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

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 (Ba lgzand) 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 bivalve s, total-biomass values were remarkably stable. Detailed data are presented on the annual variability in recruitment success of the 4 most important b ivalve species of the Balgzand area (Cerastoderma edule, Macoma balthica, M ya arenaria, and Mytilus edulis). Together these 4 species usually accounte d for more than half of the total zoobenthic biomass and largely governed i ts 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 cert ain synchrony of recruitment success between the species (within areas) and between areas (within species). Thus, over vast areas (hundreds of km), bi valve recruit densities were generally (though not invariably) high after s evere winters and low after mild winters. The resulting strong year-to-year variability in recruit numbers (with standard errors exceeding long-term m eans) was only just passed on to subsequent total-biomass values for the fo llowing 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 maxima l biomass at the same age (time elapsed after recruitment), and (3) feedbac k mechanisms (enhanced reduction of numbers at high abundance) were effecti ve in some species.