Coral reefs are declining worldwide, owing to global changes in the marine environment. Human pressures have both direct and indirect effects on ocean life, affecting the ecological functioning of coastal biological communities. The bleaching events occurring in tropical areas are raising serious questions about the prediction of the physiological response of corals to extreme scenarios, as well as on the strategies for conservation and restoration of reefs. Scleractinian corals living in coastal areas of subtropical reefs, such as the Hong Kong’s flagship species Platygyra spp., are considered to be stress-tolerant and well adapted to survive in marginal environments. These species can exhibit metabolic plasticity to environmental changes, and resilience patterns have been observed as coral holobiont responses in controlled conditions, but the metabolic performance can change across seasonal and spatial patterns, including all the environmental factors involved. However, their physiological responses and adaptation mechanisms to the exposure to abnormally high temperature and lowered salinity remains poorly understood. Therefore, the objective of this thesis was to assess the metabolic health of the coral Platygyra carnosa using non-invasive methods, in order to verify whether high temperature and low salinity conditions occurring in Hong Kong waters during the summer (e.g. wet season) were the limiting factors of metabolic resistance and survival of this species.

To address the discussion to field observations of coral health, the current status of coral reefs subjected to global and local impacts was first summarized (Chapter 1), with a focus on experimental techniques to measure coral metabolic rates and to evaluate the coral health. Through the analysis of scientific studies published in the last 30 years (1991 – 2020), the state of the art of experimental techniques used for in-situ investigations of coral health status for experimental and monitoring purposes was explored. Underwater fluorometry, respirometry chambers, and automated sensors were the main techniques used to evaluate the physiological status of corals in-situ. The photosynthetic efficiency, and dissolved oxygen and carbon dioxide fluxes were the main parameter investigated in coral ecophysiology studies as proxies for coral photosynthesis, respiration and calcification processes.

Secondly, specific protocols were developed for a new underwater respirometer so-called CISME (Community In-Situ Metabolism), designed to make routine diver-operated non-invasive measurements at coral surfaces in-situ for quantifying spatial and seasonal patterns of coral metabolism (Chapter 2). Shallow coral colonies of Platygyra spp. and Cyphastrea spp. were surveyed and metabolic profiles (respiration, photosynthesis and biocalcification), diel cycles (day and night), and photosynthesis-irradiance curves detected. Analysis of data from in-situ and laboratory-controlled conditions showed good replication among coral colonies and high precision measurements of temperature, oxygen and pH fluxes over 15-minute incubation time without noticeable detrimental effects on coral health. Moreover, marked differences were observed in coral calcification rates between estuarine-influenced and coastal marine conditions, despite the absence of significant differences in visual appearance or other health indicators, revealing the system’s potential for early detection of adverse coral conditions. Its ease of operation and rapid quantification of coral physiological status make this respirometer well suited for use by reef scientists, monitoring agencies, and stakeholders in biogenic reefs conservation efforts. The high spatial and temporal resolution of data from this underwater respirometry system will greatly facilitate understanding of how coral reefs respond to increasing stress from climate change and other anthropogenic pressures.

Thirdly, the protocols developed with CISME and other non-invasive techniques, such as diving-PAM and photographic analysis, were used to quantitatively assess the health of Platygyra carnosa during two single-stress experiments at high temperature and low salinity conditions in the laboratory (Chapter 3). Although the indicators of coral health, e.g. photosynthesis, respiration, calcification and whiteness, suggested that this coral species can survive to increasing temperature (25 - 32°C), its overall energetics were seriously diminished at temperatures >30°C. In contrast, it was well adapted to hyposaline waters (31 - 21 psu) but with reduced biocalcification, indicating short-term adaptation for expected future changes in salinity driven by increased amounts and intensities of precipitation. These findings provided useful insights to the effect of these climate drivers on P. carnosa metabolism for the first time and thus better forecast changes in their health status under future climate change scenarios.

Finally, the metabolic status of P. carnosa was assesses during monthly in-situ monitoring surveys (from April 2018 to March 2020). Using underwater respirometry combined with non-invasive techniques for photosynthetic efficiency and whiteness level, the objective was to verify which environmental factors were affecting the coral health and how coral could mitigate the metabolic stress (Chapter 4). To better understand the metabolic responses, a coral metabolic index (CMI) was developed with information collected from in-situ observations, and the biocalcification and whiteness were found playing a main role in the coral health of P. carnosa. Taken together, the seasonality of environmental parameters (dry vs wet seasons) did not affect the coral health, suggesting the capability of this coral to modulate its metabolic status. However, rainfall, pH and salinity were the main factors affecting the calcification and whiteness of this species and therefore reducing the health status as well as lowering the growth rates of corals. 

The findings discussed in this thesis showed for the first time the metabolic response of P. carnosa under natural environmental changes in a Hong Kong waters through a combination of laboratory experiments and in-situ observations. Moreover, these data would explain the high resilience pattern to natural fluctuations of seawater conditions but slow growth of massive boulder-shape corals and will be useful as a baseline in order to predict coral health in future scenarios. The need to improve in-situ studies in order to better understand the resilient capacity of coral physiology under anthropogenically driven natural fluctuations is emphasized, as is the need to outline the direct consequences due to changing climate.