EFFECTS OF CRUDE OIL EXPOSURE ON PLANKTON ASSEMBLAGES IN MARINE MESOCOSMS

Abstract

To assess the effects of crude oil spills on marine planktonic communities and environmental factors, marine mesocosms comprising 9 enclosures contained 1,000 L of seawater were manipulated over a period of 9 days at a monitoring station in the South Sea Institute of KORDI. The enclosure experiment was conducted in two different treatments: 1) A crude oil (Iranian heavy crude oil) was added at a concentration of 1,000 ppm. 2) The crude oil and a dispersant (Corexit EC9500; concentration of 100 ppm) were exposed at the ratio of 10:1 (v/v). And, a control enclosure was also prepared with no additions. Environmental factors (water temperature, salinity, pH, dissolved oxygen, nutrients, and dissolved organic carbon) and biological factors (hetero- and autotrophic bacteria, and autotrophic nano flagellates) showed no significant differences at three depths (0.5, 2.5, and 4.5 m) in each enclosed water column (p > 0.05). Most planktonic communities in oil and dispersant group responded drastically more than other groups: heterotrophic bacterial abundances increased from 0.95 ± 0.04 to 1.72 ± 0.39 x 108 cells ml-1 for the first 2 days and then decreased rapidly for the remaining days. Abundances of heterotrophic nano flagellate abundances increased quickly to the increase of bacterial cells. Phytoplankton in the treatment of oil and dispersant represented a clear decrease of its abundance and a change of its composition from predominant diatoms to flagellates. Phytoplankton abundances in the addition of oil and the control, however, were recovered gradually after rapid decrease for 1 day. Abundances of zooplankton community to oil and dispersant exposure were largely reduced, while they were less adversely affected in oil and the control groups. The present results demonstrate that planktonic ecosystem responded sensitively to massive inputs of the crude oil and treatment of dispersant. Moreover, the addition of oil and dispersant might more stimulate rapid growth of bact