2003 Volume 89 Issue 4 Pages 430-437
Human activities have increased an input of nitrogen to coastal waters. Hyper nitrogenous and phosphorous limiting condition induces red tides of harmful algae which can vitally grown on a wide variety of phosphorous compounds. Environmental quality there is then reduced by altered phytoplankton composition. We hypothesize that such hyper nitrogenous condition can be turned into healthy eutrophic condition, where desirable phytoplankton assemblage can grow, by use of a steelmaking slag as a source of phosphate and silicate. We examined the effect of enrichment of the dephosphorization slag on a natural phytoplankton assemblage in the hyper nitrogenous water with treated municipal sewage. The enrichment enhanced phytoplankton growth particularly of diatoms, such as Skeletonema costatum, while excess enrichment (200 mg/l) suppressed phytoplankton growth probably due to pH increase and/or ammonia toxicity (un-ionized NH3 form) appeared under increasing pH. These results indicate that slag enrichment would be effective at lower dose to avoid pH increase. Although 20 mg/l slag and 20% sewage was confirmed to be the best dose at 10°C, the dose should be reduced during summer since the percent NH3 increases with temperature.