Estuarine Mixing and Spatial Distribution of Phytoplankton in the Chikugo River Estuary

Abstract

Spatial distribution and temporal variation of salinity, suspended sediment concentration (SSC) and chlorophyll pigments in the macro-tidal Chikugo river estuary, Japan, were measured for two weeks and investigated the effect of salinity mixing and the estuarine turbidity maximum (ETM) on the state of phy-toplankton. The estuary was in a vertically well-mixed condition during spring tide with SSC exceeded 1,000 mg/l. The estuary changed to stratified condition during the neap tide with SSC of less than 20 mg/l. Chlorophyll-a (Chl-a) concentration was relatively low during spring tide and became high during neap. Pheophytin-a (Pheo-a) concentration was high during spring tide and it became low during neap tide. Chl-a rate in total phytoplankton concentration was 20-60% during spring tide and it reached 90% during neap tide. Peak concentration of Chl-a was located in the upper reach of the salinity front. It was found that the phytoplankton in the Chikugo River estuary mainly inhabit in brackish water below 1 psu. Good relationship between location of surface SSC maximum and Pheo-a length indicated that the detritus moved with suspended sediment, and development of the ETM and the deactivation of phytoplankton interact each other.