Proceedings 9 th International Coral Reef Symposium, Bali, Indonesia 23-27 October 2000 Keeping the wolf from the door: managing land-based threats to the Great Barrier Reef Jon Brodie 1 1 Previously Great Barrier Reef Marine Park Authority, PO Box 1379, Townsville Mail Centre, Qld 4810, Australia; current address Tropical Environmental Studies and Geography, James Cook University, Townsville, Qld, Australia. ABSTRACT Pollution of the Great Barrier Reef is dominated by runoff from the adjacent catchment. The principal land-uses contributing to this pollution are rangeland beef grazing and cropping, with lesser contributions from industrial and urban development. Runoff of sediment, nutrients and pesticides is increasing and for most pollutants the load is many times the natural amount discharged 150 years ago. Effects are now evident on inshore reefs, seagrasses and marine animals. Reduced coral cover and diversity and high levels of juvenile coral post-settlement mortality in a eutrophication gradient in the Whitsundays reef region is reported. The herbicide diuron is found in many areas of inshore seagrass at concentrations above those known to interfere with photosynthesis in seagrasses. The legislation and processes in place to manage the agricultural component of catchment pollution appear to be ineffective. To date, the Great Barrier Reef Marine Park Act has not provided effective jurisdiction on the catchment to manage pollution loads to the Marine Park. Existing Queensland legislation addressing agricultural pollution relies on voluntary codes and as yet there is no assessment of the effectiveness of the codes. Integrated catchment management strategies, also voluntary, provide some positive outcomes but are of limited success for downstream ecosystems of the Great Barrier Reef. Pollutant loads are predicted to continue to increase and it is unlikely that current management regimes will prevent this increase. Management of point source pollution from urban and industrial sources has been successful. New mechanisms to prevent continued degradation of inshore ecosystems of the Great Barrier Reef World Heritage Area caused by land-sourced agricultural pollution are urgently needed. Introduction The Great Barrier Reef (GBR) system extends along the Queensland coast of Australia for 2000 km between 9 o S and 24 o S (Fig. 1), an area of about 343,500 km 2 on the continental shelf. The width varies from 50 km in the north to 200 km in the south and the area is bounded by the coast on the west and the Coral Sea on the east. The Great Barrier Reef World Heritage Area (GBRWHA) includes islands, coral reefs, seagrass beds, mangrove forests, estuaries, soft- bottomed communities and pelagic environments. Environmental and resource management of the Great Barrier Reef Marine Park (most of the area of the World Heritage Area) is implemented by the Great Barrier Reef Marine Park Authority (GBRMPA) in association with Queensland State agencies (see Chadwick and Green this volume). The coast adjacent to the GBR (Fig. 1) consists of a large number of individual catchments, mostly small (< 40,000 km 2 ) but also two of Australia’s larger catchments, the Burdekin (133,000 km 2 ) and Fitzroy (143,000 km 2 ). Population density is generally low with a few small cities with populations up to 150,000 on the coast. There are no large cities inland from the coast. The most extensive land use in the catchment area of the GBR (hereafter ‘the GBR Catchment’) is rangeland beef grazing (77% of the area) with cropping (3%), mainly sugarcane, but also horticulture and cotton and urban/residential (1%) smaller intensive uses. Other uses include mining (coal and metalliferous) and aquaculture. The Catchment has been substantially modified since European settlement (ca. 1850). Large areas of savannah woodland and forest have been cleared or thinned (approximately 200,000 km 2 or 50% of the total watershed area) for grazing and cropping. Models of relationships between land use, erosion and sediment loss suggest nutrient and sediment runoff has increased several-fold over pre-development levels (Moss et al. 1992). The sugarcane cultivation area, located near the coast in many of the lowland areas of catchments, has increased steadily over the last 100 years with a total of 400,000 ha reached by 2000. South of Cooktown most freshwater wetlands on the floodplains have been destroyed for agricultural and urban use. These losses include 70% on the Herbert floodplain (Johnson et al. 1998), 40 to 60% on the floodplains of the Russell-Mulgrave, Tully-Murray and Johnstone rivers (Russell and Hales 1994; Russell et al. 1996a, 1996b) and 80% on the Burdekin-Haughton floodplain (Tait, pers. com.). Human populations in the region, while not large, have increased rapidly since 1950 (Gilbert 2001) with associated increases in sewage, urban stormwater and industrial discharges (Brodie 1991). Agricultural fertilizer use has increased rapidly since the 1950’s (Pulsford 1996) along with the usage of a diverse range of pesticides (Hamilton and Haydon 1996). The GBR lagoon in which the reefs and other ecosystems of the GBR are embedded, is a semi-enclosed water body lying over a shallow shelf. Water depths increase across the shelf to a maximum of 100 m before the shelf break and average about 40 m. The shelf has often been divided conveniently into three bands (Belperio 1983), the inner shelf out to a depth of about 20 metres containing the coastal and island fringing reefs and most of the seagrass beds, the middle shelf of depth 20 to 40 metres with few reefs but some areas of deep seagrass, and the outer shelf in depths of