Yemen’s coastline is about 2,200 km long, roughly one-third of which faces the Red Sea
and the remaining two-third the Gulf of Aden. Especially the Red Sea coastline represents a complex and unique tropical marine ecosystem with extraordinary biological diversity and a remarkably high degree of endemism.
The southern Red Sea supports one of the most important fisheries in the Red Sea. There is a growing understanding that nutrient transfer among coastal ecosystems is also important for fisheries and that the loss of one habitat can result in the collapse of fishing stocks. The high productivity is a result of a continuous input of surface water from the Gulf of Aden through the Straits of Bab el Mandeb which has seasonally high nutrient levels due to local upwelling processes. In addition, the broad and shallow shelf areas of this region allow re-suspension of bottom sediment by turbulent mixing, which also contributes to the raised nutrient levels. In 2001, Red Sea landings were about 40,000 tons. About 95% of the production is sold for local consumption.
Likewise, linkage between mangroves and adjacent soft-bottom habitats such as sea-grass beds and areas of sand and mud flats is very likely to contribute to the high productivity in the area. A number of organisms exploited in fisheries also complete their life cycles in more than one biotope.
The Ras Issa Peninsula appears to be an ecologically diverse area especially for its sub-tidal habitats including a coral fringing reef in the south, various coral community types all around the peninsula, sea grass beds, and macro-algal communities.
In this post, a detailed account of the marine ecology in and around Ras Issa Peninsula "Figure 1" is presented hereunder based on a site survey campaign conducted on 2005 as part of EIA study "for the proposed Refinery project" and by HMR team in which the author was part of.
Figure1. Ras Issa Peninsula Location "Yemen"
1. Methodology of Assessment
Rapid Coastal Environmental Assessment (RCEA)
The methodology used by the team for the rapid site inspections follows PERSGA standards as published in the survey manual “Standard Survey Methods for the Red Sea and the Gulf of Aden” (PERSGA 2003). RCEA allows both ecological characteristics and human impacts in an area to be rapidly assessed at the same time. The technique has been widely used for both research and coastal management work in the Red Sea, the Arabian Gulf, and other areas.
Subtidal Rapid Assessments
This method is also called “Quick Site Survey”. It was conducted at numerous sites (usually between detailed sites- see Figure 1-2) and aimed to collect rapid ‘spot check’ data for a rapid habitat and community classification or for use in calibrating and validating satellite data. These surveys are intended to be very rapid (10-15 minutes maximum), conducted either by snorkeling or using a viewing bucket over the side of the boat.
Biodiversity Surveys For Corals and Fish (Timed Swims)
Species of coral and fish were recorded on underwater writing boards during 20 minutes timed swims, performed snorkeling or Scuba diving, along a line perpendicular to the shore and then transferred into a spreadsheet program for analysis. Species which could not instantaneously be identified under water were either drawn on paper or photographed for subsequent positive taxonomic identification.
Mangrove Surveys
The survey methodology was based on the Standard Survey Methodology (SSM) and the Mangrove Monitoring program developed by PERSGA and Khalil (2002). The latter describes also statistical methods to be applied in the analysis of survey and monitoring data. A Rapid Survey Assessment (RSA) method was mainly applied, which is based on describing the physical conditions and stand characteristics together with recording presence and absence data of key species for the biotope. Additionally, disturbance and impacts were recorded and assessed describing the source and level of the impact and diagnosing the observed effects on the mangroves. Threats to the mangrove from any activities in the surrounding settlements were also recorded. These provide data for identification of the mangrove condition, concerning the level of human disturbance and habitat degradation.
2 Coastal Habitats of Ras Issa
Coral Reefs and Coral Communities
The coastal area north of Ras Issa exhibits completely different habitat types and is of ecological, as well as economic interests. With its extensive and dense sea grass meadow it provides a productive marine habitat and a nursery ground for fish and shrimp species. The terrestrial part is characterized by a wide sabkhat with no apparent vegetation and a shallow intertidal channel which is populated by a variety of shore birds including flamingos and pelicans.
Figure 1.2 presents the marine habitats and benthic communities of Ras Issa Peninsula and Kamaran Island.
Figure 1.2: Subtidal quick sites around Ras Issa and Kamaran Island.
Corals reefs in general are amongst the most productive and biologically diverse ecosystems on earth. They are critical habitat for a high number of plant and animal species which find food and shelter in the reef, settle on or attach to corrals or borrow into their skeletons, creating a complex and dynamic ecosystem. Reefs are of outstanding economic interest as a valuable source of food and various compounds of medicine, they attract large numbers of diving and snorkeling tourists which are a major source of income for coastal countries, and they protect the coastline from erosion. Corals are also important ecological indicators: due to their sensitivity towards temperature changes they are among the first systems to respond to global warming and other marine stressors.The Red Sea is most famous for its extensive and beautiful fringing coral reefs that drop steeply into deep water and are swept by very clear water. There are, in addition, many other reef types that contribute to the great diversity of this system within the region, and hence support enormous reef-associated biota. The southern Red Sea reefs in this regard have a markedly different geomorphology compared to those of the northern and central Red Sea, with comparatively limited reef development resulting from differences in bathymetry, topography, turbidity and sea temperature.
About 75% of the coastline consists of soft sediment; the scarcity of hard substrate and the high turbidity along the wide and shallow continental shelf create harsh conditions for coral growth. Nevertheless, various coral communities and some fringing reefs can be found along the southern Red Sea coast. Up to now, however, there is no detailed information available on their exact distribution, their species composition and their actual status. Available information is often contradictious and significantly influenced by the timing of the respective survey and by the survey method itself. In the 90th, the southern Red Sea was believed to support poor reef development with generally low diversity coral communities and one report (UNDP GEF 1999) even speculates that recent coral growth might have severely been interrupted. In retrospective, the latter observation can be attributed to the world bleaching event in 1998 which also affected the southern part of the Red Sea. Recent surveys reveal that most of the coral communities have unexpectedly well recovered and support a fairly high diversity of hard coral species, in many places despite a lack of extensive reef development.
Contemporary coral growth is found both as coral reefs (“true” reefs in the sense that there is considerable biogenic carbonate accretion by corals) and as coral communities on a variety of hard substrates. The true coral reefs show two types of development:
fringing reefs off the southern coast, off some offshore islands and off the southern coast of Ras Issa
semi-submerged patch reefs from offshore Al-Hodeida to the southern Farasans
Coral communities may be found either associated with red algal “reefs”, on relic Pleistocene/Holocene reef deposits or on lava flow terraces on volcanic rock pinnacles.
The variation in these forms depends largely on local environmental parameters such as water temperature, sedimentation and nutrient content. Seasonal coverings of macro-algae are a feature of shallow coral reefs and coral communities in the southern Red Sea, and they often form the major coverage of hard substrates in areas too turbid for coral growth.
Yemeni reefs and coral communities have developed in some of the most extreme environments for corals, ranging from high sea water temperature to high sedimentation rates. Any additional anthropogenic impact poses a severe threat to these habitats. Despite their limited extent, the existing reefs and communities have a high regional conservation value because of their importance for fisheries and their potential value for education and science as well as for recreation and tourism.
In the study area, the majority of the investigated sites around the Ras Issa Peninsula with available hard substrate exhibit diverse coral communities. In contrast to other coastal areas along the Yemen Red Sea coast soft bottoms are not the dominant substrate type around Ras Issa. Only a few sites were dominated by macro-algae (non-threatened or endangered) of the genera Sargassum, Turbinaria and Dictyota , whereas most communities were either dominated by stony and soft corals or exhibited a mix of all various taxa groups. It is also noteworthy to mention that the higher portion of coral mortality in the southern part of Ras Issa can be attributed to mechanical breakage of coral colonies by fishing activities, beaching of boats or impacts by the temporarily heavy surge. Likewise, the northern part of the peninsula exhibits a fairly high diversity of coral community types. While the community at site 2 "Figure 1-2" mainly consists of xeniids and Pocillopora damicornis, site 3 is dominated by tabular Acropora species and the opportunistic settler Stylophora pistillata (Figure 1. 3). Associated species are among others Porites lutea, Leptastrea sp. and Platygyra lamellina.
Figure 1.3: A. Xeniid – Pocillopora damicornis community at site 2. B. Cushion starfish (Culcita novaguineae), an abundant species in this community.
The three sites 9, 10 and 23 are all mixed stony coral communities (Figure 1. 4 B). Echinopora lamellosa, Acropora hemprichii, A. digitifera, Stylophora pistillata and Fungites sp. are only a few examples of this diverse community. Site 10 exhibits at its very shallow part an extensive area with Porites nodifera colonies (Figure 1. 4 A).
Figure 1.4: Mixed stony corals community. A. Porites nodifera colonies in shallow zones. B. Stylophora- Acropora-Echinopora-Mix.
All the above-mentioned coral communities along the northern coast are not equally distributed but rather occur in patches of various sizes. They all thrive in sheltered to semi-sheltered conditions. Patches in the vicinity of fish landing sites show relatively high abundances of the herbivore Diadema sea urchin, a phenomenon which can be attributed to high nutrient levels as a result of fishermen dumping by-catch into the water. Another consequence is elevated bio-erosion rates.
Fringing Reefs
The surveys confirmed the occurrence of a wide coral fringing reef in the south and south-west of Ras Issa. It starts a few hundred meters west of the north-western tip of the peninsula and stops east of Issa village. It extends up to 600 m into the sea. The structure of the reef flat varies depending on the exposure of the corresponding reef section and its geological features. In some areas a lagoon between the reef edge and the beach with small patch reefs is present with a depth of more than 1 m, whereas in others the reef flat consists of a massive limestone body. In correspondence with depth and consequently with the exposure to extreme habitat conditions it is covered either with living corals (in some places massive low -growing Porites nodifera colonies), a mixed community of corals, zoanthids, encrusting sponges, and macro-algae or pure brown algae communities. Figure 1.5 presents the various habitats and characteristics present in a fringing reef.
Figure 1.5: Fringing reef with breech at the south west of Ras Issa
The fringing reef (Figure 1. 6) has only one major breech which is located at the mouth of a wadi 2 km southwest of the above mentioned tip. This gap is about 260 m wide at the reef crest and widens up towards the beach. The bay-like area is sandy and due to the missing protection by the reef very exposed to the wave dynamics. Despite a high turbidity the inner perpendicular edges of the reef show living coral cover with big massive colonies at some places. In the middle of the breech at 4-5 m depth there are several coral patches with vital soft and stony coral communities.
Figure 1.6: Fringing reef Ras Issa. A. Reef section with a wide and massive reef flat. B. Reef section with a lagoon and a Turbinaria sp. community on reef blocks/patches.
Mangroves
The term ‘Mangrove’ comprises ecosystems or an ecological group of about 70 plant species from 20 families of trees or bushes, growing between the level of high water of spring tides and a level close to, but above, the mean sea level. Some genera like Rhizophora and Avicennia have developed special root systems, which support respiration and stability. Stilt roots and pneumatophores reduce erosion and slow down current velocities, which increase sedimentation of suspended material. Therefore, mangroves can progress seawards to occupy new coastal habitats and are especially important for coastal protection from erosion.
Apart from their significance for coastal protection, mangroves play a vital role for marine life and fisheries by providing food and shelter for a large and varied group of marine organisms including fish and shellfish. A diverse fauna of more than 250 species of marine invertebrates and vertebrates occurs in association with mangrove systems in the Red Sea. However, this appears to be less than comparable mangrove systems in the nearby Indian Ocean, presumably as a result of the harsh environmental conditions (related to prevalent temperature and salinity extremes) within the Red Sea. In addition to marine organisms, mangroves are used as a food source by terrestrial vertebrates and as a roosting and nesting site by many species of birds. Out welling of nutrients and organic matter in form of mangrove leaf litter contributes to fertilization of coastal waters and enriches the marine food web in the tropical coastal environments.
Four species of mangrove exist within the Red Sea and Gulf of Aden, with the two most common being Avicennia marina and Rhizophora mucronata; other species known to occur in the region are Bruguiera gymnorhiza and Ceriops tagal. In Yemen, only A. marina and R. mucronata have been recorded. In contrast to other countries along the Red Sea where mangroves exist as distinct but isolated stands, extensive stands occur in the southern Red Sea of Saudi Arabia and Yemen where the continental shelf is widest and a greater depositional environment allowing for a stable sediment layer to develop.
Mangrove stands are therefore critical habitats stabilizing near shore sediments, trapping nutrients, exporting energy to near shore subtidal habitats, and functioning as nursery grounds for a range of fish and invertebrate species.
Mangrove stands were documented at three sites inside the survey area which include the extensive stands at the north-eastern part of Kamaran island, between Al-Harounia and Al-Salif, and the smaller stand south of Al Urj (Mujeider).
Kamaran Island
The large island of Kamaran is about 20 km long and 8 km wide. The island is separated from the mainland by 2.5 km wide and less than 100 m deep channel. The mangroves cover the eastern shore of the island, which is undulating and includes several inundated lagoons and channels forming an extensive network. Two species, Rhizophora mucrunata and Avicennia marina exist there (Figure 1.7). The Avicennia form a thick belt fringing most of the peripheral channels and the middle of the main channel, surrounding a well grown stand of Rhizophora trees. The Rhizophora trees reach up to 7-9 m in height and Avicennia up to 4-5 m. Camel grazing is not significant at most parts of the stands, except of the outer landward parts are rather affected and disturbed by grazing. There is some limited cutting of the roots of Rhizophora surrounding the main lagoon. The mangroves swamps of Kamaran island are the most dense and diverse in Yemen and therefore regionally of paramount biological value.
Figure 1.7. Mangrove of Kamaran Island. A. Mixed Avicennnia marina - Rhipophora mucronata stand. B. Survey inside the mangrove stand. C. Nest with a juvenile osprey on top of an Avicennia marina bush (April 2004).
Al Mujeider
The mangrove stand at Mujeider (Figure 1.8) contains the oldest and biggest Avicennia mangrove trees in Yemen. The stand grows at a narrow shore area, fringed by palm trees inland and protected by a seaward shell-sand barrier-beach. The Avicennia trees attain up to 9-11 m in height and 200-240 cm girth at breast height (GBH). Salinity is low to moderate (around 17 ppm), perhaps favouring the massive growth of the mangrove trees. Although rich populations of terrestrial woody plants (palms and scattered Acacia trees) are present, which may provide a source of firewood for local inhabitants, cutting is still very severe at some parts of the stand, where many Avicennia trees had been felled.
Figure 1.8. Mangrove stands at Mujeider. A. Old man grove trees. B. Mixed plant communities including freshwater-dependent palm trees. C. Examples of overgrazing impacts by camels.
Al-Salif - Al-Harounia
The shoreline between Al-Harounia and Al-Salif is devoid of mangroves except for a single extensive stand fringing an elongated khor (Figure 1.9). The 3.8 km long stand is embedded in a large sabkha (including some sand and mud flat areas) which extends from the shore line to the highway connecting Al-Hodeida with Al-Salif. Its most dense section can be found along the main khor and close to the entrance to the sea where trees reach a height of up to 5 m and a GBH of 40-50 cm (Khalil 2002). The site has many seedlings, especially at undisturbed parts of the stand. Mature trees are fruiting and many seeds can be observed on the ground. Trees and bushes of the outer patches are stunted due to high salinity and camel grazing.
Importance and function of mangroves
As stated earlier, besides their significance for coastal protection, mangroves play a vital role for marine life and fisheries by providing food and shelter for a large and varied group of marine organisms including fish and shellfish. Worldwide, over 1,000 fish species of commercial importance have been recorded from mangrove areas.
Although mangroves of the Red Sea do not support as diverse associated communities as at several other tropical coasts, they play an important role as nurseries for several commercially esteemed species of fin and shellfish in the region and as natural protection of coral reefs by trapping sediment loads of the seasonal rainwater influx. As they grow in a very hostile environment the Red Sea mangroves are generally very sensitive.
Threats
In spite of the ecological importance of mangrove ecosystem and its manifold economic benefit, the loss and degradation of mangrove areas are enormous. Mangroves are used traditionally and commercially e.g. as timber and fuel wood. In many tropical areas large scale removal of mangrove forests takes place in favor of aquaculture, land reclamation and other purposes. Coastal development causing pollution, alteration of the substrate and modifying hydrological regimes also exerts serious stress on mangroves.
In Yemen, the situation of the mangroves is rather miserable. Mangrove degradation was reported in many parts of the region, mainly due to over -cutting and excessive browsing by camels, cut-off from freshwater by damming of rain -fed wadis. At present, none of the
mangrove areas in Yemen is legally protected for nature conservation purposes.
Sabkha
Sabkha-based habitats exist at the highest level of the intertidal and are usually only seasonally inundated. They are composed of sparse halophyte vegetation embedded in a sodium chloride and gypsum crust, below which a microbial/algal mat consisting of cyanophytes, bacteria and diatoms is found (Chiffings 1989). They are a productive habitat with nitrogen fixation occurring in the microbial/algal mat. A characteristic feature of sabkhas is the presence of pools which, because of the high salinity and temperatures, contain a specialized fauna of benthic invertebrates and a complex microbial community. Although little is known of the ecology and biodiversity of sabkhas, they are a significant feature and in some areas occupy an area greater than that of mangroves and salt marsh combined.
In the study area sabkhas can be found along the coastline between Ras Issa and Al-Salif and between the main road from Al-Hodeida to Al-Salif and Al-Harounia (Figure 1.9)
Figure 1.9: Sabkhas. A. Sabkha in a depression between the sand beach and sand dunes at Ras Marsa (near the lighthouse) B. Sabkha with salt ponds west of the road to Al-Salif.
Sea Grass Beds
There is an increase in abundance of sea grass beds towards the southern Red Sea, owing to the expansion of the shallow and wide continental shelf area. In Yemen, sea grass beds occur along 40% of the Red Sea coastline.
Sea grass beds generally occur in protected areas in lagoons and bays where they are an important habitat for juvenile fishes and crustaceans and a source of food for important species such as dugong and green turtles. Three major groupings of sea grass assemblages along the eastern Red Sea appear to be discernible separated by latitude, which would probably suggest three distinct biogeographic areas.
Sea grass beds in the Red Sea are inhabited by a diverse fauna, which increases from 49 species in the Gulf of Aqaba to 91 species further south. The major groups inhabiting sea grass beds include mollusks, polychaetes, crustaceans, echinoderms, and fishes, with perhaps c. 10% of species occurring in sea grass beds exclusively. The standing crop and productivity of Red Sea sea grass beds is comparable to that reported from other tropical regions of the world, and in other parts of the world, Red Sea sea grass beds stabilize near shore sediments, provide a juvenile habitat for a range of commercially important crustaceans and fishes, are a source of food for a significant number of species, and export nutrients and energy to adjacent subtidal systems.
Importance and function of sea grass beds
Sea grass beds stabilize and bind substrates and absorb nutrients from sediments. They reduce water currents by frictional forces, dampen wave energy and slow erosion processes. They are primary producers removing inorganic nutrients from the sediments and the water column and through photosynthesis convert them into organic matter. The blades are food for grazing invertebrates, fish and shore birds. The remaining dead plant matter adds substantial amounts of organic biomass to near shore deposits of detritus and benthic habitats. The detritus fuels the microbial food web which, in turn, provides food for invertebrates, fish, and birds. Sea grass beds add structural complexity and surface area to intertidal and subtidal environments. Blades become covered with an organic layer composed of microscopic plants (benthic diatoms), bacteria and grazers. They provide shelter from predation and wave action; nursery grounds for clams, fish, blue mussels, sand shrimp, lobsters, crabs, and other aquatic organisms; attachment surface for epiphytes, snails and larvae; and shading from solar radiation. Shorebirds and commercially important fish species prey on worms and invertebrates living in and feeding on sea grass. Species abundance and diversity is high compared to un-vegetated sites.
Sensitivity to disturbance and development
Sea grass beds are multi-functional productive habitats that have been classified as having a high sensitivity to disturbance and development.
Threats
Shading from physical structures: shading blocks light and reduces growth. Even temporary floats can smother and kill sea grass beds.
Removal and/or disturbance of habitat: dredging, filling, impoundment of water, sediment loading, and boating activity shades, smothers or removes sea grass and its habitat.
Re-suspension of sediments: from dredging, filling
Pollution: run-off of sediments and pollutants from upla nd construction sites, freshwater discharges, nutrient rich groundwater, industrial discharges, chlorinated effluent, oil pollution, storm water run-off, sewage, airborne pesticides from agriculture and others all damage sea grass. Eutrophication from upland point and non-point source pollution stimulates phytoplankton and algal growth (epiphytes) reducing light levels reaching sea grass beds.
Extensive sea grass beds were documented off Ras Marsa (subtidally), Al Mujeider (intertidally) and north of Ras Issa (subtidally). In all cases, the communities were composed of three to five different species which are Halodule uninervis, Halophila ovalis, Cymodocea serrulata, Cymodocea rotundata and Thalassia hemprichii.
In other sites such as the north-western tip of Ras Issa sea grass occurs as small patches in mixed communities with macro algae such as Halimeda tuna, Sargassum spp. And Dictyota spp. (Figure 1.10)
Figure 1.10 : Sea grass communities around Ras Isaa. A. Extensive subtidal community northwest of Ras Issa. B. Intertidal community together with patches of the green algae Halimeda tuna and the brown algae Sargassum sp.
Salt Marshes
Salt marshes are one of the most productive habitats on earth. Salt marshes are persistent marine near shore emergent grass habitats. They lie between the upland and intertidal flats and beaches in protected bays and coves, along tidal rivers or behind barrier beaches.
Marshland can persist at the same location for up to 4,000 years. The salinity ranges between 0.5 – 34 parts per thousand. Marshe s occur in low energy habitats with sources of fine grained sediments. Tidal marsh plants, marine and terrestrial plants and algae decompose and combine with mineral sediments to form large salt marsh peat deposits.
Salt marshes help to stabilize the surrounding soils and minimize beach erosion and the inundation of seawater. Halophytic communities are categorized according to their dominant species, height above sea level and immersion periodicity.
Salt marshes are not very widespread in the area but can be found at a few sites on alluvial deposits near the shore. On Ras Issa one salt marsh area was documented on the southern coasts along a wide khor parallel to the coastline (Figure 1. 11). The species composition of these communities depends on the amount of dew available.
Figure 1.11 : Salt marsh on Ras Issa (southern coast). A. View from east: the salt marsh is clearly visible as a narrow stretch running parallel to the coast being embedded into shrub land. B. Detail: Salt marsh section surrounding an intertidal pool.
Salt marsh communities are wide spread along the Yemeni Red Sea coast line and important as localized sources of high primary productivity. They provide food and shelter to numerous invertebrates and shore birds.
Importance and function of salt marshes Salt marshes have many biological, chemical and geological functions in marine systems.
Algal Communities
Macro-algae dominated only one survey site (24) forming extensive stands. In very shallow waters, Padina sp. was the predominant species, in deeper water Sargassum sp. Dominates (Figure 1.12).
Figure 1.12: Macro-algae communities. A. Sargassum sp.; in the foreground the sea urchin Echinometra mathaei. B. Padina sp.
REFERENCES
CIA. 2005. “The World Fact Book on Republic of Yemen.” Retrieved from http://www.cia.gov/cia/publications/factbook/print/ym.html on 13 February 2005.
Consulting Engineering Office. 1986. “Offshore Geotechnical Report.” Prepared fo Yemen Exploration & Production Company (YEPCO), Republic of Yemen.
Espey, Huston and Associates, Inc. 1989. “Baseline Marine Biology Survey of the Ras Issa Marine Terminal.” Prepared for Yemen Exploration and Production Company.
PERSGA. 2003. “Coral Reefs in the Red Sea and Gulf of Aden. Surveys 1990 to 2000: Summary and Recommendation.” The Regional Organization for the Conservation of the Environment of the Red Sea and Gulf of Aden.
University of Sana’a. 1996. “Interim Report on Oil Pollution in the Red Sea Coast of Yemen.” Sponsored by Environmental Protection Council (EPC), Dutch Support Project to Technical Secretariat EPC, Yemen. January 1996.
WB. 2001. “Strategic Action Programme for the Red Sea and Gulf of Aden – Country Reports.” The World Bank. Washington, DC.
Site survey campaign carried out on 2006 as part of EIA study for the proposed Ras Issa Refinery project.