The coral health status of Platygyra spp. at thermal and salinity tolerance limits in Hong Kong waters

Dissolved oxygen and pH fluxes are considered drivers and indicators of coral energetics dynamics, including metabolic processes of respiration, photosynthesis and calcification. The metabolic function of shallow ecosystems is threatened by severe fluctuations in temperature and salinity. The ability to accurately measure physiological rates of benthic organisms in natural conditions as a response to these fluctuations gained attention in recent years. Although Platygyra spp. corals are considered resistant species and well-adapted to survive in the marginal reef, such as Hong Kong eastern waters, the knowledge of its physiological response to short term anomalies at high temperature and low salinity is still lacking. 

In this study, we used an innovative underwater diver-portable respirometer called CISME (Coral In-Situ Metabolism) designed to make non-destructive measures of coral energetics to combine laboratory-controlled conditions and in-situ monitoring of coral health status. 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 affect the overall energetic status. These experimental results are in line with in-situ observations. Although the sea surface temperature in Hong Kong is usually below 30°C and seawater salinity above 30 psu, the increase of acute temperature and heavy rainfall events in Hong Kong waters expected in the near future will seriously affect the survival of these corals.

The application of such a method promises benefits for fast and reliable field monitoring of healthy and stressed corals. These data provide useful information on metabolism of Platygyra spp. in order to improve the accuracy of forecasting their health status under the future climate change scenarios and extreme events.