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Dynamics of macrobenthic animals were studied quantitatively during a 20-year period on tidal flats in the Wadden Sea. During this period, 4 winters were very cold (mean temperature more than 2°C below the long-term average), whereas also a period with 8 too mild winters in succession occurred. No less than 10 out of the total of 28 species studied in detail were found to be sensitive to cold winters. Their overwinter survival and numbers found after a severe winter were lower than after normal or mild winters. The 4 polychaete and 4 bivalve species among these 10 sensitive species probably died from low temperatures, whereas the 2 crustaceans moved to deeper off-shore waters. Species numbers and biomass were relatively low after cold winters, particularly at the lower tidal flats where sensitive species were numerous. At high tidal flats the share of sensitive species was low; individuals of sensitive species living there suffered more from cold winters than those at lower intertidal levels. Recovery after cold winters was rapid (within 1 or 2 years) in most species. Total-biomass values increased even faster as a consequence of generally highly successful recruitment after a cold winter. Because most invertebrate predators were scarce on the tidal flats after a severe winter, densities in several prey species could rapidly increase during the summers following cold winters. It is predicted that higher winter temperatures in the Wadden Sea area will cause higher species richness and a more stable biomass of the macrozoobenthic fauna living on the tidal flats.