4.4 Discussion 



The pelagic system is first to suffer from an oil spill. Depending 

 on local geography, distance from shores, windstress, etc., the exposure 

 of the pelagic system to oil will change over a period of time. Soon 

 after release from the tanker the oil starts to change both chemically 

 and physically, e.g., volatile fractions evaporate and soluble fractions 

 enter the water phase. The rate of this process decreases rapidly with 

 time. Combined with a continuous dilution this weathering will limit 

 the period of detectable ecological effects. 



The results from this study show only minor effects on the pelagic 

 system. Changes in species composition could not be detected either in 

 the phytoplankton or the zooplankton community. The phytoplankton 

 biomass almost certainly increased in the affected area. This may have 

 been due to decreased zooplankton grazing or increased growth rate. The 

 existing data on productivity per unit biomass indicated a normal ratio, 

 and the fact that zooplankton was found to be heavily contaminated with 

 oil (50% of specimens with visible oil droplets in the first week, 20°/ o 

 after three weeks), mainly on the feeding appendages, makes decreased 

 zooplankton grazing the more probable explanation. Similar results were 

 found after the blow-out on the Ekofisk Bravo platform (Lannergren, 

 1978). 



No significant differences in zooplankton composition or biomass 

 could be detected, except at station II immediately after the spill, 

 when two measurements from separate days (Oct. 28 and 29) showed a 

 drastically lowered biomass. Gyllenberg and Lundqvist (1976) have shown 

 that zooplankton, when exposed to oil, either try to escape or enter a 

 state of "narcosis". Either of these mechanisms could explain the very 

 low biomasses near the wreck. 



Bacterial abundance increased in the contaminated area. It is 

 however, impossible to judge whether this was a consequence of increased 

 growth rate or decreased grazing. An effort was made to estimate bac- 

 terial growth rate from the frequencey of dividing cells (Table 4.1), 

 but no clear differences were found between polluted and unpolluted 

 areas . 



Ih 



