MECHANISMS AND EFFICIENCY OF ALGAL BLOOM REGULATION BY AERATION/CIRCULATION IN RESERVOIRS

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First, impacts of aeration/circulation on the algal bloom were analyzed from previous observations in actual reservoirs. Then, structures to reduce algal bloom were classified as follows.(1)The formation of the epilimnetic mixed layr thicker than the euphotic layr, causing the light limitatin for the algal growth first, then changing the predominant species from cyanobacteria to greens and diatoms.(2)The reduction of pH, causing the change in the dominant algae from cyanobacteria to edible greens and diatoms.(3)The lowering epilimnetic temperature, reducing the algal growth rate.(4)The hypolimnetic aeration.These mechanisms were confirmed and evaluated by a numerical model, which was comprised of a reservoir model based on DYRESM,double plume circulation model, nutrients model, and biological process, including 4 types of algae.First, several experimental results were simulated, indicating process of the algal biomass change during the operation. Then, successful and unsuccessful operational conditions were parameterized by making calculations in various boundary conditions.Next, the condition for the most effcient destratification was parameterized, which shows that the destratification would be completed with less than 10% of energy being used at present.Since the important structure is not only the reduction of algal biomass but also the transion from cyanobacteria predominacy to edible species, which eventually changes the entire ecological condition, thus this bottom-up effects will be the key point. Thus, the field investigation was made in several lakes and reservoirs to understand the food web mechanisms. Also, a simple food web model was developed including the higher predators, such as zooplankton and two kinds of fish.