This dataset contains stable isotope analysis data for δ¹³C and δ¹⁵N from the study titled "Variations in organic source contributions to macrobenthic species along a sandy beach influenced by a freshwater discharge" (Seltnercich et al., unpublished manuscript). Samples were collected at an exposed sandy beach located between La Coronilla (33°50′S; 53°27′W) and Barra del Chuy (33°40′S; 53°20′W) along the Atlantic coast of Uruguay. Analysis was performed at the UC Davis Stable Isotope Facility (Davis, California, USA) using an isotope ratio mass spectrometer (IRMS). Isotope values were calculated from linear calibration curves constructed with standard reference materials of known composition. These reference materials were previously calibrated against international standards, including: IAEA-600, USGS-40, USGS-41, USGS-42, USGS-43, USGS-61, USGS-64, and USGS-6. The long-term standard deviations for internal standards used at the Stable Isotope Facility are ±0.2‰ for carbon and ±0.3‰ for nitrogen. Isotope ratios are expressed in delta (δ) notation as per mil (‰) variation, following the equation: δX = (Rsample/Rstandard -1) x1000 where R represents the ratio of heavy to light isotopes (¹³C/¹²C for carbon and ¹⁵N/¹⁴N for nitrogen), with reference to international standards: Vienna-Pee Dee Belemnite (V-PDB) for carbon and AIR for nitrogen. Samples included in the dataset: Thoracophelia furcifera from sites A (THO-A), B (THO-B) and C (THO-C). Excirolana armata from sites A (EXC-A), B (EXC-B) and C (EXC-C). Particulate organic matter from sites A (POM-A), B (POM-B) and C (POM-C). Grateloupia turuturu (GTU). Jania sp. (JAN) Grateloupia filicina (GFI). Ulva sp. (ULV). Juncus sp. (JUN). Lemna sp. (LEM). Pistia stratiotes (PST). Genidens barbus (GEN). Myliobatis sp. (MYL). Information included for each sample: Sample identification Organism/material δ¹³C values (‰, relative to V-PDB) Total carbon (µg) Carbon sample comments δ¹⁵N values (‰, relative to AIR) Total nitrogen (µg) Nitrogen sample comments Type of material analyzed Type of analysis performed Sample weight (mg)