Influence of Artificial Destratification on Limnological Processes in Lake Samsonvale (North Pine Dam), Queensland, Australia Author Littlejohn, Carla Louise Published 2004 Thesis Type Thesis (Masters) School Australian School of Environmental Studies DOI https://doi.org/10.25904/1912/3580 Copyright Statement The author owns the copyright in this thesis, unless stated otherwise. Downloaded from http://hdl.handle.net/10072/365764 Griffith Research Online https://research-repository.griffith.edu.au Influence of Artificial Destratification on Limnological Processes in Lake Samsonvale (North Pine Dam), Queensland, Australia Thesis submitted in fulfillment of the requirements of the degree of Master of Philosophy By Carla Louise Littlejohn (BappSci) Griffith University, Australian School of Environmental Studies Faculty of Environmental Sciences October 2004 ABSTRACT 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 bubble- plumes 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. i 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. ii TABLE OF CONTENTS CHAPTER 1 Introduction ............................................................................. 1 1.1 Background ...................................................................................................2 1.1.1 The problem ................................................................................. 2 1.1.2 A comparison of lakes and reservoirs........................................... 3 1.1.3 The use of artificial destratification as a management tool ........... 6 1.1.4 Artificial destratification – the Australian perspective.................... 7 1.2 Lake Samsonvale ..........................................................................................9 1.2.1 Location and study area ............................................................... 9 1.2.2 Limnological characteristics........................................................ 11 1.2.3 Artificial destratification of Lake Samsonvale.............................. 12 1.3 Study aim and objectives ...........................................................................13 CHAPTER 2 Influence of Artificial Destratification on Temperature and Dissolved Oxygen Regimes in Lake Samsonvale ....................................... 15 2.1 Introduction .................................................................................................16 2.1.1 Seasonal stratification and mixing .............................................. 16 2.1.2 Latitudinal effects........................................................................ 17 2.1.3 Climatic effects ........................................................................... 20 2.1.4 Impact of artificial destratification on stratification and mixing .... 21 2.1.5 Objectives................................................................................... 23 2.2 Methodology ................................................................................................24 2.2.1 Sampling methodology and historical data ................................. 24 2.2.2 Selection of datasets for comparison of pre and post destratification........................................................................................... 25 2.2.3 Dissolved oxygen mapping......................................................... 26 2.2.4 Data analysis .............................................................................. 28 2.3 Results .........................................................................................................29 2.3.1 Comparison of pre and post temperature ................................... 29 2.3.2 Comparison of pre and post dissolved oxygen ........................... 30 2.3.3 DO mapping September 1999 to May 2000 ............................... 31 2.4 Discussion ...................................................................................................36 iii 2.4.1 Influence of destratification on temperature and stability ............ 36 2.4.2 Influence of destratification on dissolved oxygen concentrations (pre and post comparisons) ...................................................................... 39 2.4.3 Evolution of dissolved oxygen concentrations during destratification........................................................................................... 39 2.4.4 Volume of the hypolimnion ......................................................... 40 CHAPTER 3 Influence of Artificial Destratification on Internal Nutrient Cycling in Lake Samsonvale......................................................................... 41 3.1 Introduction .................................................................................................42 3.1.1 Phosphorus and nitrogen cycling during stratification and mixing42 3.1.2 Influence of trophic status on phosphorus and nitrogen cycling . 50 3.1.3 Role of detritus in phosphorus and nitrogen cycling ................... 52 3.1.4 The role of artificial destratification in regulating internal nutrient loading 53 3.1.5 Objectives................................................................................... 55 3.2 Methodology ................................................................................................56 3.2.1 Sample collection and analysis................................................... 56 3.2.2 Sampling methodology ............................................................... 57 3.2.3 Data analysis .............................................................................. 61 3.3 Results .........................................................................................................61 3.3.1 Comparison of nutrient concentrations pre and post destratification........................................................................................... 61 3.3.2 Spatial influence of the destratifier on nutrients .......................... 62 3.4 Discussion ...................................................................................................64 3.4.1 Comparison of nutrient concentrations pre and post destratification........................................................................................... 64 3.4.2 Spatial influence of the destratifier on nutrients .......................... 66 CHAPTER 4 Influence of Artificial Destratification on Composition and Abundance of C. raciborskii and M. aeruginosa in Lake Samsonvale ...... 69 4.1 Introduction .................................................................................................70 4.1.1 Cyanobacterial responses to stratification and mixing................ 70 4.1.2 Rates of growth and succession................................................. 71 iv 4.1.3 Use of artificial destratification to control cyanobacteria ............. 72 4.1.4 Consequences of inadequate or poorly timed mixing ................. 74 4.1.5 Objectives................................................................................... 76 4.2 Methodology ................................................................................................76 4.2.1 Sampling sites and methodology...............................................