Article and photos by Dr Keith Corbett
Eastern side of Lagoon of Islands, with sand dunes, beach, recent clump of Oxylobium ellipticum
The APST walkers group enjoyed a delightful walk around Lagoon of Islands, on the Central Plateau near The Steppes, on Sunday 25 March, 2012, having previously visited the Lagoon in March 1999. The lagoon is roughly circular, about 3.5 km diameter, and is nestled within dolerite hills. A long beach on the eastern side is backed by old sand dunes which support a woodland of large old snowgums ( E. coccifera, E. pauciflora and E. rodwayi). There are farmstead ruins on the western side. The lagoon is a haven for waterbirds – particularly swans and ducks - and is much frequented by the local population of introduced fallow deer. Several scatters of artefacts in the dunes area indicate occupation by Aboriginal people.
The lagoon was noted for its unique system of floating islands of vegetation, with tea tree and bottlebrush clumps (and scattered eucalypts) growing on floating reed beds. Its importance was not recognised until too late, however, and activities by ‘the Hydro’ over several decades have greatly affected the lagoon, mostly detrimentally. The works have not really been successful, however, and in recent times there has been a change of direction towards rehabilitation, with some hope for the future. Here is a summary history of this unique and important place.
- The lake basin is quite old, geologically, with clear evidence of a much larger lake and higher water levels in the distant past, and of considerable fluctuation in water levels in more recent times. The sand dunes were probably formed during glacial times (10-20,000 years ago), when most of the highland lakes dried up and the prevailing winds eroded sand from the lake bottoms and deposited it along the downwind side. Permian sandstone and conglomerate outcrop in several places near water level around the northern side of the lagoon, suggesting that a layer of these horizontally bedded sedimentary rocks was important in determining where the lake basin originally formed. The sandstone would also have been the source of most of the sand in the dunes, although many other plateau lakes have dunes and beaches formed of dolerite sand. The sand dunes have clearly migrated downwind away from the lake over time, possibly by as much as 60 m.
- Pre-1964: Lagoon of Islands was a shallow swamp-like wetland with a nearly continuous cover by a floating mat of intertwined rhizomes of Baumea arthrophylla (fine twigsedge) and Chorisandra australis (southern bristlesedge). Scattered over this were mats colonised by Carex apressa (tall sedge), and some of the larger of these mats had developed sufficient soil to be colonised by shrubs (Leptospermum lanigerum and Callistemon viridiflorus as was – now Melaleuca virens) and even scattered eucalypts (species uncertain) to form the unique islands. The importance of this vegetation type was not recognised at the time, as noted in an article by Peter Tyler ( Tasmanian Year Book 1976) - the Lake Pedder controversy occupied most people’s attention at the time.
- 1964: The HEC built a dam on the lagoon and raised the level by 2m or so. The dam was to supply irrigation water to property owners down the Ouse system, as an alternative to water redirected when Great Lake was diverted through Poatina. It also provided a new fishing lake. The raised water level caused the gradual demise of the reed beds and death of the island vegetation. But the lake level declined again, and could not supply the riparian demand. The dead remnants of the former green islands are easily visible from the present shore.
View of Lagoon from the north showing remnants of original islands
- 1984: The HEC constructed the Ripple Canal to supply extra water to the lagoon from creeks on St Patricks Plains, to the north. This increased the yield but also increased the nutrient levels of the water (as did the decaying vegetation), to the point where it became unsuitable for irrigation. The lagoon was covered in a massive algal bloom from 1989 to 1992. Meanwhile Triglochin procerum (greater waterribbons), which had always been present, began to expand across the lagoon, and this process has continued to the point where this plant covers much of the water area.
- Some improvement in water quality was achieved for a period after 1992, but this was shortlived and by the end of 1996 the lake was again eutrophic. The supply of irrigation water ceased in 2009 after another algal bloom. On a positive note, it was observed that Baumea arthrophylla was starting to recolonise from 1998.
Two recent ‘islands’ of Baumea arthrophylla near the northern end of the lagoon.
Note also the abundant Triglochin offshore, and the dried organic matter at the shoreline.
- 2010: Ripple Canal was decommissioned, as part of a process to rehabilitate the lagoon. The Baumea continues to appear in clumps over the lake, but the Triglochin remains the dominant vegetation. The original dam and silted outflow area are to be removed in the near future, to bring the lagoon back to its original level, with the hope that the unique vegetation system may re-establish. Trout fishing is now banned. At present the lagoon is something of a sorry sight, with large amounts of black organic matter littering the beaches and foreshore – the dried remnants of the algal bloom mainly – and lots of shallow muddy water around the edges. However, much of the beauty and serenity remain, as does the special feel of the place which endeared it to so many people, no doubt including the Aboriginals.
- Reference: Carolyn Maxwell and Peter Tyler. 2006. Recolonisation of Lagoon of Islands, Tasmania, by Baumea arthrophylla: the first steps in regeneration of a unique ecosystem? Papers & Proceedings Royal Society of Tasmania, vol 140, 31-34.
Remains of wooden farmhouse, west side of Lagoon of Islands
