Salt Marshes

Salt marshes are the most abundant type of ecosystem along the Saudi Arabian Gulf coast. They usually grow on tidal mudflats in bays and other sheltered locations, where wave and current energy is low. Therefore, the sediments mainly consist of fine grained particles and a significant content of organic matter due to the high productivity of mudflats. In the study area salt marshes cover 47% or 190 km of the total coastline.

Salt marsh shores normally show extensive intertidal areas. The average is estimated around 200 to 300 metres, but in some cases the intertidal zone can reach more than 2 km width. Adjacent to salt marshes, sabkhat or sand sheets often occur on the landward side.
Being the most widespread upper shore ecosystems salt marshes were most severely affected by the 1991 Gulf War Oil Spill. The unusual high tides in March 1991 moved the oil from the mid and lower shore to the upper shore, where it still partly remains up to now.
The area between HWS (= High Water Spring) and HWN (= High Water Neap) has been covered by a continuous band of oil and tar, causing the nearly complete devastation of flora and fauna in that part of the shore. At some sites this dead zone was several hundred metres wide.

In 1992 a test site was carefully ploughed in order to improve oxygen supply for hydrocarbon degradation. In 1993 this test site showed significantly decreased hydrocarbon values compared to the non-ploughed control-site. But in 1994 the hydrocarbon concentrations of both sites were similar again, due to a stagnated degradation at the ploughed site during the last year of the experiment.

The salt marshes or tidal mudflats in the study area are mostly characterised by a series of well defined zones, each occupied by a different community of organisms. The typical vegetation zones are:

• Cyanobacteria zone
• Salicornia zone
• Arthrocnemum zone
• Halocnemum zone

And landward of the littoral fringe follow:

• Limonium zone
• Suaeda zone
• Seidlitzia zone
• Halopeplis zone

All plants growing on salt marsh shores need to be very salt tolerant, as they have to withstand the seawater. Most of the dominant species are halophytes such as Halocnemum strobiliaceum or Arthrocnemum macrostachyum. Additionally some flowering herbs and grasses are common.

The mudflats between the lower lying halophytes are often covered by extensive cyanobacteria mats. High salinity in combination with high temperatures is the most important stress factor for the vegetation in salt marsh areas. Other stress factors are the regions high diurnal (20°C) and annual (min. 3°C - max 51°C) differences in temperature.
Most of the plants in directly oil-affected areas died within a short period of time. The long term effects of the oil impact, such as toxic substances poisoned the soil. Significantly decreased oxygen availability in the soil and an increase of surface temperature led to a gradual die-off of many surviving plants.

It was generally observed, that bioturbating organisms, particularly crabs, colonise oil affected zones some years before macrophytes. Cyanobacterial mats prevent the germination of Arthrocnemum and Halocnemum.

The key species according to Jones et al. (1994) and Apel (1994) are:

The first re-settlement of burrowing animals was observed in 1992. Some polychaetes and juvenile crabs of Cleistostoma dottiliforme settled at a minor polluted salt-marsh site. Apart from that, active grab burrows were generally found near and seaward of the HWN, landward of the HWS and along tidal channels. Because of the clean sediments in them, tidal channels are the initial paths of recolonisation by crabs. Where new crab burrows occurred in the area between HWN and HWS in the following years, regeneration actively took place.

Exclusively crabs, mainly Cleistostoma dottiliforme, are able to remove both tar crusts and cyanobacteria mats. Doing that, they help prepare the ground for the re-settlement of macrophytes. If there is too much oil in the sediment or if the crusts or mats are too hard for burrowing activities, colonisation by crabs is hindered. The numbers of polychaeta remained low even until 2001.

(Barth 2002, Krupp & Kushaim 1996) (photos: F. Krupp)