Coral growth patterns result from an interplay of coral biology and environmental conditions. In this study colony size and proportion of live and dead skeletons in the cold-water coral (CWC) Lophelia pertusa(Linnaeus, 1758) were measured using video footage from Remotely Operated Vehicle (ROV) transects conducted at the inshore Mingulay Reef Complex (MRC) and at the offshore PISCES site (Rockall Bank) in the NE Atlantic. The main goal of this paper was to explore the development of a simple method to quantify coral growth and its potential application as an assessment tool of the health of these remote habitats. Eighteen colonies were selected and whole colony and dead/living layer size were measured. Live to dead layer ratios for each colony were then determined and analysed. The age of each colony was estimated using previously published data. Our paper shows that: (1) two distinct morphotypes can be described: at the MRC, colonies displayed a ‘cauliflower-shaped’ morphotype whereas at the PISCES site, colonies presented a more flattened ‘bush-shaped’ morphotype; (2) living layer size was positively correlated with whole colony size; (3) live to dead layer ratio was negatively correlated to whole colony size; (4) live to dead layer ratio never exceeded 0.27. These results suggest that as a colony develops and its growth rate slows down, the proportion of living polyps in the colony decreases. Furthermore, at least 73% of L. pertusa colonies are composed of exposed dead coral skeleton, vulnerable to ocean acidification and the associated shallowing of the aragonite saturation horizon, with significant implications for future deep-sea reef framework integrity. The clear visual contrast between white/pale living and grey/dark dead portions of the colonies also gives a new way by which they can be visually monitored over time. The increased use of marine autonomous survey vehicles offers an important new platform from which such a surveying technique could be applied to monitor deep-water marine protected areas in the future.
Mass bleaching associated with unusually high sea temperatures represents one of the greatest threats to corals and coral reef ecosystems. Deeper reef areas are hypothesized as potential refugia, but the susceptibility of Scleractinian species over depth has not been quantified. During the most severe bleaching event on record, we found up to 83% of coral cover severely affected on Maldivian reefs at a depth of 3–5 m, but significantly reduced effects at 24–30 m. Analysis of 153 species' responses showed depth, shading and species identity had strong, significant effects on susceptibility. Overall, 73.3% of the shallow-reef assemblage had individuals at a depth of 24–30 m with reduced effects, potentially mitigating local extinction and providing a source of recruits for population recovery. Although susceptibility was phylogenetically constrained, species-level effects caused most lineages to contain some partially resistant species. Many genera showed wide variation between species, including Acropora, previously considered highly susceptible. Extinction risk estimates showed species and lineages of concern and those likely to dominate following repeated events. Our results show that deeper reef areas provide refuge for a large proportion of Scleractinian species during severe bleaching events and that the deepest occurring individuals of each population have the greatest potential to survive and drive reef recovery.
Shading substantially reduced the degree of bleaching in Acropora muricata, Pocillopora damicornis and Porites cylindrica in American Samoa. Experiments were conducted outdoors at two sites on Ofu and Tutuila Islands. An aquarium experiment was set up near some reef-flat pools in the National Park of American Samoa on Ofu Island, using different levels of shading (none, 50% and 75%) early in conditions of cumulative thermal stress corresponding to NOAA's Coral Reef Watch-Bleaching Alert System. We analyzed the effects of cumulative thermal stress regarding coral growth, as well as color changes (evaluated using a standardize reference card) as a proxy for decreases in symbiontdensity and chlorophyll a content (i.e. bleaching). Thermally stressed corals grew less than controls, but corals without shading experienced a more substantial decrease in growth compared to those under 50% or 75% shade. The analysis of coral color showed that both levels of shading were protective against bleaching in conditions of cumulative thermal stress for all species, but were particularly beneficial for the most sensitive ones: A. muricata and P. cylindrica. Heavier shading (75%) offered better protection than lighter shading (50%) in this experiment, possibly because of the intense light levels corals were subjected to. Although there were limits to the extent shading could mitigate the effects of cumulative heating, it was very effective to at least Degree Heating Week (DHW) 4 and continued to offer some protection until the end of the study (DHW 8). In Tutuila, a shaded/not-shaded platform experiment was carried out in a reef pool in which corals have shown repeated annual summer bleaching for several years. This experiment was designed to investigate if shading could attenuate bleaching in the field and also if there were negative consequences to shading removal. The only factor controlled was light intensity, and our main conclusion was that overall corals on the platform became darker than field colonies in response to shading, but adjusted back to the same color level as field colonies after shade removal. However, the latter results are preliminary and need to be confirmed by future studies under more controlled conditions. As bleaching becomes more frequent and regular due to global warming, we should consider proactively using shading to help mitigate the effects of thermal stress and prolong the survival of at least some coral communities, until solutions to address global climate change become effective.
Recreational diving damages coral reefs despite heightened environmental awareness. However, divers prefer preserved coral reefs and therefore reef degradation presents an economic loss. Artificial reefs were suggested among a range of tools to mitigate and reduce divers' negative impact on coral reefs.
Coral reefs in Eilat (northern tip of the Red Sea) are among the most densely dived reefs in the world, with an estimated number of dives of up to 350,000 dives a year. At least 7 artificial reefs were deployed in the coastal waters of Eilat, however the divers' visitation on these reefs is not tracked regularly.
We found that more than one third of the total dives take place on artificial reefs in Eilat. The divers prefer to vary their diving sites and possess a desire to diversify and expand their diving experience. Thus, the divers are also willing to dive a on artificial reefs, and this is true for both novice and experienced divers. This indicates that artificial reefs can draw divers from natural reefs, thus reducing diving pressure and allowing more sustainable levels of diving on natural coral reefs. This leads us to a conclusion that artificial reefs may be useful in modern reef conservation approaches.
Predicting the impact of sea-level (SL) rise on coral reefs requires reliable models of reef accretion. Most assume that accretion results from vertical growth of coralgal framework, but recent studies show that reefs exposed to hurricanes consist of layers of coral gravel rather than in-place corals. New models are therefore needed to account for hurricane impact on reef accretion over geological timescales. To investigate this geological impact, we report the configuration and development of a 4-km-long fringing reef at Punta Maroma along the northeast Yucatan Peninsula. Satellite-derived bathymetry (SDB) shows the crest is set-back a uniform distance of 315 ±15 m from a mid-shelf slope break, and the reef-front decreases 50% in width and depth along its length. A 12-core drill transect constrained by multiple 230Th ages shows the reef is composed of an ~2-m thick layer of coral clasts that has retrograded 100 m over its back-reef during the last 5.5 ka. These findings are consistent with a hurricane-control model of reef development where large waves trip and break over the mid-shelf slope break, triggering rapid energy dissipation and thus limiting how far upslope individual waves can fragment corals and transport clasts. As SL rises and water depth increases, energy dissipation during wave-breaking is reduced, extending the clast-transport limit, thus leading to reef retrogradation. This hurricane model may be applicable to a large sub-set of fringing reefs in the tropical Western-Atlantic necessitating a reappraisal of their accretion rates and response to future SL rise.
As coral reefs continue to decline worldwide, it becomes ever more necessary to understand the connectivity between coral populations to develop efficient management strategies facilitating survival and adaptation of coral reefs in the future. Orbicella faveolata is one of the most important reef-building corals in the Caribbean and has recently experienced severe population reductions. Here, we utilize a panel of nine microsatellite loci to evaluate the genetic structure of O. faveolata and to infer connectivity across ten sites spanning the wider Caribbean region. Populations are generally well-mixed throughout the basin (FST = 0.038), although notable patterns of substructure arise at local and regional scales. Eastern and western populations appear segregated with a genetic break around the Mona Passage in the north, as has been shown previously in other species; however, we find evidence for significant connectivity between Curaçao and Mexico, suggesting that the southern margin of this barrier is permeable to dispersal. Our results also identify a strong genetic break within the Mesoamerican Barrier Reef System associated with complex oceanographic patterns that promote larval retention in southern Belize. Additionally, the diverse genetic signature at Flower Garden Banks suggests its possible function as a downstream genetic sink. The findings reported here are relevant to the ongoing conservation efforts for this important and threatened species, and contribute to the growing understanding of large-scale coral reef connectivity throughout the wider Caribbean.
Economic development and environmental conservation are often seen as opposing forces in the arena of government policy-making. With more than 7 million people and a rich diversity of marine species and habitats, Hong Kong is an excellent case study to explore this dynamic. Despite anthropogenic impacts, Hong Kong still hosts more than 90 species of stony corals within a marine area of 1650 km2. This is remarkable in light of the global plight of coral reefs, which have been reduced by ~80% worldwide in recent decades. The Hong Kong Special Administrative Region has not been immune to this negative trend with an unfortunate track of marine environmental disasters and, as such, can be viewed a harbinger for the future trajectory of coral reefs worldwide. Yet, the story is not entirely negative. Hong Kong possesses key assets, including capable government environmental agencies and competitive research led by local universities, which can bring novel and promising approaches for coral biodiversity conservation in an urbanized context. To coordinate and assist conservation efforts in Hong Kong, we here identify and prioritize major management efforts and identify knowledge gaps for coral conservation based on updated coral biodiversity distribution and a current literature review. Specifically, we propose five priorities for the most positive impact on conservation efforts: (1) reconstruct environmental baselines and establish long-term monitoring of local coral communities, (2) enhance the management and protection of local coral habitats, (3) improve water quality, (4) gain an understanding of the genetic connectivity among local and distant coral communities, and finally, (5) establish an active restoration program for local coral species/communities. In order to build progressive and integrative management strategies for coral biodiversity conservation in Hong Kong, we suggest specific ways in which these priorities be addressed and encourage a fresh dialogue between the government, the public and academia.
2014 marked the sixth and most widespread mass bleaching event reported in the Northwestern Hawaiian Islands, home to the Papahānaumokuākea Marine National Monument (PMNM), the world’s second largest marine reserve. This event was associated with an unusual basin-scale warming in the North Pacific Ocean, with an unprecedented peak intensity of around 20°C-weeks of cumulative heat stress at Lisianksi Island. In situ bleaching surveys and satellite data were used to evaluate the relative importance of potential drivers of bleaching patterns in 2014, assess the subsequent morality and its effects on coral communities and 3D complexity, test for signs of regional acclimation, and investigate long-term change in heat stress in PMNM. Surveys conducted at four island/atoll (French Frigate Shoals, Lisianski Island, Pearl and Hermes Atoll, and Midway Atoll) showed that in 2014, percent bleaching varied considerably between islands/atolls and habitats (back reef/fore reef and depth), and was up to 91% in shallow habitats at Lisianski. The percent bleaching during the 2014 event was best explained by a combination of duration of heat stress measured by Coral Reef Watch’s satellite Degree Heating Week, relative community susceptibility (bleaching susceptibility score of each taxon * the taxon’s abundance relative to the total number of colonies), depth and region. Mean coral cover at permanent Lisianski monitoring sites decreased by 68% due to severe losses of Montipora dilatata complex, resulting in rapid reductions in habitat complexity. Spatial distribution of the 2014 bleaching was significantly different from the 2002 and 2004 bleaching events likely due to a combination of differences in heat stress and local acclimatization. Historical satellite data demonstrated heat stress in 2014 was unlike any previous event and that the exposure of corals to the bleaching-level heat stress has increased significantly in the northern PMNM since 1982, highlighting the increasing threat of climate change to reefs.
Coral reefs are in decline worldwide. While coral reef managers are limited in their ability to tackle global challenges, such as ocean warming, managing local threats can increase the resilience of coral reefs to these global threats. One such local threat is high sediment inputs to coastal waters due to terrestrial over-grazing. Increases in terrestrial sediment input into coral reefs are associated with increased coral mortality, reduced growth rates, and changes in species composition, as well as alterations to fish communities. We used general linear models to investigate the link between vegetation ground cover and tree biomass index, within a dry-forest ecosystem, to coral cover, fish communities and visibility in the case study site of Bonaire, Caribbean Netherlands. We found a positive relationship between ground cover and coral cover below 10 m depth, and a negative relationship between tree biomass index and coral cover below 10 m. Greater ground cover is associated to sediment anchored through root systems, and higher surface complexity, slowing water flow, which would otherwise transport sediment. The negative relationship between tree biomass index and coral cover is unexpected, and may be a result of the deep roots associated with dry-forest trees, due to limited availability of water, which therefore do not anchor surface sediment, or contribute to surface complexity. Our analysis provides evidence that coral reef managers could improve reef health through engaging in terrestrial ecosystem protection, for example by taking steps to reduce grazing pressures, or in restoring degraded forest ecosystems.
Fossil coral reefs are valuable recorders of glacio-eustatic sea-level changes, as they provide key temporal information on deglacial meltwater pulses (MWPs). The timing, rate, magnitude, and meltwater source of these sea-level episodes remain controversial, despite their importance for understanding ocean-ice sheet dynamics during periods of abrupt climatic change. This study revisits the west coast of the Big Island of Hawaii to investigate the timing of the −150 m H1d terrace drowning off Kawaihae in response to MWP-1A. We present eight new calibrated 14C-AMS ages, which constrain the timing of terrace drowning to at or after 14.75 + 0.33/-0.42 kyr BP, coeval with the age of reef drowning at Kealakekua Bay (U-Th age 14.72 ± 0.10 kyr BP), 70 kms south along the west coast. Integrating the chronology with high-resolution bathymetry and backscatter data, detailed sedimentological analysis, and paleoenvironmental interpretation, we conclude the H1d terrace drowned at the same time along the west coast of Hawaii in response to MWP-1A. The timing of H1d reef drowning is within the reported uncertainty of the timing of MWP-1A interpreted from the IODP Expedition 310 Tahitian reef record.