Report: TR04/25

Author: Scott Stephens (NIWA), Mary de Winton (NIWA), James Sukias (NIWA), Ron Ovenden (NIWA), Aleki Taumoepeau (NIWA), Jim Cooke (NIWA)

Abstract

Lake Waikare is a large shallow lake that currently forms part of a flood-control scheme operated by Waikato Regional Council, and includes a weir at the lake outlet to facilitate water level control. The lake historically contained healthy aquatic plant populations, but is now characterised by high turbidities and low light penetration, which make it incapable of supporting submerged vegetation. Lake drawdown has been used successfully overseas to restore the light climate and submerged vegetation in shallow lakes. Drawdown can enhance rehabilitation by consolidating sediment and making it less prone to wave resuspension, by increasing light penetration to the sediment surface allowing plant growth, and by directly reducing wind-wave action and assisting coarse fish control via a smaller surface area and volume. Waikato Regional Council recognises that the weir provides an opportunity to artificially control the lake level to enhance its health, and that one option is a water level drawdown over the spring and summer months. Therefore, Waikato Regional Council contracted NIWA to assess the potential effectiveness of water level drawdown for lake rehabilitation.

Lakebed sediment was transferred to concrete troughs where it was drained and exposed for periods of up to 6 months over summer. Changes in the physical and chemical properties of the sediment were monitored and compared with an undrained control trough. The troughs were rewetted following exposure and the resistance of sediment to resuspension was measured. In addition, dried sediment from the bed of Lake Waikare was submerged and cultured for 6 months to assess the likely seed germination response to drawdown and refilling of the lake.

Water level drawdown in Lake Waikare doesn’t appear to hold significant benefits for improving the light climate through altering sediment properties. There was some consolidation of surficial sediments, but light-attenuating sediments were still resuspended and high turbidities were measured following inundation of desiccated sediments.

Establishment of aquatic vegetation to stabilise sediments and take up nutrients is crucial to rehabilitation of Lake Waikare. To do this requires a suitable light climate for aquatic plant growth. This study has shown that lake drawdown has the potential to initiate rehabilitation by creating a suitable light climate for plant growth at the sediment surface. The experiments showed that lakebed sediment is well suited to both terrestrial and aquatic plant growth. It appears that marginal/wetland and terrestrial plants are particularly likely to colonise the exposed sediment surface during drawdown, but the seedbank for aquatic plants is limited.

Although drawdown provides an opportunity to initiate plant growth in the lakebed sediments, the fate of this vegetation upon refilling the lake remains uncertain. The establishment, survival and persistence of submerged vegetation once the lake is returned to its stable level is uncertain due to the following factors:

• The aquatic plant seedbank and its germination response to drying and rewetting appears to be limited.
• Terrestrial and many marginal/wetland plants are unlikely to survive long underwater.
• Pest fish such as Koi carp are a threat to submerged vegetation. The exact relationship between coarse fish and vegetation in the lake is unknown, but other examples such as Hamilton Lake suggest coarse fish are detrimental to the establishment of aquatic plants. Assuming they are detrimental, then some consideration into methods of removal or control of pest fish is advisable before attempting rehabilitation.
• The lake is hyper-eutrophic, so algal growth and associated light climate reduction may occur following any initial improvement in light climate (Reeves et al. 2002).
• The lake forms part of a flood control scheme and during floods the vegetation will be stressed by reductions in the light climate caused by water level increase, particularly where this involves turbid inflows (Reeves et al. 2002).
• Growing vegetation will be subjected to wave disturbance on refilling the lake, and wave-induced entrainment of light-attenuating sediment will occur unless a dense coverage of aquatic vegetation has been established.

Our conclusion based on the present study and that of Reeves et al. (2002) is that lake drawdown does not provide the sole answer for the rehabilitation of Lake Waikare. The study has demonstrated that drawdown does hold some benefits for the lake, and could form part of the rehabilitation process. But it is our opinion that the number and scale of the problems the lake suffers (e.g. koi carp, large wind-wave fetch, hyper-eutrophic) make it a poor candidate for rehabilitation.

Rehabilitation of Lake Waikare : Experimental Investigations of the Potential Benefits of Water Level Drawdown [PDF, 1.7 MB]