Comparative studies of water quality in two brackish ponds for shrimp cultivation in a salt marsh

Physical, chemical and microbiological characteristics of two 'keiwais' (brackish ponds built along the coastal area for shrimp cultivation) in the Mai Po Marshes, Hong Kong, were measured and compared. One keiwai, located closer to the domestic and agricultural areas, had lower shrimp production than the other. Physical, microbiological and most of the chemical parameters of two keiwais showed no significant difference, while the keiwai closer to the domestic and agricultural areas had higher biochemical oxygen demand (BOD5), and higher levels of phosphate (PO4(3-)-P) and cadmium (Cd2+). Results of the present study indicate that these keiwais were polluted by sewage discharged from nearby domestic, agricultural and industrial areas. The possibility that water received by the less polluted keiwai was purified by surrounding mangroves and sediments in the Mai Po Marshes is discussed.