Introduction

The epoch in which we are currently living — the Anthropocene — marks a time during which species extinctions are occurring at an alarming rate (Barnosky 2014; He and Silliman 2019). Climate change is triggering an increase in temperatures and severe storm events, which, coupled with intensifying human activity, is exacerbating the decline of both terrestrial and marine ecosystems around the world (Dale 1997; Doney et al. 2012; Selwood et al. 2015). For the majority of Australia, La Niña is associated with climatic conditions with higher-than-average rainfall, which can lead to severe flood events (Bureau of Meteorology 2016). During March 2021–July 2022, coastal communities in New South Wales (NSW) experienced numerous La Niña-related flood events (Bureau of Meteorology 2021a, b, 2022). These floods impacted nearshore reefs in south-east Australia which were already exposed to a multitude of stressors, including sedimentation (Wainwright 2011); pollution (Ling et al. 2018); warming water (Wernberg et al. 2016); fishing (Stuart-Smith et al. 2008); boating activities (Glasby and West 2018); and severe wildfires (Barros et al. 2022). Consequently, threatened species reliant on these ecosystems have increasingly been impacted, with reduced opportunities to recover between disturbance events.

The ingress of large volumes of freshwater into estuaries and embayments during floods can have devastating impacts on marine-acclimatised fauna, including macroalgae (Davis et al. 2022) and corals (Coles and Jokiel 1992; Jokiel et al. 1993; Cardoso et al. 2008). Due to the hydrological characteristics of estuaries, salinities within these environments are inherently variable. However, organisms that thrive in marine-dominated zones within estuaries can experience mortality if salinity drops below critical physiological thresholds for extended periods of time (Parada et al. 2012; Saraswat et al. 2015). Whilst previous studies have investigated salinity tolerances of some marine taxa in estuaries, such as sponges (Fell et al. 1989), seagrasses (Benjamina et al. 1999), and kelp (Davis et al. 2022), no known studies have assessed the salinity tolerance envelopes for soft corals (Malacalcyonacea). Previous work with hard corals (Scleractinia) suggests that salinity tolerances vary between coral species, but there are consistently high mortality rates when exposed to salinities of ~ 17 PSU for up to 48 h (Edmondson 1928; Coles and Jokiel 1992; Moberg et al. 1997; Berkelmans et al. 2012). Floods in urbanised areas can also increase contaminants introduced to estuaries such as nitrates (Sigleo and Frick 2007), sewage (Hart et al. 2020), metals and metalloids (Conrad et al. 2020), pesticides (Laicher et al. 2022) and per-and poly-fluoroalkyl substances (PFAS) (van Leeuwen et al. 2019). Pollutants can have deleterious effects on marine organisms throughout the food web, potentially resulting in fish kills (Adams et al. 2020) and morbidity or mortality of benthic fauna (Diaz and Rosenberg 2001; Jamal et al. 2022).

During 2021–2022, three La Niña-related flood events resulted in the influx of large volumes of freshwater into embayments and estuaries on the south-east coast of Australia (Bureau of Meteorology 2021a, b, 2022). The reduction in salinity associated with these events substantially impacted shallow reef communities, including kelp habitats (Davis et al. 2022), and here we detail their impact on Dendronephthya australis, an endangered species of soft coral in Port Stephens, NSW, Australia; the only location where this species has previously been recorded in large abundances (Poulos et al. 2013, 2016).

Dendronephthya australis is a habitat-forming species, endemic to the temperate waters of south-eastern Australia (Verseveldt and Alderslade 1982). For over 50 years, aggregations of the soft coral provided habitat for a large range of marine species (Verseveldt and Alderslade 1982), including the Endangered White’s seahorse Hippocampus whitei, and juveniles of the fishery-important Australasian snapper Chrysophrys (Poulos et al. 2013; Harasti et al. 2014; Corry et al. 2018). Biodiversity of rocky reef fauna has previously been correlated with structural benthic complexity (Fulton et al. 2016; Quaas et al. 2019), particularly within estuarine environments (van Lier et al. 2017), with structure providing refuge sites for both vertebrates and invertebrates, thus contributing to a more complex food web (Alvarez-Filip et al. 2011). Soft coral aggregations not only provide structure and shelter for fishes (Poulos et al. 2013; Harasti et al. 2014), but they also host a range of epifaunal organisms such as small crustaceans and molluscs (Cúrdia et al. 2015; Davis et al. 2018; Steinberg et al. 2020; Finlay-Jones et al. 2021; Larkin et al. 2021a), which are often prey for small fishes (Hacunda 1981; Edgar and Shaw 1995).

Prior to the 2021–2022 flood events, the Port Stephens D. australis population was in rapid decline at two of the four key aggregation sites in this estuary (Larkin et al. 2021b). Subsequent to this documented decline, the geographically restricted soft coral was listed as endangered in 2021 (NSW Fisheries Scientific Committee 2021) and a top 100 priority species within the Australian Commonwealth’s Threatened Species Action Plan in 2022 (Department of Climate Change, Energy, the Environment and Water 2022). Despite the overall decline of D. australis in the Port Stephens estuary, the two remaining aggregations covered an areal extent of ~ 7,900 m2 (Larkin et al. 2021b), comprising thousands of healthy colonies.

This study details findings from a tagging programme that provided insights into natural growth and attrition rates of D. australis colonies between 2019 and 2021. Observations during this programme captured the effects of a major flood event on the soft coral population in 2021, triggering additional mapping efforts throughout 2021–2022 to document the estuary-wide flood impacts. Results from the tagging and mapping programmes detailed in this study provide a greater understanding of population dynamics and highlight the vulnerability of this endangered species to severe weather events.

Methods

Study Site

Port Stephens is a large (135 km2) estuary, located ~ 250 km north of Sydney, NSW (Fig. 1). The estuarine embayment is situated within the 4,480 km2 Karuah catchment, with the largest freshwater inflows from the Karuah River. Channels on the southern side of the Eastern Port provide a pathway for strong tidal currents, which supply food and nutrients to a plethora of suspension-feeding organisms (Poulos et al. 2013; Davis et al. 2015a). Previous studies identified four aggregations of D. australis in the embayment at Sandy Point, Redpatch Point, Pipeline and Nelson Bay Beach (Fig. 1); these are the primary focus sites for ongoing studies. Each of these locations once hosted hundreds to thousands of colonies just 4 years prior to this study (ML, Pers. Obs). As D. australis is a gonochoric broadcast spawner (Larkin et al. 2023b), clustering of colonies is important for the persistence of the species. The presence of multiple neighbouring colonies increases the chance of representation by both sexes, thus increasing the likelihood of successful sexual reproduction. The definition of an ‘aggregation’ within this study, and in Larkin et al. (2021b), is defined as a cluster with a minimum of five colonies within close proximity to each other (≤ ~ 3 m2). A video demonstrating a typical aggregation can be viewed in the Supplementary information (SM1).

Fig. 1
figure 1

Key locations inside the estuary where Dendronephthya australis aggregations were found historically (in black), and other significant locations within this study (in grey); Port Stephens, NSW, Australia

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Colony Growth and Survival

To determine lifespan and growth rates of colonies within the Port Stephens D. australis population, we carried out monthly tracking of colony growth and survival over an 18-month period (September 2019–February 2021) at the four sites with D. australis aggregations (Fig. 1). Galvanised steel pegs with cattle tag identifiers were installed amongst colonies at the four aggregation sites. A total of 99 colonies were tracked, comprising of 29 colonies at Redpatch Point, 29 at Sandy Point, 27 at Pipeline and 14 at Nelson Bay Beach. To monitor growth, all crowns of colonies immediately adjacent to the pegs were measured in a north–south, then in an east–west direction. These diameters were used to calculate the average crown diameter. As D. australis colonies have hydroskeletons that expand and contract within tidal cycles (Davis et al. 2015b), measuring colony size and growth is a challenge (Fabricius et al. 1995). To reduce this variability between surveys, diameters were recorded at slack high tide every 4–6 weeks, to determine average colony growth rates over the 18-month timeframe. Polyp counts have previously been identified as an optimal method of measuring growth rates of Dendronephthya (Fabricius et al. 1995); however, this method is difficult to apply to large colonies comprising > ~100,000 polyps (Larkin et al. 2023b), and was therefore considered as unfeasible for this study. To monitor survival, top-down photographs were obtained at each tag site (Fig. 2), and colony reference IDs were assigned to individual colonies. The depth of sand (against the tag peg), colony reference ID, and the average diameter of surviving colonies were recorded in each survey.

Fig. 2
figure 2

A top-down photograph example of the Dendronephthya australis monitoring tag C8, with a scalebar, compass, and tag in the image. Colonies were labelled with reference IDs according to their position adjacent to the tag (e.g. C8W: C = Sandy Point, Corlette; 8 = Tag 8; W = West)

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Mapping

To define the impact of floods on the areal extent of D. australis aggregations, mapping was conducted at the aggregation sites at Redpatch Point and Sandy Point, both before the flood event in February 2021 and after the floods in April 2021, November 2021 and July 2022. Aggregations at the Pipeline and Nelson Bay Beach sites were not included as they disappeared during 2019 (Larkin et al. 2021b). Mapping was conducted using diver-towed video transects, using an Olympus TG6 camera in PT-059 housing. Additionally, more remote locations were mapped using a SplashCam Deep Blue vessel-based towed video camera (www.splashcam.com/product/deep-blue-pro/). Both diver and vessel-based mapping utilised a Garmin GPS (GPSMAP 78SC) to determine GPS positions. These data were compared against transects conducted in all areas where D. australis aggregations were found, from an extensive mapping program conducted in 2019 (Larkin et al. 2021b).

Flood Events Data

To obtain salinity and temperature data at the depth and location of a thriving soft coral aggregation, a HOBO U24-002-C Conductivity Data Logger (https://www.onsetcomp.com/products/data-loggers/u24-002-c) was installed at Redpatch Point (Fig. 1) in early March 2021. Data were retrieved from the data logger from 8/3/21 to 3/4/21 to assess the impacts of the March 2021 flood event on these water parameters. As the remaining D. australis aggregations were < 2 km apart at this time, and in the same depth range (5–8 m), one logger at the site furthest from the Karuah River discharge point (Redpatch Point) was deemed sufficient to provide salinity data for this event. To analyse the magnitude of the flood events during 2021–2022, river discharge data were accessed from Water NSW (https://realtimedata.waternsw.com.au).

Water depths were obtained from 2018 bathymetric data (NSWDPIE 2019), which also formed the basis of slope and current velocity calculations. Maximum velocity values were modelled using the TELEMAC 2-D hydrodynamic analysis package (Hervouet 2000; www.opentelemac.org). Slope values were obtained using the ‘surface’ function in ArcMap 10.5.1. ‘Distance from shore’ and ‘distance from estuary mouth’ were calculated in ArcGIS Pro (v.2.9.0), using the ‘Multiple ring buffer’, and ‘Near’ functions respectively. Random points (minimum 0.5 m apart) were produced using the ‘Create random points’ function in ArcGIS Pro (v.2.9.0), simulating D. australis colony presence in 2019 and 2021. These points were used to intersect the variable rasters using the ‘Extract multi values to points’ tool in ArcGIS Pro, providing the basis for modelling.

Analyses

To compare survival of D. australis colonies by site over the 18-month study period, Kaplan-Meier survival estimation analyses were conducted in R, using ‘survival’, ‘survminer’ and ‘dplyr’ packages (RStudio Team 2020). Linear regressions of changes in the average diameter of individual colonies were plotted to visualise trends of growth between survey months for surviving colonies at each site.

Distance based linear models (DistLM) were generated in PRIMER7 (Anderson et al. 2008) to determine which variables best-explained coral losses between 2019 and 2021. Five explanatory variables were examined based on their previously identified importance to presence of D. australis (Poulos et al. 2016; Larkin et al. 2021b). These comprised depth; distance from shore; distance from estuary mouth; slope; and current velocity. All five variables were considered in models using Euclidean distance similarity matrices with the ‘BEST’ model selection procedure to identify the most parsimonious model, using AIC selection criteria (Anderson et al. 2008). The contribution and statistical significance of each variable was determined using marginal tests. All variables tested were within the acceptable co-linearity range (< 0.7, and > −0.7) (Anderson et al. 2008).

Results

Colony Growth and Survival

The average D. australis colony diameter varied between sites at the start of the soft coral monitoring program in September 2019. Site average colony diameters (cm ± SE) were Sandy Point — 16.0 ± 1.3 cm; Redpatch Point — 21.2 ± 2.0 cm; Pipeline — 10.2 ± 1.2 cm; and Nelson Bay Beach — 11.1 ± 1.4 cm.

D. australis colonies at Sandy Point and Redpatch Point survived for the entire pre-flood study period (September 2019–February 2021), and colony diameters increased over this period (Fig. 3). By contrast, colonies at Pipeline were last observed alive after 189 days of monitoring, and declined in diameter during that time. Similarly, colonies at Nelson Bay Beach were observed alive for 258 days, and diameters declined over that period (Fig. 3).

Fig. 3
figure 3

Linear regressions (+95% CI) of average % change in Dendronephthya australis colony diameter at four locations within Port Stephens estuary

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Kaplan-Meier survival analyses showed a significant difference (P < 0.001) in survival of colonies between the four study sites over the 18-month pre-flood study period. There was a gradual decline in the number of colonies at all sites due to mortality (Fig. 4). Average mortality rates per day were 0.47% at Pipeline, 0.35% at Nelson Bay Beach, 0.18% at Sandy Point, and 0.17% at Redpatch Point. At Sandy Point and Redpatch Point, mass mortality of the remaining colonies coincided with the flood event in March 2021 (Fig. 4).

Fig. 4
figure 4

Kaplan-Meier survival plot of the four key D. australis aggregation sites within Port Stephens, showing survival of colonies monitored over a 600-day period. The red arrow and brackets indicate the timing of the March 2021 flood event (surveys were not completed on the same day at each site)

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Mapping

Mapping conducted after flooding, in April–November 2021, indicates that the areal extent of D. australis aggregations had fallen to ~ 851 m2 (Fig. 5), a ~ 91% decline compared with the pre-flood areal extent in February 2021 (9,300 m2) (Larkin et al. 2021b). This equates to just 3% of the area of aggregations in 2011 (28,600 m2) (Poulos et al. 2016). Bare sand was the only visible substrate at the locations where colonies were previously found. Of the estimated 6,140 colonies that were present in the Port Stephens estuary in 2019, only ~ 185 remained after the 2021 flood event (ML, pers obs). Particularly noteworthy was the complete absence of D. australis colonies in water shallower than 8 m after the March 2021 flood. Subsequent mapping in June 2022 confirmed the complete disappearance of these remaining aggregations and, and as of September 2023, there are no longer any D. australis aggregations within the Port Stephens estuary, just isolated individual colonies. Ad hoc diving surveys conducted between June 2022 and September 2023 confirm that no new aggregations have formed.

Fig. 5
figure 5

Aggregations at (A) Sandy Point and (B) Redpatch Point showing the pre-flood Dendronephthya australis areal extent (purple shading) in Feb 2021 and post-flood in April 2021 (red shading). By July 2022, all D. australis aggregations had disappeared

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Flood Events

Data from the Port Stephens region show that the rainfall in March 2021 and the associated flooding was a historic weather event (Bureau of Meteorology 2021). The data show that Nelson Bay received a total of 649 mm rainfall over ten consecutive days (March 15–24 inclusive). Discharge data from the estuary’s largest freshwater source (the Karuah River) indicate that March 2021 was the third largest month of average daily discharge, recording a rate of 9,623 megalitres per day (ML/day) (Fig. 6). The next largest month of discharge since records began in 1969 was in 1990 with an average rate of 9,997 ML/day, and the largest 15,399 ML/day in 1971. La Niña-related rainfall continued throughout 2022, with further flooding in the Karuah catchment in March and July, as indicated by high discharge rates from the Karuah River (Fig. 6). Since records began in 1969, the 2-year Karuah River rolling average discharge rate is 1.4 million ML. The 2021–2022 period saw a record of 5.3 million ML; exceeding the previous record from 1989 to 1990 (4.1 million ML) by ~ 1.2 million ML.

Fig. 6
figure 6

Discharge rates from the Karuah River; the largest upstream river from the Port Stephens estuary during 2021–2022. Darker bars indicate the average flow rates over the months that flooding occurred (March 2021, March 2022, and July 2022)

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Prior to the flood event in March 2021, salinities from 6 m depth at Redpatch Point were essentially marine 33–35 PSU (Fig. 7). During the flood event, salinity reached a low point of 6.9 PSU (measured over a 30-min period) on 22nd March 2021, which was the seventh day of the rain event (Fig. 7). Fluctuations to levels below 10 PSU occurred for 3 days, returning to ‘normal’ (with all salinity readings > 30 PSU) from 29th March, some 14 days after the heavy rainfall began. The water temperature ranged from 18.8 to 21.9 °C during this rainfall event (Fig. 7), which was within the typical range for Port Stephens at this time of year (Davis et al. 2017).

Fig. 7
figure 7

Salinity and temperature in Port Stephens estuary over a 3-week period, which included the dates of substantial rainfall from 15th to 24th March, 2021

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Environmental Drivers of Coral Losses During Flooding

Marginal tests demonstrated that all variables except the average maximum velocity were significant, but most parameters explained a small proportion (< 10%) of the variation in coral loss — the exception being depth which explained 43.5% alone (Table 1). The most parsimonious (‘BEST’) DistLM, to explain soft coral loss following the flood in March 2021 included three variables; depth, distance to shore and slope, and accounted for 45.68% of the variation in coral loss (Table 2).

Table 1 Marginal tests for the relationship between individual environmental variables and soft coral loss between 2019 and 2021
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Table 2 Results of ‘overall best solutions’ for DistLM models assessing variables influencing coral loss between 2019 and 2021
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Discussion

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 still 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 851 m2 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 Port Stephens, 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. In contrast, within the aggregations at Redpatch Point and Sandy Point prior to the flooding, colonies increased in size and the overall areal extent remained stable between late 2019 and February 2021. 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). As salinities were measured at Redpatch Point, which was the easternmost aggregation site (furthest away from the Karuah River), it is highly likely that the salinity levels were similar, or lower, at Sandy Point (the aggregation site closer to the freshwater source, and furthest away from the ocean). These findings are consistent with the impacts recorded on kelp at shallow depths (< 10 m) at multiple sites in the Port Stephens estuary as a result of the March 2021 flood event (Davis et al. 2022). The severity of impacts on kelp decreased at sites closer to the entrance to the embayment (Davis et al. 2022), supporting the hypothesis that the freshwater ingress (and any associated contaminants) had a greater impact on the westernmost sites in the estuary.

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 h 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 information 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. As D. australis is an azooxanthellate soft coral, the impact of turbidity is likely to be much lower than for zooxanthellate corals containing symbiotic algae that are reliant on photosynthesis (Bessell-Browne et al. 2017). However, sediment movement in the water column can impact the ability of azooxanthellate corals to feed (McClanahan and Obura 1997), and can affect larval survival and settlement success (Erftemeijer et al. 2012).

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 have 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. 2023b), 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). A recent study provides evidence that D. australis is a gonochoric broadcast spawner (Larkin et al. 2023b). 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 D. australis soft coral is on the brink of extinction in the Port Stephens estuary, which previously supported by far the largest population of the species. Surveys conducted by other researchers confirmed that small aggregations previously present in Brisbane Water (Vincent Raoult, pers comm, 2022) and in Sydney (John Turnbull, pers comm, 2022), NSW also disappeared after these flood events. However, some aggregations have started to reestablish in Sydney (John Turnbull, pers comm, 2023). As of September 2023, the largest known aggregation of large (≥ 30 cm average diameter) and healthy colonies is located on the scuttled navy frigate ex-HMAS Adelaide, near Terrigal, NSW. With access to these remaining aggregations as a source of potential donor cuttings, investigations are currently underway to determine the feasibility of restoration using the transplantation of cuttings (Larkin et al. 2021a, 2023a). Preliminary trials using lab-based in vitro fertilisation techniques with gravid fragments also show promise for future remediation measures to help D. australis populations recover in the Port Stephens estuary and to ensure the species’ ongoing survival in the wild (Larkin et al. 2023b).