. Nitrogen compounds
reached their lowest values during summer, and
this coincided with change in species dominance.
In this time of the year, a low concentration of
inorganic nitrogen was followed by a high contri-
bution of Synedra sp. to the phytoplankton
biomass. This relationship was further supported
by the species biomass–environmental correla-
tions in statistical analyses.
According to the catchments morphology and
the characteristics of the surrounding area it can
be assumed that Lake Vrana receives a high
nutrient input. Despite the high external nutrient
loads, phosphorus was present in much lower
concentrations than nitrogen and might have a
limited effect on phytoplankton total biomass.
The Redfield ratio is one of the methods fre-
quently used to identify potentially limiting
nutrients, and phosphorus limitation in Lake
Vrana is evident considering the high concentra-
tion values of nitrogen in relation to phosphorus
in water column (Redfield, 1958). The influence
of phosphorus on the phytoplankton assemblage
Fig. 2 PCA ordination ofphysico-chemicalparameters showingsample grouping andgeneral trends of thoseparameters for eachgroup. Full line arrowsindicate tendencies ofmain parametersassociated with first andsecond PC axis. Dottedarrows are projection ofphytoplankton speciesdominance on axis 2
Table 3 Contribution(%) of dominant andaccompanying species infour groups and twosubgroups identified bystatistical analyses
Group I Group II Group III Subgroups III Group IV
Jul Aug Sep
Cosmarium tenue 50 87 10 15 7 29Diatoma tenue 7Gomphosphaeria sp. 30 35 22 15Gonatozygon sp. 14Navicula sp. 6 6Planktolyngbya contorta 10Synedra sp. 6 40 20 54 35Ulnaria ulna 8
342 Hydrobiologia (2007) 584:337–346
123
is shown by the high correlation between the total
phytoplankton biomass and phosphates.
Although not supported by statistical analyses,
Table 1 suggests that the lowest values of phos-
phates were recorded in group III when the
concentration of soluble reactive phosphorus was
on average below 0.01 mg l–1 during three sum-
mer months. Apart from the external nutrient
load in lake, the internal load, especially of
phosphorus, is very important in shallow lakes.
Although the phosphorus and nitrogen content in
sediment were not determined in this study, the
phosphorus exchange process over the sediment-
water interface can play important role in the
phosphorus budget in shallow lakes. There is
considerable evidence for phosphorus sediment
immobilization resulting in the phosphorus limi-
tation condition in the water column (Scheffer,
1998). Chemical adsorption of phosphorus in
sediment may be caused by a number of pro-
cesses, which can be critical to phosphorus release
and can apparently cause phosphorus limitation
for phytoplankton. Water that is rich in calcium
and carbonate, such as karstic Lake Vrana, may
buffer the pH and lead to a decrease in phospho-
rus release from the sediment (Scheffer, 1998). In
Lake Vrana, a high concentration of nitrate was
detected. It has been suggested that a high
concentration of nitrate may buffer the redox
potential of the sediment surface, preventing a
release of iron bound phosphorus in the same way
as oxygen (Scheffer, 1998). Also, at high turbu-
lence, which is frequently present in the shallow
Lake Vrana, the sediment surface becomes oxy-
genated and phosphorus can be immobilized by
iron again. The great accumulation of phosphorus
in sediment found in shallow lakes does not occur
with nitrogen (Jensen et al., 1991). Apart from
chemical absorption macrophytes can change
nutrient retention and its effect on the nitrogen
is more consistent than on phosphorus. Spring
and summer concentration of nitrogen decreased
in the presence of high macrophyte coverage. The
high uptake of nitrogen by macrophytes appar-
ently caused a nitrogen limitation for the phyto-
plankton (Van Donk et al., 1993). The low
concentration of nutrients during the summer
period in Lake Vrana followed the dynamics of
macrophyte coverage. It can be assumed that
aquatic macrophytes in Lake Vrana, due to high
coverage during summer, affect nutrient dynam-
ics in the lake and have an impact on phyto-
plankton biomass (Scheffer et al., 1993; Jeppesen
et al., 1997) and changes in the species domi-
nance. Cosmarium tenue was dominated as long
as high concentrations of inorganic nitrogen were
recorded in water column. The summer domi-
nant, Synedra sp., was present in the lake with
high biomass when nitrogen was exhausted from
the water column and it is evident that phospho-
rus limitation during the summer period for
Cosmarium tenue was not limiting for Synedra
sp. (Sommer, 1987).
Results showed a range of diversity values in this
stable, well-mixed system, inconstant with respect
to chemical features, and temporarily disturbed.
Low diversity values were strongly affected by the
most abundant species. Individual species adapta-
tions to an environment suitable to host steady-
state assemblages (Mischke & Nixdorf, 2003)
resulted in low diversity values, especially during
June. From January to June, diversity decreased
steeply, affected by the predominance of only one
species (Padisak, 1993), Cosmarium tenue. Diver-
sity was reduced to minimal levels by competitive
exclusion or some other biotic interaction that can
result in steady state assemblages (Naselli-Flores
et al., 2003; Rojo & Alvarez-Cobelas, 2003). Over-
looking the fact that steady state phases are not
frequently attained in phytoplankton succession
(Padisak et al., 2003), and the monthly (instead of
weekly) sampling dynamics during this investiga-
tion (Rojo & Alvarez-Cobelas, 2003), theoreti-
cally, this phytoplankton assemblage, is close to
establishing equilibrium (Sommer et al., 1993), as
was manifested by the monodominance of Cos-
marium tenue (Padisak et al., 2003), with more
than 80% in total biomass over 3 months. How-
ever, external factors may prevent the establish-
ment of equilibrium by favoring the dominance of a
new species (Reynolds, 1993). Changes in species
dominance and high contribution of Synedra sp.
coincided with decreases in concentrations of
inorganic nitrogen. The same values of nitrogen
yielded greater diversity values. Considering diver-
sity and disturbance as indirectly linked (Reynolds
et al., 1993) and disturbance as a force that can be
measured as a reaction, it could be suggested that
Hydrobiologia (2007) 584:337–346 343
123
the sufficient intensity of disturbance (Sommer
et al., 1993) was caused by a sudden event of low
concentration of nitrogen in the lake water.
According to BIO-ENV analysis, assemblages
were also affected by temperature and chloride
concentrations. High concentrations of chlorides
due to the artificial connection between the Lake
and Adriatic Sea were recorded in winter, while
lower and stable concentrations were observed in
summer. During stable summer temperature
conditions, changes in environmental variables,
such as nitrogen availability, produced noticeable
shifts in species dominance.
The phytoplankton assemblage in Lake Vrana
can be characterized in general as nanoplankton.
Cell size was found to be important in the deter-
mination of predominant species but could not be
considered the decisive factor for species selection
and their growth kinetics (Suttle et al., 1987;
Sommer, 1989). Species with small cell-sizes,
because of volume/surface area ratio and cell
shape, benefit from low nutrient concentration
and from phosphorus limited conditions and out-
compete larger-sized species (Smith & Kalff, 1982;
Grover, 1989). The species Cosmarium tenue in
Lake Vrana is characterized by a large mucous cell
envelope. The presence of an extracellular mucous
envelope is considered evidence indicative of
several functions suggested by Coesel (1994),
Decho (1990) and Whitton (1967). According to
Coesel (1994) mucilage sheets of desmids might be
indicatively related to the capture of scarce nutri-
ents. Apart from such results there are contradic-
tory experimental data with no clear indication of
the storage role of the extracellular mucous enve-
lope (Spijkerman & Coesel, 1998). It can be
deduced from Table 1 and statistical analyses that
species assemblage in Lake Vrana is naturally
selected according to the temperature, salinity
conditions, high conductivity, pH values and avail-
ability of CO2 (Reynolds, 1997) but also affinity for
the resource and species storage capacity (Spijk-
erman & Coesel, 1996a, b). According to Sommer
et al. (1993) phosphorus limitation of species with
high requirements becomes possible if SRP con-
centration falls below 10 lg l–1 and intracellular
stores have been depleted. Such conditions in Lake
Vrana were evident occasionally during the year
and for a period of 3 months during summer.
Species with low phosphorus demand, such as
pennate diatoms, would only become limited at
undetectable SRP concentrations (Sommer et al.,
1993). It can be assumed that the species domi-
nance in a given lake is determined by competition
for nutrients (Sommer, 1989). The species Synedra
sp. was competitively superior for phosphorus at a
low concentration of nitrogen and it became a
more productive summer species. This ability gives
a significant advantage to these organisms in
summer and Synedra sp. developed a larger pop-
ulation than Cosmarium tenue. It was able to
compete for phosphorus and reach dominance as
long as Cosmarium tenue was excluded by nitrogen
depletion (Lampert & Sommer, 1997).
In conclusion, the present study shows that in
the vegetated shallow Lake Vrana the concentra-
tion of nitrogen, like that of phosphorus, may be
important in the determination of phytoplankton
dominants. The phytoplankton annual succession
in Lake Vrana seems to be mainly controlled by
nutrients. Nutrient resources seem to be critical in
regulating phytoplankton species dominance and
total species biomass in Lake Vrana. In this way,
the phytoplankton community might be regulated
by external factors and also by competitive
interactions between the dominant species. The
influence of nutrients on the phytoplankton in
Lake Vrana was discussed in the specific lake
environment and should be considered a factor
controlling phytoplankton assemblages, biomass
and changes in dominance overlooking other
environmental features of the Lake.
Acknowledgments We would specially like to thankProfessor M. Mrakovcic, who initiated the study of LakeVrana. We, are also we are grateful to the CroatianMinistry of Science, Education and Sport, and theHungarian Scholarship Board for financial support.Special gratitude goes to the reviewers whose commentshelped us improve this paper.
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