Radiotelemetry reveals the dependence of inland tern breeding and foraging habitats on ADCP-identified sediment aggradation reaches in lowland rivers

The largest rivers in developed countries have usually been turned into waterways by straightening them and removing large bedforms hampering navigation. For river restoration and their sustainable management it is important to know how large bedforms support biodiversity, whether they could be protected and what potential conflicts in river management they can pose. We have addressed these questions by studying the role of large bedforms in supporting populations of two inland tern species Sternula albifrons and Sterna hirundo. We spatially analysed the behaviour of these two species with reference to the bedform structure mapped over a long semi-natural reach of the River Wisła (Vistula) (S. Poland). The results show that radiotagged terns breed on islands within the aggradation reaches, foraging in the adjacent shallows inhabited by populations of small fish. For Little Terns, the more complex the water line of emergent forms, the greater their foraging intensity. The islands do not pose any flood risk to human settlements. The whole geofeature forms an integral habitat for fish and birds; it is maintained by its geographic settings and so is stable over long periods of time (over 200 years). Protection of such habitats is thus feasible.