Tracing Macroalgal‐Induced Changes in Carbon Dynamics of High‐Arctic Fjords Using Biomarker Fingerprinting

Abstract Due to rising seawater temperatures and reduced sea ice extent, the coastal Arctic region is witnessing an expansion of macroalgal forests. However, changes in carbon dynamics resulting from such extensive macroalgal growth are still unknown in the high‐Arctic fjords. To trace the macroalgal signatures in Arctic fjords, bulk C‐isotopic and compound‐specific n ‐alkane distributions were studied in dominant macroalgae (brown, red, and green) ( n = 20) and also surface sediments ( n = 13) from the inner to outer regions of Kongsfjorden (Svalbard). The macroalgal species shows variable C‐isotopic signature (−17.0‰ to −28.9‰) but similar distribution of long‐chain n ‐alkanes (> n ‐C 23 ) with no predominance in carbon homologs, supporting environmental induced n ‐alkyl lipid production compared to the bulk biomass. Despite similar grain size and mineral composition of the fjord sediments, differences in C‐isotopic composition and n ‐alkanes distribution indicate contribution of organic matter from different sources. In the inner and middle fjord region, the C‐isotopic distribution (−23.3 ± 1.1‰ to −23.4 ± 1.3‰) and short‐to long‐chain n ‐alkanes signatures highlight possible mixing of contribution from phytoplankton, macroalgal‐debris, and terrestrial sources. While, in the outer fjord, significantly lower δ 13 C values (−25.6 ± 0.9‰) and n ‐alkane distribution similar to macroalgal communities suggest lipid accumulation possibly sourced from the reworking of macroalgal‐debris. High sedimentation and microbial breakdown of macroalgal debris create a low‐oxygen environment, as evidenced by higher cyclic octasulphur compounds in the inner and middle fjord region. Under future warming conditions, such oxygen‐starved regions in Arctic fjords may become more common as macroalgal forests expand and sediment influx increases.