A carbonate and terrigenous sediment budget for inshore turbid reefs on the Great Barrier Reef

Browne, Nicola (2011) A carbonate and terrigenous sediment budget for inshore turbid reefs on the Great Barrier Reef. PhD thesis, James Cook University.

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Inshore turbid zone coral reefs on the central Great Barrier Reef (GBR) are situated within 20 km of the coast where terrigenous sediments influence coral communities, carbonate production and reef growth. They exist within a range of geomorphic settings from open coastal settings to muddy coastal embayments, and include fringing and nearshore reefs and shoals. Inshore regions on the central GBR are characterised by high sediment yields and suspended sediment loads, elevated nutrients and fluctuating salinities. These marginal environmental conditions are widely viewed as unfavourable for sustained and vigorous coral reef growth, and thus it is commonly claimed that inshore turbid reefs are stressed and/or degraded. However, recent research has challenged this and demonstrates that many have high coral cover and robust coral communities, and that reefs have rapidly accreted to sea level despite exposure to elevated terrigenous sediments. The importance of terrigenous sediments for coral community composition and turbid zone reef growth has yet to be quantitatively evaluated due to a lack of detailed data and limited knowledge on sedimentary interactions and processes in these highly dynamic sedimentary settings.

The overall aim of this research was to provide a comprehensive assessment of carbonate and terrigenous sediment regimes for inshore turbid reefs on the central GBR by quantifying carbonate production and destruction together with sediment deposition, resuspension and transport across the reef. Specifically, the objectives of this research were to: 1) examine benthic community composition and distribution; 2) examine spatial variations in sediment texture and composition; 3) investigate the influence of spatial and temporal variations in turbidity on benthic cover; 4) quantify the sedimentary regime and examine its role in reef growth; 5) investigate spatial and temporal variations in coral growth and carbonate production; and 6) quantify carbonate production and destruction together with sediment import, storage and export to assess reef growth. This research focused on two inshore turbid zone reefs on the central GBR; Middle Reef, a nearshore reef situated between Magnetic Island and Townsville, a large urban area with a major port; and Paluma Shoals, approximately 30 km north of Townsville on a more exposed coastline and distal to direct anthropogenic pressures that may influence Middle Reef. These two sites were chosen to examine the influence of variable hydrodynamic and sedimentary regimes on coral community composition and distribution, and on net carbonate production and reef growth.

At both reefs coral cover was high (>30%) and diversity was moderate to high (>50 species). The coral community distribution was independent of depth and was instead driven by spatial variations in sedimentation rates and turbidity, largely controlled by reef morphological interactions with waves, currents and tides. Coral communities were dominated by either fast-growing species such as Acropora and Montipora, most abundant on the exposed windward reef edges, sediment tolerant species such as Turbinaria, Galaxea and Goniopora which dominated the leeward reef edges and were also abundant at the base of windward reef slopes, and Goniastrea which dominated the reef flats. Investigations into temporal community dynamics at Middle Reef show that coral cover on the windward reef edge (73%) has increased over the last 15 years despite a history of episodic mortality events. These data demonstrate that these coral communities are robust and resilient, and challenge perceptions that inshore turbid reefs are degraded.

Reef morphology influenced sediment composition, distribution and resuspension over both reefs. Sediments consisted of varying proportions of silt, sand and gravel, and the carbonate component was dominated by coral and mollusc fragments. The mean grain size decreased from the eastern windward reef slopes to the western sheltered leeward edge reflecting wave energy dissipation across both reefs. The mean grain size was greater at Paluma Shoals, where higher wave energy resuspended and redistributed sediments over the reef and finer sediments were winnowed away. As such, sediment composition and distribution was not significantly correlated to reef benthos. In contrast, lower wave energy and limited redistribution of sediments at Middle Reef resulted in a strong correlation between sediment composition and reef benthos. Given spatial distributions in both wave energy and sediment composition, sediment resuspension rates and turbidity also varied across both reefs. These turbidity gradients were reflected in coral community distributions with a greater abundance of heterotrophic corals in reef habitats characterised by rapid and large fluctuations in turbidity. Local wind speed data accounted for <73% and <56% in the variance in turbidity at Paluma Shoals and Middle Reef respectively, and was used to generate a site-specific turbidity model. The model will enable future researchers to direct real time management for turbidity risk assessments, identify increases in turbidity above the natural turbidity regime and assess the implications for coral communities and reef health.

A detailed quantitative assessment of the sediment regime (deposition, resuspension and removal) developed using both established and new techniques, reveals that despite high sediment flux rates (<20, 000 tonnes annually), net sedimentation rates are low (<50 g/m²/day) due to sediment resuspension and removal. Established techniques included the use of data loggers to measure spatial and temporal variations in turbidity with waves and currents, whereas sedimentation and resuspension rates were measured using 'sediment trays'. The use of sediment trays overcame the limitations of commonly used 'sediment traps' which over-estimate sedimentation rates and preferentially collect larger particles. The sediment regime was quantified across two depth zones (0.5 to -1.5 m, <-1.5 to -3.5 m at LAT), and within five geomorphic habitats (eastern, central and western windward reef edge, inner basins or reef flat and leeward reef edge) to provide data for a model that illustrated the direction and rate of sediment delivery, deposition and removal across both reefs. The model illustrated that >81% of sediments imported annually onto turbid reefs are exported as suspended sediments due to high wave energies, which corresponded to elevated turbidity (>50 mg/L). These results suggest that despite a high sediment flux rate through these reef systems, sediment deposition is limited and therefore does not impede inshore reef growth and survival within terrigenous settings.

Coral growth is influenced by environmental conditions such as sea surface temperatures (SST) and water quality, and can be used to assess coral condition as well as the rate of carbonate production. In this study the coral growth rates (linear extension, density, calcification rates) of three fast-growing corals (Acropora, Montipora, Turbinaria), common to both inshore turbid reef and offshore clear-water reefs, were studied in situ on Middle Reef to provide some of the first data used to quantify carbonate production for inshore turbid reefs. Our investigations found that Acropora growth rates (average rate of 6.3 cm/year) were comparable to those measured at similar depths on mid to offshore reefs on the GBR. Montipora linear extension (2.9 cm/year) was greater than current estimates available for both turbid and clear-water reefs, and Turbinaria, although characterised by low linear extension (1 cm/year), had a dense skeleton (1.3 g/cm³) and may be more resilient to physical damage. Spatial variations in coral growth and carbonate production rates were driven by water motion and sediment dynamics, and temporal variations indicated that coral growth was lower during the summer when SSTs (mean 29 °C) and rainfall (monthly 500 mm) were high. In summary, high contemporary growth rates on inshore turbid reefs is in accord with rapid accretion rates established from the fossil record for numerous turbid reefs on the GBR and indicate that corals on Middle Reef are resilient to their marginal environmental conditions.

This research provides the first quantitative assessment of carbonate production and destruction together with sediment import, storage and export, to evaluate the rate and mode of reef growth for inshore turbid reefs. The mean net carbonate production rate was 12 kg/m²/year at Middle Reef and 7 kg/m²/year at Paluma Shoals, although varied between habitats with lowest rates measured on shallow reef flats (>1 kg/m²/year) and highest rates at the base of reef slopes (<19 kg/m²/year). The mean net carbonate production rate was converted to a reef accretion rate, which was greater at Middle Reef (5.2 mm/year) reflecting the higher coral and Acropora cover than at Paluma Shoals (3 mm/year). The mode of reef growth for each reef habitat was determined by comparing the rate of sediment deposition to carbonate accumulation; if carbonate production was high and sediment deposition limited, it was production-dominated; if sediment accumulation was greater than carbonate production, it was import dominated; and if the rate of sediment resuspension was greater than the rate of sediment deposition, it was export dominated. The mode and rate of reef growth were used to construct a reef growth model, with accretion in deep reef habitats taken as a proxy for early reef growth. The model provides an assessment of reef growth in a terrigenous setting with depth and time, quantitatively links sedimentary processes to ecological processes over time and space, and can be used to assess how reef growth may respond to future environmental changes such as increased sediment delivery, rising sea-level and increased SSTs.

Item ID: 28079
Item Type: Thesis (PhD)
Keywords: CaCO3; carbonate destruction; carbonate production; carbonate sediments; Central GBR; coral communities; fringing reefs; GBR; Great Barrier Reef; inshore turbid reefs; marine sediments; Middle Reef; nearshore reefs; oceanography; Paluma Shoals; reef growth; sediment dynamics; sedimentary budgets; sedimentation; sediments; terrigenous sediments; turbid zone coral reefs; turbidity
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Publications arising from this thesis are available from the Related URLs field. The publications are:

[Chapter 2]: Browne, N.K., Smithers, S.G., and Perry, C.T. (2012) Coral reefs of the turbid inner-shelf of the Great Barrier Reef, Australia: an environmental and geomorphic perspective on their occurrence, composition and growth. Earth-Science Reviews, 115 (1-2). pp. 1-20.

[Chapter 3]: Browne, N.K., Smithers, S.G., and Perry, C.T. (2010) Geomorphology and community structure of Middle Reef, central Great Barrier Reef, Australia: an inner-shelf turbid zone reef subject to episodic mortality events. Coral Reefs, 29 (3). pp. 683-689.

[Chapter 6]: Browne, Nicola K., Smithers, Scott G., Perry, Chris T., and Ridd, Peter V. (2012) A field-based technique for measuring sediment flux on coral reefs: application to turbid reefs on the Great Barrier Reef. Journal of Coastal Research, 28 (5). pp. 1247-1262.

Date Deposited: 17 Jul 2013 06:08
FoR Codes: 06 BIOLOGICAL SCIENCES > 0602 Ecology > 060205 Marine and Estuarine Ecology (incl Marine Ichthyology) @ 34%
04 EARTH SCIENCES > 0403 Geology > 040310 Sedimentology @ 33%
04 EARTH SCIENCES > 0403 Geology > 040305 Marine Geoscience @ 33%
SEO Codes: 96 ENVIRONMENT > 9608 Flora, Fauna and Biodiversity > 960808 Marine Flora, Fauna and Biodiversity @ 50%
96 ENVIRONMENT > 9611 Physical and Chemical Conditions of Water > 961104 Physical and Chemical Conditions of Water in Marine Environments @ 50%
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