As the largest city in China, Shanghai is located at the mouth of the Yangtze River. There are many rivers in the city, with dense river networks and serious eutrophication. Since 1990s, the population of Eichhornia crassipes in Shanghai has shown a vicious explosion trend, and most small and medium-sized rivers have been occupied by Eichhornia crassipes, which has seriously affected the local water transport and farmland irrigation and drainage. For the Huangpu River, Suzhou River and other rivers that flow through the bustling business districts and tourist attractions in the city, the large distribution of Eichhornia crassipes population seriously affects its river landscape and has a serious negative impact on the image of Shanghai.
At present, there are two views on the source of Shanghai Eichhornia crassipes and whether it can successfully overwinter: one thinks that Shanghai Eichhornia crassipes can successfully overwinter mainly by self-reproduction; Secondly, Eichhornia crassipes can't overwinter in Shanghai, and its provenance comes from Jiangsu and Zhejiang provinces in the upper reaches of Huangpu River and Suzhou River every year. In order to effectively carry out the prevention and control of Eichhornia crassipes, it is of great significance to strengthen the study on the life history and reproductive strategy of Eichhornia crassipes in Shanghai, and to control the outbreak of Eichhornia crassipes from the source.
In view of this, this paper studies the life history characteristics of Eichhornia crassipes in Shanghai from the perspective of discussing the successful invasion mechanism and prevention, and understands whether Eichhornia crassipes can successfully overwinter in Shanghai. In addition, Eichhornia crassipes is a plant that can reproduce both sexually and asexually. Therefore, according to the actual situation of Shanghai water body, it is of great significance to explore the mechanism of its successful outbreak in Shanghai, based on the mode of transmission. Thirdly, as a floating aquatic plant, the fixed growth of Eichhornia crassipes in the river is the premise of its successful outbreak, so finding its unique fixed mechanism is an important aspect of Eichhornia crassipes control. On this basis, a feasible plan for controlling Eichhornia crassipes in Shanghai was put forward.
1 materials and methods
1. 1 Overview of the study area
Shanghai is located in the middle of the north-south coastline of China, on the eastern edge of the Yangtze River Delta, bordering the East China Sea in the east, Hangzhou Bay in the south, the Yangtze River in the north and Jiangsu and Zhejiang provinces in the west. It belongs to subtropical maritime monsoon climate, with the annual average temperature 16℃, the highest in July and August, the monthly average of 28℃, and the lowest in 65438+ 10, with the monthly average of 4℃. The frost-free period of the whole year is 230 d, and the annual average rainfall is 1200 mm. The rivers, rivers and ponds in Shanghai are densely distributed, which is a typical plain river network area. Huangpu River, as the last tributary of the Yangtze River, gathers the water system of Taihu Lake basin and crosses the urban area into the Yangtze River estuary. Affected by tides, the Shanghai River is a reciprocating flow with unstable flow direction, and the shallow sea is an irregular semidiurnal tide.
Unique climatic conditions, slow-flowing hydrological conditions and a large amount of industrial and agricultural sewage and domestic garbage discharged by local residents have created a suitable ecological environment for the growth and reproduction of aquatic plants. After Eichhornia crassipes was introduced into China, it was inevitable that it invaded Shanghai and broke out in the rivers and waters of Shanghai from the historical, environmental and humanistic perspectives.
Field investigation on life history characteristics of Eichhornia crassipes 1.2
Eichhornia crassipes is a perennial plant, and the season of rapid growth and large-scale outbreak every year is only August-65438+February. To study the occurrence law of Eichhornia crassipes, it is necessary to understand its complete life history characteristics in order to provide useful guidance for the prevention and control of Eichhornia crassipes. In view of this, from July 2002 to June 5-438+February 2003, the sampling data of Eichhornia crassipes population in various districts and counties under the jurisdiction of Shanghai and adjacent Taihu Lake basin (Jiangsu and Zhejiang provinces) were investigated. Among them, the survey is conducted every half month in summer and autumn, and once a month in winter and spring. The investigation contents mainly include: the growth status of Eichhornia crassipes population, seasonal distribution dynamics, population drift dynamics, and the physical characteristics of the river (water temperature, water depth, flow direction, velocity, channel width, turbidity, water quality, etc.). ), shoreline characteristics (uniformity, vegetation distribution), human influence and management measures (sluice, salvage, navigation, water use), and river branch characteristics. At the same time,
Study on overwintering ability and reproductive strategy of Eichhornia crassipes in Shanghai
As mentioned above, Shanghai has a subtropical maritime monsoon climate with distinct seasons throughout the year, which is suitable for the growth and reproduction of Eichhornia crassipes. The lowest temperature is 5438+ 10 month in June, and the monthly average temperature is 4℃, but the extreme minimum temperature is–7.6℃ [7]. The research results of Wu Dan et al. [8] and Yan Guoan et al. [9] show that the lowest survival temperature of Eichhornia crassipes is 0 ~ 5℃, so whether Eichhornia crassipes can overwinter naturally in Shanghai has always been a controversial issue. In the field monitoring in 2002-2003, the overwintering performance of Eichhornia crassipes was studied emphatically. In addition, in order to explore the outbreak mechanism of Eichhornia crassipes, the reproductive strategy of Eichhornia crassipes was investigated throughout the observation period.
2 Results and discussion
2. 1 Life history characteristics of Eichhornia crassipes in Shanghai
The investigation results show that the germination period of Eichhornia crassipes is from March to May every year, and some axillary buds of Eichhornia crassipes that have successfully wintered in reeds, water chestnuts and other plants begin to germinate and stand upright on the water surface. The buds are yellow-green or white at first, and gradually turn green after being exposed to sunlight. Two weeks later, the buds left the mother plant and began to grow freely. The dead mother plant of Eichhornia crassipes sank to the bottom of the water. Seedlings drift with the tide and are fixed when they meet other aquatic plants. Some seedlings germinate directly in the grass on the shore, and the ramets produced by their clonal propagation often gather together and spread around. At the same time, another alien invasive plant Alternanthera philoxeroides (commonly known as Alternanthera philoxeroides) in Shanghai waters has entered a period of mass reproduction and reproduction. Alternanthera philoxeroides is a sessile aquatic plant, which grows from shore to water through stolons. The abundant distribution of Alternanthera philoxeroides is convenient for fixing and wrapping the scattered and floating Eichhornia crassipes, creating a relatively stable growth environment for Eichhornia crassipes. Under the protection and encirclement of Alternanthera philoxeroides, the population of Eichhornia crassipes is expanding day by day.
By the middle and late July, influenced by the natural enemy Alternanthera philoxeroides, Alternanthera philoxeroides was eaten in large quantities, which led to the decline of its growth rate and rapid decline. At this time, the population of Eichhornia crassipes has expanded to a considerable scale and began to drift in the river. With the further consumption of Alternanthera philoxeroides, Alternanthera philoxeroides gradually died. It just provides a broad niche for the outbreak of Eichhornia crassipes.
In August, Eichhornia crassipes began to explode. Mature mother plants can produce a generation of seedlings in an average of 3 ~ 5 days by cloning and reproduction, while seedlings can mature and grow in 3 ~ 5 days and produce new ramets. Therefore, under the condition of infinite space, the number of ramets of Eichhornia crassipes increased almost exponentially. For a single plant, Eichhornia crassipes can usually produce 5-6 ramets, which are evenly distributed around the mother plant, so Eichhornia crassipes can produce a large number of ramets in a short time, which is not affected by resources, space, competition, death and other factors. Its calculation formula is deduced as follows:
Where Nt is the total number of ramets at time t, N0 is the initial number of base plants, a is the maximum number of ramets that can be produced by a single Eichhornia crassipes, b is the time required for Eichhornia crassipes to produce a new generation of ramets to mature and start to produce new ramets, Tt-T0 is the time from initial time to time t, and (Tt-T0)/b is the average generation algebra of Eichhornia crassipes at time t. I is 0, 1, 2, 3 ..., and its maximum value is I = (TT-t0)/B. For example, in August, the river channel where Eichhornia crassipes grows is relatively wide and there is almost no space restriction, so the maximum number of ramets that a single Eichhornia crassipes can grow after 5 weeks can be calculated, and the calculation method is as follows:
The results showed that 3906 Eichhornia crassipes could be produced after 35 days. It can be seen that the propagation speed of Eichhornia crassipes is quite amazing under the conditions of suitable temperature and infinite river space. When Eichhornia crassipes is spread all over the river, due to the limitation of water surface space and nutrient resources, the cloning and reproduction speed of Eichhornia crassipes will decrease. However, for flowing rivers, due to factors such as current, waves or human interference, it is easy to interrupt the stolons between Eichhornia crassipes branches, and these independent Eichhornia crassipes plants drift to other rivers with the current, thus releasing water surface space. Therefore, the river with relatively slow water flow has become the birthplace of Eichhornia crassipes growth and reproduction, and the tide has become the input and output power. The field observation results also confirmed this conclusion. At first, the water surface of small and medium-sized rivers and ponds with a width of 2 ~ 3 m was quickly occupied, and Eichhornia crassipes became a single dominant species. Then, with the help of the reciprocating motion of the current, Eichhornia crassipes began to expand to a wider river channel (5 ~ 6 m), and from late August to early February, a large number of Eichhornia crassipes drifted from the rural river channel to the urban river channel (table 1).
The internal cause of this phenomenon is the high asexual reproduction ability of Eichhornia crassipes, and the external cause is the serious eutrophication of Shanghai water body. The results of water sample analysis show that the total carbon (TOC) in Shanghai water is11.17 ~ 30.26 mg/L, the total N is 3.69 ~ 13.37 mg/L, and the total P is 0.08 ~ 0.58 mg/L. Unpublished data). As a water purification plant [8, 10], Eichhornia crassipes grows very rapidly under such rich nutrient conditions, and it is easy to reach the level of population outbreak, and its biomass index can reach 14.6 ~ 4 1.4 kg/m2 (fresh mass), which also confirms the previous conclusion, and its growth and reproduction ability is extremely strong.
The study also found that Eichhornia crassipes can often grow on the water surface in a relatively closed river or pond, forming a dense "straw mat". Since then, the asexual reproduction ability of Eichhornia crassipes has been limited, and the speed of producing new plants has been greatly reduced. As an adaptation to the habitat conditions, the extremely narrow stems of Eichhornia crassipes began to elongate, and more and more parts produced new plants, and the asexual reproduction of Eichhornia crassipes showed a cascade increase. At the same time, the petiole of Eichhornia crassipes grows upright and stretches constantly, and some even can't see the swollen sphere, and the leaves are increasing. In seriously polluted water, the height of leaves of Eichhornia crassipes can reach 80 ~ 90 cm, and some even reach 100 cm.
After the frost period came in June+February, 5438, the leaf tissue of Eichhornia crassipes was damaged and destroyed by freezing injury, and the edge began to wither. Then, affected by the coming cold current, the whole blade began to dry up and die. Many leaves can cover the whole plant after death and float on the water. Because the stems and leaves of Eichhornia crassipes have a colloidal layer, water can't invade immediately, so the withered Eichhornia crassipes plants float on the water for a long time, which can last until March in densely populated places. Later, due to the fluctuation of water flow, some Eichhornia crassipes plants were torn and drifted to other places, but most of them sank to the bottom of the water, and a few Eichhornia crassipes ran aground among the emergent plants on the shore.
2.2 Overwintering capacity and reproductive strategy of Eichhornia crassipes in Shanghai
Table 1 population distribution of Eichhornia crassipes in different rivers in Shanghai
Table 1 population status of water hyacinth in different rivers in Shanghai
River width/meter caused by human disturbance; Riverbank consistency; The distribution of Eichhornia crassipes accounts for% of the river width.
Municipal level? 100 sailed in an orderly way, and the solidified cement of 1~5 drifted back and forth.
The navigation at the county level 30 is orderly, and the cement solidification 7~8 is affected by the gate opening, so the navigation can be temporarily suspended.
10 There are 20 short aquatic plants on the shore of township-level agricultural fishing boats, with drifts in the middle, which can be clustered and distributed in patches.
Net cement curing 0
Village level 2~5 irrigation and drainage, irregular fishing boats, 80~ 100 water chestnut and reed evenly distributed, occupying the river space as much as possible.
Cleanliness, cement curing or manual cleaning; 0 has no source, but it was salvaged.
Drainage and irrigation in villages and ponds are irregular, with 60~90 evenly distributed, occupying as much river space as possible.
Neat, hand-cleaned, without source, or salvaged.
Anatomical studies showed that although the leaves of Eichhornia crassipes died in winter, axillary buds remained in the stems with shortened bases. These tiny axillary buds are surrounded by dead petioles, and the whole plant is covered with dead leaves, which plays a role in keeping out the cold to some extent. In addition, Eichhornia crassipes will secrete some gelatinous mucus wrapped around the axillary buds, so some axillary buds can successfully overwinter without being frozen on the water for a long time, and when the temperature gets warmer in spring, the axillary buds can germinate into new plants. So it will become the provenance of Eichhornia crassipes in the coming year.
The investigation results show that the propagation mode of Eichhornia crassipes in Shanghai is mainly asexual propagation. From March to August, Eichhornia crassipes plants produced new plants through asexual reproduction, and the population continued to expand. From September to June, when the asexual propagation of Eichhornia crassipes reached the maximum, about 1% ~ 8% of Eichhornia crassipes plants began to reproduce sexually due to the limitation of space, nutrition and other resources. After the flowering of Eichhornia crassipes, the inflorescence gradually withered, the plant growth stopped, the cloning and reproduction stopped, and the whole plant began to decline until it died. But most plants do not have sexual reproduction. Eichhornia crassipes seeds are small, but many, and each inflorescence can bear 300 seeds. When mature, the seeds are scattered on the water surface and can drift with the water. The number of sexual reproduction of Eichhornia crassipes is directly related to growth space and water nutrition. On the wide water surface (> 5 m) and rivers with good fluidity, Eichhornia crassipes mainly reproduce asexually, and few Eichhornia crassipes plants bloom, accounting for less than 1% of the total plants, and most of them cannot produce mature seeds. Only in a relatively closed water body, such as a static small pond (area10 ~1000 m2), Eichhornia crassipes has more flowering plants (3% ~ 8%). Although a few plants can form seeds, whether the seeds can germinate and produce new plants remains to be further studied.
2.3 Relationship between the outbreak of Eichhornia crassipes population and other aquatic plants
Eichhornia crassipes is a floating aquatic plant, and its life in water is floating. The major rivers in Shanghai, such as Huangpu River and Suzhou River, are all affected by tides, with high tides and low tides every day. At the same time, due to the network distribution of rivers in Shanghai, the tributaries at all levels also have reciprocating movements of water. However, most Eichhornia crassipes in small and medium-sized rivers don't go with the flow, and most of them grow fixedly, so there must be some specific mechanism to keep Eichhornia crassipes in a fixed state.
It is found that the fixation mechanism of Eichhornia crassipes in water is closely related to the uniformity of the river and the distribution of other aquatic plants in the river (table 1). Most municipal rivers with wide rivers and busy shipping have deep water bodies, few aquatic plants and neat banks, making it difficult for Eichhornia crassipes to settle down. Therefore, most Eichhornia crassipes in municipal rivers drift with the current, and its distribution is closely related to water flow, monsoon and tide. On the contrary, in village-level rivers, the banks are often irregular, and there are many aquatic plants near the shore. Such as Alternanthera philoxeroides and Zizania latifolia (Griseb). ) Tulz. EXSTAPH] (common name Zizania latifolia), Phragmites australis, Du Ying (Eleusine India (L.) Gaertn), Euphorbia fischeriana. According to the survey, Alternanthera philoxeroides is the season of outbreak and growth from March to June every year, and it is distributed on both sides of almost all rivers in Shanghai. At this time, Eichhornia crassipes has just germinated, and its plants are short and few, which is hidden in the Alternanthera philoxeroides population, thus achieving the purpose of preserving the provenance. Since July, Eichhornia crassipes began to grow explosively. At the same time, Alternanthera philoxeroides gradually declined and died, which provided a broad niche for Eichhornia crassipes. However, during August-165438+10, other aquatic plants on both sides of the river, such as reed, water bamboo, spurge, etc. Their appearance just fills the gap that Eichhornia crassipes lacks fixed obstacles after the death of Alternanthera philoxeroides, which likes drought, so that Eichhornia crassipes can grow steadily, among which Zizania latifolia and reed play the greatest role. Therefore, the outbreak of water hyacinth in Shanghai is closely related to other aquatic plants.
3 Shanghai Eichhornia crassipes harm control measures
3. 1 Strengthen the control of other aquatic plants and reduce the fixed places of Eichhornia crassipes.
At present, the government pays more attention to the existence of Eichhornia crassipes in municipal and district rivers, but not enough attention to rural rivers, which are often the source of Eichhornia crassipes. In Shanghai, especially near Dianshan Lake Basin, there are many rivers and ditches, which bring great difficulties to salvage and management. However, these water bodies are closely related to the lives of local residents. Therefore, it is necessary to mobilize the general public to unify their understanding of the harm of Eichhornia crassipes, and * * * Qi Xin will work together to control Eichhornia crassipes.
In view of the fixation effect of other aquatic plants in Shanghai river on the outbreak of Eichhornia crassipes, this study proposes to thoroughly clean the banks of small and medium-sized rivers from June to July every year, with the goal of removing weeds on both banks of the river, so that Eichhornia crassipes will lose its fixation place, thus naturally removing the external environment of the malignant outbreak of Eichhornia crassipes. In 2002, more than 95% of the rivers in Songjiang District of Shanghai were cleaned up, and the number of Eichhornia crassipes in the rivers decreased obviously. The removal rate of Eichhornia crassipes in several rivers reached 65,438 0.000%, and the treatment effect was remarkable.
3.2 Choose fishing in winter and spring to reduce provenance.
The prevention and management of Eichhornia crassipes should follow its invasion, distribution and life history, so as to get twice the result with half the effort. According to the environmental conditions in Shanghai, the outbreak season of Eichhornia crassipes is concentrated in August-65438+February every year. At present, most of the salvage control measures adopted by various districts and counties choose to be carried out at this time. However, because Eichhornia crassipes grows and reproduces very fast after entering the explosive growth season, one Eichhornia crassipes can reproduce about 250,000 new plants within 90 days [8]. According to the current conditions in Shanghai, the speed of manual salvage is 2 ~ 3 t/(d? Man), the salvage amount of mechanized salvage ship is only 100 t/(d? Ship). From August to June, 2002, all districts and counties in Shanghai invested19,388,500 yuan, with more than 7,500 people. A total of 888,904.2 Eichhornia crassipes were salvaged, but the total amount of Eichhornia crassipes in major rivers did not change much. Because the population of Eichhornia crassipes broke out quickly at this time and the time was tight, the number of its growth and reproduction greatly exceeded the speed of artificial salvage. Generally speaking, the salvage cost is high, the efficiency is low and the effect is poor. Therefore, how to take effective, reasonable and economical salvage measures is the key problem of water hyacinth control.
Combined with the continuous monitoring results of natural population of Eichhornia crassipes in this study, it was found that most Eichhornia crassipes withered and died after frost because of the temperature drop in winter. However, some axillary buds of Eichhornia crassipes are covered with dead leaves of themselves or other plants such as Zizania latifolia to survive the winter. In the next spring, these axillary buds that had successfully wintered began to germinate, but the growth was very small, basically maintaining the state of single plant. According to the life history characteristics of Eichhornia crassipes, the project team proposed that the best fishing season is from winter to spring every year, that is, 65438+ February-June the following year, with the goal of removing the provenance.
3.3 Using biological control methods to control the harm of Eichhornia crassipes.
At present, the research report shows that the natural enemies of Eichhornia crassipes are: Eichhornia crassipes, Eichhornia crassipes and Tetranychus urticae [12]. Compared with chemical and artificial control, although the speed of biological control is slow, once the natural enemy insects establish a population in the wild, they will get a good control effect, and after establishing a dynamic balance of mutual inhibition between natural enemies and Eichhornia crassipes, the control will have a strong durability. However, using artificial and chemical control, Eichhornia crassipes population is easy to break out again, so its persistence is poor. Compared with manual and chemical control year after year, the cost of biological control is very low. Another advantage of using natural enemy insects to control Eichhornia crassipes is that natural enemies can control Eichhornia crassipes not only in the release area, but also in other areas. At present, the natural enemy of successfully introduced Eichhornia crassipes in China is Eichhornia neoformans [13 ~ 15].
3.4 Strengthen cooperation with neighboring provinces and cities.
Shanghai is located at the mouth of the Yangtze River, and many rivers in Taihu Lake Basin flow through Shanghai. Therefore, Eichhornia crassipes in Shanghai not only comes from rivers in suburban counties, but also from Jiangsu and Zhejiang provinces in the upper reaches of Huangpu River and Suzhou River. Every year, a large number of foreign Eichhornia crassipes flow into Shanghai, which leads to the most serious invasion of Eichhornia crassipes in the counties of Jiangsu and Zhejiang provinces. At the same time, Eichhornia crassipes flowing into Shanghai was stranded due to sluice management and river embankment reinforcement. At present, although the salvage management of Eichhornia crassipes has been carried out in Shanghai, it has not been carried out in Jiangsu and Zhejiang provinces due to lack of funds and other reasons. As the downstream of Shanghai, even if the Eichhornia crassipes in urban rivers are cleaned up, it is difficult to completely solve the problem of Eichhornia crassipes drifting from the upstream every year. Therefore, only by strengthening cooperation with Jiangsu and Zhejiang provinces can the harm of Eichhornia crassipes in Shanghai be fundamentally solved. Change the mode of "terminal governance" in Shanghai.
4 conclusion
The life history characteristics of Eichhornia crassipes in Shanghai show that Eichhornia crassipes can successfully overwinter in Shanghai. The propagation mode of Eichhornia crassipes in Shanghai is mainly asexual propagation. The outbreak of Eichhornia crassipes population is closely related to eutrophication of water body. At the same time, the wide distribution of other aquatic plants in the river is also an important reason for the fixation, growth and outbreak of Eichhornia crassipes. Only by strengthening comprehensive management can the harm of Eichhornia crassipes be fundamentally controlled.