European rivers experience increased loading of total organic carbon (TOC) from terrestrial sources due to factors involving changes in land use, climate and soil acidity. However, little is known about how increased TOC is linked to changes in the bioreactivity of organic matter in these rivers on a continental scale. We compiled paired measurements of TOC and biological oxygen demand in 5‐day 20°C dark incubations from 3,486 EU monitoring rivers. Assuming first‐order decay and a fixed respiratory quotient, annual average TOC and biological oxygen demand values were used to calculate 11,060 values of the decay coefficient k . The k decreased by two orders of magnitude as a power function of increasing TOC. This relationship could partly be explained by carbon quality, as the C:N ratio of the organic matter was the lowest in high‐reactivity low‐TOC rivers, and vice versa. A trend analysis showed that TOC increased by 18% from 1996 to 2012, while k decreased by as much as 50%. As a consequence, the biological oxygen demand in the water decreased over time in spite of the water browning trend (increased TOC). Together, these results suggest that reactivity is low near terrestrial hot spots for TOC export, and low during years with high terrestrial TOC loading, but increases in rivers with low TOC concentrations where internal processes in the water have high relative influence on bulk TOC quality. Thus, browning of European freshwaters is linked to strong decreases in TOC reactivity on a continental scale, suggesting that the impacts of browning on microbial water deoxygenation and greenhouse gas production are less severe than previously thought. (Less)

中文翻译：

---

降低欧洲河流中的有机碳生物反应性

由于涉及土地利用、气候和土壤酸度变化的因素，欧洲河流经历了来自陆地来源的总有机碳 (TOC) 负荷的增加。然而，关于增加的 TOC 与大陆尺度上这些河流中有机物质的生物反应性变化之间的关系知之甚少。我们汇编了来自 3,486 条欧盟监测河流的 5 天 20°C 黑暗孵化中 TOC 和生物需氧量的配对测量值。假设一级衰减和固定的呼吸商，年平均 TOC 和生物需氧量值用于计算衰减系数 k 的 11,060 个值。作为 TOC 增加的幂函数，k 减少了两个数量级。这种关系可以部分用碳质量来解释，因为 C：有机质的 N 比在高反应性低 TOC 河流中最低，反之亦然。趋势分析表明，从 1996 年到 2012 年，TOC 增加了 18%，而 k 下降了 50%。因此，尽管水呈褐变趋势（TOC 增加），但水中的生物需氧量随时间下降。总之，这些结果表明，TOC 输出的陆地热点附近的反应性较低，并且在陆地 TOC 负荷高的年份中较低，但在 TOC 浓度低的河流中会增加，因为水中的内部过程对整体 TOC 质量有较高的相对影响。因此，欧洲淡水的褐变与大陆范围内 TOC 反应性的强烈下降有关，表明褐变对微生物水脱氧和温室气体产生的影响没有以前认为的那么严重。（较少的）