Phosphorus limitation of primary productivity during the spring-summer blooms in Sagami Bay, Japan

In coastal regions of Sagami Bay, Japan, we examined the nutrient limitations of primary productivity of phytoplankton blooms during spring to summer 2000, using 2 approaches; (1) dissolved nutrient concentrations and ratios, and (2) bioassay experiments. During the study period, maximum concentration of nitrate + nitrite (19.2 muM) and silicate (31.2 muM) were measured on July 10 after heavy rainfalls, where the supplies of nitrate, nitrite and silicate to the coastal regions primarily related to the increase in freshwater discharge by precipitation. However, such relationships were not found regarding the phosphate concentration. Chlorophyll (Chl) a concentration increased after the increase in freshwater discharge and Chl a peaks over 10 mg m(-3) were observed 5 times. Phosphate concentration observed at the depth of the 5 Chl a peaks was low enough to indicate probable P limitation based on criteria of nutrient concentrations and ratios. Bioassay experiments were carried out 12 times, every 8 d from May 1 to July 28. Primary productivities following nitrate and silicate additions showed no response compared to the controls. However, phosphate additions in the post-bloom period caused a significant increase in the primary productivity. On May 25, the primary productivity increased 51% relative to the control. These results suggest that phytoplankton productivity was limited by phosphate during the spring-summer blooms, and that phosphate availability in the Sagami Bay is an important factor in the termination of the blooms.