Responses of phytoplankton to the enrichment with steelmaking slags, de-Si and de-P slags, were determined by culture experiment in largvolume outdoor tanks. In the first series, chlorophyll-a concentration attained its maximum on the 11th or 12th day, whereas on the 4th or 5th day in the second series. After the maximum, cultures became nitrogen limiting in every treatment, indicating that both de-Si and de-P slags cannot supply nitrogen while phosphate and silicate can be supplied. Change in size composition was also observed for phytoplankton enriched with slags: larger fraction (> 10 μm) became dominant over smaller fraction (< 10 μm). These results indicate that elements such as P, Si and Fe released from the slags are available for phytoplankton.
