Simulation of water-bloom formation in the cyanobacterium Microcystis aeruginosa

A mechanism for buoyancy increases in the cyanobacterium Microcystis aeruginosa and the associated formation of surface water-blooms is presented. The mechanism is based on consider- ing a response time in the rate of carbohydrate accumulation. When irradiance increases, the Micro- cystis cells may require time to increase their rate of carbohydrate accumulation. If irradiance decreases before adjustment, the maximum rate of carbohydrate accumulation is not reached. Colony buoyancy increases during mixing when the time scales of the light fluctuations are shorter than the response time. To examine the mechanism, a model of Microcystis buoyancy that incorporates the response time has been coupled with a hydrodynamics model that simulates mixing. The model was applied to a shallow lake to show that a prolonged episode of intense mixing caused the simulated Microcystis colonies to become excessively buoyant. Once the mixing subsided, the colonies accumu- lated at the surface. Decreases in carbohydrate were reduced in large colonies as their size afforded buoyancy forces that could readily overcome the entraining forces of the mixing.

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