Impacts of boat-generated waves on macroinfauna: Towards a mechanistic understanding

Increasing evidence suggests that the waves produced by even low-wash vessels can have a sizeable impact on infaunal assemblages in otherwise sheltered estuaries. Although this impact is widely regarded to be a consequence of wash coarsening sediment grain-size, it may be due to a number of alternative mechanisms which include enhanced turbidity, decreased larval supply, changed resource availability and/or erosion of animals from the sediment. To increase understanding of impacts resulting from boat-wash, this study tested alternate mechanisms by which intertidal infauna along the Parramatta River, Sydney, Australia are impacted by wash from local ferries. Specifically, to discriminate between impacts of wash resulting from changes to sediment characteristics and impacts directly resulting from the altered hydrodynamic regime, cores of intertidal sediment were transplanted between a no-wash and a wash zone and within each of these zones. It was predicted that if wash indirectly determines the structure of assemblages by altering characteristics of the sediment: (1) sediment granulometry and/or mineralogy would differ between cores sourced from the wash zone and cores sourced from the no-wash zone; and (2) over a 6-week period during which sediment characteristics of transplanted cores do not appreciably change, assemblages transplanted across zones would remain more similar to assemblages at their origin than their destination. To the contrary, sediment grain-size did not differ among sites according to their location in wash or no-wash zones and the destination of sediment cores was more important than their origin in determining the structure of macroinvertebrate assemblages. Following transplant into the no-wash zone, the abundances of capitellids and nereids in cores of sediment from the wash zone rapidly increased in abundance. In cores of sediment transplanted from the no-wash zone to the wash zone, the abundances of these taxa decreased. The results of this study therefore indicate that impacts of boat-wash to infauna are not always a consequence of changes to sediment grain-size distribution. Thus, in order to successfully manage ecological impacts of boat-wash, other potential mechanisms such as erosion of sediments and changing resource availability also need to be considered.