Oil pollution and plankton dynamics. II. Abundance pattern of ciliates inside and outside enclosures and the responses of ciliates to oil during the 1980 spring bloom in lindåspollene, Norway

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Abstract
Two 13 m deep plastic enclosures (surface 0.78 m2) were used to study the responses of planktonic ciliates to oil pollution during a northern phytoplankton spring bloom. One bag was added 0.5 litre Ekofisk crude oil to the surface, the other served as a control (CB). The abundance pattern of both the heterotrophic ciliates and Mesodinium rubrum (LOHMANN) in CB differed in several respects from that observed outside the bags. The net population growth rates for heterotrophic ciliates and M. rubrum in CB were 0.32 and 0.63 divisions per day, respectively, at temperatures about 2–4° C. Hydrocarbon concentrations between 0.62 and 1.27 mg per litre caused declining densities of heterotrophic ciliates. M. rubrum appeared to be more sensitive than the heterotrophic ciliates. After the disappearance from 0.5 m, all ciliates remained absent from this depth during the nearly two months duration of the experiment.