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


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.

Planthouse and field studies were used to assess immediate and short term impacts 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.
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 marinaandRhizophora 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.
Plate 1.

Tidal waters flooding through foreshore trees ofRhizophora stylosaand someAvicennia marinaat 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.

Plate 2a, b, c, d. Studies were used to assess both longer and short term impacts of large oil spills on Australian mangroves.

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, theOceanic Grandeurlost 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.

B.Field trials in Port Curtis, Queensland, were used to quantify short term impacts of large experimental spills on mature mangroves. Plots were dominated byRhizophoratrees, 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.

C.Experimental plots, ~35 m2in 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.

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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April 7, 2010