Palaeoecological records of coral community development on a turbid, nearshore reef complex: baselines for assessing ecological change

Coral Reefs, Mar 2017

Understanding past coral community development and reef growth is crucial for placing contemporary ecological and environmental change within appropriate reef-building timescales. On Australia’s Great Barrier Reef (GBR), coral reefs situated within coastal inner-shelf zones are a particular priority. This is due to their close proximity to river point sources, and therefore susceptibility to reduced water quality discharged from coastal catchments, many of which have been modified following European settlement (ca. 1850 AD). However, the extent of water-quality decline and its impacts on the GBR’s inner-shelf reefs remain contentious. In this study, palaeoecological coral assemblage records were developed for five proximal coral reefs situated within a nearshore turbid-zone reef complex on the central GBR. A total of 29 genera of Scleractinia were identified from the palaeoecological inventory of the reef complex, with key contributions to reef-building made by Acropora, Montipora, and Turbinaria. Discrete intervals pre- and post-dating European settlement, but associated with equivalent water depths, were identified using Bayesian age–depth modelling, enabling investigation of competing ideas of the main drivers of nearshore coral assemblage change. Specifically, we tested the hypotheses that changes in the composition of nearshore coral assemblages are: (1) intrinsically driven and linked to vertical reef development towards sea level, and (2) the result of changes in water quality associated with coastal river catchment modification. Our records found no discernible evidence of change in the generic composition of coral assemblages relative to European settlement. Instead, two distinctive depth-stratified assemblages were identified. This study demonstrates the robust nature of nearshore coral communities under reported water-quality decline and provides a useful context for the monitoring and assessment of ecological change on reefs located within the most nearshore turbid-zone environments of the central GBR.

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Palaeoecological records of coral community development on a turbid, nearshore reef complex: baselines for assessing ecological change

Palaeoecological records of coral community development on a turbid, nearshore reef complex: baselines for assessing ecological change J. A. Johnson 0 1 2 3 C. T. Perry 0 1 2 3 S. G. Smithers 0 1 2 3 K. M. Morgan 0 1 2 3 N. Santodomingo 0 1 2 3 K. G. Johnson 0 1 2 3 0 School of Geography, College of Life and Environmental Sciences, University of Exeter , Exeter EX4 4RJ , UK 1 Communicated by Geology Editor Prof. Eberhard Gischler 2 Department of Palaeontology , Natural History Museum, Cromwell Road, London SW7 5BD , UK 3 College of Marine and Environmental Sciences, James Cook University , Townsville, QLD 4811 , Australia Understanding past coral community development and reef growth is crucial for placing contemporary ecological and environmental change within appropriate reef-building timescales. On Australia's Great Barrier Reef (GBR), coral reefs situated within coastal inner-shelf zones are a particular priority. This is due to their close proximity to river point sources, and therefore susceptibility to reduced water quality discharged from coastal catchments, many of which have been modified following European settlement (ca. 1850 AD). However, the extent of waterquality decline and its impacts on the GBR's inner-shelf reefs remain contentious. In this study, palaeoecological coral assemblage records were developed for five proximal coral reefs situated within a nearshore turbid-zone reef complex on the central GBR. A total of 29 genera of Scleractinia were identified from the palaeoecological inventory of the reef complex, with key contributions to reef-building made by Acropora, Montipora, and Turbinaria. Discrete intervals pre- and post-dating European settlement, but associated with equivalent water depths, were identified using Bayesian age-depth modelling, enabling investigation of competing ideas of the main drivers of nearshore coral assemblage change. Specifically, we tested the hypotheses that changes in the composition of nearshore coral assemblages are: (1) intrinsically driven and linked to vertical reef development towards sea level, and (2) the result of changes in water quality associated with coastal river catchment modification. Our records found no discernible evidence of change in the generic composition of coral assemblages relative to European settlement. Instead, two distinctive depth-stratified assemblages were identified. This study demonstrates the robust nature of nearshore coral communities under reported water-quality decline and provides a useful context for the monitoring and assessment of ecological change on reefs located within the most nearshore turbidzone environments of the central GBR. Ecological baselines; European settlement; Great Barrier Reef; Inner shelf; Palaeoecology; Water quality - Nearshore coastal environments that experience high levels of sedimentation and turbidity are widely perceived as ‘marginal’ (sensu Perry and Larcombe 2003) for coral growth and reef development. This is because sediments, especially fine-grained terrigenous sediments, can reduce coral fecundity and survival (Erftemeijer et al. 2012; Jones et al. 2015). Coral reefs located in environments exposed to regimes of naturally high sediment flux (defined by the total mass of sediment deposited and resuspended; Browne et al. 2012c) are often referred to as ‘turbid-zone reefs’. On Australia’s Great Barrier Reef (GBR), turbid-zone reefs occur across a range of geomorphic settings, located within a shallow (\20 m deep) inner-shelf zone which extends up to 20 km offshore of the mainland coast (Larcombe and Woolfe 1999b; Browne et al. 2012a). The inner shelf of the GBR is characterised by the presence of fine-grained terrigenous sediments which form an inshore sediment prism that extends offshore towards a seaward limit close to the 15-m isobath (Woolfe and Larcombe 1999). In the shallow (\5 m depth) coastal areas of the inner shelf, these finegrained sediments are frequently resuspended by waves, producing wide fluctuations in turbidity (Larcombe et al. 2001; Browne et al. 2012b). Based on the prevailing environmental conditions, a distinction has recently been made between inner-shelf reefs that occur within coastal ‘nearshore’ (\10 m isobath) areas and those located further offshore in more ‘inshore’ settings ([10 m isobath), where the local natural sedimentary background conditions are less extreme (Morgan et al. 2016a). Despite the sedimentary setting of the GBR inner shelf, the reefs located within this zone can support diverse coral communities. Regional species richness typically exceeds 60 species across the entire inner shelf (Sweatman et al. 2007) and live coral cover averages *20% (www.aims. gov.au/docs/data-centre/reef-monitoring-surveys.html), but can be as high as *40% (e.g. Browne et al. 2010; Morgan et al. 2016a). However, declining water quality is considered a major threat to the health of inner-shelf reefs across the GBR (Brodie et al. 2012). Since European settle (...truncated)


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J. A. Johnson, C. T. Perry, S. G. Smithers, K. M. Morgan, N. Santodomingo, K. G. Johnson. Palaeoecological records of coral community development on a turbid, nearshore reef complex: baselines for assessing ecological change, Coral Reefs, 2017, pp. 1-16, DOI: 10.1007/s00338-017-1561-1