Stratification and biogenic meromixis in Tasmanian reservoirs

Abstract

Part of the Mersey-Forth hydro-electric scheme, Tasmania, Lakes Rowallan and Barrington are oligotrophic, slightly humic, warm monomictic reservoirs. Their flooded vegetation, shelter and deep V-section lead to pronounced clinograde oxygen profiles despite meagre autochthonous production.

With surprising rapidity, Lake Barrington developed a monomolimnetic pool which, when destroyed by severe floods, soon reformed. Though crenogenic and triptogenic elements may participate, principal causes of this partial meromixis are flooded vegetation and basin morphometry. The monimolimnion exhibits unusually large temperature rises and considerable enrichment of such chemical species as iron, manganese, hydrogen suiphide and bicarbonate. Iron is a major factor in the meromixis and largely accounts for marked monimolimnetic rises in electrical conductivity. The sharp upper monimolimnetic boundary is an almost horizontal chemocline, above which the mixolimnion behaves as a warm monomictic lake. Winter ventilation of its hypolimnion is tardy but complete. The monimolimnion is therefore maintained by decomposition and basin morphometry, not climate.

Attendant chemical phenomena of prolonged anoxia in hypolimnetic or monimolimnetic water pose alleviable problems when that water is used domestically, industrially or for power generation.