Stronger impact of dispersant plus crude oil on natural plankton assemblages in short-term marine mesocosms

► Rapid increase in bacterial abundance as treatment of dispersant. ► Grazing effect of heterotrophic flagellates on bacteria as a food source. ► Severely adverse impact of phyto- and zooplankton as treatment of dispersant. ► In oil spill responders must conduct extensive preplanning for environmental benefit.