Marine Emironmental Research 18(1986) 93-109 The Effects of Crude Oil and Corexit 9527 on Marine Phytoplankton in an Experimental Enclosure P. J. Harrison, W. P. Cochlan, J. C. Acreman, T. R. Parsons, P. A. Thompsom H. M. Dovey Department of Oceanography, University of British Columbia, Vancouver, BC V6T 2BI, Canada & Chen Xiaolin Third Institute of Oceanography, National Bureau of Oceanography, Xiamen, The People's Republic of China (Received: 20 June, 1985) ABSTRACT The effects of a dispersant, Corexit 9527, plus Prudhoe Bay crude oil and the effects of the dispersant only on natural assemblages of marine phytoplankton in three large experimental ecoo'stem enclosures (CEEs) were studied. The oil and dispersant were added to a laver between 2 and 4m depth yielding initial concentrations of 4.5 and 2.Omg litre -t, respectively. The enclosures remained undisturbed Jor the 17-day experiment except Jor sampling at 2- or 3-day intervals. Nutrient concentrations, nitrogen transport rates, chlorophyll a, primary productivity, phytoplankton sinking rates, species composition and cell numbers were followed over the course o1 the experiment. h~ the enclosure with oil and dispersant, diatom growth was suppressed and the phytoplankton were dominated by microflagellates such as haptophytes, cho'sophytes and a prasinophyte. The diatoms appeared health v under the microscope and the total number of species was similar 93 Marine Era'iron. Res. 0141-1136/86/$03-50 ~ Elsevier Applied Science Publishers Ltd, England, 1986. Printed in Great Britain