Mangrove wetlands are superior sites for the deposition of various pollutants, but they are also more susceptible to environmental contamination. To investigate the health threats of phosphorus to mangrove ecosystems, this study presents the distribution, chemical speciation and factors influencing phosphorus (P) forms in 38 surface sediments from the mangrove wetlands of Dongzhai Harbor on Hainan Island, China. P forms were measured using the sequential extraction (SEDEX) procedures. The results showed that the total phosphorus content in the sediment was at a high level, and there was a significant spatial variability of phosphorus in each form. Among them, inorganic phosphorus accounted for 74.64% of the total phosphorus, and organic phosphorus accounted for 25.35% of the total phosphorus. Among the inorganic phosphorus, detrital limestone phosphorus and Ca-bound phosphorus were dominant, while the content of exchangeable phosphorus and Fe-bound phosphorus had a smaller percentage. The concentration of biologically available phosphorus, ranged from 1.76 umol/g to 18.57 umol/g, and the Fe-bound and organic forms of phosphorus had a controlling effect on it. Based on C_org/OP, N/P and land use surveys, it was concluded that phosphorus was mainly an exogenous input. The correlation analysis concluded that the main sources of phosphorus in the study area are agriculture and aquaculture. The development of agriculture and aquaculture led to moderate levels of phosphorus pollution in the study area as a whole, with more serious a phosphorus pollution at the wetland park in the southeast corner of Dongzhai Harbor and in the western areas, with lush vegetation and developed river systems, mainly influenced by human activities, topography, vegetation and water system conditions.

This study characterized the spatial distribution and assessed the ecological risks of carbon, nitrogen, and phosphorus in sediments of the Dongzhai Harbor mangrove wetland, Hainan, China. Analysis of key environmental indicators (grain size, pH, TOC, TN, TP) across twenty-seven sediment cores (0–100 cm depth) revealed distinct decreasing land–sea gradients and vertical stratification of nutrient concentrations. Mangrove plant debris was identified as the primary source of sedimentary organic matter. Elemental ratio analysis indicated terrestrial inputs as the dominant phosphorus source. Significant positive correlations between TOC, TN, and TP in surface sediments suggested coupled nutrient dynamics. Vertical distribution of C/N to C/P ratios increased with depth, which may be related to increased nitrogen and phosphorus inputs due to regional human activities. Pollution assessment showed significantly higher ecological risks in surface sediments (0–50 cm), particularly near inland areas and dense mangroves, indicating co-regulation by vegetation processes and human impacts. These findings highlight significant spatial heterogeneity in ecological risks, underscoring the need for enhanced monitoring and targeted management strategies in critical land–sea transition zones.

Hainan Island is the only large island located on the northern margin of the South China Sea and is surrounded by Cenozoic graben basins, including the Qiongdongnan, Yinggehai, and Beibuwan basins. The uplift and denudation history of the Jianfeng pluton on southwestern Hainan Island is significant for understanding the formation of the regional geomorphology and adjacent basin evolution. This paper presents apatite and zircon fission-track (FT) analyses conducted on the Jianfeng pluton. The zircon FT (ZFT) ages of the pluton range from 63 ± 4 to 117 ± 8 Ma, and the apatite FT (AFT) ages from 19.4 ± 1.8 to 43.9 ± 4.4 Ma. The average confined track lengths in apatite are rela-tively short (11.9–12.8 μm). An age–elevation plot indicates that two rapid cooling events occurred during 73–63 and 44–40 Ma. Thermal modeling revealed a three-stage history of uplift and denudation. From the Late Cretaceous to middle Eocene (73–40 Ma), the Jianfeng area underwent episodic rapid uplift and denudation. At the end of the Late Cretaceous (73–63 Ma), the area was affected by mid-ocean ridge spreading in the Pro-to-South China Sea. During the middle Eocene (44–40 Ma), the Yinggehai Basin un-derwent abrupt expansion and subsidence, which increased the elevation difference between the Jianfeng area and the Yinggehai Basin. From the middle Eocene to middle Miocene (40–11 Ma), the Jianfeng area underwent slow denudation and the Yinggehai Basin was rapidly infilled, which eliminated the original elevation difference between the two areas. From the middle Miocene to present (11–0 Ma), the Jianfeng area has undergone reactivated rapid uplift and denudation, driven by the remote effects of In-dia–Eurasia collision.