Andrew Kalani Carlson1, Takeshi Yoshimura1,2, and Isao Kudo1,2
1Graduate School of Environmental Science, Hokkaido University, Sapporo, Hokkaido, Japan
2Faculty of Fisheries Sciences, Hokkaido University, Hakodate, Hokkaido, Japan
Macroalgae are foundational to many Indigenous social-ecological systems, and their production of dissolved organic carbon (DOC) serves various biogeochemical roles, potentially including Blue Carbon sequestration. However, data on the mechanisms of passive and active DOC diffusion, including the role of photosynthetically active radiation (PAR), are contradictory. For this experiment, DOC release rates from the kelp Saccharina japonica var. religiosa from Oshoro Bay, Hokkaido, Japan were investigated between January 2020 and July 2021. Up to six kelp individuals were divided between a low and high PAR treatment (200 and 400 µmol photons PAR m-2 s-1) and incubated ex situ for up to 7 days, with twice-daily sampling according to a light/dark timer. There were no significant differences between the low and high PAR treatment results or the day and night release rates, with DOC accumulation largely linear throughout incubation. In addition, biomass was a poor predictor of DOC release rates, variability was high between individual kelp, and intra-annual variability was significant. These findings support the theory of passive over active DOC diffusion. They also indicate that individual and seasonal variations should be accounted for in Blue Carbon estimates, and that biomass and primary production are unreliable predictors of kelp DOC release. Moreover, the relationship between variable passive diffusion and individual condition, growth stage, or season, means DOC release could serve as a proxy for biological or ecological health. Therefore, understanding macroalgal DOC variability may be useful to Indigenous coastal managers in terms of ecosystem health and Blue Carbon funding.