ABSTRACT: Seasonal changes in abundance and species composition of phytoplankton were monitored monthly and bioassays to examine nitrogen, phosphorus and silica limitation on the phytoplankton were conducted simultaneously from 2000 to 2001 in a bay ofthe Uwa Sea where many pearl farms culture the pearl oyster Pinctada fucata martensii, whose main food is phytoplankton. Dominant phytoplankton were the diatom genera Nitzschia and Chaetoceros, and dinoflagellates dominated byGymnodinium were also abundant. Systemic and growth rate limitations of phosphorus were found only in July 2000, suggesting that P was not an important limiting element in the bay. By contrast, systemic limitation of nitrogen was detected on 8 outof 12 occasions, while growth rate limitation was detected on 5 occasions. Systemic limitation without growth-rate limitation was found in September, November and December 2000, and April and May 2001, when nitrate concentrations were relatively high. Wedetected both systemic and growth-rate limitations of nitrogen on phytoplankton in June 2000, and February and March 2001, when nitrate concentrations were low. Hence, severe nutrient limitation, or deficiency, would be found if both systemic and growthrate limitations were detected simultaneously.