BOREAL ENVIRONMENT RESEARCH 8: 135–145 ISSN 1239-6095 Helsinki 23 June 2003 © 2003 Zoobenthos of the outer archipelago waters (N. Baltic Sea) — the importance of local conditions for spatial distribution patterns Erik Bonsdorff 1) , Ari O. Laine 2) , Jari Hänninen 3) , Ilppo Vuorinen 3) and Alf Norkko 1)4) 1) Department of Biology, Environmental and Marine Biology, Åbo Akademi University, FIN-20500 Turku, Finland (e-mail: erik.bonsdorff@abo.fi) 2) Finnish Institute of Marine Research, P.O. Box 33, FIN-00931 Helsinki, Finland 3) Archipelago Research Institute, University of Turku, FIN-20014 Turku, Finland 4) Present address: National Institute of Water and Atmospheric Research, P.O. Box 11-115, Hamilton, New Zealand Bonsdorff, E., Laine, A. O., Hänninen, J., Vuorinen, I. & Norkko, A. 2003: Zoo- benthos of the outer archipelago waters (N. Baltic Sea) — the importance of local conditions for spatial distribution patterns. Boreal Env. Res. 8: 135–145. ISSN 1239-6095 The variability in species number and abundance of the soft-sediment benthic animal communities of the Archipelago Sea and western Gulf of Finland (SW Finland) is described in relation to environmental variables using multivariate analysis. The results show assemblages along the outer archipelago zone, facing the open sea, illus- trating the importance of local conditions for the functioning of the zoobenthos. Based on clustering and multidimensional scaling of species abundance, the sampling sites formed distinct groups (benthic assemblages) linked to different geographical sub- areas. The main group, dominated by the amphipod Monoporeia affinis, was mainly found between the outer islands and skerries of the Archipelago Sea. Five additional groups were identified, which were geographically scattered and dominated either by the bivalve Macoma balthica, by M. affinis, or by oligochaetes. Based on rank correla- tion between the (dis)similarity matrices of species composition and environmental characteristics, the combination of dissolved oxygen saturation, temperature and sediment organic matter content best explained the species distribution and community structure of benthic fauna. The groups could also be positioned along an environmen- tal gradient with increasing distance from the mainland, increasing depth and salinity, decreasing organic content (i.e. food availability), and declining temperature. The results are discussed in relation to environmental properties, highlighting the need for a baseline survey for future coastal monitoring, and in relation to the EC Water Frame- work Directive (2000/60/EC), stressing the relationship between environmental typol- ogy and biological (ecological) indicators of environmental health.