Anaerobic decomposition dynamics of three kelp species from the North-east Atlantic: implications for blue carbon storage

Decomposition pathways of detritus are key processes in the contribution of macroalgal habitats to natural carbon sequestration of “blue carbon”. The anaerobic decomposition of three North-east Atlantic canopy-forming kelp species was investigated using ex-situ decomposition chambers. Thallus parts (stipes, holdfasts and blades) of Laminaria hyperborea, Saccharina latissima and Laminaria digitata were incubated in still seawater in temperature-controlled dark conditions. Refractory potential (Rp), first-order decomposition rate ‘k’ and associated half-life (t1/2) were calculated. Dissolved organic carbon (DOC) and dissolved inorganic carbon (DIC) were measured in the incubation water at 0, 7, 14 and 21 days and thermal gravimetric profiles were determined at each 26 decomposition stage. Oxygen depletion occurred within 24 hours. Approximately 5 times as much DOC was released than DIC. Holdfast material produced the most DIC, while blade material released greatest amounts of DOC. Saccharina latissima released less DOC than L. hyperborea and L.digitata. The refractory potential of fragments increased from Rp = 0.46 ± 0.05 (SD) to 0.50 ± 0.04 (SD) throughout the 21-day incubations. Saccharina latissima had the highest Rp throughout. First order decomposition rates, averaged across the three kelp species, gave half-lives (t1/2) for blade fragments of 27.8 ± 2.9 days, (k = 0.025 ± 0.002) and stipes (t1/2) as 113.2 ± 21.1 days (k = 0.006 ± 33 0.001). This experiment clarifies the fate of macroalgal carbon during early decomposition and thus the processes that govern blue carbon pathways of macroalgae highlighting the differences in species and thallus parts breakdown.