August 98

Dr. Norman Duke reports


on a major research


project funded by the


Australian offshore


petroleum industry to help


reduce future impacts of


large oil spills on


mangrove habitats.

A major part of this project was to experimentally oil mature mangrove habitat whilst closely monitoring the impacts on natural ecological processes. This was necessary both to better understand these natural processes, as well as to find ways to reduce impacts and assist with post-spill recovery. It is significant that this work is largely proactive and, as such, offers opportunities to respond with greater understanding and knowledge to save mangroves both threatened and damaged by large oils spills in the future.

The project team, and its advisors on the APPEA Research Working Group, were reluctant to damage natural mangrove habitat, and this field project was only considered acceptable if two criteria were met, namely: 1) the field site was in an area already approved for destruction - a reclamation site; and 2) all possible safeguards were established to prevent loss of oil and contamination of neighbouring environments. The project also required the approval of relevant government authorities, local industry and the local port authority, as appropriate.

After considerable enquiries and consideration, the most acceptable site location was in Port Curtis, near Gladstone, Queensland. In June 1996, the Gladstone Port Authority offered the project team the use of several mangrove areas subject to already-approved and initiated reclamation projects. After a detailed site inspection, the site at Fishermans Landing was chosen and further permission to regularly access the site was obtained from Queensland Cement Limited, the local tenant.

Approvals for the field project were obtained from the Queensland Department of Primary Industries, Southern Fisheries Centre, and the project was monitored and assisted by officers of the Queensland Department of Environment Regional Office, Queensland Department of Transport, the Great Barrier Reef Marine Park Authority and the Australian Maritime Safety Authority.

The success of the project depended on this wide support and approval, but it also depended on the further innovation and dedication of the project team. To meet the second criteria, mentioned above, it was necessary to develop new experimental techniques, and to design and construct appropriate specialised equipment. No other study in the world had successfully oiled enclosed plots of mature mangroves in the natural environment - and to do this in a replicated and sound experimental design. Previous experiments had used loosely enclosed systems of floating oil spill booms. These were not successful in containing applied oil, and they relied on the remoteness of sites to lessen any neighbouring affects, impacts and controversy.

This project's success depended very much on the integrity of the experimental enclosures which were designed to allow the free ebb and flow of tidal waters whilst oil stayed inside and settled on the mangrove sediment and exposed roots. The settling process was expected to take place within two weeks, after which the enclosures could be removed.

The experimental enclosures consisted of two chief parts: 1) curtains of vinyl fabric, 1.2 metres in height, buried 0.2 metres into the sediment, and placed around the longer part of the perimeter of approximately 30 metres; and 2) a tidal oil-gate of vinyl fabric, 1.2 metres wide and also rising one metre above the sediment with 0.2 metres below. Enclosure panels were buried 0.2 metres into the sediment around the entire perimeter to prevent loss of oil treatments. The one metre high curtains were sufficient to prevent overflow of tidal waters in the chosen plot locations during the critical settling phase for the oil. Further safeguards to ensure that no oil escaped from enclosures included: installation of floating absorbant booms closely surrounding each enclosure; installation of large bags of absorbant material within 5 metres of each plot, and sufficient to soak up all oil in each plot; deployment of offshore oil spill booms within 200 metres of the site, and sufficient to contain escaping oil; and, an advisory alert to the local Harbour Master to have the oil spill skimmer vessel and other equipment on standby. These measures were further backed-up with frequent and regular monitoring of plots after applying oil treatments. Of these extra precautions taken, only the absorbant booms were used in two minor leaks (< one litre each) through the gates of plots treated with dispersed-oil.

The three study sites chosen included mature stands of 6-8 metre tall trees of Rhizophora stylosa, a common species around the northern and subtropical Australian coastline. This species is also well-known for its' above ground root structure of thick prop roots, making it quite difficult to walk within the forest. To install the enclosures, these prop roots had to be cut in a track, approximately 0.5 metre wide, along the boundaries of each plot, approximately 6 metre x 6 metre each. In all, nine plots were prepared in this way, with three each at each of three sites.

Treatments types were chosen at random from the three plots for each site, and included: oil-only, dispersed-oil, and a disturbed control. By way of further comparison, and checking for any disturbance effect, three additional sites outside of the reclamation areas were chosen as undisturbed controls where there was no cutting of mangrove roots.

The total amount of oil released into the six treatment plots was approximately 1,600 litres. This was applied in late October this year. The amount of oil applied to each plot differed based on the size of each plot, although a dosage rate around 5 litres per square metre was the objective. The oil chosen was Gippsland Light (Bass Strait Crude) oil, and the dispersant was Corexit 9527. The choice of oil was based on the common usage and transport of Gippsland Light around Australia, and based on priorities set by the APPEA Research Working Group. Oil treatments were weathered in a pond, approximately 0. 1 metre deep, for 24 hours prior to application. The site used for preparation and pre-treatment of oil mixtures was provided by BP Australia, Gladstone Terminal.

Pre-weathered oil treatments were added to the plots as the tide rose, to simulate a large offshore oil spill in each case, where weathered oil and dispersed-oil floated into mangroves. Hydrocarbon sampling of surface sediments after 40 hours, indicated that dosages, around 30,000 mg/Kg of sediment, were comparable with those found after large oil spills elsewhere. Further hydrocarbon sampling, conducted in mid November, included both surface and deeper sediments to 22 cm and this will quantify initial degradation of oil in the plots, as well as depth penetration.

Treatment plots of mature mangrove trees of Rhizophora stylosa were surrounded by fabric curtains and a tidal oil-gate to form the enclosures designed to keep oil inside while allowing tidal waters to ebb and flow naturally during each cycle.

Biological impacts are also being monitored closely. Sampling began in July this year prior to application of treatments, and it will continue, in some cases on a monthly basis, for as long as the sites are available over the next two years. The chief studies include: litterfall and productivity of mangrove tree canopies; shoot growth and productivity of mangrove leafy shoots; photosynthesis of mangrove leaves; removal of fallen leaves by crabs; presence and diversity of crabs and other mobile macro-epifauna; and the presence and diversity of burrowing worms. The data gathered so far have not been fully assessed, but there are some interesting preliminary observations.

Preliminary analysis of litterfall data suggest that dispersed-oil treatments affected trees more than other treatments since these plots had disproportionately greater leaf fall compared to oil-only treatments and controls.

Dead fauna were collected after 40 hours following treatments. These animals were chiefly crabs (Grapsids), pistol shrimps (Alpheids) and mud monsters (Thalassinids), collectively termed the mobile macro-epifauna. It was of interest that there appeared to be greater between-site differences in diversity and biomass than between treatments, indicating that there were no obvious differences between oil-only and dispersed-oil treatments. The total biomass and diversity of these usually cryptic burrowing crustaceans was also relatively high, and it is possible that this study will provide the best estimate of their presence and density in comparable mangrove ecosystems anywhere.

These and all data gathered during the project will be appropriately analysed and published in the public domain, initially as a report to APPEA and ERDC, and later in various industry and scientific research publications.

The benefits of this project will come from the lessons and experience gained by the project team from the various studies of oil-affected mangrove environments, including: the experimental programs in the field, as described above, and those in the planthouse; and, the compilation and synthesis of information regarding earlier oil spill sites around Australia. Based on these findings, it is hoped to reduce the impact of future oil spills which might otherwise alter or destroy vulnerable mangrove habitat.