Diatom response to the whole lake manipulation of a eutrophic urban impoundment

Abstract

Diatoms typically dominate during April and May in Ford Lake, a eutrophic urban impoundment in southeastern Michigan, USA, whereas cyanobacteria dominate during summer. To alter this, whole lake experiments were conducted during 2006 and 2008 in alternation with control years 2005 and 2007. In experimental years, water was withdrawn from the hypolimnion during summer to destabilize the stratified water column, promote deeper mixing, and reduce anoxia. In 2006, withdrawals occurred over a 6-week period with three discrete intervals, releasing 3 × 105 m3 d−1 for the first and third intervals, and 1.5 × 105 m3 d−1 for the second. In 2008, withdrawal occurred continuously for 6 weeks at a rate of 3 × 105 m3 d−1. In control years, water was released only from the epilimnion. Nuisance cyanobacteria replaced diatoms in control years when the lake became thermally stratified. Diatom response to experimentally induced mixing was examined with focus on 2008 as a model for future management. Selective withdrawal of hypolimnetic water destabilized the water column and promoted diatom summer blooms. Diatom success was greatest in 2008, with abundance reaching 10 mm3 L−1 and persisting through spring and summer. Dominance peaks of diatoms were punctuated when Daphnia populations approached or exceeded 3 × 105 individuals m−2.