Oil Spill Study Helps Protect Coastal Mangroves

A research project funded by Australia's offshore petroleum industry will help to reduce future impacts of large oil spills on mangrove habitats in the Great Barrier Reef World Heritage Area. While the coastline has remained relatively free from major spills in the past - with strict safeguards for special protection - most authorities are still concerned over a possible shipping accident in the future.

The project, funded by the Australian Petroleum Production and Exploration Association, is being undertaken by CRC Reef researchers working at the Australian Institute of Marine Science.

Plots of oiled mangroves near Gladstone are being studied to measure ecological changes.

A major part of the project involves short-term monitoring of a simulated oil spill on mangrove habitat. Crude oil, spread over nine small plots of mature mangrove habitat, is being studied to measure various changes on natural ecological processes. The plots, in an area already approved to be reclaimed for port development, are located in Port Curtis, near Gladstone. The 'manipulative' scientific method has been chosen over laboratory simulations to better understand how mangrove habitat responds over time to crude oil, clean-up treatments and post-spill recovery.

CRC project leader Dr Norm Duke, a research associate from AIMS, said industry advisors on the research team were reluctant to damage natural mangrove habitat. "Field work was approved on the condition that the site was already marked for destruction - a reclamation site - and all possible safeguards established to prevent loss of oil and contamination to the nearby environment," he said.

The project also required a suite of approvals from government agencies (Queensland Departments' of Primary Industries, Environment, Transport; the Great Barrier Reef Marine Park Authority and the Australian Maritime Safety Authority) in conjunction with local industry and the Gladstone Port Authority.

"There are more than 4,600 square kilometres of mangroves along the Queensland coastline which play a vital role in the biological diversity of Barrier Reef waters, and the Australian community expects a high level of protection from the threat of oil spills," said Dr Duke.

Mangrove trees are an important part of the marine food chain. Their growth and survival is essential in sustaining the life of all other organisms that live in the mangroves. The leaves fall onto the sediment providing nutrients for microorganisms, crustaceans and molluscs. Many commercial and non-commercial fish and prawn species live in mangroves, feeding on small organisms in their juvenile stages. Here they mature rapidly, with an abundant energy supply and shelter from the hungry hoards of larger fauna.

Mangroves offer a smorgasbord of food and provide a refuge from predators for many species. These coastal forests are linked to seagrass beds and reefs by legions of creatures moving between these environments. Tides and currents exchange nutrients with the mangroves, enriching the seagrass beds and coral reefs essential to animals such as dugongs and turtles. It is the diversity of mangrove environments that makes them so vulnerable to large oil spills.

Crude petroleum consists of hundreds of chemical substances, including alkanes and Polycyclic Aromatic Hydrocarbons (PAHs). These compounds have a high affinity for organically rich sediments which is why they accumulate so readily in the mangrove mud. Sand and gravel have little ability to absorb these toxins.

Some mangrove trees have complex breathing roots that grow out of organically rich and oxygen depleted sediment. Oil, washed in with tidal currents, can block the openings of the air breathing roots. The lower underground root systems can also be damaged by oil entering nearby crab and worm burrows. It coats the stems and leaves of trees interfering with their respiration and salt balance. The trees die and re-colonisation by mangrove seedlings can be severely inhibited by trapped oil. Once trapped in the mangrove sediment, these PAHs become an indivisible and malignant part in the marine ecosystem for decades. Most PAHs are toxic, mutagenic or carcinogenic, and these effects can be passed through the food chain.

Industry, regulatory bodies and scientists acknowledge that contamination of coastal ecosystems with polycyclic aromatic hydrocarbons is a serious environmental problem.

While drilling for oil is prohibited in the Great Barrier Reef World Heritage Area, crude oil and petroleum is regularly shipped through the Great Barrier Reef inner passage. The Bureau of Transport and Communications Economics estimated in 1991 that the risk of a major spill from shipping along the total Australian coastline was as high as 49% in any five year period. The statistics bear a dark prophecy as the figure increases to 93% in any 20 year period.

With grave concern over the effects of a major oil spill within the Great Barrier Reef area, the International Maritime Organisation has taken steps to protect this important region. The Barrier Reef inner passage has now been designated as a 'Particularly Sensitive Area' and all ships carrying oil through the area must be guided by Australian registered pilots. The Commonwealth Government is also legislating to make it compulsory for all large south-bound ships to travel along the outer route, outside the Reef, and updating operational systems such as navigation aids. But, despite the best intentions and practices of industry and regulatory agencies, the probability is that a major oil spill will occur in the Great Barrier Reef World Heritage Area sometime in the future.

In the event of such a spill, dispersion and clean-up procedures do not come with a guarantee. They often cannot prevent wind driven oil slicks from washing ashore. Were this to happen, immense damage could be done to intertidal and subtidal ecosystems such as coral reefs, seagrass communities and mangroves.

In the past, scientists have looked at oil spill studies from other countries to gauge impact and recovery. In 1968, millions of litres of crude oil escaped from by a stricken tanker - and again from a leaking oil refinery in 1986 - onto the Caribbean coast of Panama, devastating fringing coral reefs and mangrove forests. Dr. Duke and colleague Dr. Kathy Burns, also at AIMS, studied the oil damage to the Bahia Las Minas mangrove forests. They observed significant deforestation and, even ten years afterwards, continued to find mangroves contaminated with oil. The list of detrimental effects on corals included decrease in coral cover, low recruitment, impaired reproduction, and low growth rates. After years of chronic pollution, slicks continue oozing from oil deposits trapped in these mangrove sediments.

A number of lessons have been learnt from these tragedies but the Australian scientists are discerningly cautious - they say it would be unwise to simply transpose the findings of overseas research to the complex Great Barrier Reef ecosystem.

"The Gladstone study depends on simulating an actual spill using enclosures designed to allow free ebb and flow of tidal waters, whilst leaving oil inside to settle on mangrove sediments and exposed roots.

"The challenge is magnified by field work in the mangrove environment itself. Common mangrove species often grow in dense stands up to 6-8 meters tall. They have thick, entangled above-ground roots making headway exceedingly difficult," said Dr Duke.

Deep within the bowels of the forest study site, Dr Duke constructed nine experimental enclosures. Each enclosure consisted of two major parts: a 30 metre-long enclosed curtain of vinyl buried 20 centimeters into the sediment, and a tidal oil-gate of vinyl above and below the sediment. Floating absorbent booms surrounded each enclosure to ensure that no oil escaped from the enclosures.

Oil treatments were added to the enclosures to simulate an oil slick arriving from an offshore spill. Samples of mud are being tested regularly for toxic substances and the biological impact will be monitored carefully over the next year. Major studies include litter fall and productivity of mangrove tree canopies; shoot growth; removal of fallen leaves by crabs and the presence and diversity of crabs and other mobile macro-infauna.

Dr Duke is sharing the knowledge gained from this project to help reduce impacts of any future spills and to assist with post-spill recovery. The research is helping industry determine the usefulness of chemical dispersants and the interdependence of plants and animals in mangrove habitats. He believes the proactive nature of this work offers worldwide opportunities to respond with greater understanding and knowledge to save mangroves threatened by large oil spills.

By Deborah Lamb

Central Queensland University student, Deborah Lamb, recently won a $1,000 marine science journalism prize from the CRC Reef Research Centre Ltd for this story.