Macroalgal influence on particulate organic matter sources and early transformation in an Arctic fjord

Abstract. Accelerated Arctic warming is promoting the expansion of coastal macroalgal habitats, yet their influence on pelagic organic carbon cycling remains unresolved. This study investigate the influence of macroalgal beds on the biochemical composition of surface particulate organic matter (POM) in Kongsfjorden, Svalbard, during late summer 2023. Surface waters were sampled at four macroalgal-dominated sites (MDS) and from adjacent waters (Adj-W) located 500 m and 1500 m away. A multi-proxy approach integrating elemental composition, stable isotopes, biopolymeric fractions, monosaccharides, and amino acids was used to trace macroalgal contributions and their lateral redistribution. Concentrations of particulate organic carbon, particulate nitrogen, particulate carbohydrates, and proteins were consistently higher at MDS than in Adj-W, indicating localized enrichment of biochemically labile organic matter within macroalgal habitats. Molecular analyses revealed elevated concentrations of monosaccharides and amino acids at MDS, including macroalgal-associated sugars (glucose, galactose, fucose, mannuronic acid) and labile amino acids (Asp, Glu, Gly, Ser, Ala), demonstrating incorporation of macroalgal-bed derived matter into surface POM. Declining concentrations and composition shift in Adj-W, together with internal reorganization of biopolymeric and molecular composition, indicate efficient lateral export with selective early-stage transformation of POM. Bulk δ¹³C showed minimal spatial variation (−26.8 to −29.1 ‰), suggesting that macroalgal influence is expressed through biochemical restructuring rather than isotopic dominance. Principal component analysis identified a continuous macroalgal–pelagic gradient, with MDS occupying the macroalgal-influenced end. Overall, these findings indicate that Arctic macroalgal beds act as dynamic coastal biogeochemical hotspot, redistributing and transforming organic carbon beyond their immediate habitat.