Bac Bo Delta Estuarine Area by Vu Trung Tang Nguyen Xuan Huan Vu Ngoc Thanh Faculty of Biology Vietnam National University Hanoi

The Bac Bo Delta estuarine area, an economically important region, extends from the South of Cape Do Son to Thanh Hoa Province. Studies in this region, by a group of scientists from Vietnam National University Hanoi headed by Professor Vu Trung Tang, have continued since 1982. What follows are the results of this research program.

1. Natural Conditions

1.1 Geography and Geology

The Bac Bo estuarine area (see Figures 1 and 2) consists of many mouths of the Red and Thai Binh River systems (including the Van Uc, Thai Binh, Diem Dien, Tra Ly, Ba Lat, Ninh Co and Day Rivers), extending along the Bac Bo Delta edge for about 120 km and lying on Cainozoic bedrock, the so called 'Hanoi Valley,' over which alluvium of different origins was deposited over time. Since the late Holocene when the then existing delta was inundated by a rise in the sea level, deposition has continued to build the modern delta seaward.The ancient coast line and estuaries were also formed during the late Holocene ( ). At present, the delta continues growing seawards at a rate ranging from 15m to 100m per annum.

1.2 Climatic

This area is situated within the typically non-tropical part of Vietnam. The climate varies from a cold and dry winter with occasionally light drizzle to a warm and humid summer with heavy showers. The mean Summer temperature is 27-29deg. C and rainfall is 1500-1800 mm (which is up to 80% of total annual rainfall). In the Winter the monthly mean temperature drops to 16.3-20.9deg.C and the rainfall decreases to 115-117 mm (in November) and 26-28 mm (in January, February). Total annual radiation varies between 95-105 Kcal/cm² and the total annual thermal budget is about 8000-8500deg.C. This area withstands storms and typhoons which originate in the West Pacific and East Vietnam Sea (South China Sea) and cause considerable damage.

1.3 Hydrological

The estuarine area receives fresh water and sediment from the Red River system and from a small part of the Thai Binh system.

The Red River system is the largest in North Vietnam with main courses such as the Red, Da and Lo rivers comprising a total catchment area of 143,700 km². The mean annual discharge of the Red River system at Son Tay is 114 km³, equivalent to an average flow of 3,640 m³/sec. Some 74.4% of the river discharge occurs during the rainy season from June to October. At present peak flow during the rainy season, the river discharges 30,000m³/sec, lowering the salinity of the estuary. A tongue of fresh water may extend from the Red River mouth into the sea to a distance of 30 km. In the dry season, from November to May, the River discharge decreases to 430 m³/sec. As a result, the 1 0/000/00 isohaline penetrates 20-22 km into tributaries and the 30-31 0/00 isohalines of surface water approach the coastline with 1.7-2.5 0/00/km gradients. Salinity differences between surface water and the bottom are marked.

The mixing of fresh and marine waters also is accelerated by tidal action. The tidal regime in this area is irregularly diurnal (2 < K < 4), but is more regularly diurnal upstream (K > 10). The maximum tidal range along the coast of the Delta is approximately 4 m. The tidal transfer speed in the river mouth approaches 95-150 cm/sec. and the tidal influence extend 150-180 km from the river mouths (Nguyen Ngoc Thuy, 1982).

Water temperature varies between 27-30deg. C in Summer to 24-26deg. C in Winter. The difference between water temperature between the surface and bottom is about 1deg. C. The pH values of estuarine water reach 8.0-8.5; the pH of surface water is higher than that near the bottom by about 0.05-0.10.

The river system yearly carries a very large silt load, amounting to 115 million tons. The average silt content of the water at Son Tay is 990 g/m³ with a maximum of 6,950 g/m³. The total nutrient source carried by the river has been estimated at 21.4 million tons, of which humus concentration is 7.7-24.0 g/m³, azote = 0.9-1.8 g/m³ , P2O5 = 0.48-1.28; K2O = 1.6-2.3 g/m³ (Nguyen Viet Pho, 1985).

In estuarine water, the mean phosphate concentration varies between 2.84 - 4.20 g/m³, declining at the end of the dry season and increasing after the flood, or wet season. Bottom water is richer in phosphate than surface water (0.06-1.10 g/m³).

2. Biological Communities

2.1 Phytobenthos Community

The natural mangrove vegetation of the Bac Bo Delta was destroyed long ago and has been replaced by a much simpler plant community dominated by Sonneratia caeseolaris, Kandelia candal, Avicennia sp., Aegiceras corniculatum, Bruguiera gymnorhiza, etc. The potential surface of the tidal mud flat for planting mangroves is estimated to be 6,000-7,000 ha.

At the back of the marine dykes over 1000 ha of Cyperus fields, with a water salinity of 4-5 0/00. Rice and other vegetation is planted on land that is far from the influence of marine water, behind the Cyperus fields. Average rice production is approximately 5 ton/ha/year. In addition, on the sand dunes of the beach and the estuarine sand islands there are wild plant species such as Casuaria equisitifolia, Pandanus tonkinensis, Ipomoea pascaprae, etc.

In aquaculture ponds and swamps appears a simple vegetation including Phragmites communis, Cyperus malaccensis, Elocharnis dulsis and demersal algae such as Rhizosolenia, Chaetomorpha, Cladophora, Enteromorpha, Oedogonium.

2.2 Phytoplankton Community

The phytoplankton community has a diverse species composition including 183 species (Table 1) dominated by Diatomae, which make up 86.1% of total species. Some genera with numerous species are Chaetoceros (28 species), Coscinodiscus (18), Rhizosolenia (14), Ceratium (9), Navicula (8), and Melosira (6).Skeletonema costatum and some representatives of Pennateae are often abundant in the upper regions of the estuary where salinity is below 15 0/00.

In terms of the annual cycle, phytoplankton development changes between two seasons: declining rapidly during flood peak months (July - August) and increasing in the dry season. In July-August, the phytoplankton density ranges from 800 to 362,000 cells/m³ and the mean biomass is 130 g/m³. Later (October - November) phytoplankton explodes in number, reaching a mean density of 973,000 cells/m³ and a mean biomass of 470 g/m³. At the end of the dry season, the phytoplankton development decreases slightly, as a function of a decline in nutrients in the estuary, to a mean density of 368,000 cells/m³ and biomass of 309 g/m³.

Frequently, phytoplankton develops intensively near the Red River mouth in waters shallower than 20 m where it is controlled by tidal action. Phytoplankton density and biomass increase when the tide is high and decrease when the tide falls, reaching extremes at the times of highest and lowest tides (Table 2).

2.3 Zooplankton community

The Bac Bo estuarine area supports a rich and diverse zooplankton. A total of 185 species have been recorded, including Copepoda (107 species), Cladeocera (14), Siphonophora (8),Chaetognatha (8), Amphipoda (6), Tunicata (6), Protozoa (5), Ostracoda (4), Pteropoda - Heteropoda (3), Rotatoria (2), Cumacea (2), Sergestinae (1), Euphausidae (1) and Nauplius (18). Like the phytoplankton, the zooplankton is divided into three ecological groups, (a) freshwater, (b) estuarine, (c) euryhaline-marine. Fresh water fauna often appears in the upper parts of the estuary and is abundant in number, especially the wet season and at times of neap tide. Contrastingly, euryhaline-marine fauna occurs near the end of the estuary, is richest near Spring tide and in the dry season.

Zooplankton density and biomass vary between 6,130 - 15,500 individuals/m³ and 240 - 370 g/m³ respectively. Lowest values are in flood months, but high values are in the dry season (Khuc Ngoc Cam, 1975; Nguyen Van Khoi et al, 1980), especially at times of highest tide and during the period from midnight to 5-6:00 am.

In the Red River estuary, zooplankton density and biomass increase towards the sea, reaching maximum values at a salinity range of 30-31 0/00 then, far from the river mouth, decreasing again as water approaches the salinity of neritic water. This is similar to zooplankton development in the Mekong River system estuaries (Vu Trung Tang et al., 1981).

2.4 Zoobenthos community

Zoobenthic community in the tidal mud flats includes 130 species, representing some principal groups such as Polychaeta (34 species), Gastropoda (16), Bivalvia (23), Macrura (17) Brachyura (38). Many of these species are economically important, for example Ostrea, Meretrix, Aloides, Macta, Netica, Sanguillaria, Penaeus, Metapenaeus, Palaemon, Scylla, Portunus, etc.

The biomass of zoobenthic animals used as food by other species varies over a wide range from 4 - 96 g/m³ in the dry season and 5.9 - 11.5 g/m³ in the wet season (Dang Ngoc Thanh et al., 1991).

2.5 Fish fauna

The estuary supports a diverse ichthyofauna. A total of 233 species have been identified, belonging to 71 families and 18 fish orders. The families with numerous species are Carangidae (11 species), Cynoglossidae (14), Gobiidae (13), Leiognathidae (11), Sciaenidae (11), Teterodontidae (11), Clupeidae (9), Engraulidae (9) and Mugilidae (6).

Some fresh water fish of the families Cyprinidae and Bagridae often occur in water with a salinity below 10-12 0/00 in the upper regions of the estuary. The representatives of some Priacanthidae, Pomacentridae, Chaetodontidae are frequently found near coral reefs and some offshore fish also penetrate estuaries for feeding, for example, Elasmobranchia, Exocoetus, Sphyraena, Formio, Stromatoidae, Scombridae, but most of these are juveniles and so are of small size.

Despite the mixed origin, estuarine fish fauna of the Bac Bo Delta are related to the Tonkin Gulf fish fauna. Most representatives originated in tropical seas and became adapted to high salinity fluctuations occurring in the estuary (Vu Trung Tang et al., 1987).

The fish fauna of this area may be divided into four ecological groups (a) freshwater, (b) euryhaline-marine (c) true estuarine and (d) regularly anadromous migrants such as Clupando theissa and Hilsa reveesii.

3. Natural resources

Natural resources of Bac Bo Delta estuarine area are diverse, including land, plant and animal communities, chemical substances, heavy minerals, energy sources (natural gas, wind, tide), the capacity for navigation and potential for tourism development.

3.1 Land resources and land use

Lying on the deposition fond of Bac Bo Delta, the land of the estuary typically expands toward the sea at a rate varying from 10 to 100 m/year. The advancing delta edge has meant that the construction of marine dyke systems moved progressively forward over a long period to allow control of inundation of alluvium. The first Ninh Binh marine dyke line was build in the 5th Century AD. The Hong Duc dyke line was constructed in 1471. Between 1828-30, in order to create more virgin soil, Nguyen Cong Tru built the An River and Con Tien dyke systems to establish two new districts: Kim Son (in Ninh Binh Province) and Tien Hai (in Thai Binh Province). Since then, the marine dyke systems have continued toward the sea, for example the dyke lines built in 1899, 1927, 1934, 1950, 1960 and 1982 along the Southeast edge of the Bac Bo Delta and others built in 1892-1900, 1955-1956, 1962-1963, 1974 on the left of the Red River. Behind each successive dyke system new land was available for cultivation and villages were built (Figures 1 and 2).

Estuarine land is formed from land of different origins: riverine, marine and riverine-marine. The sediments carried by the Red River system have created fertile cultivated land. Estuarine land is also divided into three main groups: (a) alusulphate (alusulphate and salt-alusulphate), (b) sand and (c) salt.

Land reclaimed for planting rice and other crops is improved by traditional measures to "clean sourness" and "wash away saltiness". Improved land can be cultivated with two different crops, with two rice crops and one other crop alternately in a year with a mean production equivalent to 5-6 rice tons/year. However Cyperus growing on sour land can produce 12-18 dry-weight tons/year.

3.2 Biological resources

3.2.1 Mangroves. Recently, estuarine mangroves have become concentrated in scanty groves on tidal mudflats, straddling river mouths and near estuarine islands. These mangroves are being lost rapidly through the construction of marine dykes (for cultivation) or aquacultural ponds or by extraction for firewood and timber. Mangroves in this region are critically threatened.

3.2.2 Fish exploitation. At present the estuary and shallow waters below 30 metres are the region's main fishing grounds. The catch in these areas constitutes 80-90% of total landing products. The Bac Bo Delta estuary provides the local economy a production of about 16,000-18,000 tons, of which the shrimp and prawn catch is about 30%.

The fish exploited in the estuary are mainly the smaller species, most of them juveniles. Occasionally, as much as 80% of the catch is made up of juveniles. The most important commercial fish are representatives of Clueid, Engraulid, mullet, etc. Their population structure is simple, being composed of only four or five age groups. Most of these species are omnivorous detritus feeders and so form a short food chain (Vu Trung Tang, et al., 1987).

Shrimp and prawns in this area are diverse in species composition, dominated by Penaeids, and form the so called 'Cat Ba - Ba Lat shrimp ground' in front of the Red River mouth at a depth between 5 and 30 metres (mainly 5-15 metres).

The shrimp season occurs from January to October. This shrimp ground supports an annual seed stock of some 8 billion larvae and young shrimp for the brackish rearing ponds and channels of the estuary.

3.2.3 Aquaculture. The Bac Bo estuary offers good conditions for aquaculture as follows:

- Water available for aquaculture development is large, estimated at over 1,000 ha.

- Natural food sources are abundant, natural seed stock, particularly shrimp, is diverse in species composition.

- High tide level assists in the supply and drainage of water and so the reception of natural food and seed from the sea and to the sanitation of the rearing ponds.

Aquaculture remains somewhat inefficient. The effective rearing surface makes up only 20-40% of the rearing pond. Production is low, attaining only 150-250 kg/ha (50-60 kg/ha shrimp). Some smaller, well-managed ponds can produce as much as 500-800 kg/ha.

3.3 Salt and heavy minerals

In spite of the low salinity of estuarine water, the production of table salt by traditional measures in estuarine waters has been developed. Each year the salt fields of this area have provided North Vietnam a table salt production of 20,000 - 30,000 tons.

Heavy minerals also are important natural resources found on the mudflats and estuarine islands. Sites may contain 20-30 minerals, dominated by ilmenite (FeTiO3), rutile (TiO2) zircon [Zr(SiO4)] monazite [Ce(PO4)], iron sands, and gem stones. On islands in front of Tra Ly River mouth, black sands contain 30 - 40% ilmenite, 10 - 12% zircon, 1 - 1.5% monazite, 20 - 30% magnetite and 35 - 40% quartz sand of an exploitable standard for local industry.

3.4 Energy sources

Energy in the estuary is exploited through wild and tidal action, and from natural gas and oil. Wind energy in the Bac Bo Delta estuarine area has long been used to power sail boats and the water pumps used to irrigate salt fields.

In this region the wind is constant. The frequency of windless days per year is only 11% in Summer and 26% in Winter. The proportion of days with a wind speed averaging 2 - 5 m/sec is 46 - 69%, and days averaging a wind speed of 6 - 10 m/sec is 19 - 37%. This important potential has not been developed as yet.

Recently, oil exploration has increased in the Bac Bo delta estuary and associated continental shelf zones. Natural gas with a reserve of 2 to 3 billion tons, found to the North of the Red River mouth has operated a 34,000kW thermo-electric power plant since 1984.

4. Environmental impact of human activities

The population density in the Bac Bo delta estuarine area is 800 - 1,600 individuals/km². This human density has a significant environmental effect, altering estuarine ecology day by day:

- Rapid expansion of newly drained land by the construction of marine dykes. Therefore the mangrove zones have narrowed and the horizontal profile and the beach have become more sloping. These effects change the dynamic equilibrium of processes along the coast.

- Over-fishing: the estuarine area is an intensive fishing ground. The number of fishing boats is quite high, but the catch per unit is decreasing. Juvenile and females with eggs comprise the majority of the catch. Some anadromous migrants (Clupeid fish) have declined so much that it has not been possible to rehabilitate their population.

- Construction of dams, reservoirs and hydro-electric stations on the tributaries of the Red River system such as the Thac Ba and Da reservoirs, has not only caused the loss of breeding grounds of many freshwater and marine fish, but also has decreased sediment load by about 60%. This sediment is retained by the reservoir. Dams may also change deposition-erosion processes along the coastline.

- Pollution: Studies of pollution in the estuarine area are inadequate, but domestic water from densely populated ares, untreated sewage from industry and waste material from boats and ships in harbour all cause pollution of the estuarine environment.

The above mentioned issues show that the natural resources and environment of the Bac Bo Delta estuarine area face serious threat from human interaction.

The rational exploitation of natural resources, the preservation of bio-diversity and the protection of the environment of this area in particular, and all coastal zones of Vietnam in general, have become urgent issues requiring increased scientific research and international cooperation.

5. References

Khuc Ngoc Cam. 1975. Plankton and seed stock of the estuaries of Thai Binh Province. Report of the Aquatic Department of Thai Binh Province (in Vietnamese).

Nguyen Van Khoi et al. 1980. Zooplankton of the Ba Lat, Ninh Co and Day estuaries of Ha Nam Ninh Province. Bulletin of the Marine Research Institute, II-1, 111-132 (in Vietnamese).

Nguyen Viet Pho. 1984. The riverine currents of Vietnam. Hanoi: Science and Technic Publishing House 243 pp. (in Vietnamese).

Vu Trung Tang et al. 1981. The aquatic resources of the estuarine area of the Mekong River delta. Report of the 2nd National Conference on Marine Science p. 85 (in Vietnamese).

Vu Trung Tang et al. 1987. Structure of the fish fauna of the coastal-estuary area of Thai Binh Province. Bulletin of the Research Work of Universities (Agriculture-Biology-Medicine) 100-114 (in Vietnamese).

Nguyen Ngoc Thuy. 1982. The tide on the Vietnam Sea. Hanoi: Science and Technic Publishing House 261 pp. (in Vietnamese).

August, 1993.

6. Tables

Table 1.
Structure of Phytoplankton of estuarine area of the Bac Bo Delta.

Algae groups	in brackish ponds				in the estuary
	genera		species		genera		species
	abs. no.	%	abs. no.	%	abs. no.	%	abs. no.	%
Bacillariophyta:
- Centriceae	16	32	48	38.7	25	42.4	100	55.6
- Pennateae	21	42	52	41.9	21	35.5	55	30.5
Pyrrophyta	2	4	9	7.3	2	3.4	20	5.6
Cyanophyta	5	10	8	6.4	5	8.5	8	4.4
Chlorophyta	6	12	7	5.7	6	10.1	7	3.9
Total	50	100	124	100	59	100	180	100

Table 2.
Fluctuation of Phytoplankton density in relation to tidal action (cell number per cubic metre).

status of tide	dry season	rainy season
rising tide	71,090	192,175
highest tide	137,575	194,435
lowest tide	27,510	46,690
falling tide	93,430	147,870

7. Figures

Figure 1. Seheme of Bac Bo delta South - East edge development and dyke systems (after Gourou, 1936 with new dyke lines added).

Figure 2a. Seheme of Bac Bo delta edge development on the left of the Red River and the dyke systems (after Vu Trung Tang, Nguyen Hoan et al., 1985). See legend, Table 2 b.

	The coastline of the early 900s.
	The coastline before the period of the ploughing of virgin soil at the time of Nguyen Cong Tru (1825-1830).
	The coastline after the period of the ploughing of virgin soil at the time of Nguyen Cong Tru (1825-1830).
	The 1892-1900 coastline.
	The 1955-1956 coastline.
	The 1962-1963 coastline.
	The coastline after 1974.
	The area of virgin soil ploughed at the time of Nguyen Cong Tru (1828-1830).
Figure 2b. Seheme of Bac Bo delta edge development on the left of the Red River and the dyke systems. Legend.