Dynamics of Aphanizomenon and Microcystis (cyanobacteria) during experimental manipulation of an urban impoundment

Abstract

Ford Lake in southeastern Michigan has a history of midsummer cyanobacterial blooms marked by a regular succession from heterocystous Aphanizomenon flos-aquae to nonheterocystous Microcystis aeruginosa. Nitrogen (N)-fixing Aphanizomenon has an advantage following seasonal thermal stratification because nitrate is consumed by assimilative uptake in the epilimnion and by anaerobic respiration in the anoxic hypolimnion. If nitrate levels increase, however, Microcystis succeeds Aphanizomenon. Bioassays corroborated the importance of nitrate to Microcystis success in late summer. Experimental releases of hypolimnetic water from the hydroelectric outlet dam before anoxia developed in 2006 destabilized the water column without altering lake volume and promoted a diatom bloom rather than cyanobacteria. To establish that the experimental manipulation was reproducible, we followed 2006 with a control year and then a second experimental year. In 2007, experimental manipulation was not performed, water was withdrawn from the epilimnion alone, and the lake reverted to historical dominance by first Aphanizomenon and then Microcystis. In 2008 we again commenced hypolimnetic discharge at onset of thermal stratification, and a sequence of diatom blooms resulted, demonstrating that community transformation first produced in 2006 was reproducible. The algal community again became transformed from cyanobacteria to diatoms, but overall biomass did not change. Bioassays demonstrated that Aphanizomenon became P limited during diatom dominance. Cyanobacterial blooms were postponed until after termination of the whole lake experiment and subsequent diatom decline.