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

By David Williams and Miles Furnas - posted Friday, 25 January 2002


Run-off of sediment and nutrients to the Great Barrier Reef has increased several-fold as a result of past and current land-use practices. There is significant concern that coastal ecosystems in the Great Barrier Reef World Heritage Area (GBRWHA) are being adversely affected as a consequence of this increase.

Supply of Pollutants

Most pollutants from the land are delivered to the GBRWHA during major flood events. Episodic high inputs of particulate and dissolved matter during flood events are an important and natural part of the ecology of the Great Barrier Reef and the associated continental shelf and estuarine environments.

Flood plumes are generally constrained to within 20km of the coast by prevailing south-east winds. Under unusually calm or northerly wind conditions, plumes can travel further offshore but do not persist. Because of the behaviour of flood plumes and the maximum depth of sediment re-suspension by non-cyclonic waves, significant adverse effects of land-based inputs on the GBRWHA are likely to be restricted to nearshore areas – broadly within 20km of the coast and in waters less than 20m deep.

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Sediments

Run-off from the land delivers nutrients, sediment and various chemical pollutants to the GBRWHA. Because of documented impacts on reefs elsewhere in the world, most attention has focused on the potential impacts of enhanced sediment and nutrient delivery to the GBRWHA.

The most recent estimates of change in sediment run-off from the land since 1800 is a 3 to 4-fold increase. Many biologically active and toxic trace elements in agrochemical products bind to soil and sediments. Nearly half of the nitrogen (40 per cent) and most of the phosphorus (80 per cent) delivered to the GBRWHA is associated with fine sediment carried by run-off. Most of this sediment is deposited within 10km of the coast in northward facing bays (e.g. Bowling Green Bay, Trinity Bay, Princess Charlotte Bay).

The significance of the increased sediment delivery from the rivers is subject to active debate. While delivery has increased, sedimentologists argue that the increased sediment supply will not increase sediment accumulation or turbidity at most coastal coral reefs, because these factors are not currently limited by sediment supply. Turbidity in nearshore areas is primarily caused by wind-driven re-suspension of bottom sediment. Most of this sediment has accumulated over the past 5-6000 years, during which the sea has remained close to its current level.

Thriving coral reefs with high coral cover, and in some cases high diversity, occur in episodically turbid nearshore waters of the GBR. Deposition of sediments near river mouths may, however, threaten seagrasses and there are anecdotal, but unconfirmed accounts of coastal coral reefs in the Wet Tropics being buried by sediment. It’s not clear whether this happened to active reefs or reefs where corals had already died as a result of other causes.

Significant increases in sedimentation on nearshore coral reefs would be likely to cause changes in community structure and create less favourable habitats for hard corals, zooxanthellate soft corals and calcareous coralline algae. Loss of reef structure to sediment infill may lead to a reduction in numbers of herbivorous fish and a subsequent increase in macroalgae.

Nutrients

Most of the nutrients (principally N and P) required by the pelagic and reef communities of the GBR are derived from recycled biological material. Terrestrial run-off is the largest external source of nutrients to inshore regions of the GBRWHA. The most recent estimates of nutrient runoff places average annual exports of phosphorous and nitrogen at 6 to 10-fold and a 2-fold above 1800 levels, respectively. The extent to which this nutrient run-off has increased concentrations of nutrients in the marine environment, and the nearshore zone in particular, is uncertain.

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The growth of phytoplankton and seagrasses in GBR waters appears to generally be constrained by the availability of nitrogen, rather than by phosphorus or silicate. The nitrogen most immediately available to plants is dissolved inorganic nitrogen (DIN), primarily ammonia and nitrate. Land use increases the export of DIN to the GBR.

It was long thought that the increased nutrients in run-off stimulated the growth of large algae (or macroalgae) that would displace corals on nearshore reefs. We now know that many factors influence the interaction between corals and macroalgae. Algae quickly re-colonise disturbed surfaces, including corals that are killed by other factors such as freshwater, high water temperatures or crown-of-thorns starfish. If algae are established, the regrowth of hard corals can be slowed or disrupted. Algal cover on reefs is often related to the abundance (or absence) of herbivores, especially fish. Factors which control the abundance of herbivores on nearshore reefs of the GBR are not well understood, but fishing is not a direct problem as it is in other tropical regions.

Experimental studies exposing corals to artificially high levels of nutrients have demonstrated direct effects on corals including changes in coral growth and calcification, disruption of reproduction (embryo development, fertilization rates) and changes in settlement success of planulae. Most of these changes are small and the overall effect is likely to be subtle.

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.

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This paper is an edited version of a review by the senior author, available at (www.reef.crc.org.au/pdf/WQdocAugust01.pdf).



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About the Authors

Dr David McB. Williams is Deputy Chief Executive Officer (Research) at the CRC Reef Research Centre, Australian Institute of Marine Science.

Dr Miles J. Furnas is chief researcher of the Assessing Land-Based Threats & Impacts (C2) program at the the CRC Reef Research Centre, Australian Institute of Marine Science.

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