Canopy-forming kelps are foundational species in coastal ecosystems, fixing tremendous amounts of carbon, yet we know little about the ecological and physiological determinants of dissolved organic carbon (DOC) release by kelps. We examined DOC release by the bull kelp, Nereocystis luetkeana, in relation to carbon fixation, nutrient uptake, tissue nitrogen content, and light availability. DOC release was approximately 3.5 times greater during the day than at night. During the day, N. luetkeana blades released an average of 16.2% of fixed carbon as DOC. Carbon fixation increased with light availability but DOC release did not, leading to a lower proportion of fixed carbon released as DOC at high light levels. We found no relationship between carbon fixation and DOC release rates measured concurrently. Rather, DOC release by N. luetkeana blades declined with marginal significance as blade tissue nitrogen content increased and with experimental nitrate addition, supporting the role of stoichiometric relationships in DOC release. Using a stable isotope (13 C) tracer method, we demonstrated that inorganic carbon is rapidly fixed and released by N. luetkeana blades as 13 DOC, within hours. However, recently fixed carbon (13 DOC) comprised less than 20% of the total DOC released, indicating that isotope studies that rely on tracer production alone may underestimate total DOC release, as it is decoupled from recent kelp productivity. Comparing carbon and nitrogen assimilation dynamics of the annual kelp N. luetkeana with the perennial kelp Macrocystis pyrifera revealed that N. luetkeana had significantly higher carbon fixation, DOC production and nitrogen uptake rates per unit dry mass. Both kelp species were able to perform light-independent carbon fixation at night. Carbon fixation by the annual kelp N. luetkeana is as high as 2.35 kg C m-2 yr-1 , but an average of 16% of this carbon (376 g C m-2 yr-1 ) is released as DOC. As kelp forests are increasingly viewed as vehicles for carbon sequestration, it is important to consider the fate of this substantial quantity of DOC released by canopy-forming kelps.

中文翻译：

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海带释放溶解有机碳的动力学和化学计量


形成冠层的海带是沿海生态系统的基础物种，固定了大量的碳，但我们对海带释放的溶解有机碳（DOC）的生态和生理决定因素知之甚少。我们研究了公海带 Nereocystis luetkeana 释放的 DOC 与碳固定、养分吸收、组织氮含量和光利用率的关系。白天 DOC 释放量大约是夜间的 3.5 倍。白天，N. luetkeana 叶片平均释放 16.2% 的固定碳（DOC）。碳固定随着光照的可用性而增加，但 DOC 的释放却没有增加，导致在高光照水平下以 DOC 形式释放的固定碳比例较低。我们发现碳固定和同时测量的 DOC 释放率之间没有关系。相反，随着叶片组织氮含量的增加和实验硝酸盐的添加，N. luetkeana 叶片的 DOC 释放量略有下降，这支持了化学计量关系在 DOC 释放中的作用。使用稳定同位素 (13 C) 示踪方法，我们证明了无机碳在数小时内被 N. luetkeana 叶片快速固定并以 13 DOC 形式释放。然而，最近固定碳 (13 DOC) 占 DOC 释放总量的不到 20%，这表明仅依赖示踪剂生产的同位素研究可能会低估 DOC 总释放量，因为它与最近的海带生产力脱节。比较一年生海带 N. luetkeana 与多年生海带 Macrocystis Pyrifera 的碳氮同化动态，发现 N. luetkeana 具有显着更高的固碳率、DOC 产量和单位干质量的氮吸收率。两种海带物种都能够在夜间进行不依赖于光的碳固定。 一年生海带 N. luetkeana 的碳固定量高达 2.35 kg C m-2 yr-1 ，但平均 16% 的碳（376 g C m-2 yr-1 ）以 DOC 形式释放。随着海带森林越来越多地被视为固碳工具，重要的是要考虑由形成树冠的海带释放的大量 DOC 的命运。