Decline of
aquatic vegetation in Lake Donghu
: Implication for
management of shallow Chinese lakes
Donghu Experimental Station Of Lake Ecosystems, CERN, The State Key Laboratory Of Freshwater Ecology And Biotechnology, Institute Of Hydrobiology, The Chinese Academy Of Science, Wuhan 430072, P. R. China
Lakes along the middle and lower reaches of Yangtze
River are both shallow and productive. Aquatic vegetation plays an important
role for maintaining the stability of the ecosystem of the lakes. Decline of
aquatic vegetation in these lakes related to impacts of human activities occurs
commonly from 1970s. A case study conducted in Lake Donghu on the long-term
dynamics of aquatic vegetation during the process of eutrophication and increasing
annual fish production in the lake. Between 1963 and 1998, more than 30%
species of aquatic macrophytes disappeared from the lake; the cover rating of
submersed vegetation decreased from 62.3% in 1963 to 11.5% in 1994 and to less
than 2% in 1998; Marked decreases in both biomass and frequencies of 9 major
species were observed during this period; the dominant species of the
vegetation succeeded in the order of Potamogeton
maackianus, Najar major, Vallisneria natans + Myriophyllum spicatum, and Ceratophyllum demersum. Major reasons
for the decline of the vegetation are discussed: bank construction, increases
of nitrogen and phosphorus loading to the water, intensive stocking to the
lake, and a subsequent decrease of water transparency were related to the decline.
Mechanisms related were interpreted . Based on the results, management
strategies of the shallow lakes in this area were suggested for sustainable
development of the lake ecosystems.
Key words: species cover
rating, biomass, succession, and dominant species
Along the middle and lower basin of the Yangtze River in China, there are about 1760 shallow lakes covering a total area of about 3,344,000 ha. This area, characterized by is well-developed water system and fertile soil, was one of the "cradles" of Chinese civilization. The lakes were originated through obstruction of the river tributaries under the humid climate of the later Quaternary period (Liu, 1984). The worm subtropical climate in this area and fertile muddy sediment of the lakes accelerated the ecological succession of the lake ecosystems. At present, these lakes are already at the later succession stages of aquatic series indicated by well-developed aquatic vegetation. Floristic compositions of the vegetation among the lakes are very similar owing to the historic connection between the river and the lakes.
Large-scale limnological investigations on the lakes started from 1950s. Declines of aquatic vegetation and degradation of the ecosystems in many of these lakes were commonly reported thirty to forty years after the first investigation (). This decline was accompanied by intensified exploitation of these lakes and fast growing economy in this area during the last few decades. In many lakes, however, too less investigations were carried out during the period to study the process and mechanism for the vegetation change. Due to the similarity among the lakes, results from a case study may be applicable to other lakes.
Many studies showed that submersed macrophytes can control the turbidity of the water, suppress algae growth, decrease nutrient cycling rate, maintain the biodiversity and stabilize the food net of the ecosystem. Decline of aquatic vegetation, especially submersed macrophytes, may be critical for the phase changes of shallow lake ecosystems from clear water state toward turbid state (Janse, 1998; Perrow , 1999).Therefore, process and mechanism studies on the decline of the vegetation will provide scientific basis for the management of the shallow lake ecosystems of this area.
This paper presents a case study on the long-term dynamics of aquatic vegetation in relating to environment impacts in an urban Chinese lake, Lake Donghu located in the middle reaches of Yangtze River.
Material and methods
Lake Donghu (East Lake, 30o33¡¯,
and 114023') is a subtropical shallow lake with a surface area of 32 km2.
It is located 5 kilometers away from the Yangtze River, connecting with the
river through Qingshan Canal in the past. Since the later 1960s, the lake has
been separated into 3 major area, Guozhen Hu (12km2), Tanglin Hu
(5.4km2), Houhu (3.4km2) and other small parts (Fig. 1).
These major areas are the study sites.
Several rows of sandstone hillocks cut into the lake
in the south . North of the lake is the alluvial plain of the Yangtze River. A
nature levee separates the lake from the plain. Brownish clay terraces locate
on the east and west sides of the lake. The sediment of the lake mainly
consists of sapropel and ooze. Various sediment distributed patchy in the lake
(Liu, 1984). The average depth of the lake is 2.5m. The maximum and minimum
water temperature are 32oC (July and August) and 5oC
(February), respectively. Dissolved oxygen content of the water column in the
pelagic station is high (mean 7.3 mg l-1).
Owing to the increase of fish stocking density and
catching efficiency by the state farm, annual fish yield of the lake has
increased steadily from less than 100 kg h-1 to more than 1000kg h-1
from 1960s to 1998. Of the harvested fish, over 90% were comprised of the two
planktivorous species, silver carp and bighead carp. Grass carp was less than
2% of the total yield.
Nitrogen and phosphorus contents of the water are
also high (mean 3.2mg l-1 TDN and 0.3mg l-1 TDP). Mean
annual average of the daily gross primary production was 5.6 gO2 m-3
d-1 during the study years.
Samples of aquatic macrophytes were taken along
transactions across the lake and along elevation perimeters. Elevation
difference between two neighboring perimeters was 0.8m. More than 80
quantitative samples were taken for each investigation. The investigation was
conducted once a year in August before 1988 and three times a year in April,
August and December afterwards. The size of quadrates is 0.2m-2 for
submersed macrophytes and 1m-2 for emergent plants. All samples were
harvested by sickle. Two sickle blades were installed oppositely against a long
handle, which is used for quantitative sampling of submersed macrophytes by
rotating the handle above the water surface. Samples were then identified,
sorted, weighed and a portion of each species was dried in oven at 80oC.
Dry weight of the samples was converted by the fresh weight to dry weight
ratio. Frequency of each species was also calculated.
All data were from the past publication or
unpublished studies. Nutrient data were available in 1964, 1973-75, 1979-85
(Liu and Zhang, 1990), 1989-91 (Wang, unpublished), 1992-94 (Liu, unpublished).
Primary production data were available in 1964, 1973-78 (Rao and Zhang, 1984),
1981-86 (Wang, 1990), 1989-94 (Rong, 1994). Fish yield was available throughout
1951-1998 collected by Huang Gentian from the state fish farm.
Change of floristic composition
Fifty-one aquatic macrophyte species in 25 family
were recorded throughout the study (Tab. 1). Fourteen species recorded in 1960s
were found to disappear from the lake in 1990s. They are Ceratopteris thalictroides (L.) Brongn, Limnophila sessiflora (Vahl) Blume, L. aquatic L., Veronica
anagallis L., Utricularia exoleta R.
Br., U. vulgaris L., U. minor L., Potamogeton maackianus A. Been. Nymphaea alba L., N. teragona
Georgi., Polygonum amphibium L., Monochoria korsakowii Regel, M.
Vaginalis (Burm.f.) Presl. Besides
these, another 18 species recorded in 1960s were also not recorded in 1990s.
Whether or not they disappeared has not confirmed yet.
The number of species disappeared are similar among
emergent, floating-leafed and submersed macrophyte life forms.
Submersed vegetation is the major vegetation type of
the lake. Between 1963 and 1994, changes in major submersed macrophyte
association were considerable and vegetation cover rating decreased by 3 to 40
times in the 3 areas (Tab. 2), as a consequence of the disappearance of P. maackianus, the dominant species in
1960s.
Fig. 2 showed marked decreases in biomass and
frequency of all major species of the vegetation between 1963 and 1993,
indicating that all the species have grown under stress. Among these species, P. maackianus, a predominant species
with the highest biomass and frequency among aquatic macrophytes in 1960s,
disappeared from the lake. V. natans,
M. spicatum, N. major and P. crispus
are less affected species by the environmental stress. C. demersum showed very strong variation from year to year. H. Verticillata presently become very
scarce in the lake while in 1963, it showed the highest frequency as P. maackianus.
Changes of some important environmental factors
affecting the plant growth and distribution are shown in Figs. 4-6. Increase of
fish yield became very fast after 1972 (Fig. 4). With most fish stocked in
Guozhen Hu area, biomass of submersed macrophytes in the area became very low
from the beginning of the increase. The stocked fish affected macrophytes of
Tanglin Hu area progressively by entering the area through a free passage. Few
effects of stocked fish on macrophytes of Hou Hu area due to the complete
separation of this area from the other two areas.
Dissolved nitrogen and phosphorus in the water of
the lake, especially NH4-N and PO4-P species increased
continuously during the period of the study (Fig. 5). Increases of the
nutrients and fish yield in the water phase were negatively related to the
biomass trend line of submersed macrophytes, but showed low correlation
coefficient with macrophytes biomass of the two areas. Macrophyte biomass of
Guozhen Hu area related more to water transparency and primary production than
did the biomass of Tanglin Hu which related more to the yield of grass carp
(data available only during 1973-1978).
Relationships among macrophyte biomass, primary
production and water transparency of the lake were shown in Fig. 6. when the
transection biomass of submersed macrophyte were high (B = 0-326gDW m-2,
mean 140.6, 1988), it showed higher correlation with Sacchi disk depth than
when it was low (B = 0-142gDW m-2, mean 19.7, 1993) in Tanglin Hu
area. When the biomass is relatively low, seasonal changes in biomass of
submersed macrophytes and algae were also less correlated in 2 transections of
Hou Hu area (Fig.7). Both macrophyte and algae biomass showed a seasonal
increase.
Before 1960s, the catchment area of Lake Donghu was
lowly populated and undeveloped. All of the lake areas were unstocked and
joined together. The sandstone of hillocks cut into the southern basin of the
lake, allowing less colonization of emergent macrophyte along this shoreline.
The northern basin of the lake is flat and muddy, with more emergent mashes
developing along this side (Fig.1).
Several measures have been taken to modify
the lake for specific purpose, such as the close of Qingshan canal, the
separation of the lake area by artificial dikes, and the reclamation of land.
These changed the habitat of the emergent macrophytes. The close of Qingshan
Cannel in 1960 remarkably increased the water level of the lake during winter
season, which upset the germination of emergent plant in early spring. By
comparing the vegetation map of the lake in 1963 and 1994, it is clear to see
that artificial dikes built in later 1960s had destroyed the major habitat of
emergent macrophytes at the large conjunction site between Guozhen Hu and
Tanglin Hu area. Loss of emergent marshes at the upper end of Guozhen Hu and
Tanglin Hu, and the lower end of Hou Hu was due to the reclamation. Bank
construction surrounding the lake also impacts the habitat of aquatic plants by
destroying the nature slope from shoreline and adding gravel to sediment soil
during the construction. Of the 3 areas, emergent vegetation was found to
disappear in Guozhen Hu area. Compare the emergent and floating-leafed
macrophyte between 1963 and 1994, the decline of the above vegetation type was more
than 90% by distribution area. Six species of these life forms had disappeared
from the lake.
The present study showed that the
loss of habitat and relative high water level during winter season caused the
decline of emergent and floating-leafed macrophytes. Eutrophication may be
another important reason for the decline. It was reported that biomass growth
of aquatic weeds decreased at high nutrient and organic contents of the
sediment (Barko and Smart, 1986). Eutrophication also caused low resistance of
the macrophytes to environmental changes by lowering their tissue carbohydrate
reserve at high nutrient supply (Cizkova-Koncalova et al., 1992). According to
the water chemistry data of the 3 major areas of Lake Donghu, nitrogen and
phosphorus contents of Guozhen Hu area are much higher than those of the other
two areas (Tang and Xie, 1999). Thus, eutrophication is assumedly critical for
the disappearance of emergent vegetation in Guozhen Hu area.
Several reasons may
lead the decline of submersed vegetation in Lake Donghu. A rapid increase of
fish production in Guozhen Hu area may be responsible for the large-scale
decline of submersed vegetation during 1970s. This area also receives the
highest sewage loading from major inlets to the lake (Tang, 1999, Zhang et al.,
1984). It is hard to discriminate the impacts of eutrophication from the
co-effects of the increasing fish production. As fish has both disruptive
activity to the plants and the disturbing effects to the environment, its
impacts on the vegetation may be remarkable. In Tanglin Hu area, where much less fish was stocked, the
succession of dominant species and the fluctuation pattern of the vegetation
can be attributed more to the stresses of eutrophication (Best et al., 1993).
The succession pattern of the vegetation seems to be quite comparative to that
of phytoplankton communities during the progress of eutrophication (Urabe et
al., 1999; Yusoff, and McNabb, 1997). It may be applicable to succession of
dominant macrophyte species that each species is only able to sustain at
certain trophic scale and decline beyond the both ends of the scale. In
eutrophic water, only the canopy growth form and fertile resistant species are
able to sustain (Madsen, 1991; Pokorny et al., 1990). This limitation, combined
with the already much simpler species composition of submersed macrophytes than
that of phytoplankton (Wetzel, 1983), decreases the resistance and adds
unstability of the vegetation at the later stage of eutrophication. In the
present study, the vegetation is only composed of very limit species of both
low distribution rating and low resistance to the disturbance. When biomass and
frequency showed that all species grew under stresses during 1990s, the
disappearance of the vegetation became unavoidable.
The shallow Chinese lake of subtropical
zone, having received high energy inputs from the rapid developing catchment
area of the flooding plain, are very productive and high in turnover rating.
Therefore, the response of the ecosystems to environmental changes are much
faster than those in temperate zone. In recent years, more attentions have been
paid to shallow, non-stratified lakes in parallel to the biomaipulation
approach to the lake eutrophication control. Studies showed that changes in the
biotic structure of shallow lakes are more likely to result in changed water
quality than are in deep lakes (Kufel et al (eds), 1997).This case study shows
that management strategies have affected strongly the biotic structure of the
ecosystem. Bank construction has severely destroyed the habitat of the emergent
and floating-leafed macrophytes, both heavy fish stocking and sewage loading
have caused the decline of submersed vegetation. Because littoral zone is the
major part of a shallow lake and aquatic vegetation is the largest functional
component in the littoral communities, the decline of aquatic vegetation leads
to the loss of functioning of the ecosystems, the subsequent decline of other
communities, such as small fish, large zooplankton and some benthic animals
occurred, and phytoplankton came into dominance in the lakes (Gong and Xie, 2001; Rao and Zhang, 1980; Rong, 1994; Rong, et al., 1995; Tang et al., 1999;
Xie and Chen, 1999).
Alternative management strategies can be taken into
consideration: restoration of the habitat, and restoration of both emergent and
floating-leafed macrophyte after the dyke and bank construction. The
colonization of macrophytes can prevent re-suspension of sediment and diminish
strong waves which cause erosion along the bank, and thus adds possibility for
the further development of vegetation into the lake.
The stock of grass carp into the lake has to be
stopped due to difficulties in estimating its proper stocking rate. Its
recapture efficiency is low relative to other stocked species and thus hard to
estimate the amount of this fish in the lake. Its feeding rate on submersed
macrophytes is high, reportedly 1:120 (FW, Chen, 1975). Thus any
under-estimation of its impacts is easy to cause the destruction of the
vegetation. The stocking of this species showed the complete eradication of
submersed macrophytes in many occasions (Chen, 1975)
The total amount of stocked fish has to be reduced
as well in order to restore macrophytes into the lakes. The loss of the
benefits may be compensated by stocking species which are high in commercial
values, such as Mandarin Fish (Ctenopharyngodon
idella) and crab. In the present, Filtering-feeding fishes are major
stocked species in this area. Its heavy impacts on both phyto- and zooplankton
have led to a comprehensive increase of water turbidity due to the increased
internal loading of nutrients by the activities of stocked fish (Rao and Zhang,
1980; Liu, 1984; Fukushima et al., 1999). It has been suggested that beyond a
certain threshold of turbidity, the return of submersed macrophytes into
eutrophic water become impossible (Asaeda and Van Bon, 1997).
Reducing the external loading of nutrients is the
final solution for macrophyte to come into dominance (Hosper, 1994). Earlier
enclosure experiment showed that except for NO3-N,
all nitrogen and phosphorus species decreased markedly and the transparency of
the water increased significantly after the external loading was eliminated (Ni
et al., 1995). Submersed macrophytes can return to the water once the water is
not very turbid (Coops et al., 1996; Samuels and Mason, 1997). Then macrophytes gradually become dominant by efficiently lowering the
internal nutrient loading during the most time of their growing season (Barko et al.,
1991).
Regarding to the concern
that overgrowth of macrophytes may occur in eutrophic waters after the control
of fish stocking and sewage inlet, additional measures should be considered as
well. Ecologically, both top-down and bottom-up controls can be taken into
consideration. And between which, the former is more applicable than the later
because the sediment nutrient pool is still very large after the external
loading has been cut off. Harvesting the macrophyte biomass is an efficient way
to remove nutrients from lakes. In Lake Honghu, harvested biomass is used as
fodder for fishes grow in cages and pens, and change into fishery products
(Chen et al., 1995). This method should be applicable in order to remove the
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