The areas of shrimp, crab and shellfish ponds in the south China were estimated at 107.64, 86.13 and 234.09 × 103 ha, respectively, in 2006 (SOA, 2006). In south China, Guangdong has the largest aquacul- ture area (244.48 × 103 ha), followed by Fujian (109 × 103 ha) and Guangxi (58.66 × 103 ha). Hainan Province has the smallest mariculture area, at only 15.29 × 103 ha. For the entire region of south China, the total DIN and DIP flux from water exchange with mariculture ponds was 239.54 t yr− 1 and 37.03 t yr− 1 while the TN and TP flux from sed- iment wash-out was 1.7 × 105 t yr− 1 and 1.03 × 105 t yr− 1 (Table 3).
The NH+4 –N:NO−3 –N ratio in mariculture pond effluent was signifi- cantly higher than that in tidal water (Fig. 5A). Thus, pond waste dis- charge may not only increase the DIN load of downstream areas, but also shift the form of DIN. Additionally, the NH4 +–N:NO3 −–N ratio was about 10–20 higher in sediment-derived DIN than waste water (Fig. 5B), so it could be inferred that sediment-derived DIN was also an important source that could change the nutrient composition of the tidal water