Satellite imaging coral reef resilience at regional scale. A case-study from Saudi Arabia
Highlights
► Spatially estimating a proxy for coral reef resilience from remote sensing data. ► Develop indices of coral stress from satellite data and apply using GIS. ► Propose the first-basis for a remote sensed resilience index (RSRI). ► RSRI provides an estimate of expected reef resilience. ► The framework presented is flexible and may be adapted to other reef regions.
Introduction
The long term viability of even the most remote and isolated coral reefs can no longer be assured. Corals lack the capacity to escape inclement conditions and must adapt to, or resist, change. Rapid, dramatic and seemingly intractable changes to reef communities have been attributed to a host of stressors; extreme thermal events, fishing pressure, sediment load, and physical destruction (Hughes and Connell, 1999, Wilkinson, 1999). Some reefs however appear to be more resilient than others (Nyström and Folke, 2001). Understanding coral resilience has been a rising theme of reef science. Resilience has three critical components; (1) biodiversity, (2) connectivity and (3) spatial heterogeneity (Nyström et al., 2008). Biodiversity is believed to confer resilience through redundancy within important ecosystem functions and feedbacks. Connectivity between reef systems facilitates the ‘rescue’ of one reef by another following disturbance. Spatial heterogeneity refers to the uneven distribution of resilience components over a reef area.
For use in management, Nyström et al. (2008) recognize the need to develop methods to observe resilience. With this goal in mind, Obura et al. (2009) strive to measure aspects of ecosystems that are important for coral reef resilience. Building forward from previous work, we propose a means of quantifying, what we consider to be, the most accessible and important components of coral reef resilience using remote sensing data. By contrast, conventional SCUBA-based surveys are ill suited to regional-scale assessment. We believe that satellite remote sensing can deliver measures relevant to coral reef resilience across vast geographic areas.
As Obura et al. (2009) note, both positive and negative factors are important to coral reef resilience. A path towards spatial assessment of positive factors has been established, for example by work of Harbourne et al. (2006) who used remote sensing maps to access pertinent information of coral reef diversity. Dalleau et al. (2010) built on this concept to test the efficiency of reef habitats as surrogates of species-level diversity. Wedding et al. (2011) describe the use of map-derived spatial metrics to analyze patterns and processes in marine systems. Negative factors, referred to variously as ‘stress’ or ‘threat’, have also been assessed using both remote sensing and map-based techniques (Halpern et al., 2008, Maina et al., 2008, McPherson et al., 2008, Mumby et al., 2011, Burke et al., 2011). However as Halpern et al. (2008) note, such studies of marine threats suffer from a lack of context and a need for ‘research on the most basic information, such as distribution of habitat types, and whether and how different anthropogenic drivers interact’. In short, while map-based methods have been advanced for considering both positive and negative resilience factors, they have not been addressed in concert.
Herein, we use remote sensing to consider coral stress factors and seascape properties that may confer resilience. Using a geographic information system (GIS) approach, our study focuses on reef environments of the Saudi Arabian Red Sea. Freely available satellite data (NASA, Google Earth) were assembled, along with archived and tasked imagery from the QuickBird commercial satellite (DigitalGlobe Inc.). The biological and physical characteristics of reef landscapes that we consider to promote resilience were mapped. Indicators for coral reef stress were derived from satellite across a large part of the Saudi Red Sea, and used in a more focused assessment of reef resilience in five study sites. These sites span 20,000 sq km and include approximately half the shallow (<20 m) water habitat of the Saudi Red Sea. Sites encompass the spectrum of reef types encountered in the region and are distributed across the latitudinal range of the Red Sea. Coral reef habitats in the five sites were mapped. These maps break new ground as they comprise a geographic area comparable to some of the largest reef mapping efforts (Rohmann et al., 2005), but at meter-scale resolution consummate with the finest achieved from comparable sensors (Purkis et al., 2006, Roelfsema and Phinn, 2010, Scopélitis et al., 2010).
The goals of this paper are twofold. First, to develop a methodology for assimilating regional-scale data into a potential measure of remote sensed reef resilience. Second, to use this measure to assess the differences within sites spread along the 1800 km coastline of the Red Sea. Through this approach we hope to stimulate debate and new thinking about an enhanced role for remote sensing in reef monitoring.
Section snippets
Red Sea study sites
The Red Sea is oriented along an approximate north-west to south-east axis; it is ∼2000 km long, up to 350 km wide and 2000 m deep. We surveyed five regions of the Saudi Red Sea; from north to south, Ras Al-Qasabah, Al Wajh, Yanbu, Farasan Banks, and Western Farasan Islands (Fig. 1).
Measures of stress and the coral reef environment
Obura et al. (2009) state, ‘ecological resilience relates to the entire scope of positive and negative factors affecting a community’. Such a definition is clearly intractable. Any method of resilience assessment must
Remote sensed coral stress in Saudi Arabia
According to our analysis, most Saudi reefs experience some fishing pressure (RFI, Fig. 9a). It is greatest in the waters offshore Jizan and the Farasan Islands. Here, large industrial and traditional fishing fleets are moored. Moderate pressure also occurs on reefs of the central Red Sea (Um Lujh-Al Lith). As would be expected, the highest development stress is seen for reef sites offshore of the large cities Jeddah, Jizan and Yanbu (DI, Fig. 9b). Overall though, most reefs experience very
Discussion
The data presented represent the most expansive and spatially coherent assessment of reefs in the Red Sea to date. This work is underpinned by the theory that resilience of corals can be considered as a product of, what we term, ‘landscape’ and ‘stress’ factors, and that some of the most important of these can be assessed using remote sensing. With reference to principles rooted in the literature, we formulate a series of indices to capture landscape and stress factors. Terms and values used in
Conclusion
Though we have discussed avenues for further development and refinement, the primary goals of this work have been attended to. Remote sensing provides spatially continuous assessment at a regional-scale. In our study, high spatial resolution coral reef habitat mapping underpins a better understanding of the landscape. Based on factors relevant to coral resilience in the Saudi Red Sea, we have provided a flexible framework that uses GIS to rationalize stress and landscape factors within a single
Acknowledgements
Financial support was provided by the National Coral Reef Institute (NCRI) and the Khaled bin Sultan Living Oceans Foundation (KBSLOF). Invaluable logistical field support was provided by the crew of the M/Y Golden Shadow, through the generosity of HRH Prince Khaled bin Sultan. Additional in country support was provided by the Saudi Wildlife Commission (SWC) and PERSGA. Dr. Liisa Metsamaa’s work in the NCRI is supported by the Estonian Science Foundation Mobilitas postdoctoral grant. Thanks are
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