Impact of bleaching on the coral Cladocora caespitosa in the eastern Adriatic Sea

Thermal anomalies during the last 15 yr caused bleaching in the zooxanthellate scleractinian coral Cladocora caespitosa (Linnaeus, 1767) in the eastern part of the Adriatic Sea. In this study, we assessed the long-term response of this temperate coral to warming seawater in the Adriatic Sea and described, for the first time, the relationship between recurrent coral bleaching events and local seawater temperature regimes. These were also the first coral bleaching events reported for the Mediterranean Sea. Coral bleaching was recorded in the period from 1997 to 2012 in Veliko Jezero within Mljet National Park (NP), Croatia. The most severe bleaching in Veliko Jezero was observed during the summer of 2003, when C. caespitosa colonies were exposed to temperatures >29°C for 46 d. A total of 426 colonies experienced partial bleaching and 133 colonies died due to complete bleaching. Bleaching results during 2011 were compared with bleaching events in the Piran area (Slovenia) over this same temporal interval. The highest seawater temperatures were recorded in the period from August to October of 2011 (28.8°C in Piran, 30.3°C at Mljet NP). Severe bleaching was observed on 54 (29.3%) colonies within the Mljet coral bank, while 39 colonies with complete bleaching died. In the Piran area, severe bleaching was observed on 26 (34.7%) colonies, while complete bleaching occurred in 8 (10.6%) colonies. Under the present climate-warming trend, the coral bleaching reported here could lead to a severe decline in the abundance of the surveyed populations of C. caespitosa.