Multiyear destratification study of an urban, temperate climate, eutrophic lake

Abstract

Abstract Hanson D, Austin D. 2012. Multiyear destratification study of an urban, temperate climate, eutrophic lake. Lake Reserv Manage. 28: 107-119. An aeration system for destratification has been in operation since 1973 at Sweeney Lake, an urban, eutrophic lake near Minneapolis, Minnesota, USA. Water quality data are available for 19 of the 39 years, 1972–2010. Growing season (Apr–Oct) aeration was turned off in 2007–2008 to evaluate the effect of destratification on water quality. Aeration never achieved the Lorenzen and Fast (LF) complete destratification airflow criterion of 9.2 m3/min/km2. When aeration was at 51% LF, hypoxia (DO < 2.0 mg/L) occurred June–August at the lake bottom. When airflow increased to 65% LF, bottom water hypoxia was restricted to 4 weeks in late July–August. There were no systematically significant differences (p > 0.05) in median Secchi disk depths, chlorophyll a concentrations, or surface total phosphorus (TP) concentrations between destratified and stratified lake conditions, but interannual variability linked to other influences, including partial stratification and stormwater inputs, limits statistical power. The median April–October lake TP mass of 67 kg in 2007–2008 (stratified conditions) was significantly higher (p < 0.0001) than the 44 kg median mass in1998–2005 and 2009 (destratified conditions). Destratification can suppress internal phosphorus loading when properly applied.