Multi-decadal atoll-island dynamics in the Indian Ocean Chagos Archipelago
Introduction
Coastlines are dynamic environments. Their morphology is influenced by a host of controls, including sea level, on timescales of minutes to millennia. Not least because of our desire to live by the sea, the dynamics and drivers of coastal change have been studied intensively (Richmond, 1993; Kaluwin and Smith, 1997; Dickinson, 1999; Masselink and Pattiaratchi, 2001; Woodroffe, 2008; Collen et al., 2009; Webb and Kench, 2010; Rankey, 2011; Purkis and Klemas, 2011; Taylor and Purkis, 2012; Andréfouët et al., 2013; Purkis et al., 2016a; Duvat and Pillet, 2017; Tuck et al., 2019; Duvat, 2020; Newnham et al., 2020; Holdaway et al., 2021). Notwithstanding raised atolls, those that have been tectonically uplifted, atoll islands are low and flat with maximum elevations typically in the range of only a few meters. Being only slightly above sea level carries several implications. First, atoll islands are vulnerable to extreme events such as storms, as well as to global environmental change – sea-level rise, in particular. On the latter point, though, it has been suggested that some (but not all) coral islands might be morphologically resilient to rising seas (Kench et al., 2005; Woodroffe, 2005, Woodroffe, 2008; Webb and Kench, 2010; Kench et al., 2015; McLean and Kench, 2015; Beetham et al., 2017; Tuck et al., 2019), though some argue to the contrary (Hubbard et al., 2014; Storlazzi et al., 2015). In addition, atoll islands have little land area and limited natural terrestrial resources, e.g., climate-sensitive supplies of fresh groundwater (Bailey, 2015; Barkey and Bailey, 2017; Falkland and White, 2020). Finally, terrestrial plants build the base of the entire inland atoll ecosystem. The decomposition of the terrestrial plant material (leaves, stems, roots, etc.) forms a layer on the soil surface (Boberg, 2009; Coleman et al., 2017) herein referred to as ‘brown soil’. Because of its biological origin, expansion of brown soil areas will occur on timescales determined by the lifecycle of the constituent plant species. As more than 99.8% of the terrestrial plant species have a finite tolerance to salt (Flowers and Colmer, 2015), it is anticipated that repeated exposure to seawater will lead to rapid ecosystem collapse and thus ‘brown soil’ contraction will reflect rates of plant death and soil structure degradation.
Whereas atoll islands are rare in the tropical Atlantic, they are common in the Pacific and Indian Oceans, where they constitute entire countries or overseas territories of continental states, serve as home to hundreds of thousands of inhabitants, and house critical and strategic infrastructure. As summarized by Duvat (2019), out of 709 atoll islands which have so far been analyzed to quantify shoreline dynamics, 533 islands are located in the Pacific and only 176 in the Indian Ocean. Of the work accomplished in the Indian Ocean, the Maldives Archipelago has received the most attention (e.g., Aslam and Kench, 2017). Even from this portfolio of studies, only one (at the scale of entire atolls), Nadikdik Atoll in the Pacific, is an uninhabited site, thereby allowing for the dynamics of an anthropogenically-unaltered atoll to be observed (Ford and Kench, 2014). Although it has long been recognized that artificial modification of the coastal zone can strongly influence shoreline migration (Ford, 2012; Hamylton and East, 2012; Duvat and Pillet, 2017; Purkis et al., 2016a), this latter study emphasizes that even atoll islands uninhabited by humans are naturally dynamic.
The paucity of studies which consider both uninhabited and inhabited islands in the same archipelago (and therefore under the same biophysical controls) frustrates efforts to disentangle the human effects of coastline erosion triggered by inappropriate intervention, such as the construction of seawalls that can starve beaches of necessary sediment supply, from natural island dynamics. A full and thorough understanding of the natural behavior of these systems in a period of relatively static sea level, as compared to the rapid rise witnessed in the Early to Mid-Holocene, can provide a critical baseline as to the lower-end of island reconfiguration that should be anticipated in the present era of accelerated sea level rise.
In this study, we assemble a suite of archive aerial photographs (negatives) for the Indian Ocean Chagos Archipelago (Fig. 1A). These photographs were acquired in the 1960's and 1970's and have an effective resolution finer than 0.2 × 0.2 m. In this study, we consider two atolls. First, Peros Banhos (Fig. 1B), one of the very few atolls globally where natural island dynamics can be quantified at the scale of a whole atoll – withstanding just one of the 35 islands, Peros Banhos has never been settled by humans. This atoll represents an excellent natural laboratory because its islands are near-evenly distributed around the atoll rim, thereby allowing for nuanced differences between windward and leeward positions to be examined. The second atoll considered is Diego Garcia (Fig. 1C). While the majority of the coastline of Diego Garcia is unmodified, a U.S. military port and airfield complex has been constructed in the last 50 years on the western limb of the atoll which now occupies approximately 10% of its circumference (see red box in Fig. 1C). The coastline dynamics of Diego Garcia have been considered by both Hamylton and East (2012) and Purkis et al. (2016a), and therefore the trajectory of the atoll with regard to prevailing natural and anthropogenic forcings is well understood. Capitalizing on this previous work, and expanding to include the uninhabited islands of the adjacent Peros Banhos Atoll, allows the dynamics of atoll islands in their natural state (Peros Banhos) to be contrasted with those of an anthropogenically-disturbed system (Diego Garcia), while avoiding bias introduced by varying ocean climate - both sites are closely situated in the same archipelago. The comparison is further strengthened by the subequatorial position of Chagos which prevents direct impacts by cyclones. These events can cause radical local divergence of coastline dynamics, even for closely situated islands (Kench and Brander, 2006; Ford and Kench, 2016; Duvat et al., 2017). Note, however, that the Chagos atolls, like those of the adjacent Maldives (Aslam and Kench, 2017), are exposed to distant-source swells originating in the southern Indian Ocean and therefore not wholly spared cyclonic influence. Since the southern rim of Peros Banhos is poorly developed (Fig. 1B), these swells should be anticipated to impact its lagoon, and the islands which face it, more than for Diego Garcia, which has a well-developed reef rim around the entirety of the atoll (Fig. 1C).
The goals of the study are twofold: First, using remote sensing data spanning more than 50 years, to quantify whether the islands of Peros Banhos (Fig. 1D-E), an atoll which has not been disturbed by humans, behaves differently from Diego Garcia (Fig. 1F-G). Second, using a hydrodynamic model for both atolls, to explore the physical controls driving coastline change of the two systems. Answers to such questions are urgently needed – there is reasonable doubt as to whether many atoll islands globally will remain habitable in the face of rising sea level and decreasing sediment supply resulting from the wholesale collapse of coral reef systems, as wrought by ever more frequent marine heatwaves (Le Cozannet et al., 2013; Perry and Morgan, 2017; Perry et al., 2018; Sheppard et al., 2017). Atoll islands are nourished by reef-derived detritus and so their persistence is therefore closely intertwined with the fate of the reefs that surround them.
Section snippets
The Chagos Archipelago
Chagos is part of the British Indian Ocean Territory (BIOT) and lies in the center of the Indian Ocean, occupying a southern extension of the Laccadive–Maldives Ridge (Fig. 1). There are 55 atoll islands distributed atop the eight Chagos atolls. These islands are fewer and smaller than those of the Maldives or Laccadive Archipelagos. Like atoll islands globally, the Chagos Islands are low lying (only 2–5 m above sea level) and are typical coral cays constructed of limestone with underlying
Methods
This study was facilitated by the discovery of a collection of vintage aerial photographs acquired over Diego Garcia in 1963 and Peros Banhos in 1979. The former atoll was imaged in its entirety, whereas for Peros Banhos, several small islands in the southwest of the atoll were not photographed and therefore could not be included in our analyses (shaded gray in Fig. 1B). The workflow used to accomplish this study combined fieldwork, remote sensing, hydrodynamic modelling, and computational GIS.
Aggregate changes in the area of atoll islands for Peros Banhos and Diego Garcia
Data describing per-island net changes in area (Table 1) reveal that whereas some islands have expanded through time and others contracted, the decadal percentage changes are exceedingly small, at <1% when averaged across all of the considered islands. An outlier to this trend is Ile Fouquet (Peros Banhos), however, which contracted in area by −6.55% per decade of observation (Fig. 3A-D). This island, which is situated on the southern rim of Peros Banhos, lost 19.1% of its area from the
Discussion
Pairing the two sites in the same archipelago allows the dynamics of atoll islands to be contrasted while avoiding bias introduced by varying ocean climate. The atolls of Diego Garcia and Peros Banhos are geomorphologically and hydrodynamically different, which delivers dissimilar styles of sediment transport, and therefore different coastline dynamics. However, at a high level, the behavior of the islands on the two atolls is similar in the sense that total island area is almost static through
Conclusions
By compiling data describing the dynamics of atoll-island coastlines for 20 islands in Peros Banhos Atoll and for the single island atop Diego Garcia Atoll, this paper has emphasized that whether impacted by humans, or not, these islands display multi-decadal stability, if island area is the considered metric. However, if the movement of the coastlines of these islands through time is considered, they are seen to be highly dynamic, with coastlines facing prevailing wave energy eroding and those
Declaration of Competing Interest
The authors have no conflict of interest to declare and pledge that the submitted work is original.
Acknowledgments
We are indebted to the crew of the M/Y Golden Shadow, through the generosity of HRH Prince Khaled bin Sultan, for their inexhaustible help in the field. We thank Brett Thomassie and DigitalGlobe Inc. for their unwavering assistance over a decade of intense satellite image acquisition. The study would have been impossible without the assistance of Ted Morris, David Vann, and Kirby Crawford who helped secure the archive aerial photography of Diego Garcia and to Angus (Gus) Jones who kindly
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