Climate change and anthropogenic activity are contributing to species extinctions at an alarming rate (Ceballos et al. 2015). This study details the complete loss of Dendronephthya australis aggregations in the Port Stephens estuary, NSW; historically the most prolific location where this species occurred. Not only do we provide further evidence of continued declines in Port Stephens, following on from studies by Harasti (2016) and Larkin et al. (2021b), we also document the impacts of a flood event (March 2021) that caused significant mortality of the coral aggregations that were persisting prior to the flood. Significant rainfall and related flooding in March 2021 caused widespread damage to other rocky reef habitats in NSW (Davis et al. 2022), and we demonstrate that the flood impacts included mass mortality of D. australis colonies within Port Stephens. By November 2021, only 851m2 of D. australis habitat remained in Port Stephens, and by July 2022, after two additional flood events, there were no longer any D. australis aggregations remaining in the Port Stephens estuary, although a few solitary colonies persisted at isolated locations within the estuary (ML Pers. Obs.).
Prior to the March 2021 flood, loss of D. australis at the Pipeline and Nelson Bay Beach aggregation sites had already occurred. In the lead-up to the disappearance of corals at these sites, colony diameters were declining, suggesting that conditions were no longer suitable for growth and survival. As identified in Larkin et al. (2021b), it is likely that sand movement was a major contributing factor to colony decline in these areas. Contrastingly, at Redpatch Point and Sandy Point, colonies increased in size and the overall areal extent remained stable between late 2019 and February 2021 (before the flood). Kaplan-Meier analyses indicated that mortality occurred at all sites, with mortality rates for colonies at Sandy Point and Redpatch Point prior to the flood events providing an indication of the rate of natural attrition, with mortality rates averaging 0.17% per day at these sites. Monitoring data also provided important new information on D. australis life expectancy, with some colonies surviving for at least 589 days; this indicates that D. australis has the potential to live for at least 1.5 years in the wild.
Models examining the environmental variables correlated with colony mortality between February and November 2021 found that depth, distance from the shore, and slope all contributed to the ‘BEST’ model. Depth made the greatest contribution to explaining flood induced mortality, with post-flood surveys finding that only colonies present in depths > 8 m survived the March 2021 flood event. This can be explained by the density variance between fresh and sea water; when a plume of less dense fresh water combines with seawater in an estuary, it creates a lowered ‘surface salinity’ effect (Barros et al. 2014). Hence, D. australis aggregations in shallower depths were exposed to low salinities during the flood event. The influx of large volumes of low salinity water, even for brief exposure times, can have lethal impacts on marine life, including corals (Coles and Jokiel 1992; Dias et al. 2019).
Although mortality of D. australis as a result of sudden freshwater ingress has been recorded previously during an aquarium-based experiment (Poulos 2011), aggregations have persisted in the Port Stephens estuary despite prior instances of flooding over the past 50 years. Salinity has not been recorded at the soft coral aggregation sites during historical flood events, so the precise salinity tolerance levels for this species are not known. The salinity levels measured at Redpatch Point during the March 2021 flood show that there were 22 cumulative hours measured at < 10 PSU, 46 hours measured < 20 PSU, and a total of almost four days < 30 PSU (Fig. 7). The data available for local rainfall indicate that this event was unprecedented, recording the largest ever volume of rainfall in consecutive days in Port Stephens (Bureau of Meteorology (2023) ‘Climate data online – daily rainfall’ available at http://www.bom.gov.au/climate/data/). These rainfall records, coupled with the Karuah River discharge rates (Fig. 6), demonstrate the unprecedented magnitude of this event within the Port Stephens estuary.
During the past 50 years, substantial clearing of vegetation has occurred in the Karuah catchment as well as on the southern foreshore of Port Stephens (Supplementary Material SM2). Clearing of land for agriculture not only alters hydrodynamics within a catchment, it can also increase bank erosion, resulting in increased sedimentation downstream (Freeman et al. 2019). Whilst sedimentation could have contributed to the demise of D. australis aggregations during the 2021–2022 flood events, there was no evidence to suggest that the colonies were buried, as sand levels adjacent to the pegs at the monitoring sites did not change substantially (ML, Pers. Obs). However, as these measurements didn’t account for mobilised sediments in the water column, sand scour could have been another factor contributing to the colony decline.
Urbanisation on land adjacent to estuaries such as Port Stephens can lead to an increase in volume, rates, and magnitude of water runoff into waterways (Hopkinson and Vallino 1995), and also alters catchment nutrient inputs. Flood events can intensify the ingress of anthropogenically-derived pollutants into estuaries (Jeng et al. 2005; Wetz and Yoskowitz 2013), which can include: faecal-derived bacteria (Lewis et al. 2013); nutrients such as phosphorus and nitrogen (Eyre and Twigg 1997); metal(loid)s (Coates-Marnane et al. 2016); and per-and polyfluoroalkyl substances (PFAS) (Taylor 2019), amongst others. Whilst previous studies have quantified the presence of nutrients and metal(loid)s within Port Stephens-based oysters (Gifford et al. 2005), and PFAS within oysters and fish (Taylor et al. 2018), there are no known studies that elucidate the effects of flooding on these pollutants within the estuary. Similarly, historical data demonstrate that the sewage-associated bacteria enterococci can be variably present in high quantities within Port Stephens (www.environment.nsw.gov.au/beachapp/report_enterococci.aspx), though no data were recorded inside the estuary during the 2021–2022 flood events. Although the impacts of pollutants haven’t been documented for D. australis specifically, instances of morbidity and disease of other coral species has been correlated with pollutants in waterways (Kaczmarsky and Richardson 2011; Chan et al. 2012; Redding et al. 2013; Yoshioka et al. 2016). Whilst the disappearance of the majority of D. australis colonies at Redpatch Point and Sandy Point coincided with the low salinities associated with the March 2021 flood event (Fig. 7), it is possible that other factors associated with flooding, such as pollution, contributed to their mortality during 2021 and 2022.
The disappearance of D. australis aggregations in the Port Stephens estuary between 2021 and 2022 is of grave concern for this Endangered species. Without these clusters of colonies, their ability to reproduce may be hindered. Although there is evidence that D. australis can asexually propagate (Larkin et al. 2023, In Review), genetic analyses indicate that sexual reproduction is likely its primary mode of proliferation (Williamson et al. 2022), as has been found for other Malacalcyonacean species (McFadden 1997; Bastidas et al. 2001). Recent observations provide evidence that D. australis is a gonochoric broadcast spawner (Larkin et al. 2023, In Review). Consequently, the presence of numerous sexually mature colonies of different sexes within the same reef system may be critical to facilitate genetic diversity, adaptation, and resilience for the recovery of D. australis populations.
This study demonstrates that the Endangered D. australis soft coral is on the precipice of extinction in the Port Stephens estuary, which was the primary location for this species' occurrence. Surveys conducted by researchers confirm that small aggregations previously present in Brisbane Water, NSW also disappeared after these flood events (Vincent Raoult, pers comm, 2022). As of January 2023, there is only one known aggregation of large (≥ 30 cm average diameter) and healthy colonies remaining, located on the scuttled navy frigate ex-HMAS Adelaide, near Terrigal, NSW. With access to this aggregation as a source of potential donor cuttings, remediation measures to recover D. australis populations in the Port Stephens estuary are considered urgent to ensure the species' ongoing survival in the wild.