1. EFFECT OF HARD STRUCTURES ON THE SURROUNDING BENTHIC ASSEMBLAGE OF THE SOFT SEDIMENT Eugeniy Yakovis*, Anna Artemieva, Marina Varfolomeeva, Natalia Shunatova *Email: yakovis@rbcmail.ru Invertebrate Zoology Department, St.-Petersburg State University, Universitetskaya nab. 7/9, 199034, St.-Petersburg, Russia Background Epibenthic patches (EPs) dominated by barnacles, ascidians and red algaebased on shells, small stones and boulders are the primary source of topographic heterogeneity in soft bottoms of the White Sea near Solovetsky islands. Previous findings show that five of the ten top abundant macrobenthic taxa (polychaetes Aricidea nolani,Chaetozone setosa, Heteromastus filiformis, Scoloplos armiger and oligochaetes) were associated with the cores sampled close to EPs compared to the cores 25-25 cm apart of them. Manipulative experiments to reveal an effect of biogenic or non-biogenic obstacles on the surrounding benthic assemblages have been never previously arranged. assemblages close to epibenthic patches are enriched in polychaets Aricidea nolani,Chaetozone setosa, Heteromastus filiformis, Scoloplos armiger and oligochaetes 55 sq cm cores Experiments In 2005 and 2006 we placed obstacles (concrete bricks, (301510 cm) on unstructured sedimentto separate physical and biogenic distant effects of epibenthic patches (2 runs  5 bricks 2 cores before and after the manipulation). 65° 01.2’ 35° 39.7’ bricks The White Sea 3 of 10 top abundant species affected Scoloplos armiger and Heteromastus filiformis positively responded to the manipulation, i.e. treatments increased their abundance relatively to control samples, whilst Apistobranchus tullbergi demonstrated the significant negative response. Scoloplos armiger and Heteromastus filiformis significantly decreased their density in Control samples (746±76 to 294±29 and 814±83 to 542±45 m-2) and increased them in Treatment ones (407±55 to 881±73 and 497±53 to 746±67 m-2). In the surrounding sediment Heteromastus filiformis significantly increased its density in 2005-2007 (377±53 to 910±205  m-2, 2-tail Student T-test, p<0.001), whereas Scoloplos armiger insignificantly decreased one (565±61 to 432±116 m-2). Against the background of insignificant density increase in 2005-2007 (from 630±61 to 728±172 m-2, Student T-test) A. tullbergi increased its average density in Treatment cores (362±78 to 667±71 m-2) much less than in Control ones (260±40 to 1040±122 m-2). No effect on assemblages Assemblages were numerically dominated by the tube-building maldanid polychaete Rhodine loveni and oligochaetes. Repeated measures ANOVA and Wilcoxon matched pairs test revealed no significant effect of the manipulation on Bray-Curtis dissimilarity between paired Control and Impact samples. On the nMDS plot Before- and After-Control groups of samples were clearly separated, whereas the After-Treatment group was shifted in relation to the other three, but with a large overlap. ANOSIM R-s showed significant differences between Before- and After-Control groups and in pairs After-Treatment vs Before-Control, After-Control vs Before-Treatment. Closest similarity was between After-Control and After-Treatment groups. There was no effect of manipulation on the difference in biomasses between Control and Treatment samples in any of the four principal feeding guilds as well as on the corresponding difference in the ratio of biomasses of surface and subsurface deposit feeders. nMDS on Bray-Curtis dissimilarity before-control, before-treatment, after-control, after-treatment positively affected: Heteromastus filiformis, Scoloplos armiger negatively affected: Apistobranchus tullbergi