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Fate and effects
of oil and dispersed oil
on mangrove ecosystems in
Australia
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INTRODUCTION
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Oil is occasionally spilled into the sea and coastal waters in huge volumes
as a result of accidents in regular operations of the petroleum and shipping
transport industries. If this oil is washed ashore, it can cause significant
damage to coastal marine ecosystems, particularly those in the intertidal zone.
In Australia, mangroves dominate intertidal shorelines so they are the habitat
at greatest risk. Mangrove forests are extremely vulnerable to oil spills since
oil deposits on above-ground breathing roots, subsurface feeding roots, and a
myriad of associated fauna.
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Planthouse and field studies were used to assess immediate and
short term impacts
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Oiling of tree roots and sediment may result in tree
death, but oil deposits also cause depressed growth of surviving trees as well
as affecting seedling recruitment and recovery of surviving animals. In severe
cases, there are immediate and catastrophic impacts with the death of marine
fauna and trees within days and months of oiling. In the longer term, this
involves subtle and sublethal effects on both survivors and new recruits.
Sublethal impacts may persist for decades and be manifest in reduced forest
canopies (by 20-30%) and partial loss of habitat, but effects on forest function
and sustainability in the longer term are largely unknown.
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This project addresses these issues by assessing short and long term impacts
on mangroves of oils and dispersants used in Australian waters, particularly
regarding their effect on predominant mangrove species in this country;
Avicennia marina and Rhizophora stylosa. To assess long term
effects, we surveyed sites around Australia where mangroves had been affected by
larger oil spills. We used planthouse and field studies to assess immediate and
short term impacts, and to rank toxic effects.
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Plate 1.
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Tidal waters flooding through foreshore trees of Rhizophora
stylosa and some Avicennia marina at the seaward margin of Port
Curtis, Central Queensland. Mangroves are vulnerable to oil spills because oil
floats into these forests and strands amongst roots and sediments, killing and
damaging trees and fauna, and persisting in sediments afterwards.
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Plate 2a, b, c,
d. Studies were used to assess both longer and short term
impacts of large oil spills on Australian mangroves.
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A.
Surveys were
conducted around Australia using standard criteria to establish the baseline
national status of all known mangrove habitats affected by large oil spill
incidents in this country. Shipping incidents have been the greatest cause of
oil impacts on mangroves around Australia. For example, in 1970, the Oceanic
Grandeur lost around 10,000 tons of crude oil after rupturing its hull on an
uncharted rock in the shipping channel off Wednesday Island, Torres Strait. In
this area, dense mangroves occupy more than half the shoreline.
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B. Field
trials in Port Curtis, Queensland, were used to quantify short term impacts of
large experimental spills on mature mangroves. Plots were dominated by
Rhizophora trees, and the characteristic tangle of above-ground roots
made work conditions difficult but it was shady and there were plenty of seats.
Replicated oiled and control plots were monitored for two years after oiling.
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C. Experimental plots, ~35 m2 in area each, were treated with
oil and dispersed oil under conditions which simulated large oil spills by
applying approximately 200 L of oil to each plot (5 L.m-2) while
tidal waters rose.
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D.
Planthouse experiments were used to quantify the
range of short term responses of four common mangrove species to a range of oil
types and dispersed oils.
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