Manipulative field experiments were used to investigate the effects of the ascidians Botryllus schlosseri and Ascidiella aspersa and the bryozoan Bugula neritina on sessile invertebrate assemblages. Experiments were conducted on perspex plates suspended from Workshops Jetty, Williamstown, Victoria. All 3 introduced species affected the abundance of sessile marine invertebrates, but the effects were varied and not always negative or concentrated on native species. B. neritina increased the abundance of native species, A. aspersa increased the abundance of introduced species and B. schlosseri decreased the abundance of both native and introduced species. This research shows that effects of some introduced species are highly variable and do not conform to typical ideas that exist for introduced species.

This experiment was conducted to determine if an established Botryllus schlosseri colony affected the recruitment of sessile invertebrates. Small perspex plates that had been in the field for 8 weeks and then in a laboratory water table were used to test this hypothesis. Treatment plates were scraped of all organisms expect for the B. schlosseri colonies. Control and 'mimic' plates were scraped of all organisms. Mimic B. schlosseri were made from silicone and used to determine if the effects were due to the physical structure of the organism or biological attributes (e.g. post-settlement overgrowth or chemical defence mechanisms). Plates were suspended from Workshops Jetty, Williamstown for 10 days (2nd March to 12th March 2004) before being collected. At the conclusion of the experiment all species found on the plates were counted and identified.

A manipulative field experiment was used to assess the spatial variation in the effect of copper on sessile invertebrates within a marina in Port Phillip Bay, Victoria. Sessile invertebrate assemblages were grown on perspex plates for 6 weeks and then dosed with plaster blocks impregnated with copper sulfate. After 1 week of exposure to the copper the plates were collected and the numbers of sponges, polychaetes, barnacles, bryozoans and ascidians were counted on each plate. The variation in abundances of individual taxa were estimated at spatial scales of metres. Water flow was measured as a possible explanation for the variation in the effects of copper. Three of the 20 sessile invertebrates species showed significant small scale variation in the effect of copper. The effects of copper on these species varied in magnitude and direction. Water flow did not explain the variation in the effects of copper. It is suggested that variations in organic or inorganic compounds or in pH levels may provide alternative explanations for the spatially variable effects of copper in the marina.

An experiment was conducted to compare the effects of the introduced bryozoan, Bugula neritina, and the native bryozoan, Bugula dentata, on sessile invertebrate assemblages. Individuals of both species were collected from Workshops Jetty and Altona Pier prior to the experiment and transported back to the laboratory. Two colonies of either B. neritina or B. dentata were attached to each plate using sponges that expanded when wet. The plates were then transported to Workshops Jetty and attached to plates and left for 12 days. At the conclusion of the experiment plates were collected and abundances of species on the plates were counted and identified.

This thesis has used a field based approach to assess the effects of copper liberated from antifouling paints on sessile epifaunal (or fouling) assemblages in southeastern Australia. In the field the effects of dosed copper may be influenced by site conditions, or because the organism has developed tolerance to exposure. To explore these possible effects experiments were conducted inside and outside of two enclosed marinas. Marinas were located at St Kilda (Port Phillip Bay) and Hastings (Western Port Bay), Victoria. At St Kilda, sites were located inside the St Kilda Marina and at St Kilda Pier. At Hastings, sites were located inside Westernport Marina and at Hastings Jetty. Additional sites were also sampled occasionally over the study period. Studies were carried out between 1997-1999. A novel copper dosing system was developed for use in the field. Studies comparing epifaunal assemblages inside and outside the two marinas found that marinas produced different assemblages. The marinas exerted common effects on some functional groups, but effects at the species level differed at each marina. The effects of dosed copper inside and outside the two marinas were tested on: i) the early recruitment of assemblages, and ii) the growth rate and competitive ability of colonial fouling organisms. Results were characterised by high spatial and temporal variability. Copper dose affected a number of species recruiting to the plates but was rarely found to significantly alter growth rates of the colonial organisms; this may be attributed to the high variability observed in growth rates. The competitive ability of colonies was not found to be affected by copper dose or marinas, but few experiments examined this aspect. Reciprocal transplants of settlement plates between sites and copper doses showed that the effect of marinas on assemblages was more important than dosed copper, even though dose levels were well above background concentrations. This is probably due to a large number of factors associated with marinas over and above the effects of increased background heavy metal pollution. Several taxa appeared to adapt to local conditions or copper dose and some appeared to show heterogeneity in response to copper dosing or an ability to acclimate to dose. These types of findings have important implications for the manner in which we apply the findings of traditional laboratory-based ecotoxicology experiments to environmental management.