Influence of offshore cage culture on surface sediment in the Yellow Sea Cold Water Mass: Assessed with stable isotope and fatty acid analyses.

Offshore aquaculture is undergoing rapid promotion and development in China. However, the potential impacts of offshore aquaculture activities on the marine environment remain insufficiently assessed, which could affect its sustainable development. This study investigated sediments at distances of 0 m (station A), 200 m (station B), 500 m (station C), 1000 m (station D) and 2000 m (station E) from the first submersible cage, "Deep Blue 1", in the Yellow Sea Cold Water Mass. The total organic carbon (TOC) content, total nitrogen (TN) content, isotopic composition (δC and δN), and fatty acid profiles of the sediment samples were analyzed in June 2021, August 2021, December 2021, and April 2022. The results indicated that TN content at station A was significantly increased in June and August and δN was significantly enriched in August compared to station E. The mixing model analysis based on δC and δN values suggested that the relative contribution of aquaculture waste to the sources of sediment organic matter at station A was highest in August (47.2 %), followed by June (16.7 %), April (3.1 %), and December (1.9 %). In fatty acid composition, the relative contents of unsaturated fatty acids at stations A and B were significantly higher than those at station E in June and April. The fatty acid biomarkers (linoleic acid and oleic acid) were significantly enriched at station A compared to station E in June, August and April. However, the impact of aquaculture waste in December was negligible based on the assessment of all indicators. Overall, offshore cage aquaculture has an impact on marine surface sediments during active farming seasons, but this impact was mitigated with the cessation of aquaculture activities. This study emphasizes the necessity of establishing effective long-term monitoring mechanisms to ensure the sustainable development of offshore aquaculture in the Yellow Sea Cold Water Mass.