Search

Floods rarely occur in hyper-arid deserts and little is known about the magnitude and frequency of sediment delivery from their basins, despite their importance to changes to the landscape, infrastructures and engineering activities. Sediment yield from the Nahal Nehushtan watershed (11.9 km2) located in the Timna Valley in southern Israel was determined by assessing stratigraphic sections in its 60-year reservoir deposits. Stratigraphic correlation between event couplets allowed for quantification of sediment yields representing 13 former flow and flood events. Based on the sediment volume in the reservoir, the 29.8 t km−2 year−1 average specific sediment yield is one of the lowest among other studied warm deserts. Among the event layers, the thickest layer, deposited by a flash flood caused by a single short rain event, contributed 31% of the total sediment yield. Based on event reservoir sedimentation from watersheds located in several hyper-arid areas in the Middle East and North America, we demonstrate that sediment yield increases with drainage area as expected and mean annual sediment yield increases in hyper-arid areas with flood frequency. Our quantitative results, together with previous studies of hyper-arid areas, provide complementary evidence of fluvial sediment transport—the main landscape designer in hyper-arid fluvial landscapes.

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 1.5°C 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 ecosystem 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 (EPFL), can serve as the regionally inclusive, neutral organization to foster crucial regional scientific collaboration.

Acacia encompasses a keystone genus across the Middle Eastern and African drylands. This study assesses the dynamics of Acacia populations in two ephemeral stream channels (Nahal Naomi and Nahal Yael) in the hyper-arid Arava region following the establishment of a dam in the upstream channel of Nahal Yael and a long-term regional drought episode. The assessments were conducted at the individual and population levels, for a period of 45 years (during 1972, 1994 and 2017). In Nahal Naomi, the population increased by 35% during 1972–1994 (a relatively rainy period) and experienced low mortality (net change of +1.6% year−1). However, following a regional drought episode between 1995 and 2009, this population decreased by 57% (net change of −2.5% year−1). In Nahal Yael, the acacia population declined by 66% during 1972–1994 (net change of −1.6% year−1). Between 1994–2017, this population was co-affected by dam and drought, with no recruitment, and declined by 70% (net change of −2.0% year−1). By examining the tree’s specific location, species, age and state of preservation of dead individuals, we identified factors that influence tree mortality, and highlighted the adverse impacts of natural and anthropogenic disturbances on Acacia populations in hyper-arid environments.

Hydrogen peroxide (H2O2), a common reactive oxygen species, plays multiple roles in coral health and disease. Elevated H2O2 production by the symbiotic algae during stress may result in symbiosis breakdown and bleaching of the coral. We have recently reported that various Red Sea corals release H2O2 and antioxidants to their external milieu, and can influence the H2O2 dynamics in the reef. Here, we present a laboratory characterization of H2O2 and antioxidant activity release kinetics by intact, non-stressed Stylophora pistillata. Experimenting with bleached and non-bleached corals and different stirring speeds, we explored the sources and modes of H2O2 and antioxidant release. Since H2O2 is produced and degraded simultaneously, we developed a methodology for resolving the actual H2O2 concentrations released by the corals. H2O2 and antioxidant activity steadily increased in the water surrounding the coral over short periods of 1–2 h. Over longer periods of 5–7 h, the antioxidant activity kept increasing with time, while H2O2 concentrations were stabilized at ~ 1 μM by 1–3 h, and then gradually declined. Solving for H2O2 release, corals were found to release H2O2 at increasing rates over 2–4 h, and then to slow down and stop by 5–7 h. Stirring was shown to induce the release of H2O2, possibly since the flow reduces the thickness of the diffusive boundary layer of the coral, and thus increases H2O2 mass flux. Antioxidant activity was released at similar rates by bleached and non-bleached corals, suggesting that the antioxidants did not originate from the symbiotic algae. H2O2, however, was not released from bleached corals, implying that the symbiotic algae are the source of the released H2O2. The observed flow-induced H2O2 release may aid corals in removing some of the internal H2O2 produced by their symbiotic algae, and may possibly assist in preventing coral bleaching under conditions of elevated temperature and irradiance.

Ocean warming has exacerbated the severity of coral diseases, many of which are mediated by pathogenic bacteria. In response to the presence of pathogens various organisms activate an oxidative burst response, involving strong and rapid generation of reactive oxygen species (ROS) that serve both as bactericides and as signals for other defense systems. While many components of the coral immune systems are being unveiled, an oxidative burst response, triggered by bacteria proximity, has not yet been reported. Hydrogen Peroxide (H2O2), one of the ROS produced during the oxidative burst, is often monitored as an indicator of this pathway. Here we report a yet undescribed release of H2O2 from the coral Stylophora pistillata upon contact with bacteria. In a series of short term experiments we monitored H2O2 concentrations in the vicinity of S. pistillata fragments prior to and following local administration of five bacteria isolates belonging to the genus Vibrio (obtained from healthy S. pistillata). In most experiments, rapid H2O2 release was recorded at the site of interaction within one minute from Vibrio addition and persisted for the remaining five minutes of the experiment. In some experiments, discrete or continuous drop of H2O2 to below background levels were seen following Vibrio addition. This H2O2 loss was quantitatively accounted for by the bacterial antioxidants and imply that Vibrio may at times offset the coral released H2O2The total H2O2 released by the coral from the small interaction site (~10 mm2) was 200-600 pmol, which may build up to a concentration of ~20 µM in the coral diffusive boundary layer. H2O2 concentrations above 10 µM resulted in significant mortality of the coral pathogen Vibrio coralliilyticus, suggesting that the released H2O2 may act as bactericide. It is thus far unclear if this H2O2 release is part of an oxidative burst response, yet it strongly indicates that corals sense and chemically react towards bacteria.

Coral reefs are in global decline, with coral diseases increasing both in prevalence and in space, a situation that is expected only to worsen as future thermal stressors increase. Through intense surveillance, we have collected a unique and highly resolved dataset from the coral reef of Eilat (Israel, Red Sea), that documents the spatiotemporal dynamics of a White Plague Disease (WPD) outbreak over the course of a full season. Based on modern statistical methodologies, we develop a novel spatial epidemiological model that uses a maximum-likelihood procedure to fit the data and assess the transmission pattern of WPD. We link the model to sea surface temperature (SST) and test the possible effect of increasing temperatures on disease dynamics. Our results reveal that the likelihood of a susceptible coral to become infected is governed both by SST and by its spatial location relative to nearby infected corals. The model shows that the magnitude of WPD epidemics strongly depends on demographic circumstances; under one extreme, when recruitment is free-space regulated and coral density remains relatively constant, even an increase of only 0.5°C in SST can cause epidemics to double in magnitude. In reality, however, the spatial nature of transmission can effectively protect the community, restricting the magnitude of annual epidemics. This is because the probability of susceptible corals to become infected is negatively associated with coral density. Based on our findings, we expect that infectious diseases having a significant spatial component, such as Red-Sea WPD, will never lead to a complete destruction of the coral community under increased thermal stress. However, this also implies that signs of recovery of local coral communities may be misleading; indicative more of spatial dynamics than true rehabilitation of these communities. In contrast to earlier generic models, our approach captures dynamics of WPD both in space and time, accounting for the highly seasonal nature of annual WPD outbreaks.

The coral reefs of Zanzibar Island (Unguja, Tanzania) encompass a considerable proportion of the global coral-reef diversity and are representative of the western Indian Ocean region. Unfortunately, these reefs have been recently subjected to local and regional disturbances. The objectives of this study were to determine whether there are potentially non-random processes forcing the observed coral diversity patterns, and highlight where and at which spatial scales these processes might be most influential.A hierarchical (nested) sampling design was employed across three spatial scales, ranging from transects (

BACKGROUND: Transmission mechanisms of black-band disease (BBD) in coral reefs are poorly understood, although this disease is considered to be one of the most widespread and destructive coral infectious diseases. The major objective of this study was to assess transmission mechanisms of BBD in the field based on the spatio-temporal patterns of the disease. METHODOLOGY/PRINCIPAL FINDINGS: 3,175 susceptible and infected corals were mapped over an area of 10x10 m in Eilat (northern Gulf of Aqaba, Red Sea) and the distribution of the disease was examined monthly throughout almost two full disease cycles (June 2006-December 2007). Spatial and spatio-temporal analyses were applied to infer the transmission pattern of the disease and to calculate key epidemiological parameters such as (basic reproduction number). We show that the prevalence of the disease is strongly associated with high water temperature. When water temperatures rise and disease prevalence increases, infected corals exhibit aggregated distributions on small spatial scales of up to 1.9 m. Additionally, newly-infected corals clearly appear in proximity to existing infected corals and in a few cases in direct contact with them. We also present and test a model of water-borne infection, indicating that the likelihood of a susceptible coral becoming infected is defined by its spatial location and by the relative spatial distribution of nearby infected corals found in the site. CONCLUSIONS/SIGNIFICANCE: Our results provide evidence that local transmission, but not necessarily by direct contact, is likely to be an important factor in the spread of the disease over the tested spatial scale. In the absence of potential disease vectors with limited mobility (e.g., snails, fireworms) in the studied site, water-borne infection is likely to be a significant transmission mechanism of BBD. Our suggested model of water-borne transmission supports this hypothesis. The spatio-temporal analysis also points out that infected corals surviving a disease season appear to play a major role in the re-introduction of the disease to the coral community in the following season.

Floods rarely occur in hyper-arid deserts and little is known about the magnitude and frequency of sediment delivery from their basins, despite their importance to changes to the landscape, infrastructures and engineering activities. Sediment yield from the Nahal Nehushtan watershed (11.9 km2) located in the Timna Valley in southern Israel was determined by assessing stratigraphic sections in its 60-year reservoir deposits. Stratigraphic correlation between event couplets allowed for quantification of sediment yields representing 13 former flow and flood events. Based on the sediment volume in the reservoir, the 29.8 t km−2 year−1 average specific sediment yield is one of the lowest among other studied warm deserts. Among the event layers, the thickest layer, deposited by a flash flood caused by a single short rain event, contributed 31% of the total sediment yield. Based on event reservoir sedimentation from watersheds located in several hyper-arid areas in the Middle East and North America, we demonstrate that sediment yield increases with drainage area as expected and mean annual sediment yield increases in hyper-arid areas with flood frequency. Our quantitative results, together with previous studies of hyper-arid areas, provide complementary evidence of fluvial sediment transport—the main landscape designer in hyper-arid fluvial landscapes. [ABSTRACT FROM AUTHOR]

• An extreme flash flood removed trees estimated to be 200 years old. • Vegetation removal was detected using satellite images in a hyper-arid wadi. • The restricted spatial extent of the damage was demonstrated by NDVI analysis. • Over 50 % of the vegetation patches were removed; loss increased downstream. • Correlation of accumulated drainage area and tree cover loss is strong and positive. On April 10, 2023, exceptional weather conditions over southern Israel produced an extreme rainstorm. The peak precipitation, with a maximum rainfall depth of 62 mm and rain intensities exceeding 50 mm h−1, fell over the hyper-arid southern Negev and Arava Valley and generated a mega flash flood in Wadi Zihor. This ephemeral stream channel supported a comparatively dense cover of vegetation, predominated by Acacia raddiana , Acacia pachyceras , Tamarix nilotica , and Tamarix aphylla. To determine the properties of the flash flood, we used post-flood ground-based measurements and calculations of shear stress and peak discharge along the wadi. Also, we used pre- and post-flood satellite images to assess the flood's impact on woody vegetation cover along the ∼17 km-long wadi bed. Calculations based on data collected by the Israel Water Authority implied a peak discharge of 585 m3/s at the wadi outlet, indicating a low-frequency flash flood of less than once in 150 years. The pre-flood's 12.7 % vegetation cover throughout the wadi bed decreased to 7.5 % after the flood, corresponding to a 41 % net loss. To detect differences in tree removal extent along the wadi, we divided its entire length into 24 equal segments, and separately assessed the vegetation cover before and after the flood. Vegetation cover loss was 10.8 % in the wadi's uppermost segment, whereas its lowermost segment lost 51.6 %. Overall, a significant and strongly positive (r = 0.88) correlation was recorded between the accumulated watershed area (downstream) and the extent of tree cover loss. Among other factors, this effect seems to be predominantly determined by the increasing shear stress and peak discharge downstream. Forecasted climatic change scenarios, with increasing magnitude and frequency of extreme rainstorms and floods across the world's drylands, highlight the need for more data collecting and analyzing of those events impact as described in this study. [ABSTRACT FROM AUTHOR]

The ability to quickly and reliably fabricate nanoscale pore arrays in ultra‐thin membranes such as silicon nitride (SixN) is extremely important for the growing field of nanopore biosensing. Laser‐based etching of thin SixN membranes immersed in aqueous solutions has recently been demonstrated as a method to produce stable functional pores. Herein, the principal mechanism governing material etching and pore formation using light is investigated. It is found that the process is extremely sensitive to the relative content of Si over N atoms in the amorphous membrane, produced by chemical vapor deposition. Commonly, SixN membranes are made to be Si‐rich to increase their mechanical stability, which substantially reduces the material's bandgap and increases the density of Si‐dangling bonds. Hence, even minimal batch‐to‐batch variation may lead to remarkably different etch rates. It is shown that higher Si content results in orders of magnitude faster etching rates. This rate is further accelerated in an alkaline environment allowing on‐demand controlled nanopore formation in about 10 s time even at low laser radiation intensities. These results highlight that photoactivation of the SixN by the incident beam is critical to the chemical etching process and can be used to readily produce nanopore arrays at any specific location. [ABSTRACT FROM AUTHOR]

Coral reefs are in global decline, with coral diseases increasing both in prevalence and in space, a situation that is expected only to worsen as future thermal stressors increase. Through intense surveillance, we have collected a unique and highly resolved dataset from the coral reef of Eilat (Israel, Red Sea), that documents the spatiotemporal dynamics of a White Plague Disease (WPD) outbreak over the course of a full season. Based on modern statistical methodologies, we develop a novel spatial epidemiological model that uses a maximum-likelihood procedure to fit the data and assess the transmission pattern of WPD. We link the model to sea surface temperature (SST) and test the possible effect of increasing temperatures on disease dynamics. Our results reveal that the likelihood of a susceptible coral to become infected is governed both by SST and by its spatial location relative to nearby infected corals. The model shows that the magnitude of WPD epidemics strongly depends on demographic circumstances; under one extreme, when recruitment is free-space regulated and coral density remains relatively constant, even an increase of only 0.5°C in SST can cause epidemics to double in magnitude. In reality, however, the spatial nature of transmission can effectively protect the community, restricting the magnitude of annual epidemics. This is because the probability of susceptible corals to become infected is negatively associated with coral density. Based on our findings, we expect that infectious diseases having a significant spatial component, such as Red-Sea WPD, will never lead to a complete destruction of the coral community under increased thermal stress. However, this also implies that signs of recovery of local coral communities may be misleading; indicative more of spatial dynamics than true rehabilitation of these communities. In contrast to earlier generic models, our approach captures dynamics of WPD both in space and time, accounting for the highly seasonal nature of annual WPD outbreaks. [ABSTRACT FROM AUTHOR]

Background: The coral reefs of Zanzibar Island (Unguja, Tanzania) encompass a considerable proportion of the global coralreef diversity and are representative of the western Indian Ocean region. Unfortunately, these reefs have been recently subjected to local and regional disturbances. The objectives of this study were to determine whether there are potentially non-random processes forcing the observed coral diversity patterns, and highlight where and at which spatial scales these processes might be most influential. Methodology/Principal Findings: A hierarchical (nested) sampling design was employed across three spatial scales, ranging from transects (≤20 m), stations (<100 m), to sites (<1000 m), to examine coral diversity patterns. Two of the four sites, Chumbe and Mnemba, were located within Marine Protected Areas (MPAs), while the other two sites, Changuu and Bawe, were not protected. Additive partitioning of coral diversity was used to separate regional (total) diversity (c) into local a diversity and among-sample b diversity components. Individual-based null models were used to identify deviations from random distribution across the three spatial scales. We found that Chumbe and Mnemba had similar diversity components to those predicted by the null models. However, the diversity at Changuu and Bawe was lower than expected at all three spatial scales tested. Consequently, the relative contribution of the among-site diversity component was significantly greater than expected. Applying partitioning analysis for each site separately revealed that the within-transect diversity component in Changuu was significantly lower than the null expectation. Conclusions/Significance: The non-random outcome of the partitioning analyses helped to identify the among-sites scale (i.e., 10's of kilometers) and the within-transects scale (i.e., a few meters; especially at Changuu) as spatial boundaries within which to examine the processes that may interact and disproportionately differentiate coral diversity. In light of coral community compositions and diversity patterns we strongly recommend that Bawe be declared a MPA. [ABSTRACT FROM AUTHOR]

Background: Transmission mechanisms of black-band disease (BBD) in coral reefs are poorly understood, although this disease is considered to be one of the most widespread and destructive coral infectious diseases. The major objective of this study was to assess transmission mechanisms of BBD in the field based on the spatio-temporal patterns of the disease. Methodology/Principal Findings: 3,175 susceptible and infected corals were mapped over an area of 10×10 m in Eilat (northern Gulf of Aqaba, Red Sea) and the distribution of the disease was examined monthly throughout almost two full disease cycles (June 2006-December 2007). Spatial and spatio-temporal analyses were applied to infer the transmission pattern of the disease and to calculate key epidemiological parameters such as R0 (basic reproduction number). We show that the prevalence of the disease is strongly associated with high water temperature. When water temperatures rise and disease prevalence increases, infected corals exhibit aggregated distributions on small spatial scales of up to 1.9 m. Additionally, newly-infected corals clearly appear in proximity to existing infected corals and in a few cases in direct contact with them. We also present and test a model of water-borne infection, indicating that the likelihood of a susceptible coral becoming infected is defined by its spatial location and by the relative spatial distribution of nearby infected corals found in the site. Conclusions/Significance: Our results provide evidence that local transmission, but not necessarily by direct contact, is likely to be an important factor in the spread of the disease over the tested spatial scale. In the absence of potential disease vectors with limited mobility (e.g., snails, fireworms) in the studied site, water-borne infection is likely to be a significant transmission mechanism of BBD. Our suggested model of water-borne transmission supports this hypothesis. The spatiotemporal analysis also points out that infected corals surviving a disease season appear to play a major role in the reintroduction of the disease to the coral community in the following season. [ABSTRACT FROM AUTHOR]