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

The ecological importance of the tube building polychaete <i>Lanice conchilega</i> in the <i>Abra alba-Mysella bidentata</i> community (poster)

The Abra alba - Mysella bidentata community is the most diverse (average: 31 spp. 0.1 m-2) and dense (average: 6500 ind. m-2) macrobenthic community on the Belgian Continental Shelf (BCS), in which densities are dominated by polychaetes and bivalves. Numerous species of the community are an important food resource for demersal fish (e.g. cod Gadus morhua) and sea birds (e.g. common scoter Melanitta nigra). This community is mainly found in fine muddy sediments (median grain size: + 200 µm; mud content: 5-10 %) of the swales and slopes of the Western Coastal Banks and Middelkerke Bank and at some local spots along the eastern coast.Because of their ecological importance within the community, three macrobenthic species were selected for a detailed autecological investigation: (1) the bivalve Spisula subtruncata, important as a food resource for seaducks, (2) Abra alba, a highly dominant bivalve, and (3) Lanice conchilega, a habitat structuring polychaete. At this moment only information about L. conchilega is available.Lanice conchilega (family: Terebellidae) typically occurs within the A. alba - M. bidentata community (Indicator Value: 62). Its fringed tubes, with a length of up to 40 cm and extending up to about 3 cm above the sediment, are built from fine to coarse sand grains and shell fragments. When found in high densities (up to 3000 ind. m-2), the patches of tubes create a hydrodynamically benign microclimate in which suspended material is trapped. Consequently, local and patchy sediment elevations of up to 10 cm with a relatively high organic matter content are formed. The fringed tubes, stabilized sediment and increased organic matter content are illustrating the habitat structuring capacity of L. conchilega. The increased habitat complexity in dense patches of L. conchilega are believed to be responsible for the high macrobenthic diversity and density of the community. A relatively low macrobenthic density (2447 ind. m-2) and diversity (22 spp. 0.1 m-2) is found if L. conchilega is present in low densities (-2). If present in high densities, a much higher macrobenthic density (8000 ind. m-2) and diversity (36 spp. 0.1 m-2) occurs. Furthermore, the occurrence of some species (e.g. Eumida sanguinea, Anaitides mucosa, and Pariambus typicus) is almost exclusively linked to the presence of L. conchilega.At present, some information about the life history of L. conchilega is available through the ongoing macrobenthos and hyperbenthos research. Very high densities of the hyperbenthic aulophora larvae are found early May (670 ind. m-3 on average on the BCS). Settlement of L. conchilega takes place in May and is immediately followed by a fast growth till June-July. Within the framework of a PhD research, the ecological importance of L. conchilega will further be evaluated through detailed spatial and temporal investigations of all life stages (planktonic and hyperbenthic larvae and benthic juveniles and adults) from March 2002 till October 2003

Population dynamics of subtidal <i>Lanice conchilega</i> (Pallas, 1766) populations at the Belgian Continental Shelf

The paper at hand describes population dynamics, recruitment and persistence of the Lanice conchilega populations in subtidal soft - bottom sediments along the Belgian coastline. An intensive sampling campaign (monthly - biweekly from March 2002 untill September 2003), the discrimination of different benthic stages and cohort analysis of the adult population enabled the discrimination of trends in recruitment patterns of L. conchilega. Three recruitment periods were defined based on the occurrence of peaks of aulophore larvae in the water column and peaks of recently settled individuals in the benthos: (1) spring recruitment period (April - July), (2) summer recruitment period (July - September), and (3) autumn recruitment period (September - November). The spring recruitment period was the most intense and was characterized by high densities of recently settled individuals and juveniles. These high densities resulted in an adult population, which consisted of different cohorts and which was characterized by a high turn over. The summer and autumn recruitments were less intense and did not occur at every station or in every year. Spatial and temporal variations in timing and intensity of settlement and recruitment were observed and described. Results indicate that L. conchilega is an opportunistic species, characterized by a long pelagic phase, high settlement of larvae and a high turn over in the adult population. Due to this recruitment strategy and the preferential settling of the larvae in adult patches, the studied L. conchilega population was able to maintain high abundances in moderately stable patches