Japanese Journal of Water Treatment Biology

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The purpose of this study was to determine whether Microcystis blooms could be reoccurred in the enclosure constructed in the eutrophic pond in the urban park where Microcystis blooms occurred annually. The size of the enclosure was 10 m square, and the lower end of the enclosure was embedded in sediment to isolate from outside water. The water depth of this pond was about 1 m.

The occurrence of Microcystis blooms and changes of water quality items in the enclosure were investigated for two and a half years and compared with those in the urban park pond. As in the case of the urban park pond, Microcystis blooms were occurred in the enclosure in the summer for three years, but the density in the third year （1,500 colonies/mL） was decreased significantly compared to that in the first year （5,360 colonies/mL）.

In the enclosure, the water temperature and concentrations of total phosphorus and particulate phosphorus showed seasonal changes similar to those in the urban park pond over two and a half years. These results indicated that phosphorus released from the sediment as the water temperature increased caused the occurrence of Microcystis blooms. Since this mechanism was similar for the urban park pond and the enclosure, it was considered that similarity was observed in the occurrence of Microcystis blooms.

In the enclosure, concentrations of nutrients （total nitrogen, total phosphorus, particulate nitrogen and particulate phosphorus） showed seasonal changes with increasing and decreasing concentrations of chlorophyll a. These changes were similar to those in the urban park pond.

In addition, the densities of competitors （diatom） and predators （small-bodied cladocerans, copepods, and rotifers） showed similarly seasonal changes in the enclosure and the urban park pond.

These results suggest that the enclosure is useful in evaluating water purification technologies for eutrophic urban park ponds where Microcystis blooms have occurred.

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