Spatio-temporal variability within the macrobenthic <i>Abra alba</i> community, with emphasis on the structuring role of <i>Lanice conchilega</i> = Ruimtelijke en temporele variabiliteit binnen de macrobenthische <i>Abra alba</i> gemeenschap met nadruk op de structurerende rol van <i>Lanice conchilega</i>

The Belgian Continental Shelf (BCS) is situated in the southernmost part of the North Sea and is characterized by a high geomorphological and sedimentological diversity in soft - bottom habitats. The combinations of the ecological and the socio-economic values of the BCS cause conflicts between both interests. Initiatives are being taken by the scientific community to optimize the integration of the ecological value of the BCS with those of the different user functions, like shipping, dredging, sand and gravel extraction, fishing and tourism. The govemment has made a start with the implementation of Marine Protected Areas into the law (framework law of 20/01/1999): proposals for such areas have been made and are under investigation. In the mean time, it is important to provide the policy makers with a scientific basis for the development of a sustainable management plan for the natural resources of the BCS. This study focuses on the macrobenthic component of the ecosystem. This component was considered to be suitable to characterize and monitor the marine ecosystem because of its size (macroscopic), its relative immobility and its direct link with the sediment and with the processes that occur immediately above the seabed. This study aimed to investigate the macrobenthic community structure on the BCS, the spatial and temporal variability within the ecologically most important macrobenthic community (Abra alba community) and the importance and population dynamics of an ecologically important species (the sand mason, Lanice conchilega) within this community. Based on the information of a part of the marine ecosystem (macrobenthos), some remarks on the conservation of ecologically important areas or species on the BCS can be formulated.In Chapter 2, the different macrobenthic communities and their distribution on the BCS were characterized based on a large number of samples gathered between 1994 and 2000. These samples cover a diverse range of habitats: from the sandy beaches to the open sea, from the gullies between the sandbanks to the tops of the sandbanks, and from clay to coarse sandy sediments. To investigate the large-scale spatial distribution of the macrobenthos of the BCS, the data of 728 samples were combined and analysed. By means of several multivariate techniques, ten sample groups were distinguished. Each sample group is found in a particular physico-chemical environment and has a specific species composition. Four sample groups are differing drastically, both in habitat and species composition, and are considered to represent four macrobenthic communities: (1) the muddy fine sand Abra alba - Mysella bidentata community (further called the Abra alba community) is characterized by high densities and diversity, (2) the Nephtys cirrosa community occurs in well-sorted sandy sediments and is characterized by low densities and diversity, (3) very low densities and diversity typify the Ophelia limacina - Glycera lapidum community, which is found in coarse sandy sediments and (4) the Eurydice pulchra - Scolelepis squamata community is typical for the upper intertidal zone of sandy beaches. Of course these macrobenthic communities are not isolated from each other, but are linked through six transitional species assemblages. The transition between the A. alba - M. bidentata community and the N. cirrosa community, is characterized by a reduction in the mud content and is dominated by Magelona johnstoni. The transition between the N. cirrosa and the O. limacina - G. lapidum community is distinctive by decreasing densities and coincides with a gradual transition between medium to coarse sandy sediments. From the N. cirrosa to the E. pulchra - S. squamata community, transitional species assemblages related to the transition from the subtidal to the intertidal environment, were found. Each community or transitional spec

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

Macrobenthic community structure of soft-bottom sediments at the Belgian Continental Shelf

Within the frame of different research projects, a large number of sites at the Belgian Continental Shelf (BCS) have been sampled for the macrobenthos between 1994 and 2000. These samples cover a diverse range of habitats: from the sandy beaches to the open sea, from the gullies between the sandbanks to the tops of the sandbanks, and from clay to coarse sandy sediments. To investigate the large-scale spatial distribution of the macrobenthos of the Belgian Continental Shelf, the data of all these research projects -728 samples- were combined and analysed. By means of several multivariate techniques, 10 sample groups with similar macrobenthic assemblage structure were distinguished. Each sample group is found in a particular physico-chemical environment and has a specific species composition. Four sample groups differ drastically, both in habitat and species composition, and are considered to represent four macrobenthic communities: (1) the muddy fine sand Abra alba-Mysella bidentata community is characterized by high densities and diversity; (2) the Nephtys cirrosa community occurs in well-sorted sandy sediments and is characterized by low densities and diversity; (3) very low densities and diversity typify the Ophelia limacine-Glycera lapidum community, which is found in coarse sandy sediments and (4) the Eurydice pulchra-Scolelepis squamata community is typical for the upper intertidal zone of sandy beaches. These macrobenthic communities are not isolated from each other, but are linked through six transitional species assemblages. The transition between the A. alba-M. bidentata community and the N. cirrosa community is characterized by a reduction in the mud content and is dominated by Magelona johnstoni. The transition between the N. cirrosa and the O. limacine-G. lapidum community is distinctive by decreasing densities and coincides with a gradual transition between medium and coarse sandy sediments. From the N. cirrosa to the E. pulchra-S. squamata community, transitional species assemblages related to the transition from the subtidal to the intertidal environment were found. Each community or transitional species assemblages was found over a specific range along the onshore-offshore gradient, four types can be discerned: (1) almost restricted to the nearshore area, but possible wider distribution; (2) distributed over the full onshore-offshore gradient; (3) restricted to the near-shore area and (4) restricted to the sandy beach environment. The diversity pattern on the BCS follows this division, with species rich and poor assemblages in the near-shore area to only species poor assemblages more offshore. The distribution and diversity patterns are linked to the habitat type, distinguished by median grain size and mud content