Causes of environmental impacts

"Rich in aquatic flora and large in size, this Swamp has undisputed ecological value with first class assemblages of native species in parts. However, diversity and abundance of native flora in large areas of the Swamp have been and continue to be seriously reduced and harmed by many human activities in and surrounding the Swamp." (Sainty 1997)

According to Geoff Sainty of Sainty and Associates, the two largest and most important impacts on the swamp have been:

Drains cut through Swamp
Extraction of Peat

Why was this done:

To reduce unhealthy wet conditions for cattle. To improve potential for grazing. To reduce stock disease. To open up land for farming. To bring water through the Swamp for human use. To improve access to remove peat (drains were cut at close spacing in the area immediately to the south west of the peat extraction pool).

Effect on Swamp:

Increase in speed of surface flow through the Swamp. More rapid dispersal of weed seeds and weed propagules through the Swamp. Lowering of water quality by "short-circuiting" around the Swamp (i.e. water is not forced to spread right across the Swamp but allowed to follow a drain).

Rectified by:

Blocking/filling drains and redirecting flow more evenly across the Swamp.

Extraction of peat

Effect on Swamp:

The small, surface extraction (downstream of the current extraction pool) left scars that have only partially revegetated after many years. The present large scale operation has left a deep pool (down to about four metres), containing turbid water with poor aquatic plant diversity. The deep pool has altered the water flow through the Swamp; upstream has become drier and downstream wetter. There is increased speed of movement of weed seeds and propagules downstream by dispersing them across the extraction pool. Increased spread of aquatic weed into the mid region of the Swamp by the introduction of machinery carrying weed seeds and pieces (agricultural or construction machinery and boats contaminated with weed seeds/pieces is one major reason for accelerated aquatic weed problems in wetlands). None of the surface vegetation cleared from the peat has been effectively returned to the site; this has created an aesthetic imbalance in a swamp that had little open water.

Rectified by:

There is little that can be done to fully rectify the problem. The pool is too large to back fill, and if it was filled with material other than peat it would alter the Swamp sub-surface hydrology. It is too deep to revegetate with emergent species. It contains water that is evidently unsuited to many submerged plants. Once recirculation from the peat process is stopped water quality in the extraction pool may improve, and some "aquatic life" may return. The edges could be sloped and planted with transplanted aquatic vegetation but this would create a major disturbance, being difficult to physically achieve, and based on the slow regrowth in the shallow peat extraction area downstream, would be slow to respond.

Summary:

Mining peat has changed the hydrology of the Swamp immediately upstream and downstream of the extraction pool, and this can be judged by the vegetation in the vicinity. The larger and longer the extraction pool becomes, the more impact on upstream and downstream vegetation.

The peat mining pool is not a healthy place for floating, submerged and emergent macrophytes and algae; it contains none or few of the expected native aquatic plants. The depth and shape created by the present peat mining operation, and the poor water quality for aquatic life caused by mining disturbance and recirculation from processing, makes it difficult to restore. No serious attempt has been made to restore the peat excavation pool.

The main island within the peat pool is partly overrun by Willow Salix cinerea (identification subject to confirmation from flowering material) and this infestation is probably a source of spread to the rest of the Swamp. This species of Willow has produced fertile seeds elsewhere in the Sydney region; it is probable that it is setting seed in the Swamp as juveniles are scattered throughout much of the western end.

Earlier peat excavations to the south-west of the main excavation pool have been slow to regenerate surface aquatic vegetation, and serve as an example of the poor regenerative capacity of exposed peat after excavation.

Generation of peat in the future may be retarded by weed invasion— introduced weeds will not necessarily produce peat of the same quality and quantity.

The impact of mining is already significant and any increase in the size of the pool created by peat extraction will only further add to the decline of the Wingecarribee Swamp ecosystem. The proposed 7 ha extension to the north west of the present extraction pool will destroy a reasonably good mixture of primarily native vegetation; it will direct the flow even more rapidly to the north west side of the wetland, and cause the area immediately east of the extraction pool to become even drier.

In brief, the Swamp has significant areas with a good assemblage of native plants, but it also has large areas in a moderate to advanced state of change from native to exotic vegetation. The peat extraction pool is poor in macrophytes and macroalgae.

Source:

Sainty and Associates, Wingecarribee Swamp: Aquatic Vegetation Condition (Health) Restoration Issues, May 1997, pp. 2-3, 8-9.

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