Estuarine ecosystems are charac­terized by largely varying physicochemi­cal condi­tions, especially in the meso-/oligohaline zones. Knowledge of the environmental variability and related population effects on a range of spatio-temporal scales is fundamental to a better understanding of their functioning, stability, resilience and the way they are influenced by human impacts. This study combines spatial and temporal variations in macrobenthic populations (and their environment) on a brackish mudflat in the Schelde estuary (NW-Europe). From 1990 to 1999, long -term year-to-year variations were monitored on 24 sites, short -term monthly variations on two sites. The variation in macrobenthos and the physicochemical environment due to seasonal dynamics, spatial pattern, and annual as well as long-term trends, was quantified. The macrobenthic community on the mudflat was characterized by a few dominant species ( Corophium , Heteromastus , Nereis , Oligochaeta), with spatial distributions related to sediment characteristics. Considerable year-to-year variation was observed and seasonality was evident for all species. It is argued that in the meso-/oligohaline zone, where salinity shows large seasonal fluctuations, benthic communities change frequently during the year, resulting in communities that seldom progress beyond early benthic-community succession. Annual variations are less pronounced. The impact of the construction of a containerterminal (1994), adjacent to the mudflat, on the macrobentic species distributions is discussed.