Rocky Shores
 
 

In the study area rocky shores usually occur at sites of moderate to high wave energy. Only 14 kilometres of total coastline consist of pure rocky shores.
All rocky shores of the study area are developed on beach rock, which consists of consolidated marine beach sediments cemented by calcium carbonate. Also most of the supratidal rocks are built of marine beach rock sheets, forming rocky outcrops, cliffs and terraces. Most of the cliffs are below two metres high, but some reach up to nearly 20 metres.

The mainly rocky intertidal is dissected by beachrock platforms, tidal channels, crevices, sandy patches and rock pools, partly filled with tidal water. A few centimetre-thin veneer of sand or mud often lies on the rock sheets. Particularly at sheltered sites, such as crevices in between the rocks, sand accumulated. But in some cases sand layers can be found on top of rock platforms as well. Moving down to the mid eulittoral, the shore sometimes gets sandier. Rock fragments and outcrops of the continuous beachrock layer break through the sand cover.

Suaeda sp., Seidlitzia rosmarinus, Bienertia cycloptera and the parasitic Cistance tubulosa are the typical plant species at the littoral fringe.
In the mid eulittoral flat cyanobacterial mats formed on little rocks or on sandy patches. The presence of cyanobacteria is mostly the reason for the dark colour of the intertidal rocks.
Rocky shores in the study area are not as productive as other types of ecosystem. The enormous heat and desiccation during low tide in summer mean extreme stress for every kind of biota. Therefore, only various sheltered places such as crevices, rock pools, holes, and the underside of boulders are inhabited highly specialised animals.
The crab Scopimera crabricauda lays out its burrows in sandy patches. Another crab, Metopograpsus messor, is present under rock boulders.
Nodolittorina subnodosa populate rock surfaces and pools. The serpulid worm Potamoleios krausii settles at sheltered positions of beach rock boulders.
Gastropods such as Pirinella conica or Cerithium scabridum graze on cyanobacterial mats.

Most of the rocky shores in the study area were heavily affected by the 1991 Gulf War Oil Spill. At some locations life was completely destroyed. Especially at very exposed sites, large quantities of oil accumulated in the upper eulittoral zone and at the littoral fringe. Where the oil got on sandy parts or patches, it polluted the substrate down to 25 centimetres. But on the rocks it couldn’t penetrate into the ground at all and was fully exposed to wind and sun. That increased evaporation and turned the oil into a more or less thick layer of highly viscous tar. The more oil accumulated on the rocks, the longer it took for the sun and wave action to break the crust. Those physical impacts created erosive tidal channels and cracked the tar crust into a mostly polygonal pattern. By this way, the tar got removed piece by piece during high tides.

 
   
   
   
 

rockyshore

Rocky shore at

Ras al- Abkhara

in October 1991

 
  Very much of the oil was removed during the first three years after the spill. At some locations remaining oil residues were covered by new sand until 1994. Only at severely oiled sites the tar cover was still there in 1994. At those places life was very restricted. Where the fauna of the upper intertidal was completely destroyed, recruitment went on very slowly and no recovery was reached until 1993/1994. But at sites that weren’t affected that heavily, species diversity and abundance increased much faster and even reached normal values in the zone below the littoral fringe until 1995.  
 

rocky

Rocky shore at

Ras al- Abkhara

in June 1994

 
 

In general, the littoral fringe and upper eulittoral recovered quite fast. Oil residues of the 1991 Gulf War Oil Spill were present only in protected situations, but some layers of younger residues were visible. Even most severely oiled rocks were free of oil in 1999. At some sites the typical fauna of the littoral fringe area returned relatively slow. This was due to the different reproduction patterns of key species in one nutrient chain and caused significant annual fluctuations in the number of individuals.

 

(Barth 2002) (photos: F. Krupp)

 
Google
WWW www.jubail-wildlife-sanctuary.info