|The ongoing story of a special water
body with a unique vegetation system
Article and photos by Dr Keith Corbett
side of Lagoon of Islands, with sand dunes, beach, recent clump of Oxylobium
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.
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
View of Lagoon from the north showing remnants
of original islands
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
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
were removed in April 2013, 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.
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