Samples were collected seasonally from three stations in the Dardanelles between April 2016 and October 2017 to determine the effects of biological and physicochemical properties of the coastal system on the isotopic compositions of the most abundant copepod species, Acartia clausi and Calanus helgolandicus, and a cladoceran, Penilia avirostris. Plankton samples were collected horizontally with a 200-mu m-mesh plankton net and isotopic analyses were performed by mass spectrometry after identification to species level. Total zooplankton abundance reached its maximum in spring and summer. The delta C-13 values of copepods showed statistically significant differences both between stations and seasons. As a selective feeder, C. helgolandicus had the highest values in both delta C-13 and delta N-15. Being a filter feeder, P. avirostris distinctly differed from C. helgolandicus and A. clausi. Enrichment was observed in delta N-15 values of copepods in winter, when the highest values of total suspended solids and chlorophyll-a were determined. The results showed that delta C-13 values of the copepods were under the influence of environmental and biological factors. Additionally, dietary differences between herbivorous groups and other groups were clearly demonstrated, as well as differences in the diets of filter-feeding and selectively-feeding zooplanktonic species.