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Ecological monitoring of artificial destratification effects in Lake Rotoehu: 2014-2015

Lake Rotoehu is a shallow (mean depth 8 m), polymictic eutrophic lake in the Rotorua lakes district. Catchment land use intensification has contributed to a decline in water quality and persistent cyanobacterial algal blooms for the past 20 years and combines with a background of nutrient-rich geothermal inflows. Two artificial mixing devices were deployed in Lake Rotoehu in the spring of 2012 by the Bay of Plenty Regional Council with the aim of preventing lake thermal stratification. They were previously operated over the summer-autumn periods of 2012-13 and 2013-14 and their effects were monitored on a monthly basis until June 2014. The devices force compressed air through a diffuser near the lake bottom, and into three large vertical cylinders. Buoyancy, caused by the rising bubbles, draws water from the bottom of the lake up through the vertical cylinders and directs it horizontally into the surface mixed layer (epilimnion). This causes mixing of thermally distinct layers of the water column, and is designed to inhibit stratification and prevent hypolimnetic deoxygenation. Results from previous monitoring indicate that the mixing devices produced a limited, localised mixing effect, but had no influence on whole lake ecology. With a view to improving performance, modifications, including channelling all air through a single mixing device and increasing compressor output, were effected in preparation for the 2014-15 summer-autumn seasons. The University of Waikato was contracted by the Bay of Plenty Regional Council to conduct a monitoring programme to ascertain the effect of these modifications. Three sites within Lake Rotoehu were sampled monthly at similar depths from October 2014 to May 2015. Sites 1 and 3 were located approximately 30 m from a mixing device and Site 2 acted as a control site as it was considered to be unaffected by the direct effects of the mixing. Zooplankton, phytoplankton and nutrient samples (total nitrogen, total phosphorus, nitrate, ammonium and dissolved phosphorus) were collected at two different depths (0.5 m and 9.0 m), at each site. In addition, CTD (conductivity, temperature, depth) vertical profiles were also taken at each site. Temperature data were also obtained from a fixed-sensor, high-frequency monitoring buoy deployed in the lake and used to determine lake stability. The summer of 2014-15 produced conditions favourable for the formation of prolonged lake stratification. This resulted in a marked decline in water quality during January-May 2015 in comparison to the previous three years of monitoring data. Evidence of localised homogenisation by the mixing device of the dominant phytoplankton and zooplankton taxonomic groups demonstrated notable improvement in mixing device performance. Temperature profile data were more equivocal, with no clear observations of the effects of artificial mixing over an extended area of the lake. However, supplemental profiles taken from within the discharge plumes (approximately 30 m from the mixing device) demonstrated evidence of hypolimnetic water being discharged to the epilimnion. It is concluded that the current design and configuration of the artificial mixing devices is effective in drawing water from the hypolimnion into the epilimnion. However, it appears that the thermal mass of Lake Rotoehu is too large for a single mixing device to overcome and there appears to be no substantial ecological benefit in continuing to operate the devices without substantial modification. If significant, sustained improvements in water quality are to be achieved using these devices, further capital investment will be required. Further research investigating the efficiency and area of influence of the mixing devices may help to improve their design and operation.
Type of thesis
Tempero, G. W. (2015). Ecological monitoring of artificial destratification effects in Lake Rotoehu: 2014-2015 (ERI report). Hamilton, New Zealand: Environmental Research Institute, Faculty of Science and Engineering, The University of Waikato.
Environmental Research Institute, Faculty of Science and Engineering, The University of Waikato
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