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Monitoring coral cover can describe the ecology of reef degradation, but rarely can it reveal the proximal mechanisms of change, or achieve its full potential in informing conservation actions. Describing temporal variation in Symbiodiniaceae within corals can help address these limitations, but this is rarely a research priority. Here, we augmented an ecological time series of the coral reefs of St. John, US Virgin Islands, by describing the genetic complement of symbiotic algae in common corals. Seventy-five corals from nine species were marked and sampled in 2017. Of these colonies, 41% were sampled in 2018, and 72% in 2019; 28% could not be found and were assumed to have died. Symbiodiniaceae ITS2 sequencing identified 525 distinct sequences (comprising 42 ITS2 type profiles), and symbiont diversity differed among host species and individuals, but was in most cases preserved within hosts over 3 yrs that were marked by physical disturbances from major hurricanes (2017) and the regional onset of stony coral tissue loss disease (2019). While changes in symbiont communities were slight and stochastic over time within colonies, variation in the dominant symbionts among colonies was observed for all host species. Together, these results indicate that declining host abundances could lead to the loss of rare algal lineages that are found in a low proportion of few coral colonies left on many reefs, especially if coral declines are symbiont-specific. These findings highlight the importance of identifying Symbiodiniaceae as part of a time series of coral communities to support holistic conservation planning. Repeated sampling of tagged corals is unlikely to be viable for this purpose, because many Caribbean corals are dying before they can be sampled multiple times. Instead, random sampling of large numbers of corals may be more effective in capturing the diversity and temporal dynamics of Symbiodiniaceae metacommunities in reef corals. [ABSTRACT FROM AUTHOR]

As the major form of coral reef regime shift, stony coral to macroalgal transitions have received considerable attention. In the Caribbean, however, regime shifts in which scleractinian corals are replaced by octocoral assemblages hold potential for maintaining reef associated communities. Accordingly, forecasting the resilience of octocoral assemblages to future disturbance regimes is necessary to understand these assemblages' capacity to maintain reef biodiversity. We parameterised integral projection models quantifying the survival, growth, and recruitment of the octocorals, Antillogorgia americana, Gorgonia ventalina, and Eunicea flexuosa, in St John, US Virgin Islands, before, during, and after severe hurricane disturbance. Using these models, we forecast the density of populations of each species under varying future hurricane regimes. We demonstrate that although hurricanes reduce population growth, A. americana, G. ventalina, and E. flexuosa each display a capacity for quick recovery following storm disturbance. Despite this recovery potential, we illustrate how the population dynamics of each species correspond with a longer-term decline in their population densities. Despite their resilience to periodic physical disturbance events, ongoing global change jeopardises the future viability of octocoral assemblages. [ABSTRACT FROM AUTHOR]

Understanding population dynamics is a long-standing objective of ecology, but the need for progress in this area has become urgent. For coral reefs, achieving this objective is impeded by a lack of information on settlement versus post-settlement events in determining recruitment and population size. Declines in coral abundance are often inferred to be associated with reduced densities of recruits, which could arise from mechanisms occurring at larval settlement, or throughout post-settlement stages. This study uses annual measurements from 2008 to 2021 of coral cover, the density of coral settlers (S), the density of small corals (SC), and environmental conditions, to evaluate the roles of settlement versus post-settlement events in determining rates of coral recruitment and changes in coral cover at Moorea, French Polynesia. Coral cover, S, SC, and the SC:S ratio (a proxy for post-settlement success), and environmental conditions, were used in generalized additive models (GAMs) to show that: (a) coral cover was more strongly related to SC and SC:S than S, and (b) SC:S was highest when preceded by cool seawater, low concentrations of Chlorophyll a, and low flow speeds, and S showed evidence of declining with elevated temperature. Together, these results suggest that changes in coral cover in Moorea are more strongly influenced by post-settlement events than settlement. The key to understanding coral community resilience may lie in elucidating the factors attenuating the bottleneck between settlers and small corals. [ABSTRACT FROM AUTHOR]

The implications of ocean acidification are acute for calcifying organisms, notably tropical reef corals, for which accretion generally is depressed and dissolution enhanced at reduced seawater pH. We describe year‐long experiments in which back reef and fore reef (17‐m depth) communities from Moorea, French Polynesia, were incubated outdoors under pCO2 regimes reflecting endpoints of representative concentration pathways (RCPs) expected by the end the century. Incubations were completed in three to four flumes (5.0 × 0.3 m, 500 L) in which seawater was refreshed and circulated at 0.1 m s−1, and the response of the communities was evaluated monthly by measurements of net community calcification (NCC) and net community productivity (NCP). For both communities, NCC (but not NCP) was affected by treatments and time, with NCC declining with increasing pCO2, and for the fore reef, becoming negative (i.e., dissolution was occurring) at the highest pCO2 (1067–1433 μatm, RCP8.5). There was scant evidence of community adjustment to reduce the negative effects of ocean acidification, and inhibition of NCC intensified in the back reef as the abundance of massive Porites spp. declined. These results highlight the risks of dissolution under ocean acidification for coral reefs and suggest these effects will be most acute in fore reef habitats. Without signs of amelioration of the negative effects of ocean acidification during year‐long experiments, it is reasonable to expect that the future of coral reefs in acidic seas can be predicted from their current known susceptibility to ocean acidification. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

In 1983 to 1984, a mass mortality event caused a Caribbean-wide, >95% population reduction of the echinoid grazer, Diadema antillarum. This led to blooms of algae contributing to the devastation of scleractinian coral populations. Since then, D. antillarum exhibited only limited and patchy population recovery in shallow water, and in 2022 was struck by a second mass mortality reported over many reef localities in the Caribbean. Half-a-century time-series analyses of populations of this sea urchin from St. John, US Virgin Islands, reveal that the 2022 event has reduced population densities by 98.00% compared to 2021, and by 99.96% compared to 1983. In 2021, coral cover throughout the Caribbean was approaching the lowest values recorded in modern times. However, prior to 2022, locations with small aggregations of D. antillarum produced grazing halos in which weedy corals were able to successfully recruit and become the dominant coral taxa. The 2022 mortality has eliminated these algal-free halos on St. John and perhaps many other regions, thereby increasing the risk that these reefs will further transition into coral-free communities. [ABSTRACT FROM AUTHOR]

Identifying relatively intact areas within ecosystems and determining the conditions favoring their existence is necessary for effective management in the context of widespread environmental degradation. In this study, we used 3766 surveys of randomly selected sites in the United States and U.S. Territories to identify the correlates of sites categorized as "oases" (defined as sites with relatively high total coral cover). We used occupancy models to evaluate the influence of 10 environmental predictors on the probability that an area (21.2‐km2 cell) would harbor coral oases defined at four spatial extents: cross‐basin, basin, region, and subregion. Across all four spatial extents, oases were more likely to occur in habitats with high light attenuation. The influence of the other environmental predictors on the probability of oasis occurrence were less consistent and varied with the scale of observation. Oases were most likely in areas of low human population density, but this effect was evident only at the cross‐basin and subregional extents. At the regional and subregional extents oases were more likely where sea‐surface temperature was more variable, whereas at the larger spatial extents the opposite was true. By identifying the correlates of oasis occurrence, the model can inform the prioritization of reef areas for management. Areas with biophysical conditions that confer corals with physiological resilience, as well as limited human impacts, likely support coral reef oases across spatial extents. Our approach is widely applicable to the development of conservation strategies to protect biodiversity and ecosystems in an era of magnified human disturbance. [ABSTRACT FROM AUTHOR]

Most tropical coral reefs are experiencing declining coral cover, yet interpretation of this generality is tempered by spatial variation in coral cover among reefs separated over 20–200 km. This study addresses such landscape scale variation in coral reefs at 12 sites (7–10 m depth) around St. John (18° 18´ 37.04 N, 63° 43´ 23.17 W) and St. Thomas (18° 20´ 43.57 N, 65° 55´ 13.88 W), US Virgin Islands. Surveys completed from 2011 to 2021 were used to test for spatial variation in community dynamics among islands, shores, and sites. Community synchrony (φ) was used to evaluate portfolio effects in mediating changes in coral communities. From 2011 to 2021, changes in benthic communities differed among sites and times (i.e., there were site × year interactions), and, while coral cover declined at 11 sites, the decline was 2.6-fold faster around St. Thomas than St. John. The loss of coral cover was driven by multiple taxa that differed among sites, thus revealing asynchronous responses to prevailing conditions. Asynchrony suggests that coral communities at some of the sites have the capacity to exploit portfolio effects to modulate stability, yet these effects did not prevent declines in coral cover that reflect island-scale phenomena. In the US Virgin Islands, coral death is overwhelming the capacity for resilience of coral communities that historically may have benefitted from portfolio effects. Until coral assemblages are depleted through taxonomic extirpation, maintenance of the assemblage composition retains the possibility that increased resilience might emerge if environmental challenges can be alleviated. [ABSTRACT FROM AUTHOR]

This article discusses the fate of dead coral colonies on tropical reefs, specifically focusing on the reefs of Moorea, French Polynesia. The authors describe an unusual mode of decay where dead Pocillopora spp corals rapidly dissolve, leaving behind "ghosts" with hollow branches. These hollowed corals are hidden beneath a calcified façade colonized by algae, but their internal voids indicate imminent skeletal crumbling. The article raises questions about how coral reef recovery is affected by the structural legacies of dead corals and whether ghost colonies will become the new normal for reefs facing coral mortality due to ocean acidification and climate change. [Extracted from the article]

Measurements of coral recruitment can improve understanding of coral population dynamics, but in short duration studies high variance impedes interpretation of this vital rate. Here, coral recruitment in Moorea, French Polynesia, was measured over 13 yr and tested for associations with environmental conditions. Recruitment of spawning pocilloporid corals was recorded using settlement tiles immersed for ~ 6 months at 10 m and 17 m depth, biannually, and the environment was quantified through seawater clarity (Kd490), surface and bottom flow speeds, coral cover, and temperature. Using GAMs, most of the variation (63–74%) in recruitment was explained using depth and three predictors. Recruitment was high in January/February following a year of low seawater clarity (high Kd490), and in August/September following a year of high clarity. Recruitment in January/February was elevated by high coral cover, and by high surface flow during larval dispersal; recruitment was low in August/September following high temperature during gametogenesis. These effects reveal the extent to which early life stages of corals are exposed to annually variable environmental conditions that have differential consequences depending on the phenology of reproduction and recruitment. The temporal mosaic defining interactions between reproduction, recruitment, and environmental conditions create opportunities for acclimatization to affect where and when coral recruitment is successful. [ABSTRACT FROM AUTHOR]

Reef-building corals are found across > 30° of latitude from tropical to temperate regions, where they occupy habitats greatly differing in seawater temperature and light regimes. It remains largely unknown, however, how the demography of corals differs across this gradient of environmental conditions. Variation in coral growth is especially important to coral populations, because aspects of coral demography are dependent on colony size, with both fecundity and survivorship increasing with larger colonies. Here we tested for latitudinal variation in annual growth rate and survival of juvenile corals, using 11 study locations extending from 17° S to 33° N in the West and South Pacific. Regression analyses revealed a significant decline in annual growth rates with increasing latitude, whereas no significant latitudinal pattern was detected in annual survival. Seawater temperature showed a significant and positive association with annual growth rates. Growth rates varied among the four common genera, allowing them to be ranked Acropora > Pocillopora > Porites > Dipsastraea. Acropora and Pocillopora showed more variation in growth rates across latitudes than Porites and Dipsastraea. Although the present data have limitations with regard to difference in depths, survey periods, and replication among locations, they provide evidence that a higher capacity for growth of individual colonies may facilitate population growth, and hence population recovery following disturbances, at lower latitudes. These trends are likely to be best developed in Acropora and Pocillopora, which have high rates of colony growth. [ABSTRACT FROM AUTHOR]

For more than a century, coral reefs have been exposed to increasing anthropogenic disturbances that have profoundly altered their community structure. These perturbations continue to challenge coral reefs in new ways as ecological paradigms are recast in the Anthropocene Epoch 1. In recent decades, macroalgal blooms have blighted Caribbean reefs 2 , but the appearance of aggressive peyssonnelioid algal crusts (PAC) that are rapidly increasing in abundance to become dominant members of the benthos on Caribbean and Indo-Pacific reefs is a novel phenomenon in tropical seas 3. By pre-empting vacant space, overgrowing corals, deterring the settlement of coral larvae, and favouring a phase transition from coral to algae 4 , PAC are likely to accelerate the decline in dominance of corals on global reefs (Figure 1). Edmunds et al. highlight the rising threat to coral reefs of peyssonnelioid algal crusts. By pre-empting space, overgrowing corals and other organisms, and deterring recruitment, this algal functional group is likely to hasten the loss of corals on tropical reefs. [ABSTRACT FROM AUTHOR]

Coral bleaching in 2019 affected ~72% of I Pocillopora i spp. corals, killing 11-42% of Moorea colonies (French Polynesia). GLO:69VH/01jul21:bes21885-fig-0005.jpg PHOTO (COLOR): . gl Photo 5: Researcher collecting standardized images of corals, using a photoquadrat frame, to measure the size and survival of individual colonies. GLO:69VH/01jul21:bes21885-fig-0002.jpg PHOTO (COLOR): . gl Photo 2: Coral bleaching in 2019 at Moorea appeared to affect the larger colonies more than the smaller colonies of coral in the genus Pocillopora. [Extracted from the article]