The phenomenon of coral fluorescence in mesophotic reefs, although well described for shallow waters, remains largely unstudied. We found that representatives of many scleractinian species are ...brightly fluorescent at depths of 50-60 m at the Interuniversity Institute for Marine Sciences (IUI) reef in Eilat, Israel. Some of these fluorescent species have distribution maxima at mesophotic depths (40-100 m). Several individuals from these depths displayed yellow or orange-red fluorescence, the latter being essentially absent in corals from the shallowest parts of this reef. We demonstrate experimentally that in some cases the production of fluorescent pigments is independent of the exposure to light; while in others, the fluorescence signature is altered or lost when the animals are kept in darkness. Furthermore, we show that green-to-red photoconversion of fluorescent pigments mediated by short-wavelength light can occur also at depths where ultraviolet wavelengths are absent from the underwater light field. Intraspecific colour polymorphisms regarding the colour of the tissue fluorescence, common among shallow water corals, were also observed for mesophotic species. Our results suggest that fluorescent pigments in mesophotic reefs fulfil a distinct biological function and offer promising application potential for coral-reef monitoring and biomedical imaging.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Although the Gulf of Aqaba-Eilat is located in the tectonically active northern Red Sea, it has been described as low-risk with regard to tsunami activity because there are no modern records of ...damaging tsunami events and only one tsunami (1068 AD) referred to in historical records. However, this assessment may be poorly informed given that the area was formed by and is located along the seismically active Dead Sea Fault, its population is known to fluctuate in size and literacy in part due to its harsh hyper-arid climate, and there is a dearth of field studies addressing the presence or absence of tsunamigenic deposits. Here we show evidence from two offshore cores for a major paleotsunami that occurred ~2300 years ago with a sedimentological footprint that far exceeds the scarce markers of the historically mentioned 1068 AD event. The interpretation is based on the presence of a laterally continuous and synchronous, anomalous sedimentological deposit that includes allochtonous inclusions and unique structural characteristics. Based on sedimentological parameters, these deposits could not be accounted for by other transport events, or other known background sedimentological processes.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
In March 2020, an unusually intense storm system struck the Gulf of Aqaba‐Eilat, resulting in severe shoreline damage. This brief account examines post‐storm observations of inconsistent damage ...patterns and structural changes along a specific coastal stretch located at the south beach of Eilat. Certain sections of the coastline experienced direct impact from extreme waves on the south‐southeast‐facing shallow reef, resulting in areas where rocks were completely stripped of corals due to sediment backwash. Conversely, areas characterized by ridges and deep troughs saw the loss of branching corals and some massive colonies, while many small corals survived. A neighboring area with a well‐developed fringing reef suffered lesser damage. Between the severely affected shallow reef and the robust fringing reef lies an unconsolidated slope that migrated eastward by at least 2 m following the storm, incorporating numerous coral colonies dislodged by the event. We propose that this slope advances with each major storm occurrence, influencing the characteristics of nearby shores and coral reefs. This case demonstrates how storm events, in conjunction with geomorphology, have a cumulative and significant impact not only on the structure of coral communities but also on the fundamental shape of coral reefs themselves. As climate change amplifies the range, intensity, and frequency of storms, comprehending these processes becomes increasingly crucial.
Rapid ocean warming due to climate change poses a serious risk to the survival of coral reefs. It is estimated that 70-90 percent of all reefs will be severely degraded by mid-century even if the ...1.5oC goal of the Paris Climate Agreement is achieved. However, one coral reef ecosystem seems to be more resilient to rising sea temperatures than most others. The Red Sea’s reef is one of the longest continuous living reefs in the world, and its northernmost portion extends into the Gulf of Aqaba. The scleractinian corals in the Gulf have an unusually high tolerance for the rapidly warming seawater in the region. They withstand water temperature anomalies that cause severe bleaching or mortality in most hard corals elsewhere. This uniquely resilient reef employs biological mechanisms which are likely to be important for coral survival as the planet’s oceans warm. The Gulf of Aqaba could potentially be one of the planet’s largest marine refuges from climate change. However, this unique portion of the Red Sea’s reef will only survive and flourish if serious regional environmental challenges are addressed. Localized anthropogenic stressors compound the effects of warming seawater to damage corals and should be mitigated immediately. Reefs in the rest of the Red Sea are already experiencing temperatures above their thermal tolerance and have had significant bleaching, though they too would benefit from fewer local anthropogenic stressors. The countries bordering the entire Red Sea will need to cooperate to enable effective scientific research and conservation. The newly established Transnational Red Sea Center, based at the Ecole Polytechnique Fédérale de Lausanne, can serve as the regionally inclusive, neutral organization to foster crucial regional scientific collaboration.
A new relative sea level (RSL) curve is presented for the Holocene Gulf of Aqaba, northern Red Sea. The curve is based on U–Th ages of corals retrieved from leveled fossil reefs comprising the Tur ...Yam Terrace at the shore of the city of Elat, Israel and the Aqaba reef terraces, Jordan, located respectively east and west of the northern tip of the gulf. Both terrace tops are submerged under high tide and subaerial at low tide. The fossil corals are in growth position between two beachrock units. The corals comprise 100% aragonite, show pristine textures with no secondary aragonite cements and yield initial 234U/238U activity ratios of 1.146 ± 0.004, similar to the values of live corals and modern Gulf of Aqaba (GoA) waters. Sixteen U–Th ages of corals from the Tur Yam Terrace span the time interval of 6.8 to 5.5 ka, and are ordered along the growth direction of the terrace. Detailed mapping of the terrace, the distribution of corals’ genera and the order of their ages on the terrace, suggest that the exposed terrace was the back reef/lagoon zone, a very close to shore environment of a fringing reef. Identification of this reef zone was based on a detailed ecological study that we conducted on the Nature Reserve Reef (NRR), a modern fringing reef located several hundred meters to the south. A morpho-tectonic analysis of the Elat shore indicates that the Tur Yam Terrace lies on top of the footwall of the active Elat Fault and has remained at its original level, with negligible vertical shift. Thus, it appears that during the time interval of formation of the Tur Yam corals, sea level at GoA reached a maximum stand that was 1–2 m above the modern elevation. A similar figure is shown by Mid-Holocene corals from the Aqaba fossil reefs and shore indicators from the Red Sea. The relative sea level (RSL) curve deduced from the Gulf of Aqaba corals and from the Red Sea indicators resembles those of precisely-dated stable reefs off west and east Australia and a few other well dated reefs in the Indo-Pacific. This similarity calls for coeval response of the Indo-Pacific oceans to ice melting during the early to mid- Holocene, and a similar pattern of decline to the present level during the late Holocene. Such a coeval rise of distant parts of the Indo-Pacific oceans is consistent with dominance of melt waters releases over the oceans with minor effects of all other “sea level drivers".
•Reconstruction of a new relative sea level (RSL) curve for the Holocene Gulf of Aqaba.•Between 6.8 and 5.5 ka the Gulf reached a maximum stand ∼1–2 m above the modern elevation.•A similar figure is shown by other sea level markers along the Red Sea.•The Gulf of Aqaba RSL resembles those deduced from stable reefs off Australia.
Antecedent topography such as relic reef terraces as well as biogenic carbonate relief-forming deposits ~30–150 m deep, referred to as mesophotic reefs, provide structural support for diverse ...mesophotic coral ecosystems (MCEs) that may serve as coral refuges for select light-dependent species. Although terraces at mesophotic depths are found globally, an understanding of their spatial distribution, formation, and relationship with living community composition and lithology is generally lacking. Herein, 2 × 2 m resolution bathymetry from the Gulf of Aqaba (GoA) was examined to define geomorphology features spanning mesophotic depths and compare geomorphology relationships to overlying benthic and lithologic cover. Analysis led to the production of a new map categorizing 12 geomorphology features, including upper mesophotic terraces harboring thriving MCEs. Additionally, a large collection of still imagery (1726 pictures) was obtained at 94 sites and used to define eight unique habitats at mesophotic depths and lithological and biological distribution patterns over vertical and horizontal scales. Study area benthic and lithologic cover was found to be significantly different between geomorphology features and related to GoA geomorphology as well as to seafloor depth and slope, and light attenuation. While these relationships indicated modern cover could not provide a model for producing most underlying geomorphology in the study area, results provided data needed to enhance understanding of geomorphology feature formation history and reef accretion at mesophotic depths. Study results also detailed benthic cover and geomorphology features critical for better identifying and mapping unknown MCE habitats, and for recognizing mesophotic reef spatial relationships and biodiversity patterns in the GoA. These results are especially important considering most northern GoA reefs act as potential refuges, but local anthropogenic development continually stresses shallow GoA reefs and most other shallow coral reefs around the globe continue to degrade.
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•A new mesophotic reef geomorphology map was created for the northern Gulf of Aqaba.•Eight habitats, useful for future mesophotic reef management, were identified.•Mesophotic reef habitats likely cover at least 13.8% of the study area.•Depth variable partially explains community composition factored by geomorphology.•Benthic/lithologic cover mainly does not provide a geomorphology formation model.
Most of the fossil corals in the elevated reef terraces along the Gulf of Aqaba (GOA) were extensively altered to calcite. This observation indicates extensive interaction with freshwater, possibly ...when the terraces passed through a coastal aquifer that existed along the shores of the GOA, implying a wetter climate during the time of recrystallization from aragonite to calcite. Thus, dating of the recrystallization events should yield the timing of past wetter conditions in the current hyper-arid area of the GOA. In the present study, 18 aragonite and calcite corals were collected from several elevated coral reef terraces off the coast, south of the city of Aqaba. While aragonite corals were dated with the conventional closed system age equation (assuming zero initial Th), the dating of the calcite corals required the development of adequate equations to allow the calculation of both the initial formation age of the aragonite corals and the time of recrystallization to calcite. The two age calculations were based on the assumptions that each reef terrace went through a single and rapid recrystallization event and that the pristine aragonite corals were characterized by a rather uniform initial U concentration, typical for pristine modern corals. Two recrystallization events were identified at 104±6ka and 124±8ka. The ages coincide with the timing of sapropel events S4 and S5, respectively, when the African monsoon induced enhanced wetness in the desert area. Considering the age uncertainties, the times of formation of the two major reef terraces are estimated to be ∼124ka (reef terrace R2) and ∼130ka (reef terrace R3), matching the peaks in the global sea level during the last interglacial MIS 5e stage. Apparently, sea level of the GOA did not fluctuate a lot during the period between ∼130ka and ∼104ka and remained close to the Marine Isotopic stage (MIS) 5e highstand. The availability of freshwater (during the sapropel periods) and limited sea level fluctuations facilitated the recrystallization of the GOA reef corals to calcite.
Coral bleaching events are more frequent and severe as global temperatures rise. However, analysis of the surface energy fluxes that substantially affect the thermoregulation of shallow reef ...environments is rare. Previous work on evaporative cooling in a humid reef environment led to the hypothesis that in proximity to desert regions, drier air enhances evaporative cooling of water overlying coral reefs, reducing the risk of extreme high‐water temperatures. Eddy Covariance measurements made over a shallow desert fringing coral reef in the Gulf of Eilat (Israel) during summer measured evaporation of 10.3 mm d−1 which is maintaining water temperature much lower than the overlying air temperature. In addition, measurements of the most severe marine heatwave that was ever documented in the region are presented as a case study for future reference to the processes that lead to and proceed a marine heatwave, which is induced by synoptic scale events that cause a ∼50% reduction in evaporation rate. Results are compared to similar measurements from a tropical coral reef, where a lower evaporation rate suppressed by higher humidity is unable to offset the heating of water overlying the reef. We conclude that evaporative cooling is a key mechanism protecting coral reefs located in deserts from extreme high‐water temperatures, thereby representing possible thermal refugium for corals against background global warming.
Plain Language Summary
We examine the energy balance between water overlying coral reefs and the atmosphere in the desert and tropical coral reefs. We hypothesize that in drier regions higher evaporation rates will cool water overlying the reefs and by that protect the corals from extreme high‐water temperatures and bleaching. We show through rare direct measurements, that the cooling of water caused by sea water evaporation is a key mechanism in mitigating extremely high‐water temperatures that can cause coral bleaching and is especially dominant in coral reefs with strong and dry winds blowing above them. The results support the hypothesis that the meteorology of regions where coral reefs fringe arid deserts provides a thermal refugium for corals against future predicted global warming.
Key Points
Evaporative cooling is a key mechanism in mitigating extreme increase in water temperatures and preventing coral bleaching
Following a marine heatwave, surface energy partitioning varies with air humidity; in drier climates evaporative cooling takes larger role
The Gulf of Eilat (Aqaba) is identified as an evaporative thermal refugium for the preservation of coral reefs in a warming World
Abstract
Understanding the role of coral reefs on air‐sea CO
2
exchange is essential to accurately estimate carbon budgets throughout the tropical and subtropical oceans. Results from research ...conducted in humid marine environments show coral reefs are mainly net sources of CO
2
to the atmosphere due to calcification being dominant over respiration and dissolution. However, coral reefs are also found bordering deserts in the subtropical high‐pressure belt. Here, we present results from concurrent direct measurements of air‐sea CO
2
exchange over fringing coral reefs and open sea bordered by hyper‐arid deserts in the Gulf of Eilat (Aqaba). The fringing coral reefs were found to be a net sink of CO
2
at −1.62 ± 4 μmol m
−2
s
−1
sequestering 1.96 times more CO
2
than adjacent open sea. The CO
2
fluxes showed a negative correlation with some local meteorological parameters such as humidity, wind speed, and net radiation but were positively correlated to air temperature. The air‐sea CO
2
fluxes were also found to be correlated with dust, which is believed to trigger metabolic processes in bacteria and phytoplankton in the low‐nutrient, low‐chlorophyll sea of the study site. Results are compared to measurements from the humid marine setting of the Great Barrier Reef, Australia.
Plain Language Summary
Quantifying air‐sea exchange of CO
2
is critical to developing accurate carbon budgets for the world oceans. However, uncertainty remains as to whether coral reefs are net sinks or sources of atmospheric CO
2
. Here we present direct measurements of air‐sea CO
2
exchange over the desert fringing coral reefs in the northern Gulf of Eilat (Aqaba), Israel. Unlike coral reefs in humid tropical and subtropical locations, which have been found to be mainly net sources of CO
2
to the atmosphere, our results indicate that the fringing coral reefs in the northern Gulf of Eilat (Aqaba) are net sinks of atmospheric CO
2
. This net influx of CO
2
correlates with periods of increased dust concentrations in the atmosphere. The deposition of this dust into the low‐nutrient, low‐chlorophyll waters of the coral reef triggers an increase in metabolic processes in the reef ecosystem through fertilization. We conclude that dust may influence air‐sea CO
2
exchange at other coral reefs regularly impacted by dust deposition, such as in the Caribbean and northwest Australia.
Key Points
Results from the first direct measurements of air‐sea CO
2
exchange with the atmosphere over desert fringing coral reefs are presented
Fringing coral reefs in the Gulf of Eilat were found to be a net sink of atmospheric CO
2
during the summer
Fertilization of the low‐nutrient, low‐chlorophyll sea by dust was identified as a possible major influence on air‐sea CO
2
exchange
Marine particulate fluxes were studied between 2014 and 2017 in the oligotrophic Gulf of Aqaba (GOA), northern Red Sea. A bottom tethered mooring mounted with 5 sediment trap stations (KC Denmark ...Inc.) at approximately equal depth intervals between 120 and 570 m (water depth of ∼610 m) was rotated monthly. The bulk particulate fluxes were determined for the entire period, with organic C and N, CaCO3, and lithogenic fluxes determined for the first two and half years of the deployment. The results are evaluated in the context of monthly resolved records of seawater temperature, chlorophyll-a concentrations, and macro-nutrient concentrations, as well as hourly to weekly dust load records and rare fluvial events. The results are further compared to core-tops collected from varying water depths and are combined to produce a basin source-to-sink mass balance of particulate fluxes. The GOA undergoes strong seasonal changes expressed by surface water temperatures and water column stratification and mixing, which control the vertical and temporal distribution of nutrients and primary and export production. Accordingly, the seasonal variability in particulate fluxes varies over a wide range, typically displaying peak bulk fluxes in bottom waters during the winter (∼5–7 g m–2 d–1) and minimum values in shallow waters during summer (<0.5 g m–2 d–1). Organic C and N fluxes are the highest in shallow waters and display strong vertical attenuation that varies seasonally, a-priori reflecting enhanced remineralization in the warm shallow waters during summer. In contrast, particulate organic carbon and nitrogen fluxes are enhanced in bottom waters during winter, due to the combined effect of the increased presence of mineral ballasts and vertical water column mixing. The quantification of particulate fluxes in the GOA suggests that, while most of the bulk particulates are introduced into the basin via episodic fluvial events, with direct dust inputs contributing approximately an order of magnitude less material, the internal cycling of terrigenous material is complex, with a lag between the initial deposition of influxing material along shallow margins and seasonal reworking and transport of sediments to the deep seafloor. Nevertheless, the fluxes of terrigenous and organic particulates are largely independent of each other, with export production fluxes driven by water column mixing and nutrient availability in the photic zone. In addition to being the first quantitative report of bulk and export production fluxes in the region, our results provide an improved understanding of the interplay between export production and terrigenous fluxes as well as an interpretation of the paleo-record in the GOA and in comparable environments. On a wider scale, the findings reported here relate to the role of dust deposition and hemipelagic sedimentation in the oceans and their impact on export production and particle cycling in coastal regions. Combined, these findings illuminate the factors impacting marine habitats and ecosystems, the cycling and sequestration of trace elements and anthropogenic components in the oceans, and facilitate better understanding of the interplay between solid and dissolved phases in the oceans and reconstructing past oceanographic and climatic conditions from marine sediment cores.
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