Oceanographic and Biological Aspects of the Red Sea

This paper describes the present tidal regime in the Red Sea. Both the diurnal and the semidiurnal tidal amplitudes are small because of the constricted connection to the Gulf of Aden and the Indian Ocean, at the Bab el Mandeb Strait. Semidiurnal tides have a classic half-wave pattern, with a central amphidrome, zero tidal range, between Jeddah and Port Sudan. We present a high resolution numerical model output of several tidal constituents, and also model the amphidrome position in terms of ingoing and outgoing tidal Kelvin waves. We quantify the energy budgets for fluxes and dissipation.

The Gulf of Aqaba is located in the sub-tropical arid zone between 28^o–29^o30′N and 34^o30′–35^oE. It is a semi-enclosed basin that extends over a length of 180 km with a width between 5 and 25 km (average of 16 km). The deepest point in the Gulf reaches 1825 m with an average depth of 800 m. The Gulf is connected to the Red Sea by the Strait of Tiran, which has a sill depth of about 265 m. The Gulf exhibits a seasonal cycle of stratification in spring, maintenance of a shallow thermocline in summer, and subsequent deepening of the thermocline to produce deep mixed layers in winter. Much of the seasonal stratification variability is determined by exchanges with the rest of the Red Sea. Nonetheless, inter-annual variability in wintertime temperatures appears to set the depth of maximum mixing. Because of being generally warm (>21 ºC), and subject to dry winds much of the year, the Gulf is a site of high evaporation rates, estimated at 0.5–1.0 cm/day, with recent estimated values lower than earlier ones. Given a surface area of the Gulf of about 1.7 × 10⁹ m², this implies a net inflow to the Gulf of about 54 m³s⁻¹. Because the densities of the Gulf are different from the rest of the Red Sea, there are strong density-driven flows. These exchange flows through the Strait of Tiran are substantially larger than the net flows through the Straits. About 3 × 10⁴ m³s⁻¹ enters the Gulf near the surface, and leaves at depth through the Strait. The exchange varies annually with a net annual mean of 1.8 × 10⁴ m³s⁻¹. Surface water temperature may approach 28 ºC during summer months and fall to just above 20 ºC in winter. The generally weak currents (10 cm s⁻¹) in the northern Gulf of Aqaba are largely driven by the prevailing down-Gulf winds and by the semi-diurnal internal tides generated in the Strait of Tiran. The annual meteorological measurements demonstrate that the wind speed fluctuates within a range of 0–12 ms⁻¹ (mean 4.5 ± 2.4 ms⁻¹). Moreover, a harmonic change of wind speed appears during summer causing a diurnal cycle that is represented by strong winds during daytime and relatively weaker winds during the night. Meanwhile, northerly winds (NNW-NNE) dominate over the study area and represent about 85% of total measurements. Mean values of air temperature range between 32.2 ± 3.16 °C in summer and 17.6 ± 3.46 °C in winter. The minimum humidity recorded in summer is 13% compared to a maximum of 83% in winter. The maximum sea level range, with reference to Global Mean Sea Level (MSL), during the year 2013 was 154.3 cm. The highest value was 101.7 cm observed on December 12, and the lowest value was −52.6 cm recorded in April 23. The pH at coastal and offshore waters of the Jordanian Gulf of Aqaba fluctuates around 8.3 with very minor temporal and spatial variations. This is typical for all coral reef waters because these waters are always saturated with calcium carbonate, which acts as a buffer and resists change in the pH. Inorganic nutrients (ammonia, nitrate, nitrite, phosphate and silicate) are essential for marine phytoplankton productivity and growth. Higher concentrations of nutrients and chlorophyll a concentrations occur during winter that are attributed to deep water vertical mixing during winter. Cross-shore mixing (from shallow to offshore waters) due to density currents (gravity currents) has been recently documented. This process drives coastal water down slope offshore when it gets cooler at night. The increased nutrient concentrations in the euphotic zone enhance primary productivity, resulting in higher phytoplankton abundance and increased chlorophyll a concentrations. Water column stratification and high irradiance during summer result in a depletion of the inorganic nutrients in the upper waters by enhanced primary productivity at the subsurface level (50–75 m). Ammonium concentration fluctuates irregularly around 0.4 μM with a tendency to higher concentrations during the winter months (January to March). Nitrate and nitrite concentrations during the last five years showed a regular shift from a summer low (0.10 and 0.01 µM) to relatively high early winter values (0.6 and 0.25 µM). Phosphate concentrations are generally low during summer (~0.02 µM) and high during winter (~0.10 µM). Silicate concentrations show the same trend with 1.0 µM during summer and ~2.0 µM in the winter. Dissolved oxygen concentrations at the Gulf of Aqaba show a regular pattern, inversely proportional to that of temperature, with a range of 6.4 to 7.4 mgl⁻¹, indicating that the effects of the other ecosystem variables are masked by temperature. Waters of the Gulf of Aqaba are very well balanced in terms of respiration and photosynthesis and well ventilated due to the annual deep mixing with a saturation of 100%.

This chapter discusses the various input sources of extractable organic matter (EOM) compounds to the Red Sea. These are based on geochemical analyses of atmospheric dust, surface seawater particulate matter and sediment samples collected from the Gulf of Aqaba and the coasts of the Gulf of Suez, Saudi Arabia and Yemen. The samples were extracted with a dichloromethane/methanol mixture and analyzed by gas chromatography-mass spectrometry (GC-MS). The EOM compounds (lipids) in the samples are diverse and include n-alkanes, n-alkanoic acids, n-alkanols, methyl n-alkanoates, steroids, petroleum hydrocarbons and plasticizers. The steroids and n-alkanoic acids were major components of the surface seawater particulate matter samples, whereas petroleum hydrocarbons were major compounds in coastal sediments. Based on the results of the different samples, the main input sources of these lipids were from: (1) natural autochthonous microbiota (plankton and bacteria) as indicated by the presence of cholesterol and brassicasterol in the different surface seawater particulate matter and sediment samples; (2) natural allochthonous material origins from terrestrial plant detritus transported by dust as shown by the distributions of n-alkanoic acids, n-alkanols and phytosterols; and (3) anthropogenic sources (mainly petroleum) from regional oil production activities, oil tankers or shipping activities as revealed by the n-alkane distribution pattern and the presence of an unresolved complex mixture (UCM) of branched and cyclic hydrocarbons, with hopane and sterane biomarkers. Future studies of the organic and inorganic biogeochemistry on the water column, coastal areas and dust transported to the Red Sea are needed to characterize the various regional sources, transformation, and diagenetic processes of the organic matter en route to this marine environment.

The Red Sea is an oligotrophic marginal sea where the main source of nitrogen and phosphorus is the Indian Ocean water flowing through the Bab al Mandeb entrance in the south. Therefore, the nitrogen and phosphorus concentrations are expected to decrease northward. External sources resulting from urban activities enhance the level of phosphorus, nitrogen and carbon in coastal waters around major cities along the Red Sea. We utilized the results of dissolved inorganic phosphorus (reactive phosphate), dissolved inorganic nitrogen (nitrate, nitrite and ammonium) and organic carbon (dissolved organic carbon (DOC) and particulate organic carbon (POC)) along the coast of Jeddah in the eastern Red Sea, in order to understand the distribution, sources, and biogeochemical processes that control their levels in sea water. Moreover, the results were used to calculate the anthropogenic flux and contribution to the total budget of the Red Sea. The spatial distribution patterns showed very high concentrations of nitrogen, phosphorus and organic carbon in the water at the southern coast of Jeddah in comparison to the northern coast. Most of the wastewater (>300,000 m³ per day) of the city is discharged at this part of the coast. The quantity is beyond the nominal treatment capacity of the existing wastewater treatment plants, resulting in poor treatment efficiency. Further evidence of the importance of sewage discharges was obtained using salinity variations. The salinity was remarkably low at these locations and the projections of salinity against nitrogen, phosphorus and organic carbon revealed significant negative correlations. The seasonal distribution of DOC and POC reflected the seasonality of the primary productivity, showing higher values in late spring. POC showed a substantial proportion, accounting for up to 29% of total organic carbon. Ammonium was the major component in autumn, representing about 60% of the total inorganic nitrogen (TIN), while in spring nitrate became the principal component, constituting approximately 62% of the TIN. The application of various techniques revealed that nitrogen was the potential limiting element. Direct measurements and calculations indicated that the daily production of total nitrogen and phosphorus associated with sewage discharges into Jeddah coastal waters is about 21261 and 3360 kg, respectively. The inorganic forms of nitrogen and phosphorus represent 43 and 45% of the total nitrogen and phosphorus introduced into the area. Anthropogenic nitrogen and phosphorus sources at this part of the Red Sea coast represent about 0.9 and 9.9% of the deficit of the two elements through the Red Sea/Indian Ocean water exchange process at the Strait of Bab al Mandab.

The novelty of this study is the use of a multi-objective evolutionary algorithm for the automatic detection of hydrodynamic turbulent boundaries overlying coral reefs. The procedure is implemented using sequences of Flock-1 satellite data acquired in the Red Sea. The study demonstrates that implementing Pareto-optimal solutions allows for the generation of accurate coral reef-water interface patterns. This is validated by a Pareto-optimal front and the receiver-operating characteristic (ROC) curve. The Pareto-optimal front indicates a significant relationship between hydrodynamic turbulent boundaries, macroalgae, and coral reefs. The ROC curves confirm the finding of the Pareto-optimal front that hydrodynamic turbulent boundary layers and macroalgae are caused by coral reefs. The performance accuracy is identified with an under-curve area of 90%. In conclusion, the multi-objective evolutionary algorithm has the applicability for the automatic detection of hydrodynamic turbulent boundary layers to coral reef studies.

The lagoons along the Saudi Arabian coast of the Red Sea are rich in biodiversity. The Presidency of Meteorology and Environment of Saudi Arabia with the collaboration of the International Union for Conservation of Nature and Natural Resources has declared some of these lagoons as sensitive sites. These lagoons differ widely from each other and the current velocities and flushing time are a function of size, shape, tidal range and size of inlet. The water column conditions of Rabigh Lagoon and the flushing times of Shoaiba, Obhur, Ras Hatiba, Rabigh and Yanbu lagoons are determined. The water column conditions are based on the change in potential energy relative to the potential energy when the water column is mixed, and depend on the balance of heat at the air-sea interface, wind and tidal mixing. A negative potential energy change \(\frac{{\text{dv}}}{{\text{dt}}}\) develops stratification and positive potential energy change tends to mix the water column. In Rabigh Lagoon, the water column remains mixed throughout the year except in September–October, when a weak stratification develops. The flushing time of the lagoons varies from a few days to about a month. The marine environment in arid zone lagoons is under stress due to high temperature and salinities. However, the present-day flushing time scale may not exert an intolerable stress on the ecology of these lagoons. The relative important of local wind in the flushing of these lagoons can vary substantially. The changing environment of these lagoons due to rapid urbanization, industrialization and utilization of the coast may change their conditions. These lagoons are highly productive but are also stressed by anthropogenic inputs and human activities and need continuous monitoring.

Surface sediment samples from Obhur Lagoon on the Red Sea coast of Saudi Arabia were collected and analyzed to determine the levels, distribution and sources of hydrocarbon compounds. The sediments were collected using a Van Veen grab sampler, dried, extracted with a mixture of dichloromethane/methanol, and analyzed by gas chromatography-mass spectrometry. The major hydrocarbons of these lipid extracts included n-alkanes (6–645 ng g⁻¹), methyl n-alkanoates (1–311 ng g⁻¹), hopanes (3–219 ng g⁻¹), steranes (5–258 ng g⁻¹), phthalates (4–201 ng g⁻¹), and an unresolved complex mixture (UCM) (0–4571 μg g⁻¹). Anthropogenic petroleum products and plasticizers were the main sources of these hydrocarbons, with lesser amounts from biogenic sources, including natural waxes of terrestrial higher plants and marine microbial detritus. The estimated anthropogenic inputs of the total lipid hydrocarbons for all sediments ranged from 14 to 98% for petroleum products and from 2 to 18% for plasticizers, whereas biogenic inputs ranged from 0 to 10.7% for terrestrial higher plant waxes and from 0 to 68.5% for marine microbial detritus. The presence and inputs of petroleum residues and plasticizers to the coastal sediments of the lagoon likely affect the marine ecosystems and associated species groups that use the lagoon as a coastal nursery and spawning area.

Surficial sediment samples were collected from sixty stations between 23°N and 28°N latitudes in the northern Red Sea and were analyzed for 10 metals, namely Co, Cr, Cu, Fe, Hg, Mn, Mo, Ni, V, Zn and a metalloid, As. Based on mean concentrations, the order of abundance of the metals (dry weight) was: Fe (6320.61 mg kg⁻¹) > Mn (409.71 mg kg⁻¹) > Zn (38.76 mg kg⁻¹) > V (19.73 mg kg⁻¹) > Ni (15.92 mg kg⁻¹) > Cu (15.51 mg kg⁻¹) > Cr (14.6 mg kg⁻¹) > Co (6.53 mg kg⁻¹) > As (5.21 mg kg⁻¹) > Mo (1.06 mg kg⁻¹) > Hg (0.03 mg kg⁻¹). Arsenic was the only element to exhibit exceedance with 88% of the stations above upper continental crust concentrations (UCC) and 8% of the stations above threshold effect level (TEL). Sediment contamination assessment was carried out using the geoaccumulation index (I_geo) and enrichment factor (EF) and ecological hazard was assessed using the Adverse Effect Index (AEI), Potential ecological risk factor (ER) and Potential ecological index (RI). Hierarchical Cluster analysis (HAC), Principal Component Analysis (PCA) and Linear Discriminant Analysis (LDA) were used to group stations as “uncontaminated”, “minor enrichment”, “metallogenic enrichment” and “anthropogenic enrichment”.

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 depth ranged from 25.4 to 27.0 °C, whereas at deeper seawater layers (>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.

The deep-sea brines of the Red Sea are unusual extreme environments and form characteristically steep gradients across the brine-seawater interfaces. Due to their unusual nature and unique combination of physical-chemical conditions these interfaces provide an interesting source of new findings in the fields of geochemistry, geology, microbiology, biotechnology, virology, and general biology. The current chapter summarizes recent and new results in the study of geochemistry and life at the interfaces of brine-filled deeps of the Red Sea.

By the year 2025 Yemen’s per capita water availability will be around 89 m³/year and the country will be highly water stressed. As a consequence, economic status of the farmers involved in qat (also referred as khat) cultivation, a product that supports 25% of the country’s GDP, will fall below the poverty line. With declining water table, the Mesozoic–Cenozoic aquifer of Yemen will be unable to support irrigation and the geothermal reservoir too will decline due to excessive withdrawal of water. A solution to this problem is to develop the geothermal resources around Damt and Dhamar to support desalination of the Red Sea and Gulf of Aden seawater to generate fresh water to contribute to the country’s food and energy security. Damt and Dhamar silicic volcanic sites have the potential to generate more than 134 × 10⁶ kW of electricity. Fresh water generated through desalination using geothermal sources and wastewater treatment plants (WWTPs) will give the country food and energy security and reduce dependence on food imports.

The Red Sea is characterised by a unique composition of species of fishes which, based on unpublished data of the present authors, currently consists of 1166 species from 159 families whose habitats range from shallow waters to the deep sea. There is a total of 1120 species in coastal waters of the Red Sea recorded within an overall depth range 0–200 m; among them, 165 species are exclusively endemics to the Red Sea, whilst another 51 species are restricted to the Red Sea and Gulf of Aden only, and 22 species living at depths greater than 200 m are endemic. As the westernmost peripheral area of the Indo-West Pacific region, the Red Sea is at the opposite end of the distributions of many widespread coral reef organisms that range to the easternmost regions, such as the Hawaiian Islands, Easter Island, and the Marquesas Islands. It is noted that these areas exhibit high percentages of endemism among coastal fishes. The Hawaiian archipelago has 30.7% of its fishes as endemic species; Easter Island has 21.7%, the Red Sea 14.7% (19.3% when combined with the Gulf of Aden), and the Marquesas Islands have 13.7% endemic fishes. The Red Sea is 2250 km in length and it is very deep, with an average depth of 490 m, and a maximum depth of 3040 m. As expected, the fish fauna is far from homogeneous. The most divergent sector is the Gulf of Aqaba. We have noted that its entrance to the rest of the Red Sea is shallow. It has a maximum width of only 24 km, but a maximum depth of 1850 m. The shore drops off quickly to deep water. The prevailing cross wind creates upwelling, resulting in surface sea temperature at least as low as 21 ℃. Twenty-two of 46 species of Red Sea fishes living at depths greater than 200 m in the Red Sea are endemic (48% endemism). The Gulf of Aqaba has 22 endemic coastal species of fishes and eight endemic deep-dwelling species. By contrast, the neighboring Gulf of Suez, with extensive sand flats and a maximum depth of 70 m, has only seven endemic species of fishes. Of the 165 endemic Red Sea species of fishes, only two are elasmobranchs. Twenty-three families of Red Sea fishes have more than 20% of endemic species with the highest rates of endemism occurring among the Pseudochromidae, Schindleriidae (83.3% and 100% respectively) and the family Gobiidae with the greatest number of endemic species (36 of 139 recorded species). A brief summary of the history of scientific research on Red Sea fishes is provided together with complete lists of endemic species for (i) the entire Red Sea (separately for coastal and deep-dwelling fishes); (ii) the Red Sea combined with the Gulf of Aden; (iii) the Gulf of Aqaba and the Gulf of Suez; and (iv) Lessepsian migrants. Ongoing research is likely to reveal additional endemic species in the region.

The evolutionary origins of sharks date back more than 400 million years, ranking them among the oldest extant taxa of vertebrates. Sharks are found throughout the world’s oceans, inhabiting coastal waters and open seas, from the surface to depths of 3000 m. Of the more than 500 shark species described to date, only 29 are found in the Red Sea. In this chapter, I summarise the available information on life-history, habitat use, population genetics, fisheries and conservation of sharks in the Red Sea. This information is supplemented with unpublished data on reproductive parameters and shark fisheries that I collected between 2010 and 2014 along the Saudi Arabian Red Sea coast. Overall, it is apparent that, relative to other ocean basins, information on shark biology is sparse for the Red Sea. Yet, the presented data is sufficient to clearly indicate strong overfishing of Red Sea shark populations, calling for urgent regional efforts to assess the status of these species and to develop and implement effective management plans to ensure socio-ecological sustainability.

The number of cetacean species present in the Red Sea is unknown. Navigation and associated exploration of Red Sea waters dates back thousands of years, but despite relatively high levels of human activity in the basin, observations of cetaceans in the Red Sea remain sparse. However, the absence of a comprehensive record of these marine mammals in the Red Sea is not due to the absence of cetaceans. The first published report of cetaceans in the Red Sea was made by Forskål at the end of the 18th century and information about encounters with both live and stranded dolphins and whales continued throughout the 19th century. During the first 80 years of the 20th century a number of new sightings confirmed previous observations, suggesting some additions to the list. Following establishment of the Indian Ocean Whale Sanctuary in 1979, a renewed interest arose about cetacean conservation and dedicated surveys finally commenced. Information from smaller-scale projects was then collected in the waters off Egypt, Saudi Arabia, Sudan, Eritrea, Yemen and Israel, further raising the tally of cetacean species recorded in the Red Sea. The timely review presented in this chapter notes that at least 17 species of cetaceans have been observed in the Red Sea, including: Balaenoptera edeni, B. musculus, B. omurai, Megaptera novaeangliae, Delphinus delphis cfr. tropicalis, Grampus griseus, Globicephala macrorhynchus, Kogia sima, Orcinus orca, Pseudorca crassidens, Steno bredanensis, Stenella attenuata, S. coeruleoalba, S. longirostris, Sousa plumbea, Tursiops aduncus, and T. truncatus. Whilst the cetacean populations of the northern Red Sea have been recently assessed, it is a matter of concern that much less is known about the presence of cetaceans in the central and southern parts of the basin. Given the accelerating growth of human populations, together with the associated degradation of the marine environment, there is an urgent need for a more up-to-date appraisal of cetaceans, including the presence, abundance, distribution and behaviour of represented species throughout the Red Sea. The effectiveness of cetacean stock management and conservation depends on such information and there is a duty of care for governments, NGOs and academic institutions within the region to support and facilitate the research required to acquire a better understanding of the Red Sea’s whales and dolphins.

Despite the unique characteristics of the Red Sea (high temperature, high salinity, and oligotrophic conditions), only a few benthic studies have been conducted in its deep-sea environments. Hence, a study on macrobenthos was undertaken in deep-sea locations of the Red Sea within the Saudi Arabian waters. Sediment samples were collected using a box corer from 63 stations located between about 23°N and 28°N, during a cruise on R. V. Aegaeo in November 2012. The depth of the stations ranged from 96 to 1,678 m, with 85% of the stations having a depth ≥500 m. Hydrographical parameters were measured, and sediment characteristics were determined. A total of 199 taxa, including 147 polychaetes, 27 crustaceans, 19 molluscs and 6 other taxa were recorded. Density (10–2,712 ind. m⁻²), biomass (0.02–10.98 gm⁻²), species richness (1–59 taxa), and Shannon-Wiener diversity, H′ (0–5.1) were estimated. Biotic variables decreased with depth, while latitudinal variations were absent. The spatial patterns in macrobenthic variables closely match those of the organic matter, indicating the importance of food availability for the sustenance of the macrobenthic communities in the deep-sea environments of the Red Sea. Other parameters such as temperature, salinity, and dissolved oxygen were also found to influence the macrobenthos.

Seagrasses rank among the most productive ecosystems with important implications in climate change mitigation. Tropical and subtropical seas hold the largest seagrass species richness. A total of 12 different seagrass species have been reported from the Red Sea. However, there is little information on seagrass diversity and distribution along the Saudi Arabian coast of the Red Sea. This study aims to capture: (i) the distribution and composition of seagrasses from 18°N to 28°N latitudes on a broader scale, and (ii) the species composition, distribution and abundance of seagrasses by detailed investigations at three locations along the Saudi Arabian coast: Sharma, Umluj and Jazan, representing the northern, central and southern Red Sea. The most commonly observed seagrass species along the Red Sea were Halodule uninervis (17 observations), Thalassia hemprichii (13 observations) and Halophila stipulacea (11 observations). Halophila stipulacea was the most dominant species at each of the three locations studied in more detail. Syringodium isoetifolium and Thalassodendron ciliatum were found only at Umluj, while H. ovalis and T. hemprichii were found only at Jazan. H. uninervis was observed at both Umluj and Jazan. Shoot lengths of H. stipulacea and H. uninervis showed significant differences among the three locations. The average above-ground biomass of seagrasses differed significantly among locations (analysis <0.05; multiple tests), with the highest biomass for Halophila stipulacea recorded at Jazan (81 ± 24 gDW m⁻²) and an average biomass for T. ciliatum of 74 ± 16 gDW m⁻² at Umluj. The species T. ciliatum was the only taxa that exhibited significant differences (p < 0.05) in the abundance of seagrasses among the three locations. This work contributes further to our understanding of the distribution and diversity of seagrasses in the Red Sea, confirming a high seagrass richness with at least ten different species along the Saudi Arabian coast of the Red Sea.

Disease is defined as a disorder of structure or function in an organism, which produces specific signs or symptoms and is not simply a direct result of physical injury. In scleractinian corals, as in all other organisms, diseased individuals show morphologically distinct lesions, which manifest themselves in alterations of colour, shape, size, or texture. Coral diseases are routinely described by macroscopic signs, with notes on the extent of tissue loss, tissue colour, and exposure of coral skeleton aiding in the nomenclature and diagnosis. However, despite their reported significance in reefs around the world, the aetiologies of the majority of diseases are still unknown and coral epidemiology remains poorly understood. The majority of published literature suggests numerous microorganisms are capable of causing coral disease, including bacteria, viruses, fungi, and protozoa. That said, the initial trigger for disease is often recorded as being abiotic factors such as temperature, UV exposure, pollution (such as heavy metals, and agricultural chemicals) or predation scars, for example. Coral diseases have been shown to be prevalent on both well-managed (for example marine protected areas) and unmanaged reefs alike. Despite the serious negative implications associated with coral disease, not all geographic areas are as well documented as others. For example, diseases affecting corals throughout the Red Sea are one of the least well explored and documented. Here we sum up the current knowledge of the diseases described on the reefs in this area and give a brief background into what is currently known about the cause of such diseases.

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.

The black-lip-mother of pearl oyster Pinctada margaritifera has special economic value in Dungonab Bay (Sudan). It has been cultivated in the area for a long time as it has the advantage of having a definite spawning season starting at the end of June and continuing throughout July and August. Morphological and anatomical studies of this oyster are given in this piece of work including description of external feature of the shells and internal organs such as the mantle, the gills, digestive system, circulatory system, nervous system, excretory system and the reproductive organs. Morphological and anatomical differences between P. margaritifera on one hand and both the European oyster, Ostrea edulis, and the American oyster, Crassostrea gigas, are presented. These differences included the presence of a long hinge on one side of the umbo in P. margaritifera an its absence in both O. edulis and C. virginica; both valves in P. margaritifera are moderately convex and neither of them can be easily distinguished, unlike O. edulis, and C. gigas; the mantles have free edges in P. margaritifera and the omission of pedal ganglia in both C. gigas and O. edulis due to the absence of a foot in these two species.

The subclass Copepoda is an important driving force in linking the lower trophic to higher trophic levels in aquatic ecosystems. Despite their ecological importance in marine waters, very little work has been done along the Red Sea since the early 19th century. Until now, about 276 species from 76 genera, 55 families, and 6 orders of copepods have been recorded in the Red Sea. This chapter discusses the diversity, distribution and ecology of the Red Sea copepods, which show an increasing gradient of species richness and biomass from north to south. Moreover, the standing stock of zooplankton in the southern Red Sea is higher than the central and northern parts. The majority of copepods recorded are during the winter season. The epipelagic zone in the Red Sea is usually dominated by small-sized genera, especially Acrocalanus, Calocalanus, Clausocalanus, Corycaeus, Ctenocalanus, Macrosetella, Oithona, Oncaea, Paracalanus, Paraoithona and Parvocalanus. With increasing depths, microcopepods belonging to the family Oncaeidae become numerically more important than the calanoid copepods. A special focus has been provided with reference to the effect of UV-B radiation on their biology, which shows that the maximum mortality rates of copepods under ambient solar radiation levels average a five-fold increase over the average mortality in the dark. The chapter also discusses the symbiotic and parasitic relationship of copepods with other organisms, such as corals and coral-reef fishes. A preliminary report shows that symbiotic copepods attain a high diversity from scleractinian coral genera, such as Pocillopora sp., Acropora sp., Stylophora sp., Favia sp. and Fungia sp. This chapter provides a baseline introduction on copepods and possible research in different aspects of their biology, which may provide a new step in copepod research in the Red Sea.

The Red Sea zooplankton distribution pattern is characterized by a decreasing gradient in species numbers from the south to north while biomass and abundance of small-sized copepods decrease from the epipelagic zone of the southern Red Sea to the central-northern area. In the northern Red Sea, the seasonal abundance of the total zooplankton standing crop showed two peaks. The highest peak was observed in autumn with a maximum of 4990 ind/m³ in November, while the small peak was recorded during late spring-early summer, attaining a maximum (4300 ind/m³) in July. The diversity of oceanic zooplankton species in the Red Sea is relatively poor compared to other tropical seas and the number of oceanic species decreases with depth and toward the Gulfs of Suez and Aqaba. Species composition and abundance of demersal zooplankton differed considerably between the reef substrates. The highest mean density of zooplankton emerged from the living coral area (daily average: 4983 ind/m²), while the lowest mean density emerged from sand substrates (daily average: 938 ind/m²). The total number of demersal zooplankton that emerged from the coral patch was significantly higher than from sand or rubble areas. The emerged zooplankton was significantly higher in night-hours (2251 ind/m²) than in day-hours (286 ind/m²) over any substrate in the studied areas. Seasonally, the demersal zooplankton increased during summer with the maximum averages of 493 ind/m² and 3813 ind/m² during day and night, respectively. The minimum abundance of demersal zooplankton was recorded during autumn with the lowest values in October. Copepods were the most abundant group of all catches, accounting for 58.7% of the total demersal zooplankton abundance. The presence of copepod larval stages (nauplii and copepodites) all the year round indicated the continuous reproduction of copepods throughout the year. A total of 34 zooplankton species could be identified from the emergence trap fixed on different substrates (i.e., living corals, rubble and sand).

Studies on phytoplankton and primary production in the Red Sea are few and far between, and even in the few that have been conducted, most cover only a limited area. The last review of phytoplankton and primary production by Ismael (2015) reaffirmed the oligotrophic nature of the Red Sea and the north-to-south increasing trend in chlorophyll concentrations and rates of primary production. Also, in the above review the inventory of phytoplankton species was enlarged to 389 from the earlier record of 181 by Halim (1969). Since then, four research cruises undertaken in the Saudi Arabian waters of the Red Sea (2012–2015) have added a considerable amount of data on the patterns of primary production in the Red Sea and this review builds on that of Ismael (2015) by presenting the new findings. The levels of biomass and production in the Red Sea are relatively low, with a discernable north-south gradient. Their distribution is influenced by anticyclonic eddies, which entrain the nutrient-rich Gulf of Aden Intermediate Water across the Red Sea basin. Biomass and production in regions of eddy currents are twice as high as those elsewhere, suggesting that the notion that the Red Sea is oligotrophic needs to be revised. The injection of nutrients into the euphotic zone in the eddy boundary currents favours the proliferation of producers across a range of size classes rather than of a single class. As with any nutrient-poor tropical sea, the primary production in the Red Sea is supported up to 80% by nano- and picoplankton. Though the contributions of microplankton (diatoms and dinoflagellates) appear to be less significant, the phytoplankton diversity is quite high. With additional records of 74 species from the samples in the four cruises, the current inventory of phytoplankton stands at 463 species. The review also provides suggestions on prospective avenues of phytoplankton research in the Red Sea waters. These include extensive spatial and seasonal coverage of primary production, the importance of benthic production, a better evaluation of nitrogen (N) fixation by Trichodesmium spp., the role of allochthonous nutrient sources (such as dust) in increasing the productivity, additional inventories of phytoplankton species, especially those belonging to the nano- and picoplankton size classes, and the assessment of the importance of the heterotrophy and microbial loop in the food chain dynamics. Experimental studies on the physiology of phytoplankton that already live at extreme conditions of temperature and salinity in the Red Sea could also help to understand how phytoplankton in other seas would react to the effects of global warming and climate change.

Citizen science is an innovative approach that relies on non-specialists to monitor species and ecosystems over long time periods and vast geographical areas. Citizen science has been used extensively in marine science to monitor endangered species such as sharks and marine turtles, coral reefs and their associated fish species, marine mammals, invasive species and, more recently, coral bleaching and marine litter. Engaging people over the long term can be challenging but using social media, gamification, and emphasizing the value of volunteer contributions through data sharing, can help to keep communities motivated. In the Red Sea, there is enormous potential for using citizen science in monitoring endangered species and ecosystems due to the presence of a fleet of safari boats and dive centres going to sea daily. Engaging with this sector and creating long lasting partnerships for data collection through simple protocols could be a winning approach to obtain important information from remote areas and/or on rare species. In this chapter, we present the preliminary results of a citizen science program targeting marine turtles in their feeding grounds in the Egyptian Red Sea waters that was conducted from 2011 to 2013. During the study period 2,448 surveys were completed at 157 sites and included a total of 1,038 sightings of turtles. The most commonly observed species were hawksbill (Eretmochelys imbricata) and green (Chelonia mydas) turtles; however, rarer species, such as loggerhead (Caretta caretta) and olive ridley (Lepidochelys olivacea) turtles were also recorded. Among the sites that were monitored, some were considered as important for turtles (i.e., had a high probability of observing a turtle), while in others, turtles were not observed despite carrying out multiple surveys. Participants reported turtles of various sizes and ages with adults and sub-adults being the predominant observed age class. The presence of adults seemed to be related to the nesting season (May–September), which was also when the survey effort was higher. Adult male turtles were observed on various occasions, providing important input on their whereabouts during nesting and non-nesting seasons. Finally, participants detected behaviour that had not been previously described in the region, such as courting and mating. Results from TurtleWatch Egypt provided new insight in our knowledge of marine turtles in the Red Sea, especially from the largely under-studied feeding grounds.