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 phytoplankton. 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.