In situ experiments on recolonization of intertidal mudflat fauna to sediment contaminated with different concentrations of oil

Field experiments to test the recolonization response of intertidal mudflat fauna to oil (North Sea crude) contaminated sediment, were performed in experimental boxes on a mudflat in Norway. The oil was added in five different concentrations (100, 400, 1000, 4000, 10000 ppm wet weight oil in sediment). The oil content persisted at about the same level in each treatment throughout the experimental period of 4.5 months. Concentrations above 1000 ppm oil resulted in a reduction in redox-potentials in the sediments. This indirect effect of oil was probably responsible for the observed decline in abundance of the deeper burrowing species. All the most common species were present in all treatments, but most species showed decreasing densities with increasing oil concentrations. The tolerance to the oiled conditions was different among the different species. Among the meiofauna, and especially for ostracods, significant effects were found at 400 ppm and higher, while effects on macrofauna were most pronounced above 1000 ppm. However, not only tolerance to oil, but differences in exposure to the oiled sediment due to differences in behaviour or distributional patterns seems to be of importance for the species response to the oiled sediment. Although most of the species were found to recolonize defaunated mudflat areas within a short time, our results indicate that an oil spill will cause prolonged effects to this mudflat community structure due to high persistence of oil in sediment and that a few of the most abundant species have low tolerance to oil or very low dispersal.