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Numerical experiments using a 3D model of fine sediment transport in the Great Barrier Reef region indicate deposition of the bulk mass of catchment sediments from river plumes within a few tens of kilometres from river mouths. A very fine fraction of easily resuspended catchment sediment has a capacity to propagate over much greater distances reaching out into the mid-shelf and outer-shelf regions. The model suggests such particles, instrumental to the development of low density flocs in the marine environment, can play a critical role in altering optical properties of water masses over the shelf during wet years. The mid-term (4 year) impact of Great Barrier Reef catchments on the probability of suspended sediment concentration exceeding the ecologically significant trigger value of 2 mg/L is confined to inshore regions adjacent to river mouth locations., (Crown Copyright © 2018. Published by Elsevier Ltd. All rights reserved.)

The Great Barrier Reef (GBR) is founded on reef-building corals. Corals build their exoskeleton with aragonite, but ocean acidification is lowering the aragonite saturation state of seawater (Ωa). The downscaling of ocean acidification projections from global to GBR scales requires the set of regional drivers controlling Ωa to be resolved. Here we use a regional coupled circulation-biogeochemical model and observations to estimate the Ωa experienced by the 3,581 reefs of the GBR, and to apportion the contributions of the hydrological cycle, regional hydrodynamics and metabolism on Ωa variability. We find more detail, and a greater range (1.43), than previously compiled coarse maps of Ωa of the region (0.4), or in observations (1.0). Most of the variability in Ωa is due to processes upstream of the reef in question. As a result, future decline in Ωa is likely to be steeper on the GBR than currently projected by the IPCC assessment report.

Abstract The dynamics of nutrients, phytoplankton, zooplankton and water clarity of the iconic Great Barrier Reef World Heritage Area (GBRWHA) are influenced by seasonal variation in coastal and oceanic conditions, and anthropogenic inputs. We use the 3-dimensional coupled catchment-hydrodynamic-sediment-wave-optical-biogeochemical eReefs model to hindcast nutrient and planktonic responses to changes in seasonal and annual variability over a six-year period (2011–2016). In this paper we present a comparison of the eReefs model conducted against a range of in situ observations that included 24 water quality moorings, 2 nutrient sampling programs (with a total of 18 stations) and time-series of taxon-specific plankton abundance. At 14 near-coastal stations the simulated chlorophyll a (Chl a), when compared with chlorophyll extractions from water samples, had an average root mean square (RMS) error of 0.30 mg m−3 and a Willmott index of 0.59. For the same sites, simulated turbidity, when compared with turbidity sensors, had a Willmott index of 0.58 and simulated nitrate + nitrite had a Willmott index of 0.41 when compared with tri-annual water samples. Comparison with a spatially-resolved statistical model of zooplankton based on >900 observations in the region showed similarities in the relationship and spatial distribution of simulated and observed biomass, where both show zooplankton biomass is greatest inshore, biomass is highest around shallow reef areas and in the southern and central GBRWHA, and greater in summer than winter. Additionally, we investigate planktonic and nutrient responses to climatic variation. The first two years of the simulation coincided with La Niña events that drive greater than average winds, rainfall and river discharge. Throughout all years, simulated Chl a increases from north to south and from outer to inner coastal regions. Dissolved inorganic nitrogen, primary production and phytoplankton biomass were higher during La Niña in inshore and mid-shelf waters, while offshore regions were similar in all years from 2011 to 2016. Simulated Secchi depth was deeper offshore and decreased from offshore to inner coastal regions and from north to south. The eReefs simulation contributes to the understanding at a high spatial and temporal resolution of where and how nutrients and plankton originate and interact within the Great Barrier Reef. Highlights • Use of a 3D-catchment-hydrodynamic-sediment-wave-optical-biogeochemical model • Model encompasses the 2300 km long Great Barrier Reef from coast to Coral Sea. • Comparison of observations with simulated plankton & nutrients • Observations include remote sensing, moorings and water samples. • Simulated nutrients and plankton show realistic seasonal and interannual variation. [ABSTRACT FROM AUTHOR]