Reef Accretion Research Articles

Patch reefs as sources of long and continuous skeletal records of horizontal reef expansion in Dongsha Atoll, South China Sea

Atolls are annular midocean reefs with various topographical features. The reef rim of an atoll encloses a central lagoon and lagoonal patch reefs therein. Two hypotheses explaining the origins of patch reefs in atoll lagoons have been developed: the karst hypothesis and the growth hypothesis, which emphasize the role of uneven erosion and uneven timing of reef accretion (especially horizontal expansion), respectively, in the formation and growth of patch reefs. In this study, we investigated trends in the locations, depths, ages, sizes, and other features of flat-top and pinnacle-top reefs in the Dongsha Atoll in the South China Sea by analyzing bathymetric LiDAR images and performing U-Th dating on samples we collected from various reefs in the atoll. Three of our main findings were consistent with the growth hypothesis: (1) the differences in ages of the centers of different flat-top patch reefs reflected the different times at which each patch reef reached the surface and started to expand horizontally; (2) the top surfaces of individual flat-top patch reefs had old centers and young peripheries, reflecting horizontal growth; and (3) all the pinnacle-top reefs we examined had more live corals on their tips than on the surrounding bases. We observed no positive correlation between the ages of the centers and the diameters of flat-top patch reefs; that is, larger reefs did not necessarily have older centers, indicating that various complex factors affect the expansion and age patterns of patch reefs. Overall, the findings of this study provide support for the growth hypothesis and for the use of flat-top patch reefs as sources of easily accessible long-term coral skeletal records. They may therefore serve as a reference for future studies on asymmetric reef expansion and other relevant topics.

Historical declines in parrotfish on Belizean coral reefs linked to shifts in reef exploitation following European colonization

Humans have utilized the Mesoamerican Reef (MAR) for millennia but the effects of prehistorical and historical fishing on this ecosystem remain understudied. To assess the long-term coupling of reef ecosystem and human dynamics in this region, we tracked trends in the structure and functioning of lagoonal reefs within the Belizean portion of the MAR using fish teeth fossils and sediment accumulation rates within reef sediment cores. We then paired this with a timeline of demographic and cultural changes in this region’s human populations. The ∼1,300-year timeline encompassed in the core record shows that declines in the relative abundance and accumulation rate of teeth from parrotfish, a key reef herbivore, occurred at all three reef sites and began between ∼1500 and 1800 AD depending on site and metric of abundance. A causality analysis showed that parrotfish relative abundance had a positive causal effect on reef accretion rates, a proxy of coral growth, reconfirming the important role of these fish in reef ecosystem functioning. The timing of initial declines in parrotfish teeth occurred during a time of relatively low human population density in Belize. However, declines were synchronous with cultural and demographic upheaval resulting from European colonization of the New World. The more recent declines at these sites (∼1800 AD) occurred in tandem with increased subsistence fishing on reefs by multiple immigrant groups, a pattern that was likely necessitated by the establishment of an import economy controlled by a small group of land-owning European elites. These long-term trends from the paleoecological record reveal that current parrotfish abundances in central Belize are well below their pre-European contact peaks and that increased fishing pressure on parrotfish post-contact has likely caused a decline in reef accretion rates. The origins of reef degradation in the Belizean portion of the MAR began hundreds of years before the onset of modern declines resulting from the combined effects of local human disturbances and climate change.

Unraveling Moreton Bay reef history: An urban high-latitude setting for coral development

High-latitude habitats have become increasingly recognized as a potential climate refuge for coral communities, supporting both tropical and sub-tropical corals. Despite the increasing interest in the ecology of high-latitude corals, our current knowledge of their temporal dynamics is limited, especially within urbanized settings. Here, we examined the entire history of a high-latitude coral reef ecosystem in an urbanized setting. We surveyed Holocene fossil and modern coral communities along a water quality gradient in Moreton Bay, southeast Queensland, Australia, representing near-river (Wellington Point), intermediate (Peel Island) and near-oceanic (Myora Reef) environmental conditions. Reef accretion occurred during three discrete episodes from 7,400 to 5,800, 4,900 to 3,000, and 2,100 to 300 years BP, each separated by roughly 1,000-year hiatuses, where conditions were probably not favorable enough for reef accretion to occur. Episodic reef initiation and termination suggests strong environmental controls over reef development. Eastern Australian Holocene reef growth and cessation has been linked previously to sea level fluctuations and climatic regimes (e.g., ENSO). Within each reef building episode, there were few changes in coral assemblages over time. The fast growing and branching Acropora had a relative abundance greater than 90% in ten of the 13 sediment cores and all the submerged terrace excavations. However, substantial modification of adjacent coastal catchments from European colonization in the mid 1800’s resulted in increased sediment and nutrient discharge into the bay. This perturbation coincided with a greater abundance of stress-tolerant coral species (e.g., Dipsastraea, Goniastrea, and Goniopora) and the near extirpation of Acropora in the modern coral communities of near-river and intermediate sites due to poor water quality. In contrast, the modern coral assemblage at the near-oceanic site (Myora Reef) continues to be dominated by Acropora, likely due to the consistent oceanic input, resulting in lower sediment loading and higher water quality. In order for conditions for reef growth to improve, especially within the near-river portions of the bay, further sediment and nutrient runoff from anthropogenic land-use changes need to be mitigated. Given the historical abundance of Acropora, we recommend this genus be used as an indicator of natural resource management success in the bay.

The Formation of Atolls: New Insights From Numerical Simulations

AbstractSeveral theories have been proposed to explain atoll formation. While karst dissolution during glacial periods and preferential coral reef accretion along raised bank margins during deglaciations and interglacials have been invoked to explain atoll formation, the respective roles of karst dissolution and reef margin construction in atoll formation have not been adequately evaluated by simulations. In this study, we conducted three‐dimensional numerical simulations of the Quaternary development of Meiji Atoll in the southern South China Sea based on interpreted data from a 2020‐m‐deep borehole drilled on its northeast rim in 2018. Our results suggest that the origin of atolls is more likely due to spatially differential dissolution across margin and interior areas (i.e., minimal along margins and maximal in bank centers) rather than preferential reef accretion along margins of flat‐topped banks. Preferential reef accretion along margins of flat‐topped banks that can result in central lagoons and atoll morphology can hardly result in the formation of lagoonal patch reefs that reach mean sea level. Preferential reef accretion along margins is mainly predicated on the karst‐induced morphology that has a central depression surrounded by raised rims, that is, using antecedent karst morphologies. If topographic highs in the lagoon are similar in elevation to the margins, reef accretion on these topographic highs can be similar to that observed on the margins, resulting in lagoonal patch reefs that reach mean sea level. Our simulation shows that spatially differential dissolution across margin and interior areas is a critical driver of worldwide central lagoons and atoll formation.

Quaternary subsidence history of Xisha Islands (northern South China Sea): Evidences from the reef-bank system

Subsidence is a potentially important factor influencing the evolution of carbonate platforms. Yet, few studies have been carried out on the quantitative reconstruction of subsidence history of carbonate platforms in the South China Sea. Using paleo-water depth constraints from core XK-1, drilled in the Xisha Islands, northern South China Sea, the tectonic and total subsidence history of Xisha Islands during the Quaternary is quantitatively reconstructed in this study. Slow tectonic subsidence between 1.945 and 0.988 Ma was related with magmatic intrusion beneath the Xisha Islands, while the relatively high tectonic subsidence rate between 0.988 and 0.781 Ma and the subsequent gradual decline in tectonic subsidence rate between 0.781 and 0.07 Ma were linked to the thermal cooling process. A rapid tectonic uplifting process after 0.07 Ma occurred as a result of compression and volcano eruption. Total subsidence rate showed an increase from the stage of 1.945–0.988 Ma to the stage of 0.988–0.781 Ma, and a gradual decline during the stage of 0.781–0.07 Ma. After 0.07 Ma, there was a rapid uplifting in response to the tectonic movement. The high eustatic sea level and intermediate total subsidence rate from 1.945 to 1.0 Ma caused the domination of bank deposits on the Xisha carbonate platform. From 0.988 to 0.65 Ma, the long-term eustatic sea-level lowering and the relatively high total subsidence rate favored the production of thick reef units. By contrast, the low eustatic sea level and the slow total subsidence between 0.65 and 0.42 Ma forced the accommodation space to be limited, resulting in formation of thin reef-bank deposits and occurrence of exposure horizons. During the stage of slow subsidence, the high eustatic sea level around 0.4 Ma allowed the last presence of thick reef unit in the Quaternary section, after which the lower eustatic sea level reduced the accommodation space again and only thin bioclastic bank deposits and iron-stained exposure horizon occurred. Around 0.07 Ma, the Xisha Islands were continuously exposed due to the uplifting and low eustatic sea level. The strong winter monsoon caused a heterozoan factory nearby the Xisha Islands, and blew the carbonate sediments onshore forming aeolianites. Our results proves that the total subsidence has indeed greatly influenced the evolution of Quaternary carbonate system in the Xisha area. The long-term (at scale of Myr) sea-level lowering could facilitate the flourishment of corals, and a moderate total subsidence in the context of secular sea-level lowering would provide more effective accommodation space for the continuous reef accretion, favoring the formation of thick reef units in the tropical areas.

The past, present, and future of coral reef growth in the Florida Keys.

Coral‐reef degradation is driving global‐scale reductions in reef‐building capacity and the ecological, geological, and socioeconomic functions it supports. The persistence of those essential functions will depend on whether coral‐reef management is able to rebalance the competing processes of reef accretion and erosion. Here, we reconstructed census‐based carbonate budgets of 46 reefs throughout the Florida Keys from 1996 to 2019. We evaluated the environmental and ecological drivers of changing budget states and compared historical trends in reef‐accretion potential to millennial‐scale baselines of accretion from reef cores and future projections with coral restoration. We found that historically, most reefs had positive carbonate budgets, and many had reef‐accretion potential comparable to the ~3 mm year−1 average accretion rate during the peak of regional reef building ~7000 years ago; however, declines in reef‐building Acropora palmata and Orbicella spp. corals following a series of thermal stress events and coral disease outbreaks resulted in a shift from positive to negative budgets for most reefs in the region. By 2019, only ~15% of reefs had positive net carbonate production. Most of those reefs were in inshore, Lower Keys patch‐reef habitats with low water clarity, supporting the hypothesis that environments with naturally low irradiance may provide a refugia from thermal stress. We caution that our estimated carbonate budgets are likely overly optimistic; comparison of reef‐accretion potential to measured accretion from reef cores suggests that, by not accounting for the role of nonbiological physical and chemical erosion, census‐based carbonate budgets may underestimate total erosion by ~1 mm year−1 (−1.15 kg CaCO3 m−2 year−1). Although the present state of Florida's reefs is dire, we demonstrate that the restoration of reef‐building corals has the potential to help mitigate declines in reef accretion in some locations, which could allow some key ecosystem functions to be maintained until the threat of global climate change is addressed.

Recovery trends of reef carbonate budgets at remote coral atolls 6 years post‐bleaching

AbstractCoral bleaching events and resultant changes in benthic community composition and population size structure can diminish the important geo‐ecological functions reefs provide, including habitat provision and carbonate production to support reef accretion. Net reef carbonate budgets, the balance between carbonate production and erosion processes, are thus important functional indicators of reef health. This study quantifies changes in coral community composition and colony size structures, and the resultant reef carbonate budget trajectories after the 2015/2016 bleaching event in the remote Chagos Archipelago, Indian Ocean. ReefBudget surveys were conducted at 12 sites across three atolls in 2015, 2018, and 2021, with calculations of biological carbonate production and erosion supported by locally obtained calcification and bioerosion rates. Carbonate budgets (in G = kg CaCO3 m−2 yr−1) shifted from net positive states in 2015 (mean ± SD: 3.8 ± 2.6 G) to net negative states in 2018 (−2.4 ± 1.4 G) in response to bleaching‐driven mass coral mortality. By 2021, all sites were on a trajectory of recovery, but net budgets differed significantly between atolls (−2.0 ± 1.7 to 2.2 ± 1.4 G). At Salomon atoll, the threefold faster recovery of carbonate production and return to positive reef budget states only 6 yr post‐bleaching was associated with the persistence of high structural complexity and the rapid recovery of fast growing tabular Acropora spp. Inter‐atoll differences in colony size distributions furthermore illustrate that coral identity and size class are more important predictors of reef functions and post‐disturbance recovery speed than coral cover alone.

Will daytime community calcification reflect reef accretion on future, degraded coral reefs?

Abstract. Coral bleaching events continue to drive the degradation of coral reefs worldwide, causing a shift in the benthic community from coral- to algae-dominated ecosystems. Critically, this shift may decrease the capacity of degraded coral reef communities to maintain net positive accretion during warming-driven stress events (e.g., reef-wide coral bleaching). Here we measured rates of net ecosystem calcification (NEC) and net ecosystem production (NEP) on a degraded coral reef lagoon community (coral cover < 10 % and algae cover > 20 %) during a reef-wide bleaching event in February 2020 at Heron Island on the Great Barrier Reef. We found that during this bleaching event, rates of NEP and NEC across replicate transects remained positive and did not change in response to bleaching. Repeated benthic surveys over a period of 20 d indicated an increase in the percent area of bleached coral tissue, corroborated by relatively low Symbiodiniaceae densities (∼ 0.6 × 106 cm−2) and dark-adapted photosynthetic yields in photosystem II of corals (∼ 0.5) sampled along each transect over this period. Given that a clear decline in coral health was not reflected in the overall NEC estimates, it is possible that elevated temperatures in the water column that compromise coral health enhanced the thermodynamic favorability for calcification in other ahermatypic benthic calcifiers. These data suggest that positive NEC on degraded reefs may not equate to the net positive accretion of a complex, three-dimensional reef structure in a future, warmer ocean. Critically, our study highlights that if coral cover continues to decline as predicted, NEC may no longer be an appropriate proxy for reef growth as the proportion of the NEC signal owed to ahermatypic calcification increases and coral dominance on the reef decreases.

Impacts of ocean warming and acidification on calcifying coral reef taxa: mechanisms responsible and adaptive capacity.

Ocean warming (OW) and acidification (OA) are two of the greatest global threats to the persistence of coral reefs. Calcifying reef taxa such as corals and coralline algae provide the essential substrate and habitat in tropical reefs but are at particular risk due to their susceptibility to both OW and OA. OW poses the greater threat to future reef growth and function, via its capacity to destabilise the productivity of both taxa, and to cause mass bleaching events and mortality of corals. Marine heatwaves are projected to increase in frequency, intensity, and duration over the coming decades, raising the question of whether coral reefs will be able to persist as functioning ecosystems and in what form. OA should not be overlooked, as its negative impacts on the calcification of reef-building corals and coralline algae will have consequences for global reef accretion. Given that OA can have negative impacts on the reproduction and early life stages of both coralline algae and corals, the interdependence of these taxa may result in negative feedbacks for reef replenishment. However, there is little evidence that OA causes coral bleaching or exacerbates the effects of OW on coral bleaching. Instead, there is some evidence that OA alters the photo-physiology of both taxa. Tropical coralline algal possess shorter generation times than corals, which could enable more rapid evolutionary responses. Future reefs will be dominated by taxa with shorter generation times and high plasticity, or those individuals inherently resistant and resilient to both marine heatwaves and OA.

New constraints on the postglacial shallow-water carbonate accumulation in the Great Barrier Reef

More accurate global volumetric estimations of shallow-water reef deposits are needed to better inform climate and carbon cycle models. Using recently acquired datasets and International Ocean Discovery Program (IODP) Expedition 325 cores, we calculated shallow-water CaCO3 volumetrics and mass for the Great Barrier Reef region and extrapolated these results globally. In our estimates, we include deposits that have been neglected in global carbonate budgets: Holocene Halimeda bioherms located on the shelf, and postglacial pre-Holocene (now) drowned coral reefs located on the shelf edge. Our results show that in the Great Barrier Reef alone, these drowned reef deposits represent ca. 135 Gt CaCO3, comparatively representing 16–20% of the younger Holocene reef deposits. Globally, under plausible assumptions, we estimate the presence of ca. 8100 Gt CaCO3 of Holocene reef deposits, ca. 1500 Gt CaCO3 of drowned reef deposits and ca. 590 Gt CaCO3 of Halimeda shelf bioherms. Significantly, we found that in our scenarios the periods of pronounced reefal mass accumulation broadly encompass the occurrence of the Younger Dryas and periods of CO2 surge (14.9–14.4 ka, 13.0–11.5 ka) observed in Antarctic ice cores. Our estimations are consistent with reef accretion episodes inferred from previous global carbon cycle models and with the chronology from reef cores from the shelf edge of the Great Barrier Reef.

Sustained coral reef growth in the critical wave dissipation zone of a Maldivian atoll

Sea-level rise is expected to outpace the capacity of coral reefs to grow and maintain their wave protection function, exacerbating coastal flooding and erosion of adjacent shorelines and threatening coastal communities. Here we present a new method that yields highly-resolved direct measurements of contemporary reef accretion on a Maldivian atoll reef rim, the critical zone that induces wave breaking. Results incorporate the suite of physical and ecological processes that contribute to reef accumulation and show growth rates vary from 6.6 ± 12.5 mm.y−1 on the reef crest, and up to 3.1 ± 10.2 mm.y−1, and −0.5 ± 1.8 mm.yr−1 on the outer and central reef flat respectively. If these short-term results are maintained over decades, the reef crest could keep pace with current sea-level rise. Findings highlight the need to resolve contemporary reef accretion at the critical wave dissipation zone to improve predictions of future reef growth, and re-evaluate exposure of adjacent shorelines to coastal hazards.

Oyster Shell Production and Loss in the Chesapeake Bay

Oyster population demographics are used to estimate shell standing stock (g m–2) and rates of shell production (g m–2y–1) and loss by oyster, Crassostrea virginica, populations in both the Virginia and Maryland portions of the Chesapeake Bay. Source data are from long-term stock assessments whose footprints cover the majority of extant fished reefs in Virginia and a mix of fished and sanctuary reefs in Maryland. Individual longevity in extant fished populations is typically less than or equal to 5 y and truncated when compared with fossil C. virginica populations. Shell standing stock and productivity are maximal in year 1 or 2 of the progression of a year class through the population. Maintenance of the underlying reefs structure is dependent on regular shell input from mortality. The combination of truncated population structure and variable recruitment make shell addition temporally unstable. High turnover rates (≥30% y–1) of the shell substrate pool in the oxic region above the sediment water interface are present. Reef accretion rates are generally less than the combination of sedimentation and relative sea level rise in most of the Bay system. Greater individual oyster longevity is required to ensure development of self-sustaining reef structures in the Chesapeake Bay.

Water column and bottom gradients on the continental shelf eastward of the Amazon River mouth and implications for mesophotic reef occurrence

Continental margin sedimentation represents the terrestrial-marine geological transition. Coarser coastal sediments usually transition to fine-grained sediments across the shelf as the energy level decreases. As the distance from the continental source of terrigenous sediments increases, the mid-shelf mud belt transitions to a seaward organic/carbonatic sedimentation domain. However, this general pattern might change depending on the terrigenous sediment load. This is the case for the Amazon continental shelf, where the Amazon River empties and builds up the large Amazon River plume (ARP). The ARP promotes an inverse pattern with muddy nearshore prodelta sediments and siliciclastic sands offshore. This study investigates water and bottom characteristics to the east of the Amazon River mouth to understand the complex interplay that leads to simultaneous massive river plume spread and mesophotic reef development. Three cross-self profiles were performed, including physical-chemical water column measurements and bottom sample collection. The profiles were approximately 150 km apart, comprising a longitudinal gradient off of the ARP. The results reassure that there is enough light for mesophotic reefs to grow at the outer shelf and that at the mid-shelf, large-scale sand movement is likely to be the major limiting control on reef accretion. Also, mesophotic conditions (i.e., low light levels) occur in shallower waters, where a trend of reef expansion was identified, but high-energy levels and the lack of hard substrates hinder reef expansion.

The effects of coral assemblage shift on reef functions in Akumal, Mexico

Caribbean coral reefs have suffered shifts in their coral assemblages, driven by local and global stressors. These shifts have affected reef functions, ecosystem processes, and services essential to human well-being. Akumal Reef, located in the northern Mexican Caribbean, generates valuable ecosystem services, mainly linked to tourism, that depend on the persistence of the reef’s physical functionality. In this study, decadal changes in coral assemblages and their effects on functional traits decline were investigated. Coral species cover was systematically monitored at 14 sites along Akumal Reef in 2001, 2010, and 2019. These data were translated to functional traits allowing the classification of coral species into life-history strategies (competitive, stress-tolerant, and more opportunistic ‘weedy’ species). Our results indicate a significant decline in stress-tolerant species cover (mostly from the genusOrbicella)in Akumal Reef between 2001 and 2010 and between 2010 and 2019, with no significant changes in coral cover of other life-history strategies. The significant shifts away from stress-tolerant species during the time studied in Akumal Reef influenced the deterioration of functional traits related to reef physical functionality, such as calcification, rugosity, average maximum colony size, and reef functional index. Currently, Akumal reef sites are dominated by non-framework-building weedy coral species and are mostly below the 10% hard cover estimated threshold for positive reef accretion in the Caribbean. Therefore, management strategies in Akumal Reef should be integrative and oriented towards reducing local stressors while actively restoring the functional structure of coral reefs and maintaining their environmental services.

Reef accumulation is decoupled from recent degradation in the central and southern Red Sea

Reefs are biogenic structures that result in three-dimensional accumulations of calcium carbonate. Over geological timescales, a positive balance between the production and accumulation of calcium carbonate versus erosional and off-reef transport processes maintains positive net accretion on reefs. Yet, how ecological processes occurring over decadal timescales translate to the accumulation of geological structures is poorly understood, in part due to a lack of studies with detailed time-constrained chronologies of reef accretion over decades to centuries. Here, we combined ecological surveys of living reefs with palaeoecological reconstructions and high-precision radiometric (U-Th) age-dating of fossil reefs represented in both reef sediment cores and surficial dead in situ corals, to reconstruct the history of community composition and carbonate accumulation across the central and southern Saudi Arabian Red Sea throughout the late Holocene. We found that reefs were primarily comprised of thermally tolerant massive Porites colonies, creating a consolidated coral framework, with unconsolidated branching coral rubble accumulating among massive corals on shallow (5–8 m depth) exposed (windward), and gently sloping reef slopes. These unconsolidated reef rubble fields were formed primarily from ex situ Acropora and Pocillopora coral fragments, infilled post deposition within a sedimentary matrix. Bayesian age-depth models revealed a process of punctuated deposition of post-mortem coral fragments transported from adjacent reef environments. That a large portion of Saudi Arabian Red Sea reef slopes is driven by allochthonous deposition (transportation) has important implications for modeling carbonate budgets and reef growth. In addition, a multi-decadal lag exists between the time of death for branching in situ coral and incorporation into the unconsolidated reef rubble. This indicates that recent climate related degradation in the 21st century has not had an immediately negative effect on reef building processes affecting a large portion of the reef area in the Saudi Arabian Red Sea.

Spatially Modeling the Synergistic Impacts of Global Warming and Sea-Level Rise on Coral Reefs in the South China Sea

Global warming and sea-level rise (SLR) induced by rising atmospheric CO2 concentrations can cause coral bleaching, death, and submergence of the world’s coral reefs. Adopting the GIS and RS methods, we modeled how these two stressors combine to influence the future growth of the atolls and table reefs of three archipelagoes in the South China Sea (SCS), based on geomorphic and ecological zones. A large-scale survey of the coral communities in Xisha Islands in 2014, Dongsha Islands in 2014–2016 and Nansha Islands in 2007 provided zone-specific process datasets on the range of reef accretion rates. Sea surface temperature and extreme (minimum and maximum) SLR data above 1985–2005 levels by 2100 in the SCS were derived from the Intergovernmental Panel on Climate Change (IPCC) Fifth Assessment Report (AR5) models forced with the Representative Concentration Pathways (RCPs). Our model projected that: (1) the Xisha Islands and Dongsha Islands may have a better growth status, because the reef flat biotic sparse zone may be recolonized with hard coral and become a biotic dense zone; (2) the southern Nansha Islands reefs have a risk of stopping growing due to their earlier annual bleaching years. The increasing of water depths of these reefs is stronger in the RCP with more emissions. Our approach offers insights into the best-case and worst-case impacts of two global environmental pressures on potential future reef growth under a changing climate.

Climate and the latitudinal limits of subtropical reef development

Climate plays a central role in coral-reef development, especially in marginal environments. The high-latitude reefs of southeast Florida are currently non-accreting, relict systems with low coral cover. This region also did not support the extensive Late Pleistocene reef development observed in many other locations around the world; however, there is evidence of significant reef building in southeast Florida during the Holocene. Using 146 radiometric ages from reefs extending ~ 120 km along Florida’s southeast coast, we test the hypothesis that the latitudinal extent of Holocene reef development in this region was modulated by climatic variability. We demonstrate that although sea-level changes impacted rates of reef accretion and allowed reefs to backstep inshore as new habitats were flooded, sea level was not the ultimate cause of reef demise. Instead, we conclude that climate was the primary driver of the expansion and contraction of Florida’s reefs during the Holocene. Reefs grew to 26.7° N in southeast Florida during the relatively warm, stable climate at the beginning of the Holocene Thermal Maximum (HTM) ~ 10,000 years ago, but subsequent cooling and increased frequency of winter cold fronts were associated with the equatorward contraction of reef building. By ~ 7800 years ago, actively accreting reefs only extended to 26.1° N. Reefs further contracted to 25.8° N after 5800 years ago, and by 3000 years ago reef development had terminated throughout southern Florida (24.5–26.7° N). Modern warming is unlikely to simply reverse this trend, however, because the climate of the Anthropocene will be fundamentally different from the HTM. By increasing the frequency and intensity of both warm and cold extreme-weather events, contemporary climate change will instead amplify conditions inimical to reef development in marginal reef environments such as southern Florida, making them more likely to continue to deteriorate than to resume accretion in the future.

Three‐dimensional mapping reveals scale‐dependent dynamics in biogenic reef habitat structure

AbstractHabitat structure influences a broad range of ecological interactions and ecosystem functions across biomes. To understand and effectively manage dynamic ecosystems, we need detailed information about habitat properties and how they vary across spatial and temporal scales. Measuring and monitoring variation in three‐dimensional (3D) habitat structure has traditionally been challenging, despite recognition of its importance to ecological processes. Modern 3D mapping technologies present opportunities to characterize spatial and temporal variation in habitat structure at a range of ecologically relevant scales. Biogenic reefs are structurally complex and dynamic habitats, in which structure has a pivotal influence on ecosystem biodiversity, function and resilience. For the first time, we characterized spatial and temporal dynamics in the 3D structure of intertidal Sabellaria alveolata biogenic reef across scales. We used drone‐derived structure‐from‐motion photogrammetry and terrestrial laser scanning to characterize reef structural variation at mm‐to‐cm resolutions at a habitat scale (~35 000 m2) over 1 year, and at a plot scale (2500 m2) over 5 years (2014–2019, 6‐month intervals). We found that most of the variation in reef emergence above the substrate, accretion rate and erosion rate was explained by a combination of systematic trends with shore height and positive spatial autocorrelation up to the scale of colonies (1.5 m) or small patches (up to 4 m). We identified previously undocumented temporal patterns in intertidal S. alveolata reef accretion and erosion, specifically groups of rapidly accreting, short‐lived colonies and slow‐accreting, long‐lived colonies. We showed that these highly dynamic colony‐scale structural changes compensate for each other, resulting in seemingly stable reef habitat structure over larger spatial and temporal scales. These patterns could only be detected with the use of modern 3D mapping technologies, demonstrating their potential to enhance our understanding of ecosystem dynamics across scales.

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.

Role of Future Reef Growth on Morphological Response of Coral Reef Islands to Sea‐Level Rise

AbstractCoral reefs are widely recognized for providing a natural breakwater effect that modulates erosion and flooding hazards on low‐lying sedimentary reef islands. Increased water depth across reef platforms due sea‐level rise (SLR) can compromise this breakwater effect and enhance island exposure to these hazards, but reef accretion in response to SLR may positively contribute to island resilience. Morphodynamic studies suggest that reef islands can adjust to SLR by maintaining freeboard (island crest elevation above still water level) through overwash deposition and island accretion, but the impact of different future reef accretion trajectories on the morphological response of islands remains unknown. Here we show, using a process‐based morphodynamic model, that, although reef growth significantly affects wave transformation processes and island morphology, it does not lead to decreased coastal flooding and island inundation. According to the model, reef islands evolve during SLR by attuning their elevation to the maximum wave runup and islands fronted by a growing reef platform attain lower elevations than those without reef growth, but have similar overwash regimes. The mean overwash discharge Qover across the island crest plays a key role in the ability of islands to keep up with SLR and maintain freeboard, with a Qover value of O (10 l m−1 s−1) separating island construction from destruction. Islands, therefore, can grow vertically to keep up with SLR via flooding and overwash if specific forcing and sediment supply conditions are met, offering hope for uninhabited and sparely populated islands. However, this physical island response will negatively impact infrastructure and assets on developed islands.