Nicole M. Ryan

Despite decades of research, fundamental gaps remain in our understanding of coral vital rates across taxa. This is particularly true for the early life stages, which are critical for reef recovery, representing significant bottlenecks to population growth. We assessed the survival of 3334 small corals (0.3–10 cm maximum diameter) across four eastern Indian Ocean reefs through contrasting levels of heat stress. Under background conditions at multiple reefs, survival of small corals ranged between 0.43 and 0.92 yr ⁻¹ and increased with colony size. Following a major heat stress event (13 Degree Heating Weeks (°C-weeks)), which caused moderate bleaching at Ashmore Reef, survival was comparable to background levels (0.67 yr ⁻¹ HDI: 0.57–0.77). In contrast, major heat stress at Mermaid Reef (11 °C-weeks) in the same period caused severe bleaching and substantially reduced survival to 0.21 (6 _mo ) ⁻¹ (HDI: 0.10–0.32). This was associated with a collapse of size-dependent survival for structurally complex, bleaching-susceptible taxa ( Acropora, Pocillopora ). Stress-tolerant taxa ( Goniastrea, Porites ) experienced smaller reductions in survival and retained size-specific survival patterns. Spanning multiple reefs under varying levels of heat stress, this study provides taxa- and size-specific insights for improving predictions of coral population trajectories and informing reef management and restoration strategies under future climate scenarios.

Ocean warming is increasing the frequency, extent, and severity of tropical-coral bleaching and mortality. During 2014–2017, marine heatwaves caused the Third Global Coral Bleaching Event. We analyze data from 15,066 reef surveys globally during 2014–2017. Across all surveyed reefs, 80% and 35% experienced moderate or greater (affecting >10% of corals) bleaching and mortality, respectively. We assess the global extent of coral bleaching and mortality by applying bleaching response curves calibrated from surveyed reefs to predict bleaching globally, based on comprehensive remote-sensing of heat stress. These models predict that 51% and 15% of the world’s coral reefs suffered moderate or greater bleaching and mortality, respectively, during one or multiple years, surpassing damage from any prior global coral bleaching event. Our findings demonstrate that the impacts of ocean warming on coral reefs are accelerating, with the near certainty that ongoing warming will cause large-scale, possibly irreversible, degradation of these essential ecosystems. With heat stress levels during this event surpassing those observed previously, the National Oceanic and Atmospheric Administration developed more extreme Bleaching Alert levels that are now being used during the ongoing Fourth Global Coral Bleaching Event.

Climate change is restructuring natural ecosystems. The direct impacts of these events on biodiversity and community structure are widely documented, but the impacts on the genetic variation of populations remains largely unknown. We monitored populations of Acropora coral on a remote coral reef system in northwest Australia for two decades and through multiple cycles of impact and recovery. We combined these demographic data with a temporal genetic dataset of a common broadcast spawning corymbose Acropora to explore the spatial and temporal patterns of connectivity underlying recovery. Our data show that broad-scale dispersal and post-recruitment survival drive recovery from recurrent disturbances, including mass bleaching and mortality. Consequently, genetic diversity and associated patterns of connectivity are maintained through time in the broader metapopulation. The results highlight an inherent resilience in these globally threatened species of coral and showcase their ability to cope with multiple disturbances, given enough time to recover is permitted.

Coral reefs have evolved over millennia to survive disturbances. Yet, in just a few decades chronic local pressures and the climate catastrophe have accelerated so quickly that most coral reefs are now threatened. Rising ocean temperatures and recurrent bleaching pose the biggest threat, affecting even remote and well‐managed reefs on global scales. We illustrate how coral bleaching is altering reefs by contrasting the dynamics of adjacent reef systems over more than two decades. Both reef systems sit near the edge of northwest Australia's continental shelf, have escaped chronic local pressures and are regularly affected by tropical storms and cyclones. The Scott reef system has experienced multiple bleaching events, including mass bleaching in 1998 and 2016, from which it is unlikely to fully recover. The Rowley Shoals has maintained a high cover and diversity of corals and has not yet been impacted by mass bleaching. We show how the dynamics of both reef systems were driven by a combination of local environment, exposure to disturbances and coral life history traits, and consider future shifts in community structure with ongoing climate change. We then demonstrate how applying knowledge of community dynamics at local scales can aid management strategies to slow the degradation of coral reefs until carbon emissions and other human impacts are properly managed.

Novel tools and methods for monitoring marine environments can improve efficiency but must not compromise long-term data records. Quantitative comparisons between new and existing methods are therefore required to assess their compatibility for monitoring. Monitoring of shallow water coral reefs is typically conducted using diver-based collection of benthic images along transects. Diverless systems for obtaining underwater images (e.g. towed-cameras, remotely operated vehicles, autonomous underwater vehicles) are increasingly used for mapping coral reefs. Of these imaging platforms, towed-cameras offer a practical, low cost and efficient method for surveys but their utility for repeated measures in monitoring studies has not been tested. We quantitatively compare a towed-camera approach to repeated surveys of shallow water coral reef benthic assemblages on fixed transects, relative to benchmark data from diver photo-transects. Differences in the percent cover detected by the two methods was partly explained by differences in the morphology of benthic groups. The reef habitat and physical descriptors of the site—slope, depth and structural complexity—also influenced the comparability of data, with differences between the tow-camera and the diver data increasing with structural complexity and slope. Differences between the methods decreased when a greater number of images were collected per tow-camera transect. We attribute lower image quality (variable perspective, exposure and focal distance) and lower spatial accuracy and precision of the towed-camera transects as the key reasons for differences in the data from the two methods and suggest changes to the sampling design to improve the application of tow-cameras to monitoring.