Natural wetlands are recharged by rainfall or springs. In times of drought, wetlands may dry out. For many native plants and animals in southern Australia, these wet and dry periods are a necessary part of life.

Native wetland plants have adapted to cope with drying out by producing spores, cysts or tubers that can survive without water for a long time. In some natural wetland systems, the dry period may last for years.

As a wetland dries out, some of its fauna (such as insects) produce eggs or pupae; other fauna (such as mussels, yabbies, and frogs) burrow into the soil to wait for the next wet cycle. Some plant and animal species may even rely on dry periods for survival. For these reasons, wetting and drying cycles are used to manage constructed wetlands.

Imitating natural cycles of wetting and drying has a range of benefits. Dry periods help to ensure the wetland soils maintain their ability to absorb nutrients, particularly phosphorus. They also allow oxygen into the soil which aids the decomposition of plant material, increasing soil fertility.

Wetting and drying cycles are also useful in controlling bacteria, weeds and invasive fish such as European Carp. Being bottom-dwellers, Carp stir up the bed of wetland ponds and generate turbid (muddy) conditions. The muddy sediment smothers submerged plants and prevents light from reaching them. Algal blooms and plant dieback can result, which may severely damage the health of a wetland.

Historical Vegetation Associations

River Red Gum (Eucalyptus camaldulensis) open woodland dominated the gently sloping alluvial soil below the 310m contour. Wattle (Acacia sp.) and Yacca (Xanthorrhoea semiplana) were present in the midstorey, over a tussock grass and herb understorey.

Above the 310m contour, the soil type becomes Sturt Tillite comprising quartzites and tillites, locally pebbly with interbedding siltstones. Though River Red Gums were still present, the South Australian Blue Gum (Eucalyptus leucoxylon) and the Manna Gum (Eucalyptus viminalis) dominated. Manna gum (Eucalyptus viminalis) is found in the wetter parts of the hills to the west of Mt Barker and grades out below 750mm annual rainfall, while at this rainfall the rough barked Manna gum (Eucalyptus viminalis ssp. cygnetensis) grades in. The township of Mt Barker lies in this transition zone.

Current Vegetation Associations

The Laratinga site has been classified into six broad vegetation associations:

1. Creek line - Remnant Eucalyptus camaldulensis +/- Planted Mixed Woodland
2. Mixed Planted Eucalyptus sp. +/- Acacia sp. Low Closed Forest
3. Common Reed (Phragmites australis) +/- Bulrush (Typha sp.) Sedgeland
4. Exotic Grassland
5. Mixed Aquatic Reeds and Rushes
6. Scattered Trees.

Ecosystem Functions and Qualitative Water Requirements

Within the wetland, structural diversity in aquatic habitat has been achieved through marsh, rush and reed plantings ranging from 30 cm to >2 m tall. Plantings consist predominantly of emergent macrophytes including Phragmites, Eleocharis, Cyperus, Baumea, Bolboschoenus, Triglochin and Juncus. These wetland plant species are adapted to grow at different water depths, generally from 0.5 m below to 0.2 m above design water storage level. Most will grow in permanent water and tolerate periodic temporary dry conditions. 

Basin 1 features tall, dense reed beds of Phragmites australis, which acts as a water filter and provides a visual barrier.

The shallow waters of Cells 4-7 in Basin 2 and Cells 9-11 in Basin 3 are intended to support dense macrophyte beds. These cells are characterised by scattered Triglochin procera, swathe plantings of Bolboschoenus caldwellii and reed species such as Juncus spp. in the littoral (edge) zone.

Emergent and submerged plants provide habitat and shelter to support macro invertebrate populations and a growth medium for algae, in turn providing food supply for other species and shelter and breeding locations for amphibians, birds and fish. The plants and algae also assist in maintaining good water quality (through oxygenation of the water and sequestration of nutrients), thus reducing the risk of water quality related problems.

Cell 8 in Basin 2 and Cell 12 in Basin 3 are deeper open bodies of water, with a thin strip of reeds around the periphery. The fringing reeds on the outskirts of Cell 8 in Basin 2 and Cell 12 in Basin 3 allow views across these ‘open water’ habitat features to the islands in each cell, whilst discouraging the public (and predatory pest animals) from accessing the islands. The deeper, open water provides an important habitat for birds. The swathe vegetation and the thick reed cover around the outskirts of Cell 8 in particular provides an equally important protected area for birds and aquatic life and a different sensory experience for visitors.

The current distribution of wetland vegetation represents the planting history, species tolerance levels to water depth and quality and their relative dominance. The result is considered quite successful, in achieving both visual and habitat diversity.

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