Temporal and depth responses of two temperate corals, Cladocora caespitosa and Oculina patagonica, from the North Mediterranean Sea

Abstract

The effects of light and temperature on the growth and effective quantum yield (ΔF/Fm') of the scleractinian corals Cladocora caespitosa and Oculina patagonica distributed between 3 and 18 m depth was investigated for 18 mo in 2005-2006. Their growth rates, zooxanthellae and chloro- phyll concentrations generally increased from winter to spring, and decreased from early summer to autumn. O. patagonica behaved like a tropical coral with respect to growth enhancement with increasing light intensities and temperature, whereas C. caespitosa acted more as a temperate coral; its growth was enhanced by increasing temperature, not irradiance. By contrast, ΔF/Fm' did not show any relationship with environmental parameters, but was significantly lower in shallow colonies, reflecting some down-regulation of photosystem II. Mean photosynthetically active radiation received by shallow water corals during summer (1000 to 1400 µmol m -2 s -1 ) was as high as in some tropical regions. However, the diel cycle of ΔF/Fm' only showed a slight decrease at midday with a quick recovery soon afterwards, suggesting that the symbionts were either protected by coral pig- ments or adapted to light levels much higher than those usually experienced. Since C. caespitosa is mostly found in turbid environments and rarely in well-lit waters, its distribution in shallow water may be limited by factors other than light, such as competition with macroalgae. Results also showed that the 2 species inhabiting the NW Mediterranean Sea are under stress when temperatures become extremely cold or warm, during winters and summers, respectively.