Juvenile corals underpin coral reef carbonate production after disturbance

Coral geometry and why it matters

Clonal organisms like reef building corals exhibit a wide variety of colony morphologies and geometric shapes which can have many physiological and ecological implications. Colony geometry can dictate the relationship between dimensions of volume, surface area, and length, and their associated growth parameters. For calcifying organisms, there is the added dimension of two distinct components of growth, biomass production and calcification. For reef building coral, basic geometric shapes can be used to model the inherent mathematical relationships between various growth parameters and how colony geometry determines which relationships are size-dependent or size-independent. Coral linear extension rates have traditionally been assumed to be size-independent. However, even with a constant calcification rate, extension rates can vary as a function of colony size by virtue of its geometry. Whether the ratio between mass and surface area remains constant or changes with colony size is the determining factor. For some geometric shapes, the coupling of biomass production (proportional to surface area productivity) and calcification (proportional to volume) can cause one aspect of growth to geometrically constrain the other. The nature of this relationship contributes to a species' life history strategy and has important ecological implications. At one extreme, thin diameter branching corals can maximize growth in surface area and resource acquisition potential, but this geometry requires high biomass production to cover the fast growth in surface area. At the other extreme, growth in large, hemispheroidal corals can be constrained by calcification. These corals grow surface area relatively slowly, thereby retaining a surplus capacity for biomass production which can be allocated towards other anabolic processes. For hemispheroidal corals, the rate of surface area growth rapidly decreases as colony size increases. This ontogenetic relationship underlies the success of microfragmentation used to accelerate restoration of coral cover. However, ontogenetic changes in surface area productivity only applies to certain coral geometries where surface area to volume ratios decrease with colony size.

Coral recruitment: patterns and processes determining the dynamics of coral populations

Coral recruitment describes the addition of new individuals to populations, and it is one of the most fundamental demographic processes contributing to population size. As many coral reefs around the world have experienced large declines in coral cover and abundance, there has been great interest in understanding the factors causing coral recruitment to vary and the conditions under which it can support community resilience. While progress in these areas is being facilitated by technological and scientific advances, one of the best tools to quantify recruitment remains the humble settlement tile, variants of which have been in use for over a century. Here I review the biology and ecology of coral recruits and the recruitment process, largely as resolved through the use of settlement tiles, by: (i) defining how the terms 'recruit' and 'recruitment' have been used, and explaining why loose terminology has impeded scientific advancement; (ii) describing how coral recruitment is measured and why settlement tiles have value for this purpose; (iii) summarizing previous efforts to review quantitative analyses of coral recruitment; (iv) describing advances from hypothesis-driven studies in determining how refuges, seawater flow, and grazers can modulate coral recruitment; (v) reviewing the biology of small corals (i.e. recruits) to understand better how they respond to environmental conditions; and (vi) updating a quantitative compilation of coral recruitment studies extending from 1974 to present, thus revealing long-term global declines in density of recruits, juxtaposed with apparent resilience to coral bleaching. Finally, I review future directions in the study of coral recruitment, and highlight the need to expand studies to deliver taxonomic resolution, and explain why time series of settlement tile deployments are likely to remain pivotal in quantifying coral recruitment.

Coral reef structural complexity loss exposes coastlines to waves

Coral reefs offer natural coastal protection by attenuating incoming waves. Here we combine unique coral disturbance-recovery observations with hydrodynamic models to quantify how structural complexity dissipates incoming wave energy. We find that if the structural complexity of healthy coral reefs conditions is halved, extreme wave run-up heights that occur once in a 100-years will become 50 times more frequent, threatening reef-backed coastal communities with increased waves, erosion, and flooding.

Tracking changes in social-ecological systems along environmental disturbances with the ocean health index

The well-being of coastal communities is intimately tied to a healthy ocean, but coastal social-ecological systems are among the most vulnerable to global change. Improving the resilience of coastal communities requires an understanding of how local social-ecological systems respond to shocks to better inform decision-making and adapt local management interventions. However, assessments of social-ecological changes throughout a disturbance regime are scarce at the local level, although critical for efficient natural resource management and sustainable use of ocean ecosystem services. Here, we apply the Ocean Health Index (OHI) to assess the status of the marine social-ecological system of a tropical island (Moorea, French Polynesia), and track changes of the system before, during and after a disturbance regime. Our results show that while there are signs of social-ecological recovery, coastal protection was most affected along the disturbance, and that there is room for improvement toward biodiversity conservation. In addition, our study highlights some context-specific challenges associated with local OHI assessments, particularly those driven by limited fisheries data and appropriate reference point selection for coastal protection. Our results demonstrate the value of localized, regular OHI assessments through time to track changes in marine social-ecological systems, while uncovering important data gaps, to inform management at appropriate scales for decision-making.

The war of corals: patterns, drivers and implications of changing coral competitive performances across reef environments

Amidst global environmental changes, predicting species' responses to future environments is a critical challenge for preserving biodiversity and associated human benefits. We explored the original idea that coral competitive performances, the ability of corals to preempt ecological space on the reef through territorial warfare, serve as indicators of species' ecological niches and environmental windows, and therefore, responses to future environments. Our surveys indicated that coral performances varied with taxonomic identity, size and position along environmental gradients, highlighting complex interplays between life-history, warfare-strategy and niche segregation. Our results forewarn that growing alterations of coastal environments may trigger shifts in coral dominance, with the decline of major reef-building taxa like acroporids, and emphasize the importance of limiting human impacts for coastal resilience. Our empirical approach untangles the complexity of species' battle-like interactions and can help identify winners and losers in various communities caught in the interplay between ecological niches, environmental windows and global changes.

Scaling up calcification, respiration, and photosynthesis rates of six prominent coral taxa

Coral reefs provide a range of important services to humanity, which are underpinned by community-level ecological processes such as coral calcification. Estimating these processes relies on our knowledge of individual physiological rates and species-specific abundances in the field. For colonial animals such as reef-building corals, abundance is frequently expressed as the relative surface cover of coral colonies, a metric that does not account for demographic parameters such as coral size. This may be problematic because many physiological rates are directly related to organism size, and failure to account for linear scaling patterns may skew estimates of ecosystem functioning. In the present study, we characterize the scaling of three physiological rates - calcification, respiration, and photosynthesis - considering the colony size for six prominent, reef-building coral taxa in Mo'orea, French Polynesia. After a seven-day acclimation period in the laboratory, we quantified coral physiological rates for three hours during daylight (i.e., calcification and gross photosynthesis) and one hour during night light conditions (i.e., dark respiration). Our results indicate that area-specific calcification rates are higher for smaller colonies across all taxa. However, photosynthesis and respiration rates remain constant over the colony-size gradient. Furthermore, we revealed a correlation between the demographic dynamics of coral genera and the ratio between net primary production and calcification rates. Therefore, intraspecific scaling of reef-building coral physiology not only improves our understanding of community-level coral reef functioning but it may also explain species-specific responses to disturbances.

Dangerous demographics in post-bleach corals reveal boom-bust versus protracted declines

Thermal-stress events have changed the structure, biodiversity, and functioning of coral reefs. But how these disturbances affect the dynamics of individual coral colonies remains unclear. By tracking the fate of 1069 individual Acropora and massive Porites coral colonies for up to 5 years, spanning three bleaching events, we reveal striking genus-level differences in their demographic response to bleaching (mortality, growth, and recruitment). Although Acropora colonies were locally extirpated, substantial local recruitment and fast growth revealed a marked capacity for apparent recovery. By contrast, almost all massive Porites colonies survived and the majority grew in area; yet no new colonies were detected over the 5 years. Our results highlight contrasting dynamics of boom-and-bust vs. protracted declines in two major coral groups. These dangerous demographics emphasise the need for caution when documenting the susceptibility and perceived resistance or recovery of corals to disturbances.