（本文完成于1983年。）

　　Shen Tsan-Hsin

　　1　INTRODUCTION

　　A river estuary is a unique geographical unit. Located at the junction of a river and the sea; it is affected not only by the hydrology of the fresh water but also by the sea water. It has particular hydrological and developmental processes as well as agricultural and ecological structures.

　　Zhongshan University is situated in the Pearl River Delta. The faculty members have carried out numerous studies on the estuary and deltaic area in the last 40 years. However, it is only recently that more comprehensive and systematic research on the ecological balance of the area has been undertaken.

　　The author has been involved in the study of the hydrology and physical geography of the area since 1950s. Last May, the author led an investigation team in the study of the ecological balance of the Pearl River Delta. Together with experts from other fields, an in-depth investigation was made and a detailed report has also been submitted to the Provincial Government and other related institutes for consideration.

　　This paper describes the major findings of that study, with particular emphasis on the present ecological conditions, and strategies for conserving the deltaic ecosystem.

　　2　BACKGROUND OF THE PEARL RIVER DELTA

　　2.1　The Physical Environment

　　The Pearl River Delta is an important agricultural and fishing area of Guangdong Province with very favourable hydro-thermal conditions for these activities. The delta is 10,000 sq. km. in extent and covers an area between Sanshui, Shilong and Xinhui (Fig. 1). These are 8 million mu (or 1.3 million acres; a mu is one-sixth of an acre) of cropland and a population of 8 million. The main agricultural products are rice, sugar cane, silk, fruit and sea food.

　　The Pearl River discharges into the sea through eight outlets, which are, from east to west, Hu Men, Jiao Men, Hongqili, Heng Men, Jiti Men, Hutiao Men, Modao Men and Ya Men. In times of flood, the distribution of flow among these outlets is as follows:

　　The annual flow of Pearl River is 308.26 billion m3 of which 80% occurs during the high-flood season. Among the three major tributaries of Pearl River (Bei Jiang, Xi Jiang and Dong Jiang), Xi Jiang contributes the largest potion (80% of total) of flow; whilst Dong Jiang the least .Each year 85.5 million tonnes of sediments are brought down from the upper course of the Pearl River with the majority (90%) from Xi Jiang. These sediments are rich in organic substances and are valuable in improving the fertility of the croplands.

　　The underground water of the deltaic plain is also rich in ammonia-nitrogenous compound derived from the disintegration of the remains of plants and animals in the deltaic sediments and from the organic substances (600,000 tonnes per annum) brought down by the river. After dilution, this underground water can be used as irrigation water as well as some kind of fertilizer. Field experiments conducted in Shunde showed that the annual rice production could be increased by 118 catties per mu if irrigated with 25-40m3 of this “manure water”.

　　Tides in the estuary follow an irregular half-day cycle with a tidal range of 1 to 2 m. This facilitates a free irrigation/drainage system in the reclaimed fields. During high tides, sea water surges up the river channel and mixes with the fresh water from the upper course of the river. The sea-fresh water mixing zone is highly productive in sea-food. Altogether, there are more than 70 varieties of fish and other sea-food such as shrimps, crabs and oysters. 

　　The area is endowed with a very mild climate, with an average annual temperature of 21°C and an annual precipitation of about 1,600 mm. Typhoons are the main natural hazard in the area, during which strong gusty winds (12 on the Beaufort scale), heavy rain (up to 850 mm/day) and exceptionally high tides (5m) can cause damage.

　　On the whole, the Pearl River Estuary possesses the very favourable environmental conditions. If all the factors under this system are ecologically balanced, the potential for further development, especially in primary production, is very high.

　　2.2　Men as an agent in exploiting the natural environment

　　The delta has been under cultivation since the 3rd century B.C. Over the last 2,000 years, much experience has been gained, particularly in the reclamation of land for cultivation, in the formation of fish ponds, as well as in the construction of dams and “walled fields ”.

　　Cultivated lands have been reclaimed at the deltaic front for centuries. Reclamation works have to tie in with the stage of sediment deposition. When the fore-set beds have reached such a height that they begin to emerge during low tides, the farmers will start growing weeds. This practice will speed up the sedimentation process. After two to three years, this piece of newly formed land will be about -0.5 m in height. At this time, hardly any water will cover the land surface, except during high tides and reclamation can proceed. In various parts of Wan Qing Sha and Zhongshan (Fig. 1), a lot of relatively cheap, fertile, and well drained agricultural land has been reclaimed in this way.

　　As early as the 9th century A.D. fish ponds were also formed in the deltaic plains by dredging up soil from the low-lying areas. The excavated soil was then used to construct the pond dykes, on which bananas and other fruit trees were grown. It was only until the 16th century A.D. that mulberry trees were planted on the dykes. Since that time, mulberry growing, fish farming, and sericulture were integrated to form the intricate and efficient ecosystem-mulberry dyke-fish pond complex. At present, most of the mulberry dyke-fish pond complexes are found in Shunde County. The mulberry dyke-fish pond complex is a well-balanced ecosystem: mulberry leaves are used to feed the silkworms; the silkworm excreta are deposited in the fish ponds as fish food; the fish excreta and pond mud are used as manure for the mulberry trees. These three components in the system are closely related and each is essential and beneficial to the functioning of the others.

　　3　THE PRESENT STATUS OF THE DELTAIC ECOSYSTEM

　　3.1　Ecological Balance Problems

　　3.1.1　Reduction in seafood production

　　The Pearl River Delta and its estuary have long been one of the important production centres of fish and other marine products in China. The tidal forest and the littoral zone along the Pearl River Estuary provide a very good habitat for a great variety of fish, shrimps, crabs, oysters and clams. Unfortunately, the production of sea food has greatly dwindled. Zooplankton, the usual bait for fish, were previously commonly found in all distributaries in the delta, but are rarely seen today except in one or two localities near the estuary. Fish that were commonly caught in the first half of the 20th century, such as Decapterus sp., Coilia sp., Collichthys Lacida, Ilisha elongata, Scomberomorus sp., Eleutheronerna sp., Maerura ap., and Anguliia japonica are scarcely found now. Likewise, the production of oysters has also fallen drastically in recent years. Oysters have been bred in this area for over a thousand years .In the fifties, with the assistance from the government, the production of oysters increased tremendously and the oyster beds off Lingding Yang produced more than 100,000 tonnes a year .The production of oyster in the last few years is only a small fraction of the yield in the fifties. This is unfortunately also paralleled by the decrease in clam production, which is not only a delicacy but is also used as duck food. Consequently, the number of ducks has also diminished. Ducks are natural enemies to pests and insects of the paddy field. Thirty ducks raised in one mu of paddy field can help to keep pests and insects under control. Without the natural enemy, the farmers have to rely heavily on chemical pesticides and insecticides.

　　3.1.2　Imbalance of the agricultural ecosystem

　　The natural ecosystem of the delta operates in such a way that nutrients are constantly recycled. For example, water rich in organic content from the estuary is exchanged with waste water from the field; bed-load of the river and stalks of crops are returned to the fields as fertilizer; and ducks and frogs are raised to keep out pests and insects. These nutrient and material flows in the agricultural ecosystem have recently been disrupted by engineering projects designed to reclaim land for farming, which modify the natural irrigation-drainage system. Furthermore stalks of crops are now being used as raw material for industry, chemical fertilizer has to be employed to replenish soil fertility. Consequently, the soil is becoming less porous and claypans are being formed in the lower strata. As a result, the aeration processes in the soil are impeded and the number of beneficial micro-organisms reduced.

　　In the last two decades, the lack of adequate natural control on pests and insects has necessitated the extensive use of pesticides. These are highly toxic organo-chlorides and phosphates, to which the pests have unfortunately gradually built up resistance. Hence, the amount of pesticides applied is escalating, killing not only insects and pests but also frogs, shrimps and fish.

　　3.1.3　Water pollution

　　In some localities, because of extensive reclamation, industrial effluents discharged into the estuary cannot be efficiently drained and thoroughly flushed. During high tides, some of these wastes are even washed back to the fields and thus polluting the channels and making the water non-drinkable.

　　3.1.4　Siltation of navigation channels

　　Since the sixties, the amount of sediments brought down from the upper course of the river has increased appreciably. For example the amount of sediments as recorded at the Ma Kou hydrological station of Xi Jiang increased from 0.215 kg/m3 during 1960-1963 to 0.322 kg/m3 in the seventies. It is estimated that the total amount of sediments brought down by the Pearl River is about 61 million m3 per annum. As more and more dams are built ,part of these sediments (about 4 million m3) are restrained from reaching the sea and are deposited in the outlets as result of mechanical and chemical coagulation processes .

　　The rate of deposition has increased steadily. In the sixties, some places, Jiao Bei Sha beyond Modao Men, were extending at a rate of 60m/yr., but in the mid seventies, the rate was 80m/yr. Similarly, the delta bar off Jiti Men has extended by 44 m in the last years. These outlets are thus becoming shallower and shallower, and navigation is affected adversely. Some navigable outlets in the sixties, such as Er Men of San Zao Dao, are now completely silted up. 
3.2　Causes of Ecological Imbalance

　　3.2.1　Accelerated sedimentation

　　Under natural conditions, with the continuous deposition of alluvium, the river channel and the delta are gradually advancing, the rate of which is dependent on the amount of bed-load of the Pearl River and the ecological situation in the upper course.

　　Since the last century, various human activities, including deforestration, taking place in the upper course of the river have enhanced surface run-off production, soil erosion and deposition in the river basin. The tidal and flood regions are altered; and so are the living environment for aquatic life, as well as the drainage-irrigation system. To a minor extent, agricultural production and soil structure are also adversely affected.

　　3.2.2　Industrial development

　　The rapid development of industry in cities like Guangzhou, Jiangmen, Dongguan, Foshan and Zhongshan has also produced a tremendous amount of industrial wastes. These wastes, often discharged without proper treatment, are high in such heavy metals as lead, mercury, zinc, tin, copper and other pollutants like petroleum cyanides, r-666 and E-605, in amounts exceeding the International Safety Standards. Furthermore, untreated industrial effluents from paper manufacturing and brewery also cause oxygen depletion and eutrophication. Coupled with the fact that most of the fields are often contaminated by the highly toxic pesticides and insecticides, a lot of fish, shrimps, crabs, oysters, clams, frogs and even zooplankton are killed.

　　3.2.3　Over-sized fields

　　Flooding and salinization are the two major problems in the low-lying deltaic area. Because of the topography and the dense network of streams, floods are very common during rainy seasons. At the same time, tides also bring along a high salinity to the area.

　　In order to reduce the flooding and salinity hazards, small pieces of fields have been combined. In the last thirty years, more than two thousand such small fields have been joined to form two hundred and fifty bigger ones, which are further combined to become thirty-six large “walled fields”. The area stretching from Jiangmen to Xinhui (which covers 546 km2) has been set as an experimental plot for this purpose. 

　　However, without prior investigation on the resultant ecological changes, particularly that of the irrigation-drainage system, the erection of “walled fields” has caused a lot of problems. Firstly, because of their extensive size, it will not be possible to bring in organic matters freely. On the other hand, agricultural wastes are not adequately flushed, resulting in oxygen depletion. In some places, dissolved oxygen levels in streams are only 1.5 to 2.8 mg/1. Coupled with the increased application of highly toxic pesticides and insecticides, the pollution problem is thus steadily deteriorating. 

　　An over-sized “walled field” also induces a greater difference of flow rates within and outside the fields. For example, before the formation of the “walled field”, the flow rate in Jiang Men channel was fairly high (2m/s), and erosional process was predominant. The whole channel was navigable. However, after the construction project, the gate at Bei Jie has greatly restricted the water flow in the Jiang Men channel. Tremendous amount of sediments are deposited during floods, thus reducing the depth of the river channel and significantly hampering its navigability. At the same time, what happens outlets the fields is the reverse of what happens inside. The increased water flow outside the “walled field” has enhanced a much higher erosional regime. The Mu Zhou channel, for example, has been widened by 3-4 m and deepened by 2-3m in the last two decades. In time of high water, the banks are very prone to collapse, thus causing widespread disaster. Hence thorough studies should be carried out before the construction of any “walled fields” so that the optimum sizes of the fields could be ascertained.

　　3.2.4　Inappropriate reclamation

　　As the Pearl River Estuary is of the aggradation type, appropriate reclamation will not only increase the arable land, but can also help abate floods and droughts. However, some of the fields in the estuary had been reclaimed at too early a stage. As a result, a great proportion of the reclaimed field is till covered with water up to a depth of 4-5 m whilst it is impossible to carry out farming in these areas, the seashore ecological environment is also disrupted. Tidal forests are destroyed and together with the pollutants brought down from the river, valuable aquatic resources are also obliterated. For example, a reclamation plan at Ya Men was initially started with an intention to reclaim an area of 120,000 mu (or 20,000 acres) However, problems have already arisen when 30,000 mu of land were reclaimed. Consequently, the rest of the area had to be deserted and weeds have cropped up instead. As a result of such inappropriate reclamation works, more than 2,000 mu of oyster beds and tidal process is weakened, sedimentation is hastened. The usual habitat for aquatic organisms has therefore been upset and navigation is hampered.

　　3.2.5　Large scale engineering work

　　In Bai Teng, a large-scale water management engineering work has been constructed. The eastern bank measured 4,050 m in length and the western bank 1,670 m. This construction has changed the river course and water is now directed to Jiti Men for outlet. Though it offers certain improvements, particularly in the protection of the area from damages caused by 16 km silted up the Niwan outlet, the original Niwan Men has now become a lake of 32 km2. Water has to pass through Bai Teng Lake before it is finally discharged into the sea, thereby intensifying the drainage problem. Moreover, the salinity, tidal pattern and also the flow capacity of many channels in the lower course of the river have also been affected, rendering a less favourable habitat for most aquatic organisms. Because of the engineering work, fish that used to lay eggs in the upper course of the river are now unable to swim upstream. Some have died and the others have been forced to migrate to other places.

　　Since the sixties, studies on the restoration of the area were carried out and projects on rediversion of the river course were started. These have somewhat improved the hydrological conditions and have contributed to the restoration of the ecological balance.

　　3.2.6　Over-fishing

　　In the fifties, the production of sardines and pond-fish was more than 200,000 tonnes per year. However, with the introduction of the modern high-powered mechanized fishing boats, fishermen are tempted to over-fish and thereupon exhausting the precious fish resources.

　　4　SUGGESTIONS FOR RESTORING ECOLOGICAL BALANCE

　　Under the auspices of the Committee on Scientific Technology of Guangdong Province, a team of experts of different fields have undertaken a study on the following policies which should be followed:

　　(1) More research efforts should be put into the study of ways to restore the ecological balance. Factors like the natural physical conditions, ecological succession processes, energy cycles, community structure and the impact of human activities should all be taken into consideration.

　　(2) The “Environmental Protection Law” of China should be vigorously enforced. Environmental management agencies should be set up. Further research on conservation of the environment, such as the use of less toxic insecticides or pesticides, should be encouraged.

　　(3) Long-term environmental and land use plans should be formulated.

　　5　CONCLUSIONS

　　The Pearl River Delta is an important area of Guangdong Province. It has very favourable hydro-thermal conditions for farming and the soil is rich in organic matter. The shallow water along the estuary is a suitable habitat for aquatic organisms. With its hardworking and experienced labour force and efficient transportation system, the potential for further development in agriculture, fishery and even tourism is very promising. However, due to the rapid development of industry and agriculture in recent years, the natural ecosystem has been disrupted.

　　In order to conserve the environment and to formulate future development plans, it is essential to undertake an exhaustive ecological investigation that would study not only the estuary, but also the physical conditions and human activities of the upper course of Pearl River as well as the hydrological conditions of the surrounding area.

　　Human activities such as agricultural practices, fishing engineering works, industries and modes of transportation and local government policies will all affect, to a certain extent, the ecological balance of the area. Hence these human aspects should be included and studied thoroughly in any further study.