Coral is likely to occur with potential mortality. Widely

Coral
reefs around the world are under increasing pressure from anthropogenic sources
and climate change (Donner et a., 2005). Coral reefs in the western Indian
Ocean are ecosystems of high diversity, in addition to being an important part
of the  ocean-based economies and food
supplies in the region (Obura 2012; Obura et al. 2017). Coral bleaching in the
region has been observed since 1982, with severe bleaching events occurring
during the summer of 1998, 2002 and 2006 (Obura 2005; Obura 2001; McClanahan et
al. 2001). The frequency and severity of bleaching are, however, projected to
increase under global warming, thus posing a serious threat to the future state
of the world’s coral ecosystems (Hoegh-Guldberg, 1999; Hughes et al., 2003;
Donner et al., 2005). Elevated sea surface temperature (SST) is the primary
cause of mass bleaching and mortality events (Hoegh-Guldberg, 1999). Coral
reefs thrive well if SST does not exceed their maximum temperature limits but slight
increases above mean maximum monthly temperature (MMM) can cause bleaching. If bleaching
events, which are determined by how much and for how long temperatures remain
above the maximum mean summer temperature, last too long then coral mortality occurs
(Hoegh-Guldberg, 1999). The recovery of coral reefs is dependent on the
severity of the bleaching and the time between individual bleaching events, and
thus increased frequency in these events severely limits the capability of the
reefs to recover (McCook, 1999).

 

Given
the scale of coral reef systems and the availability of satellite remote sensing
data, identifying the potential for coral bleaching in a region and then
monitoring its occurrence is feasible. To identify bleaching potential at a
location, the National Oceanographic and Atmospheric Administration (NOAA)
Coral Reef Watch (CRW; Strong et al. 2004; Liu et al. 2006) uses a reference
threshold of MMM where if exceeded for a period of time, bleaching is likely to
occur with potential mortality. Widely used coral metrics for coral bleaching
prediction are Coral HotSpots and Degree Heating Weeks (DHWs) (Liu et al.
2008). Coral HotSpots measure the occurrence and the magnitude of thermal
stress potentially conducive to coral bleaching by calculating positive SST
anomalies referenced to the MMM climatology at a particular location (Liu et
al., 2003; Liu et al., 2008; Strong et al. 2006). While the Coral Bleaching
HotSpot provides an instantaneous measure of the thermal stress, there is
evidence that corals are sensitive to an accumulation of thermal stress over
time (Glynn and D’Croz, 1990). In order to monitor this cumulative effect, the
concept of coral bleaching DHWs, which are a measure of the thermal stress
accumulation that coral reefs have experienced over the past 12 weeks, was
developed (Liu et al., 2003; Liu et al., 2006). Glynn and D’Croz (1990) showed
that temperatures exceeding 1 °C above the usual summertime maximum are
sufficient to cause stress to corals (known as the bleaching threshold
temperature). Donner et al. (2005) developed similar metrics for monthly
timescales, 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 corresponded
reasonably well to the Liu et al. (2003) DHW values of 4 and 8, respectively.

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However,
the satellite based hindcast and nowcast tools only provide information as to
how bleaching thermal stress has evolved and the present likelihood of
bleaching. It is also very important to better understand how the likelihood of
coral bleaching at any given location may change in the future since global sea
surface temperatures are expected to rise by approximately 0.4 – 1.1°C by 2025 (IPCC 2015). With coral
reefs being among the most sensitive ecosystems to climate change, anticipated
increases in SSTs are likely to have large negative impacts on the numerous
goods and services provided by corals. It is therefore important to examine the
implications of this warming for coral reefs in the western Indian Ocean.

 

There
is potential for corals to adapt or acclimate to a warming ocean (Douglas,
2003; Hughes et al., 2003) by shifting to symbioses with more
temperature-tolerant species of Symbiodinium (Brown et al. 2002; Baker
et al. 2004; Coles and Brown 2003). West and Salm, (2003), for example, noted
that identifying the thermal stress and level of thermal adaptation for corals
is vital both to the conservation of coral reefs. For a given population, any
conservation strategy must consider the larval connectivity among populations
(Sale et al. 2005). Mumby et al. (2011) illustrated the potential importance of
larval connectivity across different temperature regimes in the Bahamas while
Kleypas et al. (2016) used a biophysical model to illustrate larval transport
between reefs of widely varying temperatures in the Coral Triangle. However,
the capacity of larval dispersal adapted to different temperature regimes has
not been researched in the western Indian Ocean region.

 

In this
study, thermal stress history and patterns are assessed for the western Indian
Ocean and then three extensive bleaching events (1998, 2010 and 2016) are
examined  using bleaching reports
combined with satellite-derived sea surface temperatures. Because dispersal and
connectivity of larvae between reefs is a key component of coral population
dynamics, how larval dispersal will influence acclimation and adaptation of
corals to the local maximum temperature regime is also considered.