long-range effects may be of the same magnitude as at the other stations, 

 as the residence time of the oil in the littoral system is dependent on 

 the energy in the system (Owens 1978) . 



The relatively fast recolonization of Station A may be explained by 

 the fact that tins station was exposed to oil during a comparatively 

 short period of time. Due to a change in wind direction on October 28 

 the drift of oil towards shore stopped. Thus, this station was exposed 

 to the drifting oil spill for less than 24 hrs. In December 1977 a 

 significant increase of most faunal species had already occurred at 

 station A. 



Only one genus, Iaera, did not increase during that period. This 

 may be explained by the low mobility of these small animals. At Station 

 B, in the center of ttie polluted area, recolonization of this species 

 had not started in May 1978. 



Crustaceans are without doubt sensitive to oil pollution (Notini 

 and Hagstrom 1974; Linden, 1976; Notini, 1978) and many of them must 

 have died during the acute phase of the Tsesis oil spill. The results 

 from this study indicate that the recolonization of these species to a 

 great extent is due to a horizontal migration from the unaffected areas. 



The early recolonization of the molluscs was, however, largely 

 dependent on the vertical recolonization by surviving individuals. 

 These were narcotized during the acute phase of the spill. By wave 

 action they were swept down from the Fucus plants to the bottom. Later, 

 the surviving individuals recovered and reentered the Fucus . As indi- 

 cated in Table 7.2.3.1, the mortality among small Mytilus edulis seemed 

 to be higher than that among larger individuals. Similar observations 

 have been made in previous studies (Notini, 1978). 



Bivalves have been found useful for monitoring petroleum input 

 since they reflect the concentration and relative amounts of different 

 hydrocarbons in the water (Lee, 1977). They are able to bioaccumulate , 

 but cannot metabolize hydrocarbons in their tissues. Thus, the Mytilus 

 samples from the different stations in this study are of great interest 

 since they make it possible to estimate dose and response data in situ 

 as well as the background oil contamination before the spill. These 



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