Environmental monitoring and assessment of the water quality of Lake Rotoiti's tributaries

Abstract

Lake Rotoiti is important to the Tasman District and wider community in an ecological and social sense. It is important to monitor the lake system so that an understanding of its components can lead to sustainable management strategies being developed. The Lake catchment is used for many widely varying activities including farming, skiing, swimming, tramping, boating and fishing. All these activities either effect or are affected by the water quality of the lake, with many relying on the high water quality for enjoyment. Poor water quality not only affects activities that come into direct contact with the water but also those that make use of the aesthetic qualities. This is particularly important in the tourism of the area which in turn affects the economy.

This study has been undertaken to assess the water quality of Lake Rotoiti's tributaries as a base line for future monitoring and to estimate the impact of land use on water quality of the area. Current and historical water quality information from three sites in the catchment: Buller River, Travers River and Black Valley Stream are the bases of this study. Parameters measured included flow, temperature, conductivity, dissolved oxygen, faecal coliforms, suspended solids, nutrients and invertebrate numbers.

The results show that Black Valley Stream is affected by land uses with concentrations of nutrients being about ten times higher than those at the other sites, faecal pollution is also higher. This can affect the lake, in particular the plant and algal growth and health of the water for recreational activities. The increased contamination of Black Valley Stream is likely to come from septic tank pollution and farming.

This information can be used to develop management strategies that will be sustainable to the lake system and its water quality. Currently a reticulated waste water system is being built to service the St Arnaud area, this should reduce the impact of human use in the area.

To reduce the impact of farming it would be important to fence all tributaries of Black Valley Stream to stop stock damaging banks and defecating in the stream. Planting of riparian vegetation would also help in moderating the impact of farming on the lake system by further stabilising the banks and taking up nutrients. Further monitoring programs are required to fully understand the system and evaluate management practices.