Geo-Environment Science

The purpose of this study is to evaluate the aquatic environment of eutrophic reservoirs based on the species composition of phytoplankton and zooplankton, their seasonal change, and the relationship between water quality and phytoplankton. The surveys of planktons and water qualities related to organic pollution and eutrophication were conducted from spring to autumn in 2007 at four agricultural reservoirs: Gogouchousei-ike, Hirakawa-ike, Ohsaka-ike and Ohkubo-ike, located in the Itoshima area of Fukuoka Prefecture, Japan. The observed results of phytoplankton showed that the targeted reservoirs were obviously different in the seasonal change in biomass rate and dominant genera. In Gogouchousei-ike, blue-green algae of Anabaena were generated in large quantities in summer. In Hirakawa-ike, water bloom due to Achnanthes belonging to the diatomeae was formed in spring. In Ohkubo-ike, a mass generation of blue-green algae, called Aoko, was observed for a long time from summer to autumn. In Ohsaka-ike, the floating-leaved plants overgrew the water surface, resulting in a small change in the number of algal cells and a low rate of blue-green algae. In each reservoir, the observed term was divided into several periods according to the seasonal change in the species composition of phytoplankton and vegetation of floating-leaved plants, and then correlations between the population of phytoplankton and water qualities were examined in each period. As a result, the seasonal change in dominant algae groups were found to be associated with the dynamics of water qualities related to eutrophication such as chlorophyll-a, total organic carbon, total nitrogen and total phosphorus.