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Lake Rotokawau: Water quality and sediment study
Abstract
Lake Rotokawau is a small (0.52 km2), deep (74 m), oligotrophic lake in the Rotorua Lakes District, located approximately 4.1 km east of Lake Rotorua. The lake is privately owned and managed by the local iwi, Ngati Rangiteaorere. During a site visit carried out on the 25 July 2010, surface and bottom water samples and a sediment core were collected, together with a water column profile (CTD) of temperature, dissolved oxygen, fluorescence, conductivity and depth. The CTD cast showed an anoxic hypolimnion and that the lake likely had not mixed during at the time of sampling, in mid-winter. This observation is highly unusual amongst the Te Arawa lakes and suggests that the lake may remain stratified for multiple years. The lack of mixing could be due to several factors including: presence of higher-salinity bottom waters, high water depth to area ratio, and sheltered aspect. The sediment core collected in the central basin revealed the presence of Ruapehu ash (1995-6 eruption) preserved in the upper 5 cm of the sediment record and the Tarawera Tephra (1886 eruption) located at 13 cm below the sediment water interface. The Ruapehu Ash layer is not preserved in sediment from any other Rotorua lakes and has likely been preserved due to the lack of seasonal mixing. The sediment and pore water chemistry are similar to that of the other lakes in the Rotorua region but however multiple sediment cores are necessary to fully assess variability across the whole lake area.
It is recommended that a routine monitoring programme be established so that any improvement or degradation in water quality can be detected and the mixing frequency of the lake determined. The water quality monitoring should include water column profiles of temperature, dissolved oxygen, fluorescence, conductivity and depth, nutrient analysis (ammonium, nitrate, nitrite, phosphate, silicon, total nitrogen and total phosphorus) of surface and bottom waters and water clarity readings (e.g. Secchi disk and light attenuation). Our observations point to the highly unusual nature of Lake Rotokawau in terms of mixing status and indicate a strong need to preserve significant biological and landscape features of this lake which has not been subject to problems with invasive species that characterise several other lakes in this region.
Type
Commissioned Report for External Body
Type of thesis
Series
Centre for Biodiversity and Ecology Research Reports
Citation
Date
2011-07
Publisher
Centre for Biodiversity and Ecology Research, The University of Waikato