SURVEYS OF OIL SPILL INCIDENTS AFFECTING MANGROVE HABITAT IN AUSTRALIA: A PRELIMINARY ASSESSMENT OF INCIDENTS, IMPACTS ON MANGROVES, AND RECOVERY OF DEFORESTED AREAS

Abstract

Over the past three decades, nine notable oil spills affected mangrove habitat in Australia. Six were the result of shipping incidents, and three others were land-based incidents. There were two incidents in the 1970s, three in the 1980s, and four in the 1990s. Types of oil released changed from crude oils in the 1970s to fuel and diesel oils in the 1990s. The total volume of oil released into Australian waters in mangrove-related incidents since 1970 was around 5,000 tonnes. Dispersants were used in the four major incidents involving shipping. Hydrocarbon in sediments generally decreased rapidly after spillage, leaving unrecognisable trace residual fractions in most locations after several years.

At least 220 ha of mangroves were oiled in Australia since 1970, and around 13 ha were killed. The extent of oiling depended largely on the amount of oil released. The area of deforestation however, depended on the type of oil with the greatest areas killed by crude oils and diesel, and lesser damage caused by fuel oils. Recovery of deforested sites depended chiefly on site exposure. Sites of greatest exposure experienced both rapid and slow recovery, and sometimes rapid decline. In exposed locations, sediment flushing appeared important in stimulating recovery, while greater exposure also allowed premature destruction of growing plants by strong seas and storms. Based on estimates of total above-ground biomass, mean rates of recovery matched those observed in Panama, and full biomass recovery of areas deforested by oil spills might take around 36 years on average, with 50 per cent recovery after 20 years.