Transitions in coral reef accretion rates linked to intrinsic ecological shifts on turbid-zone nearshore reefs

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. INTRODUCTION 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. The negative impacts associated with low-light and high-sedimentation conditions on coral calcification and reef ecological health are reported to be exacerbated by elevated terrigenous sediment and nutrient inputs due to coastal catchment landuse change (De’ath and Fabricius, 2010). However, 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 (or “natural”) environmental conditions under which the corals established. This is supported by core-based chronostratigraphic reconstructions from several nearshore sites (<10 m water depth) within the central region of Australia’s Great Barrier Reef, which indicate that reefs within this zone have rapidly accreted (2–7.8 mm yr–1) and supported diverse coral assemblages under conditions of terrigenous sediment influence throughout the midto late Holocene (Perry et al., 2012). Furthermore, recent research has shown high contemporary coral cover on these reefs (Morgan et al., 2016), most notably of the key reef-building taxa (branching Acropora spp.) that have reportedly declined in abundance at sites further offshore (e.g., Pelorus Island; Roff et al., 2012). On the basis of this recent work, Morgan et al. (2016) hypothesized that depth-constrained transitions in coral taxa and morphologies may act as equally 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 (<5 m depth), reduced coral diversity, and the atypical environmental conditions (i.e., low light and high turbidity) under which they form. Here, we test this hypothesis using a newly compiled chronostratigraphic record based on 31 reef cores and 142 radiometric dates, and use this to examine the growth histories of seven proximal (but discrete) reefs (i.e., reefs that grew under nearidentical environmental conditions) that are currently at different depths below sea level. 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 ultimately influence reef-building capacity and morphology. Our findings confirm that on these turbid nearshore reefs, reef morphology not only underpins habitat availability and ecological dynamics (e.g., the coral taxa and growth morphologies present), but also ultimately influences rates of vertical growth as the reefs mature. Importantly, these findings have wider relevance across other similarly lowimpacted (located further from river inputs and/ or coastal development) nearshore sites on the Great Barrier Reef where comparable ecological patterns have been reported (Browne et al., 2010). FIELD SETTING AND METHOD We compiled a core record from seven proximal nearshore reefs that form the Paluma Shoals reef complex (PSRC) located ~3 km from the coast within Halifax Bay, central Great Barrier Reef (Fig. 1; 19.1145°S, 146.5497°W). Reef surfaces span a range of water depths (-0.6 to +0.5 m relative to lowest astronomical tide [LAT]), sitting atop subtle (1-2 m elevation) rhythmically spaced arcuate ridges (see Morgan et al., 2016). These reefs experience episodic high turbidity (up to 385 mg L–1) caused by wave-driven resuspension of seafloor terrigenous sediment (Browne et al., 2013b), and periods of chronic turbidity following sustained southeast-northwest trade winds. Coral cover (June 2014) across the structures is high (mean: 38% ± 24%; Morgan et al., 2016), and coral assemblages are highly depth stratified as light attenuates rapidly through the turbid water. Our core record includes 16 new cores and 72 radiometric dates collected from four previously unsampled submerged reefs (-0.6 to -0.4 m LAT) (offshore Paluma Shoals A, B, C, and D [OPSA, OPSB, OPSC, and OPSD]; Fig. 1; Table DR1 in the GSA Data Repository1). These cores represent the first records of earlystage nearshore reef development and augment 1 GSA Data Repository item 2016335, methods and Tables DR1 and DR2, is available online at www .geosociety .org /pubs /ft2016.htm or on request from [email protected]. GEOLOGY, December 2016; v. 44; no. 12; p. 995–998 | Data Repository item 2016335 | doi:10.1130/G38610.1 | Published online 7 October 2016 © 2016 The Authors. Gold Open Access: This paper is published under the terms of the CC-BY license. 996 www.gsapubs.org | Volume 44 | Number 12 | GEOLOGY an existing record of 15 cores (70 radiometric dates) collected during earlier investigations (Paluma Shoals north [PSN] and south [PSS], Palmer et al., 2010; offshore Paluma Shoals [OPS], Perry et al., 2013; Fig. 1). Together they provide a unique data set spanning the full spectrum of nearshore reef developmental stages from incipient reef growth through to full sealevel emplacement. All cores (collected using percussion techniques with 100% recovery of undisturbed core material) penetrated the entire Holocene reef sequence, terminating in pre-reefal sedimentary units, and were logged to record biosedimentary facies information. In situ coral material was selected for accelerator mass spectrometry radiocarbon dating to determine reef chronology (see the Data Repository for methodology). Recent depth-calibrated ecological data sets of the study area (Morgan et al., 2016) were used to determine coral community distribution which were then applied to corresponding core depths. Previously, a poor understanding of nearshore ecological communities meant that such comparisons were not possible. RESULTS Core records collected from across the PSRC show that all reefs initiated directly on Pleistocene clays or intertidal marine sands and lithic gravels ca. 2000–700 calibrated (cal.) yr B.P. in contemporary water depths of between –1.5 m LAT (PSN) and –4.3 m LAT (OPSC) (Fig. 2). Three of the seven reef structures within PSRC (PSN, PSS, OPS) have recently become sealevel constrained (since <450 cal. yr B.P.), as indicated by the development of extensive semiemergent reef flats. Terrigenous muds were volumetrically important within the reef framework throughout all the cores, suggesting that naturally high levels of mud deposition were occurring long before reported increased anthropogenic sediment inputs following European settlement (ca. A.D. 1862). Mean accretion rate across all reefs that compose the PSRC was 4.5 ± 3.3 mm yr–1, but there was variability between the different stages of reef geomorphic maturity (Fig. 3A), and each stage was associated with