Monsoon-Driven Variability of Surface-Active Substances and Organic Matter in the Sea Surface Microlayer and Coastal Waters of Malaysia

Malaysia’s coastal waters experience strong monsoonal forcing, with distinct Northeast (NE), Southwest (SW), and Intermonsoon periods that influence hydrodynamics, terrestrial runoff, and atmospheric inputs. Although the sea surface microlayer (SML) plays a critical role in air-sea exchange, it remains poorly characterised in monsoon-driven tropical environments, leaving important gaps in regional biogeochemical understanding. This study investigates the concentrations and enrichment of major surface-active substances (SASs), including surfactants, dissolved monosaccharides (MCHOs), polysaccharides (PCHOs), total dissolved carbohydrates (TDCHOs), and transparent exopolymer particles (TEPs), in coastal waters off Peninsular Malaysia. The SML and underlying water (ULW, 1 m) samples were collected using the glass plate technique during the SW monsoon (August to September 2023; May to July 2024), NE monsoon (November 2023), and Intermonsoon (October 2024), and SAS components were quantified using methylene blue, TPTZ, and Alcian Blue assays. Stations influenced by anthropogenic activity showed clear enrichment of surfactants and carbohydrate species (EF > 1), while TEPs were generally depleted (EF < 1). Strong SML and ULW correlations suggest upward transport from the water column as a dominant source of SASs in the SML. During the NE monsoon, both SML and ULW were fresher than during the SW monsoon, reflecting the influence of rainfall and terrestrial runoff, which contributed to elevated carbohydrate concentrations. Overall, SAS enrichment persisted under moderate wind speeds but weakened under higher wind conditions.