Temporal distributions of biogenic elements
The vertical distributions of total organic carbon (TOC), total nitro- gen (TN),TOC/TN and δ13C in sediment core XCW of Xincun Lagoon are demonstrated in Fig. 4 and Table S2. The trends in TOC and TN in sediment core XCW exhibit similar patterns, and the distribution char- acteristics of their contents can be clearly divided into three stages from bottom to top (Fig. 4A, Fig. 4B): (Stage 1) Before 1950, both TOC and TN showed relatively stable variations, with average contents of 1.06 % ± 0.15 and 0.09 ± 0.01 %, respectively. The range of variation was be- tween 0.87 and 1.35 % for TOC and 0.08–0.12 % for TN (Table S2). Overall, the organic matter content in the lagoon was relatively low before 1950, indicating a stable environment without significant external disturbances; (Stage 2) Starting from 1950, TOC and TN exhibited noticeable increases. Until 1990, the contents continued to rise steadily, ranging from 1.46 to 2.30 % for TOC and 0.08–0.23 % for TN, with mean values of 1.91 % ± 0.30 and 0.18 ± 0.05 %, respectively (Table S2). This increase could be attributed to accelerated economic development and population growth along the lagoon ’s shores since the founding of China, leading to intensified human activities; (Stage 3) After 1990, there was a significant increase in TOC and TN contents.
This could be related to the expansion of aquaculture in the lagoon since 1980. Additionally, Lingshui County received support from the national government during the “China Seventh Five-Year Plan (1986– 1990)” period, leading to further growth in the scale of mariculture within the lagoon and rapid development of the aquaculture industry. Particularly, around 1999, 2008, and 2016, TOC and TN contents increased rapidly, even reaching peak values within certain intervals. This could be related to increased eutrophication in Xincun Lagoon since 1999 (He, 2019). Previous studies have shown that three large-scale red tide events occurred in Xincun Lagoon in 1996, 2006, and 2014 (Li et al., 2010; EEDHPC, 2015).
