Ocean warming increases the incidence of coral bleaching, which reduces or eliminates the nutrition corals receive from their algal symbionts, often resulting in widespread mortality. In contrast to extensive knowledge on the thermal tolerance of coral-associated symbionts, the role of the coral host in bleaching patterns across species is poorly understood. Here, we applied a Bayesian analysis of carbon and nitrogen stable isotope data to determine the trophic niche overlap between corals and their symbionts and propose benchmark values that define autotrophy, heterotrophy, and mixotrophy. The amount of overlap between coral and symbiont niche was negatively correlated with polyp size and bleaching resistance. Our results indicated that as oceans warm, autotrophic corals lose their competitive advantage and thus are the first to disappear from coral reefs.

Although the impacts of nutrient pollution on coral reefs are well known, surprisingly, no statistical relationships have ever been established between water quality parameters, coral biodiversity and coral cover. Hong Kong provides a unique opportunity to assess this relationship. Here, coastal waters have been monitored monthly since 1986, at 76 stations, providing a highly spatially resolved water quality dataset including 68 903 data points. Moreover, a robust coral species richness (S) dataset is available from more than 100 surveyed locations, composed of 3453 individual colonies' observations, as well as a coral cover (CC) dataset including 85 sites. This wealth of data provides a unique opportunity to test the hypothesis that water quality, and in particular nutrients, drives coral biodiversity. The influence of water quality on S and CC was analyzed using GIS and multiple regression modeling. Eutrophication (as chlorophyll‐a concentration; CHLA) was negatively correlated with S and CC, whereas physicochemical parameters (DO and salinity) had no significant effect. The modeling further illustrated that particulate suspended matter, dissolved inorganic nitrogen (DIN) and dissolved inorganic phosphorus (DIP) had a negative effect on S and on CC; however, the effect of nutrients was 1.5‐fold to twofold greater. The highest S and CC occurred where CHLA <2 μg L−1, DIN < 2 μm and DIP < 0.1 μm. Where these values were exceeded, S and CC were significantly lower and no live corals were observed where CHLA > 15 μg L−1, DIN > 9 μm and DIP > 0.33 μm. This study demonstrates the importance of nutrients over other water quality parameters in coral biodiversity loss and highlights the key role of eutrophication in shaping coastal coral reef ecosystems. This work also provides ecological thresholds that may be useful for water quality guidelines and nutrient mitigation policies.

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International audienceWe present early mid-Holocene records of Sr/Ca, δ¹⁸O and δ¹⁸O_sw from marine archives collected in Vanuatu: two Porites sp. corals (6.7-6.5 ka BP) and a Tridacna maxima giant clam (6.2-6.0 ka BP). Sr/Ca, δ¹⁸O, and δ¹⁸O_sw were used as proxies for sea surface temperature (SST) and sea surface salinity (SSS). The fossil geochemical records were compared to modern Porites sp. and T. maxima records. Reconstructed mean SSTs from the two fossil Porites sp. and from the modern coral are similar, implying that the Western Pacific Warm Pool (WPWP)' southern edge had reached its modern location by 6.7-6.5 ka BP. The post-glacial SST rise in the Southwest Pacific was thus completed by the early mid-Holocene. The two early mid-Holocene corals and the giant clam recorded saltier conditions than modern related to 1) a decoupling between the precipitation regime and the SPCZ due to a northerly position of this climatic feature and 2) an increase of the moisture transport to the extra-tropics, driven by a strengthened or extended Hadley cell. The longest δ¹⁸O coral profile displays an El Niño Southern Oscillation (ENSO) signal reduced by 20-30% compared to the period 1928-1992, in concordance with the reduced ENSO variability observed in the Pacific area during the first half of the Holocene. However, the decoupling between the SPCZ and the precipitation regime may have also contributed to the weak ENSO signal recorded in the early mid-Holocene coral δ¹⁸O profile

Historical coral skeleton (CS) δ18O and δ15N records were produced from samples recovered from sedimentary deposits, held in natural history museum collections, and cored into modern coral heads. These records were used to assess the influence of global warming and regional eutrophication, respectively, on the decline of coastal coral communities following the development of the Pearl River Delta (PRD) megacity, China. We find that, until 2007, ocean warming was not a major threat to coral communities in the Pearl River estuary; instead, nitrogen (N) inputs dominated impacts. The high but stable CS‐δ15N values (9‰–12‰ vs. air) observed from the mid‐Holocene until 1980 indicate that soil and stream denitrification reduced and modulated the hydrologic inputs of N, blunting the rise in coastal N sources during the early phase of the Pearl River estuary urbanization. However, an unprecedented CS‐δ15N peak was observed from 1987 to 1993 (>13‰ vs. air), concomitant to an increase of NH4+ concentration, consistent with the rapid Pearl River estuary urbanization as the main cause for this eutrophication event. We suggest that widespread discharge of domestic sewage entered directly into the estuary, preventing removal by natural denitrification hotspots. We argue that this event caused the dramatic decline of the Pearl River estuary coral communities reported from 1980 to 2000. Subsequently, the coral record shows that the implementation of improved wastewater management policies succeeded in bringing down both CS‐δ15N and NH4+ concentrations in the early 2000s. This study points to the potential importance of eutrophication over ocean warming in coral decline along urbanized coastlines and in particular in the vicinity of megacities.