Relation between Miniaturization of Phytoplankton And Long-Term Variation of Water Quality in Ise Bay

Abstract

The decadal change of the water quality of Ise Bay since the 1980s was studied with the use of the observation data obtained by the Wide Area Comprehensive Water Quality Survey conducted by the Ministry of the Environment of Japan. The study disclosed the trend of miniaturization of phytoplankton and the increment of DOC:TOC ratio at the surface water. The miniaturization occurred across the entire bay and the carbon content per a cell is in proportion to TN:TP ratio that has been declining according to the governmental action for reducing nutrient loads into the bay. Since the miniaturization enhances an intake rate of nutrients, phytoplankton of smaller cells could have advantage against the nutrient deficient condition at the surface water. In addition, the smaller cell is apt to enhance the primary production through its higher photosynthesis rate. The long lasting question for the continuation of high COD concentration at the surface water in spite of the prescribed governmental action could be explained by this argument. If so, the miniaturization could affect as well the generation of the hypoxic water mass above the seabed, i.e. the most serious problem of the basin. The source of DOC for the high DOC:TOC phenomenon may be the exudation from phytoplankton which releases DOC in the nutrient deficient condition at the surface water.