Pressures

Changing land use

Although population growth and urban expansion in Queensland has been rapid, the northern Queensland coast still remains relatively sparsely populated (Environmental Protection Authority 1999). Only 700,000 of the State’s 2.9 million residents live in the coastal areas adjacent to the Great Barrier Reef World Heritage Area. Despite this low population pressure, extensive land modification (land clearing) has occurred over the last 200 years since European settlement (The Condition of River Catchments in Queensland 1993). Today, 80% of the land area of catchments adjacent to the Great Barrier Reef support some form of agricultural production (Gilbert in press). To place Queensland land-use and vegetation clearing activities into perspective, more than 50% of the State’s original 117 million hectares of woody vegetation has been cleared primarily for agricultural purposes since European settlement (Environmental Protection Authority 1999). Consequently, run-off resulting from land-based agricultural activities (eg. cattle grazing, vegetation clearance and intensive cropping) and urban development is the primary anthropogenic influence on water quality in the Great Barrier Reef. (Bell 1991; Moss et al. 1992; The Condition of River Catchments in Queensland 1993; Brodie 1997).

Sugarcane cropping, Great Barrier Reef catchment.

Grazing of cattle for beef production is the largest single land use on the Great Barrier Reef catchment, with cropping (mainly of sugarcane) being a significant agricultural industry in coastal areas between Bundaberg and Port Douglas. Other significant catchment land uses include mining of coal and various metals.

Cattle grazing

Approximately 4,500,000 beef cattle graze in Great Barrier Reef catchments, with highest stock numbers in the Fitzroy and Burdekin catchments. Beef grazing on these large, dry catchments adjacent to the Great Barrier Reef Marine Park has resulted in extensive tree clearance and over-grazing, especially during drought conditions. This has resulted in widespread soil erosion (The Condition of River Catchments in Queensland 1993). The majority of the Great Barrier Reef catchment is used for rangeland beef grazing. This development has involved wide-scale clearance of woodland vegetation, particularly Brigalow, for conversion to pasture (Gilbert in press). The principal consequence for the Great Barrier Reef from the introduction of beef grazing on catchment lands stems from increased soil erosion (Ciesiolka 1987). Soil erosion increases arise from woodland removal; overgrazing, (especially in drought conditions, where vegetation cover falls below 60%); and streambank erosion when cattle have direct access to streams (Finlayson and Brigza 1993).

Estimates of the increase in soil erosion from natural conditions to modern conditions (Ciesiolka 1987; Lawrence and Thorburn 1989; Rayment and Neil 1997) range from:

  • 0.9 tonnes per hectare per year on catchments with minor gully erosion;
  • 1.6 tonnes with one active gully; and
  • 27-30 tonnes with severe gully erosion.

Intensive agriculture

The area under sugarcane cultivation in Great Barrier Reef catchments has increased steadily over the last 100 years with a total of 400,000 hectares at present (Figure 5). Cultivation areas are located near the coast in many of the lowland areas of catchments. Many environmental impacts have not been accurately quantified, but in the Herbert River catchment, clearing for sugarcane cultivation has significantly decreased the area of freshwater wetlands. Fertiliser use is closely linked to sugarcane cultivation and with continuously expanding cultivation fertiliser use has risen rapidly since 1950 (Figure 6). The use of pesticides (herbicides, insecticides and fungicides) is also significant in areas of intensive crop cultivation (Hamilton and Haydon 1996).

Figure 5. Increase in Queensland land area used for sugar cultivation from 1930 to 1996 (Gilbert in press.

 

Figure 6. Increases in the use of nitrogen and phosphorus fertiliser on the Great Barrier Reef catchment (Pulsford 1996).

In summary, sugarcane cultivation on the Great Barrier Reef catchment probably contributes on average about 20,000 tonnes of Nitrates per year to the Great Barrier Reef. This is about 25% of the total load (Moss et al. 1992; Rayment and Neil 1997). In areas of intense sugarcane cultivation, such as the Wet Tropics, it contributes the majority of the dissolved inorganic nitrogen (nitrate and ammonia) transported by the rivers (Hunter 1997; From land to river to reef lagoon 1997; Mitchell et al. 1997). This contribution is rising with increasing cane area and fertiliser rates. Elevated particulate Nitrates and nitrate concentrations in the Tully River over the last 13 years are attributed to increased soil loss in expansion areas and increased fertiliser use associated with increased cane cultivation and increased banana cultivation (Mitchell et al. 2001).

Other major crops grown on the Great Barrier Reef catchment, are cotton and horticultural crops (particularly bananas), tree crops such as mangos and lychees, and vegetable crops such as tomatoes. Nitrogen fertiliser application rates on such crops can be high, eg. for bananas approximately 400 kg/ha/year of nitrogen. Loss of fertiliser from bananas follows a similar path to sugarcane grown in the same area (Prove et al. 1997) and presents a similar, although slightly smaller source due to the smaller areas involved (From land to river to reef lagoon 1997). Cotton grown on the Fitzroy catchment uses nitrogen at rates of about 150 kilograms/ha/year and considerable loss of nitrogen from cotton cultivation has been measured downstream from the cropping areas (Noble et al. 1997).

Coastal Aquaculture

Coastal pond-based aquaculture now occupies about 450 hectares on the Great Barrier Reef coast. This area is dominated by penaid prawn cultivation with much smaller areas of finfish cultivation. The discharge from aquaculture using present techniques contains high concentrations of suspended solids and nutrients (Nitrates and Phosphates). The loss of nitrates and phosphates in the discharge per hectare of pond is about ten times that lost from an equivalent area of sugarcane cultivation (with GCTB) (Brennan 1999). The use of settlement ponds and cleanup ponds containing algae, bivalves and fish can significantly reduce the levels of suspended solids and nutrients in pond discharges (Prinsloo et al. 1999; Troell et al. 1999). With the introduction of these techniques in new aquaculture farms and the upgrading of existing farms, discharge of sediment, nitrates and phosphates from coastal aquaculture should be minimised.  

Discharge of prawn pond effluent can also lead to changes to local salinity regimes. Recent research conducted by the Cooperative Research Centre (CRC) for Aquaculture has found that regular discharge of highly saline effluent from prawn farms affects the salinity of the water in estuarine areas and in the mixing zone. The long-term effects of these changes are yet to be determined (Trott and Alongi 1999). Aquaculture also presents the risk of release of disease to the environment. The key disease risks include the accidental introduction of exotic parasites and pathogens to wild stock and other marine species; undetected importation of infectious contagions in prawns and prawn feeds; and the magnification of endemic diseases associated with the intensive culturing of aquaculture species. Pathogenic organisms from Queensland prawn farms comprise a wide variety of taxa. These include the pathogenic bacteria Vibrio spp. (Vibrio anguillarum, V. harveyi and V. alginolyticus) (Smith 1993).

Wetland loss

The area of wetlands has been dramatically reduced by land clearing activities

The watersheds of rivers in north and central Queensland have been extensively modified since European settlement through land clearing followed by forestry, mining, urbanisation and agriculture. Clearing of forest and woodland has continued throughout the last 130 years with early loss of rainforest areas in coastal lowlands and on the ranges and tablelands, as well as loss of coastal wetland forest and extensive loss of open woodland. In the Herbert catchment, Melaleuca wetlands have been reduced in area from 30,000 hectares in pre-European times to less than 5,000 hectares in 1996, while in the lower Johnstone catchment, a 78% loss occurred between 1951 and 1992. In the Fitzroy catchment, during the brigalow (Acacia harpophylla) woodland clearance schemes (1950 to 1975), approximately four million hectares of brigalow woodland were cleared for conversion to grasslands for beef cattle grazing. Forest and woodland clearing in Queensland has been assessed using satellite imagery.

Pollution impacts

Nutrients

Elevated nutrient concentrations result in a range of impacts on coral communities (Tomascik and Sander 1985; Ward and Harrison 1997; Koop et al. 2001) and under extreme situations, can result in the collapse of the coral reef community (Smith et al. 1981; Lapointe and O'Connell 1989, Van Woesik et al, 1999). There a number of ways in which elevated nutrients affect corals:

  • Elevated nutrient concentrations promote phytoplankton growth which in turn supports increased numbers of filter feeding organisms such as tubeworms, sponges and bivalves, which compete with coral for space (Smith et al. 1981).
  • Enhanced levels of nutrients may also result in blooms of macroalgae that may overgrow coral structures, out-competing the coral polyps for space and shading the coral polyps. This has been demonstrated in numerous coral reef systems around the world including the Red Sea (Walker and Ormond 1982) and in Barbados (Tomascik and Sander 1985), with the best documented example in Kaneohe Bay, Hawaii (Smith et al. 1981).
  • Excessive phosphorus concentrations can also result in coral colonies with less dense, and hence weakened skeletons, which make colonies more susceptible to damage from storm action (Wilkinson 1996). Neither macroalgae nor most filter feeders are reef building organisms, the reduction in coral reef growth will likely result in the erosion of coral reef structures.
  • Elevated nutrients have also been demonstrated to inhibit fertilisation rates and embryo formation in the corals Acropora longicyathus and A. aspera, as well as causing direct coral mortality (Ward and Harrison 1997; Koop et al. 2001).

Recent comparisons of inshore reefs in the relatively undisturbed far northern Great Barrier Reef with inshore reefs in the wet tropics region have indicated that considerable differences exist between reefs in the two areas. Coral reefs adjacent to heavily impacted catchments have lower coral biodiversity, lower rates of coral recruitment and different coral community structure compared with reefs in relatively pristine areas. This difference has been linked to changes in water quality caused by human activity.  For more information about the condition of coral reefs see Environmental status – Corals.

Sediments, turbidity and marine snow

Regardless of whether the cause is natural or the result of human activity, there is clear evidence that prolonged exposure to levels of terrestrial sediment and organic matter in excess of normal conditions, can kill affected coral reefs through:

  • smothering and burial when particles settle out (sedimentation);
  • reducing light availability (turbidity) and potentially reducing coral photosynthesis, growth and reproduction; and
  • altering the ecology and nutrient dynamics of reef surfaces (Rogers 1990; Anthony 2000; Anthony 1999).

Increased sediment loads combined with eutrophic conditions may enhance the formation of marine snow, which may also impact corals (Fabricius and Wolanski 2000).

Corals and other small sessile invertebrates have to expend considerable energy to rid themselves of large marine snow particles compared to the normal, smaller ‘clean’ sediment particles of oligiotrophic waters (Fabricius and Wolanski 2000). This creates a metabolic energy drain, which may reduce reproductive capacity, and the organisms capacity to grow and to cope with additional stress factors.

Effects on corals

Offshore coral reef environments are generally regarded as being adapted to low turbidity and low-nutrient conditions. In contrast, nearshore and coastal reef systems have evolved in relatively turbid environments where suspended sediment and turbidity are influenced more by local wind and wave regimes than by sediment supply (Larcombe and Woolfe 1999). Despite high turbidity levels and sedimentation rates, inshore reefs naturally sustain high and healthy coral cover and diversity, suggesting local adaptation to intense sediment regimes (Ayling and Ayling 1998). One reason for this may be that coral populations from inshore turbid environments have a greater capacity than offshore species to feed and thus obtain energy from sediment particles (Anthony 2000). Energy obtained in this way could balance phototrophic energy reductions caused by shading in shallow turbid waters. However, particle feeding is unlikely to provide a total alternative energy source under consistently turbid waters (Anthony 1999). Sediment smothering in inshore waters can also be prevented in more exposed, high energy areas as water movement removes excessive sediment before it harms coral (Johnson 1996), enabling successful coral growth and recruitment. For more information about the effects of water quality on coral reefs see Environmental status – Corals.

Effects on seagrass

Elevated sediment and nutrient concentrations can also negatively affect seagrass beds. Australian seagrass communities are generally characterised by low ambient nutrient loadings and increased nutrients and water turbidity can adversely affect seagrasses by lowering ambient light levels (Walker et al. 1999). Three major factors cause a reduction in light availability (Shepherd et al. 1989; Walker and McComb 1992; Abal and Dennison 1996):

  • Chronic increases in dissolved nutrients leading to a proliferation of light adsorbing algae including water column phytoplankton, benthic macroalgae or algal epiphytes on seagrass stems and leaves;
  • Chronic increases in suspended sediments leading to increased water column turbidity; and
  • Pulsed increases in suspended sediments and/or phytoplankton blooms that cause a dramatic reduction of water column light penetration for a limited time.

All these will reduce the photosynthetic capability of affected seagrass. For more information about the effects of water quality on coral reefs see Environmental status – Seagrass 

Acid Sulphate Soils

Thirty-five confirmed fish kills have been documented along the north Queensland coast between 1997 and 1998 (Sunfish Queensland Inc. 1999). Nine of these were major events and are expected to have a lasting impact on local regional fishery resources.

A majority of these have been attributed to agricultural developments, however, in some incidences urban development may be the primary cause.  Reduced dissolved oxygen concentrations in the water column was cited as the cause of all incidents. Acidic water draining from acid sulphate soils are generally poorly oxygenated, have a low pH and may contain elevated concentrations of heavy metals (Cook et al. 2000). These conditions have the potential to impact fish habitat and behaviour, although the impact on Great Barrier Reef fauna has yet to be quantified (Cook et al. 2000).

The impacts of changing land use on river flow

Loss of forests, reduction in vegetation cover and increased drainage areas, as well as road networks and hardened surfaces in urban areas continue to produce increased run-off ratios (run-off/precipitation). This causes larger river floods with greater discharge volumes as well as faster, more concentrated discharge patterns. Construction of major dams will result in alterations in the supply of sediment and water as both become trapped behind the dam wall. Generally, dams will decrease the occurrence of high flows and alter the pattern of water delivery. The impact of an altered water regime will result in longer periods of low flow, reduced variability of flows, reduced frequency of small to medium flows and poor quality water from impoundments. Throughout the world, there are numerous examples of river regulation devastating estuarine and marine fisheries resources due to:

  • greatly reduced freshwater flow drastically reducing export of nutrients that forms the basis of food chains;
  • coastal erosion and habitat loss due to reduced sediment supply; and
  • the loss of mangrove habitats due to hyper-saline conditions resulting from restricted freshwater flows.

In the Great Barrier Reef catchment there is a combination of impacts. In the dry season, the presence of dams, weirs, water regulation and irrigation result in a reduced dry season flow and may also act to moderate flows to some extent. About 10% of the average annual discharge from the Great Barrier Reef catchment (75 km3) can potentially be captured in existing large reservoirs (Gilbert in press). However with the onset of the wet season, and extreme flow events, there is the possibility of more water moving off the catchment due to loss of vegetation cover and increased run-off ratios. Downstream effects include larger floods with greater discharge volumes as well as a faster, more concentrated discharge pattern. Ongoing research into Sr/Ca ratios and d18O in corals, has quantified the changes to river run-off into the Great Barrier Reef lagoon (McCulloch et al. 2003).

Effects of Sewage Effluent

Sewage effluent contains many polluting substances including:

  • organic matter capable of causing oxygen depletion in receiving waters;
  • suspended solids capable of causing turbidity in receiving waters;
  • micro-organisms (bacteria, viruses, fungi, protozoa, parasitic worms), some of which may be pathogenic;
  • nutrients, particularly nitrogen and phosphorus compounds;
  • toxic trace metals such as lead, cadmium and chromium;
  • toxic synthetic organic substances such as pesticides and solvents;
  • petroleum oil;
  • detergents;
  • biologically active drug residues such as vitamins and steroids; and
  • litter.
Pollution from land-based sources can affect nearby marine ecosystems, such as increasing the incidence of coral disease

The nutrients - nitrogen and phosphorus - are the pollutants that most threaten the Great Barrier Reef.  Most other substances are reduced to low levels by secondary and tertiary sewage treatment or by prevention of industrial waste entering the sewage system.

The discharge of sewage effluent into the Marine Park is regulated under the Great Barrier Reef Marine Park Act 1975 and Regulations 1983 and the Sewage Discharges from Marine Outfalls into the Great Barrier Reef Marine Park Policy. Marine outfalls are regulated by the GBRMPA permit system. Under present policy, all effluent discharged into the GBRMP must be treated to a tertiary standard (i.e. nutrient removal) prior to being discharged. Secondary treated effluent can be reused for land irrigation or other activities. Outfalls discharging within the GBRMP were required to meet these standards by March 2002.

In the Great Barrier Reef catchment, the majority of urban settlements have secondary treatment facilities that discharge into waterways that drain into the GBRMP, but sewage effluent from these areas still has the potential to impact on the Great Barrier Reef. The GBRMPA has no jurisdiction over these discharges, however it is promoting the reduction of discharge volumes through increased effluent reuse and improved effluent quality. The Queensland State Coastal Management Plan, which came into effect in 2002, requires that where nutrients are identified as an environmental problem, all coastal sewage treatment plants are to upgrade the quality of discharges to coastal waters to nutrient removal standards by 2010. Local Governments must consider the Plan when revising all planning schemes.

The GBRMPA has identified present nutrient loads in inshore waters of the Great Barrier Reef as a problem . The Queensland Environment Protection Agency licenses most island and coastal facilities (those with a capacity greater than 21 equivalent persons), while the Department of Natural Resources and Mines and local government regulate smaller treatment facilities designed for less than 21 equivalent persons. Effluent seepage from smaller systems may be an issue in sandy soils. Those that provide only primary treatment may be inadequate to protect the water quality of the receiving waters of the Great Barrier Reef.

Discharge of wastewater into the Great Barrier Reef Marine Park from industrial installations is regulated under theGreat Barrier Reef Marine Park Act1975and Regulations 1983(in the GBRMP) and the Queensland Environmental Protection Act 1994. Discharges from aquaculture facilities are regulated by State and Federal legislation.

Industrial discharge

Aquaculture is a small but expanding industry that can potentially released nutrient rich discharges into the GBRMP.

The small number of major industrial sites along the Great Barrier Reef coast are generally concentrated near Gladstone and Townsville. Some of these industries discharge wastewater to the ocean and they are controlled under the Queensland Environmental Protection Act 1994 licensing system. Plants being constructed in more recent times have been required to have no ocean wastewater discharge (e.g. the zinc smelter operating south of Townsville).

Aquaculture of saltwater prawns is a small but expanding industry along the Great Barrier Reef coast. Prawns are raised in ponds near the coast and fed processed feed. Unused feed, high in organic matter and nutrients, may be discharged into the ocean. These potentially nutrient-rich discharges are controlled under the Great Barrier Reef Marine Park (Aquaculture) Regulations 2000 and the Queensland Environmental Protection Act 1994 through operating licences. New prawn farms are being required to install systems to minimise the volume of discharge and to reduce the pollutant load in the discharge. Systems using pond filtration through beds of bivalves (e.g. oysters and mussels) and algae are being used as well as filtration through mangroves.

Other Development impacts

Population growth in adjacent urban centres invariably leads to increased pressure for access to the resources of the GBRMP. This becomes a management issue when it results in overuse of certain sections of the GBRMP or where sensitive environments are exposed to excessive human impacts such as damage to corals from anchoring or interference with bird nesting and breeding areas.

The use of section zoning plans allows the GBRMPA to prescribe the allowable and permissible uses of areas of the GBRMP while providing for a variety of recreational opportunities. Plans of Management provide more detailed information on the management arrangements for high use areas or for specific issues such as dugong protection.

The development of new urban centres in sensitive areas, or the unplanned expansion of existing centres, has the potential to adversely affect marine ecosystems in the GBRMP in a variety of ways. Growth in residential nodes often leads to increased demand for marine tourism and recreation infrastructure such as marinas, ferry terminals, safe harbours and jetties, which, if not properly managed, can have adverse impacts on the GBRMP. Likewise, the scale, location and character of individual developments, such as large integrated residential and tourist resorts, or aquaculture farms can pose similar challenges to the management of the GBRMP, particularly if they are located adjacent to marine areas with a traditionally low-intensity use and high conservation values (eg some Dugong Protection Areas (DPA’s)).

Population growth in urban areas adjacent to the GBRMP invariably leads to increased pressure for access to Marine Park resources.

In addition to the broader issues of population pressure associated with coastal development, there are also a number of site-specific impacts. Sediment loss during construction and operational stages of development can reduce water quality. Dumping of dredge spoil from the construction and maintenance of canal and marina developments can effect estuarine and marine environments. Vegetation clearing associated with residential development and recreational uses often occurs adjacent to foreshore areas. There are increases in quantities of litter, especially plastics, entering the marine environment and adversely affecting marine animals including birds and mammals. Coastal development may also result in a change in the character of coastal areas, often involving a loss of the cultural or amenity values placed on the area by some users.

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