Fertility of domestic sewage and eutrophication in estuarine harbor

The concentration and partition ratio of various speciations of nutrients in domestic sewage were determined.The transformation and transportation among the speciations,as well as their biological effects during sewage-seawater mixing were simulated in laboratory.The results are compared with field observation and the following findings are:ⅰ) The suitable range of ratio DIN/DIP in seawater for growth of phytoplankton in subtropical estuary and harbor is quite wide.It could grow well even in the range of 15-55 in atom ratio,and is independ of the variation in levels of N and P.ⅱ) The transformation rate among the speciations of phosphorus is within the range of 0.5 to 1.1 μmol/d.ⅲ) Phytoplankton uptakes P prior to N during its growth.The growth rate for Skeletonema costatum (the major species) and field algae are 0.34-0.58/d and 0.30-0.31/d,respectively.ⅳ) The red tide species Prorocentrum micans could become the dominant species to a density 10⁷ cells/L after Skeletonema costatum blooms and phosphorus is almost exhausted.