Abstract
Abstract Nearshore coral communities within turbid settings are typically perceived to have limited reef-building capacity. However, several recent studies have reported reef growth over millennial time scales within such environments and have hypothesized that depth-variable community assemblages may act as equally important controls on reef growth as they do in clear-water settings. Here, we explicitly test this idea using a newly compiled chronostratigraphic record (31 cores, 142 radiometric dates) from seven proximal (but discrete) nearshore coral reefs located along the central Great Barrier Reef (Australia). Uniquely, these reefs span distinct stages of geomorphological maturity, as reflected in their elevations below sea level. Integrated age-depth and ecological data sets indicate that contemporary coral assemblage shifts, associated with changing light availability and wave exposure as reefs shallowed, coincided with transitions in accretion rates at equivalent core depths. Reef initiation followed a regional ∼1 m drop in sea level (1200–800 calibrated yr B.P.) which would have lowered the photic floor and exposed new substrate for coral recruitment by winnowing away fine seafloor sediments. We propose that a two-way feedback mechanism exists where past growth history influences current reef morphology and ecology, ultimately driving future reef accumulation and morphological change. These findings provide the first empirical evidence that nearshore reef growth trajectories are intrinsically driven by changes in coral community structure as reefs move toward sea level, a finding of direct significance for predicting the impacts of extrinsically driven ecological change (e.g., coral-algal phase shifts) on reef growth potential within the wider coastal zone on the Great Barrier Reef.
Highlights
Coral communities that form within highly turbid nearshore habitats are widely considered to be marginal environments for long-term reef building
Coral community responses and reef-building capacity under declining water quality are likely a function of the magnitude of localized sedimentation regime shifts against the longterm background environmental conditions under which the corals established. This is supported by core-based chronostratigraphic reconstructions from several nearshore sites (
This record, which for the first time spans the full spectrum of reef developmental stages, is examined together with recent field data on contemporary nearshore coral assemblages to provide insights into an intrinsic two-way relationship whereby past reef-growth history drives transitions in reef ecology, and which influence reef-building capacity and morphology
Summary
Coral communities that form within highly turbid nearshore habitats (i.e., shallow-water mesophotic settings; sensu Morgan et al, 2016) are widely considered to be marginal environments for long-term reef building. On the basis of this recent work, Morgan et al (2016) hypothesized that depth-constrained transitions in coral taxa and morphologies may act as important controls on long-term reefbuilding rates and styles in turbid environments as they do within clear-water reef settings, even despite the narrow depth ranges (
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