Mounting evidence shows dredge spoil threat to the Great Barrier Reef

Dredging at the port of Gladstone. Greens MPs/Flickr

Tonight’s Four Corners on ABC investigates dredging at ports on the Great Barrier Reef, including claims that the federal government is reportedly seeking alternatives to dumping dredge spoil at sea at Abbot Point.

Proponents for the huge expansion of coal ports along the coast of Queensland, including at Abbot Point, have asserted
that dredging and dumping of dredge spoil at sea won’t harm the Great Barrier Reef. But this runs counter to the growing weight of scientific evidence.

The latest healthcheck of the Great Barrier Reef confirmed that much of the Reef is in a bad state, and getting worse. Leaving aside the challenges of global warming and ocean acidification linked to the extraction and burning of fossil fuels, key issues for the expansion of coal ports in Queensland are the amount of dredging and the dispersal of sediment beyond dredging and disposal sites.

Dredging in the marine park is not new, but the scale of proposed dredging and dumping of sediment along the Queensland coast is vastly greater than before. Sediment from dredging at five major ports could soon exceed the amount coming from land.

Today, runoff of sediment from all of the rivers in the Great Barrier Reef catchment averages about 9 million tonnes each year, depending obviously on year-to-year variation in rainfall. Historically, before land-clearing, it was one-third that level — close to 3 million tonnes.

In comparison, proposed amounts of dredging for port expansion over the next few years along the coast from Cairns to Gladstone amount to more than 100 million tonnes, cancelling out ongoing efforts by farmers and land managers to reduce the impacts of runoff from agriculture.

Why is UNESCO concerned about ports?

The World Heritage Status of the Great Barrier Reef recognises its unique size, complexity and integrity. The size of Italy, the Barrier Reef Region is a mosaic of interconnected habitats, only some of which are covered by corals.

Consequently, the Great Barrier Reef Marine Park Act of 1975 aims to protect the “region”, not just the 7% of the Park that comprises shallow coral reefs.

A decade ago, the Great Barrier Reef Marine Park Authority collaborated with scientists to map 70 types of habitat, including 40 that have little or no corals, throughout the Park. Based on this information, the current management arrangements were legislated by the Commonwealth in 2004 to ensure that that least 20% of every habitat type was protected by green (no-fishing) zones.

The habitats of the Great Barrier Reef do not operate on their own; instead, they are vitally connected. The current sophisticated zoning explicitly recognises this. For example, some habitats such as sea grass beds serve as nurseries for species that spend their adult life on coral reefs, and many iconic species migrate in and out of areas that have, or don’t have, corals.

Proponents of dredging and dumping at sea argue that it won’t affect the coral reefs, because most of them are offshore.

The inference is that dredge spoil can be dumped on or near lesser habitats, such as coastal coral reefs, or flat bottom habitats such as sea grass beds. This argument is counter to the 1975 Act and to Australia’s commitments to maintaining the integrity and intactness of the World Heritage Area. It is one of the major reasons why UNESCO is edging towards listing the Barrier Reef as being “in danger”.

Inadequate monitoring

Long-term monitoring of dredging impacts so far has been woefully inadequate. Monitoring tends to focus on the short-term impacts at each inshore dredging site, rather than the longer-term effects of dumping sediment elsewhere.

Similarly, calculations for environmental offsets are based on the modest amount of sediments spilled during dredging, and not on the vastly greater amounts that are dumped offshore.

In the absence of adequate monitoring, it is easy to deny the environmental impacts of dredging on the Great Barrier Reef, and to claim that they are manageable.

But there is a substantial scientific literature from around the world which shows that sediment from dredging can smother and kill marine species. Sediment also reduces light levels, causes physiological stress, impairs growth and reproduction, clogs the gills of fish, and promotes diseases. The extent to which this is already occurring on the Great Barrier Reef due to dredging is poorly understood.

A case in point is the fringing reef in Geoffrey Bay on Magnetic Island, which lies just 3 kilometres from the shipping channel for Townsville port. In 1987, prior to major dredging of the channel, the Queensland Department of the Environment established a popular reef walking trail in Geoffrey Bay, designed as a recreational and educational amenity. At low tide, visitors could to walk along the reef flat and use a map with designated viewing points to experience a diverse mix of species including branching and brain corals.

Today, the walking trail has been long abandoned, and the intertidal reef is covered in a layer of mud. Whatever the complex reasons for these declines, near-shore fringing reefs and sea grass beds along the Queensland coast are fast disappearing and they can ill-afford further cumulative impacts from unprecedented dredging programs.

How far does dredge spoil travel?

The proposed offshore dredging dump site for Abbot Point is a tiny area, just 2 kilometres by 2 kilometres. However, fine sediments don’t sink like a stone, they disperse over long distances.

Dispersal of suspended particles by currents, wind and tides has been extensively studied on the Great Barrier Reef for several decades, beginning with pioneer work on the spread of the larvae crown-of-thorns starfish. No scientist would accept assertions that sediment spoil doesn’t disperse long distances.

For example, a recent modelling study predicts that fine sediments in suspension can spread up to 200 kilometres from coal ports within 90 days.

In their planning application for dredging, North Queensland Bulk Ports have used their own model of dispersion of sediments to claim that dredge spoil stays put. Among its numerous technical flaws, the model omitted any influence of the East Australia Current, or cyclones.

Because monitoring has been sparse, direct evidence for dispersal of dredge spoil on the Great Barrier Reef is scant. A study published this year established that toxic hydrocarbons from coal dust were already spread across the width of the continental shelf, in water and sediment samples collected in 2009 and 2010. Presumably levels of these chemicals will rise further as coal exports grow.

The future

The Great Barrier Reef Outlook Report, released this month by federal Environment Minister Greg Hunt, states that “there will be a need for increased capital and maintenance dredging”. However, the scientific evidence for the onward decline of the Barrier Reef Region is indisputable.

Many people would argue that it is far too risky to develop huge new coal mines, build the world’s largest coal ports, dump unprecedented levels of dredge spoil at sea, and still have aspirations to protect the Great Barrier Reef, maintain its World Heritage status, and secure reef-dependent tourism and fishing.