The geographic scale of synchronized fluctuation patterns in zoobenthos populations as a key to underlying factors: climatic or man-induced

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).