Understanding the night ecology of poisonous blue-green algae and its application to good-quality water intake system

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In the nocturnal observation conducted during August and September, small lumps of bloom stayed at a fixed depth of water between 2 and 4 a.m. without any particular motion. Blooms on the water surface started to disperse starting around 2 p.m. and they are observed only scarcely on the surface by 4 p.m. Such cycle was observed every day until late September. The rhythm of this cycle indicates that behavior of photosynthetic algae cannot be explained only by photo-taxis. We could find that bloom goes down under water during night time and stays at a certain depth during certain period. However, we could not create good-quality water intake system to intake raw water of a good quality. To realize this, the following key issues need to be addressed: 1. Vertical motions spanning a few months through the initial phase to decline phase of bloom forming; 2. Determination of the toxicity of metabolite excreted by bloom forming algae; 3. Cell behavior after the decomposition of bloom.