MAKING WAVES THE EFFECTS OF BOAT-WASH ON MACROBENTHIC ASSEMBLAGES OF ESTUARIES Melanie J. Bishop A thesis submitted in fulfillment of the requirements for the degree of Doctor of Philosophy in the University of Sydney April 2003 The work contained within this thesis, except where otherwise acknowledged, is the result of my own investigations Signed: Date: ABSTRACT 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). Only recently have studies considered the impact of wash. This is the loose water produced by a vessel as it travels through the water. Most studies have focussed on wash produced by fast-ferry services. It is, however, known that wind-generated waves, which are typically smaller and of lesser energy than many boat- generated waves, are important in determining the distribution and abundance of organisms. Thus, wash from smaller vessels may also have an impact on estuarine organisms. 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. Thus, while planting mangroves does not appear to be effective in minimizing the ecological impacts of wash on macro-invertebrates, the establishment of no-wash zones may be. 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 Abstract 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). The impact of wash on infauna may be a result of increased rates of mortality of adults and/or decreased rates of colonization. Any effect of wash may be direct or indirect. Two experiments were done to determine whether patterns between wash and no-wash zones were due to a direct or an indirect effect of wash on colonization and/or mortality. The first, which involved the deployment of units of defaunated sediment, evaluated models attributing the impact to decreased rates of colonization in the wash zone. If wash directly affects colonization, differences should be seen between the assemblages accumulating in homogeneous sediment placed in the wash zone and those in the no-wash zone. If it indirectly affects colonization via changes to characteristics of the sediment, which, in turn, determine patterns of colonization, no difference will, however, be seen in the assemblages accumulating in homogeneous units. A difference will, instead, be seen between the wash zone and the no- wash zone in the colonization of site-specific sediments. Colonization of sediment was less spatially variable in the homogeneous than in the site-specific sediment. This indicates that characteristics of the sediment are important in structuring assemblages. A difference in the colonization of the wash and no-wash zone was, however, not evident in either of the treatments. The second experiment, in which cores of sediment were transplanted within and between the zones with their associated assemblages, considered direct and indirect effects of wash on assemblages, resulting from the net effect of mortality of adults and of colonization. This experiment did not support a direct effect of wash on assemblages but, instead, indicated that characteristics of the sediment are of primary importance in structuring assemblages. Abundances of Capitellidae, Nereididae and Amphipoda did, however, increase when sediment was transplanted from the wash to the no-wash zone and decrease when the reciprocal transplant was done. This indicates that, while assemblages do not appear to be directly affected by wash, the abundances of individual taxa are affected. 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 Abstract 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. Abundances of gastropods and amphipods were smaller in the exposed places. A possible model for this pattern is that these animals are dislodged from seagrass by the flapping of the blades as waves propagate through the bed. In the case of mobile organisms, this model will only explain decreased abundances if the frequency of the disturbance is greater than the time taken to recolonize the blades, or if organisms are more susceptible to predation while displaced. If epifauna are indeed displaced by waves, the effect of wash should be immediate. It was hypothesized that if a boat were driven past patches of seagrass that are not usually subject to wash, abundances of small crustaceans and gastropods would decrease immediately after exposure to wash. This hypothesis was tested at two locations - Narrabeen and the Georges River. At Narrabeen, results supported the hypothesis. At the Georges River, in contrast, no change was seen from before to after the disturbance. This may be because this second location was exposed to strong currents, which may be much more important in structuring assemblages than wash. On their own, neither the large-scale mensurative experiments nor the small-scale manipulative experiments described above, provided much information on the role of wash in structuring assemblages. Together, however, they showed that, under certain conditions, wash can reduce the abundance of epifaunal taxa and, where this effect occurs, it is immediate. Thus, although it is often difficult or even impossible to manipulate large-scale disturbances directly, observations of these disturbances may be coupled with smaller-scale controlled manipulative experiments to identify processes that are important in determining the distribution and abundance of organisms. This study has demonstrated that the large scale of a disturbance is not a barrier to the experimental test of processes, but rather a unique ecological opportunity to be exploited. ACKNOWLEDGEMENTS First and foremost, I would like to thank Gee Chapman and Tony Underwood for the considerable contribution they have made to my professional development and the many opportunities with which they have provided me during my time at the Centre. Without them this thesis would not be here. In fact, I am fairly certain I would be a plant cell physiologist. Through their immense enthusiasm they managed to convince me that Marine Ecology was not just a subject that gave me a good timetable in third year but also an interesting and challenging science. I am particularly grateful to Gee Chapman, my primary supervisor, for controlling her excitement when handed the umpteenth piece of paper to read and for managing to keep track of almost all of them! I would also like to extend a special thank-you to Pete Peterson for allowing me to spend several months in his lab in North Carolina and providing me with invaluable advice and encouragement on that phase of my project. What with September 11, the pipe bomb at the boat ramp and the broken axel on the trailer…I am surprised he has invited me back. Thanks too to the other members of his lab for their Southern hospitality and to Hal Summerson and David Gaskill for their help in the field. To members of the Centre, past and present – I am not even going to try to list everyone who has provided assistance or made may life in some way more pleasant – thank-you all! I would, however, like to single out the remaining members of ‘the cohort’ who, after the last four years, are almost like family to me and who I am going to particularly miss. There’s Katie, with whom I have shared an office (and the ups and downs of work and social lives) and whose mountain of crap deflected attention from my not insignificant mess. Brianna, my gym buddy, who is probably the main reason why, at the end of writing this thesis, I still fit into my jeans. David - always eager to help a damsel in distress and share a beer…or six. And finally Craig – I’m glad there’s someone else out there who appreciates real football…no, not league – AFL!!! My ‘non-Marine’ friends have made sure I have always had a life outside of uni and to them I owe my sanity.