La Niña Pushes an Endangered Temperate Soft Coral Species to the Brink of Localised Extinction

Abstract

Extreme weather events such as floods are becoming more frequent, and pose a substantial threat to Australia’s nearshore marine communities. In March 2021, a 450 km stretch of the eastern NSW coastline experienced a La Niña-related rainfall event over several days, resulting in large volumes of freshwater ingress to marine systems. Port Stephens, an estuary 250 km north of Sydney, recorded its highest week of rainfall on record. This was followed by two more flood events in March 2022 and June 2022. Prior to 2021, the marine-dominant Eastern Port of Port Stephens was home to the world’s largest aggregations of Dendronephthya australis, an endangered species of soft coral endemic to the south-east coast of Australia. Using data from a 2019–2022 D. australis monitoring program in Port Stephens, spanning the unprecedented series of flood events in 2021–2022, we detail the impacts of flood events on the population. Prior to the floods, aggregations of colonies were persisting, and individuals were growing at two of the four monitored sites. However, flooding in March 2021 caused a 91% decline in the remaining areal extent of D. australis. Modelling of likely causative factors highlighted water depth as the most significant environmental variable correlated with coral loss. Corals in shallower waters experienced lower salinity and were the most impacted. Continued monitoring during 2021 and 2022 documented the loss of all remaining aggregations in the estuary after further La Niña-related rainfall events, to the point where D. australis is now on the brink of localised extinction.