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Recreation & Conservation

Can the two coexist in Thirlmere Lakes National Park? 

Ainsley Atkinson (nee Noakes)
NPA member 
PhD student with the Environmental Science Unit, University of Wollongong.

A recent university study in the Thirlmere Lakes National Park investigated the environmental history of this important ecosystem, and looked at the effect of agriculture and recreational activities on the conservation value of the park.

Thirlmere Lakes

Thirlmere Lakes National Park is located about 90 km south-west of Sydney. The 627 hectare area became a national park in 1974 due to its high conservation value based on the lake system’s unique origin and biology. Together with the adjacent 70,000 ha Nattai Reserve system, it forms part of one of the largest conservation areas in Australia. The focal point for the national park is a system of five small freshwater lakes. 

The lake system was formed over two million years ago by rare geomorphological processes and is unique in that has survived until today, unlike most lakes of similar origin and size which have filled with sediment. This remarkable level of stability has allowed the evolution of a number of rare species, including the freshwater sponge, Radiospongilla sceptroides, and the water lily, Brasenia schreberi.

The national park also forms part of the Special Area of the Warragamba Dam Catchment and provides low-impact recreation opportunities for the growing Camden-Campbelltown region.

Thirlmere Lakes study

In 1998 a joint University of Wollongong and Australian Nuclear Science and Technology Organisation (ANSTO) study was carried out on the lake system with the permission of the NPWS. The aim of the research (Noakes, 1998) was to determine the land use change and fire record of the Thirlmere Lakes area since European settlement and the effect on the lakes. Vertical sediment cores from one of the lakes, Lake Couridjah, were analysed at regular intervals for 210 Pb (lead-210) activity, charcoal, trace elements, particle size distribution and pollen of the introduced species Pinus radiata.

Sedimentation in the lake

As discussed earlier, the Thirlmere Lakes system only exists today due to its very low sedimentation  rate. Consequently, a key conservation issue for the lake system is to ensure that the rate of sedimentation is not increasing. To determine the changes in the sedimentation rate in the post-European period, the radioisotope 210 Pb was used. This naturally-occurring isotope decays at a known rate. Counting the 210 Pb activity remaining in the sediment at a given depth gives the year that the sediment was deposited in the lake. Average sedimentation rates of about 2 mm/year were calculated. This is typical for a small, natural and densely vegetated catchment. However, a period of accelerated sedimentation was observed, corresponding with the 1950s. While there had been relatively limited erosion in the catchment until then, clearing in the 1950s of vegetation adjacent to the lake for the construction of a carpark, picnic area and unsealed road, is the probable cause of erosion and the increased sedimentation. There were areas of significant disturbance in the sediment cores. One zone corresponded to the 1973 Picton earthquake, which is known to have significantly disturbed the lake sediment. Also, fluctuating water depth causes the formation and destruction of islands of peat which mix and redistribute the sediment in the lake.

Land-use change

The NPWS regard Thirlmere Lakes as a stable and unpolluted lake system. While European settlement of the area began in the early 1820s, there is currently limited rural and urban development within its catchment. However, agricultural activities - including clearing of natural vegetation, use of fertilisers and harvesting of the littoral sedges - may have resulted in accelerated erosion, chemical contamination and physical disturbance of the sediment. 

In the past the lake has been used for high-impact recreational activities such as water skiing. In the 1950s a road was constructed adjacent to the lake to improve access. Arsenic-based herbicides were used to clear natural vegetation from the lake margins, making it more attractive for watersports. At the same time the five lakes were connected by an artificial channel, increasing the effective length of the "lake" for skiers. These "improvements" resulted in accelerated erosion, increased arsenic concentrations in the sediment and sediment disturbance.


Waratahs are a common sight 
in Thirlmere Lakes NP 

Photo: Roger Lembit 

The study also found that the lake system is relatively stable yet is responsive to natural climatic and associated cycles. For example, fluctuations in water depth are caused by variations in rainfall, which in turn is related to El Niño Southern Oscillation events. During some extreme El Niño events the lake system dried completely. This affects the survival of the rare aquatic species. These effects, however, are limited in duration and frequency and are quickly restored. Therefore, the majority of change that has affected the lake system can be attributed to human activity. Thirlmere Lakes National Park needs to be protected as an area of significant conservation value. Its origin, age, resulting ecology, aesthetics and proximity to Sydney make it attractive to scientists, conservationists, and eco-recreationists alike. This unique environment’s very existence is owed to its previously undisturbed nature. While the current management of the park is a reasonable compromise between recreational access and preservation, many of the high-impact recreation activities of the past are not sustainable. Care needs to be taken by those who use and manage the park to ensure that the potential for erosion and disturbance is not increased.

Reference

Noakes, AJ (1998) A Study of the Land Use Change and Fire Record of the Thirlmere Lakes Area Since European Settlement as Provided by the Sediment Record. Unpublished B.Env.Sci. Hons. Thesis, University of Wollongong

* Ainsley Atkinson (nee Noakes) is a PhD student with the Environmental Science Unit, University of Wollongong, and a NPA member.


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