Doctor of Philosophy
School of Biological Sciences
This is the first detailed study to uncover the patterns of distribution and abundance of the sessile epifaunal invertebrate community inhabiting seagrass meadows on the south-eastern coast of Australia. It is also one of the few studies worldwide to investigate patterns and processes of sessile epifaunal invertebrate distribution in seagrass meadows. The target seagrass species, Posidonia australis, is threatened and historically has shown rapid decline in the Sydney region. The extensive seagrass meadows selected for this study exhibited negligible anthropogenic disturbances so as to construct a reliable baseline. Sessile epifaunal invertebrates inhabiting seagrass ecosystems, particularly sponges and ascidians, have been poorly studied due to their taxonomic complexity. They are considered difficult to identify because of their great morphological plasticity and a general lack of taxonomic expertise. This study constitutes a thorough and statistically robust approach to investigating patterns and processes in a poorly described community. This study addresses an important knowledge gap and widens our understanding of a community associated with a threatened seagrass ecosystem facing tremendous anthropogenic pressure. I used a step by step framework, from preliminary data to manipulative experiments, as to uncover baseline information on patterns of distribution and community dynamics.
I encountered 20 sponge and eight ascidian species within Jervis Bay, which has the most extensive and near pristine P. australis seagrass meadows of the south eastern Australian coast. The adequacy of my sampling procedures of sessile invertebrate diversity and abundance, was established for this specific assemblage through preliminary sampling and cost effective way of sampling. I established the sampling guidelines, the ideal sampling units and their dimensions, as well as the most effective experimental design. This included replication at the appropriate scales of variability whilst ensuring the efficient allocation of resources. I used a hierarchy of spatially nested scales to determine the patterns of distribution of sponges and ascidians inhabiting P. australis. This assemblage was sporadically distributed in the seagrass meadows with high variability at multiple spatial scales. A few sponge and ascidian species dominated the assemblage and were widespread across the largest spatial scale sampled. The remaining species were mostly rare and sparsely distributed. Sponges attached to a variety of substrata but most notably shells, P. australis and polychaete tubes. No obligate seagrass species were recorded although three species predominantly used P. australis as a substratum. Due to the high variability seen in the distribution of this assemblage, I assessed the robustness of my sampling design by comparing the outcome of my study with that of my initial pilot study. Overall, the pilot study was a valuable tool in the establishment of an effective sampling design. Despite intrinsic limitations and high levels of variation, the pilot study reflected the distribution of the patchy assemblage seen in the main study.
Demers, Marie-Claire A., Pattern and process in a threatened seagrass community: dynamics and habitat use by sessile epifaunal invertebrates in Posidonia australis, Doctor of Philosophy thesis, School of Biological Sciences, University of Wollongong, 2017. https://ro.uow.edu.au/theses1/43