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Fate and effects
of oil and dispersed oil
on mangrove ecosystems in
Australia
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AUSTRALIA-WIDE SURVEYS
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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.
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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.
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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.
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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.
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Plate 3a, b, c, d. Australia-wide surveys of oil spill incidents affecting
mangrove habitat showed overall impacts to be variable and site-specific.
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A. Several kilometres of a previously Rhizophora-dominated
shoreline 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.
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B. In Torres Strait, after the Oceanic Grandeur
spill 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 of Rhizophora within the larger surviving trees. Oil was not
detected in sediments in 1996.
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C. In Botany Bay, New South Wales, patches
of Avicennia marina trees killed by crude oil from the World
Encouragement spill 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.
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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
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