EFFECTS OF OILS AND DISPERSED-OILS ON MANGROVE SEEDLINGS IN PLANTHOUSE EXPERIMENTS: A PRELIMINARY ASSESSMENT OF RESULTS TWO MONTHS AFTER OIL TREATMENTS

Abstract

The effects of oil and dispersed-oil treatments on mangrove seedlings, grown in artificial tidal systems in tropical north-eastern Australia, were assessed two months after application. Seedling mortality was used as a measure of treatment toxicity. All oils and dispersed-oils were toxic to mangrove seedlings although the effect of Bunker C was quite low. Overall, oils and dispersed-oils were ordered by increasing toxicity: Bunker C fuel, Arabian Light crude, Gippsland Light crude, Thevenard crude, and Woodside condensate. Toxicity of oils correlated with viscosity, where low toxicity of the heavy fuel oil (3 per cent mortality) stood in marked contrast with high levels of mortality scored for very light crude oils, Thevenard (73 per cent) and Woodside (85 percent). Mangrove species were ordered by their overall increased vulnerability to oils and dispersed-oils as, Ceriops spp., Rhizophora stylosa, Avicennia marina and Aegiceras corniculatum. As expected, higher doses (2.0 L/m2) showed an overall increase of 30 per cent mortality compared with lower doses (0.2 L/m2). However, values for each grouping of seedlings varied widely, indicating possible synergistic effects of different environmental factors. Despite this, at least one other pattern was evident. Dispersed-oil treatments were usually less toxic to mangrove seedlings than undispersed oils, particularly for high doses. This result needs to be further assessed, but it is of interest that added dispersant did not usually result in increased seedling mortality. We will re-evaluate all findings once experiments are completed.