CHAPTER 4: IMPACT OF OIL ON THE PELAGIC ECOSYSTEM 

 (Sif Johansson) 



4. I Introduction 



Hardly any study has verified severe oil-induced effects on the 

 pelagic ecosystem following an actual spill (Sanborn 1977, Michael 1977, 

 Kuhnhold 1978) . Existing knowledge is derived mostly from laboratory 

 experiments using unrealistically high concentrations of oil. In such 

 experiments, phytoplankton clearly react in a very diverse manner. Even 

 within the same taxonomic group two species can react in a totally 

 opposite manner (Prouse et al., 1976; Winters et al., 1976; Dennington et 

 al., 1975; Mironov, 1968; and others). Depending on the composition and 

 physiological status of the phytoplankton community and the type of oil 

 used, primary productivity can be inhibited, stimulated or affected not 

 at all when exposed to realistic concentrations (H'siao et al., 1978; 

 Bender et al., 1977; Gordon and Prouse, 1973). 



Comparatively few investigations have dealt with effects on primary 

 production, biomass and species composition at the same time. In large 

 plastic bag (CEPEX) experiments, Lee et al. (1977) found that the rela- 

 tive dominance of a diatom Cerataulina bergonii , had decreased from 

 50-95% in control bags, to only 10% in oil contaminated bags in 8 days. 

 The decrease was followed by an increase of microf lagellates and, de- 

 spite a lowered total biomass, primary production increased. This was 

 explained to be a result of higher growth rate among microf lagellates . 



Conover (1971) following the wreck of the Arrow reported oil con- 

 taminated zooplankton in situ . Droplets of oil had been ingested or 

 adhered to the feeding appendages. Though as much as 10% of the total 

 oil spill was estimated to be associated with zooplankton, no apparent 

 effects could be detected. Parker and Watson (1969) observed ingestion 

 of oil by zooplankton, both experimentally and in situ and also found no 

 harmful effects. 



Lee et al. (1977) in enclosed experiments showed oil-induced minor 

 alterations, i.e., an increased abundance of ciliates and rotifers. A 



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