CHANGES IN DISTRIBUTION PATTERNS OF 0-GROUP BIVALVES IN THE WADDEN SEA - BYSSUS-DRIFTING RELEASES JUVENILES FROM THE CONSTRAINTS OF HYDROGRAPHY

The large-scale (km) distributional patterns of juvenile bivalves are established by larval settlement and subsequently changed due to activ ely initiated postlarval migrations (byssus-drifting), resuspension du ring sediment disturbance, and local differences in mortality. Repeate d mapping of O-group bivalve distribution during two summers was combi ned with simultaneous registrations of the numbers of drifting specime ns. Species differed in their susceptibility to passive resuspension a nd their activity in byssus-drifting, but the two ways of entry into t he water column were independent of each other. As a result, the relat ive magnitude of byssus-drifting and passive resuspension varied with the species. While hydrographical conditions always determined the cha nges in the distributional patterns of passively eroded species, this was not the case in active migrations of byssus-drifting bivalves. A c omparison of the distributional patterns of juveniles over two consecu tive years showed similar patterns in the tellinid clam Macoma balthic a and the razor clam Ensis americanus. In these two species spatfall m ainly occurred around mean low-tide level. Subsequently, juvenile M. b althica rapidly accumulated in the upper intertidal, whereas juvenile E. americanus accumulated subtidally. In mussels Mytilus edulis the di stribution of juveniles only changed in a longshore direction, not in the tidal level occupied. The spatial pattern of cockles Cerastoderma edule changed from aggregation of the early spat in the mid and lower intertidal towards a more uniform distribution of recruits over these tidal flats. There were only a few patches of high abundance left at t he end of summer. Finally, in juvenile clams Mya arenaria the spatial patterns of both spatfall and redistribution were unpredictable. On a km-scale, abundance of some species correlated with sediment granulome try. Presumably, this does not reflect a causal relationship but is a consequence of the correlation between grain size distribution and tid al level within the area studied. On a scale of hundreds of metres, th ere were no consistent correlations between juvenile abundance and sed iment granulometry in any species. In summary, it seems that hydrograp hy ruled the initial settlement of larvae to the sediment and strongly influenced the subsequent redistribution of juveniles caused by passi ve resuspension. The outcome of actively initiated migrations, on the other hand, was only weakly influenced by hydrography. Therefore it is suggested that hydrography plays a dominant role in the initial devel opment of km-scale distributional patterns of just-settled bivalves in the Wadden Sea, while habitat selection is delayed to the byssus-drif ting postlarvae phase.