Journal of Environmental Protection, 2014, 5, 17-28
Published Online January 2014 (http://www.scirp.org/journal/jep)
http://dx.doi.org/10.4236/jep.2014.51003
Trophic State Evaluation of a Large Medit errane an Lake
Utilizing Abiotic and Biotic Elements
George Kehayias*, Evangelia Doulka
Department of Envir onmental and Natural Resources Management, Univers ity of Patras, Agr inio, Greece.
Received September 21st , 2013; revised October 23rd, 2013; accepted November 19th, 2013
Copyright © 2014 George Kehayias, Evangelia Doulka. This is an open access article distributed under the Creative Commons At-
tribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is prop-
erly cited . In accord ance of the Creative Common s Attribu tio n License all Cop yrights © 2014 are reserved for SCIRP and the owner
of the intellectual property George Keh ayias, E vangeli a Doul ka. All Copyright © 2014 ar e guarded by law and by SCIRP as a guar-
dian.
ABSTRACT
The trophic state of a freshwater ecosystem reflects its environmental quality. This is why several trophic indi-
cators have been developed for such water bodies based on chemical, physical and biological parameters. Apart
from that, there are several biotic elements which can be used in accessing the environmental condition of a
freshwater ecosystem. Zooplankton organisms are important ele ments of the struct ure a nd functio n of lakes and
are considered useful indicators of alterations in their trophic dynamics and ecological state related to changes in
nutrient loading and climate. In accordance to the above, the present study is an attempt to assess the trophic
condition of the largest lake in Greece (Lake Trichonis) through the investigation of the physicochemical ele-
ments, along with the biotic indications provided by a three-year study of the lake’s zooplankton. The present
result s, compared with previous studies conducted between 15 and 25 years before, showed that there was an
increase in the maximum values of the concentrations of chlorophyll-α and nutrients, while there was a decrease
in water transparency. The implementation of Carslon’s trophic state index (TSI) revealed that Lake Trichonis
still remains an oligo-to mesotrophic ecosystem as it was in the past. However, although the zooplankton investi-
gation showed several features that are common in oligotrophic lakes, there are certain eutrophic characteristics
of the z ooplankton co mmunity (e .g. abundance v ariation pat tern, indicat or species, seaso nal successio n of clado-
cerans) po inti ng o ut a diffe r ent st at e o f the eco system in co mpariso n to the pa st. I n c o nclusio n, the use of a biotic
element like zooplankton revealed that Lake Trichonis is experiencing a transitional condition towards the eu-
trophic st ate and po ints out the necessity for consta nt inspection and monitoring of this ecosystem.
KEYWORDS
Lake Zooplankton; Eutrophication; Abundance; Tr i choni s Lake; Indicator Species; Mediterranean
1. Introduction
Eutrophicatio n of freshwater ecosystems such as natural
lakes and reservoirs is a major environmental problem.
Generally, eutrophication is the result of nutrient overe-
nrichment of the water originated from natural processes
such as decomposition of organic matter and sediment
nutrient release, or to land-based activities and external
nutrient loading (e.g. agricultural activities, sewage and
industrial discharges, atmospheric releases from fossil
fuel combustion, etc.). Agricultural activities are consi-
dered as the major provider of nutrients in lakes, espe-
cially in the Mediterranean region, where agricultural
sector comprises an important economic factor [1]. Eu-
trophication involves a change in a lake’s status from a
macrophyte-dominated clear water state to a phytop-
lankton-do minated turbid state, with severe effects to the
ecosystem like changes in the structure and trophic inte-
ractions among phytoplankton, zooplankton and benthic
communities.
The growing need for the control of nutrient enrich-
ment and pollution leads the European Union to the es-
tablishment of the Water Framework Directive (2000/60/
*Corresponding author.
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Troph ic State Evaluation of a Large Mediterranean Lake Utilizi ng Abio tic and Bio tic Elements
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EC) in order to achieve a “good quality” in all European
water bodies. Several indicators, indexes and models
have been developed to assess eutrophication and water
quality in aquatic ecosystems based on chemical, physi-
cal and biological parameters. Carlson’s trophic state
index (TSI) is the most widely used in freshwater bodies
[2]. The index uses phytoplankton biomass estimated
indirectly by chlorophyll-α pigment concentrations, wa-
ter transparency (by the use of Secchi depth) and total
phos phorus co ncentra tions. The TSI index range s from 0
to 100, although theoretically there are no lower or upper
boundaries. TSI values less than 40 correspond to
oligotrophic conditions, while between 40 and 50 for
mesotrophic and between 50 and 70 for eutrophic. Fi-
nally, index values greater than 70 are associated with
hypertrophic conditions.
Except from direct measurements of physicochemical
and biological elements, there are several biotic indica-
tors which can be used in accessing the environmental
quality of a water body [3]. Thus, in contrast to the in-
stant evaluation o f the state of an eco system provid ed by
the measurements of physicochemical parameters, the
biotic indicators have the advantage to show the influ-
ence of the chronic affection of the environmental dis-
turbance to the structure of the aquatic communities, as it
comes from the conditions existed before the measure-
ments. In this way, the use of biotic indicators can be
more useful in prediction of the evolution of the quality
of an aquatic ecosystem. Zooplankton organisms are im-
portant elements of the structure and function of lakes, as
they occupy a critical position between phytoplankton
and higher consumers like fishes within the trophic web
[4]. Its strategic position in the aquatic ecosystems, as
well as its sensitivity to both man-made and natural
changes, makes zooplankton quite suitable for biological
monitoring of water quality [5]. Zooplankton is still not
included as a biological quality parameter for aquatic
ecosystems according to the implementation of the EU
Water Framework Directive, though there are several
studies that have shown its usefulness as an indicator of
alterations in the trophic dynamics and the ecological
state of lakes related to changes in nutrient loading and
climate [6].
Lake Trichonis is the largest natural lake in Greece,
having a surface area of 98.6 km2, a catchment area of
421 km2 and a potential water volume of approximately
2.868 km3 [7]. It is located in the prefecture of Aito-
loakarnania in the western part of the country (38˚18'N -
38˚51'N, 21˚01'E - 21˚42'E) at an altitude of 18 m a.s.l. It
is a deep (Zmax = 57 m, Zmean = 29 m) and warm
monomictic lake, exhibiting a long period of thermal
stratification. There are various surface water supplies
(e.g. seasonal streams), as well as groundwater inflows
(approx. 30% of the total annual water inflows), which
provide an adequate quantity of water and resulted to
positive water balance [7]. The excess of the lake’s water
is discharged through a sluice gate canal to the adjacent
Lysimachia Lake to avoid potential flooding. From the
biodiversit y point of view, Lake T richonis has great eco-
logical importance and was included in the Natura 2000
protection network due to the priority habitat of calcare-
ous fe ns (Cladium mariscus). Moreover, it has also great
economic importance in respect to the use of water for
irrigation purposes of the nearby agricultural areas, as
well as to the commercial fishery of Atherina boyeri
whic h is t he do minant spec ies in the f ish co mmun ity [8].
Around the lake there are several villages with a total
population of about 35,000 people and the main eco-
nomic activities are agriculture, livestock breeding and
olive-oil refinery manufacturing. In the cathcment area of
the lake there are 150 km2 of agricultural land (mainly to-
bacco, olive trees, cotton and corn), while there are also
33 operating olive -oil refineries that dispose their wastes
untreated in septic tanks or directly in local streams. All
the above activities, in combination with the lack of a
wastewater treatment plant in the catchment area, imply
significant organic pollution loads for the lake [1].
The environmenta l elements of La ke Trichonis and its
trophic condition have been documented by several
studies [1,9-11] in the past and according to them the
lake has been classified as oligotrophic to mesotrophic.
At the more recent report of Bertahas et al. [1], which
compare data from two periods within a decade (1990-
1991 and 2001-2002), the trophic state of the lake
seemed to have altered from mesotrophic in the former
period to oligotrophic in t he latter, althou gh the nutrients’
concentrations have been greatly increased during this
decade. This paradox was attributed to the particular hy-
drology of the lake by the above authors, who concluded
that its trophic status is mainly hydrologically dependent
and thus unpredictable [1]. On the othe r hand , ther e have
been recent reports of the presence of zooplankton spe-
cies in Lake Trichonis which are considered indicators of
eutrophic conditions in other European lakes [12,13].
Consequently, all the above reports give a puzzling icon
of t he e nviro nme ntal state of t he lake and , along with the
given ecological and economic importance of this eco-
system, make the need for further investigation impera-
tive.
Cons idering the above , thro ugh a three-year investiga-
tion (between 2003 and 2006), the pr esent stud y aims to:
1) provide the most recent analytical data of the phys-
icochemical environment and of the trophic state of the
lake; 2) compare these recent data with the previous
studies to assess its environmental state progress; 3)
make an effort to combine abiotic and biotic (e.g. zoo-
plankton) data in order to draw a more accurate picture
of the present and possibly the future environmental
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quality of this ecosystem.
2. Materials and Methods
2.1. Zooplankton Sampling and in Situ
Measurements
Three sampling stations (A, B and C) with depths of 48,
35 and 25 m respectively, were selected for data collec-
tion in Lake Trichonis (Figure 1). The overall sampling
was carried out at monthly intervals during a three-year
period from September 2003 to August 2006. Zooplank-
ton samples were taken from all the sampling stations
during the first two periods (September 2003-August
2005), while due to the lack of differences among the
three stations (see bellow) and for logistic reasons, the
samples were taken only from the deepest station A in
the third period (September 2005-August 2006). The
zooplankton was collected with a conical plankton net
(40 cm in diameter, 100 cm in length, 50 μm mesh size),
which was modified to be a closing net with the addition
of a second rope and a releasing trigger, analogous to
those of the WP-2 closing net. Vertical hauls were con-
ducted at 10 m depth intervals from the surface down to
40 m, to 30 m and to 20 m for the stations A, B and C
respectively. The net was towed at a speed of approxi-
mately 0.5 m·sec−1. All samples were taken in the morn-
ing and were preserved in plastic bottles filled with 4%
neutralized formalin solution.
In situ measurements of the physicochemical parame-
ters were taken during the second sampling period (Sep-
tember 2004-August 2005) from all the three stations,
while from just station A during the third period (Sep-
tember 2005-August 2006). Temperature, dissolved
oxygen concentration (DO), pH and conductivity meas-
urements were taken at 2 m intervals from the surface
down to a maximum depth of 40 m, using WTW portable
instruments. Water transparency was measured with a
Secchi disc.
Figure 1. Lake Trichonis with the three sampling stations
(A, B and C).
2.2. Chemical Parameters and Chlorophyll-α
For the estimation of total phosphorus (TP), phosphates
(PO4), nitrates (NO3), nitrite s (NO2) and ammonia (NH4),
water samples were collected at all stations (September
2004-August 2005) and at the deepest station A (Sep-
tember 2005-August 2006) from 0, 10, 20, 30 and 40 m
with a 5 l Hydrobios water sampler. Analyses of all
chemical parameters were performed according to APHA,
AWW A & WPCF [14]. For the determination of chloro-
phyll-α concentration (Chl-α), 1500 ml of the water sam-
ples taken from the above depths was filtered through a
Whatman GF/A glass fibre filter shortly after collection.
Pigment extraction was made in 90% acetone and con-
centrations were determined spectrophotometrically [14].
Due to technica l problems, no Chl-α measurements were
taken in the period between September 2004 and Febru-
ary 2005.
2.3. Trophic Classification
The trophic classification of the lake was estimated using
Carslon’s [2] trophic state index (TSI) which utilizes
chlorophyll-a (Chl-a), Secchi disk transparency ( SD) and
total phosphorus concentrations (TP ). In particular, the
TS I ind ex for the three di ffere nt q ual ity va ria bles suc h a s
Secchi depth TSISD, chlorophyll-a TSIChl-a and total
phosphorus TSITP was calculated according to the equa-
tions:
( )
6014.41 ln
SD
TSI SD=−×
()
30.69.81 ln
Chl a
TSIChl a
−
= +×−
( )
4.15 14.42 ln
TP
TSI TP=+×
For TS I Chl-a and TSITP, the average epilimnetic (0 - 10
m) concentrations of Chl-a and TP (both as μg∙l−1) were
used, while for the TSISD, the values of the Secchi disk
(m) in each sa mpling station was used. T he overall Carl-
son’s TSI index was calculated as the average value of
TSISD, TSIChl-a and TS I TP in the thre e stations as follows:
3
SDChl aTP
TSI TSITSI
TSI
−
++
=
2.4. Zooplankton Analysis
In the laboratory, the zooplankton specimens were ex-
amined microscopically and were identified to the lowest
taxonomic level possible, using the keys of Rylov [15],
Ruttner–Kolisko [16], Korovchinsky [17], Alonso [18]
and Benzie [19]. For the abundance analysis, three
counts of 1.5 ml sub-samples from each sample were
made on a Sedwick-Rafter cell having a total volume of
100 ml [13]. The non-parametric Kruskal-Wallis te st and
the Mann-Whitney test (U-test) were used for assessing
differences in the environmental parameters and in the
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abundance of zooplankton species and groups among
sampling stations and sampling periods.
3. Results and Discussion
The temporal variations of the physicochemical parame-
ters recorded in station A in Lake Trichonis during a
two-year period (September 2004-August 2006) are pre-
sented in Figures 2 and 3. Station A is selected to be th e
most representative for the changes monitored along the
entire water column of the lake, since it is the deepest
station and considering the absence of statistically sig-
nificant differences a mong the three sampling st ations fo r
almost any of the above parameters (Kruskal-Wallis test,
p > 0.05).
3.1. In Situ Measurements
Lake Trichonis is a deep, warm monomictic lake where
Figure 2. Vertical profiles of temperature (A), dissolved
ox ygen ( B), pH (C) and con duc ti vity (D) measur ed in situ in
the deeper sampling station A of Lake Trichonis during
September 2004 - August 2006.
Figure 3. Vertical profiles of the concentrations of total
phosphorus (A), phosphates (B), nitrates (C), nitrites (D)
and ammonia (E) in the deeper sampling station A of Lake
Trichonis during Sept ember 200 4 - August 2006.
the temperature variation leads to the development of a
seasonal thermocline, causing highly stable water strati-
fication within the lake. Temperature fluctuated between
10˚C and 29.7˚C (F ig ur e 2) and there were no differenc-
es between the sampling periods and among the three
sampling stations (Kruskal-Wallis test, p > 0.05). In
comparison to the previous studies of Tafas et al. [11],
Koussouris et al. [10] and Overbeck et al. [9] the ma x i-
mum temperature value was greater by 2˚C, 3.7˚C and
5˚C respectively (Table 1). Stratification of the lake
started in April and the thermocline, which was created
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Table 1. Co mparisons of t he variation of the physicochemical parameters recorded in Lake Trichonis during the present
study with analogous reports fro m pr evious studies.
Parameter Present study Tafas et al. Koussouris et al. Overbeck et al.
(2004-06) (1997) (1993) (1982)
Temperature (˚C) 10 - 29.7 9.5 - 27.5 11.1 - 28.0 10.0 - 25.0
Dissolved oxygen (mg∙l−1) 0.3 - 14 3.7 - 12.0 0.5 - 12.2 2.0 - 12.0
pH 7.37 - 8.55 7.6 - 8.5 7.7 - 8.8 8.1 - 8.5
Conductivity (μS∙cm−1) 247.5 - 398.2 195 - 320 230 - 270 267 - 301
Transparency (m) 4 - 13 4 - 11 4 - 12.5 5.7 - 13.9
Chlorophyll-α (μg∙l−1) 0.2 - 9.4 2.2 - 8.2 2.3(1), 4.3(2) 0.5 - 0.8
TP (μg∙l−1) 1.5 - 131 <7 15(1), 66(2) 22 - 112
PO4 (μg∙l−1) 0 - 95.2 <20 - 70 1 - 14 2 - 40
NO3 (mg∙l−1) 0 - 0.718 0.05 - 0.45 0.003 - 0.125 0.02 - 0.232
NO2 (mg∙l−1) 0 - 0.058 0 - 0.005 0.0013 - 0.018 0.001 - 0.005
NH4 (mg∙l−1) 0 - 0.035 <0.01 - 0.09 0.005 - 0.13 0 - 0.062
(1): Average value, (2): Max value.
within the 10 - 20 m layer, lasted until November. Turn-
over started to appear in December where the metalim-
netic l ayer sinks to about 25 m and this gradual mixi ng of
the water column lasted until the end of February. During
turnover, and especially in January and February, tem-
perature had a uniform distribution from the surface to
the bottom of the lake (Fig ur e 2).
The water transparency fluctuated between 4 and 13 m
and presented higher values in the summer and lower in
winter. The lowest transparency value of 4 m was equal
to the respective value reported by Tafas et al. [11] and
Koussouris et al. [10], and was recorder in winter at the
shallowest station C, which is influenced by the high
amounts of water drainage entering into the lake in this
period due to increased precipitation.
The water stratification in Lake Trichonis resulted to
the presence of greater dissolved oxygen concentrations
withi n the metali mnion, a feature common in deep strati-
fied lakes [20]. The high transparency of the water dur-
ing the summer period permitted the photosynthetic ac-
tivity in the metalimnion, where the phytoplankton
gro wth was benefite d by the hig h nutrie nt concentrati ons
released by decomposition processes. Indeed, the maxi-
mum concentration of dissolved oxygen was recorded in
June 2006 (13.93 mg∙l−1) within this layer as a result of
oxygen produced by algal populations. During the strati-
fication period the DO in the hypolimnetic layer de-
creased leading to hypoxic conditions (DO < 2 mg∙l−1)
under the depth of 30 m in autumn. In contrast, during
the winter turnover the lake was well oxygenated (DO
between 8.30 and 10.85 mg∙l−1) throughout the entire
water column (Figure 2). In comparison to the previous
studies in the lake (Tabl e 1), the maximum DO concen-
tration recorded in the present study was higher and the
minimum D O value was lower .
There was little variatio n in the pH va lues which fluc-
tuated between 7.37 and 8.55 during the entire study pe-
riod, while there were no significant differences with the
pre vio us st ud i es ( Ta b le 1). In the ve rt ic al a xi s, hi gher p H
values were recorded in the epilimnion during the strati-
fied p er io d , whil e t hey we re a l most c o nstant in the who l e
water column during the mixing period (Figure 2).
Conductivity fluctuated between 247.5 and 398.2 μS∙
cm−1, with higher values in the hypolimnion during the
stratification p erio d, while during the mixing perio d there
was little co nductivity variati on with depth (Figure 2). It
must be pointed that, the values in the third sampling
period (September 2005-August 2006) were considerably
greater than the second one (September 2004-August
2005), although this was not statistically significant
(U-test, p > 0.05). Considering the previous studies of
Tafas et al. [11], Koussouris et al. [10] and Overbeck et
al. [9], there was an increase of the highest value of
conductivity by 24.4%, 47.5% and 32.3%, respectively
(Table 1).
3.2. Chemical Parameters and Chlorophyll-α
Concentrations
The concentration of total phosphorus (TP) presented the
highe st value of 0.13 1 mg∙l−1 in April 2005 in 40 m (sta-
tion A). Statistically significant differences among the
three sampling stations were found (Kruskal-Wallis test,
p < 0.05), with station C presenting higher values. This
station is in the part of the lake that is close to agricul-
tural land and activities, which probably were responsi-
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22
ble for the greater enrichment of the water with phos-
phorus. A statistically significant difference (U-test, p <
0.05) in the TP concentrations between the two sampling
periods was recorded, with higher values in the second
period (September 2004-August 2005) in comparison to
the third one (September 2005-August 2006). The same
was also true for the concentrations of phosphates (PO4)
and also the nitrates (NO3) between the two periods
(U-test, p < 0.05), while no differences among stations
concerning these two nutrients were found (Kruskal-
Wallis test, p > 0.05). The phosphates in the water fluc-
tuated between undetected values to 0.095 mg ∙l−1
(Figure 3). Nitrates (NO3) and nitrites (NO2) showed
intense temporal variation and their concentrations fluc-
tuated between undetected values to 0.718 and 0.058
mg∙l−1 in July 2005 (station B, 30 m) and in November
2005 (station A, 40 m), respectively. The concentration
of a m mo ni a ( NH 4) generally presented low values except
in October 2004 when high values reaching up to 0.035
mg∙l−1 were observed in the three sampling stations.
There were no statistically significant differences be-
tween the two sampling periods for ammonia as well as
for nitrates and nitrites (U-test, p > 0.05). Most of the
above parameters presented considerably higher concen-
trations in comparison to the previous studies of Over-
beck et al. [9] , Koussouris et al. [10] and Tafas et al. [11]
(Table 1). In particular, there was a remarkable increase
of the maximum concentration of nutrients, such as TP
(17.0% - 1771%), phosphates (36.0% - 580.0%), nitrate s
(59.6% - 474.4 %) and nitrites (222.2% - 1060%). In
addition, the present values for the average concentration
of TP, nitrates and nitrites in the water column are higher
than the respective values reported by Bertahas et al. [1]
in their survey conducted in 2001-2002, while the aver-
age concentrations of phosphates and ammonia are lower.
Ammonia has been increased by 43.4% in co mpari son to
Overbeck et al. [9] (1982), but considerably decreased by
157.1% and 271.4% in comparison to Tafas et al. [11]
and K oussouris et a l. [10], respectively.
In the vertical axis, the general trend for most of the
above chemical parameters was to present lower values
in the epilimnion and higher in the meta-or/and the
hypolimnion during the stratified period, while their
conc entra tio n wa s mo re u nifo rm dur ing t urno ve r (Figur e
3). The higher TP values in the hypolimnion were rec-
orded during stratification and especially in the period of
oxygen depletion, while during the mixing period higher
concentrations were recorded just close to the bottom of
the lake. The reason for this is probably the greater pro-
duction of phosphorus due to decomposition of organic
matter withi n the hyp oli mnion at the e nd of t he strati fica-
tion period (autumn), or/and to phosphorus release from
the bottom sediment.
The concentrations of Chl-α fluctuated between 0.227
μg∙l−1 in July 2006 at the surface to 9.37 μg∙l−1 in April
2006 in the 20 m of station A. Chl-α was always higher
in the metalimnetic layer during the stratification period,
while it was having a uniform distribution during the
mixing period (Figure 4). No differences among the
three statio ns wer e found (Kr uskal -Wallis test, p > 0.05).
The minimum Chl-α concentration reported in the
present study was lower than the respective values in all
the previous studies, while the present maximum value
overcomes these of the past (Tab l e 1).
3.3. Trophic Classification
The application of Carlson’s index (TS I) revealed values
between 25.7 - 67.5 for total phosphorus (TSITP), 23.0 -
40.0 for transparency (TSISD) and 15.2 - 47.5 for Chl-α,
(TSIChl-a) considering all sampling stations (Figure 5).
No statistically significant differences among the three
sampling stations for TSITP, TSISD and the total TSI were
found, although the TSIChl-a values for station C were
significantly higher that of the other two stations
(Kruskal-Wa llis test, p < 0.05). T he results from t he thr ee
sampling stations showed that Lake Trichonis is classi-
fied to the oligotrophic lakes according to the transpar-
ency measurements, while according to Chl-α, it is clas-
sified as oligotrophic to mesotrophic, when the use of
TSITP classifies the lake to mesotrophic and sometimes
eutrophic level. Finally, the overall Carlson’s TSI index
fluctuated between 24.3 and 45.4 having an average of
36.4 which classifies Lake Trichonis to the oligotrophic
level.
3.4. Zooplankton Composition and Abundance
The mean integrated abundance of the total zooplankton
in the water column of the three sampling stations varied
between 2.55 to 131.9 ind l−1. Statistically significant
abundance differences among the three sampling periods
were found (Kruskal-Wallis t est, p < 0.05), with the first
period (September 2003-August 2004) presenting the
lo west mean val ue. I n contra st, no differ ences a mong t he
three statio ns (at the 0 - 20 m depth range) for any of the
Figure 4. Vertical profiles of the concentrations of chloro-
phyll-α in the deeper sampling station A of Lake Trichonis
during March 2005 - August 2006.
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sampling periods were recorded (Kruskal-Wallis test, p >
0.05). This was possibly due to the strong hydrological
homogeneity along the horizontal axis of the lake, as a
result of currents, surface seiche, water inflows and out-
flows [7,21]. Generally, the abundance values recorded
in the present study were greater compared to other great
oligotrophic lakes of the southern Europe, such as Lake
Ohrid [22], Lake Tavropos [23] and Lake Bracciano [24].
On the other hand, the zooplankton abundance is lower
than other eutrophic lakes of Greece like Lake Volvi [25],
Lake Mikri Prespa [26], Lake Pamvotis [27], as well as
the nearby eutrophic Lake Lysimachia [28] and the me-
sotr ophic Lake Amvr akia [29]. The temporal variation of
the total zooplankton was characterized by a decrease of
abundance after September with lowest values in winter,
while a first increase in spring due to rotifers and a
second one in summer due to copepods. This variation
did not seem to follow the monoacmic pattern (only one
peak of abundance in summer) which is characteristic of
the oligotrop hic lakes according to the PEG model [30].
The zooplankton community in Lake Trichonis was
consisted of the groups of rotifers, copepods, cladocerans
and the larvae of molluscs (Figure 6). In particular, the
three-year investigation revealed 25 species of rotifers,
three copepod species, seven cladocerans and the larvae
of the mollusc Dreissena blanci [8,13]. Copepods, and
especially the calanoid Eudiaptomus drieschi, prevailed
in the zooplankton community accounting for an average
proportion of 50.0%, followed by rotifers (24.0%), cla-
docerans (15.6%) and the mollusc larvae (10.4%). E.
drieschi also dominated the crustacean community
(copepods and cladocerans) accounting on average for
65.6%, while the contribution of cladocerans and the
cyclopoid copepods varied between 1.5% to 67.7% and
0.1% to 14.9%, respectively (Figure 7). The domination
of calanoida among the crustaceans, which was observed
during the three-year of investigation in Lake Trichonis,
is characteristic of oligotrophic lakes [31-33]. In com-
parison to the past, however, the percentage of calanoida
in the crustacean community seems to have been de-
creased from 50% - 100% [10] to 22% - 88% in the
present study, while cladocerans increased their contri-
bution. In addition, the present species composition
within the crustacean community is different compared
to the reports of Koussouris et al. [10], with t he pre sence
of only one calanoid species, instead of three [10]. Fur-
thermore, considering that increased contribution of cla-
docerans and cyclopoid copepods in the crustacean
communities is characteristic of eutrophic lakes [34], the
Figure 5. Monthly variation of the values of Carlson’s TSI
index for total phosphorus (TS ITP), c hlorophyll-α (TSIChl-a)
and transparency (TSISD), and also the overall TSI index
(total TSI) in Lake Trichonis during September 2004 - Au-
gust 2006 for station A, and during September 2004 - Au-
gust 2005 for stati ons B and C .
Figure 6. Monthly variation of the mean integrated (0 - 40 m) abundance of the total zooplankton (ind l−1) and percentage
contribution of the main zooplanktonic groups (rotifers, cladocerans, copepods and mollusc larvae) to the total abundance
recorded during September 2003 - August 2 006 in stat ion A of Lake Trichonis .
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24
decrease in the proportions of the calanoid copepods and
the increase of cladocerans in Lake Trichonis could pos-
sibly indicate an alte r a tion in its trophic condition.
Among the cladocerans, Diaphanosoma orghidani
prevailed, accounting for an average of 50.9% in the
community, and followed by Bosmina longirostris
(25.6%) and Daphnia sp. (22.2%), while Ceriodaphnia
pulche lla , Alona sp. and Leptodora kindtii were found in
small numbers and sporadically in the samples (Figure
8). A seasonal succession between B. longirostris which
prevailed in the colder periods, and D. orghidani which
prevailed in the warmer ones, was recorded. T his typ e of
succession in the community of cladocerans is similar to
the situation described by Geller and Müller [35] for eu-
trophic lakes. However, this was apparent only during
the first year, while in the second and third year two
other cladocerans were added to the community and the
succession of species was altered.
Among the zooplankton taxa found in Lake Trichonis
during this extensive investigation, there were certain
species which are considered indicators of either
oligotrophic or eutrophic conditions (Table 2). Thus , the
rotifers Brachionus calyciflorus, Filinia longiseta, Hex-
arthra mira, Keratella quadrata and Pompholyx sulcata,
along with the cladocerans Bosmina longirostris and
Daphnia cucullata are indicators of eutrophic conditions
[36-39]. On the other hand, Kellicottia longispina and
Ploesoma hudsoni of the rotifers and the cladocerans
Daphnia galeata and Leptodora kindtii are indicators of
oligotrophic conditions [38]. It must be pointed that, the
percentage contribution of all the oligotrophic indicator
species in the zooplankton of the lake ranged between
0.6% to 8.9% in the three sampling periods, while the
respective range for the eutrophic species was 9.5% to
13.6%. This resulted to an average of 7.5% for the
oligotrophic and 10.6% for the eutrophic indicator s in the
overall sampling period. Furthermore, it is interesting to
take a closer look to the presence of the cladocerans of
the genus Daphnia sp., among which D. cucu lla ta was
the dominant species, while the abundance of D. galeata
was an order of magnitude lower and it was found spo-
radically in the samples. The two species can hybridize
and produce intermediate forms [19] and that is why we
refer to them as Daphnia sp. Both species had not been
found in Lake Trichonis according to the previous stud-
ies of Koussouris [40,41] and Koussouris et al. [10]
conducted in 1975-1977 and in 1988-1989, respectively.
Indeed, Kehayias et al. [12] were the first to report their
Figure 7. Average percentage contribution of the calanoid copepods, cladocerans and cyclopoid copepods to the crustacean
community in Lake Tricho nis (station A) during September 2003 - August 2006.
Figure 8. Monthly variation of the mean integrated (0 - 40 m) abundance of the total cladocerans (ind l−1) and percentage
contributi on of th e most i mportant s pecies t o the cla docera n’s co mmunity rec orded during September 2003 - August 2006 in
station A of Lake Trichonis.
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25
Table 2. Indicat or species of oligot r ophic and eutrophic conditions [36-39] in L ake Trichoni s.
Species Oligotrophic i n d i c ator Eutrophic indicator
ROTIFERA
Brachionus calyciflorus (Gosse 1851) x
Filinia longis eta (Ehrenb erg 1834) x
Hexarthra mira (Hudson 1871) x
Kellicottia longispina (Kellicott 1879) x
Ker at e ll a quadrat a (Müller 1786) x
Ploesom a h udso ni (Imhof 1891) x
Pompholyx sulcata (Hu ds on 18 8 5) x
CLADOCERA
Bosmina longirostris (O.F. Müller 1785) x
Daphnia cucullata ( G.O. Sars 1 86 4) x
Daphnia ga leat a (Sars 1864) x
Leptodora kindtii (Focke 1844) x
presence in zooplankton samples taken just one year be-
fore the present investigation (2002-2003). D. cucullata
is considered a typical representative of eutrophic lakes
in Europe [39], while D. galeata is an indicator of
oligotrophic conditions [38]. The characteristic body
form and shape of D. cucullata mak es a lmo st imp oss ib le
its misidentification if it had been caught in the samples
during the previous studies. Thus, if the possibility of an
accidental transfer of this species to the lake by human
activities is excluded, the only reasonable explanation for
its absence fro m the species lists in the past was that ac-
tually it was present in the lake but in undetected num-
bers, while during the last years there was an increase in
its density. One reasonable explanation for its increase of
abundance during the last decades could be the alteration
of trophic condition of the lake towards higher levels,
which favoured the development of a denser population.
On the other hand, there is also a possibility of a prey-
predator relationship, in which the diminishing of the
predation pressure on this species by its predator Ath-
erina boyeri has resulted to prey recovery. According to
the re cent i n vesti gatio n o f Do ulka et a l. [42], A. boyeri is
the main zooplanktivorous predator in Lake Trichonis,
where it exercises selective predation and D. cucullata is
among its highly selected prey. Thus, a greater popula-
tion of A. boyeri in the past could have diminished the
population of D. cucullata , while the decrease of the
former within the lake could result to the increase of the
abundance of the latter. Though, there are not enough
data to support this theory and more investigation on this
issue is required .
4. Conclusions
The present investigation showed that Lake Trichonis
seems to hold the main physicochemical features ob-
served in the previous studies of Overbeck et al. [9],
Koussouris et al. [10] and Tafas et al. [11], conducted
between 15 to 25 years before the present investigation.
However, in comparison to the above studies, there was
an increase in the maximum values of most of the phys-
icochemical parameters recorded in the present study,
although their patterns of variation in the water column
were similar. In particular, the maximum values of tem-
perature, DO, conductivity and chlorophyll-α have been
increased by 6.1% to 18.8%, 14.8% to 16.7%, 24.4% to
47.5% and 14.6% to 1075%, respectively. Moreover,
there was a considerable increase of the maximum con-
centration of nutrients, such as TP, phosphates, nitrates
and nitrites. The lowest value of transparenc y in the pre-
sent study was similar to the respective values reported
by Tafas et al. [11] and Koussouris et al. [10], but was
lower by 29.8% than the reports of Overbeck et al. [9].
Considering that Lake Trichonis is situated in an agri-
cultural area, the elevated values of nutrients in the pre-
sent study were probably related to the inflow of irriga-
tion and drainage waters ric h in residual fertilizers, which
were used in the extensive cultivations during the past
years [43]. The present higher values of Chl-α, and as a
by-product the increased DO in the metalimnetic layer,
could be also the result of this chronic influx of nutrients
into the lake. In accordance, the elevated productivity of
the basin could be also among the reasons for the in-
crease of conductivity [20], and this could have resulted
to the lower values of water transparency in comparison
to the oldest previous study of Overbeck et al. [9]. Ac-
cording to Bertahas et al. [1], phosphorus is the key nu-
trient that regulates eutrophication in Lake Trichonis as
the li mitin g nutr ient for al gae gr owth. Howe ver, i t see ms
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that it is not the quantities of phosphorous entering the
ecosystem but its hydrological regime which affects in a
great extent its trophic sta tus. In partic ular, the quantit ies
of rainfall precipitation and water recharge are responsi-
ble for regulating the water outflow from the lake and
consequently the phosphorus fl ush out of the system [1].
In this se nse, t he TP fluct ua tio n a mong ye ar s fo u nd in th e
present stud y could be o riginate d to the va ri at ion of t he s e
hydrological elements in the lake’s catchment area. On
the other hand, the strong decrease in the concentrations
of TP, phosphates and nitrates in the period of September
2005 - August 2006 in comparison to September 2004 -
August 2005, could be related to the termination of the
tobacco cultivations during the former period. Tobacco
cultivation was developed in the prefecture of Aito-
loaka rnani a at t he end of t he 19 th centur y and this p art o f
Greece became the country’s major producing area of
tobacco. However, from 1/1/2006 with the CAP reform
in relation to tobacco cultivation, the overall production
presented a significant reduction of about 80% in Greece
and, specifically, 100% red ucti on in the whol e prefecture
of Aitoloakarnania [44]. As a result, tobacco producers
either turned to other cultivations, or in most cases left
their fields uncultivated. Although this pause i n the agri-
cultural activities has not been documented in details for
the area, it is suggested that it could be among the rea-
sons of the nutrients’ decrease in the lake observed a year
after this action.
Nevertheless, the nutrients’ concentrations have been
increased from the past the trophic level of the lake has
not altered dramatically. Thus, although the previous
investiga tors [1,9-11] did not use TSI index to have di rect
compa riso ns, the y acco unted Lake T richo nis as a n oli go-
to mesotrophic aquatic ecosystem. The reason for this
stability of t he lake i mplies its large water volume a nd it s
positive water balance, as previously mentioned. On the
other ha nd, the a nthrop ogeni c impac ts on La ke Tr ichonis
can be considered as low, in the sense that there are no
heavy industrial units around or close to the lake. How-
ever, the existence of a large number of olive-oil refiner-
ies, as well as of several creameries and small livestock
facilities like sheep and pig-farms, can be considered a
constant thread for the increase of organic pollution en-
tering the lake. In addition, the absence of biological
treatment facilities for most of the surrounding villages,
whi ch probably result to undefined quantities of urban
wastewater constantly dumping into the lake, are serious
reasons of a future increase in lake’s trophicit y.
Consequently, the indications concerning the physi-
cochemical elements of Lake Trichonis and its trophic
state during the last thirty years point out a general sta-
bility and a stro ng resistance of the lake to trophic ele va-
tion. However, the present zooplankton investigation
reveals a puzzling icon of the environmental state of the
ecosystem, accumulating several indications. Thus, the
total zooplankton abundance, the dominance of the ca-
lanoid copepods (instead of the cyclopoids or/and the
rotifers) in the zooplankton community as well as among
crustaceans, and the presence of certain indicator species,
point out an oligotrophic character of the lake. On the
other hand, the pattern of the temporal zooplankton
variation and the seasonal succession pattern among
cladoceran species resemble those reported from eutro-
phic lakes of Europe. Moreover, the appearance of “new”
species, like D. cucullata, along with the disappearance
of calanoid copepod species, the increase of the propor-
tions of cladocerans among crustaceans in comparison to
the past, and the increased proportions of indicator spe-
cies of eutrophic conditions within the zooplankton
community, may suggest that certain biological parame-
ters have been altered from the past and indicate possibly
a different ecological status of the lake. The bottom line
could be that Lake Trichonis is probably experiencing a
transitional condition towards the eutrophic state and
thus, the need of constant inspection and monitoring not
only of the abiotic parameters but also the biotic ones in
the frame of a general management plan for this ecosys-
tem, is indispen sable.
Ackno wledgements
This research was funded by the European Union in the
framework of the program “Pythagoras II” of the “Op-
erational Program for Education and Initial Vocational
Training” of the 3rd Community Support Framework of
the Hellenic Ministry of Education, 25% of which was
funded from national sources, and 75% from the Euro-
pean Social Fund (ESF) .
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