Experimental plots were established at three sites within mature stands of Rhizophora stylosa (FLNS, FLSN, FLIS), in an area approved for reclamation by the Gladstone Port Authority. The sites were matched visually for tidal elevation, mangrove tree composition and condition, sediment composition and benthic invertebrate populations. Three replicate 6 m² plots were constructed at each site. Around each plot, prop roots were cut in a path about 0.5 m wide to allow installation of plastic retaining walls. The retaining walls were dug into the mud to a depth of 20 cm and supported to a height of 1 m. A gate was installed to allow tidal waters to move in and out while retaining the oil. One of three treatments was applied to each plot: no oil (control); oil; and dispersed oil. Three undisturbed control sites (FLNN, FLSS, CISS), were chosen outside the reclamation area, and plots were delineated by marking tape stretched between the trees only. A litter trap was also suspended in the trees above each plot.
Before treatment with oil, initial biological measurements were taken and each plot was sampled for sediment grain size, total organic carbon and background hydrocarbon content.
The oil used in this experiment was a medium range crude oil from the Bass Strait Basin in southern Australia and the dispersant used was Corexit 9527. Prior to application to the plots, the oil and the oil plus dispersant mixture was preweathered to simulate an oil spill arriving from seaward. Dispersant was added to the oil to achieve a 1/20 dispersant to oil (v/v) ratio in accordance with industry recommended usage. Treatments were added to the appropriate plots at high tide at a rate of 5 l/m² and pumps were used to distribute the oil evenly throughout each plot as the tide fell.
The average rate of dosing in each plot was calculated from surface sediment (0-2 cm) samples collected 40 hours after treatment. Four replicate samples, each comprised of at least 5 pooled surface samples were collected from each plot for analysis.
At 1, 7, 13 and 22 months, 4 replicate 10 cm diameter cores were collected from each plot, sliced and the 0-2, 10-12 and 20-22 cm sections from each replicate were pooled. Observations, including the presence of roots, animals, burrows and oil were made before the samples were frozen for later analysis.
The following analyses were conducted on the sediment samples: Total Extractable Organic Matter determined gravimetrically; Total oil determined by UV Fluorescence analysis (UVF); Total hydrocarbons determined by gas chromatography with flame ionisation detection (GC-FID); The percentage of unresolved hydrocarbons; Concentrations of individual isoprenoid and n-alkanes (µg/g dry weight); Sum of 218 aromatic and alkylaromatic isomers in the naphthalene/biphenyl, fluorene, phenanthrene/anthracene, benzanthracene/chrysene, fluoranthene/pyrene and benzopyrene to benzoperylene series as determined by Selected Ion Monitoring-Gas Chromatography/Mass Spectroscopy (SIM GC/MS); Ratios of specific alkyl phenanthrene isomers to illustrate selective biodegradation; Sum of triterpane biomarkers in the hopane series as determined by SIM GC/MS using m/z 191; Selected sterane and potential demethylated hopane biomarkers as determined by SIM-GC/MS using m/z 217 and m/z 177; Ratios of specific biomarkers useful as biodegradation indices.
