Cyanobacteria
 
 

Cyanobacteria are a very abundant feature in the intertidal zone of the Study Area and along the entire Gulf coast. Despite the fact that they are not biologically related to algae, they often are called “blue green algae” because of their algal-like appearance. As they are the only known bacteria (biota), that are able to perform nitrogen fixation AND oxygenic photosynthesis, cyanobacteria are quite unique organisms. If the environmental conditions are favourable cyanobacteria form leatherlike mats or crusts. The morphology of these mats depends on species composition and environmental factors like time of inundation. The cyanobacteria mats can be divided up in:

  • pinnacle mats
  • polygonal or flat laminated mats
  • folded or blister mats
  • thin flat mats
 
pinnacle
polygonal
Pinnacle mats at the lower edge of the oiled intertidal zone
Polygonal mats on oiled shores, seaward of salt-marshes
 

Cyanobacteria mats cover large areas of shores within the Sanctuary. Various microscopic animals inhabit the cyanobacteria mats. The most prominent are gastropods, ostracods, nematodes, flatworms, codepods, and oligochaete worms. Macrofauna is usually absent where cyanobacterial mats are well established.

 
 
Scaled cyanobacterial mat in the upper intertidal zone
 
 
scaledmat
 
 

The function of cyanobacteria in the ecosystem and their role in bioremediation / biodegradation of hydrocarbons is not yet exactly known. The extensive growth of cyanobacteria is favoured by the absence of burrowing animals. So the bacteria are present nearly everywhere below the HWS, where bioturbation by crabs is limited.

 
 

Schematic zonation of microbal mats:

a, before the oil spill; b, after the oil spill on unprotected shores; c, after the oil spill on protected shores.

HWS = high water springs, HWN = high water neaps.

 
 
zonation
 
 

As at many sites the bioturbating fauna, such as crabs or polychaetes, vanished due to the oil spill cyanobacteria mats spread into areas they weren’t established before. When cyanobacteria grow undisturbed they build an additional layer each year. In winter fresh sediment accumulates on top of the cyanobacteria mat. On this thin sediment layer (1-3 mm) cyanobacteria grow in summer, forming clear annual layers. Those layers prevent oxygen influx into the oiled sediment. Therefore hydrocarbon degradation is stopped beneath the mats.

 
  cyanobacteria

Dead salt-marsh vegetation with

cyanobacterial mats in Dauhat ad-Dafi

 
 

The oil load estimated for the substrate covered by ca. 14 km² of cyanobacteria mats is about 50,000 tonnes in 2001 for the Dawhat al-Musallamiya and Dawhat ad-Dafi embayment systems.

 
 

Three scenarios for the further development of the newly dominantly by cyanobacteria settled areas have been observed:

  • Desiccation, cracking and peeling of the cyanobacteria mats by sun wind and wave action.
  • Resettlement of burrowing macrofauna, such as the crab Cleistostoma dottiliforme, which prevent cyanobacteria from forming solid surface crusts and benthic animals such as the Pirinella conica or Cerithium sp., which outcompete the cyanobacteria again (i.e. grazing on them).
  • Further extensive growth of cyanobacteria building thick laminated mats, sealing the oiled sediments and thus preventing any oil degradation as well as any resettlement by macrofauna. ( pr This scenario led to completely changed ecosystems)
 
 

In March 2001 a ploughing experiment was conducted at a cyanobacteria habitat. The 1-3-cm thick laminated cyanobacteria mat was removed by try tilling without penetrating the sediment deeper than 5 cm. In April 2002 the plough marks were hardly visible anymore. The sediment was again covered by a 1 mm thick cyanobacteria layer. In summer 2002 there was already a second layer increasing the thickness of the mat. Crabs didn’t enter the ploughed area because of the oil in the substrate. As the exposure of the soil surface to oxygen did not last long enough hydrocarbon degradation wasn’t significantly accelerated.

 
(Barth 2002, Hoffmann 1996, Böer 1994, Hoffmann 1994, Jones et al. 1994) (photos: F. Krupp)
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