Limnological studies of dystrophic waters

A number of aspects of the limnology of dystrophic lentic freshwaters are covered in this thesis. Initially, studies covering a wide range of heterogeneous lakes and reservoirs from several different areas are reported, to give an overall perspective of many of the roles dissolved humic substances play in various limnological processes. More detailed case studies of a number of dystrophic lakes and reservoirs are then outlined, to demonstrate how dissolved humic substances actively influence the limnology of these individual waters. Investigations of the underwater light climates of fifty lakes and reservoirs fr om Tasmania, thirty-seven from north-east New South Wales, and of twenty-six coastal dune lakes in south-east Queensland showed that dissolved humic substances were the major attenuators of light in all three areas. Increasing gilvin concentrations led to the rapid extinction of light at shallow depths, and to changes in its underwater spectral distribution from green-yellow in the clearest waters, to red in the most humic. Turbidity and phytoplankton were important contributors to attenuation in only a few of the waters studied. Results from Tasmania allowed the construction of a predictive model for use in estimating the underwater light climates of the many remote lakes of the island where in situ measurements are impossible, from simple laboratory measurements of small water samples. The rapid attenuation of light in humic waters also strongly affects thermal stratification, leading to strong thermal gradients, shallow thermoclines, and lengthy periods of stratification. Chemical aspects were also examined. Strong negative correlations were found between the concentration of dissolved humic substances and pH in coastal dune lake waters from south-east Queensland, King and Fl inders I s l and s , and we s tern and south-wes t Tasmania.In contrast, there was no correlation between these two parameters in north-east New South Wales waters, probably due to lower humic concentrations and buffering by higher bicarbonate levels . Calcium and bicarbonate levels were also some times low in acidic, dystrophic lakes. A wide range of photo synthetic organisms were present in the sites investigated from New South Wales, Queensland, and coastal lakes of south-west Tasmania. Phytoplankton came both from the Chlorophyceae and from other algal divisions, and desmids and dinoflagellates were e specially common. Proportions of green algae decreased relative to those with accessory photosynthetic pigments, as humic concentration increased, in the highly humic western and south-west Tasmanian lagoons, but this was not apparent in the less dystrophic waters of the other two areas. However, high humic concentrations reduced the number of phytoplankton genera present from all groups. Chrysophytes dominated the phytoplankton communities of polyhumic Lake Chisholm. Studies of Lake Chisholm, the reservoirs of the Pieman River Power Development, and the meronictic lakes of the lower Gordon River, all in Tasmania, showed humic materials played an important role in their limnology. The rapid attenuation of light, and subsequent strong thermal stratification for much of the year lead to sluggish circulation, all owing near-meromictic conditions in Lake Chisholm, and the protection and resultant slowing of the demise of ectogenic meromixis in the Gordon River lakes. These features, along with enhancing the solubility of iron and manganese in the reducing, sulphide laden bottom waters of the Pieman River reservoirs, enabled their chemical stratification. However, other factors, such as basin morphometry and shelter, are also of importance in the limnology of these lakes and reservoirs.