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Ecological effects of artificial mixing in Lake Rotoehu

Lake Rotoehu is a eutrophic, moderately large (795 ha), shallow (mean depth 8.2 m), polymictic lake in the Rotorua/Te Arawa lake district. Land use intensification, water level changes, and nutrient-rich geothermal inflows are some of the factors which may have contributed to severe cyanobacteria blooms over the past 20 years. Bay of Plenty Regional Council (BoPRC) is charged with maintaining the ecosystem health of Lake Rotoehu. In 2007 BoPRC published the Lake Rotoehu Action Plan, outlining multiple restoration initiatives to reduce nutrient loading to the lake. Among these initiatives was a trial of artificial mixing to increase vertical circulation within the water column, preventing the formation of stratification. Two water column mixing devices were deployed on the lake bed of Lake Rotoehu from November 2012 to June 2014 (and are ongoing). These devices force compressed air through a diffuser near the bottom of the lake water column in the central lake basin. Buoyancy caused by the rising bubbles draws water from the bottom of the lake up through large vertical cylinders to the surface, where the entrained water is subsequently directed horizontally along the water surface. The University of Waikato was contracted to monitor the physical (temperature and dissolved oxygen), chemical (nutrients), and biological (phytoplankton and zooplankton) effects of the mixing devices on the lake. This work encompassed a range of instrumentation, laboratory analyses, and species enumeration from water samples. In addition, an evaluation of the area of effect and strength of current generated by the mixing devices was conducted by the National Institute of Water and Atmospheric Research (NIWA), the results of which are reported here. Rhodamine dye tracer injected into the northern arm of the southern mixing device revealed that cooler hypolimnetic water was being brought to the surface. A component of the water entrained in this plume flowed away from the mixing device for at least 90 m. Parts of the plume also detrained from it, sinking towards the thermocline. Plumes from the mixing devices were tracked by Acoustic Doppler Current Meters (ADCM) or Profilers. The ADCM data revealed that the plume gradually dispersed laterally with increasing distance from the mixing device, and plume velocities were not elevated above background levels at c. 70 m from the mixing device. Although current velocities measured in the plume were generally low (<1 cm s⁻¹ to 25 cm s⁻¹; average 5.5 cm s⁻¹) they could be clearly distinguished from wind driven currents. Changes to the water column thermal profile were observed adjacent to the mixing devices, however, modification of the water column temperature structure across the lake (i.e., greater than a hundred metres from the devices) was not observed. Interpretations of the effect of the aeration devices was somewhat confounded by the period prior to installation of the machines (summer 2012) being cold and windy relative to weather conditions during the period of device deployment, which were unusually hot and dry (summer 2013 and 2014). These conditions resulted in Lake Rotoehu being stratified more often and strongly than in the summer prior to device installation. Additional factors affecting the aeration device operation are discussed in the report. Analyses of nutrient concentrations, water clarity, and trophic status indicators have shown some improvement of Lake Rotoehu over recent years. However, the Trophic Level Index (TLI) has not yet met the target of 3.9 specified by the Rotoehu Action Plan. Water quality assessed by these indicators was comparatively good in 2012 - June 2014 despite the sustained periods of thermal stratification that contributed to significant oxygen depletion in bottom waters. Localised effects of mixing included homogenisation of the cyanobacterial community through the water column and a decrease in the Bray-Curtis similarity index of the plankton community between mixing sites (i.e. near the aeration device) and controls (i.e. several hundred metres away from the mixing device). These results are consistent with physical observations of localised changes in thermal structure around the aeration devices. No significant changes occurred in plankton community composition at the whole-lake scale during aerator operation during the 2013 and 2014. By contrast, plankton community composition showed large seasonal and inter-annual variability including presence of invasive species. Results suggest that the mixing devices installed in Lake Rotoehu are able to draw hypolimnetic water to the surface. The resulting horizontal plume rapidly becomes mixed with surface waters over distances of 50-60 m from the aerator but can also be tracked as it plunges towards the thermocline at these distances. Some localised effects were detected near the aerators but there were no detectable lake-scale effects. By contrast, there have been progressive improvements in water quality at a whole-lake scale over recent years and these may relate to alum dosing of the major geothermal inflow to the lake, weed harvesting and initial decreases in external nutrient inputs in response to changes in land use and land-use practices. Further monitoring will help clarify these observations, and an additional year of mixing device deployment aims to improve operational efficiency of the devices and quantify effects of proposed increases in air flow rates.
Type of thesis
ERI report
McBride, C. G., Tempero, G. W., Hamilton, D. P., Cutting, C. T., Muraoka, K., Duggan, C., & Gibbs, M. M. (2015). Ecological effects of artificial mixing in Lake Rotoehu (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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