Restoration of D’Arcy’s Lagoon: More good news for wetlands on Bruny Island (Tas)
In the newsletter last month, Bec introduced NGT’s wetland restoration program on Bruny Island, situated off the south-eastern Tasmanian coast, south of Hobart. This month, we’re sharing the story of the subsequent restoration works that have now been successfully completed at D’Arcy’s Lagoon, situated a short distance to the north of Fairyland, which was the focus of last month’s wetland restoration story.
Background story of the site
Despite being situated nearby, the wetland environment at D’Arcy’s Lagoon is completely different in character to the seepage-fed wetlands of the dune swales at Fairyland. The Lagoon is a near-coastal, landlocked, freshwater peatland, as shown below.
Closer inspection of the 1996 image shown above, indicates that – despite water being evident in the lagoon at this time – artificial drainage works appear to have commenced not long prior to this image being taken. As shown below, the coastal portion of 1996 image shows the evidence of the drain discharging a large amount of tannin-rich (brown stained) swamp water into the sea, through the low dunes. In addition to removing surface water, the process of draining a peatland usually taps into groundwater and/or subsurface moisture, as the peat substrate slowly dehydrates and artificial drainage gradually becomes fully effective.
A short time later, the drying of the peat facilitated the physical clearance of the wetland’s native vegetation and led to an attempt to convert the area for both improved livestock grazing and peat mining / harvesting. By the time the image below was taken in December 1999, we can see that the lagoon is now heavily disturbed and noticeably dehydrated heading into the Millennium Drought.
Despite these works, farming and peat mining appears to have only continued within the lagoon for less than a decade. Then, when the property changed hands around 2005, when the image below left was taken, these activities ceased. By 2009, below right, some inundation and recovery of wetland vegetation is again occurring, with conservation being the primary focus for the site since that time until today.
The gradual reversion of D’Arcy’s Lagoon to being dominated by wetland vegetation that has since occurred, is a common phenomenon in drained peatlands. This is because unless drains are fully maintained and/or deepened over time through a peatland, these areas often start to re-inundate or re-contact the water table as the peat (which is made up of lightweight organic material) subsides and compacts, causing spontaneous recovery of residual wetland flora from seed or tubers. However, given the changes that have occurred to hydrology and elevation, a slowly reverting peatland will often be different in character to its original state, and – if drainage is still active or partly effective – will remain drier and prone to further disturbances.
As a result of this drying trend, the peat becomes prone to desiccation, cracking and shrinkage (as shown), compaction and oxidation. This is because the dried out and aerated organic (plant) material that constitutes the peat is now available to either oxidise slowly via aerobic biological microbial consumption, or – in a worst case scenario – much more rapidly via burning.
This helps explain why the original vegetation community across the wetland recorded in 1978, prior to drainage and development, is so different to later surveys after these changes. In 1978, the bulk of the swamp was covered by common peat-forming and associated plants: scented paperbark (Melaleuca squarrosa) and buttongrass (Gymnoschoenus sphaerocephalus) (Kirkpatrick and Harwood, 1981), whereas after development this transitioned to a more mixed, post-disturbance suite of drier dominant vegetation types, such as areas of seasonally inundated herbland and sedgeland, with some residual or recovering areas of shrubland.
It is against this back-drop and despite partly inundating occasionally in the intervening years, such as 2011, that a significant area of the D’Arcy Lagoon peatland burned in a bushfire in 2020. Low intensity (cool) burning of the above-ground biomass in peatlands was a traditional cultural practice of Aboriginal people in some areas of south-eastern Australia, and can be important for the maintenance of floristic diversity (made possible under the right conditions because peat sediment itself does not burn when it is saturated). However, when the substrate is dry – as was the case at D’Arcy’s Lagoon in 2020 when the bushfire occurred, the peat is at risk of catching alight and is very difficult to extinguish. Indeed, in this instance, the peat continued smouldering and burning for months – causing air quality issues that significantly impacted local residents.
The large quantity of peat consumed by the fire (which released vast amounts of carbon dioxide into the atmosphere), has left a patchy, uneven surface that in places burned through the bulk of what was originally a relatively thin (0.5m – 1m) layer of peat to the underlying sandy substrate, as shown below. The shallow depth of peat indicates that D’Arcy’s Lagoon is a relatively ‘young’ wetland feature (in geological time), because the thickness of the peat layer is a reliable indicator of the length of time that organic material was able to accumulate during extended periods of constant saturation, over thousands of years prior to artificial drainage.
Setting D’Arcy’s Lagoon on a long-term recovery trajectory
As we’ve explored previously in the NGT newsletter, the importance of keeping our peatlands damp cannot be understated – given their critical role in both the carbon and water cycles, and as areas of high biodiversity. However, while preventing any further drainage of peatlands is a global priority, it is equally important to identify those places where we can enhance the hydrology of modified peatlands to improve their ability to stay saturated and trigger an associated positive biodiversity response.
Fortunately for D’Arcy’s Lagoon, the current owner is committed to the perpetual protection of the site for conservation and wanted to see the impacts of past artificial drainage on the wetland reversed. So in February this year, with the assistance of a local earthworks contractor, NGT was able to assist!
Although a lack of maintenance means that the drain has not remained quite as effectively as when it was constructed in 1996, it has continued to have an influence on the duration of inundation, by drawing water into the drain below the bed level of the wetland – and a portion still flowing downstream to seep away through the dunes. Three dams which had been cut into the bed of the peatland were also causing internal drawdown, dehydrating the surrounding peat profile.
Our restoration plan for D’Arcy’s Lagoon simply required the remediation of the lagoon’s bathymetry (i.e. the topography of the wetland bed) – where possible – by knitting the surface profile of the peat back together, using spoil material situated adjacent to the dams and drains constructed in the mid-1990s. Further, we proposed to backfill the main outlet drain along its full length, to prevent the possibility of any future discharge from the wetland seeping through the dunes.
Restoration works summary
An aerial overview of the site taken just after all the remediation works were completed in February 2025, is shown below. The dashed lines indicate the location of the drains that were continuously backfilled, while the three circles are where three dams were infilled in the bed of the wetland. To see the image without markings, please click through the second page.
A few different before and after photos, both ground-based and aerial, that will help give you a feel for the nature of the completed remedial works, are shown below.
First up is the main drain through the Lagoon… (click on images to enlarge)
And finally, here are images showing the remediation of a couple of the dams cut into the bed of the peatland… (click on images to enlarge)
As you will see from the images above, one of the key principles we adopt is to limit disturbance by ensuring that the earthworks footprint matches the zone originally disturbed when the works were first completed. So the only places above where you will see disturbed ground is either the area of the drain and/or dam being backfilled, plus the adjacent piles of spoil material from when construction occurred in the 1990s. The surrounding natural surface level is then used to guide reinstatement of the natural surface elevation through the remediated peatland.
With all the works completed, all we need now is for the rain to come and help inundate the Lagoon once more, re-saturating the peat and supporting the process of spontaneous ecological recovery. We look forward to sharing updates on this fantastic site in the future!
A huge thank you to the owner of D’Arcy’s Lagoon for their commitment to the site, as well as staff at the Department of Natural Resources and Environment Tasmania (who kindly assisted by providing some key site history information, plus processing permits and authorisation for the restoration works). Finally, this work would not have been possible without the generous support of our funding partners, outlined below.
This project was supported by the Tasmanian Government through the Landcare Action Grants (LAG) Program, Landcare Tasmania and NRM South
