Intensified Human Activities Shape the Dynamics of Sediment Organic Matter in a Highly-Disturbed Estuary

Human activities have greatly accelerated changes in the levels of organic matter within marine sediments. However, the long-term consequences of these changes on organic matter in estuarine environments remain unclear. This study investigates the impacts of human activities on sediment organic matter (SOM) dynamics in the Pearl River Estuary (PRE), a highly disturbed estuary comprising three zones: terrestrial (T), fresh-seawater mixing (M), and marine (S). Sediment cores (210Pb) were utilized to analyze physicochemical parameters, including total organic carbon (TOC), total nitrogen (TN), isotopic compositions (δ¹³C and δ¹⁵N), and fluorescence signatures of sediment dissolved organic matter, encompassing humic-like components (C1, C2, and C3) as well as protein-like components (C4, C5, and C6). The results show that in the T zone, TOC, TN, and δ15N increased, while δ¹³C, C1, and the humification index (HIX) decreased, indicating reduced terrestrial inputs and increased contributions from human activities, such as domestic discharge. In the M zone, TOC and TN decreased, but the slower reduction in the C/N ratio suggests a slower decline in TN compared to organic carbon. Additionally, C1, C6, and HIX decreased post-1980, reflecting a reduction in terrestrial sources and a significant rise in anthropogenic influence. In the S zone, the C/N ratio remained relatively stable before 1980, but a notable decline in terrestrial inputs was observed after 1980, especially post-2000. Overall, the contribution from terrestrial sources decreases along the salinity gradient over time, particularly in the M zone. Human-induced terrestrial alterations significantly impact the preservation of m SOM within the PRE, particularly in the T and M zones, amid intensified anthropogenic activities. Moreover, the increase in Fe (III) in the T zone substantially enhances the content of SOM, although intensified anthropogenic activities diminish its preservation capacity. This study provides crucial insights into the dynamics of SOM in a highly disturbed estuary over the past 40 years, emphasizing the potential effects of intensified human activities on the preservation of SOM in estuarine ecosystems.