Spatial and temporal changes in water quality in Lake Alexandrina and Lake Albert during a period of rapid water level drawdown

Abstract

Lake Alexandrina and Lake Albert (the Lower Lakes) are a set of large, shallow, fluvial lakes at the downstream end of the Murray-Darling Basin. The lakes host a number of threatened freshwater fish species; are an important nesting habitat for waterbirds; the source of water for a local irrigated agriculture economy; support a substantial fishery; and provide substantial recreational and aesthetic value to South Australia. Together with the Coorong, an estuarine-hypersaline coastal lagoon, the Lower Lakes were declared a Wetland of International Importance under the Ramsar Convention due to the abundant and diverse ecological communities within the region. From September 2001 until the end of this study (April 2008) the Murray-Darling Basin experienced a severe rainfall deficiency, the second driest seven-year period in its recorded history (MDBC 2008). This combined with the over-allocation of water within the Murray-Darling Basin and presence of the barrages separating the Lower Lakes from the Coorong, has resulted in a dramatic reduction in inflows to the Lower Lakes and rapid water level drawdown. From August 2006 to August 2008, water levels fell from 0.75 m AHD to -0.5 m AHD. This is well below the previous historical low of 0.1 m AHD in April 1968 (MDBC 2008). This report summarises three limnological studies that were conducted during the drawdown period. The overall purpose of the project was to gain a better understanding of nutrient cycling in the lakes and how it is impacted by changes in water level. The three studies were: 1) A spatially extensive water quality monitoring program, undertaken between January 2006 and April 2008; 2) An assessment of the spatial variability in sediment characteristics across the lakes; 3) A nutrient regeneration experiment simulating a drying and wetting of sediments of Lake Alexandrina. In addition to documenting ongoing changes during the drawdown, these studies are the most detailed assessment undertaken to date on the limnological properties of this system.