Spatial pattern indicates an influence of barnacle and ascidian aggregations on the surrounding benthic assemblage

Joint clusters of solitary ascidians Styela spp., Bolthenia echinata (L.), Molgula spp. and barnacles Balanus crenatus Bruguiere in the White Sea subtidal often develop on shells or stones partially buried into muddy sediment. To assess the structuring role of these suspension-feeders' aggregations, we examined the spatial patterns of the surrounding infaunal assemblage. Pairs of cores obtained close to clusters (Close ones) and 20-25 cm away from them (Distant ones) were compared, the distance corresponding to the average distance observed between aggregations. These pairs were spatially grouped in replicas (blocks) tens of meters from each other. Out of 10 dominating invertebrate species found around the clusters (84 in total), 5 demonstrated significant difference in density (N) close to aggregations and between them. All the five, including polychaets Scoloplos armiger (Muller, Orbiniidae), Aricidea nolani (Eliason, Paraonidae), Heteromastus filiformis Zachs (Capitellidae), Chaetozone setosa Malmgren (Cirratulidae) and undetermined oligochaets, were associated with Close cores. Total abundance of polychaets, oligochaets, crustaceans and bivalves was also different in Close and Distant cores and indicated significant correlations (of either sign) with biomass estimates for barnacles and ascidians in each block. Individual mean body weight (IMW) in pairs of cores was different for 3 of 10 dominants, maldanid polychaets being larger close to clusters, and cirratulids being larger between them. For several taxa the difference, observed in both parameters (N and IMW), significantly varied between the replicas.Filter-feeding activity and faeces production are regarded as the main possible factors explaining the effect of barnacles and ascidians presence, since the spatial pattern observed corresponds with feeding types of the infaunal taxa studied. Average distance between the clusters was short enough, which allows us to conclude that the spatial distribution pattern of benthic infaunal species within the research area strongly depends on barnacle and ascidian aggregations mosaic.