Physical setting of the Study Area

The Arabian Peninsula can be divided in two structural provinces: the Arabian Shield forms the western part and the Arabian Shelf the eastern part of the peninsula. The study area lies on the Shelf at the Saudi Arabian coast of the Arabian Gulf.

On top of the Precambrian plate, which shares the Arabian Shelf with the Shield, a series of continental and shallow water marine sediments accumulated from Cambrian to Pliocene times. As these layers dip away from the Shield, the thickness of the sediment increases gradually from the west to the east, reaching 11,000 m in the coastal lowlands.

The Persian Gulf Basin is the largest basin with active salt tectonism in the world. The more than 900 km long Arabian Gulf is the present-day geosynclinal expression of the 2600 km long Persian Gulf Basin. The Arabian Peninsula’s coastal region of the Arabian Gulf rises gradually inland at a rate of about one metre per kilometre. The coastline is irregular, low, and sandy. Tidal changes cause the waterfront to shift back and forth up to several kilometres.

In the coastal region north of Jubail sabkhat (salt flats) and low rolling plains, covered with a thin mantle of sand and semi-desert vegetation, are very common landscape types. A typical feature of the western and southern Gulf coast is the coastal sabkha. They reach up to 10 km inland and cover large areas. Sabkha is the Arabic term for a flat salt-crusted desert. Their sandy or finer substrate is unconsolidated and the surface level is a product of a homeostasis of deflation and aeolian sedimentation, controlled by the ground water level which forms the basis of deflation.

As a typical example for this region, the study area is widely covered by sand sheets and dunes, mostly covered by scattered perennial grasses and herbs. The vegetation cover ranges from 1 to 10%. Where the vegetation cover is lower than 2%, as a result of overgrazing, dunes are reactivated.

The climate of the Gulf coastal region is characterized by very high maximum temperatures in summer (50°C) and relatively low minimum temperatures in winter (3°C) with a mean annual temperature of 25.2°C to 26.5°C, depending on the position of the meteorological station.

Mean temperatures at the different meteorological stations within the study area. Gulf water temperatures after Hastenrath & Lamb (1979).
temperature
 

Furthermore, total precipitation is distributed only over some rainfall events between October and April. Since the annual rainfall in the area of the Sanctuary ranges from 5 mm to 277mm at Dhahran, there is aridity throughout the year anyway.

Precipitation events per rain season at the three stations.

 

Abu Kharuf

Abu Ali

Mardumah Bay

1993/1994

19

14

13

1994/1995

44

30

33

2000/2001

25

no data

30

Plants also use dew, which is deposited almost every night, especially in the winter month, on their leaves.

The potential evaporation in the Jubail coastal area is estimated to be between 2200 and 3500 mm per year. Due to the low precipitation rates, their high variability in quantity and locality and the high evaporation over the year, there’s a permanent water deficiency in the eastern part of the Arabian Peninsula. Therefore, groundwater resources, which are mainly located in underground aquifers, are of highest importance to human activities.

Because of mostly dry unconsolidated surface sediments and the sparse vegetation cover, gusts play an important role in modelling the topography. This directly affects the stability of plant roots. Furthermore strong winds, which usually occur at midday and early afternoon, increase desiccation of the surface.

The harsh environmental factors, such as temperature, limited water supply and salinity of water and soils are responsible for both low vegetation ground cover rates and a low diversity of plant life. Besides these factors the soil type (grain size distribution, mineral contents) determines the presence of certain kinds of vegetation.

In general, the flora mainly comprises thorny shrubs, therophytes, xerophytes, phraetophytes, halophytes, and some hydrophytes in the intertidal zone. For most plants summer is too hot and dry for growth. Therefore, the growth cycle usually begins in autumn or winter when conditions are more favourable.

The Arabian Gulf

The Arabian Gulf is a shallow branch of the Indian Ocean with an average depth of only 35 meters. This basin is about 1000 km long and between 200 and 300 km wide. The deepest areas are off the Iranian coast, reaching a depth of 100 metres at the entrance to the Strait of Hormuz. As the Gulf is a nearly enclosed epicontinental sea, the water circulation through the narrow Strait of Hormuz is restricted

Fresh water enters the Arabian Gulf through the Strait of Hormuz with a salinity of 36.5 – 37 ppt and circulates in a general counter-clockwise direction first northward along the Iranian coast and then southward along the Saudi Arabian shores. As evaporation is higher than the fresh water input, the salinity of the current increases on its way through the Gulf, while nutrients decrease. In open waters of the study area’s embayments the water salinity ranges between 40 and 51 ppt and in the coastal flats between 56 and 74 ppt. The salinity is a very important controlling factor for the abundance of marine life. The number of species and individuals of corals, plankton and many other organisms decrease significantly with increasing salinity.

The tidal pattern of the Arabian Gulf is complex and does not correlate with the tides of the Indian Ocean. Most of the Gulf shows a basically semi-diurnal tidal regime with a tidal cycle longer than 12 hours. In the study area the mean spring tide range is approximately 1.5 m. The mean neap tide range is about 0.8 m.

The Arabian Gulf shows high annual temperature amplitudes, which are another important controlling factor for marine life. Especially in nearshore areas the water temperature fluctuates between 16°C and 36°C over the year. This means stress to the marine communities and therefore the overall diversity of species is low.
Sea surface temperatures of the Arabian-Persian Gulf (source: FNMOC OTIS http://152.80.49.210/PUBLIC/).
st1 st2 st3

FNMOC OTIS 4.0:  SST Analysis

Optimum Thermal Interpolation System
st4 st5 st6
stl

There is no sea in the world more affected by hydrocarbon pollution than the Arabian Gulf. The estimated average oil pollution for non-war times in the last 25 years lies between estimated 100,000 to160,000 tonnes per year. During the Gulf Wars the oil spillage was certainly higher. But regarding the high biodegradation potential of the Arabian Gulf, these values are not really critical.

(Barth 2002, Barth 1998, Abuzinada & Krupp 1994, Jones et al. 1994, Mandaville 1990)
Area
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