Bab El Mandeb Research Articles

Modelling the seasonal cycle of the exchange flow in Bab El Mandab (Red Sea)

A minimum complexity, three-layer hydraulic model has been further developed to simulate the seasonal cycle of the exchange flow in Bab el Mandab (Red Sea). Unlike earlier versions our model incorporates a realistic channel cross-section. To a good approximation the model simulates observed fluxes through the strait and the layer depths at Hanish Sill. The model results indicate that the summertime intrusion of Gulf of Aden Intermediate Water into the Red Sea has been a robust feature of the exchange for the last 10 500 years. The modern intrusion acts as a dynamic barrier to the exchange in the upper and lower layers effectively, reducing the mean annual flux in each layer by 26% and 33% compared to model runs without the intrusion.

A Three-dimensional Hydrodynamic Model to Predict the Distribution of Temperature, Salinity and Water Circulation of the Red Sea

A modified version of 3D- model is applied to study the distribution of temperature, salinity and water circulation in the Red Sea. Monthly means of observed temperature, salinity and wind stress data are used as initial conditions. The results represent the contours of average temperature, salinity and water circulation for the months of January and July. The surface temperature distribution shows regional variations with an area of high temperature in the central region during winter, which shifts towards south under the influence of the north-westerly winds that dominate the entire Red Sea in summer. Other synoptic observations such as the northward increase of salinity are also successfully simulated. Vertical distributions of temperature and salinity show seasonal variability. In winter, the mixed layer is deepest whereas there is strongly stratification in summer. At depths greater than 200 m the temperature and salinity are almost constant throughout the year (22.0?C and 40.4 ppt respectively). The surface water circulation is strongly affected by the wind stress. In the southern Bab Al-Mandab region the model shows a two layers system during winter. In shallow areas, where there are transverse winds, cyclonic eddies are traced. استعمل نموذج رياضي ثلاثي الأبعاد لدراسة توزيع كل من درجة الحرارة والملوحة وكذلك حركة دوران المياه في البحر الأحمر، وذلك باستخدام المتوسطات الشهرية للمتغيرات كحدود ابتدائية . أظهرت نتائج التوزيع الأفقي لدرجة الحرارة السطحية بأن تواجد المنطقة ذات درجة الحرارة المرتفعة نسبيا تتغير خلال السنة، وأن التوزيع الأفقي للملوحة السطحية يزيد باتجاه الشمال. التوزيع الرأسي لدرجة الحرارة والملوحة بين أن طبقة المزج تصل إلى عمق أكبر خلال فصل الشتاء ولكن خلال فصل الصيف يكون تراصف الطبقات المائية أعلى. الأعماق أكبر من 200م تكون درجة حرارة المياه وملوحتها متساوية تقريبا. أما بالنسبة لحركة دوران المياه السطحية فإن تأثير إجهاد الرياح يكون فعال وفي المناطق الضحلة أظهرت نتائج النموذج بعض الدوامات المائية.

Heat and freshwater budgets in the Red Sea from direct observations at Bab el Mandeb

Direct oceanographic observations at the strait of Bab el Mandeb, which connects the Red Sea with the Indian Ocean, are used to estimate the heat and freshwater budgets of the Red Sea. The observations consist of 18-month records from transport resolving current-meter arrays and temperature–salinity chain moorings that allow accurate heat and salt fluxes through the strait to be determined. The new measurements account for the seasonally reversing surface-layer flow in the strait and the summertime intrusion of low-salinity thermocline water from the Gulf of Aden. The annual mean freshwater loss to the atmosphere over the Red Sea is estimated to be 2.06±0.22myr−1, while the annual mean heat loss is 11±5Wm−2. These results are compared with previous estimates of the Red Sea heat and freshwater budgets derived from both direct and indirect methods, which show large differences between each other and much larger uncertainties than the present estimates.

Response Characteristics of a Buoyancy-Driven Sea

The authors consider the flow in a semienclosed sea, or basin, subjected to a destabilizing surface buoyancy flux and separated from a large adjoining reservoir by a sill. A series of numerical experiments were conducted to quantify the energetics of the flow within the basin, that is, the amount of kinetic and potential energy stored within the basin and the rate at which these quantities are transported to and from the reservoir via the exchange flow over the sill. The numerical experiments were formulated at laboratory scales and conducted using a boundary-fitting, clustered grid to resolve the entrainment and mixing processes within the flow and to facilitate quantitative comparison with previous laboratory experiments. Volume and boundary integrated energetics were computed for both steady and time-varying flows. In the steady-state limit, the rate of energy flux through the surface is balanced by dissipation within the basin and advection of potential energy over the sill and into the reservoir. The analyses focus primarily on this latter quantity because it is closely related to the outflow density and volume transport in two-layered exchange flows. Scaling laws relating the energetics of the flow to the surface buoyancy flux and the geometrical scales of the basin–sill system are derived and validated using the numerical results. A second set of experiments was conducted to quantify the transient energetics in response to a sudden change in the surface forcing. These results, combined with a linear impulse–response analysis, were used to derive a general expression describing the advection of potential energy across the sill for periodically forced systems. The analytical predictions are shown to compare favorably with directly simulated flows and to be reasonably consistent with limited field observations of the seasonal variability through the Strait of Bab al Mandab.

Deep sea swimming crabs of the Charybdis miles species group in the western Indian Ocean (Crustacea: Decapoda: Portunidae)

The deep-sea species of the Charybdis miles group occurring in the western Indian Ocean are revised on the basis of recent collections obtained by German and Russian expeditions. Two new species are described, Charybdis meteor sp. n. from the Straits of Bab el Mandeb and Charybdis crosnieri sp. n. from the insular margins and seamounts of the western Indian Ocean, which has been previously identified as Charybdis sagamiensis Parisi, 1916. The C. miles group represents a lineage with a probably relatively recent tendency for submergence which culminated in the formation of C. acutidens Türkay, 1986, a characteristic species of the deep Red Sea water mass.

Hydraulic Control of Three-Layer Exchange Flows: Application to the Bab al Mandab

Two-layer hydraulic models have proven to be extremely useful in understanding a number of exchange flows through straits, for example, the Strait of Gibraltar. There are, though, some strait flows that cannot be represented by a two-layer model. A very striking example is the Bab al Mandab. In the exchange between the Red Sea and the Gulf of Aden, three distinct water masses are observed in the strait during summer. The wintertime flow is, though, very different, with only two layers observed in the strait. In this paper a three-layer model of hydraulically controlled exchange flows is described The model can represent exchange flows that are controlled with respect to either the first or second (or both) internal modes. A number of different examples are discussed. The model is able to explain the qualitative features of the seasonal variation of the flow in the Bab al Mandab. In particular, the reversal of the direction of flow of the surface layer and the intrusion into the Red Sea of cold intermediate water from the Gulf of Aden during the summer months can be explained by upwelling in the Gulf of Aden forced by the southwest monsoon.

Continuous Dynamical Modes in Straits Having Arbitrary Cross Sections, with Applications to the Bab al Mandab*

The continuous dynamical modes of the exchange flow in the Bab al Mandab are computed in an attempt to assess the hydraulic character of the flow at the sill. First, an extended version of the Taylor–Goldstein equation for long waves that accounts for cross-channel topographic variations, is developed. A series of calculations using idealized background velocity U(z) and buoyancy frequency N(z) are presented to illustrate the effects of simple topographic cross sections on the internal modes and their speeds. Next, hydrographic and direct velocity measurements from April to November 1996 using moored CTDs and a bottom-mounted ADCP are utilized to construct monthly mean vertical profiles of N2(z) and at the U(z) sill. An analytical approximation of the true topography across the strait is also constructed. The observed monthly mean profiles are then used to solve for the phase speeds of the first and second internal modes. Additional calculations are carried out using a selection of “instantaneous” (2-h average) profiles measured during extremes of the semidiurnal tide. The results are compared with a three-layer analysis of data from the previous year. Many of the authors’ conclusions follow from an intriguing observation concerning the long-wave phase speeds. Specifically, it was nearly always observed that the calculated speeds c−1 and c1 of the two waves belonging to the first internal mode obey c−1 < Umin < Umax < c1, where Umin and Umax are the minimum and maximum of the velocity profile. An immediate consequence is that neither wave has a critical level. For monthly mean profiles, each of which have Umin < 0 < Umax, the flow is therefore subcritical (the phase speeds of the two waves have opposite signs). For instantaneous profiles this relationship continues to hold, although the velocity profile can be unidirectional. Thus the flow can be critical (c−1 = 0 and/or c1 = 0) or even supercritical (c−1 and c1 have the same sign) with respect to the first mode. Similar findings follow for the second baroclinic mode phase speeds (c−2 and c2). The authors conclude that hydraulically critical flow is an intermittent feature, influenced to a great extent by the tides. It is noted that the phase speed pairs for each mode lie very close to Umin and Umax. As suggested by the analysis of idealized profiles, this behavior is characteristic of flows that are marginally stable, perhaps as a result of prior mixing. This suggestion is supported by Richardson number (Ri) profiles calculated from the monthly mean and instantaneous data. Middepth values of Ri were frequently found to be O(1) and sometimes <1/4, a result consistent with the presence of mixing over portions of the water column.

Aplanktonic zones in the Red Sea

We use micropalaeontological and stable isotope results for a series of cores from north to south through the Red Sea, to assess temporal and spatial patterns of change in planktonic foraminiferal faunas leading up to the remarkable full-glacial Red Sea aplanktonic zones. Aplanktonic zones reflect salinities in the Red Sea in excess of the lethal 49 p.s.u. limit, caused by reduced exchange transport through the Strait of Bab el Mandab due to glacial sea-level lowering. Concerning the last glacial cycle, aplanktonic conditions began at 39 ka BP in the north, where salinities eventually reached 55–57 p.s.u. Paradoxically, planktonic faunas are reported to have survived the last glacial maximum in the northern Gulf of Aqaba, suggesting a poorly understood freshwater dilution at that time. Aplanktonic conditions (S≥49 p.s.u.) reached the south-central Red Sea by 22 ka BP, while planktonic foraminiferal faunas continued, albeit in very low abundances and limited diversity (S≤45 p.s.u.), in the southernmost Red Sea. During marine isotope stage (MIS) 6, the 49 p.s.u. isohaline appears to have resided in the central Red Sea, between cores KL11 and MD921017. We observe a systematic sequence of species disappearances before all glacial maxima of the last 500 kyr. Absence of a logical relationship with sea levels suggests that the disappearance sequences are not related to a general salinity increase. Instead, we argue that the sequences were driven by complex reorganisations in hydrography (stratification), productivity (food availability) and subsurface oxygenation (reproduction-habitats). The onset is marked by dramatic basin-wide expansion of conditions that today are restricted to only the southern Red Sea, suggesting an expansion of the dominance of NE monsoonal circulation over the entire Red Sea. This expansion occurred 15 or more kyr before the aplanktonic zones of MIS-12, 6 and 2, and also before MIS-10 and 8, which never reach the aplanktonic stage. Regarding the last glacial cycle, this event occurred as early as 75 ka BP (MIS-4/5 boundary). After this major climatic reorganisation, we reconstruct progressive intensification of the new conditions, especially marked by northward expansion and intensification of the subsurface oxygen minimum zone (OMZ). During the last glacial cycle, a shallow and very distinct OMZ affected the central Red Sea as early as 68 ka BP, and the north as late as 55 ka BP. The OMZ expansion/intensification appears to have been interrupted by episodes of increased ventilation.

Hydraulic Interpretation of Direct Velocity Measurements in the Bab al Mandab*

Abstract Acoustic Doppler Current Profiler velocity measurements in the Bab al Mandab during the period June 1995–March 1996 are used to assess the hydraulic character of the exchange flow. The strait is 150 km long and contains two distinct geometrical choke points: the Hanish sill and Perim narrows. The authors use a three-layer approximation of the monthly mean velocity and density structure at the sill and narrows to calculate the phase speeds of the first and second internal, long gravity waves. The first (second) mode is generally characterized by in-phase (out-of-phase) motions of the two interfaces. The calculations take cross-strait topographic variations into consideration by using a piecewise linear representation of the actual bottom. The resulting phase speeds are used to determine whether the flow is subcritical, supercritical, or critical with respect to the first and second modes. Subcritical (supercritical) means that the two wave pairs corresponding to a given mode propagate in opposite ...

The Heat and Freshwater Budgets of the Red Sea

Abstract The heat and freshwater transports through the Strait of Bab el Mandab, connecting the Red Sea with the open ocean, are reviewed and used to test air–sea fluxes from a revised version of the Comprehensive Ocean–Atmosphere Data Set (UWM/COADS). Using historical data for the volume fluxes and water properties, the annual-mean net heat transport through the strait is found to require an average surface heat loss of 8 ± 2 W m−2, while the requirement for conservation of salt in the basin implies a net evaporation rate of 1.60 ± 0.35 m yr−1, lower than previously considered. The air–sea heat fluxes from UWM/COADS overestimate the total heat flux by nearly 100 W m−2; the discrepancy is attributed to systematic errors in the bulk formulas used to calculate the heat flux components. In particular, insolation appears to be overestimated by 36 W m−2, largely due to the neglect of aerosols. The effect of these is determined from ground stations and satellite data on the optical thickness index. The net long...

Ventilation of Red Sea Water with respect to chlorofluorocarbons

Chlorofluorocarbon (CFC) measurements were made in the northwestern Indian Ocean in August‐September 1995 as part of the World Ocean Circulation Experiment Hydrographic Program Section I1. The data in the Gulf of Aden section from this expedition provide information on the mixing processes that affect Red Sea Water (RSW) as it flows from its source into the open northwestern Indian Ocean. Combining measurements by Rhein et al. [1997] with the outcome from a two‐end‐member mixing model results in an estimate of 65–70% saturation for the CFCs in the RSW end‐member at Bab el Mandeb. A boundary condition that may be more applicable to ocean models is given by the CFC content of RSW after its initial diapycnal descent along the continental slope in the western Gulf of Aden. Entrainment of water containing lower amounts of CFCs produces RSW that has CFC saturations of 20–50% once RSW has settled at its depth of neutral buoyancy. Apparent pCFC ages at Bab el Mandeb and after the separation of RSW from the bottom are 12–13 and 18–27 years, respectively. As RSW continues eastward through the Gulf of Aden as an isopycnal flow, horizontal eddy diffusion plays an important role in further diluting RSW, which is characterized by a maximum in salinity and a positive anomaly in CFC concentrations. Horizontal eddy diffusion parameters estimated from tracer conservation in a box model of the central and eastern Gulf of Aden range from 0.2 × 104 to 2.0 × 104 m2 s−1. As other observations show, eddy diffusion is presumably enhanced in this part of the ocean.

NEW DEVELOPMENTS IN THE PRE‐RIFT PROSPECTIVITY OF THE ERITREAN RED SEA

Northern, central and southern Africa is covered by great thicknesses of Upper Palaeozoic and Mesozoic fluvio‐deltaic and marine siliciclastics, which include important source rocks, reservoirs and seals. In the Eritrean Red Sea, the importance of these rocks has not yet been properly recognised.The Red Sea initially formed by rifting of the Afro‐Arabian continent in Oligo‐Miocene times. Palaeozoic and Mesozoic rocks were downthrown into the resultant Gulf of Suez/ Red Sea Graben and were rapidly buried by syn‐ and post‐rift sediments. In the Eritrean Red Sea area, pre‐rift rocks are now only exposed in the Danakil Alps at the Straits of Bab el Mandab, on the Eritrean Plateau, in the coastal lowlands north of Massawa, and in isolated outcrops in Ethiopia.The authors believe that an as‐yet untested pre‐rift “play” is to be found in the Eritrean Red Sea contiguous with the Danakil Alps. In addition, syn‐ and post‐rift “plays” cannot be ruled out in this area. The combination of newly‐mappedpre‐salt structures, containing Jurassic source rocks and Jurassic and Cretaceous reservoir sandstones and carbonates overlain by Tertiary elastics and/or salt, embodies the primary hydrocarbon entrapment model in the area. Secondary hydrocarbon “plays” centre on deformed Miocene siliciclastic seauences in proximity to salt diapirs.

Altimetric observations of sea level cycles near the Strait of Bab al Mandab

The Strait of Bab al Mandab connects the Red Sea with the Gulf of Aden and Indian Ocean. Flow through the Strait is dominated by monsoonal forcing with surface water inflow in the winter and outflow in the summer. Analysis of TOPEX/POSEIDON altimeter sea surface height residuals at subsatellite locations in the Red Sea, the Strait and Gulf of Aden indicates a dominant annual cycle and a secondary semi-annual cycle at all locations. A combined annual and semi-annual model is then fitted to sea surface height anomalies at each of the locations. The combined model explains most of the variance in the data: from 74 per cent variance in the Gulf of Aden to 92 per cent variance in the southern Red Sea. The amplitude of the annual cycle is 18cm in the Red Sea and 13 cm in the Gulf of Aden. An analysis of coherence between altimeterderived wind stress and sea surface height residuals shows that the annual cycle is probably related to wind forcing. A weaker semi-annual cycle of 4-8 cm is probably related to the cycle of evaporation. A two-layer model of flow through the Strait shows that the dynamic sea surface height signature is only 2cm. This signature of exchange is too small to be resolved by current satellite altimeter measurements

Direct observations of seasonal exchange through the Bab el Mandab Strait

The exchange flow between the Red Sea and the Gulf of Aden‐Indian Ocean through the Bab el Mandab Strait was measured continuously for 10 months, June 1995–March 1996. ADCP and temperature‐salinity chain moorings allow an unprecedented look at the magnitude and seasonal evolution of the inflow layer from the Gulf of Aden, and the high salinity outflow layer from the Red Sea. The timing, structure, and evolution of the summer season mid‐depth intrusion of cold, low salinity water into the Red Sea from the Gulf of Aden is measured for the complete intrusion cycle of 1995. We unexpectedly find the deep outflow still strong in June 1995, with speeds of 0.6 m/sec and transport of 0.4 Sv (1 Sv = 106 m³/sec). From July to mid–September, the deep outflow persists but is attenuated to speeds of 0.2 m/sec and transport of 0.05 Sv. The dominant summer feature, the cold low salinity intermediate layer intrusion, persists for 3 months, occupies 70% of the water column in the Strait and carries approximately 1.7 × 1012 m³ of cold nutrient‐rich water into the Red Sea. The winter regime begins in mid‐September, is fully developed by early November, and continues to the end of our first observation interval in March 1996. Speeds in the lower layer are 0.8–1.0 m/sec and 0.4–0.6 m/sec in the upper layer. At maximum exchange in mid‐February, outflow transport reaches 0.7 Sv. Ubiquitous oscillations in current and salinity at synoptic and intraseasonal periods appear closely related to fluctuations in the along‐channel wind forcing and perhaps to coastally‐trapped waves.

Tracer signals of the intermediate layer of the Arabian Sea

In 1995, hydrographic and chlorofluorocarbon (CFCs, components F11, F12) measurements were carried out in the Gulf of Aden, in the Gulf of Oman, and in the Arabian Sea. In the Gulf of Oman, the F12 concentrations in the Persian Gulf outflow (PGW) at about 300m depth were significantly higher than in ambient surface water with saturations reaching 270%. These high values could not be caused by air‐sea gas exchange. The outflow was probably contaminated with oil, and the lipophilic character of the CFCs could then lead to the observed supersaturations. The intermediate F12 maximum decreased rapidly further east and south. At the Strait of Bab el Mandeb in the Gulf of Aden, the Red Sea outflow (RSW) was saturated with F12 to about 65% at 400m depth, and decreased to 50% while descending to 800m depth. The low saturation is not surprising, because the outflow contains deep and intermediate water masses from the Red Sea which were isolated from the surface for some time. The tracer contributions to the Arabian Sea for Indian Central Water (ICW) and PGW are about equal, while below 500m depth the RSW contribution greatly exceeds ICW. Modeling the CFC budget of the Arabian Sea, the inflow of ICW north of 12°N is estimated to be 1–6 Sv, depending mainly on the strength of the flow of Red Sea Water into the Arabian Sea.

Propagation of rifting along the Arabia‐Somalia Plate Boundary: The Gulfs of Aden and Tadjoura

The localization and propagation of rifting between Arabia and Somalia are investigated by assessing the deformation geometry and kinematics at different scales between the eastern Gulf of Aden and the Gulf of Tadjoura, using bathymetric, magnetic, seismological, and structural evidence. Large‐scale, southwestward propagation of the Aden ridge, markedly oblique to the Arabia‐Somalia relative motion vector, began about 30 Myr ago between the Error and Sharbithat ridges. It was an episodic process, with stages of rapid propagation, mostly at rates &gt;10 cm/yr, interrupted by million year pauses on transverse discontinuities coinciding with rheological boundaries between different crustal provinces of the Arabia‐Somalia plate. The longest pause was at the Shukra‐El Sheik discontinuity (≈45°E), where the ridge tip stalled for ≈13 Myr, between ≈17 and ≈4 Ma. West of that discontinuity, rifting and spreading took place at an azimuth (≈N25°±10°E) and rate (1.2±0.3 cm/yr) different from those of the global Arabia‐Somalia motion vector (≈N39°, ≈1.73 cm/yr), implying an additional component of movement (N65°±10°E, 0.7±0.2 cm/yr) due to rotation of the Danakil microplate. At Shukra‐El Sheik, the typical oceanic ridge gives way to a narrow, WSW trending axial trough, resembling a large fissure across a shallow shelf. This trough is composed of about eight rift segments, which result from normal faulting and fissuring along N110°–N130°E trends. All the segments step to the left southwestward, mostly through oblique transfer zones with en échelon normal faults. Only two segments show clear, significant overlap. There is one clear transform, the Maskali fault, between the Obock and Tadjoura segments. The latter segment, which encroaches onland, is composed of two parallel subrifts (Iboli, Ambabbo) that propagated northwestward and formed in succession. The most recent, southwestern subrift (Ambabbo) represents the current tip of the Aden ridge. We propose a mechanical model in which the large‐scale propagation of the ridge followed a WSW trending zone of maximum tensile stress, while the small‐scale propagation of its NW trending segments was dictated by the orientation of that stress. Oblique propagation was a consequence of passive lithospheric necking of the Arabia‐Somalia plate along its narrow section, in map view, between Socotra and the kink of the Red Sea‐Ethiopian rift, above the Afar plume. Individual ridge segments oriented roughly perpendicular to plate motion, like lithospheric cracks, were forced to jump southward because of confinement within the necking zone. Self‐sustaining, plate‐scale necking may explain why the Aden ridge did not connect with the Red Sea through Bab El Mandeb but continued straight into Afar.

Distribution and association of trace metals in soft tissue and byssus of mollusc Perna perna from the Gulf of Aden, Yemen

Concentrations of Cd, Pb, Zn, Cu, Mn, and Fe in soft tissue and byssal threads of Perna perna from the Gulf of Aden, Yemen, were determined by atomic absorption spectrophotometric (AAS) method. Significant inter-regional differences in the metal concentrations studied in both soft tissue and byssus were identified. The concentration of Cd was an order of magnitude greater in soft tissue than in byssal threads. Statistically significant correlations ( p<0.01, p<0.05) between concentrations of selected metals were observed. The spatial distribution of Cu in byssal threads was similar to that in soft tissue. The data indicate that sewage input (at Sira Island) is the most likely anthropogenic source of Cu and Mn. The concentration of tissue Cd at the Fuqum area was the highest among the other stations. Although the Fuqum area is off the harbour, water from a fishery refrigerator and some sewage outflow flux into the sea. It is assumed that the presence of the area near the strait of Bab Al-Mandab, where the water coming from both the Red Sea and the Indian Ocean mixes, especially during the seasonal monsoons, is responsible mainly for elevated tissue Cd concentrations in the molluscs.

Three hundred eighty thousand year long stable isotope and faunal records from the Red Sea: Influence of global sea level change on hydrography

Stable isotope and faunal records from the central Red Sea show high‐amplitude oscillations for the past 380,000 years. Positive δ18O anomalies indicate periods of significant salt buildup during periods of lowered sea level when water mass exchange with the Arabian Sea was reduced due to a reduced geometry of the Bab el Mandeb Strait. Salinities as high as 53‰ and 55‰ are inferred from pteropod and benthic foraminifera δ18O, respectively, for the last glacial maximum. During this period all planktonic foraminifera vanished from this part of the Red Sea. Environmental conditions improved rapidly after 13 ka as salinities decreased due to rising sea level. The foraminiferal fauna started to reappear and was fully reestablished between 9 ka and 8 ka. Spectral analysis of the planktonic δ18O record documents highest variance in the orbital eccentricity, obliquity, and precession bands, indicating a dominant influence of climatically ‐ driven sea level change on environmental conditions in the Red Sea. Variance in the precession band is enhanced compared to the global mean marine climate record (SPECMAP), suggesting an additional influence of the Indian monsoon system on Red Sea climates.

Summer distribution of micro- and small mesozooplankton in the Red Sea and Gulf of Aden, with special reference to non-calanoid copepods

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 118:81-102 (1995) - doi:10.3354/meps118081 Summer distribution of micro- and small mesozooplankton in the Red Sea and Gulf of Aden, with special reference to non-calanoid copepods Böttger-Schnack, R. From the Gulf of Aden along a transect to the central-northern Red Sea the abundance and taxonomic composition of metazoan plankton was studied during the southwest monsoon period (summer 1987). Samples were taken with 0.055 mm mesh nets down to a maximum depth of 1050 m. In the epipelagic zone, a distinct decrease in total plankton abundance was observed from south to north, which was much more pronounced in biomass (by a factor of up to 10) as compared to numbers (by a factor of 2). This could partly be explained by differences in the taxonomic and/or size composition of the planktonic fauna. Among non-calanoid copepods, 40 out of 75 species or taxa investigated decreased in abundance from south to north. Sixteen of these species were completely absent in the central-northern area. Nineteen species or taxa, however, showed the opposite feature of a higher abundance in the central-northern Red Sea. The stations were grouped according to similarities in the taxonomic composition of non-calanoid copepods in the epipelagic zone. The following 3 geographical regions could be separated: (1) Gulf of Aden and Strait of Bab al Mandab; (2) southern Red Sea; and (3) central-northern Red Sea. In the meso- and bathypelagic zones, regional differences were not evident. The results are discussed in relation to hydrographic conditions during summer 1987. Small mesozooplankton . Red Sea . South-north differences . Vertical distribution . Non-calanoid copepods . Oncaea Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 118. Publication date: March 09, 1995 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 1995 Inter-Research.

Large-Scale Variation in Assemblage Structure of Red Sea Butterflyfishes and Angelfishes

Patterns of distribution and variation in the structure of assemblages of butterflyfishes (Chaetodontidae) and angelfishes (Pomacanthidae) were studied in the Red Sea and Gulf of Aden. Fish were censused on reef-edge transects at 367 sites and species presence/absence recorded at 371 sites. Data on hard coral cover and variety were collected at most sites, and details of reef structure and zonation recorded at sixty-one. Of seven species endemic to the region, two are restricted to the central and northern Red Sea, while two species occurring in the Gulf of Aden have not colonized the Red Sea. The turbid reef-poor southern Red Sea itself, rather than the narrow Strait of Bab el Mandab, probably acts as a barrier to their further dispersal. Species richness and abundance of butterflyfishes were greatest in the central Red Sea, decreasing both to north and south. This pattern matches similar patterns in reef development, hard coral cover and coral variety. Assemblage structure was spatially consistent over large areas, but a major change in species abundance and assemblage composition occurs between the central and northern Red Sea on the one hand, and the southern Red Sea and the Gulf of Aden on the other. This change appears to be associated with a marked increase in turbidity and a sharp decrease in reef development and coral variety south of 20?N. While increased turbidity and the changed coral community may seem likely to influence assemblage structure through direct effects on post-settlement survivorship, recent reef fish assemblage studies have emphasized the importance of pre-settlement processes. We discuss the possible importance of preand post-settlement processes in generating the observed spatial variation of assemblages.