Estuarine fish dynamics associated with polychaete reefs and environmental stressors

Biogenic reefs serve as important habitats for fish, yet reef-building polychaetes and their functional role for associated estuarine fish communities is not well understood. The aim of this study was to investigate the ecological role of one of the most prominent providers of polychaete reefs, Ficopomatus enigmaticus, in a large temperate estuary in Southern Australia (the Coorong). Fish communities were compared between F. enigmaticus reef and non-reef habitats. The fish community included 22 native species, and dissimilarity between the reef versus non-reef habitats was driven by the abundances of particular estuarine species. The fish community from the reef habitat were characterised by higher abundances of estuarine species of gobies (Pseudogobius olorum, Arenigobius bifrenatus, Tasmanogobius lasti, Afurcagobius tamarensis), smallmouth hardyhead (Atherinosoma microstoma), and black bream (Acanthopagrus butcheri). The functional roles of F. enigmaticus reefs serve as either shelter or feeding habitats for estuarine fish especially during the juvenile life stage. An extreme flood event, one of the highest on record, lowered salinities throughout the estuary and lagoon. Temporal variations in fish communities of both reef and non-reef habitats from the estuary and lagoon were explained by stressors of the flood, heatwave and invasive species, and environmental conditions including the total monthly flow, water temperature and transparency. Reefs built by the polychaete F. enigmaticus deliver ecosystem functions that benefit fish communities and merit conservation.