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Loads of fine sediment, nitrogen and phosphorus from the 35 major basins of the Great Barrier Reef (GBR) catchment have increased greatly over the last 150 years associated with catchment development for agriculture and urban uses. These increased loads have caused a range of effects in the GBR resulting in loss of coral and seagrass. Major effects include the promotion of increased populations of the coral predator – the crown of thorns starfish via increased nutrient availability, now a major cause of coral mortality on the GBR. A second major effect associated with increased fine sediment and nutrient loads is an increase in GBR innershelf turbidity and a subsequent reduction of light for coral and seagrass photosynthesis. Currently coral cover on the GBR is in a state of severe decline associated with these water quality effects and the increasing effects of climate change. The water quality management plan for the GBR – Reef Plan, requires targets to be set to reduce basin pollutant loads so that marine and coastal ecosystems can be maintained in a satisfactory state. In the past targets were set for the whole of the GBR but it is now recognized that targets at the individual basin scale are required to guide management and account for the differing river plume footprints and spatial distributions of marine ecosystems along the GBR. Now as part of the Reef 2050 Water Quality Improvement Plan 2017-2022, targets for fine suspended sediment (FSS), dissolved inorganic nitrogen (DIN), particulate nitrogen (PN) and particulate phosphorus (PP) are to be set for the 35 basins to be achieved by 2025. The targets are designed such that an ecological endpoint is reached in the GBR, which represents a satisfactory ecological condition. Over the last 4 years CSIRO and partners have developed the eReefs marine modelling framework. The eReefs regional and relocatable models include hydrodynamic, sediment, wave and biogeochemistry models for the GBR ecosystem. The numerical models are capable of simulating and predicting the hydrodynamic state, sediment transport, water quality and basal ecology of the Great Barrier Reef lagoon and reef matrix and hence model the transport and fate of waterborne material, whether of oceanic or terrestrial origin, and its impact on GBR water quality. eReefs is now being used to help set the basin scale targets. eReefs has included 17 of the GBR basins in the modelling carried out in 2016. Targets for the other 18 basins were estimated using other models and techniques. The eReefs modelling used the 4 km resolution version, four years of simulation (2011 – 2014) with loads from the Source Catchments model. The final endpoint criteria chosen for the estimation of targets such that specific ecological outcomes were included in the modelling were: 1. Improved coral diversity versus macroalgae. Reducing DIN loads leads to an increase in coral diversity and a reduction in macroalgae abundance. Meeting a threshold Chlorophyll a (Chl-a) concentration is used as a proxy for nutrient status. The criteria is: Chl-a < 0.45 μg L-1, all years, all waters within river footprints. 2. Improved seagrass “health”. Reducing FSS loads leads to reduced turbidity throughout the year and improved light availability in shallow areas (< 10 m) which is a key driver of seagrass abundance. The underwater light field is a reasonably intrinsically-predictable phenomena. As the major human impact on seagrass health in GBR coastal regions is reduced light, it was considered better to directly consider bottom light than other possible criteria such as seagrass biomass. The criteria is: PAR-integrated light > 6 mol/m2/day for seagrass, all years, < 10 m, all waters within river footprints – the “chronic” case. Using scenarios that reduced fine sediment and DIN (and PN and PP in proportion with the FSS), in steps from each of the modelled basins, estimates of the loads required to meet the ecological endpoints were derived. Examples of the results include for the Tully Basin a reduction of 20 % of the current anthropogenic fine sediment load and a reduction of 50 % of the current anthropogenic DIN load are required by 2025.

Pollutant target setting for the Great Barrier Reef: Using the eReefs framework