Synchronized reproductive success in the main bivalve species in the Wadden Sea: causes and consequences

Abstract

This paper surveys data obtained during 3 decades (1969-1999) 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, total-biomass 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.