Dissolved organic carbon (DOC) concentrations and zooplankton and particulate organic matter (POM) ${delta}^{13}C$ values were measured in five reservoirs in Korea. Zooplankton ${delta}^{13}C$ and POM ${delta}^{13}C$ showed large range from -33‰ to -22‰ and a significant difference among the reservoirs. One eutrophic reservoir, Lake Masan, showed unique characteristics with the highest zooplankton density, the highest ${delta}^{13}C$, and the highest DOC. Zooplankton ${delta}^{13}C$ was similar to POM ${delta}^{13}C$, implying that zooplankton occupies substantial portion of POM or that zooplankton isotopic composition is related to selective grazing and assimilation of food sources from bulk POM. Except Lake Masan zooplankton ${delta}^{13}C$ values were negatively correlated to DOC concentration in four reservoirs with mostly forest land use. This pattern can be probably attributed to intensive agricultural land use in the watershed of Lake Masan compared to the mostly forest land use in the other watersheds. Understanding the relationship between zooplankton ${delta}^{13}C$ values and the origin of organic matter associated with watershed characteristics will be valuable to better understand trophic relationships in reservoirs in the summer monsoon region.