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Nicholas A. J. Graham - One of the best experts on this subject based on the ideXlab platform.
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Rethinking Coral Reef functional futures
Functional Ecology, 2019Co-Authors: Gareth J. Williams, Nicholas A. J. GrahamAbstract:Tropical Coral Reefs currently face an unprecedented restructuring since their extant form and function emerged ~24 million years ago in the early Neogene. They have entered the Anthropocene—an epoch where humans have become the dominant force of planetary change. Human impacts on and interactions with Coral Reefs are escalating across multiple trophic levels and scales, but we have a rudimentary understanding of what this means for their functional ecology. The overall goal of this special feature is to unpack what the Anthropocene means for the functional ecology of Coral Reefs, laying the foundations for new approaches and research directions in Coral Reef science. The collection describes the functional changes and novel dynamics that characterise Anthropocene Reefs, from variations in their taxonomy and geology through to the resulting shifts in ecosystem services they provide to humanity. Common changes to Coral Reefs are occurring that are challenging their historical functional role. These include reductions in benthic calcifiers and declining carbonate production, and benthic assemblage shifts leading to a loss of structural complexity and flattening of Reef seascapes. As Reefs as we know them are lost from some locations, range extensions and the “tropicalisation” of temperate locations present novel ecosystem configurations that are challenging ecological paradigms and our historical approach to ecosystem management. Hindering our progress, however, is a “functionality crisis.” Coral Reef functional ecology to date has lacked a clear and universal definition of the term “function,” and many assumed links between taxa and Reef processes lack empirical evidence. Moving forward, we must establish causal links between functional traits, the species that possess them, and specific ecosystem processes if we are to successfully manage Anthropocene Reefs. The functional space Coral Reefs occupy has arguably widened, presenting ethical challenges surrounding the increasingly interventionist management practices required to achieve particular functional endpoints. For us to steer Coral Reefs towards a desirable functional future will require a more mechanistic understanding between ecosystem attributes and the provision of services, acknowledging that such services are coproduced by the ecosystem and society. Ultimately, this era in Coral Reef ecology requires a new approach to Coral Reef science, one that addresses the complex socio-ecological nature of Coral Reefs. These works outline a path ahead for defining and studying the functional ecology of Coral Reefs, drive debate as to what we want their functional future to look like and call for ecosystem function to be at the heart of managing Coral Reef futures during this period of rapid transition. © 2019 The Authors. Functional Ecology © 2019 British Ecological Society
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Coral Reef ecosystem services in the Anthropocene
Functional Ecology, 2019Co-Authors: Anna Woodhead, Albert V. Norström, Gareth J. Williams, Christina C. Hicks, Nicholas A. J. GrahamAbstract:Coral Reefs underpin a range of ecosystem goods and services that contribute to the well‐being of millions of people. However, tropical Coral Reefs in the Anthropocene are likely to be functionally different from Reefs in the past. In this perspective piece, we ask, what does the Anthropocene mean for the provision of ecosystem services from Coral Reefs? First, we provide examples of the provisioning, regulating, cultural and supporting services underpinned by Coral Reef ecosystems. We conclude that Coral Reef ecosystem service research has lagged behind multidisciplinary advances in broader ecosystem services science, such as an explicit recognition that interactions between social and ecological systems underpin ecosystem services. Second, drawing on tools from functional ecology, we outline how these social–ecological relationships can be incorporated into a mechanistic understanding of service provision and how this might be used to anticipate future changes in Coral Reef ecosystem services. Finally, we explore the emergence of novel Reef ecosystem services, for example from tropicalized coastlines, or through changing technological connections to Coral Reefs. Indeed, when services are conceived as coming from social–ecological system dynamics, novelty in services can emerge from elements of the interactions between people and the ecosystem. This synthesis of the Coral Reef ecosystem services literature suggests the field is poorly prepared to understand the changing service provision anticipated in the Anthropocene. A new research agenda is needed that better connects Reef functional ecology to ecosystem service provision. This research agenda should embrace more holistic approaches to ecosystem service research, recognizing them as co‐produced by ecosystems and society. Importantly, the likelihood of novel ecosystem service configurations requires further conceptualization and empirical assessment. As with current ecosystem services, the loss or gain of services will not affect all people equally and must be understood in the context in which they occur. With the uncertainty surrounding the future of Coral Reefs in the Anthropocene, research exploring how the benefits to people change will be of great importance.
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Coral Reef ecology in the Anthropocene
Functional Ecology, 2019Co-Authors: Gareth J. Williams, Nicholas A. J. Graham, Jean-baptiste Jouffray, Albert V. Norström, Magnus Nyström, Jamison M. Gove, Adel Heenan, Lisa M. WeddingAbstract:We are in the Anthropocene—an epoch where humans are the dominant force of planetary change. Ecosystems increasingly reflect rapid human-induced, socioeconomic and cultural selection rather than being a product of their surrounding natural biophysical setting. This poses the intriguing question: To what extent do existing ecological paradigms capture and explain the current ecological patterns and processes we observe? We argue that, although biophysical drivers still influence ecosystem structure and function at particular scales, their ability to offer predictive capacity over coupled social–ecological systems is increasingly compromised as we move further into the Anthropocene. Traditionally, the dynamics of Coral Reefs have been studied in response to their proximate drivers of change rather than their underlying socioeconomic and cultural drivers. We hypothesise this is limiting our ability to accurately predict spatial and temporal changes in Coral Reef ecosystem structure and function. We propose “social–ecological macroecology” as a novel approach within the field of Coral Reef ecology to a) identify the interactive effects of biophysical and socioeconomic and cultural drivers of Coral Reef ecosystems across spatial and temporal scales; b) test the robustness of existing Coral Reef paradigms; c) explore whether existing paradigms can be adapted to capture the dynamics of contemporary Coral Reefs; and d) if they cannot, develop novel Coral Reef social–ecological paradigms, where human dynamics are part of the paradigms rather than the drivers of them. Human socioeconomic and cultural processes must become embedded in Coral Reef ecological theory and practice as much as biophysical processes are today if we are to predict and manage these systems successfully in this era of rapid change. This necessary shift in our approach to Coral Reef ecology will be challenging and will require truly interdisciplinary collaborations between the natural and social sciences. A plain language summary is available for this article.
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operationalizing resilience for adaptive Coral Reef management under global environmental change
Global Change Biology, 2015Co-Authors: Kenneth R. N. Anthony, Paul Marshall, Ameer Abdulla, Roger Beeden, Christopher Bergh, Ryan Black, Mark C Eakin, Edward T Game, Margaret Gooch, Nicholas A. J. GrahamAbstract:Cumulative pressures from global climate and ocean change combined with multiple regional and local-scale stressors pose fundamental challenges to Coral Reef managers worldwide. Understanding how cumulative stressors affect Coral Reef vulnerability is critical for successful Reef conservation now and in the future. In this review, we present the case that strategically managing for increased ecological resilience (capacity for stress resistance and recovery) can reduce Coral Reef vulnerability (risk of net decline) up to a point. Specifically, we propose an operational framework for identifying effective management levers to enhance resilience and support management decisions that reduce Reef vulnerability. Building on a system understanding of biological and ecological processes that drive resilience of Coral Reefs in different environmental and socio-economic settings, we present an Adaptive Resilience-Based management (ARBM) framework and suggest a set of guidelines for how and where resilience can be enhanced via management interventions. We argue that press-type stressors (pollution, sedimentation, overfishing, ocean warming and acidification) are key threats to Coral Reef resilience by affecting processes underpinning resistance and recovery, while pulse-type (acute) stressors (e.g. storms, bleaching events, crown-of-thorns starfish outbreaks) increase the demand for resilience. We apply the framework to a set of example problems for Caribbean and Indo-Pacific Reefs. A combined strategy of active risk reduction and resilience support is needed, informed by key management objectives, knowledge of Reef ecosystem processes and consideration of environmental and social drivers. As climate change and ocean acidification erode the resilience and increase the vulnerability of Coral Reefs globally, successful adaptive management of Coral Reefs will become increasingly difficult. Given limited resources, on-the-ground solutions are likely to focus increasingly on actions that support resilience at finer spatial scales, and that are tightly linked to ecosystem goods and services.
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extinction vulnerability of Coral Reef fishes
Ecology Letters, 2011Co-Authors: Nicholas A. J. Graham, Simon Jennings, Pascale Chabanet, Richard D Evans, Yves Letourneur, Aaron M Macneil, T R Mcclanahan, Marcus C Ohman, Nicholas Polunin, Shaun K. WilsonAbstract:With rapidly increasing rates of contemporary extinction, predicting extinction vulnerability and identifying how multiple stressors drive non-random species loss have become key challenges in ecology. These assessments are crucial for avoiding the loss of key functional groups that sustain ecosystem processes and services. We developed a novel predictive framework of species extinction vulnerability and applied it to Coral Reef fishes. Although relatively few Coral Reef fishes are at risk of global extinction from climate disturbances, a negative convex relationship between fish species locally vulnerable to climate change vs. fisheries exploitation indicates that the entire community is vulnerable on the many Reefs where both stressors co-occur. Fishes involved in maintaining key ecosystem functions are more at risk from fishing than climate disturbances. This finding is encouraging as local and regional commitment to fisheries management action can maintain Reef ecosystem functions pending progress towards the more complex global problem of stabilizing the climate.
Rebecca Albright - One of the best experts on this subject based on the ideXlab platform.
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carbon dioxide addition to Coral Reef waters suppresses net community calcification
Nature, 2018Co-Authors: Rebecca Albright, Yuichiro Takeshita, David A. Koweek, Kennedy Wolfe, Tanya Rivlin, Yana Nebuchina, Aaron Ninokawa, Jordan YoungAbstract:In situ carbon dioxide enrichment experiments show that ocean acidification poses a threat to Coral Reefs by reducing the saturation state of aragonite and the concentration of carbonate ions and that this impairs community calcification. Ocean acidification impairs Coral calcification and poses a substantial threat to tropical Coral Reef ecosystems. Rebecca Albright and colleagues exposed a natural Coral Reef community in the southern Great Barrier Reef to levels of ocean acidification that are expected to occur later this century unless deep carbon emissions cuts are made, and monitored calcification. Net community calcification was reduced by 34% in the acidified Reef. The findings suggest that acidification of the ocean will compromise Coral Reef function in the near future. Coral Reefs feed millions of people worldwide, provide coastal protection and generate billions of dollars annually in tourism revenue1. The underlying architecture of a Reef is a biogenic carbonate structure that accretes over many years of active biomineralization by calcifying organisms, including Corals and algae2. Ocean acidification poses a chronic threat to Coral Reefs by reducing the saturation state of the aragonite mineral of which Coral skeletons are primarily composed, and lowering the concentration of carbonate ions required to maintain the carbonate Reef. Reduced calcification, coupled with increased bioerosion and dissolution3, may drive Reefs into a state of net loss this century4. Our ability to predict changes in ecosystem function and associated services ultimately hinges on our understanding of community- and ecosystem-scale responses. Past research has primarily focused on the responses of individual species rather than evaluating more complex, community-level responses. Here we use an in situ carbon dioxide enrichment experiment to quantify the net calcification response of a Coral Reef flat to acidification. We present an estimate of community-scale calcification sensitivity to ocean acidification that is, to our knowledge, the first to be based on a controlled experiment in the natural environment. This estimate provides evidence that near-future reductions in the aragonite saturation state will compromise the ecosystem function of Coral Reefs.
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Carbon dioxide addition to Coral Reef waters suppresses net community calcification
Nature, 2018Co-Authors: Rebecca Albright, Yuichiro Takeshita, David A. Koweek, Kennedy Wolfe, Tanya Rivlin, Yana Nebuchina, Jordan Young, Aaron Ninokawa, Ken CaldeiraAbstract:Ocean acidification impairs Coral calcification and poses a substantial threat to tropical Coral Reef ecosystems. Rebecca Albright and colleagues exposed a natural Coral Reef community in the southern Great Barrier Reef to levels of ocean acidification that are expected to occur later this century unless deep carbon emissions cuts are made, and monitored calcification. Net community calcification was reduced by 34% in the acidified Reef. The findings suggest that acidification of the ocean will compromise Coral Reef function in the near future.
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reversal of ocean acidification enhances net Coral Reef calcification
Nature, 2016Co-Authors: Rebecca Albright, Yana Nebuchina, Aaron Ninokawa, Lilian Caldeira, Jessica Hosfelt, Lester Kwiatkowski, Jana K Maclaren, Benjamin Mason, Julia PongratzAbstract:A manipulative experiment in which a Reef is alkalinized in situ shows that calcification rates are likely to be lower already than they were in pre-industrial times because of acidification. Ocean acidification is one of several factors projected to threaten Coral Reef ecosystems, but disentangling its effects from other factors such as temperature is difficult. These authors used a manipulative experiment in which sodium hydroxide was added to seawater flowing over a natural Coral Reef community in situ. When ocean chemistry was restored closer to pre-industrial conditions, net community calcification increased. This suggests calcification rates are already lower than they were in pre-industrial times because of acidification. Deliberate alkalinization has been proposed as a geoengineering technique to offset ocean acidification, and this work suggests that the method could be effective, but only on a small scale — in protected bays or lagoons, for example. Approximately one-quarter of the anthropogenic carbon dioxide released into the atmosphere each year is absorbed by the global oceans, causing measurable declines in surface ocean pH, carbonate ion concentration ([CO32−]), and saturation state of carbonate minerals (Ω)1. This process, referred to as ocean acidification, represents a major threat to marine ecosystems, in particular marine calcifiers such as oysters, crabs, and Corals. Laboratory and field studies2,3 have shown that calcification rates of many organisms decrease with declining pH, [CO32−], and Ω. Coral Reefs are widely regarded as one of the most vulnerable marine ecosystems to ocean acidification, in part because the very architecture of the ecosystem is reliant on carbonate-secreting organisms4. Acidification-induced reductions in calcification are projected to shift Coral Reefs from a state of net accretion to one of net dissolution this century5. While retrospective studies show large-scale declines in Coral, and community, calcification over recent decades6,7,8,9,10,11,12, determining the contribution of ocean acidification to these changes is difficult, if not impossible, owing to the confounding effects of other environmental factors such as temperature. Here we quantify the net calcification response of a Coral Reef flat to alkalinity enrichment, and show that, when ocean chemistry is restored closer to pre-industrial conditions, net community calcification increases. In providing results from the first seawater chemistry manipulation experiment of a natural Coral Reef community, we provide evidence that net community calcification is depressed compared with values expected for pre-industrial conditions, indicating that ocean acidification may already be impairing Coral Reef growth.
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Coral Reef metabolism and carbon chemistry dynamics of a Coral Reef flat
Geophysical Research Letters, 2015Co-Authors: Rebecca Albright, Ken Caldeira, Jessica A. Benthuysen, Neal E. Cantin, Kenneth R. N. AnthonyAbstract:Global carbon emissions continue to acidify the oceans, motivating growing concern for the ability of Coral Reefs to maintain net positive calcification rates. Efforts to develop robust relationships between Coral Reef calcification and carbonate parameters such as aragonite saturation state (Ωarag) aim to facilitate meaningful predictions of how Reef calcification will change in the face of ocean acidification. Here we investigate natural trends in carbonate chemistry of a Coral Reef flat over diel cycles and relate these trends to benthic carbon fluxes by quantifying net community calcification and net community production. We find that, despite an apparent dependence of calcification on Ωarag seen in a simple pairwise relationship, if the dependence of net calcification on net photosynthesis is accounted for, knowing Ωarag does not add substantial explanatory value. This suggests that, over short time scales, the control of Ωarag on net calcification is weak relative to factors governing net photosynthesis.
Philip L. Munday - One of the best experts on this subject based on the ideXlab platform.
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Effects of Climate Change on Coral Reef Fishes
Ecology of Fishes on Coral Reefs, 2015Co-Authors: Morgan S. Pratchett, Shaun K. Wilson, Philip L. MundayAbstract:Climate change poses a major threat to Coral Reef ecosystems, and will affect Coral Reef fishes in three main ways. First and foremost, climate change is already contributing to widespread degradation of Coral Reef habitats, leading to declines in abundance and diversity of Reef-associated fishes. Second, increasing temperatures will have direct effects on the individual condition and fitness of some Coral Reef fishes. Unless these species can adapt to changing temperature regimes, it is likely that they will persist in either a small portion of their current geographical extent or move poleward, invading new habitats and potentially displacing other fish species. The third effect relates to rising CO2 levels and ocean acidification, which could have significant physiological and behavioral effects on fishes towards the latter part of this century. It is unequivocal that climate change will affect Reef fishes throughout this century. However, small-scale experimental studies, which are often focused on a single species and a single environmental factor, provide limited insight on expected changes in the biodiversity, productivity, and composition of Reef fish assemblages. Future research will need to assess synergistic effects of different environmental variables on not only individual species, but also on biotic interactions and community dynamics, as well as exploring the adaptive capacity of species. Global climate change has the capacity to greatly alter the biodiversity, function, and productivity of Coral Reef ecosystems [e.g. 1137]. Emerging effects of global climate change have so far been manifest mainly as increased incidence of mass bleaching and disease among scleractinian Corals and other zooxanthellate organisms, which is directly contributing to widespread degradation of Coral Reef habitats [1192], and will be increasingly compounded by effects of ocean acidification [1139]. In the Indo-Pacific, extensive depletion of scleractinian Corals and associated changes in the biological and physical structure of Coral Reef habitats has important effects on the structure and dynamics of local populations and communities of Coral Reef fishes [2023, 2027, 2707]. Consequently, climate change and ocean acidification will have significant indirect effects on Coral Reef fishes due to their effects on Coral Reef habitat.
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climate change and Coral Reef connectivity
Coral Reefs, 2009Co-Authors: Philip L. Munday, Michael J. Kingsford, Jeffrey M Leis, Janice M Lough, Claire B Paris, Michael L Berumen, Jonathan LambrechtsAbstract:This review assesses and predicts the impacts that rapid climate change will have on population connectivity in Coral Reef ecosystems, using fishes as a model group. Increased ocean temperatures are expected to accelerate larval development, potentially leading to reduced pelagic durations and earlier Reef-seeking behaviour. Depending on the spatial arrangement of Reefs, the expectation would be a reduction in dispersal distances and the spatial scale of connectivity. Small increase in temperature might enhance the number of larvae surviving the pelagic phase, but larger increases are likely to reduce reproductive output and increase larval mortality. Changes to ocean currents could alter the dynamics of larval supply and changes to planktonic productivity could affect how many larvae survive the pelagic stage and their condition at settlement; however, these patterns are likely to vary greatly from place-to-place and projections of how oceanographic features will change in the future lack sufficient certainty and resolution to make robust predictions. Connectivity could also be compromised by the increased fragmentation of Reef habitat due to the effects of Coral bleaching and ocean acidification. Changes to the spatial and temporal scales of connectivity have implications for the management of Coral Reef ecosystems, especially the design and placement of marine-protected areas. The size and spacing of protected areas may need to be strategically adjusted if reserve networks are to retain their efficacy in the future.
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effects of climate induced Coral bleaching on Coral Reef fishes ecological and economic consequences
Oceanography and Marine Biology, 2008Co-Authors: Morgan S. Pratchett, Geoffrey P. Jones, Philip L. Munday, Nicholas A. J. Graham, Joshua E. Cinner, Shaun K. Wilson, David R. Bellwood, Nicholas Polunin, T R McclanahanAbstract:Global climate change is having devastating effects on habitat structure in Coral-Reef ecosystems owing to extreme environmental sensitivities and consequent bleaching of Reef-building scleractinian Corals. Coral bleaching frequently causes immediate loss of live Coral and may lead to longer-term declines in topographic complexity. This review identifies Coral cover and topographic complexity as critical and distinct components of Coral-Reef habitats that shape communities of Coral-Reef fishes. Coral loss has the greatest and most immediate effect on fishes that depend on live Corals for food or shelter, and many such fishes may face considerable risk of extinction with increasing frequency and severity of bleaching. Coral loss may also have longer-term consequences for fishes that require live Corals at settlement, which are compounded by devastating effects of declining topographic complexity. Topographic complexity moderates major biotic factors, such as predation and competition, contributing to the high diversity of fishes on Coral Reefs. Many Coral-Reef fishes that do not depend on live Coral are nonetheless dependent on the topographic complexity provided by healthy Coral growth. Ecological and economic consequences of declining topographic complexity are likely to be substantial compared with selective effects of Coral loss but both Coral cover and topographic complexity must be recognised as a critical component of habitat structure and managed accordingly. Urgent action on the fundamental causes of climate change and appropriate management of critical elements of habitat structure (Coral cover and topographic complexity) are key to ensuring long-term persistence of Coral-Reef fishes.
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Climate change and the future for Coral Reef fishes
Fish and Fisheries, 2008Co-Authors: Philip L. Munday, Geoffrey P. Jones, Morgan S. Pratchett, Ashley J. WilliamsAbstract:Climate change will impact Coral-Reef fishes through effects on individual performance, trophic linkages, recruitment dynamics, population connectivity and other ecosystem processes. The most immediate impacts will be a loss of diversity and changes to fish community composition as a result of Coral bleaching. Coral-dependent fishes suffer the most rapid population declines as Coral is lost; however, many other species will exhibit long-term declines due to loss of settlement habitat and erosion of habitat structural complexity. Increased ocean temperature will affect the physiological performance and behaviour of Coral Reef fishes, especially during their early life history. Small temperature increases might favour larval development, but this could be counteracted by negative effects on adult reproduction. Already variable recruitment will become even more unpredictable. This will make optimal harvest strategies for Coral Reef fisheries more difficult to determine and populations more susceptible to overfishing. A substantial number of species could exhibit range shifts, with implications for extinction risk of small-range species near the margins of Reef development. There are critical gaps in our knowledge of how climate change will affect tropical marine fishes. Predictions are often based on temperate examples, which may be inappropriate for tropical species. Improved projections of how ocean currents and primary productivity will change are needed to better predict how Reef fish population dynamics and connectivity patterns will change. Finally, the potential for adaptation to climate change needs more attention. Many Coral Reef fishes have geographical ranges spanning a wide temperature gradient and some have short generation times. These characteristics are conducive to acclimation or local adaptation to climate change and provide hope that the more resilient species will persist if immediate action is taken to stabilize Earth's climate.
Serge Andrefouet - One of the best experts on this subject based on the ideXlab platform.
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spectral reflectance of Coral Reef bottom types worldwide and implications for Coral Reef remote sensing
Remote Sensing of Environment, 2003Co-Authors: Eric J Hochberg, Marlin J Atkinson, Serge AndrefouetAbstract:Abstract Coral Reef benthic communities are mosaics of individual bottom-types that are distinguished by their taxonomic composition and functional roles in the ecosystem. Knowledge of community structure is essential to understanding many Reef processes. To develop techniques for identification and mapping of Reef bottom-types using remote sensing, we measured 13,100 in situ optical reflectance spectra (400–700 nm, 1-nm intervals) of 12 basic Reef bottom-types in the Atlantic, Pacific, and Indian Oceans: fleshy (1) brown, (2) green, and (3) red algae; non-fleshy (4) encrusting calcareous and (5) turf algae; (6) bleached, (7) blue, and (8) brown hermatypic Coral; (9) soft/gorgonian Coral; (10) seagrass; (11) terrigenous mud; and (12) carbonate sand. Each bottom-type exhibits characteristic spectral reflectance features that are conservative across biogeographic regions. Most notable are the brightness of carbonate sand and local extrema near 570 nm in blue (minimum) and brown (maximum) Corals. Classification function analyses for the 12 bottom-types achieve mean accuracies of 83%, 76%, and 71% for full-spectrum data (301-wavelength), 52-wavelength, and 14-wavelength subsets, respectively. The distinguishing spectral features for the 12 bottom-types exist in well-defined, narrow (10–20 nm) wavelength ranges and are ubiquitous throughout the world. We reason that spectral reflectance features arise primarily as a result of spectral absorption processes. Radiative transfer modeling shows that in typically clear Coral Reef waters, dark substrates such as Corals have a depth-of-detection limit on the order of 10–20 m. Our results provide the foundation for design of a sensor with the purpose of assessing the global status of Coral Reefs.
Chris Roelfsema - One of the best experts on this subject based on the ideXlab platform.
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measuring Coral Reef terrain roughness using structure from motion close range photogrammetry
Geomorphology, 2015Co-Authors: Javier X Leon, Chris Roelfsema, Megan I Saunders, Stuart PhinnAbstract:Our understanding of Earth surface processes is rapidly advancing as new remote sensing technologies such as LiDAR and close-range digital photogrammetry become more accessible and affordable. A very-high spatial resolution digital terrain model (DTM) and orthophoto mosaic (mm scale) were produced using close-range digital photogrammetry based on ‘Structure-from-Motion’ (SfM) algorithms for a 250 m transect along a shallow Coral Reef flat on Heron Reef, Great Barrier Reef. The precise terrain data were used to characterise surface roughness, a critical factor affecting ecological and physical processes on the Reef. Three roughness parameters, namely the root mean square height, tortuosity (or rugosity) and fractal dimension, were derived and compared in order to asses which one better characterises Reef flat roughness. The typical relief across the shallow Reef flat was 0.1 m with a maximum value of 0.42 m. Coral Reef terrain roughness, as characterised by the three chosen parameters, generally increased towards the middle of the transect where live Coral covers most of the Reef flat and decreases towards the edges of the transect. The fractal dimension (values ranging from 2.2 to 2.59) best characterised Reef roughness, as evidenced by a closer agreement with the distribution of known Coral benthic substrates. This is the first study quantifying scale-independent roughness of a Coral Reef at benthic and biotope/patch levels (cm-m). The readily available and cost-effective methods presented are highly appropriate for data collection, processing and analysis to generate very-high spatial resolution DTMs and orthophoto mosaics of shallow and energetic Coral Reefs.
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Bottom Reflectance in Ocean Color Satellite Remote Sensing for Coral Reef Environments
Remote Sensing, 2015Co-Authors: Martina Reichstetter, Peter Fearns, Scarla Weeks, Lachlan Mckinna, Chris Roelfsema, Miles FurnasAbstract:Most ocean color algorithms are designed for optically deep waters, where the seafloor has little or no effect on remote sensing reflectance. This can lead to inaccurate retrievals of inherent optical properties (IOPs) in optically shallow water environments. Here, we investigate in situ hyperspectral bottom reflectance signatures and their separability for Coral Reef waters, when observed at the spectral resolutions of MODIS and SeaWiFS sensors. We use radiative transfer modeling to calculate the effects of bottom reflectance on the remote sensing reflectance signal, and assess detectability and discrimination of common Coral Reef bottom classes by clustering modeled remote sensing reflectance signals. We assess 8280 scenarios, including four IOPs, 23 depths and 45 bottom classes at MODIS and SeaWiFS bands. Our results show: (i) no significant contamination (Rrscorr < 0.0005) of bottom reflectance on the spectrally-averaged remote sensing reflectance signal at depths >17 m for MODIS and >19 m for SeaWiFS for the brightest spectral reflectance substrate (light sand) in clear Reef waters; and (ii) bottom cover classes can be combined into two distinct groups, “light” and “dark”, based on the modeled surface reflectance signals. This study establishes that it is possible to efficiently improve parameterization of bottom reflectance and water-column IOP retrievals in shallow water ocean color models for Coral Reef environments.
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Mapping Coral Reef Benthos, Substrates, and Bathymetry, Using Compact Airborne Spectrographic Imager (CASI) Data
Remote Sensing, 2014Co-Authors: Ian Leiper, Stuart Phinn, Chris Roelfsema, Kelly Joyce, Arnold DekkerAbstract:This study used a Reef-up approach to map Coral Reef benthos, substrates and bathymetry, with high spatial resolution hyperspectral image data. It investigated a physics-based inversion method for mapping Coral Reef benthos and substrates using readily available software: Hydrolight and ENVI. Compact Airborne Spectrographic Imager (CASI) data were acquired over Heron Reef in July 2002. The spectral reflectance of Coral Reef benthos and substrate types were measured in-situ, and using the HydroLight 4.2 radiative transfer model a spectral reflectance library of subsurface reflectance was simulated using water column depths from 0.5–10.0 m at 0.5 m intervals. Using the Spectral Angle Mapper algorithm, sediment, benthic micro-algae, algal turf, crustose Coralline algae, macro-algae, and live Coral were mapped with an overall accuracy of 65% to a depth of around 8.0 m; in waters deeper than 8.0 m the match between the classified image and field validation data was poor. Qualitative validation of the maps showed accurate mapping of areas dominated by sediment, benthic micro-algae, algal turf, live Coral, and macro-algae. A bathymetric map was produced for water column depths 0.5–10.0 m, at 0.5 m intervals, and showed high correspondence with in-situ sonar data (R2 value of 0.93).
