Nutrient economics and trophic status of Lakes Sorell and Crescent, Tasmania

Abstract

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.