Southern Red Sea Research Articles

Historia Ambiental del comanejo adaptativo en dos regiones pesqueras del noroeste mexicano

La península de Baja California es un territorio aislado destacable por varios casos de pesquerías de exportación y su organización social. En el presente escrito se describe la historia ambiental de dos sistemas de manejo pesquero (abulón/langosta y erizo) en la costa del Pacífico. El comienzo de ambos sistemas es equivalente en el manejo intensivo de pesquerías con alto valor comercial en Asia y la conversión de trabajadores en tierra (rancheros y ejidatarios) a pescadores. Un segundo momento es la organización de los pescadores en cooperativas pesqueras donde comienza el comanejo de las pesquerías de abulón, langosta en el sur y erizo rojo en el norte. Como respuesta a los cambios propios de la naturaleza, de las organizaciones sociales, institucionales y comerciales y ahora al cambio climático, los pescadores han desarrollado esquemas de comanejo adaptativo que proveen experiencias nacionales exitosas para inspirar esquemas de manejo sostenible en otras pesquerías.

Spatial patterns of cryptobenthic coral-reef fishes in the Red Sea

Surveys to document coral-reef fish assemblages are often limited to visually conspicuous species, thus excluding a significant proportion of the biodiversity. Through standardized collections of cryptobenthic reef fishes in the central and southern Red Sea, a total of 238 species and operational taxonomic units (OTUs) from 35 families were collected. Abundance and species richness increased by 60 and 30%, respectively, from north to south, and fish community composition differed between the two regions and with proximity to shore in the central region. Models suggest regional influences in fish communities, with latitudinal patterns influenced by key coral groups (Acropora, Pocilloporidae) and variation in environmental parameters (chlorophyll a, sea surface temperature, salinity). This study illustrates the limited taxonomic resolution in this group and in this region, and the need to expand baseline data for this under-studied assemblage. To assist in advancing this initiative, we have produced a catalogue of specimens, archived photographs, and established a DNA sequence library based on cytochrome-c oxidase subunit-I barcodes for all OTUs.

Birth of an ocean in the Red Sea: Oceanic-type basaltic melt intrusions precede continental rupture

The role of magmatism in continental rupture at the birth of a new ocean is poorly understood. Continental rupture can take place with voluminous volcanism, as in the southern Red Sea, or in a relatively non-volcanic mode, as in the northern Red Sea. We report geophysical, geochemical and geochronological evidence suggesting that continental rupture in the northern Red Sea is preceded by MORB-type basaltic melt intrusions that cooled at depth forming gabbros at progressively shallower crustal levels as rifting progressed towards continental separation. One of these gabbros yielded an 40Ar/39Ar age of 25±6Ma, suggesting intrusion during early rifting. A magma chamber we detected 3.5km below Thetis Deep, an axial segment of initial oceanic crust accretion, represents the final stage in this progression. Oceanic crust accretion in the Red Sea rift starts first at depth before continental rupture and initiation of sea floor spreading.

Sea-level history and tectonic uplift during the last-interglacial period (LIG): Inferred from the Bab al-Mandab coral reef terraces, southern Red Sea

Results of U-series dating of late Pleistocene raised coral reef terraces from the Bab al-Mandab area, define two distinct groups: (1) well-preserved aragonitic fossil corals recorded from the Al-Hajaja terrace (Tr3) yield ages for last-interglacial period (LIG); and (2) calcitic fossil corals recovered from Perim Island terrace (Tr1) show varying degrees of U-series open system behavior and yield coral assemblage ages of LIG and older ages. Fossil corals from Tr1 are recrystallized corals, have anomalously high initial δ234U ranged from (152 ± 2‰ to 287 ± 7‰), corresponding to ages of ∼120 ka and ∼406 ka, respectively. Applying age reliability criteria on the current data suggest majority of the ages cannot be considered reliable and all are suspected for open system behavior associated with U loss/addition that significantly affects the 230Th/U ages. The diagenesis shown by these corals occurred probably due to extensive interaction of fossil corals with freshwater during the wet periods prevailed in southern Arabia coeval with the African monsoon, which led to U loss. Post-depositional U loss suggest (230Th/238U) increase, which shift the U-Th ages to unexpectedly higher levels as it is shown here. Measured elevation at the Al-Hajaja terrace (Tr3) is ∼4 ± 2 m above present sea level (apsl) consistent with eustatic sea level changes and indicates that the Bab al-Mandab area is stable at least since the LIG period. The Perim Island terrace (Tr1) is at elevation of 7 ± 2 m apsl; its reef yields diageneticaly-altered corals of multiple ages and cannot be used for sea level reconstructions.

Recent biogenic traces from the coastal environments of the southern Red Sea coast of Saudi Arabia

A neoichnological study was done at a site west of Abha on the southern Red Sea coast of Saudi Arabia. This area covers Wadi Hali and its surroundings (180 49′35.27″N: 410 22′44.23″E). The objective of the study is to document biogenic traces that occur in the upper intertidal ripple-marked sand flats and supratidal flats that include microbial mats, mud flats, and scattered patches of mangrove stands and their muddy environments. Based on morphological descriptions, four different kinds of burrows are identified and their fossil equivalents are Psilonichnus upsilon (burrow type 1), Cylindricum (burrow types 2 and 3), and Arenicolites (burrow type 4). Tracks of land hermit crabs Coenobita clypeatus are described as ichnospecies Coenobichnus currani. Gastropod grazing trails and their fossil equivalent Planolites is identified as well. Supratidal algal mats and their microscopic contents are presented. Fossil plant roots (rhizomes) are reported from a localized fluvial deposit from a dry channel (wadi). The morphology of the traces, trace maker, and ethology and the environmental setting indicate that the ichnofauna and botanical traces belong to the Psilonichnus ichnofacies.

Climate driven variability of wind-waves in the Red Sea

Wind-wave variability in the Red Sea has been studied over a period of 32 years (1979–2010) by hindcasting the wind-waves using a third generation spectral wave model, WaveWatch III, forced with the Climate Forecast System Reanalysis (CFSR) winds. The wave model results have been validated against the satellite observations and available measurements in the Red Sea. The study reveals robust spatial and temporal variability of wind speed and significant wave height in the Red Sea. The role of climatic indices on the wind-wave variability has been assessed. The highest extreme wave heights (99th percentile) are centered at three regions; around 23° N, 19° N, and 15° N latitudes, while the 90th percentile and mean significant wave heights are scattered in the deep waters of the Red Sea. Contrasting trends of the significant wave heights such as negative trends in the northern Red Sea during summer and positive trends in the southern Red Sea during winter are the highlights, where the trends are significant and comparable with the trends at few marginal seas around the globe. A clear distinction exists between the two parts of the Red Sea in response of the variability due to climatic indices. Among the climatic indices, the North Atlantic Oscillation (NAO) and the El Niño-Southern Oscillation (ENSO) show significant positive and negative correlations with the characteristic wave heights in the northern and southern Red Sea.

Thermal refugia against coral bleaching throughout the northern Red Sea.

Tropical reefs have been impacted by thermal anomalies caused by global warming that induced coral bleaching and mortality events globally. However, there have only been very few recordings of bleaching within the Red Sea despite covering a latitudinal range of 15° and consequently it has been considered a region that is less sensitive to thermal anomalies. We therefore examined historical patterns of sea surface temperature (SST) and associated anomalies (1982-2012) and compared warming trends with a unique compilation of corresponding coral bleaching records from throughout the region. These data indicated that the northern Red Sea has not experienced mass bleaching despite intensive Degree Heating Weeks (DHW) of >15°C-weeks. Severe bleaching was restricted to the central and southern Red Sea where DHWs have been more frequent, but far less intense (DHWs <4°C-weeks). A similar pattern was observed during the 2015-2016 El Niño event during which time corals in the northern Red Sea did not bleach despite high thermal stress (i.e. DHWs >8°C-weeks), and bleaching was restricted to the central and southern Red Sea despite the lower thermal stress (DHWs<8°C-weeks). Heat stress assays carried out in the northern (Hurghada) and central (Thuwal) Red Sea on four key reef-building species confirmed different regional thermal susceptibility, and that central Red Sea corals are more sensitive to thermal anomalies as compared to those from the north. Together, our data demonstrate that corals in the northern Red Sea have a much higher heat tolerance than their prevailing temperature regime would suggest. In contrast, corals from the central Red Sea are close to their thermal limits, which closely match the maximum annual water temperatures. The northern Red Sea harbours reef-building corals that live well below their bleaching thresholds and thus we propose that the region represents a thermal refuge of global importance.

Halokinetics and other features of GLORIA long-range sidescan sonar data from the Red Sea

The Red Sea is an unusual example of a rift basin that transitioned from its evaporitic stage to fully open-ocean conditions at the end of the Miocene (∼5.3 Ma), much more recently than older Mesozoic margins around the Atlantic and Gulf of Mexico. The patterns of halokinetic deformation occurring in the Red Sea are potentially of interest for understanding more generally how evaporite deposits deform during this early stage. Relevant to this issue, a line of reconnaissance sidescan sonar data (GLORIA) collected along the Red Sea in 1979 is re-evaluated here. We first interpret the data with the aid of newly compiled bathymetry from multibeam sonars in the central and southern Red Sea. Features in the acoustic backscatter data are associated with ridges, valleys and rounded flow fronts produced by halokinetic deformation. Some areas of higher acoustic backscattering from the evaporites are suggested to relate to roughness produced by deformation of the evaporite surface. Within the volcanic (oceanic) axial valleys, areas of differing high and low backscattering suggest varied sediment cover and/or carbonate encrustations. With the benefit of the above experience, we then interpreted data from the northern Red Sea, where there are fewer multibeam data available. Rounded fronts of halokinetic deformation are present in the Zabargad Fracture Zone, a broad, shallow valley crossing the Red Sea obliquely. The presence of halokinetic deformation here is evidence that subsidence has occurred along the fracture zone. Elsewhere in the northern Red Sea, the GLORIA data reveal folds in the evaporite surface, suggesting local areas of convergence, like those implied by multibeam data from inter-trough zones further south. Some linear features are observed, many of which are likely to be ridges overlying salt walls. Interestingly, several such features are oriented along an accommodation zone that is oriented parallel to the plate spreading direction. Several rounded, corrugated features are interpreted as possible evaporite flow fronts. Overall, the impression from the data is of a strongly mobile seabed in the Red Sea because of halokinetic deformation, involving both vertical and horizontal movements. However, salt walls appear more common than in the central and southern axial Red Sea, where horizontal movements instead tend to dominate.

Physical and biological characteristics of the winter‐summer transition in the Central Red Sea

AbstractThe Central Red Sea (CRS) lies between two distinct hydrographic and atmospheric regimes. In the southern Red Sea, seasonal monsoon reversal regulates the exchange of water between the Red Sea and the Indian Ocean. In the northern Red Sea, intermediate and occasionally deep water are formed during winter to sustain the basin's overturning circulation. Highly variable mesoscale eddies and the northward flowing eastern boundary current (EBC) determine the physical and biogeochemical characteristics of the CRS. Ship‐based and glider observations in the CRS between March and June 2013 capture key features of the transition from winter to summer and depict the impact of the eddy activity on the EBC flow. Less saline and relatively warmer water of Indian Ocean origin reaches the CRS via the EBC. Initially, an anticyclonic eddy with diameter of 140 km penetrating to 150m depth with maximum velocities up to 30–35 cm s−1 prevails in the CRS. This anticyclonic eddy appears to block or at least redirect the northward flow of the EBC. Dissipation of the eddy permits the near‐coastal, northward flow of the EBC and gives place to a smaller cyclonic eddy with a diameter of about 50 km penetrating to 200 m depth. By the end of May, as the northerly winds become stronger and persistent throughout the basin, characteristic of the summer southwest monsoon wind regime, the EBC, and its associated lower salinity water became less evident, replaced by the saltier surface water that characterizes the onset of the summer stratification in the CRS.

Temporal shoreline change and infrastructure influences along the southern Red Sea coast of Saudi Arabia

Jizan is one of the Saudi Arabian coastal cities endowed with diverse natural settings, which includes Ash Shuqayq in the north, Turfah in the centre and Jizan in the South. This work analysed specific environmental characteristics, such as spits, sabkhas and wadis. Assessments used Landsat imagery to examine coastal change between 1973 and 2011. The cumulative temporal change identified regression trends given by coefficients of determination that explained a significant percentage of data variation for Jizan (R 2 = 69%) and Turfah (R 2 = 72%), while Ash Shuqayq was insignificant (R 2 = 14%). Inter-survey results predicted future change, although trends were not significant, i.e. Jizan (R 2 = 22%), Turfah (R 2 = 14%) and Ash Shuqayq (R 2 = 3 and 61% with outlying value removed). Aerial photos showed regional coastal changes, which included a maximum accretion of 36.4 m and maximum erosion of 12.9 m. These are scientifically effective techniques to monitor regional coastal change, i.e. erosion and accretion and identified rates of 0.59, 1.80 and 3.53 myr−1 for Ash Shuqayq, Turfah and Jizan. Changes were linked to infrastructure developments, e.g. tourism, port development and natural causes, e.g. spit formations and wadi outfalls.

ENSO influence on the interannual variability of the Red Sea convergence zone and associated rainfall

ABSTRACTThe Red Sea convergence zone (RSCZ) is formed by opposite surface winds blowing from northwest to southeast directions at around 18°–19°N between October and January. A reverse‐oriented, low‐level monsoon trough at 850 hPa, known as the Red Sea trough (RST), transfers moisture from the southern Red Sea to RSCZ. The positions of the RSCZ and RST and the intensity of the RST have been identified as important factors in modulating weather and climatic conditions across the Middle East. Here, we investigate the influence of the El Niño southern oscillation (ENSO) on the interannual variability of RSCZ, RST, and regional rainfall during winter months. Our results indicate that El Niño (warm ENSO phase) favours a shift of the RSCZ to the north and a strengthening of the RST in the same direction. Conversely, during November and December of La Niña periods (cold ENSO phase), the RSCZ shift to the south and the RST strengthens in the same direction. During El Niño periods, southeasterly wind speeds increase (20–30%) over the southern Red Sea and northwesterly wind speeds decrease (10–15%) over the northern Red Sea. Noticeable increases in the number of rainy days and the intensity of rain events are observed during El Niño phases. These increases are associated with colder than normal air intrusion at lower levels from the north combined with warm air intrusion from the south over the RSCZ. Our analysis suggests that during El Niño winters, warmer sea surface temperatures and higher convective instability over the Red Sea favour local storms conditions and increase rainfall over the Red Sea and adjoining regions.

Pleistocene reefs of the Egyptian Red Sea: environmental change and community persistence.

The fossil record of Red Sea fringing reefs provides an opportunity to study the history of coral-reef survival and recovery in the context of extreme environmental change. The Middle Pleistocene, the Late Pleistocene, and modern reefs represent three periods of reef growth separated by glacial low stands during which conditions became difficult for symbiotic reef fauna. Coral diversity and paleoenvironments of eight Middle and Late Pleistocene fossil terraces are described and characterized here. Pleistocene reef zones closely resemble reef zones of the modern Red Sea. All but one species identified from Middle and Late Pleistocene outcrops are also found on modern Red Sea reefs despite the possible extinction of most coral over two-thirds of the Red Sea basin during glacial low stands. Refugia in the Gulf of Aqaba and southern Red Sea may have allowed for the persistence of coral communities across glaciation events. Stability of coral communities across these extreme climate events indicates that even small populations of survivors can repopulate large areas given appropriate water conditions and time.

Assessing Pigment-Based Phytoplankton Community Distributions in the Red Sea

Pigment-based phytoplankton community composition and primary production were investigated for the first time in the Red Sea in February-April 2015 to demonstrate how the strong south to north environmental gradients determine phytoplankton community structure in Red Sea offshore regions (along the central axis). Taxonomic pigments were used as size group markers of pico, nano- and microphytoplankton. Phytoplankton primary production rates associated with the three phytoplankton groups (pico-, nano- and microphytoplankton) were estimated using a bio-optical model. Pico- (Synechococcus and Prochlorococcus sp) and Nanophytoplankton (Prymnesiophytes and Pelagophytes) were the dominant size groups and contributed to 49% and 38%, respectively, of the phytoplankton biomass. Microphytoplankton (diatoms) contributed to 13% of the phytoplankton biomass within the productive layer (1.5 Zeu). Sub-basin and mesoscale structures (cyclonic eddy and mixing) were exceptions to this general trend. In the southern Red Sea, diatoms and picophytoplankton contributed to 27% and 31% of the phytoplankton biomass, respectively. This result induced higher primary production rates (430 ± 50 mgC m-2 d-1) in this region (opposed to CRS and NRS). The cyclonic eddy contained the highest microphytoplankton proportion (45% of TChla) and the lowest picophytoplankton contribution (17% of TChla) while adjacent areas were dominated by pico- and nanophytoplankton. We estimated that the cyclonic eddy is an area of enhanced primary production, which is up to twice those of the central part of the basin. During the mixing of the water column in the extreme north of the basin, we observed the highest TChla integrated (40 mg m-2) and total primary production rate (640 mgC m-2 d-1) associated with the highest nanophytoplankton contribution (57% of TChla). Microphytoplankton were a major contributor to total primary production (54%) in the cyclonic eddy. The contribution of picophytoplankton (Synechococcus and Prochlorococcus sp) reached maximum values (49%) in the central Red Sea. Nanophytoplankton seem to provide a ubiquitous substantial contribution (30-56%). Our results contribute to providing new insights on the spatial distribution and structure of phytoplankton groups. An understanding and quantification of the carbon cycle in the Red Sea was made based on estimates of primary production associated with pico-, nano- and microphytoplankton.

Lake Afrera, a structural depression in the Northern Afar Rift (Red Sea).

The boundary between the African and Arabian plates in the Southern Red Sea region is displaced inland in the northern Afar rift, where it is marked by the Red Sea-parallel Erta Ale, Alaita, and Tat Ali volcanic ridges. The Erta Ale is offset by about 20 and 40 km from the two en echelon ridges to the south. The offset area is highly seismic and marked by a depression filled by lake Afrera, a saline body of water fed by hydrothermal springs. Acoustic bathymetric profiles show ≈80 m deep canyons parallel to the NNW shore of the lake, part of a system of extensional normal faults striking parallel to the Red Sea. This system is intersected by oblique structures, some with strike-slip earthquakes, in what might evolve into a transform boundary. Given that the lake’s surface lies today about 112 m below sea level, the depressed (minus ≈190 m below sea level) lake’s bottom area may be considered the equivalent of the “nodal deep” in slow-slip oceanic transforms. The chemistry of the lake is compatible with the water having originated from hydrothermal liquids that had reacted with evaporites and basalts, rather than residual from evaporation of sea water. Bottom sediments include calcitic grains, halite and gypsum, as well as ostracod and diatom tests. The lake’s level appears to have dropped by over 10 m during the last ≈50 years, continuing a drying up trend of the last few thousand years, after a “wet” stage 9,800 and 7,800 years before present when according to Gasse (1973) Lake Afrera covered an area several times larger than at present. This “wet” stage corresponds to an early Holocene warm-humid climate that prevailed in Saharan and Sub Saharan Africa. Lake Abhé, located roughly 250 km south of Afrera, shows similar climate-driven oscillations of its level.

Common reef-building coral in the Northern Red Sea resistant to elevated temperature and acidification.

Coral reefs are currently experiencing substantial ecological impoverishment as a result of anthropogenic stressors, and the majority of reefs are facing immediate risk. Increasing ocean surface temperatures induce frequent coral mass bleaching events—the breakdown of the nutritional photo-symbiosis with intracellular algae (genus: Symbiodinium). Here, we report that Stylophora pistillata from a highly diverse reef in the Gulf of Aqaba showed no signs of bleaching despite spending 1.5 months at 1–2°C above their long-term summer maximum (amounting to 11 degree heating weeks) and a seawater pH of 7.8. Instead, their symbiotic dinoflagellates exhibited improved photochemistry, higher pigmentation and a doubling in net oxygen production, leading to a 51% increase in primary productivity. Nanoscale secondary ion mass spectrometry imaging revealed subtle cellular-level shifts in carbon and nitrogen metabolism under elevated temperatures, but overall host and symbiont biomass proxies were not significantly affected. Now living well below their thermal threshold in the Gulf of Aqaba, these corals have been evolutionarily selected for heat tolerance during their migration through the warm Southern Red Sea after the last ice age. This may allow them to withstand future warming for a longer period of time, provided that successful environmental conservation measures are enacted across national boundaries in the region.

Introduction to the carbonate microfacies of the Farasan Islands, southern Red Sea

Microfacies analysis of the Farasan Islands sediments was carried out to unravel the depositional environment and evolution of the area. Thin section petrographic studies were carried out to expand on the field identifications, to study the composition and to classify the Farasan Islands rock units. Sedimentological characteristics and depositional settings were also established. Several fossil assemblages have been identified, including Forams, Algae, Corals, Mollusks, Ostrocods, and Ooids. The most common diagenesis processes observed were dissolution and replacement by gypsum and anhydrite. The carbonates of Farasan Islands show normal shallow marine processes. Shale and anhydrite sequences were deposited most likely in subtidal to open marine environments. Progressive shallowing of the depositional environment is illustrated by the evolution of sedimentary facies in the Farasan Islands from low energy subtidal to open marine and finally to high energy at the top of the sequence. Field observation of palaeoshorelines may reflect the control of sea-level changes on reef formations during the Plio-Pleistocene time in the Red Sea.

Food and Feeding Habits of Two-Bar Seabream, Acanthopagrus bifasciatus (Forsskål, 1775) from Southern Red Sea, Egypt

Food and Feeding Habits of Two-Bar Seabream, Acanthopagrus bifasciatus (Forsskål, 1775) from Southern Red Sea, Egypt

Cross shelf benthic biodiversity patterns in the Southern Red Sea

The diversity of coral reef and soft sediment ecosystems in the Red Sea has to date received limited scientific attention. This study investigates changes in the community composition of both reef and macrobenthic communities along a cross shelf gradient. Coral reef assemblages differed significantly in species composition and structure with location and depth. Inner shelf reefs harbored less abundant and less diverse coral assemblages with higher percentage macroalgae cover. Nutrient availability and distance from the shoreline were significantly related to changes in coral composition and structure. This study also observed a clear inshore offshore pattern for soft sediment communities. In contrast to the coral reef patterns the highest diversity and abundance of soft sediment communities were recorded at the inshore sites, which were characterized by a higher number of opportunistic polychaete species and bivalves indicative of mild disturbance. Sediment grain size and nutrient enrichment were important variables explaining the variability. This study aims to contribute to our understanding of ecosystem processes and biodiversity in the Red Sea region in an area that also has the potential to provide insight into pressing topics, such as the capacity of reef systems and benthic macrofaunal organisms to adapt to global climate change.

Taxonomic revision of the Equulites elongatus (Günther 1874) species group (Perciformes: Leiognathidae) with the description of a new species

Taxonomic analysis of a group of morphologically similar and phylogenetically related ponyfishes (Perciformes: Leiognathidae) establishes the Equulites elongatus species group comprising three valid species: Equulites aethopos sp. nov., currently known only from southern Red Sea; Equulites elongatus (Gunther 1874), known from northern Australia, Indonesia and Myanmar; and Equulites popei (Whitley 1932), known from Japan, Philippines, Malaysia (Sabah), Thailand (Gulf of Thailand), Oman, the Red Sea, and Mozambique. Although E. popei has previously been regarded as a junior synonym of E. elongatus, the present mitochondrial DNA analysis revealed that these two nominal species are different and that they constitute a monophyletic group separate from other species of Equulites. The E. elongatus species group can be defined by the following of characters: slender body (20–30% in standard length), deeply incised posterior margin of the adipose eyelid, and ventral surface of breast completely scaly. Equulites aethopos sp. nov. differs from the other two species in having smaller eyes (eye diameter 53–68% of postorbital head length vs. 78–137%) and tips of neural and hemal spines of the fourth preural centrum distinctly expanding (vs. slightly expanding). Equulites elongatus is distinguished from E. popei by: scales above and below the lateral line 5–9 and 9–14, respectively (vs. 8–13 and 12–19), anus anteriorly located (distance from the pelvic-fin insertion to the center of anus 30–42% of the distance from the pelvic-fin insertion to the anal-fin origin vs. 35–50%), and more numerous dorsolateral dark marks (1–9 ring marks and 0–14 dark spots smaller than a half of pupil diameter vs. 0–2 and 0–5).

The Gulf of Aden Intermediate Water Intrusion Regulates the Southern Red Sea Summer Phytoplankton Blooms.

Knowledge on large-scale biological processes in the southern Red Sea is relatively limited, primarily due to the scarce in situ, and satellite-derived chlorophyll-a (Chl-a) datasets. During summer, adverse atmospheric conditions in the southern Red Sea (haze and clouds) have long severely limited the retrieval of satellite ocean colour observations. Recently, a new merged ocean colour product developed by the European Space Agency (ESA)—the Ocean Color Climate Change Initiative (OC-CCI)—has substantially improved the southern Red Sea coverage of Chl-a, allowing the discovery of unexpected intense summer blooms. Here we provide the first detailed description of their spatiotemporal distribution and report the mechanisms regulating them. During summer, the monsoon-driven wind reversal modifies the circulation dynamics at the Bab-el-Mandeb strait, leading to a subsurface influx of colder, fresher, nutrient-rich water from the Indian Ocean. Using satellite observations, model simulation outputs, and in situ datasets, we track the pathway of this intrusion into the extensive shallow areas and coral reef complexes along the basin’s shores. We also provide statistical evidence that the subsurface intrusion plays a key role in the development of the southern Red Sea phytoplankton blooms.