Sediment resuspension and water quality during declining water levels in a shallow lake : a case study of Lake Alexandrina, South Australia.

Shallow lakes have a tendency to be present in one of two broad alternative states: clear or turbid. Increased demand for water by humans, drought and climatic shifts may reduce water availability and increase the frequency or magnitude of lake water level drawdown. However, there is conflicting evidence as to how shallow lakes respond to water level decline. Many lakes become clearer due to reduced nutrient inputs from inflowing rivers. Alternatively, internal processes, such as the resuspension of sediments, can exacerbate the turbid state as lower water levels attenuate less wind-induced turbulence before it reaches the sediment surface. A better understanding of responses of lake water quality to drawdown will greatly improve the ability of water managers to maximise the ecological benefits of drawdowns, while minimising the adverse consequences. Severe drought in the Murray-Darling Basin, Australia’s largest river system, resulted in extreme water level decline in the two end of system lakes, Lake Alexandrina and Lake Albert (the Lower Lakes). Water depth in Lake Alexandrina dropped from a mean of 2.9 metres in summer 2006/07 to 1.3 metres two years later, where water levels remained until April 2010 when floods refilled the lakes. This provided a unique opportunity to assess the influence of water level decline on lake characteristics. The aims of this work were to firstly elucidate the effects of water level decline on sediment resuspension and redistribution, accompanying nutrient changes and light availability. Secondly, the study sought to understand the interactions between water level decline and sediment resuspension in the context of alternative stable states during periods of drought. As water level fell in the Lower Lakes, the water quality became increasingly brackish as mean salinity rose from 0.6 g L⁻¹ to 3.7 g L⁻¹. The concentration of suspended particulate matter (SPM) and associated nitrogen and phosphorus concentrations increased as water levels declined, whereas soluble phosphorus did not. A mass balance showed that month to month variation in SPM could not be explained by the sum of inputs and outputs, suggesting that sediment resuspension was an important process in Lake Alexandrina at all water levels. Surveys of the size distribution of surface sediments suggested that focusing of fine sediments was more prevalent when water levels were higher. Analysis of gross sedimentation using sediment traps supported the hypothesis that the resuspension and redistribution of sediments had become spatially homogeneous as water levels declined. High-resolution data collected from the centre of Lake Alexandrina suggested that sediment resuspension frequently occurred below even the deepest water. At bed shear stresses greater than 0.031 N m⁻² particles with a nominal diameter of 26 μm were resuspended. Under low water levels, sediment resuspension strongly influenced the depth of light penetration into the water column (euphotic depth ranged between 0.68 and 0.91 metres). However, lower water levels led to an increase in the average irradiance through the water column compared with previous studies at higher water levels. Hydrodynamic modeling was used to quantify the influence of wind speed on sediment resuspension. This showed that wind speeds of 7.7 m s⁻¹ were required to resuspend particles with a nominal diameter of 26 μm when water levels were high. However, at low water levels, the wind speed imparting critical shear velocity at the lake-bed decreased to 2.4 m s⁻¹. The frequency distribution of wind speed suggested that sediments were being resuspended only 27% of the time when water levels were high, but over 87% of the time at the lowest water levels studied. The shallow morphology of Lake Alexandrina, as well as the high wind-induced turbulent energy at the sediment surface due to its large surface area, increased the resuspension and redistribution of fine sediments during water level drawdown. For Lake Alexandrina, the internal process of sediment resuspension overcame the drought-induced reduction in nutrient inputs to result in an increasingly turbid system. These results imply that lake management should centre on the reduction of external nutrients, as well as biomanipulation of the food webs to increase the water clarity of Lake Alexandrina and improve conditions for macrophyte development.Thesis (Ph.D.) -- University of Adelaide, School of Earth and Environmental Sciences, 201