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Coral reef environments are biodiversity hotspots that provide many services to coastal communities. They are currently facing an increasing anthropogenic pressure that jeopardises their survival. Numerical ocean models are an important tool to understand the functioning of coral ecosystems and the mechanisms ensuring their resilience. However, simulating the water circulation through reef systems is challenging because of their naturally complex topography and bathymetry. Many ocean models used for such applications have a spatial resolution coarser than the scale of individual reefs, which puts into question their suitability for the task. Here, we assess the sensitivity of a coastal ocean model's outputs to its spatial resolution when simulating the water circulation in the entire Great Barrier Reef (Australia), the largest coral reef system in the world. We consider the same model with five different resolutions near reefs, ranging from 250 m to 4 km, and compare the model outputs at different locations around reefs and in the open ocean. We also simulate the transport of passive particles released from those different locations. Our results show that the simulated tidal signal is similar for all five resolutions. However, strong discrepancies (> 10 cm/s) in the current velocity are observed near the reefs and along the rugged coastline. When using a coarse-resolution model, the amplitude of the currents is overestimated over reefs, and underestimated between them. We find that validating the model at deep water mooring sites is not sufficient to ensure it performs well close to reefs. Discrepancies in currents lead to more directional and uniform tracer dispersal patterns on coarse-resolution meshes that contrast with the more dispersive patterns observed on fine-resolution meshes. Those differences at the reef level have a large cumulative effect when simulating transport processes over several weeks. Our results suggest that ocean circulation and transport simulations in coral reef environments should be based on model resolutions finer than the reef scale, which generally means a maximum resolution of about 250–500 m. • Coastal ocean model results are sensitive to mesh resolution. • Resolution sensitivity is the largest in complex marine environments like coral reefs. • Mesh resolution mostly impacts the currents and less the sea surface elevation. • Currents discrepancies cumulatively lead to large errors in dispersal patterns. [ABSTRACT FROM AUTHOR]

Highlights: [•] Coral larval dispersal is modelled in the central Great Barrier Reef. [•] Graph theory tools are used to detect the presence of isolated reef clusters. [•] We identify important barriers to larval dispersal between clusters. [•] We find that dispersal distances may vary significantly in different clusters. [Copyright &y& Elsevier]

Abstract: An unstructured-mesh parallel hydrodynamic model of the whole Great Barrier Reef is presented. This model simultaneously simulates most scales of motion. It allows interactions between small- and large-scale processes. The depth-averaged equations of motion are discretized in space by means of a mixed finite element formulation while the time-marching procedure is based on a third order explicit Adams–Bashforth scheme. The mesh is made up of triangles. Their size and shape can be modified easily so as to resolve a wide range of scales of motion, from those of the regional flows to those of the eddies or tidal jets that develop in the vicinity of reefs and islands. The forcings are the surface wind stress, the tides and the inflow from the Coral Sea (East Australian Current, Coral Sea Coastal Current), the latter two forcings being applied along the open boundaries of the computational domain. The numerical results compare favourably with observations of both longshore currents and the local perturbations due to narrow reef passages. Comparisons are also performed with the simulations of a three-dimensional model applied to a small domain centered on Rattray Island, showing that both models produce similar horizontal flow fields. For a structured-mesh model to yield results of the same accuracy, there is no doubt that the computational cost would be much higher. [Copyright &y& Elsevier]

The 500 km long Far Northern Great Barrier Reef (FNGBR) is a semi-enclosed sea with little connectivity with the rest of the GBR further South. Its mean circulation is controlled by the prevailing southeasterly (northwestward) wind that generates a wind-driven mean longshore flow that is enabled by an inflow of oceanic water in the South. From an examination of single-point, mid-depth current meter data at ten sites over one year, it appears that there is no net current during calm weather, which implies that the northward North Queensland Coastal Current in the adjoining Coral Sea does not intrude in the FNGBR. Only about 20–40% of the wind-driven longshore transport continues northward to exit the FNGBR through Torres Strait because of blockage by reefs, shoals and islands and by the tidal friction effect in shallow waters. The remaining 60–80% of the flow appears to be deflected seaward to the Coral Sea in the North, an observation that appears to be supported by oceanographic modelling. This situation differs from that in the central and southern Great Barrier Reef where the southward flowing East Australian Current intrudes on the shelf and generates a net current even during calm weather. • The 500 km long Far Northern Great Barrier Reef (FNGBR) is a semi-enclosed sea that is flushed very slowly. • The FNGBR is little connected with the rest of the Great Barrier Reef. • During southeasterly winds, there is an inflow from the Coral Sea in the South of the FNGBR and apparently an outflow to the Coral Sea in the North. [ABSTRACT FROM AUTHOR]

The SLIM oceanographic model was used to examine the fate of hatchling flatback turtles (Natator depressus) in the first two weeks of their dispersal starting at Wild Duck Island, a major turtle-nesting site in the central Great Barrier Reef region of Australia. We ran simulations to investigate the effects of spring versus neap tides, hatchling's swimming behaviour during their first three days at sea, and the location of nesting beaches. The model predicted that up to 50% of the turtle hatchlings entering the sea from the windward, southern beach remained after 14 days in shallow, nearshore waters, irrespective of tides and swimming. These waters are turbid and may be inhospitable to hatchlings. In contrast >80% of hatchlings dispersing from the leeward, northern beach were dispersed to deeper water (10–40m) in a quasi-stationary dispersion core centred around 10km north of the nesting beach after 14 days and the offshore spread of the turtle plume was enhanced by the hatchling's seaward swimming during the first three days. This was due to the presence of a coastal boundary layer and a stagnation zone around the northern side of island, but not the southern side. The model confirmed that dispersal from eastern Queensland flatback turtle rookeries is restricted to the lagoons and coastal waters, and that water circulation and hatchling's swimming control dispersion. The model explains why more turtles nest on the northern than the southern side. This study highlights the usefulness of oceanographic models to increase knowledge about a cryptic life stage of marine turtles. [Copyright &y& Elsevier]