Contaminant release from sediments in a coastal wetland

The Mai Po Marshes Nature Reserve (Hong Kong) which is one of the most important regional wetland habitats, is under increasing pollution pressure, such that the existence and survival of the natural biota, including many endangered species, is threatened. Water quality in the tidal shrimp ponds (locally called gei wais) of the marshes was so poor, that biological impacts arising from the release of contaminants from the underlying sediments were suggested. A study was conducted to (i) investigate the level of contamination of underlying sediments in Gei wai 13, (ii) examine the effects of environmental factors on the behavior of contaminants in the sediments and (iii) study the toxicity of the sediments. Sediment in Gei wai 13 was contaminated with nutrients and heavy metals, of which nitrogen and phosphorus tend to bind to coarser particles, while copper, nickel and zinc to finer ones (silts and clays). Mobility of nitrogen and phosphorus from the sediment increased with increasing levels of salinity and temperature of the overlying water. In respect of the drying effect, the solubility of nitrogen and phosphorus of air-dried sediments was lower than that of wet sediments. Sediment extract toxicity tests indicate, that the highly contaminated sediment resulted in a low toxic level and toxic responses were detected only by bioassays of amphipods and algae, but not by the Microtox® test. The low toxicity is consistent with the low soluble content of contaminants of pore water and the low bioavailability of contaminants in sediments.