The 1991 Oil Spill was the largest oil spill to date. The area between HWS and HWN was heavily oiled. In the lower areas significant oiling occurred only locally.
At most rocky shores wave action mostly removed the oil and tar cover after only two years. In 1993 all key species were already present again.
At mangrove sites, regeneration started two years after the spill. The dense tidal channel networks led to a quick recolonisation by crabs burrowing into the soft sediments at the channel sides and then breaking the surface crust. Once the crusts had been opened, seeds could germinate.
On sandy beaches with high wave energy, oxygen and nutrients are driven into the sediment. This provides good conditions for hydrocarbon degradation. At those sites wave energy was often strong enough to rework and remove existing tar crusts within the first 2-4 years, so degradation could take off. Nevertheless at some low-energy sandy shore sites oil degradation was prevented by tar covers until 2001.
Due to their wide intertidal zones low wave energy and their sensitive vegetation, salt marshes were the most severely hit type of ecosystem. The surface has been sealed by oil residues and cyanobacterial mats. Here recovery was far from complete in 2001.
Recovery status of the different coastal ecosystem types in 2001:
ecosystem type |
damaged area in % 1991 |
out of the damaged area completely recovered in 2001 (in %) |
out of the damaged area no regeneration at all in 2001 |
salt marsh |
90 |
20 |
25 |
sand beach |
80 |
80 |
0 |
sabkha |
90 |
0 |
25 |
rocky shore |
100 |
100 |
0 |
mangrove |
50 |
80 |
0 |
The subtidal biotopes (seagrass beds, coral reef biotopes and silt sea beds) showed no evidence of being significantly affected by the oil spill.
Fish in nearshore and offshore areas were affected to some extent. Lower density of many species was observed in 1992 and 1993. This was as a result of the reduction of planktonic eggs and larvae, that couldn’t escape the oil slick.
The role of cyanobacteria in both ecosystem and hydrocarbon degradation is not totally clear yet. Concerning biodegradation of hydrocarbons the dominant presence of cyanobacteria in form of mats or crusts is ambivalent depending on the circumstances. Cyanobacteria are colonising quickly the oiled substrate, removing a thin layer of tar by desiccation, cracking and peeling off the hard crusts. If the crust is not too thick, this may help to remove some of the oil residues. But they are also covering large areas and sealing the surface, thereby preventing the transport of oxygen and nutrients into the ground, which nearly stops biodegradation.
(Barth 2002)