Dispersal of suspended sediments and nutrients in the Great Barrier Reef lagoon during river-discharge events: conclusions from satellite remote sensing and concurrent flood-plume sampling

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.