Understanding the role of induced mixing for management of nuisance algal blooms in an urbanized reservoir

Abstract

Mixed layer dynamics were studied during June to August in 2008 to 2012 in Ford Lake, Michigan, using real-time data acquisition from in situ water temperature, dissolved oxygen, and chlorophyll sensors plus a shore-side weather station. The purpose of the study was to examine the effectiveness of managed hypolimnetic discharge for inducing vertical mixing, preventing hypolimnetic anoxia, and reducing the incidences of cyanobacterial blooms. Maximum mixed layer thickness each day was deduced from vertical density profiles measured at 30-minute intervals plus estimated kinetic energy supplied by wind stress and convection. Surface heat flux and resulting convection was more important to mixing dynamics than was wind stress. Selective withdrawal of hypolimnetic water was practiced in 2008 to 2011 and resulted in enhanced vertical mixing, hypolimnetic oxygenation, and absence of nuisance cyanobacterial blooms. In 2012, drought conditions restricted upstream water availability to such an extent that selective hypolimnetic withdrawal could not be used that year. The result was more than a month-long period of stable thermal stratification during which hypolimnetic oxygen concentrations became depleted and phosphate was released from anoxic sediments. A subsequent cold weather event induced temporary destratification, and a bloom of Aphanizomenon quickly ensued, despite reduced external loading of P from the influent river. Results of this study point to mixing dynamics and internal nutrient loading as the master variables responsible for nuisance algal blooms in this system.