Impacts of terrestrial run-off on the Great Barrier Reef World Heritage Area

It has been suggested that modern crown-of-thorns outbreaks may be linked to increased run-off of terrestrial nutrients into reef waters – the "terrestrial run-off hypothesis". This could be a result of climate related variations in natural run-off or changes in land use. Evidence for a linkage between extreme rainfall events and the last three outbreaks of crown-of-thorns starfish on the Great Barrier Reef has recently increased. A relationship between outbreaks and changes in land use has neither been demonstrated nor disproven.

Impacts of Pollutants other than Sediments and Nutrients

Pollutants exported to the GBRWHA (other than unnaturally high levels of nutrients and sediment) include insecticides and herbicides, heavy metals and polyaromatic hydrocarbons. Studies to date have generally found low concentrations of these pollutants, indicative of a relatively unpolluted environment. Exceptions are found at coastal sites adjacent to ports and harbours, urban centres and intensive agricultural activity. Elevated pollutant concentrations are generally the consequence of effluent discharge, urban stormwater and agricultural and industrial runoff. There is concern, however, that much of the data on pollutants in the GBRWHA are dated and a call has been made for an update of information on the distribution and impact of potential pollutants.

The organic pollutant of greatest current concern for potential impacts on the GBR is the herbicide diuron [3-(3,4-dichlorophenyl)-1,1-dimethylurea, also known as DCMU]. Significant levels of diuron, used to control weeds, have been found in nearshore sediments and seagrass beds in the high rainfall (Wet Tropics) coast between Port Douglas and Townsville and near the Fitzroy River. The levels of diuron in some of these sediments have been experimentally demonstrated to interfere with photosynthesis of seagrasses.

Difficulties of Detecting Impacts

There are difficulties in making definitive statements regarding impacts of terrestrial runoff on GBRWHA habitats. They include: the acute and relatively frequent natural disturbance of these habitats; the relatively short duration of monitoring programs (20 years or less); the lack of unambiguous pristine controls for comparison because many of the major changes in land use occurred before monitoring of coastal and reef ecosystems was initiated; and a poor understanding of the capacity of the sediments and waters of the continental shelf to buffer and absorb cumulative changes. In particular, the fate and fluxes of nutrients are poorly understood.

Status of Coral Reefs

Most of the 3,000 reefs in the GBR are more than 20 km from the coast and unlikely to be directly influenced by run-off. However, about 750 reefs are located within 10 km of the coast in the zone strongly affected by land run-off. Nearshore reefs between Port Douglas and Gladstone are at risk, particularly those between Port Douglas and Hinchinbrook Island, and between Bowen and Mackay.

There is anecdotal information that a number of coastal reefs are ‘not what they used to be’ in the Wet Tropics region between Port Douglas and Hinchinbrook Island. A mismatch between substantial historical and limited present reef-building capacity at a number of sites on the wet tropical coast and Whitsunday Islands has been demonstrated.

Coral reefs can normally recover from a variety of acute disturbances such as cyclones, floods, bleaching and predation by crown-of-thorns starfish. One of the greatest concerns is that nearshore reefs may be stressed by impacts of run-off to a point where they cannot recover from a major disturbance. The effect of chronic, sub-lethal stresses to reef systems are difficult to recognise and not well understood. Recognizing such stresses and their impact is a major research activity at present.

Status of Seagrasses

Seagrasses grow in coastal areas and are vulnerable to run-off. Potential impacts from changes to land use include the reduction of light for photosynthesis, burial by sediment and poisoning by herbicides.

Most seagrasses in episodically turbid nearshore regions of the GBR are relatively tolerant of low underwater light levels. These seagrass beds are naturally dynamic and responsive to acute disturbances such as floods and cyclones. It is possible that enhanced run-off of nutrients, sediments and pesticides will increase chronic stress and make seagrasses more vulnerable to acute disturbances.

Because seagrass beds are naturally dynamic, there is considerable uncertainty about their status. In recent decades, there has been a major decline in the number of dugongs in the GBRWHA. Because dugongs feed exclusively on seagrass, there is concern that changes in the distribution and composition of seagrass meadows could be contributing to the loss.

Status of GBR Shelf Waters

Reef waters are normally characterised by very low concentrations of dissolved and particulate nutrients (µmoles per litre or less) and low levels (< 1 µg L^-1) of suspended particulate matter. Most of the nutrients needed by organisms on coral reefs and in the water column are obtained by recycling nutrients that are already in the environment. Nutrient and plankton concentrations vary naturally between regions of the GBR. They are typically higher in shallow coastal waters where sediments are an important source of nutrients. This is also where land run-off has its greatest effect.

Despite increasing sediment run-off and high levels of fertiliser use in catchments adjoining the GBR, studies have failed to detect increases in the nutrient and chlorophyll levels in coastal and shelf waters in the past 25 years. This suggests that natural processes are dispersing or removing nutrients delivered to the reef ecosystem at rates similar to inputs. Changes in water quality caused by run-off will be gradual and difficult to measure. These changes and their effects on the environment will be equally slow to reverse.