Collembola were the dominant groups. Station G showed a low number of 

 Porcellio scaber, only 2.0% while the gammarid Orchestia gammarellus 

 appeared at this station exclusively. 



8.2.5 Discussion 



The result of the pit-fall trap study shows that the total number 

 of systematic groups was somewhat higher at the reference station and 

 that the average number of individuals caught was 1.5 to 3 times higher 

 than at the other stations. The quantitative samples however, show that 

 the total number of systematic groups was similar for all stations 

 except station B, which had a slightly lower figure. The average number 

 of individuals per 100 g wrack was clearly higher at station G compared 

 to stations B and D, hut a little lower than at station C. This could 

 be due to the wrack at G being too wet for some animals, which would 

 explain the low number of Porcellio at this station (Backlund, 1945). 

 The fact that the gammarid Orchestia was frequent only at G and absent 

 at the contaminated stations might be a result of the oil spill, but it 

 could also be a consequence of the humidity of the wrack at G (Backlund, 

 1945). This does not explain the almost complete absence of this species 

 at C, where the wrack was as wet as at G. The differences in humidity 

 of the wrack are clearly shown by the abundance of the group Plesiopora 



(Oligochaeta) which demands humidity for thriving (.Backlund 1945). This 



2 2 



group is much more frequent at G and C (about 27,000/m and 13,000/m 



2 2 



resp., estimated values) than at B and D (about i , 100/m and 1,300/m 



resp.). The largest part of this group was Pachydrilus sp . and the rest 



Enchytraeus sp. Pachydrilus is a typical animal of the seashore and 



abundant in wrack beds, being the main food source for many animals 



living there, e.g., spiders of the genus Erigone sp. and larvae of the 



coleopteran Cercyon sp . (Backlund, 1945). 



Most of the flies (Brachycera) in the quantitative samples were 



larvae because the adults are able to fly and thus escaped capture. The 



great number of flies in the traps could be due to their being attracted 



by the smell of the detergent, as most of them are not true inhabitants 



of the seashore (Chinery, 1976). Species which truly belong to this 



185 



