Long-term variability in the <i>Abra alba</i> community: importance of physical and biological causes

The macrobenthic communities in temperate, shallow coastal waters are characterized by strong seasonal and year-to-year variations in community characteristics. These patterns are investigated in the Abra alba community on the Belgian Continental Shelf during nine years (1995 - 2003). During this investigation period, the community tended to return to its orginal state, but it never reached this point. This can be related to the replacement of Spisula subtruncata by Donax vittatus as dominant bivalve after 1997, and possibly as a result of the climatic shift of 1998. The devision of the study period in an unstable (1995-1997) and a more stable period (1999-2003) coincide with the year 1998.Different causes are believed to be responsible for shifts in the community structure during the unstable period, such as mass recruitment of the bivalve Spisula subtruncata (biological cause) and some direct physical causes, like a strong increase of the mud content and temperature fluctuations. All these causes have an effect on the macrobenthic density, diversity and species composition. The mass recruitment of S. subtruncata caused a 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. The cold winter could have been responsible for the slow recovery of the A. alba community after those disturbances. The unstable period was followed by a few years of higher stability (1999 - 2003), characterized by a cyclic seasonal pattern and the dominance of the tube building polychaete Lanice conchilega. The overall seasonal pattern in the study at hand was characterized by high macrobenthic densities in spring and summer, with a decline in autumn towards the end of the winter. Although this cycle differed quantitatively from year-to-year, the general features have been repeated throughout the stable period. The dominance of L. conchilega during the stable period probably had a positive influence on the benthos, due to its habitat structuring characteristics.This study indicates that natural causes could have a drastic impact on the normal year-to-year variability and cyclic seasonal patterns in the marine ecosystem and its ability to recover