Influence of Artificial Destratification on Limnological Processes in Lake Samsonvale (North Pine Dam), Queensland, Australia
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Artificial destratification equipment was installed in Lake Samsonvale in October 1995 to reduce the biomass of potentially toxic cyanobacteria in the reservoir. This study was undertaken to investigate the effects of the destratifier on the limnological processes occurring in the lake and to determine if operation of the destratifier has been effective at reducing the summer populations of Cylindrospermopsis raciborskii and Microcystis aeruginosa. The study showed that artificial destratification of Lake Samsonvale has been successful at reducing the surface to bottom thermal gradient and increasing dissolved oxygen concentrations at depth. While the destratifier has not resulted in the lake becoming completely 'mixed' during summer, it has weakened resistance to mixing from meteorological events, which has led to a reduction in mean concentrations of total and dissolved phosphorus during summer. Although not conclusive, it is likely that the influence of the destratifier is restricted to a narrow radial distance around the bubbleplumes during periods of strong stability, so internal loading may continue to provide a substantial source of nutrients for cyanobacterial growth, particularly in regions of the lake less influenced by the destratifier. The results for cyanobacteria are less encouraging. Despite the reduction in concentrations of dissolved phosphorus, the destratifier has not been effective at reducing summer populations of C. raciborskii and M. aeruginosa. On the contrary, there is evidence to suggest that populations have actually increased which could have serious operational consequences for the lake by mixing the previously buoyant cyanobacteria to the level of the water off-take. The growth season for C. raciborskii has been extended by up to 3 months and population onset now occurs during spring. This increase in spring populations could be a result of significantly greater baseline populations during winter, or the earlier germination of akinetes as a result of increased sediment temperatures. The seasonal successional relationship between C. raciborskii and M. aeruginosa appears to have shifted from one of alternating dominance between the two species to one of co-existence under conditions of intermediate disturbance. It was concluded that although the continued operation of the destratifier may prove useful to minimise water treatment costs through reducing internal loading of dissolved constituents, it has not been successful in achieving its original objective of controlling cyanobacterial populations in the lake.
Master of Philosophy (MPhil)
Australian School of Environmental Studies
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