Storms have long been recognized as a force that agitates sediments, even in the deep-water. In shallow water depths (<40 m) sediments are affected dominantly by wave actions, while in deeper-water (>100 m) the impact is primarily due to suspended sediment flow. While such transport events have been studied sedimentologically extensively, their impact on porewater chemistry is yet poorly understood. The March 2020 tropical storm which impacted the Gulf of Aqaba (GoA) offered a unique opportunity to investigate this particular topic. Detailed sedimentological, mineralogical and porewater analyses were carried out on two sets of short cores collected along a transect from 270 m to 700 m water depths before and after the storm on the western margins of the GoA, which represents a steep slope with a potential for high energy transport events. The cores exhibited a decrease in grain size in the mid-slope water depth and an increase at the base of the slope after the storm, with the top of the core being enriched with terrestrial material. Both Fe2+ and NOx profiles exhibited a marked shift in the aftermath of the storm. NOx concentrations increased in the top 5 cm of all the cores and the ferruginous zone migrated upwards in both the top and bottom of the slope. We postulate that the storm event remixed the top of the sediment column and infused it with fine material. This filled burrows and decreased the diffusive coefficient across the sediment-water interface, limiting the exchange of ions. As a result, the porewater trapped in the sediment has developed along the terminal electron acceptor chain.

The isotopic composition of dissolved O-2 (delta O-18) in aquatic environments is strongly affected by the preferential uptake of the lighter isotopologue during biological consumption processes. Numerous studies have shown that during incubation experiments, the isotopic effect of microorganism respiration (epsilon(organism)) is on the order of -20 parts per thousand. However, studies of the co-variations of O-2 and delta O-18 in natural environments show considerably weaker in situ fractionation (epsilon(app)). A possible explanation for this discrepancy is that a significant fraction of the O-2 consumption is diffusion-limited. Although this is a generally accepted mechanism in sediments, it cannot explain the weak fractionations observed in mid-ocean sites. Here, we analyze a time series of O-2, delta O-18, and auxiliary data from the northern Gulf of Aqaba (Red Sea). Although an incubation experiment showed strong fractionation against the heavy isotopologue (epsilon(organism) = -24.5 parts per thousand), the in situ epsilon(app) was only -14 parts per thousand in deep water isolated from the photic zone. We show that this result requires an additional O-2 consumption mechanism with weak fractionation, rather than mixing, and suggest that this mechanism is diffusion-limited respiration into aggregates of organic material. We estimate that this mechanism could be responsible for 30% of the O-2 consumption in the Gulf and suggest that it may also constitute a major O-2 consumption pathway in the world's oceans.

The effects of heterogeneous reactions between river‐borne particles and the carbonate system were studied in the plumes of the Mississippi and Brazos rivers. Measurements within these plumes revealed significant removal of dissolved inorganic carbon (DIC) and total alkalinity (TA). After accounting for all known DIC and TA sinks and sources, heterogeneous reactions (i.e., heterogeneous CaCO 3 precipitation and cation exchange between adsorbed and dissolved ions) were found to be responsible for a significant fraction of DIC and TA removal, exceeding 10% and 90%, respectively, in the Mississippi and Brazos plume waters. This finding was corroborated by laboratory experiments, in which the seeding of seawater with the riverine particles induced the removal of the DIC and TA. The combined results demonstrate that heterogeneous reactions may represent an important controlling mechanism of the seawater carbonate system in particle‐rich coastal areas and may significantly impact the coastal carbon cycle.

In situ carbon dioxide enrichment experiments show that ocean acidification poses a threat to coral reefs by reducing the saturation state of aragonite and the concentration of carbonate ions and that this impairs community calcification.

Atmospheric dust loads and chemical compositions serve as a key link between global climate patterns and marine biogeochemical cycles. The primary source of atmospheric dust in the world today is the Sahara-Arabian desert belt. Although this source was also active during the Quaternary, the interpretation of paleo-dust records and their effects on marine ecosystems is complicated by the scarcely reported atmospheric load patterns of bioavailable phases (i.e., water and acid leachable phases) and present-day contamination of anthropogenic components. This study reports a multi-annual time series of atmospheric dust loads (2006-2016) and their chemical compositions (2006-2010) collected in the north Gulf of Aqaba (north Red Sea) at a weekly to bi-weekly resolution. Major and trace element abundances in each sample are reported for three fractions: water-soluble salts, carbonates and oxides (weak acid leach), and Al-silicates.Dust loads vary seasonally from low values in late summer (similar to 20-30 lg m(-3)) to higher values in the fall, and highest values in late winter and early spring (similar to 150-250 lg m(-3)). Major and trace element abundances allow to distinguish between the sources and chemical compositions that dominate high and low dust loads in each season. The water leachable fraction (L0) is relatively enriched in Na, Ca, K and Mg, the acid-leachable fraction (L1) is enriched in Ca as well as Na, Al, Mg, Zn, Cd and Pb, and the silicate residue (L2) in Al and Fe.High dust loads occurring mainly during winter and spring months are characterized by low Mg/Ca (L1, L2), low K/Al and Na/Al (L1) and high Ca/Al (L1), high Mg/Al (L2) and relatively un-weathered (L2) contents. High dust load intervals during winter months are characterized by low passing air masses originating from the Sahara, while the ambient winter dust (low dust load) is associated with proximal source regions from the East Sahara and Arabian Peninsula. During late winter and spring months, high dust loads originate from central and west Sahara and to a lesser extent from north Sahara. Low dust loads characterize the summer with limited compositional variability relative to winter-spring months. Summer dust is generally characterized by high K/Al (L1) ratios relative to late winter and spring. It is also relatively high in anthropogenic trace elements in the L0 and L1 fractions (e.g., Zn/Al, Pb/Al, Cr/Al, Ni/Al and V/Al), whereby back trajectories indicate the source of these components is primarily from south and east Europe. The total load (ng m(-3)) of anthropogenic trace elements however, remains higher during winter and spring, stemming from the overall significantly higher dust loads characterizing this time window.The temporal load patterns of important micronutrients such as Fe, Cd, Zn, Cu, Ni and others in the bio-available phases (L0, L1) are not correlated with major nutrients or Chlorophyll-a sea surface concentrations, suggesting that the atmosphericdust plays a limited role in driving primary productivity in the oligotrophic surface waters of the Gulf of Aqaba.On a wider scale, the results provide unique chemical fingerprinting of Sahara- Arabian dust that can be applied to reconstruct past trends in dust loads recorded in deep-sea cores and other geological archives from this and other regions. (C) 2017 Elsevier Ltd. All rights reserved.

Ocean acidification has the potential to adversely affect marine calcifying organisms, with substantial ocean ecosystem impacts projected over the 21st century. Characterizing the in situ sensitivity of calcifying ecosystems to natural variability in carbonate chemistry may improve our understanding of the long-term impacts of ocean acidification. We explore the potential for intensive temporal sampling to isolate the influence of carbonate chemistry on community calcification rates of a coral reef and compare the ratio of organic to inorganic carbon production to previous studies at the same location. Even with intensive temporal sampling, community calcification displays only a weak dependence on carbonate chemistry variability. However, across three years of sampling, the ratio of organic to inorganic carbon production is highly consistent. Although further work is required to quantify the spatial variability associated with such ratios, this suggests that these measurements have the potential to indicate the response of coral reefs to ongoing disturbance, ocean acidification, and climate change.

manipulative experiment in which a reef is alkalinized in situ shows that calcification rates are likely to be lower already than they were in pre-industrial times because of acidification.

Understanding the temporal dynamics of present thermal and pH exposure on coral reefs is crucial for elucidating reef response to future global change. Diel ranges in temperature and carbonate chemistry parameters coupled with seasonal changes in the mean conditions define periods during the year when a reef habitat is exposed to anomalous thermal and/or pH exposure. Anomalous conditions are defined as values that exceed an empirically estimated threshold for each variable. We present a 200-day time series from June through December 2010 of carbonate chemistry and environmental parameters measured on the Heron Island reef flat. These data reveal that aragonite saturation state, pH, and pCO2 were primarily modulated by biologically-driven changes in dissolved organic carbon (DIC) and total alkalinity (TA), rather than salinity and temperature. The largest diel temperature ranges occurred in austral spring, in October (1.5 - 6.6°C) and lowest diel ranges (0.9 - 3.2°C) were observed in July, at the peak of winter. We observed large diel total pH variability, with a maximum range of 7.7 - 8.5 total pH units, with minimum diel average pH values occurring during spring and maximum during fall. As with many other reefs, the nighttime pH minima on the reef flat were far lower than pH values predicted for the open ocean by 2100. DIC and TA both increased from June (end of Fall) to December (end of Spring). Using this high-resolution dataset, we developed exposure metrics of pH and temperature individually for intensity, duration, and severity of low pH and high temperature events, as well as a combined metric. Periods of anomalous temperature and pH exposure were asynchronous on the Heron Island reef flat, which underlines the importance of understanding the dynamics of co-occurrence of multiple stressors on coastal ecosystems.

Measurements of community calcification (Gnet) were made during September 2008 and October 2009 on a reef flat in Lizard Island, Great Barrier Reef, Australia, 33years after the first measurements were made there by the LIMER expedition in 1975. In 2008 and 2009 we measured Gnet=61±12 and 54±13mmolCaCO3m−2·day−1, respectively. These rates are 27–49% lower than those measured during the same season in 1975–76. These rates agree well with those estimated from the measured temperature and degree of aragonite saturation using a reef calcification rate equation developed from observations in a Red Sea coral reef. Community structure surveys across the Lizard Island reef flat during our study using the same methods employed in 1978 showed that live coral coverage had not changed significantly (∼8%). However, it should be noted that the uncertainty in the live coral coverage estimates in this study and in 1978 were fairly large and inherent to this methodology. Using the reef calcification rate equation while assuming that seawater above the reef was at equilibrium with atmospheric PCO2 and given that live coral cover had not changed Gnet should have declined by 30±8% since the LIMER study as indeed observed. We note, however, that the error in estimated Gnet decrease relative to the 1970’s could be much larger due to the uncertainties in the coral coverage measurements. Nonetheless, the similarity between the predicted and the measured decrease in Gnet suggests that ocean acidification may be the primary cause for the lower CaCO3 precipitation rate on the Lizard Island reef flat.

Recent measurements have shown that holothurians (sea cucumbers) may play an important role in the cycling of CaCO3 in tropical coral reef systems through ingestion and processing of carbonate sediment. In this report, we present estimates of inorganic carbon turnover rates determined from laboratory incubations of Holothuria afro, Holothuria leucospilota and Stichopus herrmanni. The pH values of the gut lumen ranged from 7.0 to 7.6 when digestive tracts were filled with sediment compared with 6.1-6.7 in animals with empty digestive tracts. Empty gut volume estimates for H. afro and S. he; tmanni were 36 4 mL and 151 14 ml, respectively. Based on these measurements and the density and porosity of carbonate sediments of coral reefs, it is estimated that these species process 19 2 kg and 80 7 kg CaCO3 sand yr-1 per individual, respectively. The annual CaCO3 dissolution rates per H. aria and S. hei ',Lanni individual are estimated to be 6.5 1.9 g and 9.6 1.4 g, respectively, suggesting that 0.05 0.02% and 0.1 0.02% of the CaCO3 processed through their gut annually is dissolved. During incubations the CaCO3 dissolution of the fecal casts was 0.07 0.01%, 0.04 0.01% and 0.21 0.05% for H. aria, H. leucospilota and S. herrmanni, respectively. The CaCO3 saturation state in the incubation seawater decreased markedly due to a greater increase in dissolved inorganic carbon (DIC) relative to total alkalinity (AT) as a result of respiration by the animals. Our results support the hypothesis that deposit feeders such as sea cucumbers play an important ecological role in the coral reef CaCO3 cycle. (C) 2013 Elsevier Ltd. All rights reserved.

Two alternative mechanisms are suggested for nitrite accumulation in the oxygenated oligotrophic water column: (1) excretion by phytoplankton or (2) microbial oxidation of ammonium (nitrification). This study assessed the role of these 2 mechanisms, based on seasonal and high-resolution diurnal depth profiles of the dissolved inorganic nitrogen (DIN) species (nitrite, ammonium, nitrate) and chlorophyll a in the Gulf of Aqaba, Red Sea. Both mechanisms operated in the water column, but in different seasons; nitrification was the prime process responsible for nitrite accumulation during the stratified summer season and phytoplankton nitrite excretion operated during winter mixing. At the onset of summer stratification two N peaks developed below the photic zone, an ammonium maximum (AM) and below it the primary nitrite maximum (PNM). Both peaks were located at a depth range where phytoplankton are thought to be inactive and not excreting nitrite. During summer stratification, the water column deep chlorophyll maximum (DCM), AM, PNM and the nitracline were ordered by a downward increase in N oxidation state similar to the temporal order of the N-species during nitrification. This similarity, together with the diurnal stability of the PNM and its co-existence with oscillating chlorophyll profiles above the DCM, is consistent with nitrification as the key process forming the PNM. We suggest that transport and reaction control the vertical order and separation of N-species in the water column. The ratios between the rate constants for ammonification, ammonium oxidation, nitrite oxidation and nitrate assimilation were estimated by a simple box model to be 1:3:1.5:0.15, respectively. These field estimates are similar to the ratios between the rate constants measured in laboratory experiments.

During November–December 2009 community rates of gross photosynthesis (Pg), respiration (R) and net calcification (Gnet) were estimated from low‐tide slack water measurements of dissolved oxygen, dissolved inorganic carbon and total alkalinity at the historical station DK13 One Tree Island reef, Great Barrier Reef, Australia. Compared to measurements made during the 1960s–1970s at DK13 in the same season,Pg increased from 833 to 914 mmol O2·m−2·d−1 and Pg:R increased from 1.14 to 1.30, indicating that the reef has become more autotrophic. In contrast, Gnet decreased from 133 mmol C·m−2·d−1 to 74 ± 24 mmol C·m−2·d−1. This decrease stems primarily from the threefold increase in nighttime CaCO3 dissolution from −2.5 mmol·m−2·h−1 to −7.5 mmol·m−2·h−1. Comparison of the benthic community survey results from DK13 and its vicinity conducted during this study and in studies from the 1970s, 1980s and 1990s suggest that there have been no significant changes in the live coral coverage during the past 40 years. The reduced Gnet most likely reflects the almost threefold increase in dissolution rates, possibly resulting from increased bioerosion due to changes in the biota (e.g., sea cucumbers, boring organisms) and/or from greater chemical dissolution produced by changing abiotic conditions over the past 40 years associated with climate change, such as increased temperatures and ocean acidification. However, at this stage of research on One Tree Island the effects of these changes are not entirely understood.

The spring bloom in seasonally stratified seas is often characterized by a rapid increase in photosynthetic biomass. To clarify how the combined effects of nutrient and light availability influence phytoplankton composition in the oligotrophic Gulf of Aqaba, Red Sea, phytoplankton growth and acclimation responses to various nutrient and light regimes were recorded in three independent bioassays and during a naturally-occurring bloom. We show that picoeukaryotes and Synechococcus maintained a "bloomer" growth strategy, which allowed them to grow quickly when nutrient and light limitation were reversed. During the bloom picoeukaryotes and Synechococcus appeared to have higher P requirements relative to N, and were responsible for the majority of photosynthetic biomass accumulation. Following stratification events, populations limited by light showed rapid photoacclimation (based on analysis of cellular fluorescence levels and photosystem II photosynthetic efficiency) and community composition shifts without substantial changes in photosynthetic biomass. The traditional interpretation of "bloom" dynamics (i.e., as an increase in photosynthetic biomass) may therefore be confined to the upper euphotic zone where light is not limiting, while other acclimation processes are more ecologically relevant at depth. Characterizing acclimation processes and growth strategies is important if we are to clarify mechanisms that underlie productivity in oligotrophic regions, which account for approximately half of the global primary production in the ocean. This information is also important for predicting how phytoplankton may respond to global warming-induced oligotrophic ocean expansion.

We monitored the genetic diversity and phosphate (P) status of Prochlorococcus and Synechococcus populations during an annual cycle (1999-2000) in the Gulf of Aqaba, Red Sea, using DNA probes and antibodies against the protein PstS, a molecular marker of P stress. The usual spring Synechococcus bloom was not observed in this year, and the picophytoplankton was dominated by Prochlorococcus from April to December. A single high light (HL)-adapted ecotype of Prochlorococcus, HLII, dominated, and this dominance extended even to the base of the euphotic zone for much of the year. Low light (LL)-adapted ecotypes only appeared between July and October and were confined to waters deeper than 50 m. The Synechococcus population was also dominated by a single clade (clade 11), with the exception of a peak of clade III genotypes in June. Highest PstS expression from the collective Synechococcus and Prochlorococcus populations, indicative of P stress, was observed during the acutely oligotrophic summer, whereas low expression occurred during the winter and spring. Prochlorococcus abundance appeared unaffected by P stress and only decreased with the beginning of winter mixing, whereas P stress might have caused the decline of the Synechococcus population in May 1999.

Determining the nitrogen (N) status of phytoplankton is important for understanding primary production and N cycling in marine ecosystems. We assayed transcript levels of the N regulatory gene ntcA to assess the physiological N status of Synechococcus populations exposed to different N regimes in the meso- to oligotrophic Gulf of Aqaba, Red Sea. Synechococcus populations were N sufficient even in low-N environments when the ratio of dissolved nitrogen to phosphorus indicated that overall phytoplankton biomass was constrained by N. Ammonium supported Synechococcus N requirements under most conditions, but during a massive spring bloom in April 2000 alternative N sources were utilized. Evidence from ntcA clone libraries indicates changes in the genotypic makeup of Synechococcus populations under different N regimes, suggesting that the Synechococcus genotypes present in N-poor waters were those adapted for life in these environments. Thus, the success of Synechococcus in the open oceans is likely to be at least partially due to the selection of genotypes suited to life under prevailing N conditions rather than to prolonged manifestation of the N stress response, mediated by ntcA, in less well-adapted genotypes.