CHANGES IN SIZE DISTRIBUTION OF PHYTOPLANKTON IN RESPONSE TO TIDAL VARIABILITY IN THE CHIKUGO RIVER ESTUARY

Abstract

 The effect of tidal variability on the changes in phytoplankton abundance and their size distribution in the tide-dominated Chikugo River estuary was investigated in 2021. The estuary changed from a stratified condition during neap tide to well-mixed during spring tide and salinity intruded until upstream. The estuarine water was clear in the neap tide, which changed to turbid water gradually, and the estuarine turbidity maximum (ETM) zone was developed at 8-14 km in the spring tide with a maximum turbidity of ⁓900 FTU and salinity of 0.2-17. There are 159 phytoplankton species divided into 39 groups belonging to seven categories. Among them, diatoms were found as a major category and their abundance showed a similar trend to total phytoplankton abundance with 79-86% (high tide) and 76-98% (low tide). The abundance of diatoms were found the highest at the intermediate tide in both low and high tide periods. From a spatial perspective, diatoms densities decreased in the middle estuary because of the effect of ETM development. The colonial groups with branching and straight forms were dominated. The cell size of Skeletonema costatum and Nitzschia acicularis increased from neap to intermediate tides and maximum sizes were observed at the spring tide. This study concludes that intermediate tide between neap and spring tides plays an important role in diatom growth. The changes in cell size and shape are essential to retaining the phytoplankton diversity and abundance besides water mixing and ETM formation in the Chikugo River estuary.