Southern China’s coastal ecosystem displays strong monsoonal seasonality, yet the response of dissolved organic matter (DOM) and microbiome remains poorly characterized. We collected water samples (salinity 20-36) surrounding the Dapeng Peninsula encompassing a semi-enclosed embayment (BG, n=24), a river estuary (KC, n=24), and an open coastal area (OC, n=10) during wet (July-August 2023) and dry (December 2023-January 2024) seasons. The wet/summer season was characterized by higher temperature and lower salinity (30.0±1.5℃, salinity 27.7±3.3) compared to the dry/winter season (20.5±4.0℃, salinity 33.0±2.4), with comparable ranges across all three locations (BG: 15.2-32.3℃, salinity 25.2-35.5; KC: 14.1-31.7℃, salinity 19.7-35.4; OC: 23.0-30.7℃, salinity 26.0-32.9). Significant temporal but weak spatial variations confirm coherent monsoon-driven hydrological forcing in this subtropical coastal system.

DOM composition and bacterioplankton communities co-varied seasonally. Wet season samples exhibited higher dissolved organic carbon concentrations (1.39±0.74 mg/L, p=0.17), higher biological freshness index (BIX: 1.35±0.30, p<0.05) and lower humification index (HIX: 0.44±0.14, p<0.01) than those of the dry season (DOC: 1.06±0.16 mg/L, BIX: 1.25±0.47, HIX: 0.62±0.10), indicating a shift from more humified and terrestrially-dominated DOM in winter to freshly-derived and microbially-processed DOM in summer. Enhanced marine biological productivities were likely attributable to higher land-sourced nutrient inputs and optimal growth conditions in wet months. Consistently, wet season bacterioplankton displayed lower Shannon diversity and evenness (Wilcoxon test, p<0.001), with dominance of select taxa including Cyanobacteriota (21.2±11.7% versus 12.8±5.9% in the dry season). Despite these pervasive seasonal shifts, DOM composition exerted a significant, season-independent effect on bacterial community structure (partial Mantel r=0.205, p=0.002), confirming genuine DOM–microbe coupling. 

Metabolic partitioning of bacterioplankton communities into 16S autotrophic, 16S heterotrophic, and 18S non-heterotrophic eukaryotes revealed a functional divergence. Heterotrophic bacteria were strongly coupled to DOM quality, particularly tyrosine-like and humic-like fractions, whereas autotrophs principally tracked physicochemical parameters (dissolved oxygen, temperature, pH). Piecewise structural equation modeling confirmed a producer–DOM–consumer cascade in which seasonal forcing directly structured autotrophic communities (p<0.001) and subsequently shaped DOM properties through primary production and exudation – an effect amplified during the wet/summer season. DOM quality, in turn, exerted a direct effect on heterotrophic community compositions (p=0.001), with seasonal asymmetry indicating stronger DOM-heterotroph coupling also during the wet/summer season. The seasonally modulated DOM–bacterioplankton coupling should be resolved for accurate coastal carbon dynamics.