Oceanological and Hydrobiological Studies
International Journal of Oceanography and Hydrobiology
Vol. XXXVIII, No.1
Institute of Oceanography
ISSN 1730-413X
(3-15)
2009
DOI 10.2478/v10009-009-0002-z
Original research paper
University of Gdańsk
eISSN 1897-3191
Received:
Accepted:
November 03, 2008
January 14, 2009
Spatial distribution and seasonal variability in chlorophyll
concentrations in the coastal Lake Gardno (Poland)
Dariusz Ficek1, Magdalena Wielgat-Rychert2
1
Institute of Physics, Pomeranian University in Słupsk
ul. Arciszewskiego 22, 76-200 Słupsk, Poland
2
Institute of Biology and Environmental Protection
Pomeranian University in Słupsk
ul. Arciszewskiego 22, 76-200 Słupsk, Poland
Key words: chlorophyll concentration, phytoplankton fluorescence, coastal
lakes
Abstract
In 2006 the spatial distribution and seasonal variations in chlorophyll concentration were
measured, at about two-week frequency, in Lake Gardno. In general, chlorophyll concentrations
in the central part of the lake were high throughout the growth season. The minimum chlorophyll
concentration was recorded in March (7.5 mg m-3), and the maximum value in September
(303 mg m-3). Higher chlorophyll concentrations and lower temporal variability were measured in
the central part of the lake, compared to lower concentrations and higher variability in the vicinity
of the Łupawa River input to the lake. Chlorophyll concentrations were measured
fluorometrically along several vertical and horizontal profiles, enabling direct observations of the
dynamics of changing chlorophyll concentrations in Lake Gardno throughout the year.
1
Corresponding author: [email protected]
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D. Ficek, M. Wielgat-Rychert
INTRODUCTION
Lake Gardno is a very complex system in terms of its hydrology. As a result
of the lake being shallow, directly connected to the sea and receiving riverine
water inputs, the phytoplankton biomass and abundance fluctuate strongly
(Mudryk 2003) (Fig. 1). High nutrient concentrations and mixing of the entire
water column enable intense phytoplankton growth. The lake area can be
divided into three parts (Trojanowski et al. 1991): one part is under the
influence of land-derived factors, the second one is under the influence of
marine waters and the third (central) part is of “intermediate” character. In the
central part (which covers the majority of lake area) conditions for
phytoplankton development are rather uniform. Frequent winds mix the entire
water column to the bottom, resulting in almost no thermal or oxygen
concentration stratification. In the area under marine influence the lake water is
enriched in chloride ions, which cause an increase in water density throughout
the lake. High variability of hydrological parameters (and in consequence high
phytoplankton biomass variability) is expected in the area under the influence of
land-derived factors, especially in the vicinity of the Łupawa River mouth. The
Łupawa River discharges 90% of the fresh water inputs into the lake (Cyberski
& Jędrasik 2003). It is difficult to determine the exact area of the river water
plume. Sometimes, during windless weather, the river water plume can reach far
into the central part of the lake, with the riverine waters preserving, to a large
extent, their physicochemical characteristics.
This study was undertaken in order to assess two aspects of the lake
functioning. The first was to observe seasonal changes in chlorophyll
concentrations over the course of an annual cycle. The second was to observe
variations in the spatial distribution of chlorophyll. A submersible PrimProd
fluorometer was used to collect the measurements, registering in situ
fluorescence of phytoplankton photosynthetic apparatus. Lake Gardno is one of
nine coastal lakes that have rather unique characteristics, classified as a separate
group in the Polish typology according to the requirements of the EU Water
Framework Directive (Kolada et al. 2005). This group of lakes has been
relatively little studied as compared to other types of Polish lakes, and so this
study is in response to a need for basic data.
MATERIALS AND METHODS
The results presented in this paper are based on measurements collected in
2006. Material was collected at one fixed location (G0), and many other
locations in different parts of the lake. In addition, the fluorescence properties of
phytoplankton and water temperature were measured directly every five
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Spatial distribution and seasonal variation in chlorophyll a…
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Fig. 1. Gardno Lake location.
seconds in surface water, along seven short and nine longer transects. At some
locations such measurements were also conducted along vertical profiles.
The first measurement was taken on March 16th 2006, when the lake was
covered with ice 35 cm thick. Other measurements were conducted from April
28th to December 2nd 2006, and were taken every two weeks, weather
permitting. Good weather conditions were essential for making measurements
along the long transects. Chlorophyll concentrations were measured according
to standard spectrophotometric methods (Jeffrey and Humphrey 1975, Jeffrey et
al. 1997). Water samples were collected at a depth of 20 cm, and filtered
through Whatman GF/F filters with a suction pressure not exceeding 35 kPa.
The volume of water filtered depended on the chlorophyll concentration in the
sample. In the case of the riverine waters, about 1-1.5 liters of water were
filtered. Lake water from the central part of Lake Gardno contained more
chlorophyll, so in that case 0.2–0.5 liters of water were filtered. Chlorophyll
extractions were performed by incubation with 90% acetone for 24 hours.
Samples were stored in the dark at 4°C. Extracts were centrifuged for 20 min at
2320 x g. Absorption was measured with a Hitachi U 2810 spectrophotometer.
The fluorescence properties of marine phytoplankton were measured in situ
using a submersible fluorometer Pump Probe type (PrimProd-Ekomonitor),
constructed to measure chlorophyll concentrations based on the method created
by Falkowski and his research team (Kolber & Falkowski 1993, Ostrowska
2000). Fluorometers measure the in vivo phytoplankton fluorescence, F0’,
induced by a weak flash in the dark-adapted state. Chlorophyll concentration is
a major factor determining the intensity of artificially induced fluorescence.
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Figure 2 presents the relationship established for fluorescence F’0 and
chlorophyll concentration Ca, based on 86 samples collected in water bodies of
different trophy in northern Poland. It can be seen that fluorescence increases
with increasing chlorophyll concentration. With regression analysis the
following relationship between chlorophyll concentration and fluorescence, F’0,
was found:
C a = 7.27 ⋅ (F'0 ) 0.871
(1)
This relationship was used for the calculation of chlorophyll concentrations
from subsequent fluorescence, F’0, measurements. It should be mentioned that
the constant values used in this equation depend on the fluorometer construction
and must be determined separately for each instrument.
Errors of approximation were also calculated. To do so, chlorophyll
concentrations measured using the spectrophotometric method, Ca,M, were
compared to concentrations calculated using the equation (1) from the
fluorometric data, Ca,C. The results of these verifications are shown in Table 1.
The statistical error, σ+, of these calculations is relatively low, up to about 24%.
Therefore, calculations based on the equation (1) gave satisfactory
approximations of empirically measured values, and the fluorometric method
can be used for calculation of realistic chlorophyll concentrations.
chlorophyll a concentration
Ca [ mg.m -3 ]
1000
100
10
1
0.1
1
10
100
fluorescence F0' [ relative units ]
Fig. 2. Relationship between measured fluorescence, F0’, and chlorophyll a
concentrations measured using the spectrophotometric method, Ca.
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Spatial distribution and seasonal variation in chlorophyll a…
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Table 1
Errors in calculations of chlorophyll concentrations, Ca, as determined using
the equation (1).
Arithmetic statistics
Logarithmic statistics
systematic
statistical
systematic Standard error factor
statistical
<ε> [%]
σε [%]
<ε>g [%]
x
σ- [%] σ+ [%]
1.57
22.1
0.0757
1.24
-19.51 24.25
where:
ε = (Ca ,C - Ca , M ) / Ca , M
ε
σε
ε
- errors,
- arithmetic mean of errors,
- standard deviation of errors (statistical error),
g
= 10
[ log (C
a ,C
/ C a ,M
log(Ca ,C Ca , M )
σ log
)]
− 1 - logarithmic mean of errors,
- mean of
- standard deviation of
log(Ca ,C Ca , M ) ,
log(Ca ,C Ca , M ) ,
σ
x = 10 log - standard error factor,
1
σ − = − 1 and σ + = x − 1 .
x
RESULTS AND DISCUSSION
The chlorophyll concentrations recorded at station G0 in 2006 are shown in
Figure 3. In winter (under ice) chlorophyll concentrations were low, at 7.5 mg
m-3. Between April and May chlorophyll concentrations exceeded 100 mg m-3
and remained high until November, except two dates when lower values were
noted on June 16th (30 mg m-3) and October 7th (44 mg m-3) (Table 2). The high
values indicate intense development of phytoplankton during the whole growth
season, which has also been observed by other authors (Mudryk 2003,
Trojanowski et al. 1991). Maximum chlorophyll concentrations were seen in
August and at the beginning of September, at 166 mg m-3 and 303 mg m-3
respectively. The chlorophyll concentration measured at the beginning of winter
(before ice formed) was also high, exceeding 30 mg m-3 at station G0. The
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chlorophyll a concentration
Ca [ mg .m -3 ]
300
200
100
0
M
A
M
J
J
A
months
S
O
N
D
Fig. 3. Seasonal variability of chlorophyll a concentrations at station G0,
measured using the spectrophotometric method.
Table 2
Chlorophyll concentrations measured using the spectrophotometric method at
different stations shown on maps in Figure 4.
Date
16 March
28 April
15 May
a*
30 May
a
16 June
a
3 July
17 July
17 August
6 September
7 September
22 September
6 October
7 October
20 October
16 November
17 November
a
1 December
2 December
a
Chlorophyll a concentration
-3
Ca [mg tot. chl a m ]
G0
G1
G2
G3
7.5
106
115 26.3 87.2
112
61.6 25.1 63.4
30.1 19.4 11.2 25.5
45.7
30.7 50.1
70.6 66.2 50.8 73.7
166 13.4 197
173
303
165
69.6
9.7
66.7 71.8
52.9
43.7
99.7
3.0
46.8 71.8
86.9
8.6
115
112
88.7
48.0
23.2
32.9
- Not shown in Figure 4.
* - Location of stations close to the locations as at 28.04.2006.
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Spatial distribution and seasonal variation in chlorophyll a…
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seasonal pattern of chlorophyll concentrations observed in this study can be
considered as representative for Lake Gardno, and is in agreement with the
seasonal pattern and range of values observed by Antonowicz in the 1998–2002
period (Antonowicz 2006).
Significant differences in chlorophyll concentrations were observed when
comparing values measured on subsequent days: i.e. on the 6th and the 7th of
September, on the 6th and the 7th of October, and on the 1st and the 2nd of
December (Table 2). A particularly large difference was recorded between the
6th and the 7th of September, when the chlorophyll concentration dropped by
half, from 303 to 165 mg m-3, in one day. The very high concentration measured
on the 6th of September occurred at a time when the wind had ceased after a
strong two-day storm, indicating that the chlorophyll concentration in the water
column could depend on mixing influencing the proportion of microalgal cells
suspended in the water and settled on the bottom (Gerbersdorf et al. 2004).
The chlorophyll concentrations measured using spectrophotometric
methods, presented in Table 2, reveal a low spatial heterogeneity of chlorophyll
concentrations in the central part of the lake, and high spatial heterogeneity in
the vicinity of the Łupawa River mouth (point A on map shown in Figure 4).
Fluorometric
methods,
complemented
with
spectrophotometric
measurements, were used to assess the spatial distribution of chlorophyll
concentrations in Lake Gardno (Figure 4a-f). In 2006 the research focused on
the southeastern area of the lake, where the Łupawa River enters the lake, and
as a result of the input a high variability of parameters was seen in this area. In
the vicinity of the Łupawa River mouth, riverine water gradually mixes with
lake water, depending on the hydrological and climatic conditions. Riverine
waters contain much lower chlorophyll concentrations than, and usually have a
different temperature to, the lake water. The results show that chlorophyll
concentrations increased gradually from the river mouth to the central part of
the lake. The strong influence of the Łupawa River on chlorophyll
concentrations in the lake was visible along transects close to the river mouth,
where riverine water, lake water, and water mixed to various degrees, were
observed within a short geographical distance. Sometimes chlorophyll
concentrations differed even by a factor. The riverine water plume is visible in
Figure 4. Generally the riverine water loses its properties (different temperature
and lower chlorophyll concentrations) quite fast on entering the lake, due to
wind-driven mixing. Sometimes, however, during windless periods, chlorophyll
and temperature profiles indicate that the riverine water maintains its properties,
even in the central part of the lake, e.g. in the vicinity of station G2 (Figure 4e).
Many authors recommend that a sampling point should be located at the deepest
point of a lake. In Lake Gardno the deepest point is located in the
“intermediate” part of the lake, close to the Łupawa River input. Therefore,
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10
160
14
120
13
80
12
40
0
G0
G1
G3
G2
temperature t [ 0C ]
chlorophyll a concentration
Ca [ mg m-3 ]
a. Transect 28.04.2006.
B
A
G3
G0
G2
G1
11
time
26
60
25
40
24
20
23
0
G0
G2
G1
G3
temperature t [ 0C ]
80
B
temperature t [ 0C ]
chlorophyll a concentration
Ca [ mg m-3 ]
b. Transect 17.07.2006.
B
G2
A
G1
G3
G0
22
time
chlorophyll a concentration
C a [ mg m-3 ]
c. Transect 17.08.2006.
200
25
160
23
120
21
80
19
40
0
17
G0
G1
G2
G3
G2
A
G3
G1
G0
15
time
Fig. 4. Chlorophyll a (solid line) and temperature (dotted line) along transects
outlined on the maps shown to the right of the graphs. Chlorophyll
concentrations were calculated from fluorescence according to equation (1).
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Spatial distribution and seasonal variation in chlorophyll a…
11
60
19
40
17
20
0
G0
G1
G2
G3
temperature t [ 0C ]
21
80
B
temperature t [ 0C ]
100
B
temperature t [ 0C ]
chlorophyll a concentration
Ca [ mg m-3 ]
d. Transect 22.09.2006.
B
G2
A
G3
G1
G0
15
time
chlorophyll a concentration
Ca [ mg m-3 ]
e. Transect 20.10.2006.
14
160
13
120
12
80
11
40
0
10
G0
G1
G3
G2
G2
A
G3
G1
G0
9
time
chlorophyll a concentration
Ca [ mg m-3 ]
f. Transect 16.11.2006.
10
160
120
9
80
8
40
0
G0
G1
G2 G3
A
G2
G3
G1
G0
7
time
G0, G1, G2, G3 – station locations
A – the Łupawa River mouth
B – canal connecting Lake Gardno with the Baltic.
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D. Ficek, M. Wielgat-Rychert
results from that location are not relevant to the central part of the lake, as
indicated by the graphs in Figure 4.
The strong influence of the Łupawa River weakens further from the river
mouth, as the water became more homogeneous. However, water was not
necessarily fully mixed in the central part of the lake; even when measurements
were carried out under windy conditions, areas of water of different
characteristics (e.g. local differences in temperature and chlorophyll
concentrations) could be distinguished. Such differences were not necessarily
large, and in the case of chlorophyll concentrations seldom exceeded 50% (see
Table 2). The range of variability in the fluorometric data is presented in
Figures 4 b-e, from data sets collected along extended transects covering the
central part of the lake. It is worth noting that there were differences between
values measured at the beginning and end of the cruise. Such differences in
values could result from changes in hydrological conditions that took place
within the few hours of the cruise duration.
In general, it can be expected that the amplitude of short-term changes, in
the central part of the lake, is lower than that presented in this work because it
was possible to carry out fluorometrical measurements only during good
weather conditions, that is, on windless or low wind days. Whilst for the
majority of the year the lake is strongly affected by wind mixing the entire
water column (Cyberski & Jędrasik 2003).
Fluorometric measurements turned out to be not only a good indicator of
short-term phytoplankton fluctuations, but also enabled other conclusions to be
drawn. Comparing chlorophyll values at station G0 with values in the central
part of the lake, as presented in Figure 4, it can be seen that values measured at
G0 are generally representative for the majority of the lake, with seasonal
variations observed at G0 (Figure 3), applicable to the whole lake area.
Measurements along many vertical profiles were carried out. Results
confirmed the authors’ expectations that the water column of the lake is almost
homogenous and differences in vertical distribution of chlorophyll
concentrations are rather small. Distinct vertical stratification was present only
in areas where waters of different characteristics were brought together, that is
in the areas of inflows of riverine or sea water. It can be expected that, taking
into account the density gradient linked to salinity, riverine waters penetrate into
the lake basin on the surface and seawater at the bottom. However, depending
on the thermohaline situation of lake waters, riverine water can move at
different depths. An example of a vertical profile, recorded on July 3rd 2006 and
shown in Figure 5a, illustrates the situation when riverine water which is cold,
and thus of high density, and low in chlorophyll, incurs into the lake at the
bottom of the water column.
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Spatial distribution and seasonal variation in chlorophyll a…
temperature t [ 0C ]
18
22
24
temperature t [ 0C ]
26
8
0
a
3.07.2006
0.4
depth z [ m ]
depth z [ m ]
0
20
0.8
1.2
8.4
8.8
9.2
b
16.11.2006
0.4
0.8
1.2
0
20
40
chlorophyll a concentration
Ca [mg tot. chl a m-3]
60
0
20
0
0
c
0.2
60
80
temperature t [ 0C ]
19 19.2 19.4 19.6 19.8 20
0
40
chlorophyll a concentration
Ca [mg tot. chl a m-3]
temperature t [ 0C ]
1
2
3
4
d
22.09.2006
16.03.2006
depth z [ m ]
depth z [ m ]
13
0.4
0.6
0.2
0.4
0.8
0.6
1
0
20
40
60
chlorophyll a concentration
Ca [mg tot. chl a m-3]
80
0
2
4
6
8
chlorophyll a concentration
Ca [mg tot. chl a m-3]
10
Fig. 5. Examples of vertical profiles of chlorophyll a (solid line) and temperature
(dotted line). Chlorophyll concentrations were calculated from fluorescence
according to equation (1):
a)
b)
c)
d)
at the vicinity of the Łupawa River mouth, station G1, 3.07.2006,
at the vicinity of the Łupawa River mouth, station G1, 16.11.2006,
in the central part of the lake, station G3, 22.09.2006,
under ice cover, station G0, 16.03.2006.
An interesting situation was observed in measurements made on November
16th 2006 at station G1 (Fig 5b), where the water level in the lake was higher
than the average level by 70 cm as a result of northerly winds banking up
seawater and pushing it into the lake. Distinct vertical stratification was visible
where on the surface warm water with high chlorophyll concentration was seen,
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D. Ficek, M. Wielgat-Rychert
beneath that a mid layer of colder water with lower chlorophyll, and beneath
that a deep layer with an even lower temperature and higher chlorophyll
concentration (Fig 5b). The authors assume that these results indicate a layer of
low chlorophyll riverine water sandwiched between a deep, cold, saline lake
water layer of high density (high chlorophyll concentration), and a surface,
warm lake water (high chlorophyll concentration) layer.
During sunny, windless days, weak thermal stratification was observed.
Figure 5c illustrates the situation recorded on September 22nd 2006, at station
G3, when the temperature gradient differed by almost 1°C, but the chlorophyll
concentration gradient was very weak. It should be noted, however, that such
situations were rare.
Measurements taken at station G0 under 35-cm thick ice cover on March
16th 2006 show a distinct thermal stratification of the water column, presumably
as a result of the ice cover preventing wind mixing of the water column (Fig
5d). The water at the surface was recorded as being close to 0°C, while water at
the lake bottom was at about 4°C. At that time the chlorophyll concentration
was low throughout the water column at about 7 mg m-3.
CONCLUSIONS
The results show geographical and temporal distributions of chlorophyll
concentrations over the course of a year in Lake Gardno. Considerable
variations of chlorophyll concentrations were observed over both the short and
long term. Spatial heterogeneity of chlorophyll distribution was observed
throughout the water body, with the scale of the heterogeneity depending both
on sample location and on external factors, such as wind conditions. The
smallest variations were seen in the central part of the lake, and the largest in
the “intermediate” areas, that is in the areas close to riverine or saline water
inflows. The seasonal variability observed at station G0 was representative of
most of the lake. Chlorophyll concentrations remained high during the whole
growth period, from April to December, a feature typical of eutrophic water
bodies (Korzeniewski 1992, Burchardt 2004).
Variability in the dynamics of coastal lakes calls for the use of instruments
that can allow continuous recording of water parameters. For chlorophyll
concentration measurements fluorometric methods seem especially useful.
Fluorometric methods allow good resolution in space and time, making
monitoring data more effective than traditional methods based on
spectrophotometric measurements, since without continuous measurements, fast
and frequent changes are difficult to detect.
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