Abstract
The surface microlayer (SML) is a fundamental control of energy transfer, biogeochemical processes, and climate-active gases flux at the water-atmosphere interface. Organic matter in the SML has an important role in marine environments but inland waters have received far less attention. This results in large unknowns of SML variability and its impact on boundary layer exchanges. In this study, we investigate variations in dissolved organic carbon (DOC) and dissolved organic matter (DOM) composition in SML and subsurface water (SSW) using size exclusion chromatography in a Northern Amazonia rainforest headwater during the dry to wet season transition. During the dry season, we observed higher DOC concentrations with greater DOM compositional variability, characterised by lower humic substance (HS) abundance and elevated contributions of biopolymers (BP), building blocks (BB), and low-molecular-weight neutrals and acids (LMWN, LMWA). River discharge and DOC flux increased in the wet season. However, the overall DOC concentration decreased accompanied by DOM homogenisation, a dominance of HS, and the disappearance of LMWA. The SML and SSW exhibited similar DOC concentrations but distinct compositional differences, evidenced by divergent HS-DOC relationships and HS nominal molecular weight (M(n)) characteristics. In the dry season, high M(n) variability in the SML suggested preferential accumulation of higher-molecular-weight HS, which diminished as wet-season conditions stabilised DOM distribution. Enrichment factors (EFs) for all DOM compound groups between the SML and SSW displayed strong temporal variability. These patterns likely reflect shifts in hydrodynamic regimes, terrestrial inputs, and microbial or photochemical processing. Notably, rainfall events did likely not directly trigger EF pulses but indirectly influenced DOM composition from local sources through terrestrial mobilisation. Our results demonstrate that tropical river DOM dynamics are fundamentally controlled by seasonal hydrological regimes, with discharge-driven processes exerting stronger influence on molecular composition and vertical heterogeneity (particularly in the SML) than direct rainfall effects. This hydrological dominance shapes DOM distribution through three interconnected mechanisms: source variability during low-flow periods, transport-mediated homogenisation during high discharge, and in situ processing that differentially affects boundary layer composition.