Reef Growth Potential Research Articles

Growing at the limit: Reef growth sensitivity to climate and oceanographic changes in the South Western Atlantic

Whilst the impacts of climatic and oceanographic change on lower latitude reefs are increasingly well documented, our understanding of how reef-building has fluctuated in higher latitude settings remains limited. Here, we explore the timing and longevity of reef-building through the mid- to late Holocene in the most southerly known reef (24°S) in the Western Atlantic. Reef core data show that reef growth was driven by a single coral species, Madracis decactis, and occurred over two phases since ~6000 calibrated (cal.) yr B.P.. These records further indicate that there was a clear growth hiatus from ~5500 to 2500 cal. yr B.P., and that there is no evidence of reef accretion on the Queimada Grande Reef (QGR) over the past 2000 yrs. It thus presently exists as a submerged senescent structure colonized largely by non-reef building organisms. Integration of these growth data with those from sites further north (18°S and 21°S) suggests that Intertropical Convergence Zone (ITCZ), South Westerlies Winds (SWW) and El Niño-Southern Oscillation (ENSO) variability and shifts during the Holocene drove changes in the position of the Brazil-Falklands/Malvinas Confluence (BFMC), and that this has had a strong regional influence on the timing and longevity of reef growth. Our results add new evidence to the idea that reef growth in marginal settings can rapidly turn-on or -off according to regional environmental changes, and thus are of relevance for predicting high latitude reef growth potential under climate change.

Re-evaluating mid-Holocene reef “turn-off” on the inshore Southern Great Barrier Reef

In the face of changing global climate the future of corals reefs is uncertain. High latitude reefs may offer potential refugia for corals under projected increasing sea surface temperatures (SSTs). To understand the reef growth potential of modern high latitude reefs it is first necessary to understand past reef growth and response to climatic and environmental changes. The history of the Great Barrier Reef (GBR) has been shown to be punctuated by a multi-millennial Mid-Holocene reef “turn-off” or initiation hiatus in the Northern and Southern GBR (∼5500–2500 years before present [yr. BP]). Here we present the results of chronologically constrained reef matrix cores from five continental island fringing reefs and coral communities in the Keppel Islands to revaluate the timing, extent and possible drivers of the Mid-Holocene hiatus in the Southern GBR. Earliest initiation occurred at Wedge and Halfway Islands at 7773 ± 19 yr. BP and 7455 ± 20 yr. BP, respectively. Following initiation, vertical reef accretion at Halfway was comparable to previously reported rates for the GBR of ∼3.4 mm yr−1, increasing to ∼8.0 mm yr−1 between 6100 and 5500 yr. BP. Conversely, the coral community at Wedge Island, located closer to mainland terrestrial influence, accreted <1.0 mm yr−1 to 5500 yr. BP after which a 3000-year stratigraphic hiatus is observed. A negative relative sea level oscillation at 5500 yr. BP coincides with the transition of Halfway reef from vertical accretion to lateral progradation and the “turn-off” of the Wedge Island coral community. The reef at Middle Island appears to have initiated in the midst of the Mid-Holocene hiatus period at ∼3500 yr. BP, with vertical accretion at both Middle and Halfway Islands being the highest for the region’s history at 12.5 ± 3.3 mm yr−1 and 15.0 ± 6.0 mm yr−1, respectively, between 3500 and 3000 yr. BP. A proposed late-Holocene relative sea level highstand of ∼1 m at ∼2000 yr. BP coincides with the first dates identified at near-shore Divided Island, and the re-initiation of coral growth at Wedge Island, although both showed limited net vertical accretion. These near-shore sites “turned-off” again between 1300 and 900 yr. BP, likely associated with a rapid relative sea level fall. Reef progradation also slowed at Halfway Island and Middle Island after 1200 yr. BP until a recent increase in reef growth at Middle Island since the late 1970’s associated with modern increases in average SSTs. Our data suggests that although reef-scale “turn-off” events occurred, there is no evidence for a regional scale hiatus in coral growth or initiation comparable to that found further north on the GBR. A comparison with other Southern and south-Central GBR reef cores suggests that relative sea level oscillations in conjunction with changes to climate, appear to have driven varying, but synchronous, modifications to reefs at 5500, 4600, 2800 and 1200 yr. BP.

A U‐Th Dating Approach to Understanding Past Coral Reef Dynamics and Geomorphological Constraints on Future Reef Growth Potential; Mazie Bay, Southern Great Barrier Reef

AbstractReconstructing coral reef histories provides a window of understanding into reef response to changing environmental and climatic conditions over various temporal scales. Here we present the results of 117 U‐Th dates from emergent reef flat and slope cores and surface death assemblages, combined with previously published fossil microatoll data, to capture the entire sequence of reef growth at Mazie Bay, inshore Southern Great Barrier Reef (GBR). Coral U‐Th dates indicate that Mazie Bay reef initiated ~6,900 years before present (yr. BP) quickly filling accommodation space. While rates of vertical reef accretion (5.3 ± 1.0 mm year−1) were comparable to the GBR average during the mid‐Holocene (~5.0 mm year−1), reef flat progradation occurred at a rate 1.5‐ to 6‐fold previous GBR rates until 5,100 yr. BP (~70.4 cm year−1). Average progradation slowed to ~7.1 cm year−1 in the subsequent ~4,000 years and reef slope cores indicate this reef had largely “turned‐off” by 400 yr. BP, with modern coral communities existing as a veneer over the largely senescent framework. Death assemblage dates highlight coral disturbance and recovery regimes in response to increased cyclone activity 1960–1985 AD and recent extreme sea surface temperature and flood events post 2000 AD. U‐Th dating of mid‐Holocene to modern coral deposits from Mazie Bay reef provides a unique insight into past reef development, response to recent disturbance regimes, and potential for future reef growth. In the case of Mazie Bay, our data suggest limited accommodation space and increased occurrence of sea surface temperature extremes will restrict future reef growth at this site.

Changing geo‐ecological functions of coral reefs in the Anthropocene

Abstract The ecology of many coral reefs has changed markedly over recent decades in response to various combinations of local and global stressors. These ecological changes have important implications for the abundance of taxa that regulate the production and erosion of skeletal carbonates, and thus for many of the geo‐ecological functions that coral reefs provide, including reef framework production and sediment generation, the maintenance of reef habitat complexity and reef growth potential. These functional attributes underpin many of the ecosystem goods and services that reefs provide to society. Rapidly changing conditions of reefs in the Anthropocene are likely to significantly impact the capacity of reefs to sustain these geo‐ecological functions. Although the Anthropocene footprint of disturbance will be expressed differently across ecoregions and habitats, the end point for many reefs may be broadly similar: (a) progressively shifting towards net neutral or negative carbonate budget states; (b) becoming structurally flatter; and (c) having lower vertical growth rates. It is also likely that a progressive depth‐homogenisation will occur in terms of these processes. The Anthropocene is likely to be defined by an increasing disconnect between the ecological processes that drive carbonate production on the reef surface, and the net geological outcome of that production, that is, the accumulation of the underlying reef structure. Reef structures are thus likely to become increasingly relict or senescent features, which will reduce reef habitat complexity and sediment generation rates, and limit reef potential to accrete vertically at rates that can track rising sea levels. In the absence of pervasive stressors, recovery of degraded coral communities has been observed, resulting in high net‐positive budgets being regained. However, the frequency and intensity of climate‐driven bleaching events are predicted to increase over the next decades. This would increase the spatial footprint of disturbances and exacerbate the magnitude of the changes described here, limiting the capacity of many reefs to maintain their geo‐ecological functions. The enforcement of effective marine protection or the benefits of geographic isolation or of favourable environmental conditions (“refugia” sites) may offer the hope of more optimistic futures in some locations. A &gt;plain language summary is available for this article.

Reef Habitat Type and Spatial Extent as Interacting Controls on Platform-Scale Carbonate Budgets

A coral reefs carbonate budget strongly influences reef structural complexity and net reef growth potential, and thus is increasingly recognised as a key “health” metric. Despite this, understanding of habitat specific budget states, how these scale across reef platforms, and our ability to quantify both framework and sediment production values remains limited. Here we use in-situ census data from an atoll rim reef platform in the central Maldives to quantify rates of both reef framework and sediment production and loss within different platform habitats, and then combine these data with high-resolution habitat maps to quantify contributions to platform wide carbonate budgets. The net reef framework budget for the entire platform is extremely low (0.12 G, where G = Kg CaCO3 m-2 yr-1), with a very high proportion (143,745 kg or 65.1%) of total framework production generated within the platform margin reef zones, despite these comprising only ~8% of platform area. Net platform-scale sediment budgets are higher (1.04 G), but most is produced in the reef and platform margin hardground habitats, of which ~80% derives from parrotfish bioerosion. Significant quantities of new sediment (up to ~1 G derived from the calcareous green algae Halimeda) are produced only in one habitat. All lagoonal habitats have negative or neutral net carbonate budgets. These data demonstrate the marked inter-habitat differences in reef carbonate budgets that occur across reef platforms, and the major dampening effect on overall platform scale budgets when rates are factored for habitat type and size. Furthermore, the data highlights the disproportionately important role that relatively small areas of reef habitat can have on the maintenance of net positive platform scale budgets. Because of the intrinsic link between carbonate production rates and reef-associated landform development and maintenance, these findings also have implications for understanding reef-associated landform stability. In this context the reef island at this site has been highly mobile over the last ~40 years, and we hypothesise that such instability may be being exacerbated by the measured low overall rates of framework and sediment generation.

Drivers and predictions of coral reef carbonate budget trajectories.

Climate change is one of the greatest threats to the long-term maintenance of coral-dominated tropical ecosystems, and has received considerable attention over the past two decades. Coral bleaching and associated mortality events, which are predicted to become more frequent and intense, can alter the balance of different elements that are responsible for coral reef growth and maintenance. The geomorphic impacts of coral mass mortality have received relatively little attention, particularly questions concerning temporal recovery of reef carbonate production and the factors that promote resilience of reef growth potential. Here, we track the biological carbonate budgets of inner Seychelles reefs from 1994 to 2014, spanning the 1998 global bleaching event when these reefs lost more than 90% of coral cover. All 21 reefs had positive budgets in 1994, but in 2005 budgets were predominantly negative. By 2014, carbonate budgets on seven reefs were comparable with 1994, but on all reefs where an ecological regime shift to macroalgal dominance occurred, budgets remained negative through 2014. Reefs with higher massive coral cover, lower macroalgae cover and lower excavating parrotfish biomass in 1994 were more likely to have positive budgets post-bleaching. If mortality of corals from the 2016 bleaching event is as severe as that of 1998, our predictions based on past trends would suggest that six of eight reefs with positive budgets in 2014 would still have positive budgets by 2030. Our results highlight that reef accretion and framework maintenance cannot be assumed from the ecological state alone, and that managers should focus on conserving aspects of coral reefs that support resilient carbonate budgets.

Bleaching drives collapse in reef carbonate budgets and reef growth potential on southern Maldives reefs

Sea-surface temperature (SST) warming events, which are projected to increase in frequency and intensity with climate change, represent major threats to coral reefs. How these events impact reef carbonate budgets, and thus the capacity of reefs to sustain vertical growth under rising sea levels, remains poorly quantified. Here we quantify the magnitude of changes that followed the ENSO-induced SST warming that affected the Indian Ocean region in mid-2016. Resultant coral bleaching caused an average 75% reduction in coral cover (present mean 6.2%). Most critically we report major declines in shallow fore-reef carbonate budgets, these shifting from strongly net positive (mean 5.92 G, where G = kg CaCO3 m−2 yr−1) to strongly net negative (mean −2.96 G). These changes have driven major reductions in reef growth potential, which have declined from an average 4.2 to −0.4 mm yr−1. Thus these shallow fore-reef habitats are now in a phase of net erosion. Based on past bleaching recovery trajectories, and predicted increases in bleaching frequency, we predict a prolonged period of suppressed budget and reef growth states. This will limit reef capacity to track IPCC projections of sea-level rise, thus limiting the natural breakwater capacity of these reefs and threatening reef island stability.

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

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.

Remote coral reefs can sustain high growth potential and may match future sea-level trends.

Climate-induced disturbances are contributing to rapid, global-scale changes in coral reef ecology. As a consequence, reef carbonate budgets are declining, threatening reef growth potential and thus capacity to track rising sea-levels. Whether disturbed reefs can recover their growth potential and how rapidly, are thus critical research questions. Here we address these questions by measuring the carbonate budgets of 28 reefs across the Chagos Archipelago (Indian Ocean) which, while geographically remote and largely isolated from compounding human impacts, experienced severe (>90%) coral mortality during the 1998 warming event. Coral communities on most reefs recovered rapidly and we show that carbonate budgets in 2015 average +3.7 G (G = kg CaCO3 m−2 yr−1). Most significantly the production rates on Acropora-dominated reefs, the corals most severely impacted in 1998, averaged +8.4 G by 2015, comparable with estimates under pre-human (Holocene) disturbance conditions. These positive budgets are reflected in high reef growth rates (4.2 mm yr−1) on Acropora-dominated reefs, demonstrating that carbonate budgets on these remote reefs have recovered rapidly from major climate-driven disturbances. Critically, these reefs retain the capacity to grow at rates exceeding measured regional mid-late Holocene and 20th century sea-level rise, and close to IPCC sea-level rise projections through to 2100.

Regional‐scale dominance of non‐framework building corals on Caribbean reefs affects carbonate production and future reef growth

Coral cover on Caribbean reefs has declined rapidly since the early 1980's. Diseases have been a major driver, decimating communities of framework building Acropora and Orbicella coral species, and reportedly leading to the emergence of novel coral assemblages often dominated by domed and plating species of the genera Agaricia, Porites and Siderastrea. These corals were not historically important Caribbean framework builders, and typically have much smaller stature and lower calcification rates, fuelling concerns over reef carbonate production and growth potential. Using data from 75 reefs from across the Caribbean we quantify: (i) the magnitude of non-framework building coral dominance throughout the region and (ii) the contribution of these corals to contemporary carbonate production. Our data show that live coral cover averages 18.2% across our sites and coral carbonate production 4.1kg CaCO3 m(-2) yr(-1) . However, non-framework building coral species dominate and are major carbonate producers at a high proportion of sites; they are more abundant than Acropora and Orbicella at 73% of sites; contribute an average 68% of the carbonate produced; and produce more than half the carbonate at 79% of sites. Coral cover and carbonate production rate are strongly correlated but, as relative abundance of non-framework building corals increases, average carbonate production rates decline. Consequently, the use of coral cover as a predictor of carbonate budget status, without species level production rate data, needs to be treated with caution. Our findings provide compelling evidence for the Caribbean-wide dominance of non-framework building coral taxa, and that these species are now major regional carbonate producers. However, because these species typically have lower calcification rates, continued transitions to states dominated by non-framework building coral species will further reduce carbonate production rates below 'predecline' levels, resulting in shifts towards negative carbonate budget states and reducing reef growth potential.

Changing dynamics of Caribbean reef carbonate budgets: emergence of reef bioeroders as critical controls on present and future reef growth potential.

Coral cover has declined rapidly on Caribbean reefs since the early 1980s, reducing carbonate production and reef growth. Using a cross-regional dataset, we show that widespread reductions in bioerosion rates-a key carbonate cycling process-have accompanied carbonate production declines. Bioerosion by parrotfish, urchins, endolithic sponges and microendoliths collectively averages 2 G (where G = kg CaCO3 m(-2) yr(-1)) (range 0.96-3.67 G). This rate is at least 75% lower than that reported from Caribbean reefs prior to their shift towards their present degraded state. Despite chronic overfishing, parrotfish are the dominant bioeroders, but erosion rates are reduced from averages of approximately 4 to 1.6 G. Urchin erosion rates have declined further and are functionally irrelevant to bioerosion on most reefs. These changes demonstrate a fundamental shift in Caribbean reef carbonate budget dynamics. To-date, reduced bioerosion rates have partially offset carbonate production declines, limiting the extent to which more widespread transitions to negative budget states have occurred. However, given the poor prognosis for coral recovery in the Caribbean and reported shifts to coral community states dominated by slower calcifying taxa, a continued transition from production to bioerosion-controlled budget states, which will increasingly threaten reef growth, is predicted.

Carbonate and terrigenous sediment budgets for two inshore turbid reefs on the central Great Barrier Reef

Inshore turbid zone reefs on the Great Barrier Reef (GBR) occur within 20 km of the mainland coast under marine environmental conditions (with respect to sedimentation rates, turbidity and water quality) that are generally considered marginal for reef growth. Despite this, data from various benthic habitat assessments report high (> 30%) coral cover in these environments and reef core records show them to be characterised by relatively rapid rates of vertical accretion (2–8 mm/year), a long-term trend indicative of high net carbonate productivity and in-situ carbonate framework accumulation. However, the lack of quantitative data on terrigenous sediment input and flux rates, and on carbonate production rates has inhibited understanding of both ecological timescale rates of carbonate production and the aggregated long-term net impacts of sediments on reef growth. To address this knowledge gap a modern carbonate budget and terrigenous sediment model, that quantified allochthonous sediment inputs onto, within and off reef, was developed at two inshore reefs: Middle Reef and Paluma Shoals. Both are located within the central region of the GBR and are subjected to high terrigenous sediment load (> 11,000 tonnes/year) and fluctuating turbidity (5 to > 100 mg/L) regimes. Based on sediment dynamic modelling, over 81% of sediments delivered were transported off reef, with net sediment accumulation limited to sheltered reef habitats. Net carbonate production was high (> 6.9 kg/m 2 /year) due to high coral cover (> 30%), high coral calcification rates ( Acropora average 6.3 g/cm 2 /year), and low bioerosion rates (0.3 to 5 kg/m 2 /year), but varied spatially with highest net carbonate production (> 10 kg/m 2 /year) within deep (>− 2 m at LAT) windward reef zones. High carbonate framework production has enabled Middle Reef and Paluma Shoals to vertically accrete rapidly: Middle Reef establishing at depths of ~ 4 m, Paluma Shoals at ~ 3 m depth and both reaching sea level in < 1200 years. Carbonate and terrigenous sediment inputs were used to develop a reef growth model with time and depth that illustrates how rates and modes of reef growth varied temporally as the reefs approached sea level. Both Middle Reef and Paluma Shoals are still actively accreting, although vertical reef growth potential is increasingly constrained as the reef flats infill at present sea level. • We examine carbonate production and destruction with terrigenous sediment fluxes. • High carbonate production enables rapid reef accretion despite high sediment inputs. • Sediment transport maintains low sedimentation rates where sediment yields are high. • Carbonate production and sediment dynamics were used to develop a reef growth model. • The reef growth model is in excellent accord with radiocarbon dating of reef cores.

Caribbean-wide decline in carbonate production threatens coral reef growth

Global-scale deteriorations in coral reef health have caused major shifts in species composition. One projected consequence is a lowering of reef carbonate production rates, potentially impairing reef growth, compromising ecosystem functionality and ultimately leading to net reef erosion. Here, using measures of gross and net carbonate production and erosion from 19 Caribbean reefs, we show that contemporary carbonate production rates are now substantially below historical (mid- to late-Holocene) values. On average, current production rates are reduced by at least 50%, and 37% of surveyed sites were net erosional. Calculated accretion rates (mm year−1) for shallow fore-reef habitats are also close to an order of magnitude lower than Holocene averages. A live coral cover threshold of ~10% appears critical to maintaining positive production states. Below this ecological threshold carbonate budgets typically become net negative and threaten reef accretion. Collectively, these data suggest that recent ecological declines are now suppressing Caribbean reef growth potential.

Sequence organization and growth patterns of late Mesoproterozoic stromatolite reefs: an example from the Burovaya Formation, Turukhansk Uplift, Siberia

The latest Mesoproterozoic Burovaya Formation is a 600–1000 m thick stromatolite/mud dominated carbonate unit demarcated by an erosional unconformity into two subformations containing stromatolite reefs of variable size and geometry. The lower subformation consists of two distinct stratigraphic sequences. The lower sequence records shelf to ramp evolution and includes small-scale stromatolite reefs developed within an upper subtidal shelf settings at the transgressive systems tract and overlying mud-dominated highstand deposits accumulated in lower subtidal to peritidal environments. The second sequence of muddy and fine-grained sediments makes up a lowstand systems tract and documents a progressive structural and environmental differentiation of the Burovaya platform. The upper subformation records a third stratigraphic sequence, containing transgressive and highstand systems tracts represented by reef-bearing and reef-cap facies complexes. The reef-bearing complex characteristic of the transgressive and the bulk of the highstand systems tracts contains a giant (20–25 km wide, 10+ km long, up to 550 m thick) stromatolite reef and surrounding deep-water carbonate mudstones containing sporadic slump block deposits. The reef was located on the distally steepened part of the ramp and contains distinct internal facies organization defined by particular stromatolite microstructure and morphology and prevailing environments. Late highstand development of the Burovaya platform was marked by the termination of reef accretion, infilling of basinal relief by shallow-water carbonate and shale sediments of the cap complex, and subsequent subaerial exposure. The history of the Burovaya reef is indicative of the interplay between reef growth potential and platform geometry defined by regional tectonic and sea-level dynamics.