Ripped and torn off: insights into autumn carbon loss in shallow subarctic sugar kelp

Kelp forests, recently recognized for their potential to capture and store carbon, remain relatively underexplored in their actual contributions to carbon budgets. Here, we investigated the carbon release dynamics of the sugar kelp, Saccharina latissima , a dominant kelp species of shallow subarctic zones, which is seasonally impacted by ice scour. Our study focused on autumn and winter, combining field assessments with laboratory experiments to quantify and compare mechanisms of biomass and carbon loss, and thus contributing new insights into seasonal kelp-derived carbon flows in these environments. Daily dislodgement rates were comparable to temperate regions, with higher rates in autumn than winter, contrary to our initial hypothesis about ice scouring effects. Apical blade erosion rates aligned with previous regional studies but were lower than temperate sites, possibly due to reduced bryozoan encrustation and colder waters. Laboratory measurements revealed higher dissolved organic carbon (DOC) release compared to other kelp species, while particulate organic carbon (POC) loss was 8-fold higher than DOC release. This study underscores the need for comprehensive annual measurements to refine carbon budgets, especially in subarctic kelp forests, where carbon contributions are likely underestimated, particularly in shallow, seasonally dynamic environments. • Subarctic sugar kelp had 3-fold more dislodged biomass in autumn, likely due to strong currents and waves, than in winter. • Carbon lost by apical erosion in autumn in shallow dynamic habitats was probably underestimated by not considering grazing. • POC released was more than 8-fold greater than DOC in the laboratory in autumn. • Rates of DOC release were in the high range of previously reported rates.