Phytoplankton Population Succession in Hyper-Eutrophic Stabilization Ponds Located at a Sea Based Dredged Sludge Disposal Site.

Abstract

Field studies of water quality and phytoplankton community at the stabilization ponds of Osaka North Port Solid Waste Disposal Site (ONPSWDS) were carried out to analyze the relationship between eutrophication and the succession of phytoplankton of the red tide.Water temperature of the stabilization ponds varied from 6.5°C in winter to 30.0°C in summer. Its water salinity ranged from 27.2 to 29.2. The ponds were found to be hyper-eutrophic because they contained much total nitrogen (5 to 15mg/l) and total phosphorous (0.5 to 2mg/l) .Chlorophyll a concentrations showed a stepwise increase and decrease pattern with values varying at about 20, above 90 and 50, μg/l in the first, second and last step, respectively. pCOD variation followed the same pattern as chlorophyll a concentration with values varying at about 5, above 10 and 10 mg/l. Phytoplankton cell number varied from 103cells/ml level in the first twenty months of reclamation to 105cells/ml level in the advanced stage of reclamation.The flora of Bacillariophyceae species was abundant and diversified at the earlier stage of reclamation. Other groups such as Flagellata and Chlorophyceae became dominant as reclamation and eutrophication proceeded. It was also observed that when Bacillariophyceae occurred in abundance, Flagellata or Chlorophyceae disappeared or appeared in low number conversely.The increase in Bacillariophyceae cell number was followed by the increase in silica concentration. These findings indicate that silica plays important roles as a thriving compound for Bacillariophyceae.