Fore Reef Research Articles

Chronic low-level nutrient enrichment benefits coral thermal performance in a fore reef habitat

Chronic low-level nutrient enrichment benefits coral thermal performance in a fore reef habitat

Response diversity in corals: hidden differences in bleaching mortality among cryptic Pocillopora species.

Variation among functionally similar species in their response to environmental stress buffers ecosystems from changing states. Functionally similar species may often be cryptic species representing evolutionarily distinct genetic lineages that are morphologically indistinguishable. However, the extent to which cryptic species differ in their response to stress, and could therefore provide a source of response diversity, remains unclear because they are often not identified or are assumed to be ecologically equivalent. Here, we uncover differences in the bleaching response between sympatric cryptic species of the common Indo‐Pacific coral, Pocillopora. In April 2019, prolonged ocean heating occurred at Moorea, French Polynesia. 72% of pocilloporid colonies bleached after 22 d of severe heating (>8oC‐days) at 10 m depth on the north shore fore reef. Colony mortality ranged from 11% to 42% around the island four months after heating subsided. The majority (86%) of pocilloporids that died from bleaching belonged to a single haplotype, despite twelve haplotypes, representing at least five species, being sampled. Mitochondrial (open reading frame) sequence variation was greater between the haplotypes that experienced mortality versus haplotypes that all survived than it was between nominal species that all survived. Colonies > 30 cm in diameter were identified as the haplotype experiencing the most mortality, and in 1125 colonies that were not genetically identified, bleaching and mortality increased with colony size. Mortality did not increase with colony size within the haplotype suffering the highest mortality, suggesting that size‐dependent bleaching and mortality at the genus level was caused instead by differences among cryptic species. The relative abundance of haplotypes shifted between February and August, driven by declines in the same common haplotype for which mortality was estimated directly, at sites where heat accumulation was greatest, and where larger colony sizes occurred. The identification of morphologically indistinguishable species that differ in their response to thermal stress, but share a similar ecological function in terms of maintaining a coral‐dominated state, has important consequences for uncovering response diversity that drives resilience, especially in systems with low or declining functional diversity.

Niche differences in co-occurring cryptic coral species (Pocillopora spp.)

Cryptic species that are morphologically similar co-occur because either the rate of competitive exclusion is very slow, or because they are not, in fact, ecologically similar. The processes that maintain cryptic local diversity may, therefore, be particularly subtle and difficult to identify. Here, we uncover differences among several cryptic species in their relative abundance across a depth gradient within a dominant and ecologically important genus of hard coral, Pocillopora. From extensive sampling unbiased toward morphological characters, at multiple depths on the fore reef around the island of Mo’orea, French Polynesia, we genetically identified 673 colonies in the Pocillopora species complex. We identified 14 mitochondrial Open Reading Frame haplotypes (mtORFs, a well-studied and informative species marker used for pocilloporids), which included at least six nominal species, and uncovered differences among haplotypes in their relative abundance at 5, 10, and 20 m at four sites around the island. Differences in relative haplotype abundance across depths were greater than differences among sites separated by several kilometers. The four most abundant species are often visibly indistinguishable at the gross colony level, yet they exhibited stark differences in their associations with light irradiance and daily water temperature variance. The pattern of community composition was associated with frequent cooling in deeper versus shallower water more than warmer temperatures in shallow water. Our results indicate that these cryptic species are not all ecologically similar. The differential abundance of Pocillopora cryptic species across depth should promote their coexistence at the reef scale, as well as promote resilience through response diversity.

Coral Reef Restorations Can Be Optimized to Reduce Coastal Flooding Hazards

Coral reefs are effective natural coastal flood barriers that protect adjacent communities. Coral degradation compromises the coastal protection value of reefs while also reducing their other ecosystem services, making them a target for restoration. Here we provide a physics-based evaluation of how coral restoration can reduce coastal flooding for various types of reefs. Wave-driven flooding reduction is greatest for broader, shallower restorations on the upper fore reef and between the middle of the reef flat and the shoreline than for deeper locations on the fore reef or at the reef crest. These results indicate that to increase the coastal hazard risk reduction potential of reef restoration, more physically robust species of coral need to be outplanted to shallower, more energetic locations than more fragile, faster-growing species primarily being grown in coral nurseries. The optimization and quantification of coral reef restoration efforts to reduce coastal flooding may open hazard risk reduction funding for conservation purposes.

Fore reef location influences spawning success and egg predation in lek-like mating territories of the bird wrasse, Gomphosus varius

Many fish spawn in aggregations, but little is understood about the dynamics governing the success of spawning interactions. Here, we evaluate the influence that location of lek-like mating territories has on spawning interactions of Gomphosus varius. We used direct observations of spawning and egg predation events as well as local population counts to compare the rates of spawning, spawning interruptions, and predation on the eggs of G. varius at Finger Reef, Apra Harbor, Guam. We hypothesized that spawning rates would be highest among seaward locations that facilitate transport of pelagic larvae from reefs and that those territories would subsequently experience higher densities of egg predators, egg predation rates, and spawning interruptions. Male spawning success was highly skewed by mating territory location, with holders of the outer, seaward mating territories being more successful than those males holding territories in the middle and inner areas of the aggregation site. Within the outer territories, male mating success was also skewed by location. Egg predation was observed occasionally and increased linearly with bird wrasse spawning frequency. The population densities of egg predators were distributed equally across the study area. Spawning interruptions occurred most frequently within the inner zone of the spawning aggregation due to greater male-male aggression in intraspecific competition for females and territories. This study provides evidence that reef location influences the spawning success, egg predation rates, and spawning interruption rates of fishes that reproduce using lek-like mating territories.

The Sedimentology of the Late Campanian–Maastrichtian Sequence, Southwestern Iraq

Petrography, diagenesis, and facies analyses as well as the depositional environments of the late Campanian-Maastrichtian sequence in southwestern Iraq are studied in five keyholes. The sequence incorporates parts of the Hartha, Shiranish and Tayarat Formations. The Hartha Formation comprises creamy and organodetrital dolomite, grey dolomitic marl, and evaporites. The Shiranish Formation is composed of grey marl and claystone, whereas the Tayarat Formation is composed of grey ash, along with tough and fossiliferous dolomitic limestone inter-bedded with grey mudstone layers and/or wisps. Several diagenetic processes affected the sequence, such as neomorphic replacement, dissolution, dolomitization, and sulphate development. Some of these processes obliterated the primary textures.
 The late Campanian-Maastrichtian sequence consists of three microfacies (Dolomitic Intraclastic Limestone, Dolomitized Biomicrite, and Biomicrosparite Microfacies) and two lithofacies (Mudrock and Sulphates-Rock Lithofacies), in addition to Fine- to Medium-Crystalline Dolomite Lithotype.
 The Hartha Formation is evaporitic, possibly with supratidal sabkha deposits. The overlying Tayarat and Shiranish Formations reflect deposition in a warm tropical to subtropical reefal and open marine conditions, as deduced from faunal assemblages. Some effects of deep marine condition are evident by the presence of Shiranish facies. The sequence represents deposition in the central reef- fore reef area. The absence of isolated back-reef lagoon facies suggests that the reef was patchy without isolation of water in the middle shelf region. However, at the top of the sequence, i.e. at the end of the Cretaceous Period, restricted lagoons seem to have dominated the studied succession.

Fish predation on corals promotes the dispersal of coral symbionts

BackgroundThe microbiomes of foundation (habitat-forming) species such as corals and sponges underpin the biodiversity, productivity, and stability of ecosystems. Consumers shape communities of foundation species through trophic interactions, but the role of consumers in dispersing the microbiomes of such species is rarely examined. For example, stony corals rely on a nutritional symbiosis with single-celled endosymbiotic dinoflagellates (family Symbiodiniaceae) to construct reefs. Most corals acquire Symbiodiniaceae from the environment, but the processes that make Symbiodiniaceae available for uptake are not resolved. Here, we provide the first comprehensive, reef-scale demonstration that predation by diverse coral-eating (corallivorous) fish species promotes the dispersal of Symbiodiniaceae, based on symbiont cell densities and community compositions from the feces of four obligate corallivores, three facultative corallivores, two grazer/detritivores as well as samples of reef sediment and water.ResultsObligate corallivore feces are environmental hotspots of Symbiodiniaceae cells: live symbiont cell concentrations in such feces are 5–7 orders of magnitude higher than sediment and water environmental reservoirs. Symbiodiniaceae community compositions in the feces of obligate corallivores are similar to those in two locally abundant coral genera (Pocillopora and Porites), but differ from Symbiodiniaceae communities in the feces of facultative corallivores and grazer/detritivores as well as sediment and water. Combining our data on live Symbiodiniaceae cell densities in feces with in situ observations of fish, we estimate that some obligate corallivorous fish species release over 100 million Symbiodiniaceae cells per 100 m2 of reef per day. Released corallivore feces came in direct contact with coral colonies in the fore reef zone following 91% of observed egestion events, providing a potential mechanism for the transfer of live Symbiodiniaceae cells among coral colonies.ConclusionsTaken together, our findings show that fish predation on corals may support the maintenance of coral cover on reefs in an unexpected way: through the dispersal of beneficial coral symbionts in corallivore feces. Few studies examine the processes that make symbionts available to foundation species, or how environmental reservoirs of such symbionts are replenished. This work sets the stage for parallel studies of consumer-mediated microbiome dispersal and assembly in other sessile, habitat-forming species.

Ecological consequences of Stony Coral Tissue Loss Disease in the Turks and Caicos Islands

Coral reefs are suffering global declines due to climate change, natural disasters, pollution, and diseases. Coral disease events have increased in frequency and severity in the past several decades and have nearly wiped out populations of some Caribbean coral species. Stony coral tissue loss disease (SCTLD) is a novel white plague that infects many species, is highly contagious, and causes rapid mortality. SCTLD was first documented in the Turks and Caicos Islands in 2019. This study described the trends in coral community composition, cover, richness, and diversity from 2012 to 2020 on the South Caicos fore reef, to contextualize the impact of the ongoing disease outbreak in 2020. Monitoring sites were assessed using photo quadrats along permanent transects and CPCe for analysis. Coral cover, richness, and diversity remained consistent from 2012 to 2018. Coral cover was significantly lower in 2020 after the outbreak (1.14% ± 0.65) than in any other year (2.97% ± 1.12%, mean 2012–2018), a 62% loss. Richness reached its lowest point and was significantly lower in 2020 (6.33 ± 1.37 taxa) than in all other years except for 2013 and 2016, and diversity reached its lowest point in 2020 as well. Coral community composition differed by year and particularly in 2020, with significant differences between 2013/2014 and 2020. In 2020, rare and susceptible taxa Dichocoenia spp., Eusmilia fastigiata, and Meandrina meandrites were absent from all surveys for the first time. Trends in relative cover of “weedy” coral taxa varied, with Agaricia spp. reaching its lowest relative cover (24.2%) in 2020 and Porites astreoides reaching its highest (18.9%). Ecological shifts associated with SCTLD in South Caicos are the first reported in the Turks and Caicos Islands and support other studies in the Caribbean which demonstrate that SCTLD significantly deteriorates reef composition.

Stratigraphic development of a Late Oligocene Reef Complex under strong fluviatile influence in the Tertiary Piedmont Basin (Liguria, NWItaly)

The modern-day decline of coral reefs due to bleaching events has been recognized as one of the major consequences of man-driven climate change. However, also eutrophication has been highlighted as an equally great danger for coral reefs and as such for biodiversity hotspots. In the latest years this phenomenon has moved to the forefront in the scientific community. Fossil reefs play a key role in studying the emergence, development and faunal/floral diversity of reef environments under eutrophic conditions. Their importance as valuable data sources for studying long-term changes of coral reef environments and their resilience cannot be disputed, especially since they may record the complete life cycle of a reef complex. In this study, nine sections nearby the town of Dego (Savona Province, NW Italy) are presented and discussed with regards to their lithostratigraphic and paleontological contents. Due to the extensive amount of data, the original morphology of a fringing reef, consisting of core, flank and fore reef, under strong fluviatile influence could be reconstructed. This study emphasizes the importance of the coralline red algae association in such biocarbonatic build-ups as major constituent and as substrate stabilizers. The sections record the original colonization event of the local basement by the builder community, the emergence of the coral reef and finally the suffocation by the fluviatile sediments. The variation of the red algae association reflects a deepening trend and is possibly correlated to enhanced fluvial input, which tends to deteriorate ecological conditions and functions as a major trigger for initial reef suffocation.

High resolution spatiotemporal patterns of seawater temperatures across the Belize Mesoamerican Barrier Reef

Coral reefs are under increasingly severe threat from climate change and other anthropogenic stressors. Anomalously high seawater temperatures in particular are known to cause coral bleaching (loss of algal symbionts in the family Symbiodiniaceae), which frequently leads to coral mortality. Remote sensing of sea surface temperature (SST) has served as an invaluable tool for monitoring physical conditions that can lead to bleaching events over relatively large scales (e.g. few kms to 100 s of kms). But, it is also well known that seawater temperatures within a site can vary significantly across depths due to the combined influence of solar heating of surface waters, water column thermal stratification, and cooling from internal waves and upwelling. We deployed small autonomous benthic temperature sensors at depths ranging from 0–40 m in fore reef, back reef, and lagoonal reef habitats on the Belize Mesoamerican Barrier Reef System from 2000–2019. These data can be used to calculate depth-specific climatologies across reef depths and sites, and emphasize the dynamic and spatially-variable nature of coral reef physical environments.

Influence of Freshwater Discharges on Biogeochemistry and Benthic Communities of a Coral Reef Ecosystem (La Réunion Island, Indian Ocean)

We investigated the influence of freshwater discharges on biogeochemistry and benthic communities of La Saline fringing reef, the largest coral reef system of La Reunion Island (France, Indian Ocean). Sampling was conducted in February-March 2012 along a land-ocean continuum, including rivers, groundwater, reef waters, and oceanic waters. Our results indicated the discharges of basalt groundwater (enriched in nitrates) in the southern part of the reef, and the discharges of sand groundwater and river waters (enriched in organic matter) in sites located northward. These discharges mainly impacted fore reef surface waters, while groundwater resurgences were more diffuse and not clearly detected in depth. We highlighted the relationship Si(OH) 4 /NO 3 − to identify basalt groundwater discharges. We also put forward the relationships humic-like fluorescence/NH 4 + and tryptophan-like fluorescence/PO 4 3− to identify sand groundwater discharges and wastewater pollutions, respectively. We observed the eutrophication of the southern part of the reef through higher phytoplankton biomasses, a massive presence of fleshy algal formations and a low coral coverage. The spatial homogeneity of the benthic community structures onto the entire slope contrasted with the spatial heterogeneity of chemical variables in water. This reflected the different spatial and temporal scales involved in biological and chemical processes in coral reef ecosystems.

Modelling three-dimensional flow over spur-and-groove morphology

Spur-and-groove (SAG) morphology characterizes the fore reef of many coral reefs worldwide. Although the existence and geometrical properties of SAG have been well documented, an understanding of the hydrodynamics over them is limited. Here, the three-dimensional flow patterns over SAG formations, and a sensitivity of those patterns to waves, currents, and SAG geometry were characterized using the physics-based Delft3D-FLOW and SWAN models. Shore-normal shoaling waves over SAG formations were shown to drive two circulation cells: a cell on the lower fore reef with offshore flow over the spurs and onshore flow over the grooves, except near the seabed where velocities were always onshore, and a cell on the upper fore reef with offshore surface velocities and onshore bottom currents, which result in depth-averaged onshore and offshore flow over the spurs and grooves, respectively. The mechanism driving this flow results from the net of the radiation stress gradients and pressure gradient, which is balanced by the Reynolds stress gradients and bottom friction that differ over the spur and over the groove. Waves were the primary driver of variations in modelled flow over SAG, with the flow strength increasing for increasing wave heights and periods. Spur height, SAG wavelength, and the water depth at peak spur height were the dominant influences on the hydrodynamics, with spur heights directly proportional to the strength of SAG circulation cells. SAG formations with shorter SAG wavelengths only presented one circulation cell on the shallower portion of the reef, as opposed to the two circulation cells for longer SAG wavelengths. SAG formations with peak spur heights occurring in shallower water had stronger circulation than those with peak spur heights occurring in deeper water. These hydrodynamic patterns also likely affect coral and reef development through sediment and nutrient fluxes.

Coral Microbiomes Demonstrate Flexibility and Resilience Through a Reduction in Community Diversity Following a Thermal Stress Event

Thermal stress increases community diversity, community variability, and the abundance of potentially pathogenic microbial taxa in the coral microbiome. Nutrient pollution such as excess nitrogen can also interact with thermal stress to exacerbate host fitness degradation. However, it is unclear how different forms of nitrogen (nitrate vs. ammonium/urea) interact with bleaching-level temperature stress to drive changes in coral microbiomes, especially on reefs with histories of resilience. We used a 13-month field experiment spanning a thermal stress event in the Austral summer of 2016 on the oligotrophic fore reef of Mo’orea, French Polynesia to test how different forms of nitrogen (nitrate vs. urea) impact the resistance and resilience of coral microbiomes. For Acropora, Pocillopora, and Porites corals, we found no significant differences in diversity metrics between control, nitrate- and urea-treated corals during thermal stress. In fact, thermal stress may have overwhelmed any effects of nitrogen. Although all three coral hosts were dominated by the bacterial clade Endozoicomonas which is a proposed beneficial coral symbiont, each host differed through time in patterns of community diversity and variability. These differences between hosts may reflect different strategies for restructuring or maintaining microbiome composition to cope with environmental stress. Contrary to our expectation, post-stress microbiomes did not return to pre-stress community composition, but rather were less diverse and increasingly dominated by Endozoicomonas. The dominance of Endozoicomonas in microbiomes 10 months after peak sea surface temperatures may suggest its ability to utilize host metabolic products of thermal stress for a sustained competitive advantage against other microbial members. If Endozoicomonas is a beneficial coral symbiont, its proliferation after warm summer months could provide evidence of its ability to mitigate coral holobiont dysbiosis to thermal stress and of resilience in coral microbiomes.

Sea Level Rise Will Drive Divergent Sediment Transport Patterns on Fore Reefs and Reef Flats, Potentially Causing Erosion on Atoll Islands

AbstractAtoll reef islands primarily consist of unconsolidated sediment, and their ocean‐facing shorelines are maintained by sediment produced and transported across their reefs. Changes in incident waves can alter cross‐shore sediment exchange and, thus, affect the sediment budget and morphology of atoll reef islands. Here we investigate the influence of sea level rise and projected wave climate change on wave characteristics and cross‐shore sediment transport across an atoll reef at Kwajalein Island, Republic of the Marshall Islands. Using a phase‐resolving model, we quantify the influence on sediment transport of quantities not well captured by wave‐averaged models, namely, wave asymmetry and skewness and flow acceleration. Model results suggest that for current reef geometry, sea level, and wave climate, potential bedload transport is directed onshore, decreases from the fore reef to the beach, and is sensitive to the influence of flow acceleration. We find that a projected 12% decrease in annual wave energy by 2100 CE has negligible influence on reef flat hydrodynamics. However, 0.5–2.0 m of sea level rise increases wave heights, skewness, and shear stress on the reef flat and decreases wave skewness and shear stress on the fore reef. These hydrodynamic changes decrease potential sediment inputs onshore from the fore reef where coral production is greatest but increase potential cross‐reef sediment transport from the outer reef flat to the beach. Assuming sediment production on the fore reef remains constant or decreases due to increasing ocean temperatures and acidification, these processes have the potential to decrease net sediment delivery to atoll islands, causing erosion.

Dugong (Dugong dugon) movements and habitat use in a coral reef lagoonal ecosystem

Little is known about how the Vulnerable dugong Dugong dugon uses coral reef lagoons despite the importance of these habitats throughout much of its vast range. We used GPS satellite tracking systems to explore the space use of 12 dugongs at 3 locations in the coral reef lagoons of the main island of New Caledonia in the southwest Pacific: Cap Goulvain, Ouano and Nouméa. The movements of the tracked dugongs varied among individuals and all except one animal undertook large-scale movements (>15 km; mean [±SE] 37.7 ± 5.2 km) from their capture location (maximum waterway distance range: 13.8 to 72.9 km). The straight-line distances between the furthest GPS locations during each animal’s tracking period ranged from 21.3 to 74.5 km. We identified areas used intensively by dugongs in all 3 study areas, some of which were areas where seagrass presence has not been verified, or where dugongs have not been observed during past aerial surveys. Dugongs spent most of their tracking time within the lagoons, with 99.4% of GPS locations found inside the barrier reef. Nonetheless, where the lagoon was narrow and confined, 3 tracked dugongs used the fore reef shelf outside the barrier reef in the open ocean to commute between bays. Our findings can inform conservation and management initiatives in New Caledonia as well as other countries within the dugong’s range which have similar habitat geomorphology but where dugongs occur in numbers too low to be tracked and are considered Critically Endangered.

Enhanced and fine seismic recognition of organic reefs: a case study of the Changxing Formation, eastern Sichuan, China

Organic reefs are developed in the Changxing Formation (Fm.) in the southeastern end of the Kaijiang-Liangping ocean trough, eastern Sichuan. Due to the ambiguous seismic reflection characteristics of organic reefs, application of organic reef recognition methods that have been successfully employed in other areas is limited in the study area, but these methods can still indicate development of favorable facies zones. Accordingly, supplemented by seismic reflection characteristics analysis, seismic attributes analysis and time-difference analysis, frequency decomposition and reconstruction interpretation technique, high-frequency uncompressing method, and dominant azimuth lightening method were optimized to be the core technologies of enhanced and fine depiction technique system of organic reefs. The technique system transforms seismic data from the time and reflection angle domain to the frequency and azimuth-offset domain; increases the differences in seismic reflection characteristics in and out of organic reefs, especially patch reefs of small volume and low recognition; enhances seismic reflection characteristics of both the outline especially the lateral boundary and interior of organic reefs; and achieves precisely depicting the boundary of organic reefs and fine recognition of the internal structure of platform margin facies reefs such as reef core, fore reef, and back reef. This technique system provides a new technical idea for organic reef recognition, has produced optimistic recognition effects in the study area, and is of certain guidance significant to the research into recognition of organic reefs with ambiguous seismic reflection characteristics.

Spatial distribution of microplastics around an inhabited coral island in the Maldives, Indian Ocean

Small plastic particles are considered environmental pollutants and are highly concentrated in marine sediments. However, knowledge about plastic abundance within coral reef habitat and beach sediments surrounding remote inhabited coral islands is scarce. In this study, microplastic accumulation was investigated on a small inhabited coral island located in the Maldives. Sediments from 22 sampling sites across fore reef, reef flat, and beach environments were analysed for plastic particles <5 mm. Density separation and microscope enumeration revealed a total of 1244 individual microplastic pieces, in filamentous (49%) and fragmented (51%) forms, found across all sampling sites. High concentrations were recorded at all sites, however, there was no significant relationship between microplastic concentration or size across regions (inner atoll and outer atoll) or environments (fore reef, reef flat, and beach). Furthermore, concentrations of microplastic fragment and filament forms, total concentration, and the microplastic community, showed little correlation with sediment particle size. Our findings show microplastics are ubiquitous in marine sediments around a remote coral island, at sizes ingestible by marine organisms, raising concerns about potential effects of microplastic ingestion by coral reef species.

Stable nitrogen isotopes in octocorals as an indicator of water quality decline from the northwestern region of Cuba.

Eutrophication is one of the causes of the degradation of reefs worldwide. The aim of this research is to determine if sewage discharge reaches the fore reefs at northwest of Cuba using δ15N in tissues of the octocorals Eunicea flexuosa and Plexaura kuekenthali and the concentration of microbiological and physical-chemical variables. Thirteen reefs at 10-m depth were selected near river basins and far from the urban and industrial development of Havana City. Branch tips of both species were collected, the concentrations of nutrient and microorganisms in water samples were quantified, and horizontal visibility in the water (Vis) was determined. Overall, δ15N of E. flexuosa ranged from 1.5 to 6.3‰ and P. kuekenthali from 1.7 to 6.7‰. The tissue of both species was significantly enriched in 15N in reefs near polluted watersheds compared with reefs far from pollution by anthropogenic activities. The δ15N of both species showed a positive and significant correlation with the concentration of fecal and total coliform bacteria, heterotrophic bacteria, and NH4+ and a negative and significant correlation with the Vis. The δ15N of the two species and microbiological and physical-chemical variables evidenced water quality decline by sewage discharge that reached reefs near polluted watersheds.

Trends and variability in spectral diffuse attenuation of coral reef waters

Knowledge of water clarity is an essential component of coral reef ecology. While qualitative trends are well known, there are currently few published records of spectral—varying as a (relatively) continuous function of wavelength—water optical properties for these systems. The purpose of this study is to quantify trends in the range and variability of the spectral diffuse attenuation coefficient (Kd) in coral reef and adjacent waters. Using a Biospherical PRR-800, 199 vertical profiles of downwelling spectral irradiance were collected across the reefs and nearby optically deepwaters of Bermuda and Hawaii. A single spectral Kd was calculated for each profile. Results reveal water types ranging from clear oceanic to turbid coastal, with Bermuda and Hawaii showing similar patterns. Kd roughly correlates with reef geomorphic zonation, consistent with well-known reef water characteristics: (a) Suspended sediments are ubiquitous on coral reef flats and in lagoons, chiefly comprising calcium carbonate, which is effectively spectrally flat and has the effect of increasing the overall magnitude of Kd; (b) reefs generate large amounts of dissolved organic matter, which becomes apparent in some lagoonal Kd; and (c) the Kd data exhibit little indication of chlorophyll, which is typically very low in the water column above reefs. These patterns appear reproducible when compared with prior data from French Polynesia. The depth of the 1% light level varies accordingly, approximately 30 m, 45 m, 75 m, and 105 m for lagoon, reef flat, fore reef, and offshore, respectively. The data presented here provide some insight into trends in the spatial context of reef geomorphology and lay the foundation for a more quantitative understanding of reef water clarity and color and their importance to reef ecology.

Prior exposure to elevated pCO2 does not affect calcification of a tropical scleractinian when returned to ambient pCO2

Coral reefs experience biologically-driven pCO2 oscillations that are predicted to become more extreme in magnitude and duration under ocean acidification (OA) regimes. Understanding the plasticity of responses in common reef-building corals to oscillations in pCO2 will allow for better predictions of their function in future seawater conditions. This study explored the effects of variation in seawater pCO2 on coral calcification using experiments conducted over one month between 9 April 2018 and 18 May 2018. Branches (~4-cm long) of Acropora retusa were sampled from colonies at 10-m depth on the fore reef of Mo'orea, French Polynesia (17° 28′ 53.9004" S, 149° 49′ 50.5992" W). We tested the hypothesis that depressed calcification caused by elevated pCO2 (~1000 μatm) is relaxed (i.e., calcification increases) upon return to ambient pCO2 (~400 μatm). Corals first were incubated in ambient or elevated pCO2 for 19 days, with the result that calcification integrated over this period was reduced by 31% under elevated pCO2. The same corals were then incubated at ambient pCO2 for 11 days, during which calcification was independent of the experimental pCO2 exposure history. Our results suggest that a quick relaxation of pCO2-depressed calcification in A. retusa following cessation of high pCO2 indicates that corals are capable of a reversible plastic response of calcification when confronted by pCO2 oscillations.