Graduate School of Environmental Science, Hokkaido University, Sapporo, Hokkaido, Japan.
Faculty of Fisheries Sciences, Hokkaido University, Hakodate, Hokkaido, Japan.
J Phycol. 2024 Aug;60(4):980-1000. doi: 10.1111/jpy.13483. Epub 2024 Jul 19.
Macroalgae influence local and global biogeochemical cycles through their production of dissolved organic carbon (DOC). Yet, data remain scarce and annualized estimates are typically based on high growth periods without considering seasonal variability. Although the mechanisms of active exudation and passive leakage need clarifying, ecophysiological stress is known to enhance DOC release. Therefore, DOC leakage from seasonally senescent macroalgae may be overlooked. This study focuses on the annual kelp Saccharina japonica var. religiosa (class Phaeophyceae) from Oshoro Bay, Hokkaido, Japan. Three years (2020-2022) of seasonal data were collected and analyzed, with least squares mean DOC release rates established for kelp (n = 88) across 16 incubation experiments (t ≥ 4 d, DOC samples ≥1 · d) under different photosynthetically active radiation (PAR) treatments (200, 400, 1200, or 1500 μmol photons · m · s). Differences in PAR, dry weight biomass (g DW), 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 · d ± standard error between n kelp) higher during the autumn "late decay" period (0.71 ± 0.10, n = 27) compared to the winter "early growth" period (0.14 ± 0.025, n = 10) and summer "early decay" period (0.25 ± 0.050, n = 24). This relationship between seasonal senescence and macroalgal DOC release is further evidence that long-term, place-based studies of DOC dynamics are essential and that global extrapolations are premature.
大型藻类通过产生溶解有机碳 (DOC) 来影响本地和全球生物地球化学循环。然而，相关数据仍然有限，且年度估算通常基于高生长时期，而没有考虑季节性变化。尽管主动分泌和被动泄漏的机制仍需澄清，但已知生态生理压力会增强 DOC 的释放。因此，季节性衰老的大型藻类可能会忽略 DOC 的泄漏。本研究以日本北海道大沼湾的季节性褐藻昆布（class Phaeophyceae）为研究对象。收集并分析了三年（2020-2022 年）的季节性数据，通过 16 次培养实验（t ≥ 4 d，DOC 样本 ≥1 d），对 88 个海带样本进行最小二乘均数 DOC 释放率分析，实验条件为不同的光合有效辐射（PAR）处理（200、400、1200 或 1500 μmol 光子·m·s）。PAR、干重生物量（g DW）、海表温度或盐度的差异无法解释 DOC 释放率的变化，个体海带之间的差异很大。相反，存在显著的年内差异，秋季“晚期衰退”期的平均 DOC 释放率（mg C·g DW·d ± n 个海带的标准误差）（0.71 ± 0.10，n = 27）明显高于冬季“早期生长”期（0.14 ± 0.025，n = 10）和夏季“早期衰退”期（0.25 ± 0.050，n = 24）。这种季节性衰老与大型藻类 DOC 释放之间的关系进一步证明，对 DOC 动态进行长期、基于地点的研究至关重要，而全球外推法还为时过早。
