unchanged in the deeper, oxygen-free layers for at least five to ten 

 years (Krebs and Burns, 1977). This persistent oil continues to present 

 a hazard to the biological community, preventing its return to pre-spill 

 status, and constituting a potential source of slow, continuous oil 

 leakage to surrounding areas (Vandermeulen and Gordon, 1976). 



6.1.2 The spill area 



The Tsesis spill occurred in an area where benthic data have been 

 collected since 1972 by Ulf Larsson, Asko Laboratory, in connection with 

 a study of the environmental impact of the large modern sewage plant 

 "Himmerf jardsverket" . Pre-spill macrofauna data were already available, 

 while meiofauna samples are stored and will be sorted later if funding 

 becomes available. Furthermore, the benthos of the Asko area has been 

 treated in detail in a number of earlier papers (Cederwall, 1977, 1978; 

 Ankar, 1977; Ankar and Elmgren, 1976; Elmgren, 1976) so that the back- 

 ground knowledge for the study of the oil spill was exceptionally good. 



Stations 20 and 21 (see map, Fig. 6.1) were generally sampled for 

 macrofauna once a year in October-November (i.e. at about the time of 

 the spill) starting in 1972. The depth of the stations were: station 

 15: 44-45 m, station 20: 32-33 m, station 21: 28-29 m, and the bottom 

 substrate, mud at all the stations. Stations 20 and 21 are located in 

 an area, "Svardsf jarden" , which is cut off from exchange of deep water 

 with the open Baltic by a sill of about 20 m depth between Asko and 

 Toro, whereas Station 15 is outside this sill. 



6.2 Methods 



6.2.1 Sediment sampling 



Cores for oil analysis of surface sediments were collected using 

 either a modified Kajak core sampler (Kajak et al., 1965) of 80 mm 

 internal diameter (10 Nov. 1977, station 1 and 2; 17-31 August 1978, 

 station 14, 15, 20, 20C, 20D) or the Asko corer, also used for meiofauna 

 sampling (inner diameter 22 mm, used for all other samples). The top 

 2 cm of the sediment were extruded into hexane-washed glass bottles and 

 kept deep-frozen until analyzed. On 4 December 1978, further cores 



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