Present Limits to Heat-Adaptability in Corals and Population-Level Responses to Climate Extremes

Climate change scenarios suggest an increase in tropical ocean temperature by 1–3°C by 2099, potentially killing many coral reefs. But Arabian/Persian Gulf corals already exist in this future thermal environment predicted for most tropical reefs and survived severe bleaching in 2010, one of the hottest years on record. Exposure to 33–35°C was on average twice as long as in non-bleaching years. Gulf corals bleached after exposure to temperatures above 34°C for a total of 8 weeks of which 3 weeks were above 35°C. This is more heat than any other corals can survive, providing an insight into the present limits of holobiont adaptation. We show that average temperatures as well as heat-waves in the Gulf have been increasing, that coral population levels will fluctuate strongly, and reef-building capability will be compromised. This, in combination with ocean acidification and significant local threats posed by rampant coastal development puts even these most heat-adapted corals at risk. WWF considers the Gulf ecoregion as “critically endangered”. We argue here that Gulf corals should be considered for assisted migration to the tropical Indo-Pacific. This would have the double benefit of avoiding local extinction of the world's most heat-adapted holobionts while at the same time introducing their genetic information to populations naïve to such extremes, potentially assisting their survival. Thus, the heat-adaptation acquired by Gulf corals over 6 k, could benefit tropical Indo-Pacific corals who have <100 y until they will experience a similarly harsh climate. Population models suggest that the heat-adapted corals could become dominant on tropical reefs within ∼20 years

Decision framework for assisted migration modified and appended from [<b>16</b>] which clearly indicates that Gulf corals are prime candidates.

<p>Decision framework for assisted migration modified and appended from <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0024802#pone.0024802-HoeghGuldberg3" target="_blank">[<b>16</b>]</a> which clearly indicates that Gulf corals are prime candidates.</p

Degree heating days (DHD) and heating rate (HR) at sites in Qatar and Abu Dhabi.

<p>The peak in DHD in August suggests onset of bleaching in that month. DHD and HR are lower in Abu Dhabi since the site has ∼1°C higher long-term summer mean temperatures than Qatar, thus requiring less heat to reach the bleaching threshold.</p

The 2010 bleaching event in the Arabian/Persian Gulf.

<p>(a) Modis SST composite for September 2010. The Gulf was the hottest part of the world's ocean. Conditions comparable to the Gulf will prevail throughout the tropics in 2099. (b) Mean (+standard deviation) bleaching incidence of the most common coral genera across SE Gulf coral monitoring sites (identified by circles in Fig. 1c) in October 2010, during regeneration from the main bleaching event. Bleaching was worst in <i>Anomastraea irregularis</i> (a rare species), <i>Acropora</i> spp. and <i>Porites harrisoni</i> (the most common framework-builders) (c) HadISST and Pathfinder (Modis) data showing the greatest heat in the central and SE Gulf. SST averages mentioned in the text were calculated from areas within the white boxes. Circles show monitoring sites (empty circle = coral monitoring only, full circle = coral monitoring plus temperature monitoring). Datasource: <a href="http://badc.nerc.ac.uk" target="_blank">http://badc.nerc.ac.uk</a>; <a href="http://www.nodc.noaa.gov" target="_blank">http://www.nodc.noaa.gov</a>.</p

Seasonal variability of hydrography off the east coast of Qatar, central Arabian Gulf

Seasonal variability of hydrography along a nearshore-offshore transect in the eastern part of the EEZ of Qatar has been analyzed using in situ measurements carried out during 5 different months. The study reveals distinct features in temperature, salinity, density, dissolved oxygen (DO), and chlorophyll fluorescence. The sea surface temperature (SST) varies from nearshore to offshore with a positive gradient during January, June, and August, of the order of 1.0–2.0 °C, and with a negative gradient during October and April, of the order of 1.0 °C. Thermal stratification began in June, reached a well-defined thermocline with a vertical difference in temperature of about 10 °C during August, and started to de-stratify during October. The low salinity and low-density inflow of IOSW is evident in the surface layer of the middle of the transect during August, which has enriched the DO in the surface layer up to 20 m depth, while hypoxia prevailed below 50 m depth. The lowest chlorophyll fluorescence was measured in April 2013 (~ 1.0 μg l−1), moderate in June 2013 (~ 1.5 μg l−1), and relatively high in August 2013 (~ 6.0 μg l−1).This work has been supported by the QAFCO project (Grant no. QUEX-ESC-QAFCO-20/21-2

An assessment of Qatar's coral communities in a regional context

Qatar's once extensive coral communities have undergone considerable change in recent decades. We quantitatively surveyed three coral assemblages in Qatar to assess current status, and compared these against 14 sites in Bahrain and the United Arab Emirates to evaluate Qatar in a larger biogeographic context. Umm Al-Arshan had the highest species richness of 17 sites examined in the southern Arabian Gulf, as well as the highest coral cover and the only Acropora observed on sites in Qatar. Coral cover and richness were more modest at Fuwayrit and Al-Ashat, reflecting greater impacts from earlier stress events. Two distinct communities were identified across the southern Gulf, with Umm Al-Arshan clustering with high-cover, mixed merulinid/poritid assemblages that were less impacted by earlier bleaching and long-term stress, while Fuwayrit and Al-Ashat grouped with a lower-cover, stress-tolerant community characteristic of more extreme environments in the southern Gulf. We recommend implementation of a nation-wide baseline assessment of coral communities to guide development of an MPA network and long-term coral monitoring program for Qatar