Intensified human activities shape the dynamics of sediment organic matter in a highly-disturbed estuary

Human activities have significantly altered organic matter dynamics in marine sediments. However, the long-term consequences of these alterations on organic matter within estuarine environments remain ambiguous. This study examines the sustained sediment organic matter (SeOM) dynamics in the anthropogenically disturbed Pearl River Estuary (PRE) spanning terrestrial (T), fresh-seawater mixing (M), and marine (S) zones, with an emphasis on the effects of human activities. 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, which includes humic-like components (C1, C2, and C3) and protein-like components (C4, C5, and C6). The results reveal that within the T zone, total organic carbon (TOC: 0.79∼1.22 %), total nitrogen (TN: 0.081∼0.123 %), and δ¹⁵N values (4.03∼6.60 ‰) exhibited an increasing trend, while δ¹³C values (−24.89 ∼ −24.57 ‰), C1 content (28.97∼32.64 %), and humification index (HIX: 0.60∼0.83) displayed a decreasing trend. These patterns collectively indicate a decline in terrestrial organic matter inputs and a concurrent rise in contributions from anthropogenic sources, such as domestic wastewater discharge. In the M zone, TOC content (0.67∼1.14 %) and TN content (0.07∼0.10 %) exhibited a decrease trend; however, the gradual reduction in the C/N ratio (9.94∼12.38) implies a slower decline in TN relative to organic carbon. Furthermore, C1 content (20.43∼28.69 %), C6 content (0.42∼30.35 %), and HIX value (0.51∼0.78 %) diminished after 1980, indicating a decrease in terrestrial sources and a substantial increase in anthropogenic influence. In the S zone, the C/N ratio (7.71∼8.77) remained relatively stable prior to 1980; however, a significant decline in terrestrial inputs was noted after 1980, particularly following 2000. Human-induced terrestrial alterations and domestic discharge have a substantial impact on the preservation of SeOM within the PRE, particularly in the T and M zones. The increase in Fe (III) within the T zone significantly enhances the content of SeOM; however, intensified anthropogenic activities compromise its preservation capacity. This study offers critical insights into the dynamics of SeOM within a significantly disturbed estuary over the past four decades, underscoring the potential effects of intensified human activities on the preservation of SeOM in estuarine ecosystems.