Coralreefs around the world are under increasing pressure from anthropogenic sourcesand climate change (Donner et a., 2005). Coral reefs in the western IndianOcean are ecosystems of high diversity, in addition to being an important partof the ocean-based economies and foodsupplies in the region (Obura 2012; Obura et al.
2017). Coral bleaching in theregion has been observed since 1982, with severe bleaching events occurringduring the summer of 1998, 2002 and 2006 (Obura 2005; Obura 2001; McClanahan etal. 2001). The frequency and severity of bleaching are, however, projected toincrease under global warming, thus posing a serious threat to the future stateof the world’s coral ecosystems (Hoegh-Guldberg, 1999; Hughes et al., 2003;Donner et al., 2005). Elevated sea surface temperature (SST) is the primarycause of mass bleaching and mortality events (Hoegh-Guldberg, 1999).
Coralreefs thrive well if SST does not exceed their maximum temperature limits but slightincreases above mean maximum monthly temperature (MMM) can cause bleaching. If bleachingevents, which are determined by how much and for how long temperatures remainabove the maximum mean summer temperature, last too long then coral mortality occurs(Hoegh-Guldberg, 1999). The recovery of coral reefs is dependent on theseverity of the bleaching and the time between individual bleaching events, andthus increased frequency in these events severely limits the capability of thereefs to recover (McCook, 1999). Giventhe scale of coral reef systems and the availability of satellite remote sensingdata, identifying the potential for coral bleaching in a region and thenmonitoring its occurrence is feasible. To identify bleaching potential at alocation, the National Oceanographic and Atmospheric Administration (NOAA)Coral Reef Watch (CRW; Strong et al. 2004; Liu et al.
2006) uses a referencethreshold of MMM where if exceeded for a period of time, bleaching is likely tooccur with potential mortality. Widely used coral metrics for coral bleachingprediction are Coral HotSpots and Degree Heating Weeks (DHWs) (Liu et al.2008). Coral HotSpots measure the occurrence and the magnitude of thermalstress potentially conducive to coral bleaching by calculating positive SSTanomalies referenced to the MMM climatology at a particular location (Liu etal., 2003; Liu et al., 2008; Strong et al.
2006). While the Coral BleachingHotSpot provides an instantaneous measure of the thermal stress, there isevidence that corals are sensitive to an accumulation of thermal stress overtime (Glynn and D’Croz, 1990). In order to monitor this cumulative effect, theconcept of coral bleaching DHWs, which are a measure of the thermal stressaccumulation that coral reefs have experienced over the past 12 weeks, wasdeveloped (Liu et al., 2003; Liu et al., 2006).
Glynn and D’Croz (1990) showedthat temperatures exceeding 1 °C above the usual summertime maximum aresufficient to cause stress to corals (known as the bleaching thresholdtemperature). Donner et al. (2005) developed similar metrics for monthlytimescales, defining Degree Heating Months (DHM) as the sum of monthly HotSpots> 0°C over a rolling period of 4 months. In their study, they found that DHM values of 1 and 2 correspondedreasonably well to the Liu et al. (2003) DHW values of 4 and 8, respectively. However,the satellite based hindcast and nowcast tools only provide information as tohow bleaching thermal stress has evolved and the present likelihood ofbleaching.
It is also very important to better understand how the likelihood ofcoral bleaching at any given location may change in the future since global seasurface temperatures are expected to rise by approximately 0.4 – 1.1°C by 2025 (IPCC 2015). With coralreefs being among the most sensitive ecosystems to climate change, anticipatedincreases in SSTs are likely to have large negative impacts on the numerousgoods and services provided by corals. It is therefore important to examine theimplications of this warming for coral reefs in the western Indian Ocean.
Thereis potential for corals to adapt or acclimate to a warming ocean (Douglas,2003; Hughes et al., 2003) by shifting to symbioses with moretemperature-tolerant species of Symbiodinium (Brown et al. 2002; Bakeret al.
2004; Coles and Brown 2003). West and Salm, (2003), for example, notedthat identifying the thermal stress and level of thermal adaptation for coralsis vital both to the conservation of coral reefs. For a given population, anyconservation strategy must consider the larval connectivity among populations(Sale et al. 2005). Mumby et al.
(2011) illustrated the potential importance oflarval connectivity across different temperature regimes in the Bahamas whileKleypas et al. (2016) used a biophysical model to illustrate larval transportbetween reefs of widely varying temperatures in the Coral Triangle. However,the capacity of larval dispersal adapted to different temperature regimes hasnot been researched in the western Indian Ocean region. In thisstudy, thermal stress history and patterns are assessed for the western IndianOcean and then three extensive bleaching events (1998, 2010 and 2016) areexamined using bleaching reportscombined with satellite-derived sea surface temperatures. Because dispersal andconnectivity of larvae between reefs is a key component of coral populationdynamics, how larval dispersal will influence acclimation and adaptation ofcorals to the local maximum temperature regime is also considered.