Numerous studies have examined ecological impacts of boating resulting from scarring by propellers, the discharge of pollutants and sewage, noise, anchoring and the infrastructure associated with boating (e.g. marinas and wharves). The impact of boat generated waves (wash) on organisms is, however, largely unknown. This thesis considered: (i) any impact of wash from RiverCat ferries - 35 m, low-wash vessels that operate on the Parramatta River, Sydney, Australia ? on intertidal assemblages and (ii) the effect of wash on epifauna associated with seagrass blades.

The collapse of seawalls and the erosion of river-banks was observed following the introduction of RiverCat ferries to the Parramatta River, Sydney, Australia. Several strategies of management ? establishing no-wash zones, where ferries must slow to minimize wash and planting mangroves, which may dissipate wave-energy ? have consequently been implemented. These were used in mensurative experiments, examining the effects of wash on infauna. If the establishment of no-wash zones and planting of mangroves are both effective in minimizing any ecological impact of wash, there should be a greater difference between assemblages in wash zones (where speed is unrestricted) from those in no-wash zones when mudflats are sampled than when sampling is done amongst pneumatophores of mangroves. Along the upper Parramatta River, assemblages of infauna differed between the zones, regardless of whether sampling was done on mudflats or amongst pneumatophores. The difference was no greater for organisms in mudflats. Along the lower Parramatta River, where there is generally less compliance with wash restrictions, no difference was seen. During the 2000 Sydney Olympic Games, ferry services were suspended for 5 weeks along the western section of the Parramatta River. This managerial decision provided the manipulation for an experiment to determine whether patterns between the wash and no-wash zones of the upper Parramatta River were due to differences in the intensity of wash. If patterns are due to wash, it was hypothesized that, following the removal of the disturbing force: (i) the assemblages of the wash and the no-wash zones would become more similar and (ii) abundances of taxa in the wash zone would increase to match abundances in the no-wash zone. Results supported hypothesis (i) but not hypothesis (ii). Further experiments ascertained that the effect of wash on macrofauna was most likely to be via changes to characteristics of the sediment (organic content, grain size).

The second section of this thesis examined the effect of wash on epifauna associated with seagrass blades through a series of experiments in North Carolina, USA and the Greater Sydney Metropolitan Region. In North Carolina, assemblages of epifauna differed between places exposed to wash from vessels traveling along the Atlantic Intracoastal Waterway and places that were sheltered from this disturbance due to smaller abundances of epifunal organisms in exposed places. I hypothesized that these patterns were due to the dislodgement of epifaunal organisms from seagrass by the flapping of the blades as waves propagate through the bed. This hypothesis was tested using small-scale manipulative experiments in Australia where waves were produced at previously undisturbed sites using a small vessel. Changes in the abundances of organisms in response to this disturbance occurred only at sites where background current velocities were relatively small.

In summary, this thesis used a combination of large-scale mensurative experiments and small-scale manipulative experiments to examine the ecological effects of wash. Such an approach clearly demonstrates that the large scale of a disturbance should not be seen as a barrier to the experimental test of processes.