|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.
- 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
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.
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.
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
Remains of wooden farmhouse, west side of Lagoon of Islands