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ENVIRON SCIENCE AND TECHNOLOGY IN ROMANIA—ESTROM •SHORT ORIGINAL COMMUNICATION
Eutrophication of Lake Tasaul, Romania—proposals
for rehabilitation
Mihaela Laurenta Alexandrov &Jürg Bloesch
Received: 25 April 2008 /Accepted: 24 October 2008 / Published online: 9 January 2009
#Springer-Verlag 2008
Abstract
Lake Tasaul on the Black Sea coast is highly eutrophic, but not
strongly contaminated (heavy metals, PAHs, and organochlo-
rine pesticides). Cyanophytes dominate phytoplankton by 67–
94% and form frequent algal blooms. High primary produc-
tion (up to 270 mg C
ass
/m
2
.h) and algal biomass (maximum
chlorophyll aconcentration 417 μg/l) may be controlled by
light, as Secchi depth is often below 1 m. The main tributary,
Casimcea River, provides high quantities of suspended
matter and about 3 tons TP/year and 660 tons TN/year.
Based on chemical and biological analysis as well as fishery
investigations, we provide recommendations for Lake Tasaul
rehabilitation.
Keywords Biomass .Black Sea .Contamination .
ESTROM .Eutrophication .Fishery .Lake management .
Lake Tasaul .Nutrient load .Primary production .Romania .
Tasaul project
1 Background, aim, and scope
Black Sea coastal lakes are important and threatened
ecosystems. In the 1920s, Lake Tasaul (area 23.35 km
2
,
maximum depth 4 m) near Constanta has been transformed
by technical structures from an open coastal lagoon (salt
water) to a freshwater lake (no outlet, shallow, turbulent,
and eutrophic), classified as heavily modified water body
according to the EU Water Framework Directive. Lake
Tasaul has been used mainly for aquaculture and fish
production. We hypothesize that the drastic decline in fish-
catch in 1994–1995 from 180 to 50 t, and further to 5 t until
2000 (Alexandrov et al. 2008), was the direct response of
the lake ecosystem to a trophic change, in combination with
diminished fish reproduction.
2 Materials and methods
Our limnological survey was performed from May 2005
to June 2007. Figure 1shows the sampling sites.
Sediment cores were taken by a gravity corer (90 mm;
Uwitec, Mondsee, Austria, No. 019001). Commonly used
methods (most of them standardized for the Black Sea
area) were applied for chemical water and sediment
analysis, biological investigations, and species identifica-
tion (NIMRD 2006). Primary production was measured
with the
14
C technique using acid bubbling (Gächter and
Mares 1979). Nutrient load of tributaries was calculated by a
regression model (Fourier-polynom-function) based on
discharge measured by limnigraphs and nutrient concentrations
from discharge-proportional samples (IGKB 1989).
Environ Sci Pollut Res (2009) 16 (Suppl 1):S42–S45
DOI 10.1007/s11356-008-0071-7
DO00071; No of Pages
ESPR Special Issue-ESTROM
Responsible editor: Walter Giger
M. L. Alexandrov (*)
National Institute for Marine Research and Development,
‘Grigore Antipa’,
Bd. Mamaia 300,
900581 Constanta, Romania
e-mail: laurenta@alpha.rmri.ro
J. Bloesch
Eawag, Swiss Federal Institute of Aquatic
Science and Technology,
Überlandstrasse 133,
CH-8600 Dübendorf, Switzerland
e-mail: bloesch@eawag.ch
3 Results
Nutrient concentrations (SRP mean lake concentration
17 μg/l, NO
3
–N 370 μg/l) reflected the eutrophic state of
Lake Tasaul (Table 1). The N/P ratios (mean 125) exceeded
the Redfield’s ratio (16:1) significantly and point toward P-
limitation, although light may play a crucial role at times of
high turbidity (see below). Horizontal differences did not
show a consistent pattern and were smaller than seasonal
variation (Alexandrov 2008, in preparation). The water
column was well mixed throughout the seasons, and oxygen
concentrations revealed a routine oversaturation up to 210%.
However, some oxygen depletion was observed occasionally
near the lake bottom (minimum 3.9 mg O
2
/l and 62%
saturation). The sediments showed anoxic black layers
below the surface indicating strong gradients of dissolved
components and a high potential for internal nutrient
loading. Consistent, wind-induced sediment resuspension
fosters intensive nutrient cycling. Significant external
nutrient load from Casimcea River (as the main tributary),
was estimated to be about 3 tons TP/year and 660 tons TN/
year (these are minimum values as some high flow events
contributing the main proportion to the annual load could
not be sampled as planned). The input of Sibioara and
Dalufac Rivers were relatively low, as these small rivers
have a much smaller catchment than Casimcea River.
High nutrient concentrations in spring favored green
algae (Scenedesmus), blue-greens (Oscillatoria, Microcys-
tis), and brackish species, while diatoms were scarce
(salinity is in the range of 0.2–2.7‰). Phytoplankton
showed frequent blooms (Cyanophyta, Euglenophyta).
The species lists showed a high biodiversity (Phytoplankton
69 species; zooplankton 35 species). Zooplankton was
dominated by rotifers with a low diversity and productivity.
There was a lack of Cladocerans. The biomass (fresh
weight) of various biological compartments peaked at
38 mg/l (phytoplankton), 9.5 mg/l (zooplankton), and
39 mg/m
2
(benthos). The total number of bacteria was 5×
10
3
–1.6×10
6
/ml, and two stations with anthropogenic
influence showed increased bacteria concentrations including
pathogenic forms. The low benthos abundance (no Mysids
and Amphipods) can be explained by anoxic sediments
providing unsuitable habitat for bottom fauna. A check of
persistent contaminants such as PAHs, PCBs and heavy
metals showed significant temporal and spatial variation, but
no severe pollution as mostly below critical threshold values
in water and in sediments stipulated in national regulations:
Total hydrocarbons showed a mean concentration of 381 μg/
l in water and 2,126 μg/l at the surface of sediments,
polyaromatic hydrocarbons (15 components) in total were
702 ng/g at sediment surface, organochlorine pesticides (nine
components) showed a total mean concentration of 1,793 ng/
l in water and 254 ng/g in sediments, trace heavy metals
showed a maximum concentration for Cu in water
(12.57 μg/l, range 3.07–48.32 μg/l) and Pb in sediments
(73.95 μg/g d.w., range 26.90–125.87 μg/g d.w.; Alexandrov
2008, in preparation).
Hourly, primary production amounted to 80–2,712 mg
C
ass
/m
2.
h and was concentrated in the top meter of water
column during a large part of the year according to the
Table 1 Lake Tasaul chemistry (selected parameters), May 2005–
November 2007
Parameter Mean value (±SD) Range
Salinity (‰) 0.95± 0.389 0.24–2.67
Alkalinity (meq/l) 5.6± 0.37 4.4–6.7
pH 8.7± 0.46 7.6–9.6
O
2
(mg/l) 12.4± 2.41 3.9–17.1
O
2
saturation (%) 132± 26.1 62–210
SRP (mg/l) 0.017± 0.0121 0.003–0.070
TP (mg/l) 0.106± 0.1435 0.023–0.899
NO
3
–N (mg/l) 0.37± 0.419 0.02–1.71
TN (mg/l) 1.884± 0.641 0.852–3.784
TOC (mg/l) 16.318 ± 3.763 8.430–23.050
Chl a(μg/l) 164.3± 122.6 8.6–417.0
Atomic N/P ratio 125±108 4.7–451
During May 2005–March 2006, surface samples from eight stations;
during April 2006–November 2007, vertical profiles at mid-lake
station (see Fig. 1)
Fig. 1 Sampling sites in Lake Tasaul and major tributaries (ASibioara
River, BDalufac River, CCasimcea River, DPiatra River). Stations
1–8: sampled in 2005–2006 at the lake surface; station marked with
black point (deepest site): sampled by depth profiles in 2006–2007;
stations A–C near the river mouths: sampled during 2006–2007 (D
gauging only)
Environ Sci Pollut Res (2009) 16 (Suppl 1):S42–S45 S43
measured light profiles and low transparency (Fig. 2).
Hence, algal production may be limited not only by
nutrients, but also by self-shading effects of dense algal
biomass increasing turbidity, which is also caused by
resuspended sediments. The calculation of daily and annual
primary production is in progress.
Fish stock in Lake Tasaul was assessed in 2005 for two
out of eight species (Carassius gibelio [giebel carp] and
Rutilus rutilus [roach]) from a total of 581 specimens
investigated (Alexandrov et al. 2008). FAO-related meth-
ods were applied by using “age-structured”models based
on population parameters such as length frequency
diagrams and length–weight relationships. Biomass in
2005 was 23.6 tons for giebel carp and 34.2 tons for
roach. Since the fishing mortality coefficient was greater
than the natural mortality coefficient, fishery is claimed
not to be sustainable (i.e., as a result of overexploitation).
The maximum sustainable yield, according to the model based
on standard fish catch of 10 tons/year, would be 10.9 tons/year
for giebel carp and 12.2 tons/year for roach.
4 Discussion
Our limnological investigation proved the highly eutrophic
state of Lake Tasaul. It is likely that the biota is not biased
by contamination and bottom up controlled by nutrients
originating mainly from internal and external loading.
Shallow lakes are naturally eutrophic systems. Internal
nutrient loading triggered by bottom sediment resuspension
may encompass external loading from point and diffuse
sources. Since Lake Tasaul is mainly used for fisheries,
general measures for lake protection/restoration are the
basic prerequisites for fish reproduction. It is not likely that
contaminants bias fish life in Lake Tasaul. We could not
quantify the role of turbidity and nutrients (bottom up
control) vs significant water fowl as fish predators (top
down control). However, lack of scientifically based
management, reduced fish reproduction due to unfavorable
conditions, and overfishing by man may be at least partly
responsible for the decline of fish yield. Hence, apart from
the classical limnological remedies such as nutrient input
reduction, the fishery management strategy must be in the
focus of restoration measures (stocking programs, regulation
of mesh-size of fishing nets for professional fishery,
regulation of sport fishery and licenses, control of poaching,
optimization of the role of fish hatchery, etc).
On the basis of our experience, a nutrient mass balance
of Lake Tasaul can only be achieved if the sampling
strategy can be optimized and lake output and ground
water interactions can be assessed by investigating the
complex hydrology at the south shore of Lake Tasaul
(outlet now closed by abandoned channels, unpredictable
groundwater exfiltration) and measuring an accurate
nutrient lake output.
5 Conclusions
Recommendations toward reduction of eutrophication and
a more sustainable fishery management in Lake Tasaul
are: (1) to reduce nutrient input of Casimcea River by
fighting point sources; (2) to continue the general
monitoring, hydrology, nutrients, primary production,
and fish production in particular; (3) to perform a detailed
monitoring of yearly restocking to quantify fishing effort/
input; (4) to perform detailed statistics about net catches
and angling, and estimate poaching, to quantify fishing/
output.
6 Recommendations and perspectives
Restocking may sustain fish stocks, but biomanipulation
would be very difficult. The goal to rehabilitate Lake
Fig. 2 Typical temperature (°C), oxygen (mg/l), chlorophyll a(μg/l),
light PAR (in % of Io) and
14
C (mg C
ass
/m
2.
h) profiles in Lake Tasaul
(upper panel: winter, lower panel: summer). Note the scale/tenfold
higher concentration of chl. ain summer
S44 Environ Sci Pollut Res (2009) 16 (Suppl 1):S42–S45
Tasaul’s water quality and natural resources may be
achieved best by implementing a management plan for the
whole catchment. Applying GIS systems and maps based
on a data base, which is continuously improved, can be
very supportive.
Acknowledgement This work was financed by the Swiss
National Science Foundation, the Swiss Agency for Development
and Cooperation and the Romanian Ministry for Education and
Research within the framework of the Swiss-Romanian coopera-
tion program on “Environmental Science and Technology in
Romania—ESTROM”. The reported study was performed in the
project TASAUL, focusing on the assessment of anthropogenic
impacts on Tasaul Lake, Romania, and ecosystem rehabilitation.
We greatly acknowledge Daniela Mircea-Rosioru, Dan Vasiliu,
Razvan Mateescu, Victoria Smocov, Victoria Piescu, Valentina
Coatu, Andra Oros, Adriana Cociasu, and NIMRD-Constanta staff
for their cooperation, as well as Irina Cernisencu from DDNIRD,
Andreas Kohler, Hanspeter Hodel, Alessandro Grasso, Bernhard
Luder, Daniel Wyder (FOEN-Landeshydrologie), Peter Bossard,
Daniel Steiner, Mike Sturm, Heinrich Bührer (Eawag) for their
support.
References
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