Ecological Patterns and Processes of the South Taupo Wetland, North Island, New Zealand, With Special Reference to Nature Conservation Management

Abstract

The South Taupo Wetland (STW), situated on the southern shore of Lake Taupo, is one of the largest continuous wetlands in North Island, New Zealand. Its extent depends on the artificial lake level regulation, and parts of the STW were farmed in the past. The present study aims to establish the major ecological processes acting within and between vegetation, soil, and groundwater of the STW, and to identify the ecological values and functions of, and threats to specific areas of the STW. Recommendations for the conservation management of the STW are derived from the study's findings. Quantitative floristic data were collected along five transects in the STW, and samples classified using Two Way Indicator Species Analysis (TWINSPAN) (resulting in 14 plant communities), as well as ordinated by means of Detrended Correspondence Analysis (DCA). The vegetation data were correlated to environmental parameters employing Canonical Correspondence Analysis (CCA). The vegetation of the STW was found to be strongly related to surface water height (ranging from 0-40 cm above ground surface), soil moisture (15-93%), and organic matter content of the soil (l-73%). Soil nutrient status (P: 0-56 mg/l; K: 0-81 mg/l) was the second most important gradient determining the distribution of plant communities. The vegetation of the STW was mapped using Semi-Quantitative Measurements of Canopy Composition, which resulted in twelve vegetation classes similar to the sample groups derived by TWINSPAN. Approximately half of the mapped wetland area is covered by plant communities that are dominated by native plant species, the other half is covered by plant communities dominated by exotic species. Stump Bay (central STW) was found to be the area with the lowest abundance of exotic vegetation, while the Tongariro River Delta had the highest concentration of exotic plant communities. A total area of 183 ha was found to be infested by the exotic tree species Salix fragilis, while S. cinerea had invaded a total area of 432ha. S. fragilis started to colonise the STW in the 1940's and has stopped spreading in the 1980's, while S. cinerea has been spreading since the 1970's at an increasing rate. The hydrology and hydrochemistry study is based on water samples collected seasonally from piezometers installed in Stump Bay. The hydrology is characterised by through-flowing or discharging groundwater, with a main flow direction towards the lake. Higher dissolved oxygen concentrations (0.3-6.1 mg/l) and slightly higher temperatures (8-18.9ºC) in the western part of Stump Bay suggests the presence of Tongariro River seepage water in this area. The main chemical characteristics of Stump Bay's groundwater is the high concentration of total dissolved iron (up to 145 g/m3). Iron concentrations appeared to be determined by the presence or absence of a clay layer within the top metre of peat/sediment. The reduction of iron controlled the groundwater's conductivity (0.07-0.6 mS/cm), alkalinity (18-423 g/m3), and pH (6-7.2). The main ecological functions and conservation values of the STW include the provision of a habitat for wetland plant and animal species, the improvement of the water quality of water passing through the wetland, and the provision of recreational activities. Natural and human induced disturbances identified as threats to the ecological balance of the STW are the invasion by Salix cinerea, the potential reduction of mean lake levels, and fire.