The physiological response to high temperature and low salinity of Hong Kong's Platygyra spp. corals by underwater respirometry observations

Physiological rates of corals have been an object of investigation for a long time and different tools have been developed both for field and laboratory studies. The use of new diver-portable respirometers designed to measure coral respiration and photosynthesis by in-situ analysis of dissolved oxygen, pH and temperature variations is now under investigation. Hong Kong coral communities are frequently exposed to acute temperature and heavy rainfall events during the wet season (summer). However, they could be adapted to live at the edge of their environmental and physiological tolerances, even with slow growth and physiological rates.
Here we show the first results from our strategy to evaluate the coral health by combining classic scientific diving techniques with novel technologies. We selected the temperature of 30°C as physiological limit over which corals go under stress. Gross photosynthesis (P_g, p < 0.01) and calcification rates (G, p < 0.05) significantly decreased at high temperature (30-32°C). In few cases, photosynthetic efficiency (Fv/Fm) was also reduced. As result, the coral energetics (P_g / R) was seriously affected by increasing temperature (p < 0.01). On the other hand, the decrease of salinity (up to 21 psu) caused a decrease of calcification and whiteness (p < 0.05) but did not affected the status of energetics. These experimental results are in line with in-situ observations. Although the sea surface temperature is usually below the 30°C threshold limit (as yearly average) and seawater salinity above 30 psu, the increase of acute temperature and heavy rainfall events in Hong Kong waters is expected in the near future and will seriously affect the survival of these corals.
Our aim is to provide a better definition of coral health by integrating the complexity of the coral holobiont and the co-influence of biotic and abiotic factors in a changing climate scenario.