Macroalgae are foundational to the health of many Indigenous social-ecological systems, and their production of dissolved organic carbon (DOC) serves various biogeochemical roles. Improved understanding of seasonal variations in DOC release as an ecophysiological response could therefore help Indigenous stewards balance these implications. However, multi-year seasonal studies of macroalgal DOC release are few and the underlying roles of passive and active DOC diffusion need clarifying. This study focuses on the kelp Saccharina japonica var. religiosa (class Phaeophyceae) from Oshoro Bay, Ainu Mosir (Hokkaido). The conclusions are supported by three years (2020–2022) of data, including 1091 DOC samples from 16 incubation experiments (t = 4–9 days) comparing individual kelp (n = 88) to in situ seawater control tanks (n = 31) under different photosynthetically active radiation (PAR) treatments (200, 400, 1200, or 1500 µmol photons · m⁻² · s⁻¹). Differences in PAR, dry weight biomass, sea surface temperature, or salinity could not explain DOC release rate variability, which was high between individual kelp. Instead, there were significant intra-annual differences, with mean DOC release rates (mg C · g DW⁻¹ · d⁻¹) (± standard error between n kelp) higher (p < 0.05) during the autumn “late decay” period (0.82 ± 0.12, n = 27) compared to the winter “early growth” period (0.20 ± 0.028, n = 10) and summer “early decay” period (0.34 ± 0.066, n = 24). Monitoring this relationship between seasonal decay and macroalgal DOC release may therefore help inform Indigenous stewardship strategies.