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Asian Journal of Water, Environment and Pollution, Vol. 4, No. 1, pp. 37-46. Biogeochemical Variability of Vietnamese Coastal Waters Influenced by Natural and Anthropogenic Processes Nguyen Tac An* and Phan Minh Thu Institute of Oceanography, 01 Cau Da, Nha Trang, Vietnam ! ngtacan@dng.vnn.vn Received January 15, 2006; revised and accepted November 5, 2006 Abstract: The paper focuses on an analysis of the main characteristics of Vietnamese coastal zones and their biogeochemical cycles. Spatial and temporal variability in the distribution of chlorophyll, primary production, carbon, nitrogen and phosphorus are discussed. Biogeochemistry depends on hydrodynamics, especially on the upwelling from the seas, and on anthropogenic processes from the land. Key words: Biogeochemical processes, coastal zone, Vietnam, chlorophyll, primary production, balance. Introduction More than 50% of a population of about 82 million live in Vietnam’s coastal provinces that cover 41.30% of the country’s total area (GDS, 2004). Their activities, especially those that are related to food production such as the development of aquaculture and fishing have brought enormous pressure on Vietnam’s coastal environment. For example, the accumulation of waste from aquaculture has caused soil degradation, pollution and eutrophication in water bodies in general and, red tide blooms in coastal waters. Agricultural run-off and inputs of domestic and industrial waste have further contributed to changes in the distribution and behaviour of elements and cycling of carbon and other nutrient elements in coastal waters. Thus, not just the socioeconomic but also the regulatory functions of the coastal waters are under threat. A better knowledge on the processes involved and their interactions with the overall structure and functioning of coastal ecosystems is a prerequisite for developing management measures towards the wise use and the protection of coastal marine * Corresponding Author resources. This paper gives a brief overview of the available information on the inputs from natural and landbased human activities to Vietnamese coastal waters and their influence on the biogeochemistry of coastal marine systems. The Main Characteristics of Vietnamese Coastal Zones Vietnamese Coastal Zones Vietnam has a coastline of a 3260 km in length that is very sinuous. As a result, many bays and lagoons have been formed along the coast. On an average, for every 20 km of coastline, an estuary or a bay is found. The landward boundary of Vietnam’s coastal zones from coastal zone management programmes is set by the limit beyond which there is no influence of tidal or marine storms. The seaward boundary is set by the Exclusive Economic Zone (EEZ) which is up to 200 nautical miles from the coast. These boundaries vary from country to country and are dependent on the specific local conditions including geographical features and the legislative framework (Table 1). 38 Nguyen Tac An and Phan Minh Thu Table 1: Examples of coastal area boundaries from coastal zone management programmes Country or State Landward boundary Seaward boundary Comment New Jersey/USA 30 m-30 km Tidal, bay and ocean state waters Rhode Island 200 feet from shoreward boundaries of coastal features + specified actions likely to damage coastal environments All land except state forest reserves Territorial sea (three miles) excluding fishery State Coastal Programme State Coastal Programme Hawaii Brunei Indonesia Malaysia The Philippines All land and water areas 1 km inland from MHWM and areas inundated by tides any time of the year Administrative and selected environmental units District boundaries State waters From MHWM to 200 m isobaths State Coastal Programme ASEAN/US CRMP 60 m isobaths ASEAN/US CRMP Up to 20 km off shore to include islets off Mersing 100 fathom isobaths ASEAN/US CRMP ADB Singapore Boundaries of coastal municipalities + inland municipalities with brackish-water aquaculture Inner regions on marine dependant systems or 1 km whichever is the greatest Entire island Thailand Costa Rica District boundaries 200 m from MHWM Outer reaches of fisheries resource systems which are associated with or influenced by the coast Territorial waters and offshore islands Shallow continental shelf n/a Sri Lanka 300 m from MHWM 2 km from MLWM Ecuador Variable line depending of issues in five special management areas. Limit which is not impacted by tidal or marine storms (about 100 km from coastline) n/a The Philippines Vietnam Up to 200 nautical miles, Economic Exclusive Zone ASEAN/US CRMP ASEAN/US CRMP ASEAN/US CRMP National Coastal Programme Law of the Marine and Terrestrial Zone 6043 URI CRMP. Coast Conservation Act 1981 URI CRMP Institute of Oceanography, Vietnam Source: modified from Lunkapis, 1998 Like many other coastal states, Vietnam’s coastal zones are also characterized by the presence of ecosystems such as mangroves, coral reefs, sea grass meadows, which provide goods and services to coastal communities. They are also the preferred sites for urbanization. Coastal zones thus play a major role in the national economy. There is however enormous competition for land and sea resources and space by various stake holders that result in conflicts and the degradation of coastal ecosystems. Further challenges to the coasts come from: (1) erosion and siltation in coastal areas, (2) increasing population growth (growth rate of population in 2004 is 1.44%, that means population is 1.25 million people higher than in 2003) and (3) the dangers of natural hazards. Meteorological and Hydrologic Settings Vietnam is located in the Indo-Chinese Peninsula and covers tropical monsoon climatic zone. In the north, annual rainfall is approximately 2,000 mm while temperatures remain relatively constant—on average between 25-35∞C. Most of the rainfall is between August and November. In the south, the annual rainfall is about 1560 mm with most occurring between May and November. The cities Ho Chi Minh in the south and Ha Noi in the north represent major economic centres within the coastal zone. Different from the north and the south, rainy season in central Vietnam is from August to January. The water balance of Vietnam given in Table 2 shows that about 50% of the rainfall is removed as surface and Biogeochemical Variability of Vietnamese Coastal Waters Influenced by Natural and Anthropogenic Processes coasts and have been increasing in frequency over the past few years. However, damage from them have fortunately remained less severe relative to other regions of SE Asia. But they do have an influence on marine dynamics. Table 2: The components of water balance in Vietnam Volume (km3) Components Rainfall water Runoff water Ground water Surface water Evaporation water 39 640 313 94 219 327 Source: CMESRC (2004) groundwater runoff. There are about 2345 rivers—all longer than 10 km—discharging varying quantities of freshwater, sediments, nutrients and metals to the sea, account for a density of rivers of 0.6 km/km2 (CMERSC, 2004, Table 3). Because of the short distances between the sources of most of these rivers and their receiving marine waters, their impact on coastal seas is, in most cases, relatively fast and because of the intense human activities in their drainage basins, very severe. These important factors need to be considered in studying the ecology and biogeochemistry of the coastal zone. Between July and November, Vietnam is hit by random, violent typhoons that develop off the coast in the East Sea. They typically hit the central and north Socioeconomic Setting About 25.4% of the population of Vietnam is urban and the rest rural. Because of the rather slow rate of urbanization, the growth of urban population has not been significant (GDS, 2004). GDP (Gross Domestic Products) in 2003 was 7.24% higher than in 2002, in which the growth rate of agriculture forestry—aquaculture, industry and the services sector contributed, 3.2%, 10.34%, and 6.57% respectively (GDS, 2004). Agriculture and aquaculture sectors remain however the major sectors affecting directly and indirectly the coastal waters (Minh, 2003). Between 2002 and 2003, there has been a slight reduction in the area where agriculture is practiced because of the conversion of nonproductive agricultural land for aquaculture. Product-wise however, there has been an increase in both agriculture and aquaculture sectors. The development of agriculture—aquaculture, consumer products industry as well as mining and maritime industry all have an effect on material runoff Table 3: Annual discharge (tonnes year-1) of some rivers in Vietnam Factor Catchment area (km2) Runoff (km3) TSS (106) COD NO3 NH4 TN PO4 TP Fe Mn Zn Cu Pb As Cd Co Hg Red river Thai Binh river 298050 28200 200.00 114 46400 24602 352 46.26 14860 2015 2817 730 448 118 254 11 Dinh river (Khanh Hoa province) 0.679 0.062 Cai River (Khanh Hoa province) 2.535 0.150 45700 10466 706 3776 58 1029 79 13 6 0.4 108 2147 160 74 22 3 9888 3352 1100 154 120 164 20 16.5 Source: National project KT03.07, 2001 Han river (Da Nang City) Thu Bon river (Quang Nam) Dong Nai river Mekong river 10590 47280 830780 5.676 0.194 3236 2475 14.0 50.5 62 99600 79570 573.1 160 52000 27941 6602 36.3 62 1782 126 79 37 16 28 8613 16 265 2849 192 10220 1470 2921 500 102 12775 1825 190 982 128 25.6 <13 62 16 Vietnam 21739 18084 2063 2407 1082 503 134 40 Nguyen Tac An and Phan Minh Thu from land to the coastal waters and affect the ecology and biogeochemistry of the coastal zones. The Fluxes of Material Impact on Element Distribution in Coastal Waters Enhanced nutrient inputs from intensive aquaculture to coastal waters have stimulated development of harmful algal blooms. Seven species of harmful algae were found in industrial shrimp farms in Do Son (Hai Phong) associated with such algal blooms. In many cases, environmental conditions in areas receiving discharge from intensive aquaculture reveal drastic deviation from allowed environmental standards in Vietnam. These have also led to formation of hypoxia; for example, H2S content in Nghe An and Thanh Hoa was found to be 1.7 mg/L indicating an oxygen content of <2.0 mg/L. Furthermore, human activities and development of industry also account for environmental pollution. The annual solid waste production of Vietnam is higher than 15 million tonnes, in which more than 80% is domestic wastes (MRNE, 2004, Table 4). Together with aquaculture, waste from shipping and navigation has increased the frequency of undesired impacts on coastal water environments and destroyed ecological balances in their ecosystems. Oil pollution also appears to be a major problem in Vietnamese waters. According to CMESRC (2004), the average annual discharge of oil to the sea from Dong Nai river system is 2.7–3.3 × 103 tonnes, from Mekong river systems it is 27.5–55.0 × 103 tonnes, and from Red river system it is 7–8 × 103 tonnes. The total of oil discharged to the ocean was about 17.65 × 103 tonnes in 2000 (CMESC, 2004). In addition, the Vietnam coastal waters are touched by major Europe-Asia shipping routes and are affected by the heavy traffic. In addition, construction of dams for hydroelectric power and irrigation purposes also have their impacts on coastal seas. On the one hand, retention of sediments in reservoir behind dams reduces the amount of sediments reaching coastal seas and thus affecting hydrodynamic systems and productivity in coastal waters (Milliman, 1997; Humborg et al., 1997; Ittekkot et al., 2000; Chen, 2000). Duc (2000) showed that building dams is an unsustainable way of development causing changes of ecosystem structures also within reservoirs behind dams mainly due to changing nutrient and sediment inputs. For example, with Hoa Binh reservoir near Hanoi, the constructed reservoir led to a reduction of forest coverage from 41% down to 17%; more than 167 million m3 of sediment were trapped , which otherwise could have been discharged to lower catchments or to coastal waters. Studies on the impact of changing river sediment inputs on coral reefs have been scarce. In one such study, An and Thu (2001) report that from November 29, 1998 to January 28, 1999, the coral reefs in southern part of Nha Trang Bay were covered by 0.52 g of sediment cm-2 with an estimated annual sedimentation rate of about 1.0– 1.7 cm year-1. This appeared to have led to the destruction of the reefs. Similar processes are probably occurring elsewhere along the coast. Distribution of Elements and Biogeochemical Cycles in Vietnamese Coastal Waters The Chlorophyll and Primary Production in the East Sea There is strong spatial and temporal variability in the distribution of biogeochemical entities in Vietnamese coastal water. This related to the seasonality in material fluxes from land as well as in the prevailing Table 4: Assessment of wastes discharged into marine environment of main areas Area Ha Long Ha Noi Da Nang Dong Nai catchments Mekong Kind of waste Volume (m3 day-1) Domestic Industry Domestic Domestic Industry Domestic Industry Domestic Industry 8500 59000 270000 44200 9855 815205 153851 220312 19216 Source: CMESRC (2004) BOD (kg day-1) COD (kg day-1) TSS (kg day-1) TN (kg day-1) 10158 16455 334585 22535 21879 1350 107587 24880 109054 2632 38454 3144 202773 39666 236835 6131 47515 2188 87773 37030 236835 4266 32832 8487 19828 Biogeochemical Variability of Vietnamese Coastal Waters Influenced by Natural and Anthropogenic Processes hydrodynamic conditions. This is best seen in the distribution of chlorophyll and primary productivity in the East Sea (Bien Dong) (Figures 1 and 2). The chlorophyll content in the East Sea including upwelling regions ranges from 0.01 to 3.00 mg/m3 while the primary production from 1 to 700 mgC/m3/day. These values are higher than those available at the National Oceanographic Data Centre (NODC, 2002). Generally, 41 chlorophyll-a content in coastal waters of the East Sea is higher than in offshore areas. Higher values along the coast are found in regions with large material fluxes from land such as off the mouths of the Red, Mekong and Dong Nai rivers and within some bays in central Vietnam (Figure 1). The places with high chlorophyll-a content are areas of high primary production (Figure 2). Especially high concentrations of chlorophyll and Figure 1: Distribution of Chlorophyll-a (mg/m3) in the East Sea during the Northeastern (left) and Southwestern monsoon (right) seasons. (Source: National project: KC 09.02, 2004, unpublished). Figure 2: Distribution of primary production (mgC/m3, day) in the East Sea during the Northeastern (left) and Southwestern monsoon (right) seasons. (Source: National project: KC 09.02, 2004, unpublished). 42 Nguyen Tac An and Phan Minh Thu primary production are also found in upwelling areas in southwestern parts of the East Sea (An and Son, 2004; An et al., 2004). They exhibit average primary production of about 1980 mg C m-2 day-1 (An, 2003). Material Cycles in Coastal Waters of Vietnam Studies on the river systems of Vietnam based on the model in Box 1 (Gordon et al., 1996) show that the sources and sinks of C, N or P determine the biogeochemical cycling of elements in coastal waters. Furthermore, the current sources of elements from outside the ecosystem appear to balance biogeochemical budgets. Data presented in Table 5 show that estuarine systems are autotrophic ecosystems while bays or lagoons can be ranked from autotrophic to heterotrophic ecosystems. Cluster analysis shows however that only three rivers— Tien river (rainy season), Red river and Thu Bon river— are strictly autotrophic and exhibit the capacity to assimilate nitrogen (P < 0.05). In contrast, the other studied systems lack these characteristics (Box 2). In a study of the Mekong River and the adjacent deltaic, estuarine, and coastal environment, An and Son (1998) could show differences in the elemental cycling between the river, front and plume, and shallow waters (Figure 3). They demonstrated that the estuarine systems in Vietnam are more eutrophic areas while the river and ocean can be ranked from eutrophic to oligotrophic. An and Son (1998) also could show that this model calculation was not influenced by sinks and sources of detritus from within the system. The model needs a standing production of 570 mgC/m 2/d for which a nutrient-consumption in the order of 118 mgN/m2/day is required. Box 1 Material balance within a system according to the model by Gordon et al. (1996) For conservative material balance: d (VS ) = Â Vin Sin - Â Vout Sout dt Expanding this equation: dS dV +S = Â Vin Sin - Â Vout Sout dt dt where SVin and SVout represent all of the hydrographic inputs and outputs and Sin and Sout are salinity of those water masses. For non-conservative material balances: V dY dV +Y = Â Vin Yin - Â Vout Yout + DY dt dt where Y is material concentration in waters and DY = (Sources – Sink). V Table 5: Nonconservative fluxes and N-P biogeochemical cycles in Vietnamese coastal waters Location Hau river Mekong river Tien river Mekong river Phan Thiet Bay Nha Trang Bay Van Phong Bay Xuan Dai Bay Cu Mong Bay Thu Bon river Cau Hai lagoon Red river Residence time (day) DDIP mmol m-2 d-1 DDIN mmol m-2 d-1 p-r mmol m-2 d-1 nfix-denit mmol m-2 d-1 Reference 14* 4** 11* 1** 55* 6** 67* 37** 61* 43** 29* 5** 71* 22** 4 51* 9** - +0.05 +0.40 +0.1 -2.2 +0.02 +0.22 +0.05 +0.26 -0.0002 +0.0002 -0.08 -0.14 -0.004 +0.013 -25.58 -0.01 -0.09 +0.96 -0.02 +10.40 +1.2 +23.6 -0.3 -9.8 -0.99 -0.35 +3.2 +0.4 +0.009 +1.78 -0.057 +7.658 -67.2 -0.39 +0.56 +7.68 -5 -42 -10 +233 -2 -23 -6 -30 +0.03 -0.03 +7.95 +15.41 +0.37 -1.43 +271 +9.77 +1.17 +288 -0.8 +4.1 -0.5 +58.8 -0.7 -13.3 -1.9 -5.0 +3.2 +0.4 +1.21 +4.10 -0.007 +7.45 +342 -0.21 +0.65 +17.35 Thu (2000) *: dry season, **: rainy season p-r: Net ecosystem metabolism, nfix: nitrogen fixation and denit: denitrification Huan and Thu (2000) Huan et al. (2000) Huan (2000a) Huan and An (2000) Huan and Long (2004) Huan and Long (2002) Huan (2000b) Huong (2000) Wosten et al. (2003) Biogeochemical Variability of Vietnamese Coastal Waters Influenced by Natural and Anthropogenic Processes Box 2 Classification of N and P budgets in coastal waters in Vietnam Hierarchical Cluster Analysis Dendrogram using Average Linkage (Between Groups) Rescaled Distance Cluster Combine C A S E 0 5 10 15 20 25 Label Num +——+——+——+——+——+ VP R -+ CM D -¦ CH R -¦ PT D -¦ VP D -¦ Hau D -¦ NT D -¦ Tieu D -¦ CM R -¦ XD D -¦ CH D -¦ XD R -¦ PT R -+---------------------+ NT R -¦ +-------------------------+ Hau R -+ ¦ ¦ Tien R -----------------------+ ¦ Red -------+ ¦ ThuBon -------------------------------------------------+ VP: Van Phong Bay, CM: Cu Mong Bay, XD: Xuan Dai Bay, PT: Phan Thiet Bay, CH: Cau Hai lagoon, NT: Nha Trang Bay, D: Dry season, R: Rainy season River Front + Plume 43 Factors Affecting the Biogeochemical Cycles of Ecosystems in Coastal Waters It must be noted that the East Sea which receives most of the fluxes is a semi-enclosed marginal deep sea with its own seasonal dynamics, and this certainly has an impact on its biogeochemical processes. While being influenced by land-derived fluxes and near-coastal processes, biogeochemical processes driven by the internal East Sea dynamics in turn influence the former. This makes investigations of the processes difficult and there is a scarcity of information. In the following text we restrict ourselves to the three upwelling centres in the East Sea. In the East Sea, the seasonally reversing monsoon system controls the surface circulation (Wyrtki, 1961). In summer, when the southwest monsoon prevails, winds blow primarily from south to north, and Indian Ocean surface water flows into the East Sea. In winter, these flow patterns are reversed by the northeast monsoon, and surface water enters the East Sea mostly from the western Pacific Ocean (Wyrtki, 1961). This brings with it a certain amount of nutrients which fuel primary production in the East Sea. Total production in the northern East Sea has been determined to be ~38 mmol C m-2 d-1 (456 mg C m-2 d-1) (Diego-McGlone et al., 1999) with particulate organic carbon export of about 2 mmol Cm-2 d-1 (12 mg C m-2 d-1) (Michaels et al., 1994; Karl et al., 1996) and total export production about 5 mmol Cm-2 d-1 (60 mg C m-2 d-1) (Emerson et al., 1997), while the primary production Ocean Figure 3: Carbon cycle for eutrophic zone of river, front + plume, ocean in Mekong Delta (unit: mg C/m2/day). (An and Son, 1998) 44 Nguyen Tac An and Phan Minh Thu in waters off Vietnam was 569 mg C m-2 d-1 (An, 1995), in the continental shelf waters it was 776 mg C m-2 d-1, and in the upwelling regions of southern central Vietnam it was 1980 ± 1969 mg C m-2 d-1 (An, 1997; An et al., 2004). The East Sea is characterized by three upwelling areas: one between 16∞ and 19∞N 100 km offshore the northwestern Philippines during winter (L area) (Chao et al., 1996; Shaw et al., 1996) and two centres located in the coastal areas of Vietnam during summer (Wiesner et al., 1996; Lanh, 1997) (V and S areas) (Figure 4). Lanh (1997) found that maximum upwelling velocity in upwelling areas of coastal Vietnam is 13 × 10-4 cm s-1 at the 125 m water layer. The vertical upward fluxes in the East Sea play an important role in supporting the nutrient requirements in oligotrophic water bodies. In the coastal upwelling areas of Vietnam upwelling supplies about 0.05–0.10 mmol P m-2 d-1; the upward phosphate flux in the Philippines upwelling area is 0.019 mmol P m-2 d-1. However, the vertical fluxes of nitrate and phosphate to the euphotic zone contribute towards a molar N/P ratio significantly higher than the normal Redfield ratio of 16 throughout the region. It is 35 off the Philippines (Cai et al., 2002) and increases from 27 to 72 in the Vietnam upwelling regions. These values show that the upwelling does not play a role in relieving P limitation (Thom, 1997). Hence, it is implied that the East Sea is a P-limited system; that means the low productivity is due to the low vertical fluxes of phosphate (Cai et al., 2002). Conclusions The biogeochemical characteristics of Vietnamese coastal waters discerned from the distribution and behaviour of elements suggest that they are determined by human activities that control the material inputs from land as well by the prevailing hydrodynamics in coastal waters. Based on the observed biogeochemical characteristics, Figure 4: Three centres of upwelling in East Sea with high sea-surface chlorophyll concentrations. (modified from Lanh et al., 1997, Chao et al., 1996; Shaw et al., 1996) Biogeochemical Variability of Vietnamese Coastal Waters Influenced by Natural and Anthropogenic Processes the coastal waters can be classified into types: autotrophic and heterotrophic systems. Most of the larger estuaries are autotrophic systems whereas the others can change from autotrophic to heterotrophic systems or be heterotrophic systems. These findings have important implications in developing measures for the protection of environment and natural resources of coastal ecosystems of Vietnam. Acknowledgements We would like to thank the University of Jenderal Soedirman at Purworketo, Indonesia, Federal Ministry for Education and Research of Germany and the Rector of UNSOED, Prof Rubiyanto Misman for support to present and discuss the paper at the International Workshop on Aquatic Ecosystems of the Monsoon Asia region. References An, Nguyen Tac (1995). Biological productivity of Vietnam marine waters. 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