Caulerpa taxifolia is a marine alga notorious for its introduction and subsequent colonization of the Mediterranean Sea. It is recognized as one of the 100 worst invasive species, and it is suggested that much of its expansion may have been at the expense of native seagrass beds. To date, the bulk of research on C. taxifolia has centred on quantifying expansion and methods of eradication. There are few quantitative data on the relationship between C. taxifolia growth and environmental characteristics (e.g. light, temperature, nutrients). Furthermore, once C. taxifolia has been introduced to a system it is exceptionally difficult and expensive to eradicate. Accordingly the implications, both positive and negative, of this new habitat type must be considered in the context of the new habitat mosaic into which it fits. Australia is unique in that it has both native and invasive populations of C. taxifolia. These populations offer not only an opportunity to examine the dynamics of C. taxifolia beds and their associated communities at different latitudes and temperature and light regimes, but especially in the context of a high diversity marine coastal environment. The objectives of this thesis were to use native (Moreton Bay, Queensland) and invasive (Pittwater, New South Wales and Port River, South Australia) populations of C. taxifolia to: 1) quantify the relationship between environmental drivers (light, temperature, nutrients) and C. taxifolia growth, and 2) examine differences in habitat use and function between seagrass, C. taxifolia, unvegetated substrate. Most of the locations in Australia where large C. taxifolia beds occur are adjacent to urban areas that have a degraded water quality. Manipulative experiments in Moreton Bay demonstrated that nutrients stimulate C. taxifolia growth, however, light availability and seasonality appear to influence the response of C. taxifolia growth to nutrients. Short-term manipulative experiments were conducted across a range of seasons and locations, to capture the effects of temperature on growth. Temperature was the dominant factor affecting rate of stolon extension in both native and invasive locations. Colonization potential of C. taxifolia appears to be driven by ambient water quality (light and nutrients) and bed expansion is driven by temperature in systems where nutrients are saturating. Epifaunal communities sampled by beam trawl were dominated by the families Palaeomonidae, Terapontidae, Scorpaenidae, Monacanthidae, Syngnathidae, Gobiidae, and were largely similar between seagrasses and C. taxifolia; however, syngnathids were absent from C. taxifolia beds. I examined habitat use patterns between seagrass (Zostera muelleri), C. taxifolia, and unvegetated substrate. Fish preferred seagrass to C. taxifolia; however, in the absence of a seagrass fish used C. taxifolia. While C. taxifolia may have similar structural benefits to some seagrasses, there are fewer food resources available within C. taxifolia beds. Furthermore, grazing may be limited to a few specialist grazers. Within the habitat mosaic, C. taxifolia will provide some benefit over an unvegetated substrate; however, that benefit might mask losses in system quality or resilience by decreasing the threshold level for change within the community. Therefore, should a perturbation occur (e.g. sudden drop in water temperature, filamentous algal bloom) a system comprised solely of seagrass could withstand such stress; however, a habitat mosaic of seagrass and C. taxifolia could have a rapid and dramatic loss in its ability to sustain a diverse faunal community. Ultimately, it is most important to protect the system from anthropogenic degradation so it is more resilient to environmental changes.

We examined effects of the macroalgae Laurencia and Palisada spp. on epifauna within seagrass beds of Thalassia testudinum. First we conducted a field survey of Thalassia testudinum beds with varying densities of Laurencia and Palisada spp. Second, we conducted a field experiment, manipulating natural and simulated Laurencia and Palisada spp. In the field survey, we found that total faunal biomass (g m-2) increased significantly with Laurencia and Palisada spp. cover. In the experiment, natural Laurencia and Palisada spp. supported an increased density (No. m-2) of fauna, however, faunal values were highest in the simulated algae treatment. This suggests that the mechanism of increased density/biomass due to Laurencia and Palisada spp. is structural complexity. Since habitat quality in Thalassia testudinum beds may be mediated by associated macroalgae, these small-scale habitat factors need to be incorporated in habitat and ecosystem conservation plans.