Metabolic performance and thermal and salinity tolerance of the coral Platygyra carnosa in Hong Kong waters

Marine Pollution Bulletin 153 (2020) 111005

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Metabolic performance and thermal and salinity tolerance of the coral Platygyra carnosa in Hong Kong waters

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Walter Dellisantia,b,c, , Ryan H.L. Tsangd, Put Ang Jrd, Jiajun Wua,c, Mark L. Wellse,f, ⁎ Leo L. Chana,b,c, a

State Key Laboratory of Marine Pollution, City University of Hong Kong, Hong Kong, China Department of Biomedical Sciences, City University of Hong Kong, Hong Kong, China c Shenzhen Key Laboratory for the Sustainable Use of Marine Biodiversity, Research Centre for the Oceans and Human Health, City University of Hong Kong, Shenzhen Research Institute, Shenzhen, China d Marine Science Laboratory, Chinese University of Hong Kong, Shatin, Hong Kong, China e School of Marine Sciences, University of Maine, Orono, USA f State Key Laboratory of Satellite Ocean Environment Dynamics, Second Institute of Oceanography, Ministry of Natural Resources, 36 Baochubei Road, Hangzhou 310012, China b

A R T I C LE I N FO

A B S T R A C T

Keywords: Coral physiology Energetics Hong Kong Climate change Metabolic performance

Stress-tolerant coral species, such as Platygyra spp., are considered to be well adapted to survive in marginal reefs, but their physiological response to short term exposure to abnormally high temperature and lowered salinity remains poorly understood. Using non-invasive techniques to quantitatively assess the health of Platygyra carnosa (e.g. respiration, photosynthesis, biocalcification and whiteness), we identified the plasticity of its energetics and physiological limits. Although these indicators suggest that it 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. Our findings provide useful insights to the effect of these climate drivers on P. carnosa metabolism and thus better forecast changes in their health status under future climate change scenarios.

1. Introduction Anthropogenic CO2 emissions are affecting marine ecosystems worldwide and temperature sensitive benthic organisms such as corals, in particular (IPCC, 2013). Seawater temperature has increased globally by ~0.11 °C per decade since the pre-industrial period (Rhein et al., 2013) and by ~0.25 °C between 1971 and 2010 (Levitus et al., 2009). Heat stress has emerged as the most dangerous threat for corals (Hughes et al., 2017; Spalding and Brown, 2015) with increases of only 1 °C above the long-term summer maximum in some cases leading to catastrophic impacts (Cramer et al., 2014). Moreover, corals are subjected to more intense marine heatwaves during El Niño anomalies (Genevier et al., 2019), causing oxidative stress, production of reactive oxygen species, increased activity of protective enzymes as well as lipid peroxidation (Dias et al., 2019) and leading to reduced energy store and relative reproductive success (Ward et al., 2002; Szmant and Gassman, 1990) and making them more susceptible to corallivory and bioerosion

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events (Kayal et al., 2012). Increasingly frequent large scale severe coral bleaching (Hughes et al., 2017) and coral mortalities (Hoegh-Guldberg, 2014; De'ath et al., 2012) have attracted the wide attention of conservation management authorities, scientists and the general public, and are attributed to the combination of global scale climate change as well as local impacts, such as pollution, overfishing and coastal development (Cheal et al., 2017; Halpern et al., 2015). Ocean warming is affecting the metabolic efficiency of some marine organisms; ocean acidification is weakening the ability of biocalcifiers to generate their calcium carbonate skeletons (Anthony, 2016; Dove et al., 2013); and changes in the ocean heat content are destabilizing the hydrological cycle, generating impacts on the timing, intensity and duration of local precipitation events (Schewe et al., 2014). As a consequence, these shifts in net precipitation and evaporation are generating regional-scale salinity fluctuations of coastal waters (Rhein et al., 2013). The effects of climate change on coral ecosystems are widely

Corresponding authors at: State Key Laboratory of Marine Pollution, City University of Hong Kong, Hong Kong China. E-mail addresses: [email protected] (W. Dellisanti), [email protected] (L.L. Chan).

https://doi.org/10.1016/j.marpolbul.2020.111005 Received 5 August 2019; Received in revised form 11 February 2020; Accepted 19 February 2020 0025-326X/ © 2020 Elsevier Ltd. All rights reserved.

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in the western region is attributed to cultural eutrophication and hypoxia associated with the Pearl River Estuary, while water quality in the eastern coastal region has been affected by coastal development, with increases in dissolved inorganic carbon, phosphate and particulate suspended matter (Duprey et al., 2017). These changes, as well as the effects from harmful dinoflagellate blooms (Lu and Hodgkiss, 2004) are the main causes for declining diversity of the coral communities in the eastern region. Nevertheless, some hard coral species are widely distributed in eastern waters and only minor bleaching events have been observed (Xie et al., 2017). Among these, P. carnosa is a common species found along the coasts of the SCS (Veron, 2000) and shows an apparently healthy pattern in a dominant massive structure in Hong Kong waters (Qiu et al., 2014). Here, we investigated the metabolic response of P. carnosa in high temperature and low salinity conditions using outdoor mesocosms. We used non-invasive techniques to evaluate the effect of these two environmental factors on the physiology of P. carnosa. Our results provide new insights to coral metabolism and energetics (e.g. respiration, photosynthesis, calcification and whiteness) under the threshold physiological limits of this species in Hong Kong waters.

debated (Mumby and van Woesik, 2014; Camp et al., 2018), and while climate predictions forecast steadily increasing seawater temperatures globally (IPCC, 2013), temperature change at regional levels will be much more variable. For example, Yuan et al. (2019) found higher than predicted rates of increasing temperature (0.038–0.074 °C/year) in the tropical and subtropical waters of the South China Sea (SCS). This acceleration is of concern because the region has been expected to serve as a coral refuge from ocean warming in the future (Keppel et al., 2012), in part because coral communities in subtropical waters experience a greater seasonal temperature range and thus might be better adapted to tolerate thermal perturbation than tropical corals (Brown et al., 2000). In addition to temperature fluctuations, terrestrial runoff associated with heavy rainfall events are projected to increase in frequency and intensity in subtropical regions (IPCC, 2013), and these events can lead to coral bleaching and massive coral death (Genevier et al., 2019; Xie et al., 2017). Our investigation began to address the need to better characterize the temperature and salinity tolerances of subtropical corals by studying Platygyra carnosa, a species that is widely distributed in the SCS and recognized for its resilience to environmental fluctuations. Metabolic measurements enable the quantification of energy expenditure in biological systems and provide insight to organism health. The measurement of the physiological rates in corals has long been of great interest (e.g., Gattuso and Jaubert, 1990; Gattuso et al., 1998) and several tools and strategies have been developed to obtain these measurements in both field and laboratory studies. The metabolism of benthic ecosystems has been studied using environmental chambers to measure changes in oxygen and pH over time as a proxy for biological processes (Bates et al., 2010). More recently, the technology for obtaining these measurements has been improved by reducing the size and logistics of the experimental apparatus. Underwater respirometers have been developed to quantify coral respiration and photosynthesis through measurements of dissolved oxygen, pH and temperature (Camp et al., 2015; Long et al., 2013; Murphy et al., 2012). However, while these measures clearly give insight to coral metabolism, calibrating these rate measurements to assessments of coral health remains highly uncertain. Marginal reefs are considered to be where coral communities live at the threshold limit for reef development (Kleypas et al., 1999; Camp et al., 2018) exhibiting reduced growth rates and variable recruitment success (Chui et al., 2016), as well as slow recovery from bleaching disturbance (Yang and Goodkin, 2014; Harrison et al., 2011). In addition, the ability of corals in urban reefs to successfully accommodate multiple simultaneous anthropogenic stressors (Heery et al., 2018) may be exacerbated by additional climate-associated pressures (Beger et al., 2014). Until recently, higher latitude (cooler) coral systems have been viewed as potential refuges to help sustain coral diversity, however, the protection afforded by these potential sanctuaries remains uncertain with continued climate change (Camp et al., 2018). The SCS, adjacent to the north-western border of the Coral Triangle, hosts a wide diversity of reef corals (Huang et al., 2015). Within this area, subtropical Hong Kong waters are influenced by strong seasonal changes in water temperature and salinity, with a distinct wet season (May – October) and dry season (November – April). The wet season is characterized by high seawater temperature (maximum 30 °C) and heavy rainfall. In contrast, the dry season brings lower seawater temperatures (minimum 13 °C) and higher salinity levels (Ang Jr et al., 2005). These seasonal fluctuations, combined with lack of reef accretion (Goodkin et al., 2011), hamper the rapid growth of local corals needed to develop into true shallow-water reef structures (Duprey et al., 2016). However, the corals in Hong Kong waters are still highly diverse relative to other subtropical coral communities, hosting at least 84 species of hard corals (Chan et al., 2005). Many studies show a decline in biodiversity over the last 30 years in the Hong Kong regional waters (Wong et al., 2018 and references therein). The overall decrease of marine diversity and reef complexity

2. Materials and methods 2.1. Survey site and coral collection A shallow area (2–4 m depth) located in Port Island, Mirs Bay (Hong Kong, 22.502° N – 114.356° E) was selected where P. carnosa colonies were abundant and easily accessible. Previous monitoring datasets from this site are provided by Wong et al. (2018). Twelve fragments (surface area 30–45 cm2) were collected using SCUBA from apparently healthy independent colonies on 13th April 2018, tagged and shipped within 2 h in transparent zip-lock bags to the Simon F.S. Li Marine Science Laboratory, the Chinese University of Hong Kong. There, corals were allowed to recover for three weeks in an outdoor tank (300 L) equipped with natural flow-through seawater (1 L min−1) from the adjacent Tolo Harbour under shaded light conditions (Supplementary Table 1) prior to experimental manipulations. The seawater conditions in Tolo Harbour were similar to those at the collection site near Port Island (Supplementary Table 2) and prior experience has demonstrated that they are well within the tolerance range of P. carnosa. 2.2. Physical and chemical parameters Seawater conditions (temperature, salinity, pH, dissolved oxygen and turbidity) were monitored in situ and in experimental tanks using a YSI multiparameter sonde (YSI Exo2 Water Sonde) calibrated before each experiment with standard materials (Xylem Ltd). To provide better precision, measurements in tanks were taken before, during and after each sampling. The pH was measured on the total hydrogen ion scales (pHT) calibrated in Tris-buffered seawater (CRM #T32) (Supplementary Table 2). Light intensities were measured continuously at a regular time interval (5 min) using the Onset HOBO light-meter data logger (Supplementary Table 3). Discrete seawater samples were collected each second day of the experiment for the determination of AT. Samples were immediately poisoned with a saturated HgCl2 solution (0.08% final concentration) after collection and stored in the dark (+4 °C) until analysis. A fixed subsample volume (20 g) was analyzed using the open-cell potentiometric acid-titration method given in the Best Practices for Seawater CO2 Measurements (Dickson et al., 2007) using a G20s Mettler-Toledo automatic titrator equipped with a DGi115-SC electrode. Analytical precision was typically within ± 3 μmol kg−1 while the accuracy, calculated as the average ( ± 1 std) offset from a certified reference material (A. Dickson Laboratory, Scripps Institution of Oceanography), was ± 4.2 (n = 7) μmol kg−1. 2

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subsequent rate of oxygen production by the photosynthetic endosymbionts under simulated natural light (10 min) while the chamber water is continuously recirculated. The LED intensity in the chamber was adjusted to match the mid-day in-situ irradiance at the site of coral collection (~460 μmol photons m−2 s−1). All measurements were made on the coral fragments during mid-day (10:30 am to 1:30 pm). Gross photosynthesis (Pg) was calculated by adding the absolute value of R to Pn quantified as oxygen fluxes during the incubations. At the end of each light incubation, a sample of the recirculated chamber water was collected for measurement of AT. Rates of coral calcification rates (CA) were determined using the alkalinity anomaly technique (Schoepf, 2017) normalized to the coral surface area (24.5 cm2). Overall coral energetics were quantified as the ratio of oxygen production through photosynthesis (Pg) to consumption through respiration (R). Digital photographs were taken of the coral surfaces every two days for colorimetric analysis to quantify the levels of whiteness as a measure of bleaching (Chow et al., 2016). The whiteness value was measured as the dissimilarity percentage in color composition between treated and control corals using the SIMPER tool of Primer 5.0 software (Primer-E Ltd). The photosynthetic capacity of the symbiont was measured as maximum quantum yield (Fv/Fm) using a pulse amplitude modulated (PAM) fluorometer (diving-PAM, Heinz Walz, Effeltrich, Germany) equipped with a standard glass-fiber optic probe (Ralph et al., 1999). PAM measurements were taken on randomly selected polyps for each fragment after 30 min dark acclimation every second day of the experiment.

2.3. Experimental design The effect of increasing temperature and decreasing salinity on coral metabolic measurements was investigated in two independent timeseries experiments. Corals were exposed to the experimental conditions for 8–14 days with a 3-week recovery period at ambient low stress conditions being used before each experiment to ensure metabolic recovery. Outdoor tanks were equipped with a flow-through water system in order to have constant fresh unfiltered seawater. The seawater conditions in Port Island and Tolo Harbour were similar (Supplementary Table 2). We used a 3-weeks recovery time before and between experimental treatments in order to allow corals to recover from any stress and to reduce bias in the measurements of coral metabolism (Supplementary Fig. 1). The experimental durations (and recovery period) were chosen to avoid induced bleaching or mortality. During the recovery period, all parameters were analyzed weekly to keep a record of the metabolic performance (Supplementary Fig. 1). Three fragments were randomly selected and moved into one of the four independent tanks in a closed system (i.e., no seawater replacement) prior to the beginning of the experiment. 2.4. Temperature experiment Water temperature was controlled in four 200 L tanks deployed in an outdoor closed system under shaded light conditions (~75% intensity lower than in-situ). Two tanks were maintained as controls at the seawater temperature (25.35 ± 0.29 °C) of the Tolo Harbour source water at the beginning of the experiment (i.e., the same temperature that the corals experienced in the recovery phase). The remaining two treatment tanks were progressively warmed above this temperature by ~1 °C every two days up to 32.12 ± 0.60 °C for a total experimental duration of 14 days (Supplementary Table 2). The temperature in the tanks was controlled by a combination of cooling and heating systems. Coral physiological rates (see below) were measured in both the control and treatment tanks every second day after warming began.

2.7. Data analysis To avoid variability due to naturally changing outdoor conditions, such as light, changes from the control fluxes were calculated allowing for comparisons between incubations that were carried out at the same time. Fluxes in the control chambers were averaged (n = 6) and subtracted from the fluxes in each treatment chamber. The results were expressed as mean ± standard error of the mean (SE), unless mentioned otherwise. All data were processed using the Dixon's Q test to identify and reject a single outlier (two-sided test) prior to further statistical analysis. Metabolic variables measured among and between control and treatments were compared using a paired t-test (p < 0.05). Differences between treatment values and control means were used for a simple linear regression to describe the relationships of metabolic rates. In order to verify the stress at high temperature the data were split between 25 and 30 °C and 30–32 °C ranges prior to verifying the significance of slopes using linear regression. The coefficient of variation (CV%) was used to compare the variability of measurements in tanks and between tanks and calculated as ratio of standard deviation to the mean values of measurements. The comparison of variability between tanks was investigated using one-way analysis of variance (ANOVA). The effect of variables measured in tanks during experiments was investigated using the distance linear model (DistLM). The measurements taken in each experimental group were compared using a two-way permutational analysis of variance (PERMANOVA) based on Euclidean distances and visualized with a principal coordinate analysis (PCoA). The analyses were performed with IBM SPSS Statistics v 26.0 (IBM Corp., Armonk, USA) and PRIMER v 6 & PERMANOVA+ (Anderson et al., 2008).

2.5. Salinity experiment To study the effects of decreasing salinity on coral metabolism, four 200 L tanks were deployed as described above under shaded light conditions (~50% intensity lower than in-situ). In this case the temperature was kept stable at 27.78 ± 0.14 °C (the recovery phase seawater temperature from Tolo Harbour at the beginning of the experiment). The salinity in the two control tanks was maintained at 31.22 ± 0.45 over the experiment (Tolo Harbour salinity) while salinity in the two treatment tanks was lowered every two days by ~3 psu with additions of deionized water until the salinity reached 21.09 ± 0.01, equivalent to the lowest sea surface salinity recorded after heavy rainfall in the region (Chui and Ang Jr, 2015). Physiological rates were measured on each second day after the freshwater input. Total duration of the experiment was eight days (Supplementary Table 4). 2.6. Coral physiological measurements Respiration (R) and net photosynthesis (Pn) rates were measured at the coral surfaces using an underwater respirometer (CISME Instruments LLC) as previously described by Dellisanti et al. (in review). CISME is a diver-operated instrument designed to compare net oxygen production rates measured under controlled irradiance with net oxygen utilization rates under dark conditions. It comprises an electronic control module, a tablet and an incubation chamber head. This smallchamber device seals on the coral surface and quantifies the rate of oxygen consumption during dark incubation (5 min), and the

3. Results 3.1. Temperature experiment The temperature of the control and treatment tanks was regulated at 25 °C for two days at the beginning of the experiment. The control tanks were maintained at 25.35 ± 0.29 °C during the entire 14-days experiment, while the temperature in the treatment tanks was raised 3

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Fig. 1. Physiological parameters measured during the temperature experiment: a) respiration (R); b) gross photosynthesis (Pg); c) calcification (CA); d) photosynthetic efficiency (Fv/Fm); e) whiteness %. Differences from the control means are shown and defined by linear regression.

0.56 μmol O2 cm−2 h−1 to a maximum of 1.57 μmol O2 cm−2 h−1 up to 30 °C (Δ0.87 ± 0.08 μmol O2 cm−2 h−1, p < 0.01) but was unchanged between 30 and 32 °C (Δ0.92 ± 0.17 μmol O2 cm−2 h−1, p = 0.86; Fig. 1a). The rate of Pg also increased linearly with rising temperature to 30 °C (Δ0.84 ± 0.02 μmol O2 cm−2 h−1, p < 0.01) but then decreased significantly at 32 °C (Δ0.03 ± 0.12 μmol O2 cm−2 h−1, p < 0.01; Fig. 1b). The photosynthetic efficiency was slightly stimulated up to 30 °C (Δ0.06 ± 0.01 Fv/fm, p < 0.05), but decreased by ~0.5 Fv/fm in some coral fragments at temperatures above 30 °C (Fig. 1d). Measures of coral whiteness showed a generally increasing trend across all temperatures, although the variability

stepwise up to 32.12 ± 0.60 °C by 0.5 °C/day (Supplementary Table 2). Dissolved oxygen remained high (98.26 ± 3.24% s.d.) and the seawater pH remained stable (7.94 ± 0.06 s.d.) in both the control and treatment tanks over the experiment (Supplementary Table 2). However, the salinity increased slowly from 32.25 ± 0.16 psu at the beginning to 35.75 ± 1.60 psu by the end of the experiment, owing to evaporation from the tanks. Turbidity also fluctuated somewhat over the course of the experiment reaching a maximum of 2.17 ± 1.73 FNU in the tanks subjected to high temperature (Supplementary Table 2) owing to a mucoid matrix being released by the corals. Respiration (R) increased linearly with increasing temperature from 4

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Fig. 2. Physiological parameters measured during the salinity experiment: a) respiration (R); b) gross photosynthesis (Pg); c) calcification (CA); d) photosynthetic efficiency (Fv/Fm); e) whiteness %. Differences from the control means are shown and defined by linear regression.

3.2. Salinity experiment

among fragments was high (Fig. 1e). In contrast, the rates of CA were unchanged between 25 and 30 °C (p = 0.07) but then decreased significantly between 30 and 32 °C (Δ-79.62 ± 34.46 μmol CaCO3 cm−2 h−1, p < 0.05) (Fig. 1c).

The seawater salinity at the start of the experiment was ~31.5 psu and was kept stable in the control tanks at 31.22 ± 0.45 psu while every second day of the experiment the salinity was decreased stepwise 5

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to 21.09 ± 0.01 psu at a rate of – 1.25 psu/day (Supplementary Table 4). The temperature was regulated to ~28 °C in all tanks over the course of the experiment. The pH remained stable at (7.92 ± 0.09 s.d.) over the first six days but then decrease significantly between Day 6–8 to 7.78 ± 0.02 at the lowest salinity (Supplementary Table 4). The rates of R and Pg were not affected by the changes of salinity (p = 0.62 and p = 0.32, respectively; Fig. 2a, b). CA was reduced at the lowest salinity (Δ-0.66 ± 0.03 μmol CaCO3 cm−2 h−1, p < 0.05) (Fig. 2c), while no significant change was observed in photosynthetic efficiency (p = 0.52; Fig. 2d). The degree of coral whiteness increased slightly at the lowest salinity level (Δ + 7.72 ± 5.68%, p < 0.05; Fig. 2e).

metabolic parameters measured at each increase of temperature, we summarized the Euclidean distances using principal coordinate analysis (PCoA). This analysis clustered the metabolic measurements with the temperature groups. In general, the effect of temperature was significative between 25.15, 27.01–28.9 and 30.2–31.12 °C groups (PERMANOVA, Supplementary Table 5), suggesting that the increase of temperature from ~27 to ~29 °C did not bring any detectable effect on coral metabolism (Fig. 4a), similarly to what observed with Pg/R (Fig. 3). Moreover, the effects on coral metabolism were similar on R, Pg and CA, but not on Fv/fm and whiteness (Fig. 4b), although some changes were observed (Fig. 1d, e). To analyze differences between metabolic parameters measured at decreasing salinity, we used the same approach with principal coordinate analysis (PCoA). This analysis clustered the metabolic measurements with the salinity groups. The results of PERMANOVA (Supplementary Table 6) showed the effect of salinity between each group (Fig. 4c), indicating that the decrease of salinity by 1.25 psu/day affected the coral metabolism, but mainly on CA and whiteness (Fig. 4d) as observed in Fig. 2 c, e.

3.3. Holobiont health The net effect of environmental change on coral metabolism can be expressed as the ratio of Pg/R, a proxy for metabolic “health” or status of the holobiont. In terms of temperature effects, the Pg/R remained stable at ~2.5–3.0 at temperatures between 25 and 30 °C, within the range measured in the controls at 25 °C, but then decreased progressively between 30 and 32 °C to roughly half that in the control (Δ -1.7 ± 0.21, p < 0.01; Fig. 3a). In contrast, Pg/R did not change across the full salinity range tested, although values in the controls and treatments were slightly lower than those measured a few weeks earlier during the temperature experiment (Fig. 3b). The variation of replicated fragments in both experiments, quantified as the coefficient of variation (CV) of six measurements, generally was better than 20% for all parameters, with exception of CA rates and Whiteness (CV < 40%). The variation between tanks in both experiments resulted as not significant for all metabolic parameters measured (One-way ANOVA of n = 4 tanks). To analyze differences between

4. Discussion Our findings illustrated the difficulty in assessing the status of coral health through measurement of a few biological parameters. Increasing temperatures from 25 to 30 °C led to higher respiration rates of the holobiont relative to the control, often taken as a sign of coral stress, but the rates of Pg by the endosymbionts also increased, consistent with improving metabolic status (Fig. 1a, b). This increase in photosynthesis could not be attributed to improved photosynthetic efficiencies (Fig. 1d), or to increased production of chlorophyll (Fig. 1e), so presumably was associated with temperature effects on photorespiration or other metabolic efficiencies that we could not test with our data. The Pg/R ratio of the corals remained essentially constant across the 25–30 °C range (Fig. 3a), indicating that the overall coral metabolism was well adapted to this temperature range such that the corals could take advantage of the increased energy production for growth. Indeed, the rates of biocalcification increased slightly in most of the coral fragments with increasing temperatures (Fig. 1c), consistent with the indications of increased energy reserves at higher temperature. Negative impacts on coral metabolism became apparent at temperatures above 30 °C. Although there was no increase in R rates, Pg, photosynthetic efficiencies and rates of CA all trended downwards between 30 and 32 °C. These decreases were accompanied by an upwards shift in coral whiteness. Taken together, these collective data indicate coral metabolism was becoming less efficient. That view was supported by the marked decreases in Pg/R between 30 and 32 °C (Fig. 3a). It is well recognized that the calcification and growth of corals can be affected by seawater warming (Lough and Barnes, 2000; McNeil et al., 2004). Generally, the optimum rate for calcification coincides with that for photosynthesis (25–28 °C) for tropical marine calcifiers (Jokiel and Coles, 1977; Swart, 1983) and decline above 28 °C (Howe and Marshall, 2002) in both autotrophic (Comeau et al., 2016) and heterotrophic (Marshall and Clode, 2004) calcification. Our results showing lower calcification and photosynthesis above 30 °C (Fig. 1c) were similar to the threshold limit found by Lantz et al. (2019), and very near the average summer maximum temperatures at the study site (Tsang and Ang, 2015). The salinity in the experimental tanks increased from 32.25 to 35.75 psu (+ 3.5 psu) from Day 0 to Day 14, owing to evaporation from the tanks (Supplementary Table 2). Although the increase of 0.25 psu/ day was significant (p < 0.05), it would not bring any effects in metabolism detectable by our methods in the short-term (p = 0.69, 0.97% prop., DistLM, Supplementary Table 7). We observed the production and release of a mucoid matrix on the coral surface in temperatures above 30 °C, which significantly reduced the water quality in the high temperature tanks (Supplementary Table 2). Indeed, the increased turbidity could have a significant effect

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Fig. 3. Coral energetics measured as gross photosynthesis to respiration (Pg/R) during the temperature (a) and salinity (b) experiment. 6

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Fig. 4. Results of PCoA. Distances between each point based on Euclidean distances. a) PCo plot showing the temperature groups; b) PCo vector showing the coral metabolic parameters in the temperature experiment; c) PCo plot showing the salinity groups; d) PCo vector showing the coral metabolic parameters in the salinity experiment.

change in pH potentially decreased carbonate concentrations (CO3−) by roughly half, which likely was the reason for the observed inhibition of biocalcification at lower salinity. Although the salinity was the main variable affecting coral metabolism, the decrease of pH could have contributed to the observed coral response, as well as other variables measured (p < 0.01, DistLM, Supplementary Table 7). The negative effects of ocean acidification on coral biocalcification is well described (Gattuso et al., 1998; Langdon et al., 2000). A local example in SCS waters is the significant negative interaction between pH and temperature leading to reduced calcification rates in Pocillopora damicornis (Yuan et al., 2019). In damaged corals, elevated temperature can result in higher net dissolution rates (Trnovsky et al., 2016). Our salinity response findings have important ramifications for Hong Kong waters. Hyposaline events (decreases to 19 psu) have been recorded frequently in Hong Kong coastal waters over the past few decades (McCorry, 2002; Ang Jr et al., 2005), typically coinciding with extraordinary heavy rainfall and massive bleaching events (Xie et al., 2017). Salinity sensitivities of corals are likely to vary among different coral species (Berkelmans et al., 2012) and the tolerance to a hyposaline environment has been documented in the early stage of P. acuta in Hong Kong, suggesting a physiological adaptation to salinity stress (Chui and Ang Jr, 2015). Our findings showed that short term exposure of P. carnosa to these lower salinity episodes is likely to have little effect on coral health but can restrict coral growth until the event passes. Even so, multi-stressors effects (e.g. combined high temperature and low salinity) might generate markedly different indications of the metabolic resistance of these corals. Indeed, major anomalies appeared in massive corals in Hong Kong waters under multistressor conditions. These include the high level of eutrophication (e.g. chlorophyll-a,

on coral metabolism (p < 0.01, 13.67% prop., DistLM, Supplementary Table 5). Mucus secretion by stressed corals has been recognized as a particle trap (Hadaidi et al., 2019) as well as functioning as UV protection or substrate exchange for bacteria and microorganisms (Wild et al., 2004; Brown and Bythell, 2005; Ritchie, 2006). Regardless of its role, the production of mucus was consistent with the quantified metabolic indicators in showing that P. carnosa began experiencing temperature-related stress above 30 °C. On the other hand, salinity is an important environmental factor for osmoregulation of corals (Hoegh-Guldberg and Smith, 1989; Vermeij et al., 2006). Our findings showed that decreasing the salinity from 31 to 21 psu had no detectable negative effects on the metabolic performance of R or Pg rates relative to the control (Fig. 2a, b). There also was no significant change in endosymbiont photosynthetic efficiency (Fig. 2d) or in Pg/R (Fig. 3b); all consistent with P. carnosa remaining in good metabolic health across this range of salinity variation. These findings contrasted with the increased whiteness generally observed at lower salinity (Fig. 2e), which was ~80% more than in the control conditions. There also was a significant change in color of the coral surface, from dark-brown typical of P. carnosa in healthy conditions to green/light green. Although it could not be demonstrated with our data, the color change might be due to a reduced concentration of chlorophyll which might be responsible of reduced light calcification. Despite the apparent health of P. carnosa at lower salinity, there was a marked decline in biocalcification rates particularly evident at the lowest salinity (Fig. 2c) where CA rates decreased by ~70% relative to the control. Moreover, pH decreased in the experimental tanks by −0.13 relative to the control at the lowest salinity treatment, due at least partly to the dilution of seawater with deionized water. This small 7

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References

dissolved inorganic nitrogen and phosphorous concentration) negatively correlated with coral species richness and coral cover (Duprey et al., 2016) and bioerosion events caused by outbreaks of the sea urchin Diadema setosum (Qiu et al., 2014), polychaetae bioeroders like Spirobranchus tetraceros and bivalve Lithophaga sp. (Xie et al., 2016), or gastropod Drupella rugosa (Lam et al., 2007). In summary, our study showed the plasticity and the high tolerance of the stress-resistant P. carnosa to short-term abiotic stressors of elevated temperature (> 30 °C) and lowered salinity (21); extreme conditions reported that occur in Hong Kong waters (Xie et al., 2017). We found that P. carnosa might survive at elevated fluctuations of temperature caused by short heatwaves (> 30 °C), even though exhibiting physiological stress, but the predicted ocean warming could be fatal in the long-term if any physiological or evolutionary adaptation occurs. In contrast, hyposaline events (down to salinity 21 psu) related to runoff from heavy rainfall events had little impact on the metabolic health of P. carnosa, although growth (biocalcification) rates were impeded. However, these threshold limits may not reflect the resilience of this species to prolonged conditions. These finding add new insight to coral metabolism in the Hong Kong and the SCS region and underline the need the viability of using in-situ monitoring with real-time underwater respirometry to quantify the physiological status of corals species.

Anderson, M.J., Gorley, R.N., Clarke, K.R., 2008. PERMANOVA+ for PRIMER: Guide to Software and Statistical Methods. PRIMER-E, Ltd, Plymouth. Ang, P.O. Jr, Choi, L.S., Choi, M.M., Cornish, A., Fung, H.L., Lee, M.W., Lin, T.P., Ma, W. C., Tam, M.C., Wong, S.Y., 2005. Hong Kong, in: Japan Wildlife Research Center, Ministry of the Environment, Government of Japan, (Ed), Status of Coral Reefs of the East Asian Seas Region: 2004. Global Coral Reef Monitoring Network, Ministry of Environment, Tokyo, pp. 121–152. Anthony, K., 2016. Coral reefs under climate change and ocean acidification: challenges and opportunities for management and policy. Annu. Rev. Environ. Resour. 41, 59–81. https://doi.org/10.1146/annurev-environ-110615-085610. Bates, N.R., Amat, A., Andersson, A.J., 2010. Feedbacks and responses of coral calcification on the Bermuda reef system to seasonal changes in biological processes and ocean acidification. Biogeosciences 7, 2509–2530. https://doi.org/10.5194/bg-72509-2010. Beger, M., Sommer, B., Harrison, P.L., Smith, S.D., Pandolfi, J.M., 2014. Conserving potential coral reef refuges at high latitudes. Divers. Distrib. 20, 245–257. https://doi. org/10.1111/ddi.12140. Berkelmans, R., Jones, A.M., Schaffelke, B., 2012. Salinity thresholds of Acropora spp. on the great barrier reef. Coral Reefs 31 (4), 1103–1110. https://doi.org/10.1007/ s00338-012-0930-z. Brown, B.E., Bythell, J.C., 2005. Perspectives on mucus secretion in reef corals. Mar. Ecol. Prog. Ser. 296, 291–309. https://doi.org/10.3354/meps296291. Brown, B.E., Dunne, R.P., Goodson, M.S., Douglas, A.E., 2000. Marine ecology: bleaching patterns in reef corals. Nature 404, 142–143. https://doi.org/10.1038/35004657. Camp, E.F., Krause, S.L., Santos, L.M.F., Naumann, M.S., Kikuchi, R.K.P., Wild, D.J.C., Suggett, D.J., 2015. The “flexi-chamber”: a novel cost-effective in situ respirometry chamber for coral physiological measurements. PLoS One 10, e0138800. https://doi. org/10.1371/journal.pone.0138800. Camp, E.F., Schoepf, V., Mumby, P.J., Hardtke, L.A., Rodolfo-Metalpa, R., Smith, D.J., Suggett, D.J., 2018. The future of coral reefs subject to rapid climate change: lessons from natural extreme environments. Fron. Mar. Sci 5, 4. https://doi.org/10.3389/ fmars.2018.00004. Chan, A.L.K., Choi, C.L.S., McCorry, D., et al., 2005. Field guide to hard corals of Hong Kong. In: Cosmos Books and Friends of the Country Parks, Hong Kong. Cheal, A.J., MacNeil, M.A., Emslie, M.J., Sweatman, H., 2017. The threat to coral reefs from more 42 intense cyclones under climate change. Glob. Change Biol. 23 (4), 1511–1524. https://doi.org/10.1111/gcb.13593. Chow, M.H., Tsang, R.H.L., Lam, K.Y., Ang Jr., P., 2016. Quantifying the degree of coral bleaching using digital photographic technique. J. Exp. Mar. Biol. Ecol. 479, 60–68. https://doi.org/10.1016/j.jembe.2016.03.003. Chui, A.P.Y., Ang Jr., P., 2015. Elevated temperature enhances normal early embryonic development in the coral Platygyra acuta under low salinity conditions. Coral Reefs 34, 461–469. https://doi.org/10.1007/s00338-014-1247-x. Chui, A.P.Y., Yeung, C.W., Tsang, R.H.L., Leung, Y.H., Ng, T.Y., Chai, K.H., Ang, P., 2016. Lowered temperature and reduced salinity retarded development of early life history stages of Acropora valida from the marginal environment. Reg. Stud. Mar. Sci. 8 (3), 430–438. https://doi.org/10.1016/j.rsma.2016.04.004. Comeau, S., Carpenter, R.C., Lantz, C.A., Edmunds, P.J., 2016. Parameterization of the response of calcification to temperature and pCO2 in the coral Acropora pulchra and the alga Lithophyllum kotschyanum. Coral Reefs 35, 929–939. https://doi.org/10. 1007/s00338-016-1425-0. Cramer, W., Yohe, G.W., Auffhammer, M., Huggel, C., Molau, U., da Silva Dias, M.A.F., Solow, A., Stone, D.A., Tibig, L., 2014. Detection and attribution of observed impacts. In: Field, C.B., Barros, V.R., Dokken, D.J., Mach, K.J., Mastrandrea, M.D. (Eds.), Climate Change 2014: Impacts, 33 Adaptation, and Vulnerability. Part A: Global and Sectoral Aspects. Contribution of Working Group 34 II to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, pp. 979–1037. De’ath, G., Fabricius, K.E., Sweatman, H., Puotinen, M., 2012. The 27-year decline of coral cover on the great barrier reef and its causes. P. Natl. Acad. Sci. USA 109 (44), 17995–17999. https://doi.org/10.1073/pnas.1208909109. Dellisanti, W., Tsang, R.H.L., Ang, P., Wu, J., Wells, M.L., Chan, L.L., 2020. A Diver Portable Respirometry System for Coral In-Situ Monitoring. (In review). Dias, M., Madeira, C., Jogee, N., Ferreira, A., Gouveia, R., Cabral, H., Diniz, M., Vinagre, C., 2019. Oxidative stress on Scleractinian coral fragments following exposure to high temperature and low salinity. Ecol. Indic. 107 (105586), 12. https://doi.org/10. 1016/j.ecolind.2019.105586. Dickson, A.G., Sabine, C.L., Christian, J.R., 2007. Guide to Best Practices for Ocean CO2 Measurement. PICES Special Publication 3 North Pacific Marine Science Organization, Sidney, British Columbia 191 pp. http://hdl.handle.net/11329/249. Dove, S.G., Kline, D.I., Pantos, O., Angly, F.E., Tyson, G.W., Hoegh-Guldberg, O., 2013. Future reef decalcification under a business-as-usual CO2 emission scenario. P. Natl. Acad. Sci. USA 110 (38), 15342–9 15347. https://doi.org/10.1073/pnas. 1302701110. Duprey, N.N., Yasuhara, M., Baker, D.M., 2016. Reefs of tomorrow: eutrophication reduces coral biodiversity in an urbanized seascape. Glob. Chang. Biol. 22, 3550–3565. https://doi.org/10.1111/gcb.13432. Duprey, N.N., McIlroy, S.E., Ng, T.P., Thompson, P.D., Kim, T., Wong, J.C.Y., Wong, C.W.M., Husa, S.M., Li, S.M.J., Williams, G.A., Baker, D.M., 2017. Facing a wicked problem with optimism: issues and priorities for coral conservation in Hong Kong. Biodivers. Conserv. 26, 2521–2545. https://doi.org/10.1007/s10531-017-1383-z. Gattuso, J.P., Jaubert, J., 1990. Effect of light on oxygen and carbon dioxide fluxes and on metabolic quotients measured in situ in a zooxantheallate coral. Limnol. Oceanogr.

CRediT authorship contribution statement Walter Dellisanti:Conceptualization, Methodology, Validation, Formal analysis, Investigation, Writing - original draft, Visualization.Ryan H.L. Tsang:Methodology, Validation, Formal analysis, Investigation, Writing - review & editing.Put Ang:Supervision, Writing review & editing.Jiajun Wu:Supervision, Writing - review & editing, Project administration, Funding acquisition.Mark L. Wells:Supervision, Formal analysis, Writing - review & editing.Leo L. Chan:Supervision, Writing - review & editing, Project administration, Funding acquisition. Declaration of competing interest The authors declare that they have no competing interests. Acknowledgements The authors thank all the persons who provided support for the experiments, in particular Mr. Oliver Petterson Stubbs to help collecting coral fragments, Mr. Kelvin So and Ms. Zoe Wong for the assistance during laboratory work, Mr. Sam Yiu for statistical analysis and Prof. John Hodgkiss for proof-reading. Funding sources This study is supported by the Collaborative Research Fund (C101215G) of the Hong Kong Research Grants Council and the Internal Research Project of the State Key Laboratory of Satellite Ocean Environment Dynamics, Second Institute of Oceanography, State Oceanic Administration (No. SOEDZZ1702). Data availability statement The authors will make available data and associated protocols promptly on request. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.marpolbul.2020.111005. 8

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waters. Hydrobiologia 512 (1–3), 231–238. Marshall, A.T., Clode, P., 2004. Calcification rate and the effect of temperature in a zooxanthellate and an azooxanthellate scleractinian reef coral. Coral Reefs 23, 218–224. https://doi.org/10.1007/s00338-004-0369-y. McCorry, D., 2002. Hong Kong's Scleractinian Coral Communities: Status, Threats and Proposals for Management. PhD Thesis. The University of Hong Kong, Hong Kong. McNeil, B.I., Matear, R.J., Barnes, D.J., 2004. Coral reef calcification and climate change: the effect of ocean warming. Geo. Res. Lett. 31, L22309. https://doi.org/10.1029/ 2004GL021541. Mumby, P., van Woesik, R., 2014. Consequences of ecological, evolutionary and biogeochemical uncertainty for coral reef responses to climatic stress. Curr. Biol. 24 (1), R413–R423. https://doi.org/10.1016/j.cub.2014.04.029. Murphy, B.A., Mazel, C.H., Whitehead, R., Szmant, A.M., 2012. CISME: a self-contained diver portable metabolism and energetics system. Oceans 2012, 1–7. https://doi.org/ 10.1109/oceans.2012.6405075. (Hampton Roads, VA). Qiu, J.W., Lau, D.C., Cheang, C.C., Chow, W.K., 2014. Community-level destruction of hard corals by the sea urchin Diadema setosum. Mar. Pollut. Bull. 85, 783–788. https://doi.org/10.1016/j.marpolbul.2013. Ralph, P., Gademann, R., Larkum, A.W.D., Schreiber, U., 1999. In situ underwater measurement of photosynthetic activity of coral zooxanthellae and other reef-dwelling dinoflagellate endosymbionts. Mar. Ecol. Prog. Ser. 180, 139–147. Rhein, M., Rintoul, S.R., Aoki, S., Campos, E., Chambers, D., 2013. Observations: ocean. In: Stocker, T.F., Qin, D., Plattner, G., 35 Tignor, M., Allen, S.K., Boschung, J., Nauels, A., Xia, Y., Bex, V., Midgley, P.M. (Eds.), Climate 33 Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment 34 Report of the Intergovernmental Panel on Climate Change. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, pp. 255–316. Ritchie, K.B., 2006. Regulation of microbial populations by coral surface mucus and mucus-associated bacteria. Mar. Ecol. Prog. Ser. 322, 1–14. https://doi.org/10.3354/ meps322001. Schewe, J., et al., 2014. Multimodel assessment of water scarcity under climate change. P. Natl. Acad. Sci. USA 111 (9), 3245–3250. https://doi.org/10.1073/pnas. 0709640104. Schoepf, V., et al., 2017. Coral calcification under environmental change: a direct comparison of the alkalinity anomaly and buoyant weight techniques. Coral Reefs 36, 13–25. https://doi.org/10.1007/s00338-016-1507-z. Spalding, M.D., Brown, B.E., 2015. Warm-water coral reefs and climate change. Science 350 (6262), 769–771. https://doi.org/10.1126/science.aad0349. Swart, P.K., 1983. Carbon and oxygen isotope fractionation in Scleractinian corals. Earth Science Review 19, 51–80. Szmant, A.M., Gassman, N.J., 1990. The effects of prolonged ‘bleaching’ on the tissue biomass and reproduction of the reef coral Montastrea annularis. Coral Reefs 8, 217–224. https://doi.org/10.1007/BF00265014. Trnovsky, D., Stoltenberg, L., Cyronak, T., Eyre, B.D., 2016. Antagonistic effects of ocean acidification and rising sea surface temperature on the dissolution of coral reef carbonate sediments. Front. Mar. Sci. 3, 211. https://doi.org/10.3389/fmars.2016. 00211. Tsang, R.H.L., Ang, P., 2015. Cold temperature stress and predation effects on corals: their possible roles in structuring a nonreefal coral community. Coral Reefs 34 (1), 97.108. https://doi.org/10.1007/s00338-014-1210-x. Vermeij, M.J.A., Fogarty, N.D., Millner, N.W., 2006. Pelagic conditions affect larval behavior, survival and settlement patterns in the Caribbean coral Montastrea faveolata. Mar. Ecol. Series 310, 119–128. https://doi.org/10.3354/meps310119. Veron, J.E.N., 2000. Corals of the World. 1–3 Australian Institute of Marine Science and CRR, Queensland, Australia. Ward, S., Harrison, P., Hoegh-Guldberg, O., 2002. Coral bleaching reduces reproduction of scleractinian corals and increases susceptibility to future stress. In: Moosa, M.K., Soemodihardjo, S., Soegiarto, A., Romimohtarto, K., Nontji, A., Suharsono, S. (Eds.), Proceedings of the Ninth International Coral Reef Symposium’, Volume 2, 23–27 October 2000, Bali. Ministry of Environment, Indonesian Institute of Sciences, International Society for Reef Studies, Bali, pp. 1123–1128. Wild, C., Rasheed, M.Y., Werner, U., Franke, U., Johnstone, R.W., Huettel, M., 2004. Degradation and mineralization of coral mucus in reef environments. Mar. Ecol. Prog. Ser. 267, 159–171. https://doi.org/10.3354/meps267159. Wong, K.T., Chui, A.P.Y., Lam, E.K.Y., Ang, P., 2018. A 30-year monitoring of changes in coral community structure following anthropogenic disturbances in Tolo Harbour and Channel, Hong Kong. Mar. Poll. Bull. 133, 900–910. https://doi.org/10.1016/j. marpolbul.2018.06.049. Xie, J.Y., Wong, J.C., Dumont, C.P., Goodkin, N., Qiu, J.W., 2016. Borehole density on the surface of living Porites corals as an indicator of sedimentation in Hong Kong. Mar. Poll. Bull. 108, 87–93. https://doi.org/10.1016/j.marpolbul.2016.04.055. Xie, J.X., Lau, D.C.C., Kei, K., Yu, V.P.F., Chow, W.K., Qiu, J., 2017. The 2014 summer coral bleaching event in subtropical Hong Kong. Mar. Poll. Bull. 124 (2), 653–659. https://doi.org/10.1016/j.marpolbul.2017.03.061. Yang, T.T., Goodkin, N.F., 2014. Wet season upwelling and dry season chlorophyll-a describe interannual growth rates of Porites in Southern China. PLoS One 9 (6), e99088. https://doi.org/10.1371/journal.pone.0099088. Yuan, X., Guo, Y., Cai, W., Huang, H., Zhou, W., Liu, W., 2019. Coral responses to ocean warming and acidification: implication for future distribution of coral reefs in the South China Sea. Mar. Poll. Bull. 138, 241–248. https://doi.org/10.1016/j. marpolbul.2018.11.053.

35, 1796e1804, doi.org/104119/lo.1990.35.8.1796. Gattuso, J.P., Frankignoulle, M., Bourge, L., Romaine, S., Buddemeier, R.W., 1998. Effect of calcium carbonate saturation of seawater on coral calcification. Glob. Planet. Change 18, 37–46. https://doi.org/10.1016/s0921-8181(98)00035-6. Genevier, L.G.C., Jamil, T., Raitsos, D.E., Krokos, G., Hoteit, I., 2019. Marine heatwaves reveal coral reef zones susceptible to bleaching in the Red Sea. Glob. Change Biol. 25 (7), 2338–2351. https://doi.org/10.1111/gcb.14652. Goodkin, N.F., Switzer, A.D., McCorry, D., Devantier, L., True, J.D., Hughen, K., Angeline, N., Yang, T.T., 2011. Coral communities of Hong Kong: long-lived corals in a marginal reef environment. Mar. Ecol. Prog. Ser. 426, 185–196. https://doi.org/10. 3354/meps09019. Hadaidi, G., Gegner, H.M., Ziegler, M., Voolstra, C.R., 2019. Carbohydrate composition of mucus from Scleractinian corals from the Central Red Sea. Coral Reefs 38 (1), 21–27. https://doi.org/10.1007/s00338-018-01758-5. Halpern, B.S., Frazier, M., Potapenko, J., Casey, K.S., Koenig, K., Longo, C., Lowndes, J.S., Rockwood, R.C., Selig, E.R., Selkoe, K.A., Walbridge, S., 2015. Spatial and temporal changes in cumulative human impacts on the world’s ocean. Nat. Commun. 6, 7615. https://doi.org/10.1038/ncomms8615. Harrison, P.L., Dalton, S.J., Carroll, A.G., 2011. Extensive coral bleaching on the world’s southernmost coral reef at Lord Howe Island, Australia. Coral Reefs 30 (3), 775. https://doi.org/10.1007/s00338-011-0778-7. Heery, E.C., Hoeksema, B.W., Browne, N.K., Reimer, J.D., Ang, P., Huang, D., Friess, D.A., Chou, L.M., Loke, L.H.L., Saksena-Taylor, P., Alsagoff, N., Yeemin, T., Sutthacheep, M., Vo, S.T., Bos, A.R., Gumanao, G.S., Hussein, M.A.S., Waheed, Z., Lane, D.J.W., Johan, O., Kunzmann, A., Jompa, J., Suharsono, T.D., Bauman, A.G., Todd, P.A., 2018. Urban coral reefs: degradation and resilience of hard coral assemblages in coastal cities of East and Southeast Asia. Mar. Poll. Bull. 135, 654–681. https://doi. org/10.1016/j.marpolbul.2018.07.041. Hoegh-Guldberg, O., 2014. Coral reefs in the Anthropocene: persistence or the end of the line? Geol. Soc. Lond., Spec. Publ. 395 (1), 167–183. https://doi.org/10.1144/ SP395.17. Hoegh-Guldberg, O., Smith, G.J., 1989. The effect of sudden changes in temperature, light and salinity on the population density and export of zooxanthellae from the reef corals Stylophora pistillata Esper and Seriatopora hystrix. J. Exp. Mar. Biol. Ecol. 129, 279–303. https://doi.org/10.1016/0022-0981(89)90109-3. Howe, S., Marshall, A.T., 2002. Thermal compensation of metabolism in the temperate coral Plesiastrea versipora (Lamark, 1816). J. Exp. Mar. Biol. Ecol. 259 (2), 231–248. https://doi.org/10.1016/S0022-0981(01)00230-1. Huang, D., Licuanan, W.Y., Hoeksema, B.W., Chen, C.A., Ang, P., Huang, H., Lane, D.J.W., Vo, T.S., Waheed, Z., Affendi, Y.A., Yeemin, T., Chou, L.M., 2015. Extraordinary diversity of reef corals in the South China Sea. Mar. Biodivers. 45, 157–168. https:// doi.org/10.1007/s12526-014-0236-1. Hughes, T.P., et al., 2017. Global warming and recurrent mass bleaching of corals. Nature 543 (7645), 373–23 377. https://doi.org/10.1038/nature21707. IPCC, 2013. Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge, United Kingdom. Cambridge University Press, New York, NY, USA. Jokiel, P.L., Coles, S.L., 1977. Effects of temperature on the mortality and growth of Hawaiian reef corals. Mar. Biol. 43, 201–208. Kayal, M., Vercelloni, J., Lison De Loma, T., Bosserelle, P., Chancerelle, Y., Geoffroy, S., Stievenart, C., Michonneau, F., Penin, L., Planes, S., Adjeroud, M., 2012. Predator crown-of-thorns starfish (Acanthaster planci) outbreak, mass mortality of corals, and cascading effects on reef fish and benthic communities. PLoS One 7, e47363. https:// doi.org/10.1371/journal.pone.0047363. Keppel, G., Niel, K.P.V., Wardell-Johnson, G.W., Yates, C.J., Byrne, M., Mucina, L., Schut, A.G.T., Hopper, S.D., Franklin, S.E., 2012. Refugia: identifying and understanding safe havens for biodiversity under climate change. Glob. Ecol. Biogeogr. 21, 393–404. https://doi.org/10.1111/j.1466-8238.2011.00686.x. Kleypas, J.A., McManus, J.W., Menez, L.A.A., 1999. Environmental limits to coral reef development: where do we draw the line? Amer. Zool. 39, 146–159. https://doi.org/ 10.1093/icb/39.1.146. Lam, K., Shin, P.K.S., Hodgson, P., 2007. Severe bioerosion caused by an outbreak of corallivorous Drupella and Diadema at Hoi Ha Wan Marine Park, Hong Kong. Coral Reefs 26 (4), 893. https://doi.org/10.1007/s00338-007-0288-9. Langdon, C., Takahashi, T., Sweeney, C., Chipman, D., Goddard, J., Marubini, F., Aceves, H., Barnett, H., Atkinson, M.J., 2000. Effect of calcium carbonate saturation state on the calcification rate of an experimental coral reef. Glob. Biogeochem. Cy. 14 (4). https://doi.org/10.1029/1999GB001195. (639–354). Lantz, C.A., Schulz, K.G., Eyre, B.D., 2019. The effect of warming and benthic community acclimation on coral reef carbonate sediment metabolism and dissolution. Coral Reefs 38 (1), 149–163. https://doi.org/10.1007/s00338-018-01752-x. Levitus, S., Antonov, J.I., Boyer, T.P., Locarnini, R.A., Garcia, H.E., Mishonov, A.V., 2009. Global Ocean heat content 1955–2008 in light of recently revealed instrumentation problems. Geophys. Res. Lett. 36, L07608. https://doi.org/10.1029/2008GL037155. Long, M.H., Berg, P., de Beer, D., Zieman, J., 2013. In situ coral reef oxygen metabolism: an eddy correlation study. PLoS One 8 (3), 1–11. https://doi.org/10.1371/journal. pone.0058581. Lough, J.M., Barnes, D.J., 2000. Environmental controls on growth of the massive coral Porites. J. Exp. Mar. Biol. Ecol. 245, 225–243. https://doi.org/10.1016/S00220981(99)00168-9. Lu, S., Hodgkiss, I.J., 2004. Harmful algal bloom causative collected from Hong Kong

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