Unsteady continuous culture of the diatom Skeletonema costatum, which is a dominant species forming red tides during the warm season in Mikawa Bay, was carried out. From the reported field observations, silicate concentration was considered to be the most plausible factor governing the biomass of S. costatum in the surface layer of the stratified water column. The two possible major processes involved in Si supply, i.e. from the bottom layer by wind mixing and by fresh water runoff, occurring in situ during this season were established by reciprocal replacement with a Si-sufficient media, with or without an accompanying salinity decrease of 5 ‰ to the Si-limited culture of S. costatum. The uptake rate of nitrate and phosphate per cell increased with Si limitation. Silicate limitation also caused several noticeable changes in the morphology of S. costatum. One of these morphological changes, cell elongation, made the cell quota of N and P larger than that of Si. On the other hand, silicate uptake rate per cell showed a sharp increase with increased Si supply and this elevated Si uptake rate was 5-9 fold that under Si-limited conditions. The degree of elevation of Si uptake was higher without an accompanying salinity decrease than with a salinity decrease, indicating that osmotic pressure may not be responsible for the uptake mechanism. Furthermore, Si-limited S. costatum showed higher growth rates when supplied Si with an accompanying salinity decrease than without a salinity decrease. Judging from these results, it is supposed that Si-depleted S. costatum grows faster in situ with Si supplied through river discharge, if the salinity is optimum for S. costatum, than through overturning of the water column due to wind mixing.