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 investigates 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 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, nitrogen, 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 further revealed elevated concentrations of macroalgal-associated sugars (glucose, galactose, fucose, mannuronic acid) and labile amino acids (Asp, Glu, Gly, Ser, Ala) reinforcing macroalgal-derived contributions to surface POM. While δ13CPOC showed minimal spatial variation (−26.8 ‰ to −29.1 ‰), the biochemical and molecular signatures indicated a decreasing macroalgal contribution towards Adj-W, along with internal reorganization, suggesting lateral transport of macroalgal-derived POM with selective early-stage transformation. Overall, these findings indicate that Arctic macroalgal beds act as dynamic coastal biogeochemical hotspots, redistributing and transforming organic carbon beyond their habitat.