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Case study: The Great Barrier Reef Lagoon

Concern over reports of locally raised nutrient and sediment concentrations within the inshore parts of the Great Barrier Reef Lagoon and near to a number of island resorts led to a review and recommendations to GBRMPA (Baldwin 1990). This review concluded that locally nutrients had reached concentration levels that have caused detrimental effects to corals elsewhere, but that damage to coral on the Reef could not conclusively be assigned to this cause. Thus further research was needed to clarify this issue.

Point-source pollution from discharges at island and coastal resorts and pontoons within the World Heritage Area was detailed by the review, which proposed that these be dealt with by regulation. The State of the Environment

Table 3.9 Estimated average sediment yields (million tonnes/yr) to Great Barrier Reef Lagoon

Catchment

Area (km2)

Moss et at. (1993)

Neil & Yu (1996), natural

Neil& Yu (1996), current

Ratio of current to natural

Burnett-Kolan

39470

0.724

0.641

2.381

3.7

Curtis Coast

9225

0.374

0.133

0.476

3.6

Fitzroy

142645

1.774

2.57

10.466

4.1

Shoalwater Bay

11270

0.924

0.166

0.568

3.4

Pioneer-O'Connell

3925

0.657

0.064

0.24

3.8

Proserpine

2485

0.349

0.039

0.132

3.4

Don

3985

0.175

0.057

0.225

3.9

Burdekin-Haughton

133510

2.711

2.116

8.52

4.0

Ross-Black

2890

0.265

0.043

0.133

3.1

Tully-Murray

2825

0.66

0.069

0.124

1.8

Herbert

10130

0.624

0.155

0.536

3.5

Johnstone

2330

0.582

0.056

0.159

2.8

Mulgrave-Russell

2020

0.521

0.05

0.137

2.7

Barron

2175

0.137

0.035

0.094

2.7

Mossman-Daintree

2615

0.528

0.06

0.096

1.6

North-east Cape York

43300

2.387

0.649

2.008

3.1

Total

450515

15.25

7.35

28.03

3.8

Source: Wasson 1997

Report for the Reef 1998 (p.110) reported that as a result of regulation 'most outfalls within the Park' do not release effluent that is not tertiary treated to the marine environment.

The review saw terrestrial run-off as a major external source of nutrients to Reef waters, and stated that further investigation and regulation would also be needed in this area. Moss et al. (1993), Neil & Yu (1995), and Mitchell and Fumas (1997) have undertaken further work on sediment and nutrient discharge to the Lagoon, but variability between the different sets of monitoring and estimates remains large. Wasson (1997) suggested that further monitoring is needed to achieve focused and targeted catchment management. It is clear that land use practices and catchment management have a significant impact on the water quality of the rivers discharging to the Great Barrier Reef Lagoon, but the key areas contributing to the sediment and nutrient levels have yet to be defined.

Sixteen sizeable coastal catchments between Bundaberg and Cape York drain to the Lagoon (table 3.9).

It appears that the current sediment yield is about two to four times the natural yield, and that catchments with the largest pristine and timbered areas (Tully-Murray and Mossman Daintree both about 60% uncleared) show the least increase. The major land use in the catchments listed above is cattle grazing. The two very large catchments (the Burdekin and the Fitzroy together make up over half the area draining to the reef), which are both used almost entirely for grazing, show an increase of about 300% in sediment yield.

Wasson (1997) noted that there was a variation of approximately three times in contemporary estimates of quantities of phosphorus and nitrogen carried by rivers to the Reef, and concluded that ongoing monitoring is essential. One difficulty in assessing change and trends in river discharge and sediment and nutrient concentrations is the great natural variability. There is a large variation in total seasonal rainfall, and in the frequency of intense rainfall events, over the years, so long-term monitoring is necessary to clarify change as well as to calibrate models of catchment behaviour.

Locally there are some indications of changed water quality within the Lagoon, but there appears to be no clearcut evidence of degradation as a result of land based discharges, except at specific locations or associated with particular events. The State of the Environment Report for the Great Barrier Reef (Wachenfeld et al. 1998, p. 31) stated that raised nutrient levels can lead to sea- grass destruction, but concluded that more detailed monitoring was needed to show the extent of this. The report also noted (p. 42) that low salinity and high turbidity from storm river flows can kill coral: 'In general, terrestrial run off is considered to be one of the greatest potential threats to the Great Barrier Reef, however there are very little data to demonstrate that coral communities have been directly impacted by this threat.' A similar conclusion for macroalgae was reached (p. 35): 'It has been suggested that the abundance of macroalgae on inshore reefs is unnaturally high and is a sign of eutrophication and reef degradation, due to increased sediment and nutrient inputs from the land. However, in the absence of good historical data, it is still uncertain whether current abundances are natural or human induced.' Management implications of this report provide a challenge to all in the careful interpretation of the precautionary principle.

The lagoon is naturally characterised by low nutrient levels. Phytoplankton have been shown to be productive in Reef waters, and nutrients for this productivity are recycled through the mineralisation of organic matter. Nitrogen is added to the system by the N-fixing cyanobacterium Trichodesmium, but the extent of this input has not been quantified. Rivers are the major external source of nutrients to the lagoon. The indications are that river waters do not mix extensively through the lagoon: wind-induced currents tend to trap these near to the shore, where they are driven north along the shore to accumulate in embayments. Although the freshwater and sediment plume of very large storm events may rarely extend to the mid to outer reef in low wind conditions (as happened, for example, during Cyclone Sadie in 1994), storm plumes are more usually held near the coast by strong southeast winds.

In inshore waters the prevailing south-east trades, as well as storm events may resuspend sediments within the water column (Wachenfeld et al. 1998, p. 13); nutrient release from re-suspended sediment stimulates phytoplankton growth and chlorophyll concentrations may be raised to three times normal background levels during strong trades and 30 times following cyclones. Measurements of suspended solids, phosphate and nitrate levels in coastal waters near Cairns from 1989 to 1995 show distinct variability over time, and no trend. Chlorophyll concentration in the central Reef, measured from 1975 to 1995, is also highly variable, but shows no trend up or down over that time. The outcome of a decade of monitoring and debate is the adoption in 2002 of a 'Water Quality Action Plan' by the Great Barrier Reef Marine Park Authority and the Commonwealth and Queensland governments. This sets pollution reduction targets for 2012 for all reef catchments and processes to achieve the targets.

 
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