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Spur and groove features occur on the seaward reef slope of coral reefs around the world. They are believed to act as important natural breakwaters, regulating the hydrodynamic energy and nutrients received by reef platforms. They also represent one of the most diverse and productive zones of modern reefs. However, the formation processes and morphodynamics of spur and groove systems are poorly understood, particularly in the Great Barrier Reef (GBR). This paper constitutes the first broad scale analysis of spur and groove systems in the Capricorn Bunker Group (CBG) in the southern GBR. It uses remotely sensed imagery coupled with ground-truthed data to measure groove length at four reefs (Wreck, Heron, One Tree and Lady Elliot). A total of 2621 grooves were digitised across the four study reefs. Groove length was found to vary both between and within the study reefs. The maximum groove length was 536 m. Average groove length ranged from 93 m at Wreck Reef to 32 m at Heron Reef. This data was compared to relative wave exposure estimates derived from the fetch scenario at each reef. Strong positive correlation was found with groove length increasing as wave exposure increased. Groove length was highly spatially dependant and varied around the reef platforms according to the degree of wave exposure. The longest grooves were found on the most exposed, eastern sides of all reefs. These results provide valuable insight into spur and groove function, formation and likely response to future environmental changes in the CBG and further afield. [ABSTRACT FROM AUTHOR]

Sea surface salinity (SSS) is critical for studying biological and physical processes in the ocean, such as the global water balance, ocean currents, and evaporation rates. The water and heat fluxes associated with precipitation and evaporation over global oceans are fundamental in regulating climate and weather. Yet measurements of global SSSs are sparse and do not show the required temporal and spatial variability of SSS distributions. Airborne microwave radiometers, such as the Scanning Low-Frequency Microwave Radiometer (SLFMR) and the Salinity, Temperature, and Roughness Remote Scanner (STARRS), have been used successfully to map SSS and its variability, but only in estuaries and coastal waters. Since 2009, SSS has been measured from satellite orbit by the European Soil Moisture and Ocean Salinity satellite, which is designed to provide synthesized SSS maps with a high accuracy. Other salinity-related satellites are being developed, such as Aquarius, which will provide the global view of salinity variability needed for climate studies. The objectives of this paper are to review remote sensing techniques for mapping SSS and to demonstrate the application of microwave radiometry by presenting two case studies. [ABSTRACT FROM AUTHOR]

Fellowes, T.E.; Gacutan, J.; Harris, D.L.; Vila-Concejo, A.; Webster, J.M., and Byrne, M., 2017. Patterns of sediment transport using foraminifera tracers across sand aprons on the Great Barrier Reef. Sediment dynamics exert large control over coral reef geomorphological evolution and are vital to understanding past and present geomorphic responses. Large benthic foraminifera (LBF) live in the algal reef flats, and their tests (shells) are transported post-mortem by waves and currents onto back-reef environments, including sand aprons. This study investigated the patterns of transport linking surficial and downcore sediments in samples from three sand aprons with different wave exposures at One Tree Reef on the southern Great Barrier Reef (Australia). Six LBF genera represented up to 32% of the sediments analysed. Lagoonward transport increased LBF test abrasion and sediment bulk density. Sediment grain size and LBF abundance in sediments also decreased with lagoonward transport. Sediment transport patterns indicated by LBF species used as tracer were consistent with the prominent E-SE wave environment. A novel taphofacies approach was used to describe stratigraphic layers in downcore sediments based on LBF test abrasion and abundance. Varied sediment deposition rates did not affect the LBF test abrasion signature downcore. It appears that Baculogypsina sphaerulata has been the dominant species for at least 3 ka. Tests that were deposited slowly exhibited less or the same levels of abrasion than those that were rapidly deposited. It appears that test abrasion is primarily determined by the distance travelled rather than the influence of increased age or chemical dissolution. [ABSTRACT FROM AUTHOR]

Inshore turbid reefs on the Great Barrier Reef (GBR) are exposed to high and fluctuating sediment loads normally associated with poor reef growth, but many have high coral cover (>30%) and diversity (>50 species). Previous assessments of sediment regimes on these reefs have largely relied on sediment trap data, which overestimate sedimentation rates and may not accurately reflect sedimentary conditions. A new approach, based on paired sediment trays, is described here that allows the sedimentation rate, sediment resuspension, and total mass of mobile sediments transported on to and off of a site per unit time and area (termed the two-way total sediment flux) to be measured or calculated. The sediment trays were deployed on Middle Reef and Paluma Shoals, two inshore turbid reefs on the GBR where the two-way total sediment flux ranged from 34 g/m2/d in protected reef habitats to more than 640 g/m2/d in higher-energy settings. Mean sedimentation rates, calculated using data from four sites across these reefs, of less than 122 g/m2/d are considerably lower than published rates estimated for nearby coral reefs, largely because sediment traps limit sediment resuspension. At each tray installation, sediments were collected every 4 to 6 weeks to measure variations in net sedimentation through the year, and resuspension rates were calculated by comparing 100 g of preanalysed sediments placed on trays at deployment to sediments recovered 2 weeks later. These data demonstrate that despite high sediment delivery rates, net sedimentation may still be relatively low and potentially less of a threat to benthic communities on turbid reefs than previously assumed. Sediment trays provide a comprehensive assessment of sediment regimes that, together with ecological assessments of coral cover, improve our understanding of the sedimentary pressures affecting inshore turbid reefs and their ability to tolerate sedimentation. [ABSTRACT FROM AUTHOR]

Australian reef flats on the Cocos (Keeling) Islands atoll, Indian Ocean; Warraber Reef, Torres Strait; and Lady Elliot Island, Great Barrier Reef vary greatly in morphology (width, elevation) and hydrodynamic setting (wave and tidal regime). This study describes results from detailed wave and current measurements, under nonstorm conditions, along five reef flat transects on these reef systems and examines implications for surface geomorphic processes. Results show that wave frequency and transformation varies between reefs in a consistent manner dependent on tidal elevation, reef elevation, and reef width. A nondimensional reef energy window index (Ψ) is developed that incorporates these critical factors (water depth at spring high tide and reef width). A statistically significant relation (95% confidence interval) between Ψ and the proportion of time that wave energy propagates across reefs illustrates the index ability to characterise the wave process regime of reef flats and provide a physically meaningful descriptor of the efficacy of geomorphic processes on reefs. High values of Ψ indicate narrow and low-elevation reef flats, which are exposed to high wave energy and are geomorphically active. Low values reflect wide and high-elevation reef flats associated with less active wave and geomorphic processes. Results show that while incident energy is undoubtedly an important factor for reef geomorphology, the nature of wave modification across reef flats is equally important in governing levels of geomorphic activity that control development of surface geomorphic features on reef platforms. [ABSTRACT FROM AUTHOR]

Several modes of coral reef growth are found along the edge of Australia's Great Barrier Reef (GBR), determined by the morphology and slope of the shelf edge, especially between -50 m and -100 m, and the velocity of tidal currents near the surface. The simplest forms are the flood tide deltaic reefs and ribbon reefs of the far north. The shelf margin of the central GBR is characterized by lines of submerged reefs that continue on the ocean (northeastern) side of the Pompey Reefs, which are the largest and most complex in the entire GBR. Combining interpretations of reef evolution from the simpler marginal reefs with data collected from the Pompey Complex, a model of evolution on a stepped continental shelf margin is developed, involving initiation as ribbon reefs, formation of both ebb and flood tide deltas (which have formed the foundation for further reef growth), and incorporation of at least one line of previously submerged reefs on the open ocean side by progradation of the deltaic structures to form large lagoonal reefs. Although the reefs cover a smaller area than the extensive reefs of the continental shelf, which could have grown only at higher Quaternary sea levels, the smaller area of shelf-marginal reefs may contain a longer record of coral growth than that of the better-known shelf reefs. [ABSTRACT FROM AUTHOR]