Coral reefs in the Red Sea belong to the most diverse and productive reef ecosystems worldwide, although they are exposed to strong seasonal variability, high temperature, and high salinity. These factors are considered stressful for coral reef biota and challenge reef growth in other oceans, but coral reefs in the Red Sea thrive despite these challenges. In the central Red Sea high temperatures, high salinities, and low dissolved oxygen on the one hand reflect conditions that are predicted for ‘future oceans’ under global warming. On the other hand, alkalinity and other carbonate chemistry parameters are considered favourable for coral growth. In coral reefs of the central Red Sea, temperature and salinity follow a seasonal cycle, while chlorophyll and inorganic nutrients mostly vary spatially, and dissolved oxygen and pH fluctuate on the scale of hours to days. Within these strong environmental gradients micro- and macroscopic reef communities are dynamic and demonstrate plasticity and acclimatisation potential. Epilithic biofilm communities of bacteria and algae, crucial for the recruitment of reef-builders, undergo seasonal community shifts that are mainly driven by changes in temperature, salinity, and dissolved oxygen. These variables are predicted to change with the progression of global environmental change and suggest an immediate effect of climate change on the microbial community composition of biofilms. Corals are so-called holobionts and associate with a variety of microbial organisms that fulfill important functions in coral health and productivity. For instance, coral-associated bacterial communities are more specific and less diverse than those of marine biofilms, and in many coral species in the central Red Sea they are dominated by bacteria from the genus Endozoicomonas. Generally, coral microbiomes align with ecological differences between reef sites. They are similar at sites where these corals are abundant and successful. Coral microbiomes reveal a measurable footprint of anthropogenic influence at polluted sites. Coral-associated communities of endosymbiotic dinoflagellates in central Red Sea corals are dominated by Symbiodinium from clade C. Some corals harbour the same specific symbiont with a high physiological plasticity throughout their distribution range, while others maintain a more flexible association with varying symbionts of high physiological specificity over depths, seasons, or reef locations. The coral-Symbiodinium endosymbiosis drives calcification of the coral skeleton, which is a key process that provides maintenance and formation of the reef framework. Calcification rates and reef growth are not higher than in other coral reef regions, despite the beneficial carbonate chemistry in the central Red Sea. This may be related to the comparatively high temperatures, as indicated by reduced summer calcification and long-term slowing of growth rates that correlate with ocean warming trends. Indeed, thermal limits of abundant coral species in the central Red Sea may have been exceeded, as evidenced by repeated mass bleaching events during previous years. Recent comprehensive baseline data from central Red Sea reefs allow for insight into coral reef functioning and for quantification of the impacts of environmental change in the region.
Posts Tagged 'Red Sea'
Physicochemical dynamics, microbial community patterns, and reef growth in coral reefs of the central Red Sea
Published 13 December 2018 Science ClosedTags: chemistry, corals, field, prokaryotes, Red Sea
Calcite and aragonite saturation levels of the Red Sea coastal waters of Yemen during early winter and expected pH decrease (acidification) effects
Published 12 December 2018 Science ClosedTags: chemistry, field, Red Sea
Seawater samples from different depths of eight stations along the Red Sea coast of Yemen were collected during early winter for the determinations of the temperature, salinity, pH value and total alkalinity profiles. The seawater surface temperature at 100 m) it ranged from 21.7 to 22.1 °C. The salinities were found to range from 36.32 to 37.36‰ at surface seawaters and from 40.27 to 40.35‰ at >100 m depths. The pH ranged from 7.983 to 8.198 at surface seawater and from 7.960 to 8.052 at deeper layers. The total alkalinities were found to range from 2.3268 to 3.6159 meq kg−1 at surface layers and from 2.4082 to 2.9659 meq kg−1 in seawater layers deeper than 100 m. The results showed that the surface seawater layers were several-fold supersaturated with respect to both calcite and aragonite, where the percent degree of saturation values ranged from 511 to 852% with respect to calcite and from 340 to 567% with respect to aragonite. At >100 m depth the percent degree of saturation ranged from 327% to 396% and from 221% to 268% with respect to calcite and aragonite, respectively. The results suggest that low magnesian calcite and aragonite are likely the major carbonate solid phases formed under current saturation levels. Recent studies show that the present oceanic pH values may drop by 0.1 and 0.4 units in 50 and 200 years, respectively. Thus, a projected change of −0.1 pH unit decreases the saturation levels to 426–710% for calcite and 283–473% for aragonite in surface waters and to 286–327% for calcite and 196–221% for aragonite at >100 m depth. A drop of −0.4 pH unit decreases the calcite saturation levels of surface and deep waters to 243–406% and 155–189%, respectively, whereas the saturation levels for aragonite reduce by 184–210% for surface waters and 105–120% for deep waters. These drops will affect the morphology and mineralogy of calcium carbon deposits as well as the distribution of calcifying organisms in the region. Further studies are warranted to investigate the occurrence, distribution and mineralogy of corals and the effects of physical and chemical parameters upon their growth in the region.
Coral reef carbonate budgets and ecological drivers in the central Red Sea – a naturally high temperature and high total alkalinity environment
Published 29 October 2018 Science ClosedTags: abundance, biogeochemistry, biological response, BRcommunity, chemistry, corals, dissolution, field, morphology, Red Sea
The structural framework provided by corals is crucial for reef ecosystem function and services, but high seawater temperatures can be detrimental to the calcification capacity of reef-building organisms. The Red Sea is very warm, but total alkalinity (TA) is naturally high and beneficial for reef accretion. To date, we know little about how such detrimental and beneficial abiotic factors affect each other and the balance between calcification and erosion on Red Sea coral reefs, i.e., overall reef growth, in this unique ocean basin. To provide estimates of present-day reef growth dynamics in the central Red Sea, we measured two metrics of reef growth, i.e., in situ net-accretion/-erosion rates (Gnet) determined by deployment of limestone blocks and ecosystem-scale carbonate budgets (Gbudget), along a cross-shelf gradient (25km, encompassing nearshore, midshore, and offshore reefs). Along this gradient, we assessed multiple abiotic (i.e., temperature, salinity, diurnal pH fluctuation, inorganic nutrients, and TA) and biotic (i.e., calcifier and epilithic bioeroder communities) variables. Both reef growth metrics revealed similar patterns from nearshore to offshore: net-erosive, neutral, and net-accretion states. The average cross-shelf Gbudget was 0.66kg CaCO3m−2yr−1, with the highest budget of 2.44kg CaCO3m−2yr−1 measured in the offshore reef. These data are comparable to the contemporary Gbudgets from the western Atlantic and Indian oceans, but lie well below optimal reef production (5–10kg CaCO3m−2yr−1) and below maxima recently recorded in remote high coral cover reef sites. However, the erosive forces observed in the Red Sea nearshore reef contributed less than observed elsewhere. A higher TA accompanied reef growth across the shelf gradient, whereas stronger diurnal pH fluctuations were associated with negative carbonate budgets. Noteworthy for this oligotrophic region was the positive effect of phosphate, which is a central micronutrient for reef building corals. While parrotfish contributed substantially to bioerosion, our dataset also highlights coralline algae as important local reef builders. Altogether, our study establishes a baseline for reef growth in the central Red Sea that should be useful in assessing trajectories of reef growth capacity under current and future ocean scenarios.
Water chemistry reveals a significant decline in coral calcification rates in the southern Red Sea
Published 13 September 2018 Science ClosedTags: biological response, BRcommunity, calcification, chemistry, corals, field, Red Sea
Experimental and field evidence support the assumption that global warming and ocean acidification is decreasing rates of calcification in the oceans. Local measurements of coral growth rates in reefs from various locations have suggested a decline of ~6–10% per decade since the late 1990’s. Here, by measuring open water strontium-to-alkalinity ratios along the Red Sea, we show that the net contribution of hermatypic corals to the CaCO3 budget of the southern and central Red Sea declined by ~100% between 1998 and 2015 and remained low between 2015 and 2018. Measured differences in total alkalinity of the Red Sea surface water indicate a 26 ± 16% decline in total CaCO3 deposition rates along the basin. These findings suggest that coral reefs of the southern Red Sea are under severe stress and demonstrate the strength of geochemical measurements as cost-effective indicators for calcification trends on regional scales.
Eutrophication may compromise the resilience of the Red Sea coral Stylophora pistillata to global change
Published 11 May 2018 Science ClosedTags: biological response, corals, laboratory, multiple factors, nutrients, Red Sea, temperature
- • Interactive effects of stressors are variable; coral reefs should be managed on a local scale in accordance with local data.
- • Additive effects of nutrients and global stressors result in changes in coral symbionts leading to shifts in overall health.
- • Gulf of Aqaba corals may be resilient to OA and warming, yet a rise in nutrients would severely impede the reef.
Environmental stressors are adversely affecting coral reef ecosystems. There is ample evidence that scleractinian coral growth and physiology may be compromised by reduced pH, and elevated temperature, and that this is exacerbated by local environmental stressors. The Gulf of Aqaba is considered a coral reef refuge from acidification and warming but coastal development and nutrient effluent may pose a local threat. This study examined the effects of select forecasted environmental changes (acidification, warming, and increased nutrients) individually and in combination on the coral holobiont Stylophora pistillata from the Gulf of Aqaba to understand how corals in a potential global climate change refugia may fare in the face of local eutrophication. The results indicate interactions between all stressors, with elevated nutrient concentrations having the broadest individual and additive impacts upon the performance of S. pistillata. These findings highlight the importance of maintaining oligotrophic conditions to secure these reefs as potential refugia.
The Red Sea simulator: a high‐precision climate change mesocosm with automated monitoring for the long‐term study of coral reef organisms
Published 10 May 2018 Science ClosedTags: biological response, field, mesocosms, methods, Red Sea
Experimental systems that enable the controlled perturbation of environmental parameters toward future scenarios are in high demand and becoming increasingly advanced. Herein, we describe the design and assess the performance of a large‐scale, flow‐through, mesocosm system. Located in the northern Gulf of Aqaba, the Red Sea simulator (RSS) was constructed in order to expose local coral reef organisms to future ocean scenarios. Seawater temperature and pH are typically set to a delta from incoming seawater readings and thus follow the diel range. This is achieved through automated monitoring (sensor‐carrying robot) and feedback system and a remote‐controlled user interface. Up to six different temperatures and four pH scenarios can be concomitantly operated in a total of 80 experimental aquaria. In addition, the RSS currently facilitates the manipulation of light intensity, light spectra, nutrient concentration, flow, and feeding regime. Monitoring data show that the system performs well; meeting the user‐defined environmental settings. A variety of reef organisms have been housed in the system for several months. Brooding reef building and soft coral species maintained in the simulator for many months have released planulae in synchrony with field colonies. This system boasts a high degree of replication, potential for multistressor manipulation, typical physiochemical environmental variability, and remotely controlled monitoring and data acquisition. These aspects greatly enhance our ability to make ecologically relevant performance assessments in a changing world.
Coral reef carbonate budgets and ecological drivers in the naturally high temperature and high total alkalinity environment of the Red Sea
Published 14 March 2018 Science ClosedTags: abundance, biogeochemistry, biological response, calcification, chemistry, corals, dissolution, field, otherprocess, Red Sea
The coral structural framework is crucial for maintaining reef ecosystem function and services. Rising seawater temperatures impair the calcification capacity of reef-building organisms on a global scale, but in the Red Sea total alkalinity is naturally high and beneficial to reef growth. It is currently unknown how beneficial and detrimental factors affect the balance between calcification and erosion, and thereby overall reef growth, in the Red Sea. To provide estimates of present-day carbonate budgets and reef growth dynamics in the central Red Sea, we measured in situ net-accretion and net-erosion rates (Gnet) by deployment of limestone blocks to estimate census-based carbonate budgets (Gbudget) in four reef sites along a cross-shelf gradient (25 km). In addition, we assessed abiotic (i.e., temperature, inorganic nutrients, and carbonate system variables) and biotic (i.e., calcifier and bioeroder abundances) variables. Our data show that aragonite saturation states (Ω = 3.65–4.20) were in the upper range compared to the chemistry of other tropical reef sites. Further, Gnet and Gbudget encompassed positive (offshore) and negative (midshore-lagoon and exposed nearshore site) carbonate budgets. Notably, Gbudget maxima were lower compared to reef growth from undisturbed Indian Ocean reefs, but erosive forces for Red Sea reefs were not as strong as observed elsewhere. In line with this, a comparison with recent historical data from the northern Red Sea suggests that overall reef growth in the Red Sea has remained similar since 1995. When assessing reef sites across the shelf gradient, AT correlated well and positive with reef growth (ρ = 0.9), while temperature (ρ = −0.7), pH variation (ρ = −0.8), and pCO2 (ρ = −0.8) were weaker negative correlates. Noteworthy for this oligotrophic sea was the positive effect of PO43− (ρ = 0.7) on reef growth. In the best-fitting distance-based linear model, AT explained about 64 % of Gbudget. Interestingly, parrotfish abundances added up to 78 % of the explained variation, further corroborating recent studies that highlight the importance of parrotfish to reef ecosystem functioning. Our study provides a baseline for reef growth in the central Red Sea that will be particularly useful in assessing future trajectories of reef growth capacities under current and future ocean warming and acidification scenarios.
Ocean acidification in the Middle East and North African region
Published 28 February 2018 Science ClosedTags: Mediterranean, modeling, Red Sea, regionalmodeling, socio-economy
After examining the current state of knowledge about ocean acidification in Middle East and North African (MENA) countries, we model the socio-economic impacts of disasters, ocean acidification and ecological risk. We use Extreme Value Theory and Peak Over Threshold concept to define the critical threshold point for ocean pH value as an Ornstein-Uhlenbeck process, initially with Gaussian noise. We define the benchmark pH based on time series observations which exhibit moderate to large variations and use Monte Carlo simulations and also model non-Gaussian cases to examine the probability of disasters.
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Stylophora pistillata in the Red Sea demonstrate higher GFP fluorescence under ocean acidification conditions
Published 24 January 2018 Science ClosedTags: biological response, corals, laboratory, molecular biology, physiology, Red Sea
Ocean acidification is thought to exert a major impact on calcifying organisms, including corals. While previous studies have reported changes in the physiological response of corals to environmental change, none have described changes in expression of the ubiquitous host pigments—fluorescent proteins (FPs)—to ocean acidification. The function of FPs in corals is controversial, with the most common consideration being that these primarily regulate the light environment in the coral tissue and protect the host from harmful UV radiation. Here, we provide for the first time experimental evidence that increased fluorescence of colonies of the coral Stylophora pistillata is independent of stress and can be regulated by a non-stressful decrease in pH. Stylophora pistillata is the most abundant and among the most resilient coral species in the northern Gulf of Eilat/Aqaba (GoE/A). Fragmented “sub-colonies” (n = 72) incubated for 33 days under three pH treatments (ambient, 7.9, and 7.6), under ambient light, and running seawater showed no stress or adverse physiological performance, but did display significantly higher fluorescence, with lower pH. Neither the average number of planulae shed from the experimental sub-colonies nor planulae green fluorescent protein (GFP) expression changed significantly among pH treatments. Sub-colonies incubated under the lower-than-ambient pH conditions showed an increase in both total protein and GFP expression. Since extensive protein synthesis requires a high level of transcription, we suggest that GFP constitutes a UV protection mechanism against potential RNA as well as against DNA damage caused by UV exposure. Manipulating the regulation of FPs in adult corals and planulae, under controlled and combined effects of pH, light, and temperature, is crucial if we are to obtain a better understanding of the role played by this group of proteins in cnidarians.
Taking the metabolic pulse of the world’s coral reefs
Published 10 January 2018 Science ClosedTags: biogeochemistry, biological response, calcification, chemistry, corals, dissolution, field, Indian, North Atlantic, North Pacific, physiology, primary production, Red Sea, South Pacific
Worldwide, coral reef ecosystems are experiencing increasing pressure from a variety of anthropogenic perturbations including ocean warming and acidification, increased sedimentation, eutrophication, and overfishing, which could shift reefs to a condition of net calcium carbonate (CaCO3) dissolution and erosion. Herein, we determine the net calcification potential and the relative balance of net organic carbon metabolism (net community production; NCP) and net inorganic carbon metabolism (net community calcification; NCC) within 23 coral reef locations across the globe. In light of these results, we consider the suitability of using these two metrics developed from total alkalinity (TA) and dissolved inorganic carbon (DIC) measurements collected on different spatiotemporal scales to monitor coral reef biogeochemistry under anthropogenic change. All reefs in this study were net calcifying for the majority of observations as inferred from alkalinity depletion relative to offshore, although occasional observations of net dissolution occurred at most locations. However, reefs with lower net calcification potential (i.e., lower TA depletion) could shift towards net dissolution sooner than reefs with a higher potential. The percent influence of organic carbon fluxes on total changes in dissolved inorganic carbon (DIC) (i.e., NCP compared to the sum of NCP and NCC) ranged from 32% to 88% and reflected inherent biogeochemical differences between reefs. Reefs with the largest relative percentage of NCP experienced the largest variability in seawater pH for a given change in DIC, which is directly related to the reefs ability to elevate or suppress local pH relative to the open ocean. This work highlights the value of measuring coral reef carbonate chemistry when evaluating their susceptibility to ongoing global environmental change and offers a baseline from which to guide future conservation efforts aimed at preserving these valuable ecosystems.
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Reproductive and trans-generational effect of ocean acidification and warming on the coral Stylophora pistillata in the Gulf of Aqaba
Published 8 November 2017 Science ClosedTags: adaptation, biological response, corals, laboratory, mortality, multiple factors, otherprocess, physiology, Red Sea, reproduction, temperature
Global warming is threatening 75 % of the world’s coral reefs. The reproduction of corals is a driver for the development of the whole reef ecosystem. Then, it is essential to better understand the transgenerational mechanisms in the response of parents and offspring to elevated temperature and lowered pH. Colonies of Stylophora pistillata from the Gulf of Aqaba during their reproduction period were exposed to a 4°C increase in temperature and a pH of 7.6 for 36 days, then a 6°C increase for six days. Planulae were counted on seven consecutive nights, two times during the experiment period. Physiological characteristics of adult and planulae were assessed on four and five sampling points respectively, as well as the behaviour of the planulae through their incubation. Results show no effect of OWA on the reproduction, parents, and planulae physiology. They suggest that the natural resistance of corals in the Gulf of Aqaba is transmitted from parent to offspring. Data on planulae quantity, survival, settlement, and metabolism provides additional and useful information to understand the biology of this coral, specially in its early-life stage. This study’s outcome is adding evidences of the future development of corals reefs in this region, unlike several other tropical reefs in the world.
Common reef-building coral in the Northern Red Sea resistant to elevated temperature and acidification
Published 19 May 2017 Science ClosedTags: biological response, corals, laboratory, multiple factors, photosynthesis, physiology, primary production, protists, Red Sea, temperature
Coral reefs are currently experiencing substantial ecological impoverishment as a result of anthropogenic stressors, and the majority of reefs are facing immediate risk. Increasing ocean surface temperatures induce frequent coral mass bleaching events—the breakdown of the nutritional photo-symbiosis with intracellular algae (genus: Symbiodinium). Here, we report that Stylophora pistillata from a highly diverse reef in the Gulf of Aqaba showed no signs of bleaching despite spending 1.5 months at 1–2°C above their long-term summer maximum (amounting to 11 degree heating weeks) and a seawater pH of 7.8. Instead, their symbiotic dinoflagellates exhibited improved photochemistry, higher pigmentation and a doubling in net oxygen production, leading to a 51% increase in primary productivity. Nanoscale secondary ion mass spectrometry imaging revealed subtle cellular-level shifts in carbon and nitrogen metabolism under elevated temperatures, but overall host and symbiont biomass proxies were not significantly affected. Now living well below their thermal threshold in the Gulf of Aqaba, these corals have been evolutionarily selected for heat tolerance during their migration through the warm Southern Red Sea after the last ice age. This may allow them to withstand future warming for a longer period of time, provided that successful environmental conservation measures are enacted across national boundaries in the region.
Mediterranean versus Red sea corals facing climate change, a transcriptome analysis
Published 2 March 2017 Science ClosedTags: algae, biological response, corals, individualmodeling, laboratory, Mediterranean, molecular biology, multiple factors, photosynthesis, physiology, Red Sea, temperature
The anthropogenic increase in atmospheric CO2 that drives global warming and ocean acidification raises serious concerns regarding the future of corals, the main carbonate biomineralizers. Here we used transcriptome analysis to study the effect of long-term gradual temperature increase (annual rate), combined with lowered pH values, on a sub-tropical Red Sea coral, Stylophora pistillata, and on a temperate Mediterranean symbiotic coral Balanophyllia europaea. The gene expression profiles revealed a strong effect of both temperature increase and pH decrease implying for synergism response. The temperate coral, exposed to a twice as high range of seasonal temperature fluctuations than the Red Sea species, faced stress more effectively. The compensatory strategy for coping apparently involves deviating cellular resources into a massive up-regulation of genes in general, and specifically of genes involved in the generation of metabolic energy. Our results imply that sub-lethal, prolonged exposure to stress can stimulate evolutionary increase in stress resilience.
Spatial competition dynamics between reef corals under ocean acidification
Published 12 January 2017 Science ClosedTags: biological response, BRcommunity, communityMF, communitymodeling, corals, growth, laboratory, modeling, multiple factors, performance, Red Sea
Climate change, including ocean acidification (OA), represents a major threat to coral-reef ecosystems. Although previous experiments have shown that OA can negatively affect the fitness of reef corals, these have not included the long-term effects of competition for space on coral growth rates. Our multispecies year-long study subjected reef-building corals from the Gulf of Aqaba (Red Sea) to competitive interactions under present-day ocean pH (pH 8.1) and predicted end-of-century ocean pH (pH 7.6). Results showed coral growth is significantly impeded by OA under intraspecific competition for five out of six study species. Reduced growth from OA, however, is negligible when growth is already suppressed in the presence of interspecific competition. Using a spatial competition model, our analysis indicates shifts in the competitive hierarchy and a decrease in overall coral cover under lowered pH. Collectively, our case study demonstrates how modified competitive performance under increasing OA will in all likelihood change the composition, structure and functionality of reef coral communities.
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Environmental controls on daytime net community calcification on a Red Sea reef flat
Published 1 June 2016 Science ClosedTags: biological response, BRcommunity, calcification, corals, field, primary production, Red Sea
Coral growth and carbonate accumulation form the foundation of the coral reef ecosystem. Changes in environmental conditions due to coastal development, climate change, and ocean acidification may pose a threat to net carbonate production in the near future. Controlled laboratory studies demonstrate that calcification by corals and coralline algae is sensitive to changes in aragonite saturation state (Ωa), as well as temperature, light, and nutrition. Studies also show that the dissolution rate of carbonate substrates is impacted by changes in carbonate chemistry. The sensitivity of coral reefs to these parameters must be confirmed and quantified in the natural environment in order to predict how coral reefs will respond to local and global changes, particularly ocean acidification. We estimated the daytime hourly net community metabolic rates, both net community calcification (NCC) and net community productivity (NCP), at Sheltered Reef, an offshore platform reef in the central Red Sea. Average NCC was 8 ± 3 mmol m−2 h−1 in December 2010 and 11 ± 1 mmol m−2 h−1 in May 2011, and NCP was 21 ± 7 mmol m−2 h−1 in December 2010 and 44 ± 4 mmol m−2 h−1 in May 2011. We also monitored a suite of physical and chemical properties to help relate the rates at Sheltered Reef to published rates from other sites. While previous research shows that short-term field studies investigating the NCC–Ωa relationship have differing results due to confounding factors, it is important to continue estimating NCC in different places, seasons, and years, in order to monitor changes in NCC versus Ω in space and time, and to ultimately resolve a broader understanding of this relationship.
Ocean acidification in the Arabian Sea and the Red Sea – factors controlling pH
Published 9 May 2016 Science ClosedTags: chemistry, field, Indian, Red Sea
The CO2 increase in the ocean due to uptake of anthropogenic CO2 and the companying lowering of ocean pH is of major concern. In this study we investigated the variability of CO2 system parameters, focusing particularly on the pH and how it changes with changes in other parameters like: temperature (T), salinity (S), total dissolved inorganic carbon (CT ), and total alkalinity (AT). For Arabian Sea the data from the United States Joint Global Ocean Flux Study (US{JGOFS) in 1995 were used. For the Red Sea data from the Geochemical Ocean Section Study (GEOSECS) in 1977 and the Mer Rouge (MEROU) cruises in June and October 1982 were used.
The seasonal and spatial variations in pH and therefore also for calcium carbonate saturation (Ar for aragonite and Ca for calcite) are controlled by biological and physical processes that in turn are driven by the in uence of monsoonal seasons. In winter season the surface average pH, Ar, and Ca in the Arabian Sea were 8.070.01, 3.90.1 and 5.90.2, respectively. A relatively high biological
production, due to the winter cooling and mixing caused by the northeast monsoonal winds increases the pH. During summer season, Southwestern monsoonal winds caused upwelling along the coast of Oman, resulting in extremely low pH values (7.9) and lower saturation for aragonite (Ar 2.36) and for calcite (Ca 3.62). Because of the strong change in pH, this area might serve as a natural laboratory for studies of ocean acidication.
For comparison, in the Red Sea, the surface average pH was 8.10.02 during winter with higher values in the north due to lower temperatures and high A(T) and C(T). The Ar and Ca were around 4.120.02 and 6.20.15, respectively, with highest values in the central part of the basin caused by higher temperatures. Summer surface pH was 8.070.03, with higher values in the north and the south due to relatively low temperature. In the central of the Red Sea, pH was low due to the convergence (high temperature). The Ar and Ca were averaged to 4.60.3 and 6.950.35, respectively, with higher values in the south and north. This is attributed to the high biological productivity in the south and the high temperature in the center of the Red Sea.
The vertical distributions of Ar, and Ca showed that the Arabian Sea is undersaturated with respect to aragonite below 600 m and calcite below 3500 m, whereas the Red Sea is supersaturated throughout the water column. In both seas pH was higher in the surface layers due to the consumption of CO2 by photosynthesis, but decreased rapidly in subsurface waters due to the release of CO2 by respiration processes. Between about 100 and 1500 m in the Arabian Sea pH is nearly constant due to the counteracting eects of decreasing temperatures and oxidation of the organic matter. The temperature effect on pH is about 0.015 units per 1C both in the Arabian Sea and Red Sea. Thus, the 0.5C warming reported for the Arabian Sea between 1904 and 1994, theoretically would result in a pH reduction of about 0.007, but the temporal coverage of the available data is unfortunately too short to verify this.
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Calcite and aragonite saturation states of the Red Sea and biogeochemical impacts of excess carbon dioxide
Published 8 April 2015 Science ClosedTags: chemistry, Red Sea, review
This chapter discusses the saturation states of the Red Sea with respect to both calcite and aragonite and their possible biogeochemical impacts as a result of ocean carbonate chemistry changes. The saturation levels of the Red Sea surface waters are several-fold supersaturated with respect to calcite and aragonite; they range from 634 to 721 % and from 446 to 488 %, respectively. The saturation levels of the deep waters range from 256 to 341 % with respect to calcite and from 177 to 230 % with respect to aragonite. They generally increase from south to the north. The lowest values of seawater supersaturation with respect to both calcite and aragonite were found at water depths >1,400 m. Changes in the seawater acid–base chemistry due to excess CO2 emission and oceanic acidification affect the saturation states of calcium carbonate. Based on reported results of the excess CO2 sink in the northern part of the Red Sea (Krumgalz et al. 1990), the estimated degree of saturation with respect to calcite and aragonite was higher by 1.9 ± 0.4 % at >200 m, 4.9 ± 0.7 % at 200–600 m, and 2.5 ± 0.1 % at >600 m in preindustrial times than in 1982. A projected drop in pH by a 0.1 unit decreases the saturation level by a factor of 1.2, whereas a drop by 0.4 pH unit decreases the saturation level by a factor of 2.1. These changes in saturation levels will have major impacts on the calcifying pelagic and benthic organisms as well as the distribution and depth of coral reefs. Low magnesian calcite and pure calcite are expected to be the dominant carbonate minerals at these low supersaturation levels.
Lithium isotopes in foraminifera shells as a novel proxy for the ocean dissolved inorganic carbon (DIC)
Published 19 January 2015 Science ClosedTags: biological response, chemistry, laboratory, otherprocess, paleo, protists, Red Sea
Past ocean pH and pCO2 are critical parameters for establishing relationships between Earth’s climate and the carbon cycle. Previous pCO2 estimates are associated with large uncertainties and are debated. In this study, laboratory cultures of the foraminiferan genus Amphistegina were performed in order to examine the possible factors that control the Li isotope composition (δ7Li) of their shells. δ7Li is insensitive to temperature and pH variations but correlates positively with the Dissolved Inorganic Carbon (DIC) of seawater. Li/Ca ratio in the shells shows negative correlation with δ7Li, consistent with published data for planktonic foraminifera from core tops and from short periods during the Cenozoic. We propose that the sensitivity of δ7Li and Li/Ca ratio to DIC is a biological phenomenon and is related to biomineralization mechanisms in foraminifera. We used the published foraminiferal δ7Li records, and our experimental results, to determine the paleo-ocean DIC and pH for the last glacial–interglacial cycle. The results are consistent with published estimates of pH and pCO2 based on boron isotopes and ice cores. We suggest Li and its isotopes may serve as a new complementary proxy for the paleo-ocean carbonate chemistry.
Basin-scale estimates of pelagic and coral reef calcification in the Red Sea and Western Indian Ocean
Published 21 November 2014 Science ClosedTags: biogeochemistry, biological response, calcification, chemistry, corals, field, Indian, modeling, phytoplankton, Red Sea, regionalmodeling
Basin-scale calcification rates are highly important in assessments of the global oceanic carbon cycle. Traditionally, such estimates were based on rates of sedimentation measured with sediment traps or in deep sea cores. Here we estimated CaCO3 precipitation rates in the surface water of the Red Sea from total alkalinity depletion along their axial flow using the water flux in the straits of Bab el Mandeb. The relative contribution of coral reefs and open sea plankton were calculated by fitting a Rayleigh distillation model to the increase in the strontium to calcium ratio. We estimate the net amount of CaCO3 precipitated in the Red Sea to be 7.3 ± 0.4·1010 kg·y−1 of which 80 ± 5% is by pelagic calcareous plankton and 20 ± 5% is by the flourishing coastal coral reefs. This estimate for pelagic calcification rate is up to 40% higher than published sedimentary CaCO3 accumulation rates for the region. The calcification rate of the Gulf of Aden was estimated by the Rayleigh model to be ∼1/2 of the Red Sea, and in the northwestern Indian Ocean, it was smaller than our detection limit. The results of this study suggest that variations of major ions on a basin scale may potentially help in assessing long-term effects of ocean acidification on carbonate deposition by marine organisms.
Tough as a rock-boring urchin: adult Echinometra sp. EE from the Red Sea show high resistance to ocean acidification over long-term exposures
Published 1 September 2014 Science ClosedTags: biological response, echinoderms, laboratory, morphology, Red Sea, reproduction
Ocean acidification, a process caused by the continuous rise of atmospheric CO2 levels, is expected to have a profound impact on marine invertebrates. Findings of the numerous studies conducted in this field indicate high variability in species responses to future ocean conditions. This study aimed at understanding the effects of long-term exposure to elevated pCO2 conditions on the performance of adult Echinometra sp. EE from the Gulf of Aqaba (Red Sea). During an 11-month incubation under high pCO2 (1,433 μatm, pHNBS 7.7) and control (435 μatm, pHNBS 8.1) conditions, we examined the urchins’ somatic and gonadal growth, gametogenesis and skeletal microstructure. Somatic and gonadal growths were exhibited with no significant differences between the treatments. In addition, all urchins in the experiment completed a full reproductive cycle, typical of natural populations, with no detectable impact of increased pCO2 on the timing, duration or progression of the cycle. Furthermore, scanning electron microscopy imaging of urchin tests and spines revealed no signs of the usual observed effects of acidosis, such as skeletal dissolution, widened stereom pores or non-smoothed structures. Our results, which yielded no significant impact of the high pCO2 treatment on any of the examined processes in the urchins studied, suggest high resistance of adult Echinometra sp. EE to near future ocean acidification conditions. With respect to other findings in this area, the outcome of this study provides an example of the complicated and diverse responses of echinoids to the predicted environmental changes.
