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
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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)