Restoration of benthic macrofauna promotes biogeochemical remediation of hostile sediments; An in situ transplantation experiment in a eutrophic estuarine-hypersaline lagoon system.

Estuarine ecosystems have very high ecological and economic value, and also act as a buffer for coastal oceans by processing nutrient inputs from terrestrial sources. However, ongoing pressures from increased urbanisation and agriculture, overlaid by climate change, has reduced inflows and increased nutrient loads that challenge the health and buffering capacity of these ecosystems. This study aimed to investigate whether restoring the bioturbating activity of Simplisetia aequisetis (Polychaeta: Nereididae) and other macrofauna could improve biogeochemical conditions in 'hostile' (i.e. hypersaline, sulfide-rich) sediments. To achieve this aim, we conducted an in situ experiment in the Coorong estuarine-lagoon ecosystem, translocating hostile hypersaline sediments, devoid of bioturbating macrofauna, to a 'healthy' (lower salinity) location where macrobenthic fauna naturally occur, and manipulating the S. aequisetis density in the sediments. Porewater, solid-phase, and diffusive equilibrium and diffusive gradient in thin-films (DET/DGT) measurements showed that bioturbation by macrobenthic fauna significantly influenced sediment biogeochemistry and remediated hostile conditions in sediment within a short time (four weeks) irrespective of S. aequisetis density. Bioturbation promoted sediment oxygenation, while salinity and the concentrations of total organic carbon and porewater sulfide, ammonium, and phosphate all decreased over time at all sediment depths. This research highlights the importance of macrobenthic communities and their functional traits for improving sediment conditions, promoting resilience to eutrophication, providing a nature-based remediation option, and in general ensuring healthy functioning of estuarine ecosystems.