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This paper surveys data obtained during 3 decades of monitoring the macrobenthic fauna of a 50 km 2 tidal-flat area in the Wadden Sea (Balgzand) and compares the annual figures with similar long-term data series from other parts of the Wadden Sea (Groningen, Norderney). Despite enormous year-to-year variability in annual recruitment of particularly the bivalves, totalbiomass values were remarkably stable. Detailed data are presented on the annual variability in recruitment success of the 4 most important bivalve species of the Balgzand area (Cerastoderma edule, Macoma balthica, Mya arenaria, and Mytilus edulis). Together these 4 species usually accounted for more than half of the total zoobenthic biomass and largely governed its year-to-year fluctuations. Recruit numbers of the 4 species observed in 27 summers showed similar relationships with the character of the foregoing winter. Such relationships were also observed in other Wadden Sea areas in The Netherlands and Germany. These common relationships resulted in a certain synchrony of recruitment success between the species (within areas) and between areas (within species). Thus, over vast areas (hundreds of km), bivalve recruit densities were generally (though not invariably) high after severe winters and low after mild winters. The resulting strong year-to-year variability in recruit numbers (with standard errors exceeding long-term means) was only just passed on to subsequent total-biomass values for the following 3 reasons: (1) the biomass of any species was generally composed of more than 1 year class, (2) the various species did not reach their maximal biomass at the same age (time elapsed after recruitment), and (3) feedback mechanisms (enhanced reduction of numbers at high abundance) were effective in some species.

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Densities of zoobenthic species observed at different locations tend to fluctuate in parallel even when these sites are far apart. Climatic factors can synchronize population changes over wide geographic areas if they have a direct effect on recruitment or mortality. Several examples indicate that severe winters represent a major synchronizing factor among many of the zoobenthos species of tidal flats, both by immediately enhanced mortality rates in species sensitive to low temperatures and by enhanced recruitment in bivalve species some months later. Long-term monitoring in three areas of the Wadden Sea (Balgzand, Groningen, and Norderney) revealed synchronous baseline patterns in the fluctuations of several species. Examples are shown for the polychaete Nephtys hombergii and the tellinid bivalve Macoma balthica. Local departures from such common patterns indicate local disturbing factors. Examples are shown on scales of about 1 km (a waste-water discharge) and several tens of kilometres (eutrophication in the westernmost part of the Wadden Sea).1996 International Council for the Exploration of the Sea