Plate 2.1 The coastline of the Western Arabian Gulf. 20 Marine Atlas of the Western Arabian Gulf

Transcription

1 Plate 2.1 The coastline of the Western Arabian Gulf. 20 Marine Atlas of the Western Arabian Gulf

2 Mohammed Ali Qurban, Krishnakumar, P. K., Joydas, T.V., Mohamed Ashraf, T.T., Manikandan, K. P., Research Institute - KFUPM, Dhahran, Saudi Arabia Khaled Al-Abdulkader and Ronald A. Loughland Environmental Protection Department, Saudi Aramco

3 Introduction The Western Saudi Arabian Gulf Coast and offshore islands support a variety of coastal and marine habitats, such as mangroves, coral reefs, and seagrass beds. The Saudi Arabian Gulf Coast from Ras Al-Khafji in the north to Ras Abu Qamees in the south is divisible into several relatively homogeneous areas in terms of the variety and distribution of coastal and marine habitats including three major habitats: Supra tidal flats, intertidal and subtidal regions. Tidal flats are an important part of the intertidal environment. The tidal flats in the Western Arabian Gulf are characterized by a series of well defined zones and may include mangroves, salt marshes and sabkhas. Mangroves are found in association with tidal flats and are represented by a single species, Avicennia marina. Coral reefs are important subtidal habitats especially around the offshore islands being highly productive and are important for many organisms because they provide habitat structure and feeding resources. The Western Arabian Gulf is a sub-tropical sea in a temperate setting being warm in the summer and more temperate in the winter than most seas of equivalent latitude. The surface water undergoes rapid temperature changes, ranging from less than 10 oc in winter to over 35 oc in summer. These conditions along with elevated salinities of up to 70 (Sheppard, 1993) create a harsh natural setting, which affects habitat distribution. 22 M a r i n e A t l a s o f t h e W e s t e r n A r a b i a n G u l f Plate 2.2 Natural mangroves, Avicennia marina, in Tarut Bay are subject to regular flooding during high tides. C h a p t e r 2 : O V E R V I E W O F T H E G U L F M A R I N E H A B I T A T S 23

4 Major Coastal Habitats The habitat map presented as Figure 2.1 illustrates the major coastal habitats of the Gulf Coast of Saudi Arabia. The three major habitats are tidal flats, intertidal and subtidal regions, and their components are illustrated in Table 2.1. Map 2.1 Habitat Map of Jubail and Abu Ali on the Saudi Arabian Coast of the Arabian Gulf. 24 Marine Atlas of the Western Arabian Gulf

5 Plate 2.3 Wide Sabkha areas in Abu Ali. Supratidal Flats Tidal flats occupy 30%-40% of embayments of the coastline (Basson, et al., 1977). The greater part of these tidal flats consists of mud or very fine sand, deposited in sheltered locations where wave energy is low. Salt marsh Salt marshes mostly occur above the average high tide zone (supra-tidal region). Many of the salt marsh plants do not appear to be water stressed for most of the time, although they grow on what appears to be dry, often loose sand. While they obtain a portion of their moisture from dew, their deeply penetrating roots commonly extend through the dry surface layer, where the salinity of the limited interstitial moisture is high, and into a layer where the moisture has lower salinity (Sheppard, et al., 1992). The salt marsh shelters populations of invertebrates and birds, especially during the cooler months (i.e., bird migration). Sabkha (Salt Flat) In general, Sabkha is composed of sand deposits mixed with silt and clay and is mixed with evaporites. Most coastal Sabkha is normally above the intertidal zone, and whether directly connected or not, is usually immersed at least occasionally. Their salts are related mostly to the evaporation of the continental groundwater that contains little carbonate but has abundant gypsum and anhydrite. During dry periods, the salt encrusted surface of the Sabkha is strong and durable whereas when the surface is wet due to rain or high tides, the Sabkha is soft and muddy. Chapter 2: OVERVIEW OF THE GULF MARINE HABITATS 25

6 Plate 2.4 Halophytic shrubs on beach dunes at Ras Abu Qamees. Intertidal Habitats Intertidal habitats are typically regarded as open ecosystems in which energy and materials are regularly imported and exported. Extensive intertidal habitats exist along the Arabian Gulf shores due to the extremely low topographic relief that has an average slope of 35 cm/km. Intertidal habitats include mangroves open-coast sand beaches, patchy rock shorelines, and protected tidal flats associated with coastal embayments and lagoons. Plate 2.5 Shells in intertidal muds. Mangroves Mangroves are conspicuous and important biological components occupying the upper parts of suitable shorelines (Sheppard, et al., 1992). Avicennia marina, the only mangrove species existing in the Gulf, can withstand extreme environmental conditions, including wide variations in surface water temperature, salinity, and dissolved oxygen. Most mangroves are subjected to human induced environmental stresses from land reclamation, dredging, domestic animal grazing, and liquid or solid waste disposal. Mangrove reforestation has been carried out since the early 1990s and A. marina has been transplanted successfully in Abu Ali and Tanajib Bays. These efforts continue at embayment systems along the coast. Sandy intertidal The beaches of the Gulf Coast of Saudi Arabia are composed primarily of carbonate sand of biological origin with fragments of shells and coral. Zonation of organisms is less distinct than that found in other intertidal habitats (Basson, et al., 1977). Basson, et al., (1977) divided this zone into supratidal and intertidal. The upper supratidal zone has several characteristic species of terrestrial origin, while the lower intertidal zone is composed of typical marine species. Basson et al., (1977) and McCain (1984a) reported the occurrence of over 200 species of macrofauna from the exposed sand beaches. These species included snails, bivalves, polychaetes, amphipods, isopods, and decapods. Plate 2.6 Crabs are one of the many species living in the intertidal muddy pools. Rocky intertidal Rocky habitats are naturally limited in the Arabian Gulf. Where rocky habitats do exist, e.g., Ras Abu Qamees, there is a mosaic of micro habitats rich with fauna and flora that is seasonally influenced by water temperature. 26 Marine Atlas of the Western Arabian Gulf

7 * (Sabkha) is a popular term used in the Arab countries to denote salt flat. Table 2.1 Overview of the major coastal habitats of the Gulf Coast of Saudi Arabia. Plate 2.7 Characteristic rocky shore in Ras Abu Qamees. Chapter 2: OVERVIEW OF THE GULF MARINE HABITATS 27

8 Subtidal Habitats Habitats found below the lower intertidal zone are included under this category. The most important factor determining which organisms occur in these habitats is the physical nature of the substrate that may consist either of rock or rock-like materials such as corals or soft sediments (Basson, et al., 1977). Important subtidal habitats, such as seagrass beds, coral reefs, sandy, muddy and rocky habitats, are summarized below. Seagrass beds Seagrass beds occur principally in shallow coastal areas. Three seagrass species occur off the Saudi coast in the Arabian Gulf viz. Halodule uninervis, Halophila stipulacea and H. ovalis. A progressive increase in the seagrass cover towards the north has been observed (Price and Coles, 1992). These beds are the most important communities in the overall biological economy of the Gulf (Basson, et al., 1977), providing habitat for a wide variety of both adult and developing juvenile organisms. High benthic density has been recorded from seagrass beds compared to adjacent sand/silt habitats (McCain, 1984b; Coles and McCain, 1990). Offshore Islands Coral islands such as Jana, Jurayd, Karan, Kurayn, Harqus and Al Arabiyah are located in the Arabian Gulf about km off the Coast of Saudi Arabia (Map 2.2). Some descriptions on the biological and physical features of these islands are provided in Chapter 6 of this Atlas and are highlighted in Table 2.2 and Figure 2.3. Coral reefs Coral reefs are one of the most diverse ecosystems on earth. Along the Gulf Coast of Saudi Arabia, coral reefs occur both in the nearshore, and the offshore. Reefs occurring offshore are more diverse because temperature, salinity, turbidity, and sedimentation are less extreme (McCain, et al., 1984). A total of 57 species of corals are found in the Arabian Gulf, with most of these species occurring on offshore islands (Burchard, 1979). Sandy In the Western Arabian Gulf the biological community associated with the sandy subtidal zone is quite diverse. Macroalgae, diatoms and benthic fauna are abundant. The deeper sand bottoms (15-30 m) surrounding the offshore coral reef islands are comprised mostly of carbonate particles from corals. Muddy In offshore deeper areas of the Western Arabian Gulf beyond the influence of wave action fine muddy substrates occur (Wagner and Van der Togt, 1973). Muddy habitats are also characterized by a diversity of organisms, such as Polychaetes, Amphipods, Ostracods, Isopods and Mollusks. Rocky Shallow subtidal rocks occur as patches along the Saudi Coast of the Arabian Gulf. The subtidal rocks provide suitable substrata and physical conditions to support coral attachment. Many species of corals compete with algae for attachment space on the rock surfaces. The irregular surfaces of rock also provide habitat for Polychaetes, shrimp, crabs and fishes. Figure 2.1 Sea Surface temperature in the Arabian Gulf during summer (left) and winter (right) (Source: NASA s Goddard Earth Sciences (GES) Data and Information Services Center (DISC)). Plate 2.9 Assemblages of stony corals on outcrops in the Gulf. The Brain Coral is Platygyra. the Tables and Branching Corals are of the genus Acropora. Also present are the genera Coscinaraea, Favites and Porites. Ras Abu-Qamees, located in the southern part of the Gulf Coast of Saudi Arabia is unique with rocky formations of horizontally bedded sedimentary rocks. Macro algae, coralline algae, gastropods, bivalves and serpulid polychaetes are the major flora and fauna attached to rocky substrate. Solitary hard corals also occur. Common fishes in the rock pools are Scolopsis ghanum, Lutjanus fulviflamma and Plectorhinchus sordidus. Patchy seagrasses occur in subtidal areas. Migratory and resident birds, such as Herons, use rocky shore habitats for breeding and feeding purposes. Plate 2.8 Coral reefs such as those occurring around Jana Island are very rich subtidal habitats. 28 Marine Atlas of the Western Arabian Gulf Chapter 2: OVERVIEW OF THE GULF MARINE HABITATS 29

9 Temperature and Salinity Stresses in the Arabian Gulf The geographical location of the Arabian Gulf between large, arid land masses (Arabia and Asia) and bordering high mountain chains of Iran make the climate tropical in summer and temperate during winter. Consequently, the atmospheric temperature and sea surface undergoes wide seasonal fluctuations. The Arabian Gulf is connected with the adjacent Indian Ocean by a narrow passage at the Strait of Hormuz. The restricted water exchange and high evaporation rates results in high salinities, and coupled with wide temperature fluctuations provides for a harsh marine setting. Water temperature and its seasonal variation The surface waters and coastal shallows of the Arabian Gulf undergo wide rapid temperature changes in response to daily and seasonal cycles of heating and cooling. Offshore sea surface temperatures vary from 16 C in winter to 32 C in summer while coastal bays and lagoons have an even wider temperature fluctuation range from 10 C to as high as 40 C. Most Gulf biotopes are therefore subjected to far greater temperature fluctuations than similar marine environments elsewhere in the world (Basson, et al., 1977). Strong wind results in frequent and thorough mixing of the entire water column and vertical temperature gradients are usually small especially in coastal waters. This can place stress on subtidal habitats, such as coral reefs. An important feature of the Western Arabian Gulf is the temperature variation in the north-south direction particularly in coastal waters, with the northern portion of the Gulf being colder than the central and southern parts during most seasons. Comparatively cooler water occurs in the coastal waters and inner bays of the Gulf during late fall, winter and spring seasons. The Gulf has an increasing trend in temperature toward offshore waters; whereas, a reverse trend is found during the summer season with warmer water near the coast (Figure 2.2). Water salinity and its seasonal variation The lack of significant river runoff, a high rate of evaporation (~2 m/yr) as well as limited interchange with the Indian Ocean creates overall salinities in the Arabian Gulf up to 42 offshore and over 60 in some restricted shallow coastal areas of the Western Arabian Gulf (KFUPM/RI 2003a, 2008, 2010). Generally, higher salinity occurs within the coastal waters, and decreases offshore. Seasonal variations in surface salinities are also attributed to: (i) nature of the inner bays causing limited water flushing and a subsequent increase in salinity, (ii) wind stress and associated evaporation, (iii) high temperatures in summer with subsequent evaporation (iv) intrusion of low saline surface water from the Indian Ocean, and (v) the Tigris-Euphrates River flow through the Shatt Al-Arab (Figure 2.3). Figure 2.2 Sea surface salinity along the west coast of the Arabian Gulf during summer (above) and winter (below) (data from KFUPM/RI studies (KFUPM/RI 2003a, 2008, 2010). 30 Marine Atlas of the Western Arabian Gulf

10 Plate 2.10 Coastal vegetation at Al Uqair along the Gulf of Salwa. These plant species indicate a source of freshwater. Stress of temperature and salinity on the marine system Characterized by temperature extremes and by high and often fluctuating salinities, the marine life in the Arabian Gulf inhabit a stressful environment. High salinity is one of the most important environment factors controlling and limiting the occurrence and distribution of marine life in the Gulf. As a result, the biota is believed to be restricted with many organisms being unable to tolerate the harsh conditions. The diversity of biological communities in the Gulf is often low compared with similar communities living under similar conditions elsewhere (Basson, et al., 1977). These conditions limit distribution of some species. The low temperatures in winter, are outside the optimum range for many tropical species, and for example reef coral development; and Arabian Gulf coral reefs tend to be less developed than those of other parts of the world. The Gulf of Salwa is located between Saudi Arabia and the Qatar Peninsula, and is nearly cut off from the main water body of the Arabian Gulf by the Island of Bahrain and shallow water flats extending on either side. These shallow flats form a tidal barrier to water movements, and the tidal amplitude within the Gulf of Salwa is much reduced. The Gulf of Salwa is therefore a good example of how temperature and salinity impact on habitats and species assemblages. Salinities are high ranging from about 55 at the entrance to the Gulf of Salwa to 70 in the south (Basson, et al., 1977). The elevated salinity and temperature restrict coral growth, and macroalgae replace the corals as dominant biota (Sheppard, et al., 1992). Summer temperature along Gulf of Salwa varies from 28 C to 40 C and winter temperature has a range from 17 C to 25 C (Figure 2.2). The seasonal amplitude of surface temperature is ~23.0 C, which is huge when compared to other parts of the Arabian Gulf. Similarly summer salinity varies from 57 to 63 with an average salinity of 59.5 (Figure 2.3). The winter salinity ranges from 55 to 62 with a mean salinity of 57.5 [KFUPM/RI, 2008]. As environmental conditions become increasingly saline further within the Gulf of Salwa with greater temperature extremes, coral growth and reef development deceases (Sheppard, et al., 1992). Coral reef growth is dependent on a number of physical conditions including salinity, temperature, and sedimentation (Sheppard, et al., 1992). In Tarut Bay, salinities reach 55, seasonal temperatures fluctuate greatly, and a considerable amount of the marine bottom is composed of soft or muddy substrate, all of which limit coral reef growth. The solubility of oxygen in seawater decreases with increasing salinity resulting in environments that are low in available oxygen. Marine habitats in this environment usually have a low faunistic diversity. Very few animal species are found at salinities higher than 70 (Clarke and Keij, 1973). The extreme environmental stresses lead to marine community changes as opportunities for species succession follow the demise of communities that cannot tolerate the harsher conditions (Sheppard, et al., 1992). Chapter 2: OVERVIEW OF THE GULF MARINE HABITATS 31

11 All animals in the marine environment have a thermal range - the temperature range at which they can most efficiently grow, reproduce, and live. For example, the Manifa area was subjected to two months of continuous exposure to temperatures of 11 C to 16 C during early This long duration of cool temperatures resulted in the mortality of several species of corals, especially those near the surface and on the reef flats (Coles and Fadlallah, 1991). The dominant environmental factor controlling the occurrence of plants and animals in the intertidal zone is the alternate exposure and submergence of the habitats that subjects organisms living there to drastic and sudden changes in temperature and salinity. Fish follow a similar pattern of decreasing species richness with increasingly extreme environmental conditions (Sheppard, et al., 1992). Water temperature has a negative correlation with the abundance of many benthic organisms (KFUPM/RI, 2003a). Dissolved Oxygen (DO) has a negative correlation with water temperature reflecting the declining trend in DO from a colder to warmer season (KFUPM/RI, 2003a). Plate 2.11 Coral heads occur either as discrete outcrops or as part of reef systems and support an array of marine species. 32 Marine Atlas of the Western Arabian Gulf

12 Over the last 50 years, global warming has caused a sea surface temperature (SST) rise of about 0.2 C per decade, though this has accelerated to about 0.45 C per decade in the last 20 years (Sheppard and Loughland, 2002). This increasing temperature adversely affects marine organisms. The general warming of the SST could possibly allow groups of coral species that were not previously dominant to gain dominance and there may be a more permanent change in the assemblages of corals (Sheppard and Loughland, 2002). Coral mortality has been reported along the southern Arabian Gulf following the SST rise in 1998 especially in shallow waters (<3 m). Tremendous stress on the coastal fringe of the Western Arabian Gulf has been caused by coastal development and urbanization. Anthropogenic stresses can often result in detrimental effects to aquatic life or irreversible degradation of the coastal ecosystems. High temperatures of around 40 C were recorded in the outfall areas of power stations and desalination plants (KFUPM/RI, 2008). This local, elevated temperature and increased salinity minimizes the overall concentration of dissolved oxygen in the seawater that restricts the diversity of marine organisms residing therein. Man-made marine structures like the Abu Ali Causeway and the Saudi Bahrain Causeway restrict flow resulting in limited water exchange that lead to cumulative increases in water salinities within the back bays (KFUPM/RI, 2008). Plate 2.12 Osprey nesting on the rocky shore at Ras Abu Qamees. Chapter 2: OVERVIEW OF THE GULF MARINE HABITATS 33

13 Most of these islands are low and flat with an elevation of around 2-3 m above spring high tide levels. The larger islands are covered by salt tolerant vegetation belonging to the genera Suaeda and Salsola that are also common on salt flats along the mainland coast. The smallest island Harqus, is very low and lacks any terrestrial vegetation. These coral islands occupy a unique and irreplaceable status in the Gulf s ecosystem, mainly because they provide a safe breeding site for important animals, especially two species of marine turtles and three species of terns (Basson, et al., 1977). Coral reefs of Jana, Kurayn and especially Karan Island are rich in diversity of species when compared to other reef systems in the Arabian Gulf, and support a wide variety of invertebrate species, including coral reef fish (KFUPM/RI 1994, 2003b). The coral structure and sand islands provide a varied habitat for many species of invertebrates, algae, fish, birds, and turtles. Coral reefs growing on the leeward side of these islands typically are steep and vertically compressed; whereas reefs located on the windward side are gradually sloping (Basson, et al., 1977). Light availability plays an important role in coral reef formation around the offshore islands and contributes to a depth limit of around m with continuous coral cover (KFUPM/RI, 1994). Maximum hard coral coverage (Figure 2.4), coral fish diversity and associated fauna have been reported from Karan and Jana Islands (Sheppard, et al., 1992; KFUPM/RI, 2003b). Plate 2.13 Table Acropora Coral growing together with Platygyra Brain Coral. According to Basson et al., (1977), the coral reef structure underlying each island is composed of three major regions: reef platform, reef slope and reef base. The reef platforms of the islands are dominated by corals such as Stylophora sp., Cyphastrea sp., Poites sp., Platygryra sp., and seasonal occurrence of few coralline algae (Basson, et al., 1977). The reef slope is the most diverse region of the coral islands and is usually dominated by corals such as Goniopora sp., Porites sp., Acropora sp., Montipora sp., Hydnophora sp., Turbinaria sp. and Acanthastrea sp. The reef base is usually located at a depth of 15 m and generally limited light intensity does not permit the more dominant species, and luxuriant coral growth observed closer to the surface. Specialized coral types such as Turbinaria sp. are generally dominant in the base region. Usually during the winter season, the reef platforms are covered with coralline alga such as: Pocockiella sp., Porolithon sp., and Dictyota sp.. The offshore islands are important nesting grounds for both hawksbill turtles Eretmochelys imbricata and green turtles Chelonia mydas (Miller, 1989b). Birds of the offshore islands have been studied by several researchers (Symens and Evans, 1993; Symens and Suhaibani, 1994; Newton and Symens, 1994). Most of these islands are considered as breeding sites for several birds (BLI, 2009). The offshore islands are considered to be the most important breeding sites for the Tern Sterna bengalensis in the world. Other breeding species include crested lark Galerida cristata, bimaculated lark Melanocorypha bimaculata (irregular) and greater short-toed lark Calandrella brachydactyla (irregular). The islands are used as a stopover site by considerable numbers of passage migrants (mainly passerine bird species), especially in spring. Table 2.2 Characteristics of the main Offshore Islands of Saudi Arabia (Arabian Gulf). 34 Marine Atlas of the Western Arabian Gulf

14 Figure 2.3 Mean percentage cover of hard corals at different depths on Karan, Jana and Jurayd Islands (bars represent Standard Errors) Reported During (Modified from KFUPM/RI, 2003b). Coral reefs of these offshore islands survived the largest oil spill on record remarkably unscathed with no visible signs of immediate or long-term effects up to 20 years after the Gulf War (Vogt, 1995; KFUPM/RI, 2003b). As part of the Gulf War impact study, growth characteristics of Porites from two of the islands (Karan and Jurayd) were assessed and were reported as normal as predicted from the average water temperature of the region (Poulsen, et al., 2006). Plate 2.14 Seagrass meadows grow in shallow waters such as Halophila stipulacea. Chapter 2: OVERVIEW OF THE GULF MARINE HABITATS 35

15 Plate 2.15 Rocky headland at Ras Abu Qamees. 36 Marine Atlas of the Western Arabian Gulf Chapter 2: OVERVIEW OF THE GULF MARINE HABITATS 37

16 Map 2.2 Overview Map of the Habitats of the Northern Region.

17 Map 2.1 Habitats of the Northern Region of the Western Gulf. Map 2.3 Habitat Map of the Tanajib - Manifa Embayment.

18 Map 2.4 Habitat Map of the Abu Ali Embayment.

19 Map 2.5 Habitat Map of Tarut Bay.