Field experiments on colonization and movements of stream invertebrates

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