This study provides the most comprehensive physico-chemical and phytoplankton data yet available for Australian dune lakes, which are among the world's most naturally acidic and oligotrophic freshwaters. Seasoal and spatial variations were examined in Blue Lagoon and Lake Freshwater, two 'water-table window' lakes in south-east Queensland. Like other dune lakes, they are acidic (minimum pH 4.20 and 4.55, respectively), polymictic water bodies with low concentrations of marine-derived major ions and almost undetectable levels of trace metals. While limnologically similar in winter, during spring-summer Lake Freshwater has significantly higher levels of chlorophyll-alpha, total phosphorus (TP) and turbidity than Blue Lagoon and other dune lakes, indicating seasonal mesotrophy. The key nutrient is TP, which has recently increased to a maximum of 17 mu-g cntdot l-1, due either to inputs from recreational souces, or to the death and decomposition of littoral vegetation resulting from falling water levels over the last decade. Inorganic nitrogen, though present only in small amounts, does not appear to limit the eutrophication process because of a shift in phytoplankton dominance from the usual desmids and dinoflagellates to N-2-fixing blue-green algae. A chlorophyll - TP linear regression derived for dune lakes indicates that at TP lt 20 mu-g cntdot l-1 chlorophyll 'yield' is higher than in other lake types represented by regression from the literature. This may be due to a more efficient utilization of the limited available phosphorus by dune lake algae which have adapted to the naturally oligotrophic environment. The implications of these findings for lake management are discussed.
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