Decline of aquatic vegetation in Lake Donghu

: Implication for management of shallow Chinese lakes

 

 

Leyi Ni

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

 

 

Abstract

 

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

 

 

Introduction

 

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

 

Features of the lake

 

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.

 

Sampling methods of aquatic macrophytes

 

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.

 

Data sources of nutrient concentration, primary production and fish yield

 

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.

 

 

Results and discussion

 

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.  

 

Quantitative changes of the submersed vegetation

 

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.

 

Succession of dominant species

 

Succession sequence of the dominant species of the aquatic vegetation in Tanglin Hu area was shown in Fig. 2. The dominant species was P. maackianus of 1960s, N. major in mid 1970s, V. natans and M. spicatum in early 1990s, and C. demersum in mid 1990s. The dominant species in 1980s is unknown as no investigation was carried out during 1983-1987. Total biomass of the vegetation showed a strong fluctuation and a decreasing trend line throughout 1962-1998 (Fig. 3). Almost all dominant species showed a subsequent decline after it came into dominance.

 

Environmental changes and their effects to the biomass of submersed macrophytes

 

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.

 

Decline of emergent and floating-leafed vegetation with the habitat changes

 

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.

 

Mechanisms for the decline of aquatic vegetation

 

      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.

 

Management implications of shallow Chinese lakes

 

     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 extra biomass of macrophytes in the lakes.

 

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