Effects of oil and dispersed-oil on mature mangroves in field trials at Gladstone

In late 1996, oil and dispersed-oil treatments were released into six experimental plots of mature mangroves near Gladstone, Queensland. Local stands of mangroves are dominated by Rhizophora stylosa up to 10 m tall. Each plot enclosed approximately 20 trees in plots of approximately 40m2. The object of the experiment was to simulate a large oil spill flooding mangroves at high tide, and to ascertain the relative effects of treatments on sediments, trees and animals, and to monitor short term recovery over one year. The oil used was weathered Gippsland Light, a waxy crude oil. This was applied on its own and in combination with the dispersant, Corexit 9527, at the same dosage of 5L/m2. A large proportion, if not all, crustaceans in the mangrove plots died within two days of oiling. Trees died in some plots after 2-3 months. In this preliminary assessment, there was no simple reason why trees died in some plots and not in others. For example, the amount of hydrocarbon in sediments was not correlated with effect. However, there were indications of synergistically-related factors, including presence of dispersant, sediment porosity, the number of burrowing crabs, and the presence of the mud lobster, Thalassina squamifera. Furthermore, after one month, the treated plots were partially recolonised by leaf-eating crabs. We present a preliminary assessment of results and observations using current data on hydrocarbon levels in sediments, growth and canopy condition of mangrove trees, and the presence and quantity of leaf-eating crabs