Effluent, nutrient and organic matter export from shrimp and fish ponds causing eutrophication in coastal and back-reef waters of NE Hainan, tropical China

Global aquaculture has grown at a rate of 8.7% per year since 1970. Particularly along the coasts of tropical Asia, aquaculture ponds have expanded rapidly at the expense of natural wetlands. The objectives of this study were (i) to characterize the extent and production process of brackish-water pond aquaculture at the NE coast of Hainan, tropical China, (ii) to quantify effluent and organic carbon, nitrogen and phosphorus export from shrimp and fish ponds and (iii) to trace their effect on the water quality in adjacent estuarine and nearshore coastal waters harboring seagrass meadows and coral reefs. During two expeditions in 2008 and 2009, we determined dissolved inorganic nutrients, dissolved organic carbon (DOC) and dissolved organic nitrogen (DON), chlorophyll a (chl a) and particulate organic matter (POM) in aquaculture ponds, drainage channels and coastal waters in three areas varying in extent of aquaculture ponds. From the analysis of satellite images we calculated a total of 39.6 km² covered by shrimp and fish ponds in the study area. According to pond owners, there is no standardized production pattern for feeding management and water exchange. Nutrient and suspended matter concentrations were high in aquaculture ponds and drainage channels, but varied considerably. The calculated annual export of total dissolved nitrogen (TDN) and particulate nitrogen (PN) from pond aquaculture into coastal waters was 612 and 680 t yr−1, respectively. High concentrations of dissolved inorganic nitrogen (DIN), phosphate and chl a at the majority of the coastal stations point at eutrophication of coastal waters, especially close to shore. Coastal eutrophication driven by the introduction of untreated aquaculture effluents may be especially harmful in back-reef areas, where estuarine retention and mixing with open ocean water is restricted thus threatening seagrasses and corals.

► Areal extent, operation and water quality in aquaculture ponds in NE Hainan were studied. ► Nutrient and organic matter export from ponds to estuarine and coastal waters was calculated. ► Coastal back-reef areas were high in nutrients and chlorophyll a due to inputs of aquaculture effluents. ► Effluents initially released to estuarine lagoon had lower effects on coastal water quality than those directly to coastal waters. ► Direct release of aquaculture effluents to back-reef areas is particularly harmful.