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To better understand the long-term survival of modern coral reefs under global warming, the history of past coral bleaching events prior to modern marine ecological surveys must be investigated. Here, 11 dead Porites and 2 Porites with growth hiatuses growing in the 20th century were collected from the Nansha Islands, southern South China Sea (SCS) for monthly Sr/Ca, δ18O and δ13C analyses to explore the climatic conditions and metabolic information corresponding to these growth structures. Among the 13 episodes, the results showed that 8 mortalities occurred in the summers, coral MJ8 died in spring, and the other 2 mortalities and 2 growth hiatuses occurred in the cooling season. In addition to the dead surface of the coral mortalities, stress bands with significantly reduced growth rates could also be observed in the corals. Negative shifts in δ13C occurred prior to 5 coral mortalities and most stress bands were caused in summer, suggesting a sharp reduction in photosynthesis by decreased symbiotic zooxanthellae or photosynthetic pigments. Overall, the mortalities can be concluded to have occurred in summer and stress bands with abrupt δ13C decreases in the last century were mainly caused by thermal coral bleaching in the southern SCS. This in turn suggests that the repeated coral bleaching and mortalities occurred much earlier before the 1980s, which is a longer-term response to modern global warming during the last century. [ABSTRACT FROM AUTHOR]

The healthy status of corals in the isolated atolls of the central South China Sea (SCS) remains unclear. Symbiodiniaceae density (SD) can effectively reflect the thermal tolerance and health of hard corals. Here, the SDs of 238 samples from the Huangyan Atoll (HA) were analyzed. The results revealed significantly intergeneric and geomorphological differences in SD. Intergeneric variation may reflect that corals with high SD have stronger thermal tolerance. Geomorphic analysis showed that the SDs at the outer reef slope were higher than in the lagoon. Hydrodynamics and sea surface temperature were likely the main influencing factors. Most notably, corals in SCS HA had higher SDs than those at neighboring reefs, indicating that their thermal tolerance were strong, which may be related to HA's local upwelling. These results suggest that the HA has the potential to serve as a refuge for corals, but increasing human disturbance limit its function. Unlabelled Image • Coral Symbiodiniaceae density (SD) shows intergeneric and spatial differences. • Corals with high SD in South China Sea (SCS) have stronger thermal tolerance. • Corals at outer reef slope of SCS atoll possess higher SDs than those at lagoon. • Corals in SCS Huangyan Atoll have higher SDs than those at neighboring reefs. • Human disturbances may limit the function of potential refuges for SCS corals. [ABSTRACT FROM AUTHOR]

• Distribution coefficients were species dependent, indicating some biologic controls. • No significant correlation between D values of metals except Cd and SST was found. • Metal in coral skeleton is a reliable proxy for dissolved metal in surface seawater. Scleractinian corals provide an excellent archive of geochemical proxies that allow the reconstruction of the oceanographic and climatic changes in the oceans. However, limited knowledge regarding the distribution coefficients (D) of trace metals between coral lattices and seawater has complicated efforts to obtain accurate quantitative data for seawater chemistry. Here we estimated the D values of five trace metals (Cu, Zn, Pb, Cd and Cr) for three coral species (Favia palauensis , Porites lutea , and Pavona decussata) and in situ seawater around Weizhou Island in the northern South China Sea. Despite several variations that could not be explained solely by species, our results were within the approved ranges of the previous reported estimates, and provide the most accurate D values for the these species to date. We also focused on the possible effects of surface seawater temperature (SST) on D vaules, finding no significant relationship. Nevertheless, the effects of SST, which could impact the distribution behavior of trace metals (especially Cd) should be paid more attention in the context of global climate change. The more precise D values for corals provided here represent important basic data for future research on coral reefs and expanded our knowledge of chemical signatures in biogenic lattices with regard to ocean composition. [ABSTRACT FROM AUTHOR]

As an efficient indicator of coral reef health, live coral cover (LCC) is regularly surveyed and recorded by many coral reef documents. However, there usually exist some blanks for the historic records, while current in-field surveys are impossible to fill the blanks. To overcome such difficulties, we focus on exploiting the potential of optical satellite images. The purpose is to fill the blanks of the records over the past and further estimate the LCC in future. As historic records were usually lack of accurate geographical locations to match to the satellite images, a spectral index was defined based on the mean of the subsurface remote sensing reflectance. The index was then used to link the LCC with the satellite images by a cubic polynomial function. Thereafter, the LCC and the coefficients of the polynomial function were finally estimated by simultaneously combining the mean subsurface remote sensing reflectance, the historic LCC records, and the constraints among LCC in adjacent years. Experiments on a series of Landsat images of Luhuitou fringing reef (1973 to 2018) demonstrated that the proposed method is effective and feasible, where the introduction of the satellite images can greatly improve the accuracy. The Root Mean Square Error (RMSE), Mean Absolute Error (MAE), and Mean Relative Errors (MRE) of the LCC were able to reach 5.4%, 4.0%, and 15.9% respectively. This is regarded as the first test on LCC estimation by combining such a long-term LCC records with a series of satellite images. [ABSTRACT FROM AUTHOR]

Coral reefs are well known for playing an important role in the global carbon cycle, however little research has been done into their potential for documenting carbon cycles of the past. Here we report the δ13C of coral reef carbonate from a 928.75-m-long core (CK2) from the northern South China Sea (SCS). The chronological profile of CK2 has been well established based on Strontium isotopes and magnetostratigraphy. Our aim is to explore the potential for δ13C in coral reef carbonate to act as a proxy for the seawater δ13C in the northern SCS and related regional carbon cycle in the geological time scale. Our results demonstrate that variations in CK2 δ13C values were generally consistent with changes in seawater δ13C levels in the northern SCS (as inferred from planktonic foraminifer in the northern SCS) over the past 1.8 Ma. This indicates that the δ13C in our CK2 coral reef carbonate was influenced by seawater δ13C levels. The δ13C values of CK2 also presented obvious periodic variations corresponding to glacial-interglacial cycles, with values being higher during glacial periods and lower during interglacial periods. We speculate that the higher δ13C values during glacial periods indicate higher seawater productivity, i.e., increased photosynthesis intensity in coral reefs. In addition, we also found a coherent relationship between changes in CK2 δ13C values and changes in the East Asian terrestrial ecosystem, and we suppose that the CK2 δ13C values maybe partially influenced by the East Asian terrestrial ecosystem through its regulation of SCS seawater δ13C levels. Therefore, we suggest that δ13C values of coral reef carbonate from the northern SCS represent a potential proxy for the seawater δ13C in the northern SCS and related carbon cycle changes in the East Asia. • The δ13C of coral reef carbonate from northern South China Sea can well document changes in seawater δ13C and the regional carbon cycle in the geological time scale. • The evolution of East Asian terrestrial ecosystem could partially influenced CK2 δ13C values through regulate the seawater δ13C levels. • Higher CK2 δ13C values during glacial periods but lower during inter-glacial periods, indicating that seawater productivity, i.e. photosynthesis intensity in coral reefs was higher during glacial periods than during inter-glacial periods. [ABSTRACT FROM AUTHOR]

Despite organophosphate esters (OPEs) are widely prevalent in the environment, however, limited information is available regarding their occurrence, trophodynamics, and exposure risks in coral reef ecosystems. In this study, 11 OPEs were investigated in a tropical marine food web (7 fish species and 9 benthos species) from the Xisha (XS) Islands, South China Sea (SCS). The ∑ 11 OPEs were 1.52 ± 0.33 ng/L, 2227 ± 2062 ng/g lipid weight (lw), 1024 ± 606 ng/g lw, and 1800 ± 1344 ng/g lw in seawater, fish, molluscs, and corals, respectively. Tris (2-chloroisopropyl) phosphate (TCIPPs) were the dominant OPEs in seawater, fish, and molluscs, while tris (2-butoxyethyl) phosphate (TBOEP) predominated in coral tissues. Abiotic and biotic factors jointly affect the OPEs enrichment in marine organisms. Trophic magnification factors (TMFs) (range: 1.31–39.2) indicated the biomagnification potency of OPEs. A dietary exposure risk assessment indicated that OPEs at current levels in coral reef fish posed a low risk to human health but were not negligible. Overall, this study contributes to a further understanding of the environmental behaviors of OPEs in coral reef ecosystems. [Display omitted] • OPEs profiles in seawater and marine organisms are different. • Fish tend to enrich Cl-alkyl OPEs and benthos prefer to accumulate aryl OPEs. • Abiotic and biotic factors jointly affect the OPEs enrichment in marine organisms. • OPEs pose a low risk to human health by coral reef fish ingestion. • Emerging pollutants are dominating the pollution status in the Xisha Islands. [ABSTRACT FROM AUTHOR]

As one of the most abundant metals in heavy oils, Ni has suffered so notably increasing impacts from industrial and traffic activities that anthropogenic Ni emissions have altered natural geochemical processes. The coral Ni/Ca has become a reliable proxy for characterizing marine pollution, but this potential has been unexploited for highlighting oil pollution. Here, we utilized a high-resolution record of geochemical parameters (Ni/Ca, δ18O, and δ13C) in a Porites coral of an offshore island in the northern South China Sea to reconstruct of Ni distribution patterns in surface seawater from 1984 to 2015. The coral Ni/Ca ratios exhibit minor fluctuations, except for multiple mutation peaks (0.20 ± 0.42 μmol/mol) during the period from 1984 to 1993. The ratio was low and stable (0.10 ± 0.09 μmol/mol) from 1994 to 2008, and then increased rapidly with significant variations (1.60 ± 4.56 μmol/mol) from 2009 to 2015. The coral Ni/Ca ratios captured all significant Ni discharges, and this demonstrates its potential for recording oil spill episodes. The historical variations in the contributions of Ni indicate that industrial and traffic activities should be responsible for changes in the anthropogenic input. The leaks and consumptions of petroleum likely account for the primary Ni emission sources. [Display omitted] • Anthropogenic Ni input has altered the geochemical processes of oceanic Ni. • Coral Ni/Ca records have potentials to be novel proxies for surface winds. • Temporal emissions of anthropogenic Ni can be captured by coral Ni/Ca records. [ABSTRACT FROM AUTHOR]

Holocene coral reef development is valuable to understanding the behavior and impacts of paleoclimate and sea level changes. This study focuses on the Holocene carbonate sequences of a drill core extracted from Well CK2 in the northern South China Sea (SCS). This drill core consists of an 873.55 m thick carbonate layer overlying 55.20 m of volcanic basement. High precision U-series dating of 22 coral samples suggests that the Holocene coral reef, which has a thickness of 16.7 m, initiated at ~7800 yr BP (years before 1950 CE) and stopped vertical accretion at 3900 yr BP. The mean vertical accretion rate in the Holocene in Well CK2 was 3.48 m kyr−1, varying from 6.44 m kyr−1 during 7800–6000 yr BP to 0.87 m kyr−1 during 6000–3900 yr BP. After 3900 yr BP, the vertical development of the reef ceased, indicating that the reef development likely involved lateral accretion. The Holocene section mostly consists of unconsolidated coarse sediments, mainly including corals, coralline algae, large benthic foraminifera, and mollusks. Based on the relationship between coral reef development and sea level, together with the age profile, we determined that the relative sea level near Chenhang Island rose rapidly between 7800 and 6000 yr BP, slowed at ~6000 yr BP, and reached a position of about 2 m above the present-day mean sea level by 3900 yr BP. After 3900 yr BP, the sea level was stable or fell, resulting in a cessation in the reef's upward development. • 22 230Th ages of a SCS reef core suggest the Holocene reef initiated at 7800 yr BP. • Vertical development decreased from 6.44 to 0.87 m kyr−1 at 3900 yr BP. • Sea level at 7800 yr BP was ~13.8 m below present and rose rapidly. • Sea level has stabilized/fallen since 3900 yr BP, as indicated by reef development. [ABSTRACT FROM AUTHOR]

As global warming threatens their existence, there are widespread concerns regarding the persistence of lime-sand islands and the future of mid-ocean atoll nations. To investigate how climate change has affected lime-sand islands, changes in vegetation and other characteristics of such islands in the South China Sea (SCS) were investigated from 1989 to 2019 using 67 satellite images. First, boundaries of the lime-sand islands and their vegetation were extracted using an active contour extraction procedure called the gradient vector flow snake model. Afterwards, the spatial extents were estimated by enclosing the extracted boundaries, and the digital shoreline analysis system was used to calculate beach widths. Finally, area growth rates and speeds were used to evaluate areal changes in the lime-sand islands and their respective vegetation. Based on the estimated area growth rates, area growth speeds, and beach widths, the lime-sand islands in SCS eroded over the past three decades whereas their vegetation expanded. Further analysis suggested that direct inundation caused by sea-level rise might not be clearly identified from the satellite images. However, other climate change-related factors were most likely responsible for the observed island erosions. These factors included higher wave energy, stronger typhoon intensity and destructiveness, and accelerated coral reef degradation. In addition, the observed expansion of vegetation on the lime-sand islands was likely due to the increase in precipitation in a warming world. The results show that 1) the lime-sand islands will continue to erode whereas vegetation will continue to expand; 2) As vegetation growth is significantly inhibited by salt water when it is adjacent to the ocean, vegetation areas on lime-sand islands may start to continuously decline. Overall, this study is the first to quantitatively examine changes in SCS lime-sand islands due to global warming. [ABSTRACT FROM AUTHOR]

Eight fish species were sampled from a coastal coral reef ecosystem near Weizhou Island, South China Sea, to investigate the composition profiles and bioaccumulation of organochlorines (OCs). The total concentrations of 18 organochlorine pesticides (OCPs) and 22 polychlorinated biphenyls (PCBs) were found to be 26.5–452 ng/g lw and 0.87–19.8 ng/g lw, respectively. The contaminant distribution pattern indicated that agrochemical sources were more important than industrial sources, and that historical residues remain the primary source of OCs in Weizhou Island. Bioaccumulation factors (BAFs) indicated that dichlorodiphenyltrichloroethanes was bioaccumulating with log BAFs ranging from 3.53 to 5.21. Some congeners diverged from the general trend predicted by the logarithm octanol-water partition coefficient (log K ow); this was mainly attributable to differences in the bioaccumulation potentials of these congeners in the studied samples. Trophic magnification factors demonstrated that aldrin, endrin, and dieldrin undergo significant trophic dilution, while the other six OC compounds undergo trophic magnification in the food chain. The presence of OCP congeners was also probably affected by their metabolism in fish tissues. The estimated daily intakes of OCPs via fish consumption by residents ranged from 0.05 to 5.45 ng/kg body weight/day, which is below the acceptable daily intake recommended by the FAO/WHO. Image 1 • We studied OCs in the coastal coral reef ecosystems of the South China Sea.. • Organochlorines from historical agricultural sources persisted in the fish muscle. • The TMFs for organochlorines were lower than those from high latitude regions. • Consumption of the coral reef fish may not pose health risk to human at present. [ABSTRACT FROM AUTHOR]

Most studies on coral bleaching alerts use common Degree Heating Week (DHW) thresholds; however, these may underestimate historical patterns of heat stress for coral reef ecosystems. Taking an optimized DHW threshold for coral bleaching alerts for Coral Reef Watch (CRW) and Coral Reef Temperature Anomaly Database (CoRTAD) products, we analyzed the precise spatial and temporal pattern of heat stress on China's coral reefs from 2010 to 2021 in the South China Sea (SCS) and the Beibu Gulf (BG). We compared acute heat stress using common and optimized thresholds. Results indicated that the ocean warming rate in 2010–2021 was approximately 0.43 ± 0.22 °C/10a, showing a significant increase in the northern SCS and the BG. More severe bleaching events were predicted by the optimized thresholds and the high-frequency areas were mainly in the northern SCS. The number and intensity of years with severe heat stress anomalies was in the order 2020 > 2014 > 2010 > 2015. Heat stress duration was the longest in the Xisha Islands among offshore archipelagos, and longest in 2020–2021 in Weizhou Island in BG in the relative high-latitude inshore reefs. These abnormal events were mainly caused by El Niño, but La Niña was also involved in 2020. • The most intense severe heat stress anomaly years were 2010 > 2014>2020 > 2015. • The anomaly duration of offshore islands was Xisha > Dongsha > Zhongsha > Nansha Islands. • High-latitude reefs were predicted not to be future thermal refuges for SCS reefs. • The optimized DHW threshold more accurately predicted spatiotemporal heat stress. [ABSTRACT FROM AUTHOR]

Nutrient enrichment is a widely recognized threat to coastal coral reefs. Yet its impact on open ocean reefs is largely unknown due to the lack of long-term observations at remote sites and the entanglement of various environmental factors affecting coral growth. Here we determined growth rates and the carbonate chemistry of calcifying fluid (CF) of massive corals over the last 550 years at a remote South China Sea reef ecosystem. We show that large growth variations cannot be fully explained by changes in seawater pH or temperature due to the antiphase relationship between growth rate and pH cf as well as small sea surface temperature variations before the 1960s. Instead, coral growth rate decreases proportionally with past seabird populations over the past centuries. We suggest that excess nutrient loads through monsoon pumping and seabird migration played an important role in controlling coral growth at this remote reef ecosystem. Specifically, episodic eutrophication as indicated by a ∼ 3 per mil increase in coral δ13C has depressed coral growth by ∼30% during 15th to 18th centuries, which is comparable to the modern decline. Thus, accurate projections of future coral growth for remote reefs under ongoing human activities and climate change needs to take into account the history of past nutrient inputs into these ecosystems under pre-anthropogenic conditions, which has been overlooked previously due to a lack of historical observations. • Coral cores from an open ocean reef that grew from ∼1450 to present were analyzed. • Temperature affects multi-decadal variability in coral calcifying fluid DIC but not pH. • Reef-water pH modified by monsoon controls calcifying fluid pH and [CO 3 2−] • Eutrophication caused by monsoon and seabirds depressed coral growth during the Little Ice Age • An analog to the responses of corals to the projected rise in anthropogenic nutrient inputs. [ABSTRACT FROM AUTHOR]