Temporal variability in the Abra alba community determined by global and local events

Macrobenthic communities in temperate, shallow coastal waters are characterised by strong seasonal and year-to-year variations in community characteristics. These temporal variations were investigated in the Abra alba community on the Belgian Continental Shelf over a period of nine years (1995 - 2003). During this period, the community did not show a cyclic pattern, but a shift between the years 1995 - 1997 and 1999 - 2003 that was possibly triggered by changes in the hydroclimatic state of the North Sea and was reflected by a small shift of the dominant species in the A. alba community. In the years 1995 - 1997, the temporal pattern was dominated by strong year-to-year differences, coinciding with different successive events (strong recruitment, sedimentological changes, cold winters). Therefore, these years were characterised as the unstable period. Such events may affect the macrobenthic density, diversity and species composition. The mass recruitment of S. subtruncata may have been responsible for an initial decrease in the density and diversity of the macrobenthos, whereas the increase of mud content was responsible for a crash of the species richness and macrobenthic density in the winter of 1996. After those events, the A. alba community needed time to recover (1996 - 1998). The recovery was possibly retarded by a slow amelioration of the habitat conditions, unsuccessful recruitment and the occurrence of a cold winter. This period was followed by some years in which the seasonal dynamics in the community exceeded the lower year-to-year variability and therefore these years were characterised as the stable period. The seasonal dynamics in the study were characterised by high macrobenthic densities and diversity in spring and summer, followed by declines in the autumn-winter period. The conclusion is that mainly local factors were responsible for the short-term variations within the community, whereas global events (hydroclimatic, cold and mild conditions) caused the long-term changes.