Coral reefs are one of the most biodiverse and economically important ecosystems in the world, but they are rapidly degrading due to the effects of global climate change and local anthropogenic ...stressors. Reef scientists are increasingly studying coral reefs that occur in marginal and extreme environments to understand how organisms respond to, and cope with, environmental stress, and to gain insight into how reef organisms may acclimate or adapt to future environmental change. To date, there have been more than 860 publications describing the biology and/or abiotic conditions of marginal and extreme reef environments, most of which were published within the past decade. These include systems characterized by unusually high, low, and/or variable temperatures (intertidal, lagoonal, high-latitude areas, and shallow seas), turbid or urban environments, acidified habitats, and mesophotic depth, and focus on reefs geographically spread throughout most of the tropics. The papers in this special issue of Coral Reefs, entitled
Coral Reefs in a Changing World: Insights from Extremes
, build on the growing body of literature on these unique and important ecosystems, providing a deeper understanding of the patterns and processes governing life in marginal reef systems, and the implications that these insights may have for the future of tropical coral reefs in our rapidly changing world.
Coral reef ecosystems contain exceptionally high concentrations of marine biodiversity, potentially encompassing millions of species. Similar to tropical rainforests and their insects, the majority ...of reef animal species are small and cryptic, living in the cracks and crevices of structural taxa (trees and corals). Although the cryptofauna make up the majority of a reef’s metazoan biodiversity, we know little about their basic ecology. We sampled motile cryptofaunal communities from both live corals and dead carbonate reef framework across a gradient of increasing erosion on a reef in Pacific Panamá. A total of 289 Operational Taxonomic Units (OTUs) from six phyla were identified. We used species-accumulation models fitted to individual- and sample-based rarefaction curves, as well as seven nonparametric richness estimators to estimate species richness among the different framework types. All procedures predicted the same trends in species richness across the differing framework types. Estimated species richness was higher in dead framework (261–370 OTUs) than in live coral substrates (112–219 OTUs). Surprisingly, richness increased as framework structure was eroded: coral rubble contained the greatest number of species (227–320 OTUs) and the lowest estimated richness of 47–115 OTUs was found in the zone where the reef framework had the greatest vertical relief. This contradicts the paradigm that abundant live coral indicates the apex of reef diversity.
This study examined fluctuations in an eastern Pacific reef fish assemblage as it varied with coral recovery over 30 yr. Concurrent fish and coral monitoring were conducted at Uva Island reef, which ...lies within the boundaries of Coiba National Park, Panama, in an area that has received virtually no fishing pressure or watershed development over the past 80 yr. Coral and fish monitoring spanned the 1982–1983 and 1997–1998 El Niño disturbances—anomalous warming events that selectively killed reef-building corals. While no fish mortalities were observed during the 1982–1983 El Niño event, live coral cover was reduced to nearly 0% at the study reef. From 1984 to 1990, live coral (Pocillopora spp.) cover was extremely low (< 5%), but demonstrated steady recovery to ~35% by 2010. By quantifying disturbance-related, long-term changes in coral reef resources and relating these to fish trophic group responses, several functional relationships emerged. A total of 63 fish taxa were observed, and reef fish density (all taxa combined) remained relatively stable. Multivariate analysis of species abundances revealed a strong overlap between seasons and a clustering of community composition in the years following bleaching. Fish species richness increased significantly as live coral cover rose from near 0 to 15–20% and then demonstrated a decreasing trend to 35% cover. Benthic invertivores showed a significant parabolic increase in density peaking at ~20% live coral cover. A pattern of decline was apparent for the mixed diet feeders guild as coral cover increased, whereas an asymptotic relationship with coral cover emerged for the facultative corallivore guild. No clear patterns in herbivore, piscivore and planktivore abundance were apparent with increasing coral cover. The varying responses of invertivore, corallivore and mixed diet feeders guilds demonstrated strong associations with coral cover, probably reflecting changes in the availability of their respective trophic resources during reef recovery. Thus, variations in coral cover probably influence fish communities through trophic pathways involving invertebrate food sources.
Coral reefs along the Florida Keys portion of the Florida Reef Tract (FRT) have undergone a dramatic decline since the 1980s. Since the 1997–1998 El Niño event, coral cover on offshore reefs of the ...FRT has been ≤5% and continues to decline. Mortality of the framework-constructing coral in the Orbicella (formerly Montastraea) annularis species complex has driven this recent loss in overall coral cover. One exception to this decline occurred on the inshore patch reefs of the Florida Keys, where coral cover has remained relatively high. We examined the growth and calcification of Orbicella faveolata, an ecologically important subspecies of the O. annularis complex, at both an inshore and an offshore reef site representing this dichotomy of present-day coral cover. The period examined (2004 to 2013) encompasses the Caribbean-wide 2005 mass coral bleaching, the 2009–2010 catastrophic cold-water bleaching, and a warm-water bleaching event in 2011. Extension and calcification rates were higher inshore every year from 2004 to 2013 except when there were thermal stress events that solely impacted inshore reefs (as in 2009–2010 and 2011). Inshore growth rates recovered quickly from cold and warm-water stress. These higher calcification rates and their quick recovery after thermal stress are likely important factors in the persistence of high coral cover inshore.
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Marine organisms are often subject to numerous anthropogenic stressors, resulting in widespread ecosystem degradation. Physiological responses to these stressors, however, are ...complicated by high biological variability, species-specific sensitivities, nonlinear relationships, and countless permutations of stressor combinations. Nevertheless, quantification of these relationships is paramount for parameterizing predictive tools and ultimately for effective management of marine resources. Multi-level, multi-stressor experimentation is therefore key, yet the high replication required has remained a logistical challenge and a financial barrier. To overcome these issues, we created an automated system for experimentation on marine organisms, the Sequential Treatment Application Robot (STAR). The system consists of a track-mounted robotic arm that sequentially applies precision treatments to independent aquaria via syringe and peristaltic pumps. The accuracy and precision were validated with dye and spectrophotometry, and stability was demonstrated by maintaining corals under treatment conditions for more than a month. The system is open source and scalable in that additional treatments and replicates may be added without incurring multiplicative costs. While STAR was designed for investigating the combined impacts of nutrients, warming, and disease on reef-building corals, it is highly customizable and may be used for experimentation involving a diverse array of treatments and species.
Ocean acidification (OA) threatens coral reef persistence by decreasing calcification and accelerating the dissolution of reef frameworks. The carbonate chemistry of coastal areas where many reefs ...exist is strongly influenced by the metabolic activity of the underlying benthic community, contributing to high spatiotemporal variability. While characterizing this variability is difficult, it has important implications for the progression of OA and the persistence of the ecosystems. Here, we characterized the carbonate chemistry at 38 permanent stations located along 10 inshore‐offshore transects spanning 250 km of the Florida Coral Reef (FCR), which encompass four major biogeographic regions (Biscayne Bay, Upper Keys, Middle Keys, and Lower Keys) and four shelf zones (inshore, mid‐channel, offshore, and oceanic). Data have been collected since 2010, with approximately bi‐monthly periodicity starting in 2015. Increasing OA, driven by increasing DIC, was detected in the mid‐channel, offshore, and oceanic zones in every biogeographic region. In the inshore zone, however, increasing TA counteracted any measurable OA trend. Strong seasonal variability occurred at inshore sites and included periods of both exacerbated and mitigated OA. Seasonality was region‐dependent, with greater variability in the Lower and Middle Keys. Elevated pH and aragonite saturation states (ΩAr) were observed in the Upper and Middle Keys, which could favor reef habitat persistence in these regions. Offshore reefs in the FCR could be more susceptible to global OA by experiencing open‐ocean‐like water chemistry conditions. By contrast, higher seasonal variability at inshore reefs could offer a temporary OA refuge during periods of enhanced primary production.
Plain Language Summary
Elevated carbon dioxide (CO2) input into the atmosphere is causing the acidification of the oceans, hindering the ability of corals to grow their hard skeleton and thus affecting coral reef habitat persistence. However, shallow ecosystems such as coral reefs can experience strong temporal and spatial variability in carbonate chemistry (such as pH and CO2), which may both mitigate or exacerbate exposure to ocean acidification (OA). From 2010 to 2021, we sampled seawater carbonate chemistry at 38 permanent stations along and across the Florida Coral Reef to assess its variability among seasons, years, and reef areas. OA was detected in most of the Florida Coral Reef, including the mid‐channel and offshore reefs. However, there were no OA trends at inshore reefs, where seasonal variability in the carbonate parameters was the greatest. Among the regions, the Upper and Middle Keys had higher pH values compared with the Lower Keys, suggesting more favorable conditions for reef persistence in the first two regions. This temporal and spatial variability may have important implications for coral reef resilience to OA.
Key Points
Interannual acidification trends were detected in the mid‐channel, offshore, and oceanic zones of Florida's Coral Reef
At the inshore reefs, strong seasonal variability in carbonate chemistry and increasing TA mitigated or obscured acidification trends
Higher pH and aragonite saturation states occur in the Upper and Middle Keys, which could favor reef habitat persistence in these regions
Coral reef cryptofauna, which live hidden within reef framework structures, are considered to be the most diverse group of coral reef metazoans. They likely comprise more biomass than all surface ...fauna, providing food sources for fishes and playing important roles as predators, herbivores, detritivores, filter feeders, and scavengers. In an era of global change, it is necessary to determine how these communities are structured across reef habitats as well as to understand how reef framework degradation will impact the cryptofauna and, by extension, ecosystem function. Artificial reef framework units were constructed from coral rubble to approximate framework substrates. Forty replicates were subjected to treatments of differing porosity, flow, and coral cover in a fully crossed ANOVA design. After 2 moin situ, all motile cryptofauna (>2 mm) were counted, weighed, and identified to the lowest possible level. A total of 11 309 specimens were collected, comprising >121 species from 6 separate phyla. Cryptofaunal abundances and biomass were higher in low-porosity crypts and biomass was greater in slow-flow environments, highlighting the importance of sheltered low-porosity habitats, such as back-reef rubble plains. The presence of live coral was not found to have a significant effect on the motile cryptofauna occupying the dead coral framework below it, suggesting a high degree of resilience in how framework-dwelling fauna respond to coral mortality. These data support the assertion that artificial reefs are capable of facilitating the accumulation of a diverse cryptic community, independent of live coral, provided they contain suitably porous crypts.
Naturally acidified environments, such as those caused by volcanic CO
2
venting, reveal how complex coral reef ecosystems may respond to future ocean acidification conditions. Few of these sites have ...been described worldwide, and only a single such site is known from the Caribbean. Herein, we have characterized an area of volcanic acidification at Mayreau Island, St. Vincent and the Grenadines. Despite localized CO
2
enrichment and gas venting, the surrounding area has high hard and soft coral cover, as well as extensive carbonate frameworks. Twice daily extremes in acidification, in some cases leading to undersaturation of aragonite, are correlated with tidal fluctuations and are likely related to water flow. Corals persisting despite this periodic acidification can provide insights into mechanisms of resilience and the importance of natural pH variability on coral reefs.
The effects of light and elevated
p
CO
2
on the growth and photochemical efficiency of the critically endangered staghorn coral,
Acropora cervicornis
, were examined experimentally. Corals were ...subjected to high and low treatments of CO
2
and light in a fully crossed design and monitored using 3D scanning and buoyant weight methodologies. Calcification rates, linear extension, as well as colony surface area and volume of
A. cervicornis
were highly dependent on light intensity. At
p
CO
2
levels projected to occur by the end of the century from ocean acidification (OA),
A. cervicornis
exhibited depressed calcification, but no change in linear extension. Photochemical efficiency (
F
v
/
F
m
) was higher at low light, but unaffected by CO
2
. Amelioration of OA-depressed calcification under high-light treatments was not observed, and we suggest that the high-light intensity necessary to reach saturation of photosynthesis and calcification in
A. cervicornis
may limit the effectiveness of this potentially protective mechanism in this species. High CO
2
causes depressed skeletal density, but not linear extension, illustrating that the measurement of extension by itself is inadequate to detect CO
2
impacts. The skeletal integrity of
A. cervicornis
will be impaired by OA, which may further reduce the resilience of the already diminished populations of this endangered species.
The reproductive ecology of
Tubastraea coccinea
Lesson, an azooxanthellate tropical scleractinian coral, was studied over various periods from 1985 to 2006 at four principal eastern Pacific locations ...in Costa Rica, Panamá, and the Galápagos Islands (Ecuador). This small (polyp diameter 0.8–1.0 cm), relatively cryptic species produced ova and planulae year round, including colonies with as few as 2–10 polyps. Of 424 colonies examined histologically, 13.7% contained both ova and sperm. Mature ova varied in diameter from ∼300 to 800 μm and the time from spawning and fertilization of oocytes to release of brooded planulae was about 6 weeks. Planulae were 0.5–1.5 mm long and they settled and metamorphosed on a variety of substrates after 1–3 days. Spermaries, though more difficult to distinguish in histological sections, were present throughout the year. Spent spermaries were never observed in sections, but several colonies in Panamá and the Galápagos Islands released sperm from night one to night five after full moon, indicating the potential for cross-fertilization among colonies. Planula release was observed at Uva Island (Panamá) in March, May, June, and July, and in general planula presence was higher at warm ocean temperatures at all sites, whether or not the sites were influenced by seasonal upwelling. Annual fecundity estimates for
T. coccinea
are comparable with other high fecundity brooding species, including the zooxanthellate
Porites panamensis
, with which it co-occurs in Panamá.
Tubastraea coccinea
is widely distributed in the tropical Indo-Pacific and has colonized substrates in the western Atlantic. In addition to the reproductive characteristics described in the present study, other features of the biology of
T. coccinea
, such as an ability to withstand conditions that produce bleaching and mortality in zooxanthellate species, may account for its widespread, low-latitude distribution in multiple oceans.
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