Pollution studies on mangroves in Hong Kong and Mainland China

Mangroves are unique inter-tidal wetland ecosystems found along the sheltered estuarine shores of tropical and subtropical regions, and provide a wide variety of goods and services to people. Mangroves act as a transit zone between terrestrial and marine environments, contaminants transported from rivers and land-based sources as well as those in tidal water are therefore accumulated in mangrove ecosystems. In the past, mangroves especially those in developing countries have been used as convenient waste disposal sites (Clough et al., 1983) and are sinks or reservoirs of various man-made pollutants (Harbison, 1986; Tam and Wong, 1995a, 1999a; Zheng et al., 2000). Since the 1970s, researchers worldwide have published more than 3,500 SCI papers on various aspects of mangroves, ranging from mangrove distribution and geographical patterns, climatic changes and sea level rise, general ecology, community structure and function, population dynamics, physiological and molecular studies of individual species, fisheries, aquaculture and other economic values, restoration, management and conservation of mangroves. However, environmental pollution and its impacts on mangrove ecosystems especially those in China have received little attention. Mangroves are under increasing pollution pressure from human activities because of rapid industrialization and urbanization in our coastal areas. Refuse disposal, sewage discharge, urban emission, and accidental spillage of toxic pollutants are significant anthropogenic inputs. For instance, Deep Bay at the mouth of the Pearl River delta has the sixth largest mangrove forest in China, but mangroves around Deep Bay including Mai Po RAMSAR in Hong Kong and Futian in Shenzhen (Figure 1) have suffered from discharge of untreated domestic sewage, livestock and industrial wastewater in the catchment. Since the 1980s, Deep Bay region especially the Shenzhen Economic Special Zone has become one of the rapidest developing cities and the mangrove becomes the only one located at the heart of a modern city in China. The contaminants in the polluted water including nutrients, heavy metals and toxic organic pollutants are adsorbed onto suspended particles and subsequently accumulated in the mangrove sediments which act as a sink (Tam and Wong, 1993, 1995a, 1999b; Tam et al., 2001). On the other hand, evidence suggests that mangrove forests are generally nutrient deficient, and mangrove plants such as Bruguiera gymnorrhiza and Avicennia marina have shown positive growth responses to added nitrogen (e.g. Naidoo, 1990; Wong et al., 1997a; Ye and Tam, 2002). Recent studies suggest that mangrove wetlands could be employed for treating domestic, municipal, livestock, and shrimp pond effluents, and solve water pollution problem. The potential of developing a sustainable mangrove wastewater treatment facility should not be overlooked. In this paper, the degree of pollution including nutrients, heavy metals and persistent organic pollutants in mangrove ecosystems in China, in particular, Hong Kong, Shenzhen and Fujian were reviewed. The feasibility of using mangrove wetlands as a sustainable wastewater treatment facility was discussed.