Part of the global DOC versus AOU (dissolved organic carbon/apparent oxygen utilization) data compilation, Southern Ocean

Recent evidence that dissolved organic carbon (DOC) is a significant component of the organic carbon flux below the photic layer of the ocean (1), together with verification of high respiration rates in the dark ocean (2), suggests that the downward flux of DOC may play a major role in supporting respiration there. Here we show, on the basis of examination of the relation between DOC and apparent oxygen utilization (AOU), that the DOC flux supports ~10% of the respiration in the dark ocean.
The contribution of DOC to pelagic respiration below the surface mixed layer can be inferred from the relation between DOC and apparent oxygen utilization (AOU, µM O2), a variable quantifying the cumulative oxygen consumption since a water parcel was last in contact with the atmosphere. However, assessments of DOC/AOU relations have been limited to specific regions of the ocean (3, 4) and have not considered the global ocean. We assembled a large data set (N = 9824) of concurrent DOC and AOU observations collected in cruises conducted throughout the world's oceans (fig. S1, table S1) to examine the relative contribution of DOC to AOU and, therefore, respiration in the dark ocean. AOU increased from an average (±SE) 96.3 ± 2.0 µM at the base of the surface mixed layer (100 m) to 165.5 ± 4.3 µM at the bottom of the main thermocline (1000 m), with a parallel decline in the average DOC from 53.5 ± 0.2 to 43.4 ± 0.3 µM C (Fig. 1). In contrast, there is no significant decline in DOC with increasing depth beyond 1000 m depth (Fig. 1), indicating that DOC exported with overturning circulation plays a minor role in supporting respiration in the ocean interior (5). Assuming a molar respiratory quotient of 0.69, the decline in DOC accounts for 19.6 ± 0.4% of the AOU within the top 1000 m (Fig. 1). This estimate represents, however, an upper limit, because the correlation between DOC and AOU is partly due to mixing of DOC-rich warm surface waters with DOC-poor cold thermocline waters (6). Removal of this effect by regressing DOC against AOU and water temperature indicates that DOC supports only 8.4 ± 0.3% of the respiration in the mesopelagic waters.

近期研究证据表明，溶解有机碳（dissolved organic carbon, DOC）是海洋透光层下方有机碳通量的重要组成部分（1），结合对暗海洋高呼吸速率的验证结果（2），可推知DOC的向下通量或许在维持该区域呼吸作用中扮演核心角色。本研究基于DOC与表观氧利用度（apparent oxygen utilization, AOU）的关联分析，证实DOC通量可支撑暗海洋中约10%的呼吸作用。

表面混合层下方的远洋呼吸对DOC的贡献占比，可通过DOC与表观氧利用度（AOU，单位为µM O₂）的关联推导得出：AOU是用于量化水体自末次与大气接触以来累计耗氧量的变量。然而，此前针对DOC/AOU关联的评估仅局限于特定海域（3,4），尚未覆盖全球大洋。我们整合了全球大洋科考中获取的大型同步观测数据集（样本量N=9824），包含DOC与AOU的同步观测数据（fig. S1、table S1），以此分析DOC对AOU的相对贡献，进而明确其在暗海洋呼吸作用中的占比。

在100米深的表面混合层底部，AOU的平均值（±标准误，SE）为96.3±2.0 µM，至1000米深的主温跃层底部升至165.5±4.3 µM；与此同时，平均DOC浓度从53.5±0.2 µM C降至43.4±0.3 µM C（Fig. 1）。与之形成对照的是，当水深超过1000米后，DOC浓度未出现显著下降（Fig. 1），这表明通过翻转环流输送的DOC对海洋内部呼吸作用的支撑作用相对有限（5）。

假设摩尔呼吸商为0.69，则1000米以浅海域内DOC浓度的下降可解释AOU变化的19.6±0.4%（Fig. 1）。但该估算值仅为上限，因为DOC与AOU之间的相关性部分源于富DOC的暖表层水与贫DOC的冷温跃层水的混合作用（6）。通过将DOC对AOU与水温进行回归以消除该混合效应后，结果显示DOC仅能支撑中层海水中8.4±0.3%的呼吸作用。