Effects of Ekofisk crude oil on an enclosed planktonic ecosystem

Abstract

There has been increasing concern about effects of petroleum hydrocarbons and their derivatives on the highly productive North Sea waters. This study reports on possible effects of Ekofisk crude oil on planktonic communities enclosed in medium-scale controlled ecosystems. A 5-mm layer of oil was added to 1 of 2 parallel bags filled before the diatom bloom started in March, and the development followed for 4 wk. Population sizes of bacteria, algae, and zooplankton were estimated; chlorophyll a, mineral nutrients and hydrocarbons were analysed, and primary productivity was measured. In the oil-polluted bag, growth of diatoms and copepods was inhibited, while planktonic bacteria, choanoflagellates and tintinnid ciliates increased. Phosphate probably lim~tedth e growth of both algae and bacteria, and was rapidly reduced to very low levels in the bags. The measured concentration of petroleum hydrocarbons and alteration products avcraqed 470 pg I-'. This level may be considered acutely toxic to diatoms. The reasons for the bacterial developn~entso bserved, their role in hydrocarbon mineralization, and the predominance of flagellates and tintinnid ciliates are discussed. It is concluded that results from bag experiments may provide important clues to fate and effects of oil pollution at sea.