PHYTOPLANKTON HABITATS RESPONSE TO THE VARIABILITY OF ESTUARINE HYDRODYNAMIC CONDITION IN THE MACROTIDAL CHIKUGO RIVER ESTUARY

The effects of hydrodynamic and mixing conditions on the variation of phytoplankton composition and their habitats were investigated in the tide-dominated macrotidal Chikugo River estuary during a neap-spring tidal cycle in 2021. The estuary changed from stratified to well-mixed conditions during a neap-spring transition. The river discharge was < 60 m³s^-1 during the study period. Seawater intruded towards 17 km (upstream) during neap tide and until 16 km during spring tide. Surface suspended sediment concentration (SSC) was low during neap tide and maximum (∼400 mg/L) during spring tide corresponding with changes in mixing and an estuarine turbidity maximum (ETM) was developed between 8-12 km during spring tide. Marine habitats diatoms were the major group with 20-89% of total phytoplankton. During a neap-spring tidal cycle, the maximum abundance of freshwater green algae was found during neap tide while diatoms (both marine and freshwater habitats) was in two days after neap tide and freshwater blue-green algae was in intermediate tide in response to changes in mixing conditions. The distribution of marine habitat diatoms and freshwater blue-green algae were positively correlated with salinity whereas freshwater green algae and freshwater habitat diatoms were negatively correlated with salinity. This study concludes that saltwater intrusion and mixing conditions driven by tidal forcing mainly controlled the species composition and their habitats in the Chikugo River estuary.