Effect of Freshwater Due to Heavy Rain on Phytoplankton Growth in Hiroshima Bay

• Published in: Journal of Japan Society on Water Environment Vol. 19; no. 12; pp. 995 - 1003
• Main Authors: LEE, Young Sik, YAMADA, Takahisa, SEIKI, Tohru, MUKAI, Tetsuo, TAKIMOTO, Kazuto, OKADA, Mitsumasa
• Format: Journal Article
• Language: Japanese
• Published: Japan Society on Water Environment 10.12.1996
• Subjects: chlorophyll a concentration; DIN/DIP ratio; freshwater; heavy rain; limiting nutrient
• ISSN: 0916-8958; 1881-3690
• DOI: 10.2965/jswe.19.995

Growth of phytoplankton and its limiting nutrient were investigated to elucidate the mechanism of phytoplankton growth caused by inflow of freshwater. Limiting nutrient of phytoplankton growth was nitrogen on little volume of freshwater inflow. However, the limiting nutrient was shifted from nitrogen to phosphorus after heavy rain. This may be due to the inflow of freshwater of DIN/DIP ratio higher than Redfield ratio 16 : 1. Chlorophyll a concentration was very low on little volume of freshwater inflow. On the other hand, after high volume inflow of fresh water, high value of the chlorophyll a concentration was recorded. The occurrence of marked phytoplankton growth after heavy rain can be explained as follows. First, inflow of freshwater that has high concentration of nitrogen induced the growth of phytoplankton. Second, phytoplankton growth was controlled by phosphorus loaded from fresh water, because the limiting nutrient was shifted from nitrogen to phosphorus after heavy rain.