The Nakdong River, located to the south-eastern part of the Korean Peninsula (Figure 1), is the second largest river in South Korea (second to the Han River, which flows through Seoul), with a total length of 530 km, and a drainage area of 24,000 km² (approximately 2,400×10³ ha). The estuary of the Nakdong River is located between 35.03° and 35.13°N, and 128.48° and 129.00°E. The estuarine area acts as a buffer between fresh and salt water, and contains the largest nesting grounds for migratory birds in eastern countries due to the abundance of phytoplankton and zooplankton, mollusks, seaweeds and aquatic plants (Lee et al. 1993; Park et al. 1986)). Average annual temperature is between 12°C and 14°C, and annual precipitation is 1,160 mm. The variation of annual precipitation is very high. About two thirds of annual precipitation is concentrated in three months, between July and September.

The average flow rate is 470 m³ sec^-1or about 41,000×10³ m³ d^-1. The Nakdong River estuary is heavily polluted and also eutrophicated due to the discharge of sewage from domestic and industrial sources (Pusan Institute of Health and Environmental Research 1993). Sewage from domestic sources is 2,000×10³ m³ d^-1 comprising 74% of the total; the other 26%, 700×10³ m³ d^-1, comes from industrial sources. An estuarine dike system 2 km in length was constructed in 1989 to conserve freshwater for drinking, agricultural and industrial usage. However, this dike construction prevents the free exchange of water in the estuary and leads to reduction in the water quality of both the freshwater lake upstream and the coastal environment downstream.

The LOICZ Biogeochemical Modelling Guidelines (Gordon et al. 1996) were used to calculate the stoichiometrically linked water-salt-nutrient budgets. A mean pan evaporation of 3 mm d^-1 (for the estuary; V_E = -300×10³ m³ d^-1) for both wet season (June to September) and dry season (October to May) was obtained from the local weather office in Pusan. This rate was multiplied with the area (96 km²) to get V_E. The average depth of the estuary used in this calculation is 5 m. No pan correction factors were used. Precipitations were 13 mm d^-1 (for the estuary; V_P = 1,300×10³ m³ d^-1) for the wet season (June to September) and 3 mm d^-1 (for the estuary V_P = 300×10³ m³ d^-1) for the dry season (October to May), based on 1988-1998 data from weather office stations in Pusan.

During the wet season, salinity values used for the budget calculation are 28.9 psu for the ocean and 3.7 psu for the estuarine system. During the dry season, these values were 33.2 psu and 31.7 psu for the ocean and estuary, respectively.

The difference between the precipitation and evaporation was very great. The volume of groundwater and wastewater input is 170×10³ m³ d^-1 for the rainy season and 30×10³ m³ d^-1 for the dry season. Water exchange time for the estuary was calculated to be 12 days in the wet season and 5 days in the dry season. Table 1 summarizes the water budget and water exchange time for the bay for both seasons. Figure 2 represents the water and salt budgets for the wet and dry seasons.

Figure 3 illustrates the DIP budgets for the wet and dry seasons. The waste load of DIP in each season was calculated from the riverine flux and from the industrial sources. The riverine flux accounted for most of the DIP load in this region.

Figure 4 represents the DIN budget for the wet and dry seasons. The waste load of DIN in each season was calculated from the riverine flux and from the industrial sources. As for the phosphate budget, the riverine flux accounted for most of the DIN load in this region. The DIN balance is strongly dominated by waste discharge in all of the budget boxes.

The carbon equivalent of net P flux (p-r) is +10×10⁶ mol C d^-1 or +0.1 mol C m^-2 d^-1 in the wet season and +3×10⁶ mol C d^-1 or +0.03 mol C m^-2 d^-1 in the dry season, indicating a net autotrophic system. Estimated (nfix-denit) was -22×10⁶ mol N d^-1 or about -0.2 mol m^-2 d^-1 in the wet season and -8×10⁶ mol N d^-1 or about -0.1 mol m^-2 d^-1 in the dry season, indicating a net denitrifying system.

Figure 2. Salt and water budgets for the Nakdong River estuary in the wet (a) and dry (b) seasons. Water in 10⁶ m³ d^-1 and salt flux in 10⁶ psu-m³ d^-1.