Despite growing attention on the contribution of macroalgae to carbon cycling and sequestration (blue carbon), more ob-servational data is needed to constrain current estimates. In this study, we estimate the floating macroalgal carbon flux within and beyond a large sub-Arctic fjord system, Nuup Kangerlua, Greenland, which could potentially reach carbon sinks. Our study estimates 1) the fjord-scale area with macroalgal coverage and barrens caused by sea urchin grazing, 2) the floating macroalgal biomass in the fjord, and 3) the annual export flux of floating macroalgae out of the fjord system. ROV surveys documented that macroalgal habitats cover 32 % of the seafloor within the photic zone (0-30 m) with an average coverage of 39.6, 22, and 7.2 % in the depth intervals 0-10, 10-20, and 20-30 m, respectively. 15 % of the area suitable for macroalgae was denuded by sea urchin grazing. Floating macroalgae were common with an average biomass of 55 kg wet weight km(-2). Densities and species composition varied seasonally with the highest levels after storms. The floating biomass was composed of intertidal macroalgal species (58 %) (Fucus vesiculosus, Fucus distichus, and Ascophyllumnodosum) and kelps (42 %) (Saccharinalongicruris, S. latissima, and Alaria esculenta). We deployed surface GPS drifters to simulate floating macroalgal trajectories and velocity. Data indicated that 80 % of the floating biomass is retained in the fjord where its fate in relation to long-term sequestration is unknown. Export beyond the fjord was limited and indicated an annual floating macroalgal export beyond the fjord of only 6.92 t Cyr(-1), which is equal to similar to 0.02 % of the annual net primary production. Our findings suggest that floating macroalgae support a limited blue carbon potential beyond this fjord and that future research should focus on the fate of retained floating macroalgae and subsurface export to resolve the connectivity between macroalgal habitats and long-term carbon sinks.

Despite growing attention to the potential contribution of macroalgae to Blue Carbon, there is a gaping lack of evidence of their export to carbon sinks in the deep ocean and marine sediments. In this study, we quantified the annual carbon export of floating macroalgae in a large sub-arctic fjord system, Nuup Kangerlua, Greenland, from April to August 2021 and trajectories of exported biomass within and beyond the fjord. We identified extensive macroalgae communities within the fjord system, which sustained 55.44 kg wet weight km -2 of floating macroalgal biomass on average in the fjord system. Using GPS drifters we identified transport pathways and the likelihood of macroalgal export out of the fjord to sinks in the deep ocean. The annual export of floating macroalgae beyond the fjord amounted to 445.73 t C yr -1 . Our observation suggests that most of the floating biomass is retained in the fjord where its fate in relation to long-term sequestration is unknown. The floating macroalgal biomass was composed of intertidal macroalgae species (58%) ( Fucus vesiculosus, Fucus distichus, and Ascophyllum nodosum) , and kelps (42%) ( Alaria esculenta , Saccharina latissima & Saccharina longicruris) . Macroalgae densities and species composition varied seasonally with the highest levels after storms. Dominance of intertidal species in the spring suggests that ice scouring related to ice melt is an important driver of export. Sea urchin barrens occupied 15% of the potential macroalgae habitats of the fjord and, hence, limited standing biomass and associated blue carbon potential.