Trophic Status and Primary Production in Lake Choghakhor,
Chaharmahal-Bakhtiyari Province, Islamic Republic of Iran
R. Mousavi Nadushan
S.M. Reza Fatemi
In this study, trophic state indicators, total phosphate,
algal chlorophyll and Secchi disk transparency and zooplankton community
of Lake Choghakhor was studied monthly between May 2003 and April 2004.
This lake is a shallow ecologically and economically important water body
in eastern part of Iran. Crop farming and recreational activities are
examples of the human impact around and within the lake, leading to a
loading of DIN (Dissolved Inorganic Nitrogen) and TP (Total Phosphate)
into the lake. Now submerged plants especially Myriophyllum spicatum
has covered almost the entire lake and dense macrophyte beds (Polygonom
amphibium), located on the East Southern end of the lake appear to
act as a sink for these nutrients. Lake Choghakhor appeared to be in a
macrophyte dominated clear water state with low TP (annual mean: 24 ±15 μg L-1) and chlorophyll a (annual mean: 3 ±1.28
μg L-1) concentrations and very high Secchi depth. The
grazing pressure of dominant pelagic filtering zooplankton Daphnia
longespina did not seem to be significant in determining the low phytoplankton
crop expressed as chlorophyll a. We expect that sequestering of nutrients
by submerged plants and associated epiphytes are the dominant stabilizing
mechanisms suppressing the phytoplankton crop of Lake Choghakhor.
The primary productivity of shallow lakes is characterized by mixed populations
of phytoplankton and submerged aquatic vegetation in the open water, fringed
by various species of emergent vegetation. Research in other parts of
the world has indicated that the relative proportion of submerged macrophyte
and phytoplankton depends on nutrient loading, water depth, basin slope
and size and herbivore (Wetzel, 2001). Some shallow lakes appear to be
dominated by submerged aquatic vegetation, whereas other appears to be
dominated by dense phytoplankton blooms (and little submerged macrophyte),
a finding that has been labeled alternative stable states (Scheffer, 1997).
According to this theory at low nutrient levels phytoplankton becomes
nutrient limited, which leads to clear water and dominance of macrophytes.
At high nutrient levels, phytoplankton dominance will lead to insufficient
light for submerged macrophytes and a stable turbid state. At intermediate
nutrient levels, however, either macrophytes or phytoplankton can dominate
and shifts between the two states are possible. A clearwater state is
largely characterized by submerged plants and sufficient piscivorous fish
(e.g., Pike and predatory Perch) biomass to extent strong control on planktivorous
fish (e.g., Bream, Carp, Roach, Tench) enabling zooplankton (e.g., Daphnia)
to control phytoplankton and snails to control epiphytes on the plant
surfaces (Jeppesen et al., 1997; Moss et al., 1998; Scheffer,
1997). Nutrient uptake (Wetzel, 2001), reduction of sediment resuspension,
improving conditions for micro-macro invertebrate epiphyton filtrates
(Cottenie et al., 2001), enhancement of denitrification (v) may
also contribute to the effect of aquatic macrophytes on water clarity.
The growth of submerged aquatic macrophytes may be limited by lack of
light due to high turbidity (from phytoplankton) (Jeppesen et al.,
1998), lack of light due to water depth (Moss et al., 1994), or
low nitrogen (Carpenter et al., 1998). Nutrient loading (bottom-up
control), predatory fish and invertebrates (top-down control), climatic
events (flood/drought), or some combination of external and internal factors
may regulate these alternative states (Scheffer et al., 2001).
A change in abundance of fish has been credited with changing the trophic
state of shallow lakes (Jeppesen et al., 2000; Zimmer et al.,
2001; Tatrai et al., 2003). The role of fish can be particularly
large in shallow lakes, because the biomass of fish per cubic meter of
water is higher in shallow lakes than in deeper lakes (Jeppesen et
al., 1997) and because they have access to alternative food resources
that may facilitate the top-down control of zooplankton.
Lake Choghakhor used to be a natural wetland known for its
reed mats. In 1991 an earthen dam was constructed at the exit canal of
the lake increasing its depth and turning it into a permanent shallow
lake. Subsequently, Regional Fisheries Office stocked the lake with Chinese
carp species mainly with Cyprinus carpio, Hypophthalmichthys molitrix
and Ctenopharyngodon idella for a few years. It is regarded as
a wetland by the Department of the Environment because of its natural
importance mainly high diversity of aquatics and also a wintering site
for waterfowls and migratory birds. Lake Choghakhor is an important ecosystem
in the region. It has a great recreational value and also supports waterfowl,
local agriculture, tourism and fisheries. In spite of these importance
and a few exemptions, there is great deficiency towards the water chemistry
and biological aspects. The early studies before construction of the dam
indicates that the lake was not entirely covered by macrophytes and their
extension was restricted mainly to the littoral belt dominating by Juncus
(Bagheri, 1997). Today, the lake is completely dominated by submerged
aquatic vegetation making great problem for the recreational and fisheries
activities. The present study, part of a comprehensive project covering
all environmental, biological and socio-economical aspects, aimed to carry
out through limnological study in order to draw a clear picture about
the prominent environmental conditions, trophic status and primary production
of the lake.
MATERIALS AND METHODS
Site description: Lake Choghakhor is located in eastern part of
Iran (31°, 54´-31°, 56´N; 50°, 40´-56°, 14´E) Chaharmahal - Bakhtiyari
Province and is about 2300 m above sea level. It is a shallow lake (mean
depth = 2.5 m) and occupies an area of 14 km2. The total volume
of water varies between 25x106 m3 in summer and
35x106 m3 in winter. This lake surrounded by 8 small
villages, agriculture and livestock main activities. The runoff of agricultural
lands ends into the lake. There are also about 5 considerable permanent
springs` originations from calcareous mountains to the south (Fig.
Schematic map of Lake
Choghakhor (not to scale) with the 5 sampling stations(shown as+)
and locations of inflows(shown as) and outflow (dam). Station
5 is the deepest part of the lake
predominant climate of the area is Mediterranean with very
cold winter and mild summers (-20 and 30°C) with an annual precipitation
of 800 mm (Annual Statistic of the Country, 2003).
Five stations of Lake Choghakhor were sampled monthly from
May 2003 to April 2004. Sampling was not done for two months, November
and February, because of weather and freezing problem. Water samples were
collected by using Ruttner hydrobiological water sampler of 2 L capacity.
Water samples for chemical analysis were taken at surface and for physical
analysis in two depths (surface and bottom).
Sites were selected to represent different hydrodynamic
environment and different diversity and density of submerged macrophytes.
Analytical procedures for the chemical analysis were done according to
APHA (1992). Water temperature and dissolved oxygen were measured with
a YSI 57(Yellow Springs Instruments) digital oxygenmeter. Water transparency
was estimated using a Secchi disk. Samples for chlorophyll analysis were
filtered onto Whatman GF/F filters within 24 h of collection. Chlorophyll
was extracted in 95% acetone and analyzed with a spectrophotometer at
750, 665 and 649 nm. (APHA, 1992).The Chl a concentration was used as
a proxy for phytoplankton biomass. The ratio of biologically available
nitrogen (NH4+, NO3–)
to biologically available phosphorus (SRP) was calculated to provide an
estimate of nutrient limitation (Redfield ratio).
At least 30 L of water was sampled for zooplankton. Samples
were collected by polyethylene tube and 45 µm mesh-size nylon plankton
net. The samples for identification were stored in a 4% formaldehyde solution
until microscopic - stereoscopic counting (Smith, 2001).
||Physical-chemical data of Lake Chaghakhor, May 2003-April
|s: Surface, b: Bottom
Lake Choghakhor did not undergo stable thermal and oxygen
stratifications and lake water was mixed throughout the study. The surface
water temperature varied from 3°C (in January) to a maximum of 26°C (in
July) and bottom temperature followed the same variation (i.e., from 4°C
in January to 19.6 in July) (Table 1). A weak thermal
stratification was noticed from June through October while the water mixing
began early in autumn. The lake`s mixing regime is polymictic.
Dissolved oxygen in surface waters fluctuates between 8.2
and 11.2 mg L-1, appearing the minimum values through the warm
period (Table 1). Diurnal stratification was measured
throughout the late spring and summer, while the water column was isothermal
in winter (Table 1). The maximum concentration, 11.2
mg L-1 was recorded in January during the isothermal conditions.
Decomposition processing and sediment oxygen demand were sufficient to
cause lower dissolved oxygen values, near-bottom, from June to October
Secchi-disk transparency varied between 0.9 and 3.4 m through
the year (Fig. 2). The highest value (summer) may be
related to low phytoplankton biomass, while there was low seasonal variability
throughout the year. The Secchi depth was found to be the minimum in early
spring and winter when submerged plants were absent (Fig.
There was a significant inverse correlation between transparency
and Chlorophyll a concentrations. Annual mean Chlorophyll a
concentration was 3±1.28 µg L-1 and the Chlorophyll a
concentrations were <=3 µg L-1 throughout the summer (Fig.
In Lake Choghakhor, the annual mean of total phosphorous
(TP) and soluble reactive phosphorous (SRP) concentrations were 24±15
and 8±7 µg L-1, respectively that the concentrations were lower
throughout the summer than in spring and winter (Fig. 3a).
Concentrations of TP had a seasonal range of 0.007 ~ 0.03 mg L-1
and exhibited higher values during the spring and winter.
Fluctuations in Chlorophyll a
concentration in Lake Choghakhor, May 2003-April 2004
Changes in (a) soluble reactive
phosphorous (SRP) and total phosphorus (TP) and (b) dissolved
inorganic nitrogen (DIN) concentrations of Lake Choghakhor, May
The annual mean DIN concentration, which is the sum of the nitrite, nitrate
and ammonium concentrations, was 670 µg L-1
). Of the two forms of inorganic nitrogen analyzed, NO3
was dominant, with an annual mean of 508 µg L-1
and the concentrations
peaked in spring (Fig. 3b
). The NH4
were low (annual mean: 160 µg L-1
) during (throughout) the
||Density of large bodied zooplankton (ind L-1)
in Lake Choghakhor, May 2003-April 2004
sampling period except December and January 2003 (maximum:
560 µg L-1), which coincided with rotting of the submerged
plants (Fig. 3b).
Large-bodied filtering zooplankton:
In Lake Choghakhor, the densities of Cladoceran zooplankters
were low and the following genera were recorded: Daphnia longespina,
Bosmina longirostris and Chydorus sphaericus. The density
of D. longespina were very low during the study period and a small
increase was recorded in spring 2003 (7 ind L-1) (Table
2). The density of Chydorus was also low, but it was observed
throughout the sampling period, with an increase in summer and autumn
(6.6 ind L-1 in December 2003). The density of Bosmina was
insignificant being lower than 1 ind L-1 during the sampling
period. The Calanoid Copepod Cyclops sp. was the dominant copepod
in Lake Choghakhor. However, the density of Cyclops was also low
(2.7 ind L-1) throughout the sampling period, with an increase
in autumn (7.1 ind L-1 in October, 2003).
Nutrient availability, particularly of phosphorous (Schindler,
1978) determines potential phytoplankton biomass, while the grazing rate
of zooplankton determines how much of this potential realized (Schriver
et al., 1995). At low nutrient levels (<50 µg TP L-1)
phytoplankton becomes nutrient limited, which leads to clear water and
dominance of macrophytes. In this situation if P/N>>10, a decline in macrophyte
species diversity and dominance of nutrient tolerant species occurs and
subsequently the most abundant species will be tolerant and canopy forming
species like Myriophyllum spicatum and Potamogeton pectinatus
(Korner, 2002). At high nutrient levels, (>100 µg TP L-1)
phytoplankton dominance will lead to insufficient light for submerged
macrophytes and a stable turbid state. At intermediate nutrient levels
(50-100 µg TP L-1), however, either macrophytes or phytoplankton
can dominate and shifts between the two states are possible (Scheffer
et al., 1993; Korner, 2002; Bachman et al., 2002).
According to nutrients and Chlorophyll a concentrations
measured during this study, Lake Choghakhor is classified as oligo-mesotrophic
lake (Vollenweider and Kereks, 1982; OECD, 1982). The ratio of DIN/SRP
(where DIN = NO3– + NH4+,
NO2– was found negligible) reflects the limiting
factor, controlling the primary production (Reynolds, 1984). In our case,
this ratio was greater than Redfield ratio of 16, suggesting a phosphorous
limitation (the ratio ranged between 50-500 with a mean value of 227).
Although there was not sufficient information about the
trophic status of the lake Choghakhor, the results of this study revealed
that this lake with low TP (annual mean: 24 µg L-1) and Chlorophyll
a concentrations (annual mean: 3 µg L-1), very high Secchi
depth (found to be the bottom) and submerged plants (Myriophyllum spicatum)
covering almost the entire lake is in a pristine condition, macrophyte
dominated clear water state.
Myriophyllum spicatum is a highly competitive freshwater
macrophyte that produces and releases algicidal and cyanobactericidal
polyphenols but existence of allelopathy has been a subject of ongoing
debate, since its significance under field conditions is still too poorly
evidenced (Gross, 2003). Therefore in Lake Choghakhor luxury nutrient
uptake by submerged plants as well as associated epiphytes may impose
nutrient limitation on phytoplankton (Wetzel, 2001).
The increased grazing by pelagic zooplankton hiding in macrophytes
during daytime (Sondergaard and Moss, 1998), as well as the grazing by
macrophyte-associated zooplankton species (Lauridsen and Buenk, 1996),
has been often considered among the most important factors controlling
phytoplankton biomass in macrophyte dominated lakes (Schriver et al.,
1995; Jeppesen et al., 1998). In Lake Choghakhor density of large
bodied grazer Daphnia longespina (2 ind L-1) was low
throughout summer. In this lake, carp and tench were found to spawn from
May to August and their diet largely involved Cladocera, especially Daphnia
(Sondergaard et al., 1990). The entire coverage of observed submerged
plants and high densities of juvenile fish schooling in the lake (personal
observation) throughout the summer might have led to the very low densities
of the large grazers. Plant-associated cladocerans, less vulnerable to
fish predation than pelagic species, can have high grazing impact on phytoplankton
(Jeppesen et al., 1998; Balaya and Moss, 2004). Chydorids scrape
on epiphytic algae and are considered less effective (Lovgren and Persson,
2002). In Lake Choghakhor Chydorus sphaericus as a plant associated
grazer was observed in all samples, especially when the density of Daphnia
was minimum. The biomasses of Chydorus sphaericus increased with
macrophytes and were likely even higher than observed, since water sampling
underestimated the numbers of plant associated species (Vakkailainen and
Submerged plants are thought to negatively affect phytoplankton
crops in the temperate zone by a number of mechanisms, including nutrient
and light limitation. Macrophytes may reduce turbidity by both sequestering
algal growth-limiting nutrients and by restricting turbulent resuspension
of bottom deposits (Hamilton and Mitchel, 1996). Lake Choghakhor is a
large shallow lake with open wind induced resuspension and strong winds
in late autumn, winter and spring. In the absence of submerged plants,
powerful upward flux of nutrients in the lake, caused mainly by resuspension,
could enhance phytoplankton growth in March and April 2003-2004 (5.8 µ
Chl. a L-1).
In Lake Choghakhor the growth and dominancy of macrophytes
has been increasing for the last 10 years, so resulting more or less into
the coverage of the whole lake nowadays. Recent climate trends with periodic
(droughts and) lowering of water tables (especially in summer) may foster
shallow water favoring submerged plants. Studies have suggested that macrophytes
will become established within a few months, even for short time, if propagules
are readily available (Perrow et al., 1997). Another factor that
might have been significant in the expansion of the submerged macrophytes
is the lack of grass carp (0.3%), which is the case with this lake after
the release of this species, was stopped.
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