Freshwater unionid mussels have low mobility and filter-feed largely on suspended fine particulate organic matter (SFPOM). Since food web in lentic systems are commonly based on energy originating from SFPOM, mussel tissue can contain valuable information regarding food-web structure and human pressure. Stable isotope ratios of tissues (δ15N & δ13C) can imply the presence of anthropogenic nitrogen loading and provide a baseline of trophic levels. We examined if C/N stable isotope ratios of tissues of unionid mussels can serve as a indicator of eutrophication and the measure of a baseline for food chain length (FCL). Mussel tissues and water sampling were conducted in four occasions on December 2015, May, July and November 2016 in 42 floodplain lakes along Ishikari River. There was no significant correlation between tissue δ15N and any of eutrophication measures of water (e.g., NO3). δ15N and δ13C significantly varied with season, but the medians seasonally changed by less than 1‰ which was much less than the ranges of FCL estimates among lakes (i.e., a median of 5‰). This suggests the usefulness of mussel δ15N as a trophic baseline. Possible effects of denitrification and/or flexibility of food resource choices of mussels might have precluded the detection of eutrophication.