Assessing soft sediment biodiversity associated with shellfish reefs using eDNA & morphological sampling methods

Shellfish reefs face enormous threats from anthropogenic impacts. Globally, an estimated 85% of reefs have been lost, primarily through overfishing, disease and parasitism. Port Philip Bay, Victoria has been the site of recent reef reconstruction (2017 & 2018) by The Nature Conservancy. However, much of the post-reef monitoring has focused on pelagic and epibenthic organisms rather than soft sediment assemblages. A current knowledge gap is whether restored reefs influence nearby soft-sediment assemblages via the supply of organic matter (i.e. ‘reef spill over’). The current study aims to assesses whether soft-sediment assemblages directly adjacent to restored reefs differ from nearby non-reef sediment. Traditional morphological sorting and modern eDNA metabarcoding methods were used to test this aim. Two restored reef types consisting of mussels (Mytilus edulis planulatus) and mud oysters (Ostrea angasi), together with three control sites (non-reef, bare sediment) were sampled. These methods, using presence/absence data, revealed no significant differences in soft-sediment assemblages among reef types or controls. However, abundance data associated with the traditional morphological sorting detected significant site-to-site variability, which was associated with differences at one of the control sites, 9ft bank. Methods were significantly different in terms of unique taxa identified. Polychaetes were the most diverse taxa, with seventeen unique families revealed across the three habitat types. Nemerteans (ribbon worms) were the most abundant taxon at mussel and oyster reef sites, whereas Syllidae (Polychaeta) was the most abundant taxon at control sites. These results suggest that the recently restored reefs are yet to influence adjacent soft-sediment assemblages, and will provide important baseline data for future monitoring efforts. The restoration of these reefs is still at an early stage, so further reef monitoring will be important for assessing future change.