Mar Biol (2010) 157:1271–1282 DOI 10.1007/s00227-010-1407-7 123 ORIGINAL PAPER Impact of Arenicola marina (Polychaeta) on the microbial assemblages and meiobenthos in a marine intertidal Xat Yanli Lei · Karen Stumm · Nils Volkenborn · Stephen A. Wickham · Ulrike-G. Berninger Received: 10 October 2008 / Accepted: 3 February 2010 / Published online: 6 March 2010  Springer-Verlag 2010 Abstract The benthic microbial food web can be respon- sible for a large proportion of benthic carbon cycling yet there are few data on the trophic interactions between this food web and macrobenthos. A large-scale Weld experiment was conducted to investigate eVects of eliminating the polychaete Arenicola marina on benthic microbes (pro- karyotes, heterotrophic and autotrophic protists) and meta- zoan meiofauna in a marine intertidal Xat of the North Sea, Germany. Over a period of 2 years, quantity and composi- tion of micro- and meiobenthos from unmanipulated sites were compared to those from sites deplete of lugworms. These grazer treatments were cross-classiWed with diVerent sediment characteristics (low- and mid-intertidal areas). Lugworm removal resulted in an initial increase in abun- dance of prokaryotes and nanoXagellates, which became less pronounced in the second year. Ciliates were not aVected quantitatively, but in the absence of lugworms, diversity and the proportion of carnivorous forms increased. Meiobenthos (nematodes, ostracods and cope- pods) were aVected only moderately. The observed changes are probably due to a combination of release from grazing/ predation pressure, changes in the species composition of higher trophic levels (namely large polychaetes) and altered environmental conditions (such as depth of the oxygenated layer and sediment grain size). Spatial diVerences between sites of diVerent tidal exposure/grain size appeared to be as large as temporal diVerences during the 2 years following the manipulation of the system. We conclude that in inter- tidal sediments, indirect eVects due to habitat transforma- tion are as important as direct biological interactions (grazing pressure and competition) for the dynamics of the benthic microbial food web. Introduction It has long been suggested that the microbenthic food web (prokaryotes, protists in the size range of 2–200 m, i.e. nano- and microorganisms, and small metazoa, such as copepods, ostracods and nematodes) is responsible for a large fraction of the cycling of organic matter in aquatic sediments and should be regarded as a complete functional unit. These organisms are characterized by their small body sizes, relatively short life spans and a small total biomass compared to the macrofauna community. However, due to Communicated by P. Kraufvelin. Y. Lei · S. A. Wickham · U.-G. Berninger Department of Organismic Biology, University of Salzburg, Hellbrunner Str. 34, 5020 Salzburg, Austria Y. Lei (&) Institute of Oceanology, Chinese Academy of Sciences, 266071 Qingdao, People’s Republic of China e-mail: leiyanli@qdio.ac.cn K. Stumm Alfred Wegener Institute for Polar and Marine Research, Am Handelshafen 12, 27570 Bremerhaven, Germany Present Address: K. Stumm Department of Microbiology and Bioprocess Technology, University of Leipzig, Johannisallee 21-23, 04103 Leipzig, Germany N. Volkenborn Alfred Wegener Institute for Polar and Marine Research, Wadden Sea Station Sylt, Hafenstrasse 43, 25992 List, Germany Present Address: N. Volkenborn Department of Biological Sciences, University of South Carolina, Columbia, SC 29208, USA