Hydrobiologia 474: 125–130, 2002. © 2002 Kluwer Academic Publishers. Printed in the Netherlands. 125 Field experiments on colonization and movements of stream invertebrates in an Apennine river (Visone, NW Italy) Stefano Fenoglio, Paolo Agosta, Tiziano Bo & Marco Cucco University of Eastern Piedmont, Dept. Sciences and Adv. Tech., via Cavour 84, 15100 Alessandria, Italy Tel: 131-283-835. Fax: 131-254-410. E-mail: fenoglio@unipmn.it Received 15 June 2001; accepted 31 January 2002 Key words: macroinvertebrates, lotic environments, recolonization movements Abstract Macroinvertebrates continuously redistribute themselves in the riverbed. A knowledge of the colonization mech- anisms and movement patterns is very important for an understanding of processes of restoration of lotic environments, particularly of inland waters with severe pollution. We tested the colonization patterns of stream macroinvertebrates in the Visone River, a tributary of the highly contaminated Bormida River (NW Italy). We placed six groups of traps in the riverbed, each group consisting of three traps: the C trap allowed colonization from all directions, while the D and U traps allowed access only from downstream and upstream respectively. The C traps were the most colonized substrates, both in number of individuals and taxa. The U traps were more colonized than the D traps, demonstrating the great importance of movements directed downstream. We report data on taxonomic and seasonal differences in the colonization process. Introduction Benthic macroinvertebrates are a central element of lotic environments, playing an important role in the processing of allochthonous and autochthonous or- ganic substances and in the river’s self-purifying ca- pacity (Vannote et al., 1980). Furthermore, their community structure is widely used as an indicator in environmental quality monitoring (Ghetti, 1997). It has long been known that freshwater invertebrate populations continually redistribute themselves over the stream bed. Environmental alterations can deplete or destroy a benthic biocenosis, but a recolonization process begins as soon as conditions are restored. Many studies have shown that macroinvertebrates can quickly colonize new or disturbed substrates from source areas. Recolonization studies in running water habitats have shown that animals promptly reappear in affected areas (Williams, 1980). Colonization depends on many elements, like invertebrate mobility, substrate texture (Wise et al., 1979) and associated food sup- plies, competition, predation, life history (Mackay, 1992) and season (Williams, 1980). Williams & Hynes (1976) showed that four main sources of animals con- tributed to the recolonization of denuded substrates. These were vertical movements from within the sub- strate, aerial sources (e.g. oviposition), upstream and downstream migration (drift). Downward movement toward the hyporrheic zone plays an important role especially in the recolonization of temporary streams (Delucchi, 1989). The upstream flight of ovigerous lotic insects is also a principal component of the col- onization cycle of many taxa (Bird & Hynes, 1980; Müller, 1982). Drift is the downstream transport of aquatic organisms in the current. There are different types of drift, which present seasonal and diel period- icity patterns and differ in qualitative and quantitative characteristics (Brittain & Eikeland, 1988). Upstream movement within the water has been recorded for many benthic invertebrates (Bishop & Hynes, 1969; Söderström, 1987). In permanent streams, all four sources contribute substantially, but positive rheotaxis and especially drift appear to be the primary com- ponents. Several studies have described recolonization mechanisms in North American, northern European and tropical streams, but until now there have been no similar investigations in Italian lotic environments.