Nutrient and faecal contamination and retention in wetland enclosures (geiwais) in the Mai Po Marshes, Hong Kong

Nutrient and faecal contamination is an increasing problem to the shrimp pr oductivity and wildlife conservation at the internationally important wetla nd ecosystems of the Mai Po Marshes (Hong Kong, P.R. China). The present st udy examined the nutrient status and faecal bacteria loading and potential retention capacity of contaminants of two wetland enclosures. Water in the wetland enclosures was eutrophicated with high concentrations of dissolved inorganic nitrogen (inorg-N-diss= 15.0 mg l(-1)) and orthophosphate phospho rus (o-P = 1.89 mg l(-1)) and was loaded with high levels of faecal colifor ms (172 x 10(3) cfu in 100 ml) and faecal streptococci (1.94 x 10(3) cfu in 100 ml). The pattern of nutrient enrichment of two wetland enclosures is r elated to a north-to-south pollution gradient from the Shenzhen River to th e wetlands. By retaining tidal water in the wetland for an 8-day period, wa ter quality was greatly improved; NH4-N was removed by 83%, o-P by 45% and faecal bacteria by 100%. This implies a self-purification capability of the wetland enclosures and a potentiality of using them as an alternative sewa ge treatment.