Annual environmental changes in large yellow croaker aquaculture in Dachen Island, China

Deep-sea mariculture, particularly copper (Cu) alloy fence mariculture, is increasingly regarded as a sustainable model for future mariculture development. However, limited research has been conducted on its impact on seawater and sediment. To investigate variations in environmental factors induced by large-scale enclosure aquaculture of the large yellow croaker ( Larimichthys crocea ), a comprehensive quarterly analysis was conducted in the sea area near Dachen Island, China, from 2021 to 2023. This study focused on changes in pH, dissolved oxygen (DO), Cu, chlorophyll a , dissolved inorganic nitrogen (DIN), and labile phosphate ( PO 4 3 - ) in seawater. Additionally, variations in total nitrogen (TN), total phosphorus (TP), and total organic carbon (TOC) content and pollution levels in the sediment were examined. The Nemerow index was used to assess the quality of both seawater and sediment. In 2021 and 2022, the highest DIN content in seawater was observed during the summer, reaching 0.491 ± 0.161 mg·L −1 and 0.485 ± 0.101 mg·L −1 , respectively. Contrastingly, in 2023, the peak DIN concentration occurred in autumn, escalating to 0.597 ± 0.054 mg·L −1 . Over the three years, the content of PO 4 3 - in seawater exhibited elevated levels during summer and autumn, with the highest concentration of 0.087 ± 0.009 mg·L −1 observed in autumn 2023. Notably, the Cu content in spring 2023 was 4.09 ± 0.36 μg·L −1 , making the highest quarterly value within the three-year period. This data indicated a discernible accumulation trend for DIN, PO 4 3 - , and Cu in seawater. Additionally, TN, TP, and TOC levels in the sediment demonstrated an increasing trend over the three years. The maximum TN content recorded in the sediment was 0.69 ± 0.15 mg·L −1 in 2021, 0.74 ± 0.12 mg·L −1 in 2022, and 0.90 ± 0.15 mg·L −1 in 2023. For TP, the highest content was 0.66 ± 0.17 mg·L −1 in 2021, 0.60 ± 0.18 mg·L −1 in 2022, and 0.85 ± 0.12 mg·L −1 in 2023. The maximum TOC content was 0.74 ± 0.15% in 2021, 0.74 ± 0.13% in 2022, and 0.95 ± 0.12% in 2023. The Nemerow index for both seawater and sediment showed a consistent increasing trend over the three-year period. Although seawater quality remained at Class III (mild pollution) for most of this period, it approached Class IV (moderate pollution) during the summer and autumn of 2023. During these three years, sediment quality reached Class II (relatively clean) only in the winter of 2021 and the spring of 2022; however, it remained C