Water clarity in two shallow lake systems of the Central Plateau, Tasmania, Australia

Lake Crescent and Lake Sorell are two adjacent, large, shallow, mesotrophic lakes of the Central Plateau, Tasmania, Australia. Both lakes are in the same catchment and have similar geological and morphological characteristics. Early limnological work by Cheng and Tyler found trophic characteristics to be substantially different, despite their physical similarities. Historically, each lake exhibited strongly contrasting 'stable-states': Lake Sorell was a macrophyte-dominated clear water system, while Lake Crescent was turbid and dominated by phytoplankton. Cheng and Tyler dubbed this a "limnological paradox". Since the late 1990s, the quality of the trout fishery declined, nutrient and algal concentrations increased markedly and water clarity declined dramatically. These changes coincided with unprecedented low water levels due primarily to severe drought and competition for water by various users. The rapid decline in water clarity prompted this investigation to determine the underlying processes responsible for the degradation and to recommend management strategies to improve water quality. Variables limiting light attenuation, turbidity and water clarity were measured from April 2000 to August 2002, and modelled using multiple linear regression. Regression coefficients were used to estimate the relative importance of each water quality component, and this analysis showed that high levels of inorganic suspensoids were largely responsible for the decline in water clarity. Although there were increases in nutrients and suspended sediment in these lakes in the late 1990s, detailed analysis of inputs to and outputs from these lakes suggested that these increases were derived from internal sources as inputs from the surrounding catchment were negligible. The historical record oflake levels was analysed to quantify the areas oflake bed that would be in contact with the wave base under differing wind conditions and water levels. This showed that both lakes were more prone to wind effects after 1998 owing to the lower water levels. Further, prior to 1999 Lake Crescent was, on average, more turbulent and more prone to wind-driven resuspension events than Lake Sorell, which suggests a potential mechanism underlying Cheng and Tyler's "limnological paradox". The physical disturbance of sediments from wind-driven waves was further investigated by calculating shear stress from wave theory, and quantifying relationships between shear stress and suspended sediment concentration. Shear stress characteristics across the lake basins were modelled under various lake levels and wind speeds, and the magnitude of shear stress increased dramatically at lower lake levels. DYRESM-CAEDYM was used to develop a sediment resuspension model relating wind, lake-level and sediment flux that was then calibrated and verified against field observations. (The ecosystem model CAEDYM (ComputAtional Ecosystem Dynamics Model) is coupled with the hydrodynamic driver DYRESM (DYnamic REservoir Simulation Model) to accomplish these simulations). The model was used to ascertain the benefits of managing water levels in the lakes to ameliorate the affects of sediment resuspension and improve water quality. This modelling suggested that the "degraded" state of the lakes from the late 1990s was initiated and sustained by low lake levels leading to increased shear stress acting on the sediments. The increased nutrient concentrations and algal biomass were also found to result from low lake levels because external nutrient loading was insignificant. Conversely, the modelling showed that raising water levels would dramatically improve water clarity. Alternative, trophically-based explanations of the differences between lakes Crescent and Sorell were examined by investigating the biotic interactions that influence water quality and ecosystem function. The aim was to determine if trophic cascades and stable-state theory would help explain the contrasting phycology of these two lakes. The historical biological data from lakes Sorell and Crescent was reviewed and reanalysed, and contemporary data collected to compare the trophic structure of the "degraded" status at the end of the 1990s with the historical record. The strong contrasts in phytoplankton productivity and community composition evident between the lakes in the past were still prominent. The algal community of Lake Crescent has concentrations up to 100 x those of Lake Sorell, and is still dominated by diatoms while green algae dominate Lake Sorell. The zooplankton of Lake Crescent is dominated by small cladocerans and copepods, while Lake Sorell has more frequent occurrences of larger cladocerans such as Daphnia. Lake Crescent also has an order of magnitude greater biomass of the zooplanktivorous fish, Galaxias auratus, than Lake Sorell, which leads to a much greater (up to 30 fold) predation pressure on large zooplankters. By contrast the pattern in biomass of the introduced piscivorous brown trout (Salmo trutta) between the two lakes is reversed. While some of these patterns are consistent with differences in the nature of top-down trophic cascades between the lakes, the accumulated evidence suggests that such relationships break down at the link between zooplankton and phytoplankton. The empirical evidence collected suggests that zooplankton grazing had little effect on limiting phytoplankton productivity in either lake for any significant period of time, while the greater dominance of meroplanktonic diatoms and the greater susceptibility of Lake Crescent to wind-driven resuspension suggests a more parsimonious explanation of the persistent phytoplankton dominance in this lake. DYRESM-CAEDYM was then employed to investigate plankton and meroplankton dynamics in lakes Sorell and Crescent, since this technique can be use to test 'N-P-Z' (nutrients-phytoplankton-zooplankton) models. The hypothesis tested was that developed above: that differences in resuspension combined with contrasts in the proportions of meroplanktonic phytoplankton were sufficient to explain the differences between the two lakes. Modelling of plankton and meroplankton dynamics in both lakes returned significant contrasts in algal productivity that were driven largely by contrasting sediment resuspension dynamics between the lakes. It was concluded that the differing phytoplankton communities of the lakes are a result of contrasting sediment resuspension dynamics between the lakes, with a limited influence from contrasting levels of zooplankton grazing pressure