Two approaches were used to study coupling between seagrass production and bacterial processes in the rhizosphere. In the first approach, stable carbon-isotope ratios of bacteria, sediment organic matter and plants were determined to infer sources of organic carbon used by bacteria in the sediments of 4 European Zostera marina and 2 Z. noltii meadows. Bacterial isotope ratios were derived from bacteria-specific polar lipid-derived fatty acids (PLFA), mainly methyl-branched i15:0 and a15:0. Bacterial δ 13 C ratios in the sediment from both vegetated and unvegetated sites were mostly similar and did not resemble Z. marina ratios, suggesting that seagrass material was of limited importance as a bacterial carbon source. Bacterial ratios were in most cases similar to benthic macroalgae and did correlate well with ratios of diatom biomarkers. Sediment organic matter inside the meadows had δ 13 C ratios similar to those of nearby unvegetated sites, and ratios were clearly different from the material produced by seagrasses, indicating that little seagrass material accumulated. Results from the 2 Z. noltii sites were less conclusive, as there was no clear difference in δ 13 C ratios between the potential source materials. In addition, bacterial δ 13 C ratios were highly variable at one Z. noltii site. In the second approach, cultured Z. marina was labeled with 13 C-bicarbonate to study the short-term transfer of label from plants to bacteria in the rhizosphere. However, no label was detected in bacterial PLFA after 20 h of incubation. In conclusion, a close coupling between macrophyte production and bacterial carbon cycling could not be detected in the sediment of Z. marina meadows, and benthic production by algae was probably the main carbon source for bacterial growth.KEY WORDS: Seagrass meadows · Bacterial carbon sources · Sediment · Stable carbon isotopes · Biomarkers · Zostera marina · Zostera noltii Resale or republication not permitted without written consent of the publisher