LAUSR

The introduction and spread of alien mollusc species is strongly related to human activities such as connecting river basins through canal construction and shipping. Economic growth has caused an increase in commercial and recreational navigation on rivers and led to the development of extensive networks of waterways. Ships alter flow velocity in littoral zones via water displacement and propeller jet streams, thereby affecting structure and functioning of riverine mollusc communities and their ecosystem services, such as water purification and nutrient cycling. A literature review was performed to derive data for determining field based upper flow velocity occurrences for 37 native and 8 alien mollusc species present in the rivers Rhine and Meuse. Next, these upper flow velocities were used to construct species sensitivity distributions (SSDs) representing the species assemblages of native and alien molluscs in the littoral zone of these rivers. The SSDs were used to derive the potentially occurring fractions (POFs) of both species assemblages in groyne fields or in channels behind longitudinal training dams (LTDs), due to shipping induced changes in flow velocity conditions. POFs were calculated for various types of ships, in three river Rhine distributaries (Nederrijn: impounded; Waal and IJssel: free flowing) and the river Meuse (impounded). The SSDs of native and alien species assemblages did not differ significantly. Alien species with the lowest and highest tolerances were Musculium transversum (Say, 1829) and Dreissena polymorpha (Pallas, 1771), respectively. Valvata cristata Müller, 1774 and Radix balthica (Linnaeus, 1758) were the native species with the lowest and highest flow velocity tolerance, respectively. Freight ships were associated with the lowest POF in impounded rivers (0.76) as well as in free-flowing rivers (0.61). Shipping was associated with lower POFs in groyne fields of free-flowing rivers than those of impounded rivers. The highest POFs were found in channels behind an LTD in a free-flowing river. Shipping is associated with a shift of the mollusc species assemblage towards flow resistant species and could thereby affect ecosystem functioning and services.