Maucieri, D. G. | Search bioRxiv

bioRxiv2025

Spatial variation in benthic community composition on a minimally disturbed coral reef in the years following a prolonged marine heatwave

With the increased frequency of marine heatwaves and related coral mass mortality events, it is imperative that we improve our understanding of coral reef recovery processes including how benthic community compositions can change over time. Coral reef benthic communities are influenced by many abiotic factors, but on most reefs local anthropogenic disturbances overshadow these factors thus obscuring their influence. Here, we leverage a dataset from a coral reef with very minimal local anthropogenic disturbance - the uninhabited southern coast of the worlds largest atoll (Kiritimati) - to assess spatial variation in benthic community composition three years after the mass coral mortality event driven by the 2015-2016 El Nino. Across forereef sites ranging from 7 to 22 m, scleractinian coral cover remained very low (6.9 {+/-} 0.4 % SE) while soft coral cover was < 1%. Coral cover was highest at deep sites (18-22 m compared to sites at 7-10 m depth) and exposed locations, where stress-tolerant corals likely made up a larger proportion of the coral community before the mass mortality event. Higher cover of crustose coralline algae at shallow exposed sites, fleshy macroalgae at deep sites, and turf algae at exposed locations were consistent with taxa-specific preferences for light, wave action, and sedimentation. The abundances of the most common genera of juvenile coral (Acropora and Pocillopora) were still low (< 1 juvenile colony per genus per site) and varied only with depth. These findings demonstrate how variation in pre-mortality coral composition can lead to differences in post-mortality benthic communities on low disturbance coral reefs.

ecology↗

bioRxiv2022

Transformation of coral communities subjected to an unprecedented heatwave is modulated by local disturbance

Corals are imminently threatened by climate change-amplified marine heatwaves. Yet how to conserve reef ecosystems faced with this threat remains unclear, since protected reefs often seem equally or more susceptible to thermal stress as unprotected ones. Here, we disentangle this apparent paradox, revealing that the relationship between reef disturbance and heatwave impacts depends upon the focal scale of biological organization. We document a heatwave of unprecedented duration that culminated in an 89% loss of coral cover. At the community level, losses hinged on pre-heatwave community structure, with sites dominated by competitive corals--which were predominantly protected from local disturbance--undergoing the greatest losses. In contrast, at the species level, survivorship of individual coral colonies typically decreased as local disturbance intensified, illustrating that underlying chronic disturbances can impair resilience to thermal stress at this scale. Our study advances understanding of the relationship between climate change and local disturbance, knowledge of which is crucial for coral conservation this century.

ecology↗