As a result of the impending Tricolor oil pollution in the Southern Bight of the North Sea the Zwin nature reserve was blocked from the North Sea by use of a sandbarrier. This method of protection has an important consequence for tidal flat ecosystems: the absence of the tide. The effects on the ecological very important bottom-life of the tidal flats could not be estimated beforehand. All species that were present before the damming up were still present afterwards. During the emersion period Talitrus saltator and Orchestia gammarellus were found in high densities while these species were absent before and after the emersion. Strikingly was the strong decline in abundance of all species which were present in high densities in a sampling station and the abundance of Aphelochaeta marioni and Pygospio elegans declining in all sampling stations during the period of emersion.