Reef coral assemblages are highly dynamic and subject to repeated disturbances, which are predicted to increase in response to climate change. Consequently there is an urgent need to improve our understanding of the mechanisms underlying different recovery scenarios. Recent work has demonstrated that reef structural complexity can facilitate coral recovery, but the mechanism remains unclear. Similarly, experiments suggest that coral larvae can distinguish between the water from healthy and degraded reefs, however, whether or not they can use these cues to navigate to healthy reefs is an open question. Here, we use a meta-analytic approach to document that coral larval swimming speeds are orders of magnitude lower than measurements of water flow both on and off reefs. Therefore, the ability of coral larvae to navigate to reefs while in the open-ocean, or to settlement sites while on reefs is extremely limited. We then show experimentally that turbulence generated by fine scale structure is required to deliver larvae to the substratum even in conditions mimicking calm back-reef flow environments. We conclude that structural complexity at a number of scales assists coral recovery by facilitating both the delivery of coral larvae to the substratum and settlement.

Many organisms have a complex life‐cycle in which dispersal occurs at the propagule stage. For marine environments, there is growing evidence that high levels of recruitment back to the natal population (self‐recruitment) are common in many marine organisms. For fish, swimming behavior is frequently invoked as a key mechanism allowing high self‐recruitment. For organisms with weak‐swimming larvae, such as many marine invertebrates, the mechanisms behind self‐recruitment are less clear. Here, we assessed whether the combination of passive retention of larvae due to re‐circulation processes near reefs, and the dynamics of settlement competence, can produce the high levels of self‐recruitment previously estimated by population genetic studies for reef‐building corals. Additionally, we investigated whether time to motility, which is more readily measurable than competence parameters, can explain the between‐species variation in self‐recruitment. We measured the larval competence dynamics of broadcast‐spawning and brooding corals and incorporated these in a model of larval retention around reefs to estimate the potential for self‐recruitment and assess its variation among species and reefs. Our results suggest that the larvae of many corals, even those with an obligate planktonic phase, develop with sufficient rapidity to allow high levels of self‐recruitment, particularly for reefs with long water retention times. Time to motility explained 77–86% of the between‐species variation in potential self‐recruitment in scenarios with a realistic range of retention times. Among broadcast spawners, time to motility was strongly and positively correlated with egg size, i.e., broadcast spawner species with small eggs developed more rapidly and exhibited greater potential for self‐recruitment. These findings suggest that, along with water retention estimates, easy‐to‐measure species traits, such as egg size and time to motility, may be good predictors of potential self‐recruitment, and therefore may be used to characterize the spectrum of self‐recruitment in corals.

Increased temperature and CO₂ levels are considered key drivers of coral reef degradation. However, individual assessments of ecological responses (calcification) to these stressors are often contradicting. To detect underlying drivers of heterogeneity in coral calcification responses, we developed a procedure for the inclusion of stress–effect relationships in ecological meta‐analyses. We applied this technique to a dataset of 294 empirical observations from 62 peer‐reviewed publications testing individual and combined effects of elevated temperature and pCO₂ on coral calcification. Our results show an additive interaction between warming and acidification, which reduces coral calcification by 20% when pCO₂ levels exceed 700 ppm and temperature increases by 3°C. However, stress levels varied among studies and significantly affected outcomes, with unaffected calcification rates under moderate stresses (pCO₂ ≤ 700 ppm, ΔT < 3°C). Future coral reef carbon budgets will therefore depend on the magnitude of pCO₂ and temperature elevations and, thus, anthropogenic CO₂ emissions. Accounting for stress–effect relationships enabled us to identify additional drivers of heterogeneity including coral taxa, life stage, habitat, food availability, climate, and season. These differences can aid reef management identifying refuges and conservation priorities, but without a global effort to reduce CO₂ emissions, coral capacity to build reefs will be at risk.