Fate and effects of oil and dispersed oil
on mangrove ecosystems in Australia

AUSTRALIA-WIDE SURVEYS

Assessment of longer term impacts involved the survey of major spill sites around Australia where oil affected mangrove habitat over the last 30 years. We report on the fate and current status of previously oiled mangrove habitat in this country. Incidents were systematically described and the sites mapped, using a standard set of criteria covering the range of biological and physical site characteristics.

Approximately 5,000 tons of oil were released, resulting in the oiling of approximately 220 ha of mangrove habitat and the deforestation of 13 ha Our site visits chiefly involved assessments of forest composition and biomass, and the concentration and characteristics of residual oil in sediments. We recorded nine notable incidents from these surveys. Six were related to shipping activities while three were essentially land-based, and none were related to offshore operations. In all, approximately 5,000 tons of oil were released, resulting in the oiling of approximately 220 ha of mangrove habitat and the deforestation of 13 ha.
We describe the value in reporting tree mortality as a means to rank severity of particular incidents. Severity was dependant on oil type, in the first instance. We ranked crude oil spills as the most damaging, while bunker fuel oils caused the least impact on mangroves. Dispersants were used during the four major shipping incidents, but in neither instance were the environmental benefits, or otherwise, reported.
The response strategy of applying dispersants to oil slicks offshore did not appear to detrimentally influence mangroves impacted later by remaining oil which deposited on roots and sediments Our assessment of relative biological impacts found that dispersant use did not appear to add to the affect of oil on mangrove habitat. In most instances, the application of dispersants on oil slicks was reportedly made offshore, away from mangroves. The response strategy of applying dispersants to oil slicks offshore did not appear to detrimentally influence mangroves impacted later by remaining oil which deposited on roots and sediments.
Plate 3a, b, c, d.Australia-wide surveys of oil spill incidents affecting mangrove habitat showed overall impacts to be variable and site-specific.

A.Several kilometres of a previouslyRhizophora-dominatedshoreline near Cape Flattery, Queensland, had little recovery five years after oiling. There were indications of early recovery with good recruitment, but at the time of this survey, the site was degrading rapidly since trees killed by the original oiling had decomposed and become mobile with tidal and wave movements along the exposed shoreline. The moving debris were scouring the surface and killing the seemingly-established young seedlings, disrupting any chance of recovery.

B.In Torres Strait, after theOceanic Grandeurspill in 1970, trees were not killed along foreshore margins but there were odd patches of trees killed. In these few instances, forest recovery appeared to have been complete after 25 years since the canopy had reformed with younger trees ofRhizophorawithin the larger surviving trees. Oil was not detected in sediments in 1996.

C.In Botany Bay, New South Wales, patchesof Avicennia marinatrees killed by crude oil from theWorld Encouragementspill in 1979, and others, were colonised by numerous recruits in 1996. At the time, oil was detected in sediments but the origin of this oil was uncertain. In any case, height growth appeared stunted by the presence of oil in sediments.

D.An example of an incident map for the Botany Bay spill. Incident maps were prepared for each site, showing among other things, the extent of mangroves, areas of oiling (as far as recorded), the areas of dead trees, and any sample sites.

-View incident map in greater detail

- Contents page

April 7, 2010