Nutrient and phytoplankton dynamics in Hong Kong and their response to sewage abatement

The Hong Kong government has a 15-year time series of ambient nutrients and phytoplankton dynamics in Victoria Harbor and its vicinity. Ammonium and phosphate are good indicators of sewage inputs where screened, untreated sewage was discharged into the harbor until 2001. Total inorganic nitrogen, nitrate and chlorophyll have been increasing and dissolved oxygen has decreased during this 15-year period. The phytoplankton biomass was usually dominated by fast growing centric diatoms and chlorophyll ranged from 5 to 20 mu g/L. During the last four years, 70% of the sewage is now receiving primary treatment at a sewage treatment plant located several kilometers away. We have been following the recovery in Victoria Harbor with a focus on nutrients, dissolved oxygen, chlorophyll and phytoplankton dynamics. Ammonium, phosphate and total inorganic nitrogen concentrations have declined since sewage treatment has begun. However, increasing Chl-a and decreasing dissolved oxygen in the bottom waters near the new sewage treatment plant indicates that the potential eutrophication problem may have been moved from the harbor to the vicinity of the sewage treatment plant