Coral reefs provide integral services to social, economic, and ecological systems. They support more than 500 million livelihoods worldwide and account for 15% of gross domestic product in more than 20 countries. A quarter of all marine species on planet Earth, representing 28 of the 35 animal phyla, can be found in coral reefs, and novel compounds derived from these organisms provide numerous useful medicinal applications. However, over the past three decades coral reefs have suffered from and continue to face unprecedented declines due to the cumulative impacts of multiple local and global stressors. This highlights the need to use scientific technological advances to better monitor and maintain coral reefs for future generations.

Many threats faced by coral reefs are anthropogenic. On a local scale, coastal development, dredging, deforestation, and agriculture can cause sedimentation, nutrient enrichment and pollution affecting coral health (Burke et al. 2011). Overfishing can change the balance of coral reef food webs and destabilise these complex ecosystems, especially when key functional groups such as grazing parrotfish or top predators are removed. However, on a larger scale, increasingly common marine heatwaves threaten the persistence of entire swathes of coral reef. This has been recognised as the most significant threat to coral reefs worldwide. Due to our reliance on fossil fuels and increasing carbon dioxide emissions, Sea Surface Temperatures (SST) of the Indian, Atlantic, and Pacific Oceans have increased by 0.65, 0.41, and 0.31 °C respectively since the 1960s and are projected to rise by a further 1-4 °C by 2100 (Hoegh-Guldberg et al. 2014). The amount of warming we are committed to, and the impacts it will have on ecosystems will be determined by social and political decisions made in the coming decades. There is compelling evidence that a great proportion of the world’s coral reefs have been lost already, and more than half of reefs are under severe immediate risk (Hoegh-Guldberg et al. 2018). Thus, policymakers and managers need to have access to detailed up-to-date information about the status of coral reefs and high-resolution risk forecasts for multiple coral reef stressors. Critical information on the global area, extent, and composition of remaining coral reefs is needed to effectively monitor them through time, forecast coral reef stressors, and provide realistic management and conservation targets.

Coral reefs and how to find them

Global reef mapping started with the work of Charles Darwin in his 1842 book, “The Structure and Distribution of Coral Reefs”, which aimed to derive some basic global distribution estimates. However, the first spatially explicit catalogue of the world’s coral reefs made using satellite imagery by the United Nations Environment Program World Conservation Monitoring Centre (UNEP-WCMC) coral reef mapping project (Spalding M.D. et al. 2001). Space technologies and remote sensing have allowed scientists to monitor changes in coral reef ecosystems in a cost-effective way at large spatial and temporal scales. Computing techniques have been developed to use remote sensing imagery in the visual and infrared range to measure sea surface temperature, to estimate water quality indicators (i.e. sediment plumes, algae blooms, pollutant spills, and plastics), and to map habitat features (i.e. live coral, seagrass, bare sand, or rock). Current research initiatives are focused on comprehensively mapping the area and composition of the world’s coral reefs, and their major threats, in near-real time using these remote sensing products.

Coral bleaching and heat stress

Temperature fundamentally impacts everything in the natural and modern world, from the everyday choices we make, like “what should I wear today?”, to the productivity of whole agricultural and ecological systems. Temperature affects our weather systems, the storms we must endure, the annual seasonal cycle, and regional climates across the world. Concerningly, long-term changes in temperature due to ever-increasing carbon emissions has huge ramifications for how climate changes across large areas, and the frequency and intensity of extreme weather events. Thus, being able to measure temperatures across the world at a high resolution is very important. Satellites record this information using multispectral imaging technologies that measure the intensity of electromagnetic radiation across the spectrum using instruments like the Advanced Very-High-Resolution Radiometer (AVHRR). This is achieved by measuring the infrared radiation emitted from objects on Earth's surface, based on the principle that as matter becomes hotter, the molecules within move faster and emit more infrared radiation, and if they are hot enough, even emit visible light (i.e. glowing metals). Global maps of temperature thus provide important information to help science understand natural systems, and support decision-making and adaptation processes.

Like many other taxa in the animal kingdom, corals are highly sensitive to changes in temperature. Their life cycles even rely on seasonal temperature cues, for instance, rapid springtime increases in temperature are used by many species to time their reproduction synchronously and produce new generations. However, extreme thermal stress causes a breakdown in the symbiosis that reef-building corals have with photosynthetic microscopic algae called Symbiodinium (analogous to the symbiosis we have with our gut microbiome). Excessive thermal stress causes coral bleaching, or the expulsion of these pigmented Symbiodinium algal cells from the coral host which can later lead to coral mortality if the symbiosis is not re-established within a short period of time of a few weeks. Due to ever-more frequent and intense marine heatwaves (Oliver et al. 2018), coral bleaching en-masse has resulted in coral reef decline across large spatial scales, bringing the future of coral reef ecosystems into question.

Global ocean temperature datasets that can be used to understand ecological impacts and implications of warming events are extremely useful, however, the spatial and temporal coverage of satellite-derived temperature data from single satellites is patchy. Thus, the National Oceanographic and Atmospheric Administration (NOAA) Coral Reef Watch (CRW) has worked to provide a blended reanalysis of SST datasets that provides daily global coverage (Skirving et al. 2018). This is achieved using SST data from various satellites from multiple nation states together with algorithms to interpolate spatial or temporal gaps. CRW resources are calculated in near-real time and provide an early-warning system for coral bleaching by providing risk maps based on the Degree Heating Week (DHW) metric. DHW is a measure of accumulated heat stress over the last 12 weeks and above a physiological temperature stress threshold relevant to corals. It is important to note that this metric of heat-stress has limitations as coral sensitivity to heat stress is highly variable among genera, with some groups showing much higher heat-tolerance (e.g. Goniastrea spp.) than others (e.g. Acropora spp.). Nonetheless, such remote sensing products are critical resources to better understand the impact of temperature on marine ecosystems, and for predicting real-time bleaching risk.

Habitats maps and monitoring change

Managing coral reefs sustainably on large spatial scales requires long-term monitoring and prior knowledge of ecosystem health. This is where habitat maps showing the extent and composition of coral reef communities are of key importance. The Allen Coral Atlas initiative is leading the drive toward a global coral reef habitat map showing areas of live coral or algal cover. This project uses combinations of high-resolution satellite imagery, ground-truthing field data and machine learning to create these habitat maps. Focusing only at the Great Barrier Reef (GBR), the most detailed resource that describes all GBR reefs has been a reef outline map, giving no further information on community structure which can be highly variable and provide a diverse range of ecological services. A project is under way to map coral reef habitats across the Southwest Pacific and GBR region encompassing approximately 140,000 km2 of reef (Lyons et al. 2020) based on geomorphic zonation (reef crest, windward reef slope, outer reef flat, small reef, patch reef), and benthic cover (coral, algae, sand). Additionally, this project will also define habitats by coral growth form (branching, massive, plate). These large-scale high taxonomic resolution maps will allow coral reef managers to make informed management decisions such as where to conduct Crown-of-Thorns starfish culls or coral restoration programmes.

Coral reefs are dynamic three-dimensional structures created by myriad of habitat-forming species with different growth forms and ecological functions. As we consider reefs on progressively smaller scales, 3D structure and complexity increase greatly. While the bathymetrically justified habitat maps mentioned previously are highly valuable, the fine-scale 3D reef structure is still missing. On a reef, tabular tightly branched coral colonies provide essential microhabitat refuge for obligate associates like damselfish, crabs and other crustacea. Massive colonies create large island-like formations in the reefscape that can become cleaning stations for marine megafauna such as manta rays or resting places for turtles. Capturing this three-dimensional structure at scale is highly important (Calders et al. 2020). Laser Distance and Ranging (LiDAR) can be used to create extremely high-resolution 3D models of ecosystems. Altimetry data from satellite-attached LiDAR systems is freely available from NASA ICESat and ICESat-2, however, these 3D data products do not have ubiquitous coverage globally. Thus, there is limited application of such satellite-borne LiDAR data for coral reef monitoring. Instead, 3D structure and complexity of coral reefs is better monitored aircraft-borne LiDAR or structure-from-motion (SfM) photogrammetry. In recent years, increasing computational resources, and open sharing of SfM algorithms have allowed a democratisation of these technologies, and their application in monitoring the 3D structure is becoming evident. Such studies can provide novel insights at multiple ecologically significant scales, from individual-level measures of 3D coral growth, to reef-scale assessments of structural complexity. Currently, a lack of spatially continuous satellite-derived LiDAR altimetry datasets is preventing large-scale assessment of coral reef 3D structure, however, with ever improving technologies, this may be possible in the future

Conservation targets and future outlook

Publicly available coral reef maps have the potential to support informed decision-making and ecosystem conservation efforts. Remote-sensing products can provide the baseline information to sustainably managing coral reef systems. Temperature stress maps using SST time series can assist identify areas of potential risk during bleaching events, while habitat maps can be used for many purposes but critically to: a) locate suitable sites for restoration programs at ecologically relevant spatial scales, b) identify resilient reefs with high coral cover that are a priority for conservation, c) model ecosystem services (e.g. wave attenuation, fish nurseries), and e) detect detrimental changes in reef environments using time series analysis. Additionally, 3D maps can provide more detailed information on reef structure and function but are less applicable to broad spatial scales. Semi-automated mapping methods and publicly available outputs will be fundamental for stakeholders, governments, and NGO’s to protect coral reefs on large spatial scales.

The implementation of habitat and risk maps in conservation will depend on overcoming various methodological limitations. Remote sensing products are not accessible to users from all socio-economic backgrounds, and the technical expertise needed to create these maps is highly advanced, limiting their use in smaller management and conservation monitoring programmes. The development of user-friendly methodologies and products such as the UNESCO Bilko project (Green et al. 2000) will facilitate better collaboration between remote sensing specialists and practitioners and will be essential for the implementation of habitat maps in management actions. Ideally, future mapping frameworks will be able to access and process multisource sensor data within a single analysis platform, have the ability to update outputs when new sensor or training data is made available, and use a public analysis platform with minimal local computing resources requirements. Advances in the use of remote sensing in coral reef science is providing powerful new tools for management agencies and is widening the portfolio of interventionist measures that could be mobilized in response to the coral reef crisis.

Learn more about the Coralassist Lab at Newcastle University, Newcastle upon Tyne here www.coralassistlab.org