To improve our understanding of high copepod productivity in the estuarine turbidity maximum (ETM) of the macrotidal Chikugo River estuary in southwestern Japan, we determined stable carbon isotope ratios (delta C-13) in the sympatric oligohaline copepods Sinocalanus sinensis and Pseudodiaptomus inopinus from 2005 to 2006. Terrestrial-plant and phytoplankton detritus always accumulated in the ETM (salinity 0.1-3), whereas outside the ETM phytoplankton dominated especially in the warm season (> 20 degrees C). In contrast to the year-round concentration of S. sinensis in the ETM, P. inopinus occurred widely along the upper estuary under phytoplanktondominated conditions. Terrestrial-plant detritus was characterized by relatively constant delta C-13 (similar to -24 parts per thousand), suggesting that significant spatiotemporal variability in copepod delta C-13 was attributable to the feeding of copepods on phytoplankton and/or its detritus. Both copepods had relatively depleted delta C-13 values in the ETM,