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Experiments with chemically and ultrasonically dispersed Arabian light crude oil and a dispersant (Finasol OSR 5) were carried out on an intertidal sand flat in the Wadden Sea (German Bight). “Bremerhaven Caissons,” flow through mesocosms for intertidal field experiments, allowed pollutant addition to the enclosed water during submersion time. Reiterated contaminations over a period of 12 successive tides of low concentrations of oil (10 ppm) and dispersant made it possible to study penetration and alteration processes of the dispersed oil in the sediment. Sublethal and lethal effects upon micro-phytobenthos, meiofauna, and macrofauna were observed. Oil reduced the activity of microbenthic algae and the food uptake of filter feeding bivalves and a polychaete. Nematodes showed a lower diversity and decreasing abundances in some groups. No major differences between the effects of chemically and ultrasonically dispersed oil on the benthos were observed. Application of dispersant alone had no clear effects when compared with controls.