Intense wet-season rainfall in January 2005 caused rivers in the Mackay–Whitsunday region of Queensland, Australia, to produce large discharges to the Great Barrier Reef (GBR) lagoon. The regional land use is dominated by sugarcane cultivation, beef grazing and urban uses. The high nutrient (nitrogen and phosphorus) fluxes from these land uses via river runoff produced a massive phytoplankton bloom in the GBR lagoon, which, after 9 days, had spread 150 km offshore. The plume and algal bloom surrounded inner-shelf reefs of the GBR such as Brampton Island Reef and its spread was tracked with a variety of satellite sensors including MODIS, SeaWiFS and Landsat over the 9-day period. The ability to be able to access imagery from a large number of satellite sensors allowed almost daily estimates of the extent of plume to be made, despite periods of cloud. Analysis of water samples from the plume revealed elevated (2–50 times higher) concentrations of Chlorophyll a (and hence phytoplankton biomass), up to 50 times higher than in non-flood conditions, nutrients (2–100 times higher) and herbicide residues (10–100 times higher) compared with GBR lagoon waters in non-discharge conditions. The concentration data from the samples and estimated exposure periods from the satellite images allowed estimates of the exposure of GBR marine ecosystems (coral reefs, the pelagic community, seagrass beds and mangrove forests) to the terrestrial contaminants to be made.

Abstract. To assist in the development of the Tully Water Quality Improvement Plan, a subcatchment water quality monitoring program was undertaken to identify the pollutants of concern and their land-based sources. Monitoring of suspended sediments, nutrients and pesticides in subcatchment waterways was conducted during the 2005-06 and 2006-07 wet seasons, which both had above average annual flows. We found distinct water quality signals from the basin's major land uses (forest, grazing, urban, sugarcane and banana cultivation), except for suspended sediment concentrations, which were low across all land uses when compared with neighbouring river catchments. This reflects the high ground cover of the basin and the location of intensive agriculture on low sloping areas of the floodplain, minimising the potential for erosion. Nitrate concentrations were elevated in streams draining sugarcane, indicating fertiliser export from intensive agricultural landscapes. Residues of the herbicides diuron and atrazine were detected at sites draining sugarcane, and on occasion exceeded national ecological protection trigger values, which highlights a potential threat to downstream wetlands of recognised national significance. Herbicides were also detectable offshore in flood plumes of the Tully-Murray Rivers, with some concentrations of diuron above lowest observable effect concentrations for specific species of seagrass and corals. Run-off of nitrate and diuron were identified as key water quality issues in the Tully-Murray basin.