Nutrients in Coastal Waters of the Pearl River Estuary

1 Nutrients in Coastal Waters of the Pearl River Estuary 

8b and c show the variation in DIN concentrations in a vertical direction in the ebb and flood tides respectively. The results indicated that both in the northern area and southern area of the estuary, the variation was not apparent in a vertical direction, while the concentration in the surface layer was higher than that in the bottom layer in the middle area. The reason is that the middle area is influenced by the invasion of the briny wedge, the freshwaters from nmoff mainly concentrate on the surface, and the seawater is distributed at the bottom, while the water system in the southern and northern areas is well mixed in a vertical direction. This also indicated that the runoff of the Pearl River was the chief source of the DIN from another point of view. 

8d and e indicate the form variation of DIN in the ebb and flood tides respectively. It can be seen that NO3-N was the main form in most sea areas except station 6 both in the ebb and flood tides, and the concentration of NO3-N was generally over 60% of DIN, it was even over 90% in some stations. The second one was NH4-N, and the third one was NO2-N (below 10%). However, in the area near station 6, NH4-N was the main form of DIN, and NO3-N was the next one. This indicated that this area was mainly influenced by domestic wastewater. It may be related to the influence of domestic wastewater in Shenzhen (including the submarine wastewater discharge project in the west of Shenzhen) and Hong Kong near this area. 

The spatial-temporal variation patterns of phosphate in the flood and ebb tides are shown in Fig. 5.9a (Huang et al., 2003). The results indicated that the phosphate concentration in the seawater near the area of Shenzhen Bay was the highest, which was over 0.030 mg/L. This implied that land-based pollutants near Shenzhen Bay contribute greatly to phosphate. Phosphate concentration in the other area was 0.010 0.015 mg/L in the ebb tide, 0.007 0.017 mg/L in the flood tide. What should be emphasized was that the concentration of phosphate in the southern seaward area was obviously higher than that in the northern riverward area in the flood tide, while the difference in concentrations in the northern part and southern part was not obvious in the ebb tide (except the middle area near Shenzhen Bay). This result implied 

9b and c show the variation in phosphate concentrations in a vertical direction in the ebb tide and flood tide respectively. This proved further that the contribution of phosphate from river runoff was not large in the sea area of the PRE. 

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