Lakes Sorell and Crescent—A Tasmanian Paradox

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Abstract
Lakes Sorell and Crescent are closely adjacent shallow lakes on the Tasmanian Central Plateau. They have similar morphometry and similar climate, geology, soils and vegetation in their catchments. They are polymictic, oxygen‐saturated and colourless but turbid. They have soft water with major ions Na, Ca, Mg, Cl, HCO3present in almost equi‐ionic quantities, and a slightly alkaline reaction. Chemically they are alike, the major difference being a 20% higher salinity in Crescent from 1967–69. During 1969–71 heavy rains reduced this difference. Water frequently flows from Sorell to Crescent in any year.Despite these similarities their phytoplankton populations differ markedly in every respect—species composition, population structure, population stability and total biomass. Lake Crescent has a standing crop 10 X that of Sorell. The former is eutrophic, the latter mesotrophic. Well marked seasonal cycles do not occur but sporadic fluctuations of biomass are brought about by hydrologic or other events.The pronounced differences in every aspect of plankton populations in two so similar and connected lakes cannot yet be explained. They remain a limnological paradox.