Great Barrier Reef Continental Shelf Research Articles

Changes in water clarity in response to river discharges on the Great Barrier Reef continental shelf: 2002–2013

Water clarity is a key factor for the health of marine ecosystems. The Australian Great Barrier Reef (GBR) is located on a continental shelf, with >35 major seasonal rivers discharging into this 344,000 km2 tropical to subtropical ecosystem. This work investigates how river discharges affect water clarity in different zones along and across the GBR. For each day over 11 years (2002–2013) we calculated ‘photic depth’ as a proxy measure of water clarity (calibrated to be equivalent to Secchi depth), for each 1 km2 pixel from MODIS-Aqua remote sensing data. Long-term and seasonal changes in photic depth were related to the daily discharge volumes of the nearest rivers, after statistically removing the effects of waves and tides on photic depth. The relationships between photic depths and rivers differed across and along the GBR. They typically declined from the coastal to offshore zones, and were strongest in proximity to rivers in agriculturally modified catchments. In most southern inner zones, photic depth declined consistently throughout the 11-year observation period; such long-term trend was not observed offshore nor in the northern regions. Averaged across the GBR, photic depths declined to 47% of local maximum values soon after the onset of river floods, and recovery to 95% of maximum values took on average 6 months (range: 150–260 days). The river effects were strongest at latitude 14.5°–19.0°S, where river loads are high and the continental shelf is narrow. Here, even offshore zones showed a >40% seasonal decline in photic depth, and 17–24% reductions in annual mean photic depth in years with large river nutrients and sediment loads. Our methodology is based on freely available data and tools and may be applied to other shelf systems, providing valuable insights in support of ecosystem management.

Reef-scale partitioning of cryptobenthic fish assemblages across the Great Barrier Reef, Australia

Onshore to offshore gradients in marine assemblages have been well documented on coral reefs, with most studies showing a distinct separation between onshore and offshore locations. Here we use enclosed anaesthetic sampling of small, cryptobenthic reef fishes to assess changes in assemblage composition across the Great Barrier Reef continental shelf. The cryptobenthic fishes exhibited fine-scale partitioning across the shelf. Three dominant species of goby accounted for over 55% of all fishes collected, with 1 species characterising each of the 3 key shelf positions: inner-, mid- and outer-shelf. Multivariate analyses of assemblage composition revealed further separation of reefs within the inner- and mid-shelf positions, highlighting the exceptional sensitivity of cryptobenthic reef fish assemblages to shelf position, with a progressive separation of individual reef assemblages with distance from the shore. These among-reef patterns contrast markedly with other reef fish taxa which invariably show 2 broad assemblages across the continental shelf (inner-vs. a composite mid- and outer-shelf community). As a result of this exceptional sensitivity to environmental conditions, cryptobenthic reef fish communities may represent good subjects for high-resolution monitoring of disturbances on coral reefs.

The net water circulation through Torres strait

The extensive existing data set on the oceanography of Torres Strait (separating Australia and Papua New Guinea) is reviewed and is used to show that the long-term east to west water flux through Torres Strait is about 0.04Sv. To explain this finding, a high resolution, finite element, depth-integrated (2D) model of Torres Strait is set-up. The model is driven by the wind and the open boundary conditions that are prescribed in the Coral Sea, the northern Great Barrier Reef continental shelf, the Gulf of Papua, and the Gulf of Carpentaria. The latter/open boundary conditions were taken from field data. The model suggests that the wind alone over Torres Strait generates negligible net currents; the net currents are instead generated by the wind setting up a water slope across Torres Strait through its opposing influence on the mean sea levels in the Gulf of Carpentaria and in the Coral Sea. The model predicts that the long-term, net east–west flow through Torres Strait is small to negligible, in agreement with field data. The model also reveals that large scale flow dynamics are largely restricted to reef passages and open waters, and that areas of shallow waters and areas densely populated with reefs and islands are poorly flushed. Finally, the model also reveals the prevalence of highly energetic tidal flows around shoals, reefs, islands and reef passages, and these swift tidal currents are responsible for the small net flow through Torres Strait through the sticky water effect. These findings enable us to propose a general water circulation model for the whole Great Barrier Reef during southeast winds.

Submerged banks in the Great Barrier Reef, Australia, greatly increase available coral reef habitat

Abstract Harris, P. T., Bridge, T. C. L., Beaman, R. J., Webster, J. M., Nichol, S. L., and Brooke, B. P. 2013. Submerged banks in the Great Barrier Reef, Australia, greatly increase available coral reef habitat. – ICES Journal of Marine Science, 70: 284–293. Anthropogenic global ocean warming is predicted to cause bleaching of many near-sea-surface (NSS) coral reefs, placing increased importance on deeper reef habitats to maintain coral reef biodiversity and ecosystem function. However, the location and spatial extent of many deep reef habitats is poorly known. The question arises: how common are deep reef habitats in comparison with NSS reefs? We used a dataset from the Great Barrier Reef (GBR) to show that only about 39% of available seabed on submerged banks is capped by NSS coral reefs (16 110 km2); the other 61% of bank area (25 600 km2) is submerged at a mean depth of around 27 m and represents potential deep reef habitat that is spatially distributed along the GBR continental shelf in the same latitudinal distribution as NSS reefs. Out of 25 600 km2 of submerged bank area, predictive habitat modelling indicates that more than half (around 14 000 km2) is suitable habitat for coral communities.

Deposition, mineralization, and storage of carbon and nitrogen in sediments of the far northern and northern Great Barrier Reef shelf

The flow of carbon and nitrogen in sediments of the far northern and northern sections of the Great Barrier Reef continental shelf was examined. Most of the organic carbon (81–94%) and total nitrogen (74–92%) depositing to the seabed was mineralized, with burial of carbon (6–19%) and nitrogen (8–20%) being proportionally less on this tropical shelf compared with other non-deltaic shelves. Differences in carbon and nitrogen mineralization among stations related best to water depth and proximity to river basins, with rates of mineralization based on net ∑CO 2 production ranging from 17 to 39 ( mean=23) mmol C m −2 d −1. The overall ratio of O 2:CO 2 flux was 1.3, close to the Redfield ratio, implying that most organic matter mineralized was algal. Sulfate reduction was estimated to account for ≈30% (range: 6–62%), and denitrification for ≈5% (range: 2–13%), of total C mineralization; there was no measurable CH 4 production. Discrepancies between ∑CO 2 production across the sediment–water interface and sediment incubations suggest that as much as 5 mmol m −2 d −1 (≈25% of ∑CO 2 flux) was involved in carbonate mineral formation. Most microbial activity was in the upper 20 cm of sediment. Rates of net NH 4 + production ranged from 1.6 to 2.7 mmol N m −2 d −1, with highly variable N 2 fixation rates contributing little to total N input. Ammonification and nitrification rates were sufficient to support rapid rates of denitrification (range: 0.1–12.4 mmol N m −2 d −1). On average, nearly 50% of total N input to the shelf sediment was denitrified. The average rates of sedimentation, mineralization, and burial of C and N were greater in the northern section of the shelf than in the far northern section, presumably due to higher rainfall and river discharge, as plankton production was similar between regions. The relative proportion of plankton primary production remineralized at the seafloor was in the range of 30–50% which is at the high end of the range found on other shelves. The highly reactive nature of these sediments is attributed to the deposition of high-quality organic material as well as to the shallowness of the shelf, warm temperatures year-round, and a variety of physical disturbances (cyclones, trawling) fostering physicochemical conditions favorable for maintaining rapid rates of microbial metabolism. The rapid and highly efficient recycling of nutrients on the inner and middle shelf may help to explain why the coral reefs on the outer shelf have remained unscathed from increased sediment delivery since European settlement.

Phosphorus speciation in the sediment and mass balance for the central region of the Great Barrier Reef continental shelf (Australia)

Solid phase P speciation has been determined in sediments from a transect across the central section of the continental shelf and slope of the Great Barrier Reef (GBR) lagoon. This region is characterized by a gradient of riverine aluminosilicate clay and silt nearshore, seawards of which biogenic carbonate sediment predominates. Phosphorus speciation results show large variations along this transect. Organic P and authigenic (apatite) P are the major chemical forms of phosphorus in the central GBR continental shelf sediments. Post-depositional reorganization of P was also observed, converting organic P and iron bound P (Fe–P) to authigenic (apatite) P. Phosphorus burial rate was estimated from measurements of total P concentration and excess 210Pb sediment mass accumulation rates. Burial efficiency varies significantly over the shelf. Inshore areas showed significant P remobilization from sediments to the water column (up to ∼50%). The mid and the outer shelf showed little evidence for remobilization (except for coral reef platform sediments), with more of the sediment P being in the less reactive authigenic apatite phases. An appreciable fraction of this non-labile authigenic apatite phase was identified as fish bone. P sources and sinks over the central part of the GBR shelf were quantified using a mass balance approach. This showed that Coral Sea shelf edge upwelling events are essential to satisfy the large P nutrient demand of the whole GBR lagoon. P inputs due to upwelling events were greater than those contributed by local rivers over an average year.

Bedload sediment transport dynamics in a macrotidal embayment, and implications for export to the southern Great Barrier Reef shelf

Keppel Bay is a macrotidal embayment on a tectonically stable, tropical coast, which links the Fitzroy River with the Great Barrier Reef continental shelf. Estuaries and deltas act as conduits between catchments and inner shelf environments. Therefore, understanding sediment transport pathways in these complex systems is essential for the management of ecosystems such as coral-reefs that are potentially vulnerable to enhanced river sediment loads. Furthermore, the morphology and sediment dynamics of subtidal sand ridges and dunes are relatively poorly characterised in macrotidal estuaries, particularly in turbid, episodic systems such as the Fitzroy River and Keppel Bay. Our sedimentological analysis of seabed samples, shear-stress modelling and three-dimensional acoustic imaging reveals that Keppel Bay is a mixed wave- and tide-dominated estuarine system. Areas of sediment starvation and shoreward transport characterise the offshore zone, whereas a complex of both active and relict tidal sand ridges, and associated subaqueous dunes, dominate the relatively protected southern Keppel Bay. Transport within this region is highly dynamic and variable, with ebb-dominated sediment transport through tidal channels into the outer bay where there is a switch to wave-dominated shoreward transport. Ultimately, bedload sediments appear to be reworked back inshore and to the north, and are gradually infilling the bedrock-defined embayment. Our characterisation of the Keppel Bay system provides a detailed example of the physiography of the seaward portion of a tide-dominated system, and shows that sediment transport in these areas is influenced by a variable hydrodynamic regime as well as relict channels and bedrock topography.

Carbon Burial Rates in Sediments and a Carbon Mass Balance for the Herbert River Region of the Great Barrier Reef Continental Shelf, North Queensland, Australia

The largest century scale carbon reservoir in the Herbert River sector of the Great Barrier Reef continental shelf is carbonate carbon. This carbonate carbon is produced by corals, coralline algae, and other benthic organisms, and is stored on less than 13% of the area of the shelf. Maximum organic carbon burial rates (1–28molm−2yr−1) occur within a wind-protected <20m water depth embayment (<1% area of the shelf), where the highest bulk sedimentation rates (1–12kgm−2yr−1) were determined from 210Pb and137 Cs profiles in 1–4m cores. Sediment, organic carbon (OC), and carbonate carbon (CC) accumulation rates are very low in the Great Barrier Reef lagoon shelf and slope, where most annual productivity (8–15molOC m−2yr−1) is decomposed by algal and microbial respiration. A carbon mass balance indicated that approximately 1% of combined river and marine organic carbon production was preserved in across-shelf sedimentation, but 3% of river and mangrove organic carbon input was preserved in this small wind-protected embayment. An across-shelf carbon mass balance model predicted average shelf organic matter respiration correctly, and the ratio of organic carbon fixation to respiration (corrected for burial losses) was 1·06.

Tidal current variability in the Central Great Barrier Reef

With tidal data from the literature and the field, a two-dimensional (depth-averaged) numerical model was formulated to simulate the dominant semi-diurnal tidal hydrodynamics of the Central Great Barrier Reef continental shelf. Importantly, the individual mesh dimensions of the numerical scheme were set at approximately 2 × 2 km which was sufficient resolution to incorporate the topography of each reef within the matrix. The model provided a new detailed understanding of the influence of the reef matrix on the tidal currents of the outer shelf. In particular, the model demonstrated that the spatial variability in the tidal current's speed and direction exists down to the scale of the 2 km grid size. The model also demonstrated the significant tidally-induced residual currents that result from the interaction with the complex topography of the reef matrix. The advective effect of these tidal currents would be significant as the tidally-induced residual currents are of similar magnitude to the non-tidal currents of the region. Further, the spatial variability in the modelled and observed tidal currents suggests highly spatially variable advective processes operate within the reef waters.

Late Quaternary sedimentation on the Great Barrier Reef continental shelf and slope east of Townsville, Australia

Cores collected in a shelf to basin transect from northeastern Australia document the stratigraphic development of late Quaternary deposits. Low sea-level (to − 120 m) slope sedimentation is dominated by prograding fluvial deposits with wave-dominated deltas forming on the shelf edge and upper slope. The onset of sea-level transgression into the excavated shelf river channels on the outer shelf resulted in estuarine sedimentation along an indented coastline (15,000–11,000 yrs B.P.), reducing terrigenous sediment supplies to the slope. On the outer shelf, Halimeda muddy sandy gravels were deposited over a Pleistocene limestone erosional surface from about 10,500 yrs B.P. Transgression submerged the limestone outcrops (and estuaries) of the middle shelf where coral reef complexes formed; the outer and middle parts of the shelf thus became dominated by carbonate sedimentation while the inner shelf received fluvial input mixed with locally derived carbonates. The sequence is bounded by an unconformity, clearly marked by erosional down-cutting. The results of the study are combined with those of previous workers to develop transgressive and continued high sea-level system tract models. The latter (continued high sea level) model is used to extrapolate present high sea-level sedimentation patterns into the future, providing a model for comparison with ancient sequences which may also have formed in such a depositional environment.

Benthic processes across mixed terrigenous-carbonate sedimentary facies on the central Great Barrier Reef continental shelf

The shallow continental shelf of the central Great Barrier Reef province is characterized by an across-shelf gradient of terrigenous to carbonate sedimentary facies, and is a region subjected to physical disturbances from commercial prawn trawling and, less frequently, from summer cyclones and storms. The benthic regime was examined during austral spring and summer to delimit changes in relation to the sedimentary transition. Standing stocks of bacteria ( x = 1.2;range= 0.4-2.7 × 10 10cells g −1 dry wt), protozoa ( x = 1.6;range= 1.0–2.1 × 10 6cells m -2) and meiofauna (abundances: x = 1955;range= 810–3255ind.10 cm −2; biomass: x = 2028;range= 694–3130μg dry wt 10 cm −2) generally did not vary significantly across the shelf and are very abundant compared to their counterparts on other shelves. Bacterial productivity ( 3H thymidine incorporation into DNA) and specific growth rates (μ) ranged from 0.4 to 2.6 g C m −2 d −1 ( x = 1.5) and from 0.1 to 0.8 d −1 ( x = 0.3), respectively, in surface sediments and generally did not decline significantly with sediment depth. Standing amounts of chlorophyll a ( x = 0.6;range= 0.1–2.0 μg g −1dry wt) and phaeopigments ( x = 3.9;range= 1.0–1.6 μ g −1dry wt) were low compared with pigment levels in adjacent slope and bathyal trough sediments, indicating low deposition rates of phytodetritus on the shelf proper. Macroinfaunal densities ( x = 3056;range = 2060–5406 ind. m −2) were moderate to high, but biomass ( x = 1.96;range = 0.94-3.37 g ash-free dry wt m −2) was low and decreased significantly across the shelf. Small, tube-building deposit and suspension-feeding polychaetes and amphipods dominated the infauna with a notable absence of bivalve molluscs. X-radiography revealed sedimentary profiles characteristic of oligotrophic seafloors experiencing low and variable rates of sedimentation and infaunal bioturbation, but subjected to episodes of physical reworking. The sediment fabric was extensively bioturbated mainly in the upper 2 cm with compact (<40% water) sediments devoid of large equilibrium infauna below this depth. Levels of organic carbon, total nitrogen, total phosphorus and porewater (NH 4 +, NO 2 − + NO 3 − Si(OH) 4, PO 4 3−) nutrients were low compared to other shelf sediments and generally varied little with sediment depth indicating some vertical mixing, but little burial of reactive organic matter. Dissolved inorganic nutrient fluxes measured in incubated ☐corers were low (ΣN = 0–2591; PO 4 3−; = 0–142; Si(OH) 4 = 1100–5790 μmol m −2 d −1) compared to other benthic environments suggesting low rates of detrital input. Oxygen consumption rates ranged from 13 to 40 mmol O 2 m −2 d −1. Nutrient fluxes were not directly related to surface respiration, bacterial growth or temperature. The average stoichiometric ratios of solid-phase (atomic C:N:P = 23:5:1) nutrients and measured fluxes (Si:P = 66:1; N:P = 9:1; O:N = 39:1) differed substantially from the Redfield ratio. Departure from the predicted ratios was attributed to denitrification and preferential retention of P due to binding with Fe and Mn oxyhydroxides and CaCO 3. Benthic standing crops and processes on the central shelf appear to be regulated by a variety of factors, including low and intermittent inputs of detritus, continually warm temperatures and physical disturbances caused by occasional cyclones and by frequent prawn trawling. Such disturbances and oligotrophic conditions perpetuate the dominance of a pioneering benthic regime as well as the cycle of low plankton production and sedimentation to the seafloor. Nutrient release from the benthos contributes, on average, 13 and 24% of the annual N and P requirements of the shelf phytoplankton, suggesting that a moderate coupling exists between benthic and pelagic regimes on this shallow tropical shelf.

Long-term observations of currents on the central Great Barrier Reef continental shelf

Time-series are presented of wind, sea levels, currents and temperature in the central region of the Great Barrier Reef continental shelf over the period 1980 to 1982 (inclusive). Except for occasional periods of calm weather or during the passage of tropical cyclones, the wind was alternately southwestward or northwestward, with periods of several days to several weeks. The fluctuations of the low-frequency sea level, longshore current and wind components, but not temperature, were highly coherent over distances >400 km longshore and over the shelf width. The temperature and the atmospheric pressure controlled much of the seasonal changes in the sea level. A simple model for wind-driven circulation using a non-linear bottom friction law is able to account for most of the variance of the currents with perios <20 days. An additional poleward base flow was present at all sites, probably driven by the circulation in the Coral Sea. There were also large interannual variations, possibly associated with the El Nino Southern Oscillation phenomena. These observations imply that the advection and dispersion over the Great Barrier Reef of fish eggs and larvae, coral planulae etc., and hence reef recruitment, experience considerable variability at time scales of days of years.

Wind-driven circulation on the northern Great Barrier Reef continental shelf in summer

Observations of wind, atmospheric pressure, sea levels and current are presented for the northern (9°14′S) Great Barrier Reef continental shelf for November 1981 to May 1982. Strong low-frequency non-tidal oceanic fluctuations were observed, resulting in alternating northward and southward transport and a weak ‘mean’ circulation, and are the result of long ‘arrested’ topographic waves driven by a quasi-steady longshore wind stress and damped by turbulent bottom friction. The coefficient of friction for low-frequency currents is found to be proportional to the tidal velocities. The coefficient is also found to be much larger than that in the central (15–19°S) region of the Great Barrier Reef continental shelf, and this difference is attributed to the greatly enhanced energy dissipation by secondary circulation around coral reefs in the reef-studded northern region. Sough of Cape York (10·5°S), the primary effect of the cross-shelf wind is to complement the geostrophic set-up due to the longshore current. Intense tidal currents in the vicinity of Cape York combined with a dissipative western boundary in Torres Strait appear to prevent long wave propagation north of Cape York where a steady-state analytical model is used to show that both cross- and longshore wind components generate the reversing currents observed through Bligh Entrance.

River plumes, Coral Reefs and mixing in the Gulf of Papua and the northern Great Barrier Reef

The mean annual freshwater discharge in the Gulf of Papua, principally from the Fly, Kikori and Purari rivers, is estimated to be 13000 m3 s−1. Water from these rivers forms a low salinity surface layer in the Gulf, where it has a residence time of about two months and flows generally eastward. A small fraction of this water intrudes through Bligh Entrance on the Great Barrier Reef continental shelf. Horizontal patchiness and mixing of these intruding waters with shelf waters are considerably enhanced by secondary circulation (wakes) around coral reefs. The wakes shapes, visible in enhanced LANDSAT imagery, are similar to those around cylinders and plates in laboratory experiments at low Reynolds numbers. Topographically enhanced mixing may explain why the cross-shelf gradients of temperature and of the structure of fish communities are smaller in the northern region than in the central region of the Great Barrier Reef.