PHYSICAL AND CHEMICAL CHARACTERISTICS OF SEDIMENTS FROM PRAWN FARMS AND MANGROVE HABITATS ON THE CLARENCE RIVER, AUSTRALIA

The culture of Penaeus monodon in earthen ponds produces a black, glut inous sediment which has caused degradation of waterways in some count ries and is suspected of reducing farm productivity. Superficially, th e sediment is similar to that found in mangrove habitats, however, the re have been few studies which compare these materials. This study des cribes the physical and chemical characteristics of sediment from four Australian prawn farms and compares results with those from nearby si tes in intertidal mangrove habitats of the Clarence River, Australia. At the farms, sediment accumulated in the centre of ponds and around d rains at variable rates (20 x 10(3)-180 X 10(3) kg ha(-1) per crop), d epending on the stability of pond walls, prevailing winds, type of soi l and aeration rates. The main source of the sediment was soil eroded from the pond walls and from the periphery of pond bottoms. The accumu lating sediment was a mixture of silt and clay size particles, with a median particle size of 13 mu m. Chemical analysis showed it consisted of (by weight): quartz, feldspar and clay minerals (70-80%); amorphou s oxides of silicon, iron and aluminium (5-10%); organic matter and vo latile compounds (5-10%); elevated levels of trace metals and nutrient s. Inputs of food, faeces and decaying plankton enriched the sediment, increasing the organic carbon content from 13.9 mg g(-1) at the perip hery of ponds to 22.8 mg g(-1) in the centre. Fresh and dried sediment had neutral pH, although soil in the pond walls was often acidic. In the centre of ponds, total nitrogen (2.25 mg g(-1)) and total phosphor us (690 mu g g(-1)) were significantly higher than at the periphery. T here were abundant microalgal fragments and high levels of amorphous s ilica (10-15% by volume) on the bottom of ponds, and calculations sugg est that weathering of clay provided 87% of the silicon used by diatom s. The sediment in intertidal mangrove habitats was physically and che mically similar to sediment from the periphery of prawn ponds, while s ediment from inland forest mangrove habitats was similar to sediment w hich accumulated in the centre of prawn ponds.