Long-Term Responses of the Endemic Reef-Builder <i>Cladocora caespitosa</i> to Mediterranean Warming

<div><p>Recurrent climate-induced mass-mortalities have been recorded in the Mediterranean Sea over the past 15 years. <i>Cladocora caespitosa,</i> the sole zooxanthellate scleractinian reef-builder in the Mediterranean, is among the organisms affected by these episodes. Extensive bioconstructions of this endemic coral are very rare at the present time and are threatened by several stressors. In this study, we assessed the long-term response of this temperate coral to warming sea-water in the Columbretes Islands (NW Mediterranean) and described, for the first time, the relationship between recurrent mortality events and local sea surface temperature (SST) regimes in the Mediterranean Sea. A water temperature series spanning more than 20 years showed a summer warming trend of 0.06°C per year and an increased frequency of positive thermal anomalies. Mortality resulted from tissue necrosis without massive zooxanthellae loss and during the 11-year study, necrosis was recorded during nine summers separated into two mortality periods (2003–2006 and 2008–2012). The highest necrosis rates were registered during the first mortality period, after the exceptionally hot summer of 2003. Although necrosis and temperature were significantly associated, the variability in necrosis rates during summers with similar thermal anomalies pointed to other acting factors. In this sense, our results showed that these differences were more closely related to the interannual temperature context and delayed thermal stress after extreme summers, rather than to acclimatisation and adaption processes.</p></div

A. SST mean annual cycle in the Columbretes Islands (1991–2012).

<p>B. Mean summer SST (June-September, 1991–2012).</p

Map of the study site.

<p>A. The Columbretes Islands (NW Mediterranean, Spain). B. Illa Grossa Bay.</p

Thermal environment at a depth of 5 to 20 m.

<p>A. The 2007–2012 annual average B. Data from June to November in a summer with negative SST anomaly, 2007. C. Data from June to November in a summer with highly positive SST anomaly, 2009.</p

Mortality of <i>C. caespitosa</i>.

<p>A. <i>C. caespitosa</i> colony showing partial necrosis. B. Totally affected colony. C. The necrosis process in the polyps of <i>C. caespitosa</i>.</p

A. Percentage of necrosis (mean ± SD) detected in each mortality period.

<p>B. Percentage of necrosis (mean ± SD) detected in the second mortality period in colonies that were either affected or unaffected in the first mortality period.</p

<i>C. caespitosa</i> necrosis rates (2002–2012) and summer SST anomalies (1991–2012).

<p><i>C. caespitosa</i> necrosis rates (2002–2012) and summer SST anomalies (1991–2012).</p

Annual thermal regime (2003–2012) and average SST for the data series 1991–2012.

<p>Dotted vertical lines delimit the summer period.</p

Mortality and temperature descriptors.

<p>Note that necrosis is given in reference to the remaining living colony area.</p

“Colony-on-colony” recruitment in a necrosis-affected <i>C. caespitosa</i> colony.

<p>Scale bar: 5 cm.</p