Information about the effects of oil and oil products upon planktonic organisms is much sparser than for nekton or benthos because of the problems of quantitative plankton analysis. The data available derive from three sources: laboratory experiments, studies with enclosed ecosystems and test organisms (e.g. Cepex, phytoplankton cages) and from field observations made in oil-affected areas. Laboratory experiments have tended to be conducted at unrealistically high hydrocarbon concentrations upon planktonic species that are amenable to laboratory conditions. However, such investigations have shown that the early oil dispersants were very toxic and revealed the great differences between the toxicities of crude oils from various oil fields. Sublethal studies have shown that hydrocarbons, especially the high aromatic fractions, can damage development and alter behaviour and physiology in planktonic organisms. Biochemical investigations have demonstrated both accumulation and depuration of hydrocarbons (including carcinogens) in plankton. Enclosed ecosystem experiments at low hydrocarbon concentrations (less than 40 ng g<latex>$^{-1}$</latex>) have demonstrated stimulation of microflagellates and small zooplankton (tintinnids and rotifers), whereas diatom populations were reduced and large zooplankton little affected. At higher concentrations (ca. 100 ng g<latex>$^{-1}$</latex>) phytoplankton production was little affected but copepod and predator populations collapsed. Field studies have revealed no lasting damage to planktonic ecosystems caused by oil. Typically, oil spills are followed by rises in bacterial and yeast numbers (though the latter may be inhibited by oils with high aromatic fractions), temporary falls in zooplankton densities and increases in phytoplankton production. Chronically polluted inshore areas have been little studied; neustonic, arctic and coral reef ecosystems also merit further investigation. Cautious optimism is expressed about the usefulness of enzyme ratio and adenylate charge measurements in future field studies upon plankton.

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