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, C1, HC03 present in almost equi-ionic quantities, and a slightly alkaline reaction. Chemically they are alike, the major difference being a 200/,, higher salinity in Crescent f r m i 1967-69. During 1969-71 heavy rains reduced this difference. Water frequently flows from Sxell to Crescent in any year.Despite these similarities their phytoplankton populations differ markedly in every respectspecies corriyosition, 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 liydrologic or other events.The pronounced differences in every aspect of plankton populations in two so similar and connected lakes cannot get be explaincd. They remain a limnological paradox.

No abstract

The primary productivity of two turbid, shallow lakes on the Tasmanian Central Plateau was determined by the C 14 technique from half-light day incubations in situ. Graphical integration of depth-rate curves gave estimates of areal day rates of production and of annual rates .The 2 lakes are closely adjacent and very similar physically and chemically, but have very different phytoplankton populations . Lake Crescent has ten times the standing crop biomass of Lake Sorell but its greater turbidity restricts light penetration, and production per unit of surface per day and per year is only 2.6 times that of Sorell .With day rates of 25-(44)-93 mgCm 2 and annual production of 16 .9 gCm2 Lake Sorell could be regarded as oligotrophic . Consideration of standing crop biomass and morphometry however indicates oligo-mesotrophy . Lake Crescent with day rates of 35-(115)-250 mgCm2 and annual production of 45 gCm z is moderately eutrophic .Incubations in constant light demonstrated considerable variation in production rates in different parts of Lake Crescent .

Effects of various nutrients on short-term photosynthetic carbon uptake and longer-term yield of algae were assayed for two similar, connected lakes (Lake Sorell flowing into Lake Crescent) with contrasting plankton populations. Phosphorus (P) and phosphorus plus nitrogen (P+N) were the only nutrients to stimulate 14C uptake, and more for Lake Crescent than for Lake Sorell, over 3-h incubations, and P+N was only marginally more effective than P alone. Added SiO2 was inhibitory in short-term but not in long-term incubations. During 3-day incubations, P, N and P+N increased 14C uptake for Lake Sorell but usually not for Lake Crescent. Micronutrients and chelators also accelerated 14C incorporation for Lake Sorell but not for Lake Crescent. No nutrient tested singly increased the ultimate yield of algae, but P+N, Na2CO3 and SiO2 added incrementally produced marked, incremental increases in yield, and more so in Lake Sorell than in Lake Crescent. Addition of micronutrients had no further stimulatory effect. Removal of tripton from lake samples seriously impaired yields, even in P+N-enriched cultures. Incremental replacement of tripton produced incremental restoration of yields. Volume for volume, Lake Crescent water contained more tripton and a more organic tripton than Lake Sorell water. Weight for weight, Lake Crescent tripton stimulated faster growth than Lake Sorell tripton but ultimate yields were the same. Micronutrients and chelators together could substitute for native tripton, the role of which seems to be the maintenance of micronutrients in solution in the oxidizing milieu of the polymictic lakes. The greater productivity and greater phytoplankton biomass of Lake Crescent rests on its greater tripton load, derived from greater scouring of sediments, its flow through from Lake Sorell and an abundant fringing marshland. It is likely that the distinctive floras of the two lakes are adapted to the different nutrient conditions, so that flow-through inocula of Lake Sorell algae cannot become established in Lake Crescent. As Lakes Sorell and Crescent are already mesotrophic and eutrophic respectively, and since additions of P and P+N considerably increase productivity and yield, management plans for this recreational area should seek to limit nutrient inputs.