Growth and photosynthesis of two Mediterranean corals, Cladocora caespitosa and Oculina patagonica, under normal and elevated temperatures

Abstract

The Ligurian Sea (NW Mediterranean) experienced warm summers in 1998, 1999 and from 2003 to 2005. The temperature was 1-3°C higher than the mean summer value (24°C) and remained high over a long period. During these summers, mass-mortality events, affecting several sessile benthic species, were reported. In the present study, we tested the long-term (3-7 weeks) effect of different temperatures (20°C measured in spring and autumn, 24°C observed in summer, and 26°C and 28°C abnormal summer values) on two Mediterranean corals, Cladocora caespitosa and Oculina patagonica. Growth rate, photosynthetic efficiency (F v /F m ), relative electron transport rate (ETR), zooxanthellae and chlorophyll (chl) contents were measured during 48 days incubation. At 20°C, all parameters remained constant during the whole experiment for both species. At higher temperatures, most physiological parameters were affected by only 2-5 weeks at 24°C, and were severely depressed at higher temperatures. Small replicate samples (nubbins) of O. patagonica significantly decreased their zooxanthellae and chl concentrations at all temperatures, after 2 weeks of incubation. Their F v /F m values, as well as their growth rates, were also gradually reduced during the incubation at all temperatures. However, only a few nubbins maintained at 28°C showed signs of tissue necrosis after 34 days, and these gradually recovered tissue when temperature was returned to normal. In nubbins of C. caespitosa, chl and zooxanthellae concentrations decreased only after 34 days of incubation at 26°C and 28°C. At the same time, tissue necrosis was observed, explaining the loss of the symbionts. F v /F m was reduced only after 34 days of incubation at the different temperatures, and growth rate was first enhanced, before collapsing by 30% at 24°C and by 90-100% at 26°C and 28°C. All samples maintained at 26°C and 28°C had died, due to tissue necrosis, by the end of the experiment. Results obtained suggest that O. patagonica is more able than C. caespitosa to resist high temperature conditions because of its rapid bleaching capacity. In contrast, it seems that C. caespitosa is living close to its thermal limit during the summer period; therefore, a long-term increase at 24°C or above could be lethal for this coral, just as was observed in situ during the recent warm summers.

Response of the temperate coral <i>Cladocora caespitosa</i> to mid- and long-term exposure to <i>p</i>CO<sub>2</sub> and temperature levels projected for the year 2100 AD

“…A very sensitive marker of thermal stress in symbiotic corals is the decrease in photosynthetic efficiency ( F /F m ) after exposure to elevated temperatures (Warner et al, 1996). In this experiment, and in agreement with Rodolfo-Metalpa et al (2006), F /F m decreased after four weeks at 25 • C. If the elevated temperature was maintained for longer (>6 weeks), this decrease could have triggered a cascading effect leading to the death of the corals (Rodolfo-Metalpa et al, 2006, 2008c. Coral mortality has become increasingly frequent in the first 10-20 m because colonies are exposed to prolonged periods of high summer temperatures (mean 25-26 • C).…”

“…The effect of seawater warming has been extensively studied on corals because it is the primary cause of bleaching of tropical species (e.g. Brown, 1997) and of the mortality of warmtemperate Mediterranean species (Rodolfo-Metalpa et al, 2006, 2008c.…”

“…Colonies were subject to: (1) mid-term perturbations (1 month) in summer and winter conditions of irradiance and temperature, and (2) a long-term perturbation (1 year), mimicking the seasonal changes in temperature and irradiance. Because shallow-water corals already live (during summer) near their upper thermal limits in the north Mediterranean Sea and suffer mass-mortality during abnormal long periods of elevated temperatures (Rodolfo-Metalpa et al, 2006, 2008c, the experiments were carried out on colonies collected at 20 to 30 m depth, where the seasonal thermocline generally limits the incidence of summer thermal anomalies.…”

“…The fragments were carefully cleaned of epiphytes before each measurement. The calcification rate was estimated as the difference between the two weights normalized per day (Davies, 1989) and unit surface area measured according to Rodolfo-Metalpa et al (2006).…”

“…Chl a and c 2 were determined according to the equations of Jeffrey and Humphrey (1975) using a spectrophotometer. The details of both methods can be found in Rodolfo-Metalpa et al (2006).…”