Effects of freshwater flow on environmental factors and copepod density in the Chikugo estuary, Japan

This study describes the effects of freshwater flow on the major environmental parameters (temperature, salinity, turbidity, chlorophyll-a and phaeopigment) and the density of the two most dominant copepods, a calanoid copepod Sinocalanus sinensis and a cyclopoid copepod Oithona davisae, in the Chikugo River estuary in Ariake Bay, Japan. Freshwater flow negatively and significantly influenced salinity and phaeopigment but the influence on temperature, turbidity and chlorophyll-a was not significant. Freshwater flow also negatively and significantly influenced the distance of turbidity maximum from the river mouth. Density of S. sinensis which is distributed in the upstream areas was negatively and significantly influenced by freshwater flow while the density of O. davisae which is distributed in the downstream areas was not influenced by freshwater flow. The effects of freshwater flow appear to be associated with the spatio-temporal patterns of the dependent parameters. The strongest influences of freshwater flow occurred at the upper part of the estuary within 1–7 days and the influences weakened with space and time.