ECOLOGICAL EFFECTS OF OIL VERSUS OIL PLUS DISPERSANT ON THE LITTORAL ECOSYSTEM OF THE BALTIC SEA

Abstract

ABSTRACT The effects of a North Sea oil with or without the addition of dispersant were studied in a model of the littoral ecosystem of the Baltic Sea. Experiments were carried out in six pools with a volume of 8 m3 each, with flowthrough seawater and an ecosystem of the shallow rocky Baltic archipelago. All major fauna and flora were transferred into the pools in normal numbers and proportions. Two of the pools were exposed to oil alone. The amount of oil was equivalent to 20 ppm assuming total mixture. Two other pools were exposed to the same amount of oil and an oil dispersant (Corexit 9550, Exxon), and two pools served as controls. The effects studied were those on abundance of heterotrophic bacteria, periphyton photosynthesis, growth of bladder wrack, phytoplankton growth, zooplankton abundance and diversity, benthic fauna, physiological responses of certain crustaceans and molluscs, and the growth of blue mussels. In addition, the total photosynthesis and respiration of the ecosystem was studied. Concentrations of oil in water and in blue mussels were monitored. The experiments showed that almost all the measured parameters were affected. When comparing the effects between the pools, several of the results indicated a stronger response for oil alone compared to oil and dispersant. This was particularly obvious when monitoring the total production and respiration of the ecosystems. The explanation may be that the ecosystems in the pools exposed to oil and dispersant were exposed less time compared to those in the pools where oil alone was added. The oil and dispersant mixture obviously left the system much faster due to the water exchange compared to the oil without dispersant. In the latter case the oil adhered to surfaces and detritus and thus tended to stay longer in the environment. These results may provide valuable information for decision makers faced with an oil spill in shallow waters and who have an option to use oil spill dispersant.