Download LAKE CARGELLIGO – ENVIRONMENTAL ASSETS

LAKE CARGELLIGO – ENVIRONMENTAL
VALUES
The Lake Cargelligo system is made up of a number of different ecological
components which add to its value as a wetland. The main ecological
components include:
Open Water areas
(eg. Curlew Water and the front recreation area of
Lake Cargelligo)
River Red Gum Swamps
(eg. the back area of Lake Cargelligo)
Black Box Woodlands
(eg. located between Lake Cargelligo and the
Lachlan River)
Reed Beds
(eg. located in the back area of Lake Cargelligo)
Mudflat areas
(eg. Sheet of Water, McInnes Basin and Lake
Cargelligo);
Lignum Swamps
(eg. back area of Lake Cargelligo)
Permanent River Channel (Lachlan River).
The characteristics and functions of these assets are described below.
Open Water
There are two significant open water lakes in the Lake Cargelligo system.
Curlew Water and the front recreational area of Lake Cargelligo provide open
water habitats to 3-6 m in depth. Under normal conditions, both lakes are
filled by the delivery of river water via the inlet channel, with water first
flowing through Curlew Water, via Sheet of Water, and then onto the main
lake. Throughout the history of regulation, both these areas have generally
remained permanently inundated, however, under severe drought
conditions, they may be drawn-down to dead storage levels.
These areas provide important habitat for large bodied fish species and
waterbirds and, as they tend to be more permanent water bodies within a
variable landscape, they can provide drought refuge for a variety of plant
and animal species. Open water areas, or pelagic zones, can be divided into
the euphotic zone, measured from the surface to where light ceases to
penetrate, and the profundal zone, where light does not penetrate. Algae
tend to dominate in the euphotic zone, where light penetrates, and provide
food for zooplankton and fish. The benthic zone, or lake bottom, also
provides food and habitat through sediments and accumulated detrital
material.
River Red Gum Swamps
Swamps occur throughout the floodplain of the lower Lachlan and are often
distinguished by the presence of forest or woodland, generally dominated by
river red gum (Eucalyptus camaldulensis). River red gum woodlands provide
important functions for the lower Lachlan as they encourage the growth of
understorey plants, largely through shading, and supply habitat and food for
many animal species. Riparian river red gums also influence nearby aquatic
areas through shading and by inputs of litter and woody debris. River red
gum woodland also influences water flow and aids channel stability.
River red gums tend to occur along the margins of channels and at higher
elevations than reed beds. While mature trees can tolerate prolonged
flooding (up to 4 years), seedlings are less tolerant, particularly if they are
completely submerged for more than a few weeks. For this reason, it is
generally thought that a flooding frequency of once every one to three years
and inundation periods of 4-7 months is ideal to maintain the health of
mature trees, support successful regeneration and limit encroachment by
other vegetation types.
Changes to hydrology are likely to impact on the health, distribution and
extent of river red gums. Over the last few years of drought it is likely that
reduced water availability has led to the decline in river red gum health and
recruitment. Changes to grazing regimes can also influence the long term
health and extent of these communities, particularly by reducing recruitment
success. Red gums are also harvested throughout the lower Lachlan.
Black Box Woodlands
Black box (Eucalyptus largiflorens) woodlands tend to be less dense and age
diverse than that of river red gum forests. The understorey of black box
woodlands are often dominated by a high diversity of chenopod shrubs
including old man saltbush (Atriplex nummularia), nitre goosefoot
(Chenopodium nitrariaceum), curious saltbush (Dissocarpus paradoxus),
ruby saltbush (Enchylaena tomentosa), black bluebush (Maireana
pyramidata), creeping saltbush (Rhagodia spinesens) and Sclerolaena spp
(eg. Roly-Poly).
Like reed beds and river red gum swamps, black box distributions are
controlled by flow patterns. They are more tolerant of water stress than river
red gums and less tolerant of long periods of flooding. Black box woodlands
are therefore found in more elevated areas and appear to be better suited to
areas with flood frequencies of one in three to one in five years, with a
wetting period of two to four months.
Black box woodlands provide a habitat for understorey plants and terrestrial
animals but have less interaction with aquatic communities as they are less
frequently flooded and are usually a greater distance from river channels. As
mature black box trees have a greater resistance to drought, they have a
greater ability to survive reduced water availability frequentlyly experienced
in the lower Lachlan.
Reed Beds
Common reed (Phragmites australis) is one of the dominant aquatic species
in the lower Lachlan. Reed beds are absent from the deeper areas of the
lake system where other aquatic macrophytes such as Typha and ribbon
weed may dominate. The upper limit of Phragmites distribution varies with
flood history, and their distribution will expand and contract in response to
water availability. However, as Phragmites most commonly re-establishes
via growth from rhizomes rather than seed, re-establishment following
drying may take some time. Expansion is most rapid when flooding occurs
early in the growing season.
Reed dominated areas are productive and contribute to primary productivity
within a system directly through their own growth or indirectly by providing
substrate for attached algae. Reeds also reduce flow velocity, thereby
reducing available nutrients and turbidity in the water column. Reed beds
also provide habitat for aquatic animals, as they are structurally complex
and reduce water turbulence, making them important nursery areas for fish
by providing protection from predation and strong currents.
Reed beds also provide feeding and breeding habitat for waterbirds,
particularly following large floods. These areas are commonly used by
Australasian bittern, spoonbills and ibis. They also provide pasture for cattle
and are known to help recharge groundwater systems. This recharge service
provides a mechanism for the export of salts that may otherwise accumulate
in soils as a result of evaporation. Reed beds also reduce erosion through
lowering flow velocity and binding sediments.
Threatening processes such as grazing, deteriorating water quality (eg.
salinity) and burning have the potential to reduce the extent of this
community and perhaps accelerate any changes occurring in response to
hydrological change.
Mudflats
Mudflats are unvegetated areas in shallow or receding waters, which
increase in occurrence during the wetland’s drying phase. These areas
support high levels of production by benthic micro-algae (eg. diatoms) and
invertebrates, nutrient regeneration, decomposition of organic matter,
thereby providing feeding habitat for wading birds.
The primary production of shallow-water algae that lives on the sediment,
has been found to equal that of the overlying water column, indicating that it
plays an important role in keeping wetlands productive. Micro-algae are also
crucial to nutrient cycling, particularly carbon cycling and are important in
maintaining sediment stability.
Some of the migratory waders known to utilise Lake Cargelligo wetlands,
particularly the mudflats, include the red-necked stint, curlew sandpiper,
pectoral sandpiper, sharp-tailed sandpiper, Latham’s snipe, painted snipe,
bar-tailed godwit and black-tailed godwit.
Lignum Swamps
Lignum (Muehlenbeckia florulenta) occurs both as an understorey plant in
open woodlands and as the dominant species in small patches of shrubland
near water-ponding areas. These shrublands can be particularly important
for waterbird breeding following flooding.
During dry periods, few understorey species may be present in areas of
lignum swamps. Those that have been recorded include branching groundsel
(Senecio cunninghamii), Medic (Medicago spp.) and canary-grass (Phalaris
paradoxa). Understorey diversity and abundance is likely to be significantly
greater, however, following flooding of these communities.
Lignum communities appear to tolerate longer dry periods and respond more
quickly to wetting than other communities such as reed beds, and are
capable of withstanding longer periods of flooding than river red gum.
Lignum areas are particularly important for breeding amongst straw-necked
ibis and glossy ibis and provides habitat for aquatic fauna during wet periods
and other terrestrial fauna during dry times.
Change resulting from water resource development or climate change would
be expected to alter the distribution of lignum as an understorey species in
river red gum woodlands, where reduced flooding frequency may result in
the replacement of lignum by saltbush and other chenopods as often found
associated with black box woodlands.
Permanent River Channel
The lower reaches of the Lachlan River contain a variety of habitat types
including pools, riffles, gravel beds and some sand beds. The channel has an
extremely low gradient and greatly meanders. The Lachlan below Wyangala
Dam has been declared an Endangered Ecological Community (DPI 2005)
and many features of the riverine system have been identified as having
high conservation value.
Endangered Ecological Community
It should be noted that the lower Lachlan River is currently listed as an
endangered ecological community. This includes all fish and aquatic
invertebrates within all natural rivers, creeks, streams and associated
lagoons, billabongs, lakes, wetlands, paleochannels, floodrunners, effluent
streams and the floodplains of the Lachlan River, including Lake Cargelligo.