Abstract
Field observation of water quality, plankton biomass and bacteria mass was carried out to analyze and discuss the characteristics of changes in water quality and N : P ratio on the growth yield of phytoplankton at a sea based dredged soil disposal site, which has stabilization ponds (Osaka North Port Solid Waste Disposal Site, South District) . The stabilization ponds were always in a red tide condition and hyper-eutrophic.
The PO4-P concentration decreased, and the organic-phosphorus concentration increased during the phytoplankton exponential growth phase. The particle-COD concentration was closely related to phytoplankton biomass, which was low in summer due to active predation by zooplankton, but high in spring and autumn because of little predation and phytoplankton exponential growth phase. The cause of the high COD concentration was attributed to phytoplankton biomass.
The increase of T-P and PO4-P concentration in summer may be attributed to an increase of elution from the bottom sediment with the rise in water temperature and shortage of dissolved oxygen. The increase of T-N and NH4-N concentration in winter may be attributed to the following factors ; (1) the nutrient release for resuspension of the sediment when the depth was shallower than 2.4m, (2) the increase of nutrient elution due to increase in volume of dredged soil which was mixed with sea water.
The N : P ratio of phytoplankton was about 8 and had no correlational coefficient with the N : P ratio of environmental nutrients (DIN/DIP) . The N : P ratio of each phytoplankton group, such as Bacillariophyceae, Flagellata and Chlorophyceae, was not statistically different.
