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.

 
   
 
intertidal
 
saltmarsh

Devastated salt marsh

in February 1992

 
  Until 2001 there was some improvement in the lower intertidal, but the upper intertidal was still in a very bad condition. The littoral fringe and the upper eulittoral were most severely affected by the oil. Particularly in this zone there is the trend that the oiling of substrate remained constant through 10 years of observation. If there was a decline of hydrocarbons in the upper sediment, most of the times the oil residues just moved into deeper soil layers, but where conditions were favourable, oil degradation took place to some extent. “The main processes of hydrocarbon degradation occur through oxidative abiotic chemical reactions and enzyme controlled reactions by a variety of organisms.” (Atlas 1981)

Furthermore oxygen and nutrients such as nitrogen and phosphorus need to be available to provide the conditions for hydrocarbon degradation. When the concentration of such nutrients is too low, oil degradation is limited. Below a soil depth of 30 cm the substrate was usually free of oil. Only crab burrows allowed the oil to penetrate deeper (up to 50 cm) into the sediment.

 
   
 

burrows

Burrows of Cleistostoma

dotilliforme in the upper

intertidal zone

 

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
 
   
  Schematic profile of a salt marsh at the Gulf coast  
 
profile
 
 

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 Halocnemum are the most salt-tolerant and settle the area round the littoral fringe. This zone is only occasionally flooded in winter and shows the highest salinities. The Arthrocnemum dominates the intermediate zone between the littoral fringe and the eulittoral. Salicornica or the mangrove Avicennia marina sometimes occur further towards the sea. Not all dominant species are present at each site.
 
   
saltmarsh

Intertidal area in Dauhat ad-Dafi with

Arthrocnemum salt-marsh destroyed

by the Gulf War oil spill.

(August 1992)

 
 

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:

 
 

 

Crustacea

gastropods

barnacles / bivalves

littoral fringe / upper eulittoral

Cleistostoma dottiliforme, Metopograpsus messor, Scopimera crabricauda

Pirinella conica, Mitrella blanda

Euraphia sp.

mid eulittoral

Macrophthalamus depressus,
Ilyoplax frater

Pirinella conica, Mitrella blanda, Cerithium cingulata

Dosinia hepatica

lower eulittoral

Metaplex indiaca, Macrophthalamus depressus,
Ilyoplax frater

Pirinella conica Cerithium cingulata, Mitrella blanda

Dosinia hepatica, Solen vagina, Macrocallista umbonella

 

 

In the following two years after the oil spill all macrobiota were absent in the upper intertidal between HWS und HWN.
Millions of individuals of the burrowing crab Cleistostoma dottiliforme were killed. The few who survived disappeared until winter 1991/1992. Some of the Cleistostoma burrows were inhabited by Metopograpsus messor in the following years. (1993-2000). Besides Metopograpsus messor individuals of Macrophthalamus depressus were observed in the eulittoral zone. Scopimera crabricauda was missing on all polluted beaches.
 
  crab Macrophthalmus depressus  
 

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)

 
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