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Marine heatwaves are increasing in frequency and duration, threatening tropical reef ecosystems through intensified coral bleaching events. We examined a strikingly variable spatial pattern of bleaching in Moorea, French Polynesia following a heatwave that lasted from November 2018 to July 2019. In July 2019, four months after the onset of bleaching, we surveyed > 5000 individual colonies of the two dominant coral genera, Pocillopora and Acropora, at 10 m and 17 m water depths, at six forereef sites around the island where temperature was measured. We found severe bleaching increased with colony size for both coral genera, but Acropora bleached more severely than Pocillopora overall. Acropora bleached more at 10 m than 17 m, likely due to higher light availability at 10 m compared to 17 m, or greater daily temperature fluctuation at depth. Bleaching in Pocillopora corals did not differ with depth but instead varied with the interaction of colony size and Accumulated Heat Stress (AHS), in that larger colonies (> 30 cm) were more sensitive to AHS than mid-size (10–29 cm) or small colonies (5–9 cm). Our findings provide insight into complex interactions among coral taxa, colony size, and water depth that produce high spatial variation in bleaching and related coral mortality. [ABSTRACT FROM AUTHOR]

For many long‐lived taxa, such as trees and corals, older, and larger individuals often have the lowest mortality and highest fecundity. However, climate change‐driven disturbances such as droughts and heatwaves may fundamentally alter typical size‐dependent patterns of mortality and reproduction in these important foundation taxa. Working in Moorea, French Polynesia, we investigated how a marine heatwave in 2019, one of the most intense marine heatwaves at our sites over the past 30 years, drove patterns of coral bleaching and mortality. The marine heatwave drove island‐wide mass coral bleaching that killed up to 76% and 65% of the largest individuals of the two dominant coral genera, Pocillopora and Acropora, respectively. Colonies of Pocillopora and Acropora ≥30 cm diameter were ~3.5× and ~1.3×, respectively, more likely to die than colonies <30‐cm diameter. Typically, annual mortality in these corals is concentrated on the smallest size classes. Yet, this heatwave dramatically reshaped this pattern, with heat stress disproportionately killing larger coral colonies and equalizing annual mortality rates across the size spectrum. This shift in the size‐mortality relationship reduced the overall fecundity of these genera by >60% because big corals are disproportionately important for reproduction on reefs. Additionally, the survivorship of microscopic coral recruits, critical for the recovery of corals following disturbances, declined to 2%, over an order of magnitude lower compared to a year without elevated thermal stress, where 33% of coral recruits survived. While other research has shown that larger corals can bleach more frequently than smaller corals, we show the severe impact this phenomenon can have at the reef‐wide scale. As marine heatwaves become more frequent and intense, disproportionate mortality of the largest, most fecund corals and near‐complete loss of entire cohorts of newly‐settled coral recruits will likely reduce the recovery capacity of these iconic ecosystems. [ABSTRACT FROM AUTHOR]

Nitrogen pollution increases the susceptibility of corals to heat-induced bleaching. However, different forms of nitrogen (nitrate vs. ammonium/urea) may have different impacts on thermal tolerance of corals. We used an 18-month field experiment on the oligotrophic fore reef of Moorea, French Polynesia, to test how different forms of nitrogen (nitrate vs. urea) impacted coral bleaching. The experiment spanned two moderate thermal stress events in 2016 and 2017. Nitrate increased bleaching prevalence in Acropora by up to 100% and in Pocillopora by up to 60% compared to control corals. Urea exposure often had intermediate effects on bleaching (not different from either control or nitrate-exposed corals) in both taxa. Importantly, nitrate prolonged bleaching in both Acropora and Pocillopora as nitrate-exposed corals remained bleached even after thermal stress ended, while control and urea-exposed corals had mostly recovered. Nitrate exposure also increased the prevalence of partial mortality in Pocillopora colonies and more than tripled the number of colonies that completely died. Our data are the first to show contrasting effects of different forms of nitrogen on coral bleaching and mortality in a natural reef environment, linking previous patterns from large-scale correlative studies with results from more mechanistic laboratory experiments. Most importantly, we showed that corals exposed to nitrate exhibited more frequent bleaching, bleached for longer duration, and were more likely to die than corals in low nitrogen conditions. Exposure to excess nitrogen, particularly anthropogenic nitrogen, may lower the temperature threshold at which corals bleach, triggering bleaching events on polluted reefs even when typical thermal stress thresholds have not been crossed. [ABSTRACT FROM AUTHOR]

Climate change is increasing the frequency and magnitude of temperature anomalies that cause coral bleaching, leading to widespread mortality of stony corals that can fundamentally alter reef structure and function. However, bleaching often is spatially variable for a given heat stress event, and drivers of this heterogeneity are not well resolved. While small-scale experiments have shown that excess nitrogen can increase the susceptibility of a coral colony to bleaching, we lack evidence that heterogeneity in nitrogen pollution can shape spatial patterns of coral bleaching across a seascape. Using island-wide surveys of coral bleaching and nitrogen availability within a Bayesian hierarchical modeling framework, we tested the hypothesis that excess nitrogen interacts with temperature anomalies to alter coral bleaching for the two dominant genera of branching corals in Moorea, French Polynesia. For both coral genera, Pocillopora and Acropora, heat stress primarily drove bleaching prevalence (i.e., the proportion of colonies on a reef that bleached). In contrast, the severity of bleaching (i.e., the proportion of an individual colony that bleached) was positively associated with both heat stress and nitrogen availability for both genera. Importantly, nitrogen interacted with heat stress to increase bleaching severity up to twofold when nitrogen was high and heat stress was relatively low. Our finding that excess nitrogen can trigger severe bleaching even under relatively low heat stress implies that mitigating nutrient pollution may enhance the resilience of coral communities in the face of mounting stresses from global climate change. [ABSTRACT FROM AUTHOR]

Coral reefs experience biologically-driven p CO 2 oscillations that are predicted to become more extreme in magnitude and duration under ocean acidification (OA) regimes. Understanding the plasticity of responses in common reef-building corals to oscillations in p CO 2 will allow for better predictions of their function in future seawater conditions. This study explored the effects of variation in seawater p CO 2 on coral calcification using experiments conducted over one month between 9 April 2018 and 18 May 2018. Branches (~4-cm long) of Acropora retusa were sampled from colonies at 10-m depth on the fore reef of Mo'orea, French Polynesia (17° 28′ 53.9004" S, 149° 49′ 50.5992" W). We tested the hypothesis that depressed calcification caused by elevated p CO 2 (~1000 μatm) is relaxed (i.e., calcification increases) upon return to ambient p CO 2 (~400 μatm). Corals first were incubated in ambient or elevated p CO 2 for 19 days, with the result that calcification integrated over this period was reduced by 31% under elevated p CO 2. The same corals were then incubated at ambient p CO 2 for 11 days, during which calcification was independent of the experimental p CO 2 exposure history. Our results suggest that a quick relaxation of p CO 2 -depressed calcification in A. retusa following cessation of high p CO 2 indicates that corals are capable of a reversible plastic response of calcification when confronted by p CO 2 oscillations. • Coral reefs experience biologically-driven p CO 2 oscillations • Calcification of A. retusa with two p CO 2 exposure histories differed. • When subsequently placed in common p CO 2 environment, calcification was similar. • Some corals are capable of a reversible plastic response of calcification. [ABSTRACT FROM AUTHOR]