Influence of restored oyster and mussel reefs on the biodiversity of adjacent soft-sediment bacterial assemblages in Port Phillip Bay

Shellfish reef provide many ecosystem services so their decline over several decades has led to modern restoration efforts. Within Port Phillip Bay, Victoria, The Nature Conservancy Australia (TNC) are leading restoration efforts, using historically and culturally significant species. Flat oysters, Ostrea angasi and blue mussels, Mytilus edulis planulatus are being used to seed and restore or replace degraded reefs. Ecosystem services provided by shellfish reefs include important biogeochemical processes that are mediated by benthic bacteria. These biogeochemical processes are altered by the host filter feeding mechanisms and associated by-products (e.g. faeces) which can influence adjacent soft sediments. This area lacks research due to the infancy of many restoration projects and complex ubiquity of bacteria. The diversity of marine prokaryotes is, as yet, not well understood due to the lack of molecular tools. Therefore, this study consisted of two aims: 1) describe bacterial assemblages directly adjacent to restored reefs compared to nearby controls (absent of reef); and 2) assess the influence of oysters and production of faecal outputs on underlying sediment. The first aim involved a field study comparing bacteria in sediments adjacent to restored shellfish reefs verses non-reef controls. Next generation revealed 9,140 unique bacterial ASVs, however multivariate analysis revealed no significant differences in assemblage diversity between reefs and controls. This lack of difference may be explained by the age of the reefs (3-4 years post restoration) and the need to increase the number of reef sites. A laboratory experiment tested aim 2, which showed that the presence of a single oyster can significantly alter sediment bacteria over a very short time-period (i.e. 6 days). This is the first study, to my knowledge, that has attempted to characterise bacterial assemblages associated with shellfish reefs in Port Phillip Bay. Therefore, these results provide important baseline data for future restoration efforts and provide a starting point for further understanding the functional role of reef-associated bacteria as technical advances improve our ability to study marine microbiomes.