Table3_Organic Matter Processing on Dry Riverbeds is More Reactive to Water Diversion and Pollution Than on Wet Channels.docx

Rivers are severely affected by human activities and many are simultaneously impacted by multiple stressors. Water diversion for hydropower generation affects ecosystem functioning of the bypassed reaches, which can alternate between periods with natural discharge and others with reduced flow that increase the surface of dry riverbeds. In parallel, urban pollution contributes a complex mixture of nutrients, organic matter, heavy metals, pesticides, and drugs, thus becoming an important stressor in rivers. However, there is little information on the interaction between both stressors on ecosystem functioning and, particularly, on organic matter processing, a key process linked to the input of energy to food webs. To assess the impact of water diversion and urban pollution on organic matter processing, we selected four rivers in a pollution gradient with a similar diversion scheme and compared reaches upstream and downstream from the diversion weirs. We measured leaf-litter decomposition and carbon dioxide (CO2) fluxes in both the wet channel and the dry riverbed. Water diversion and pollution in the wet channel did not affect CO2 fluxes but reduced microbial decomposition, whereas in the dry riverbed, their interaction reduced total and microbial decomposition and CO2 fluxes. Thus, both stressors affected organic matter processing stronger in dry riverbeds than in the wet channel. These results show that dry riverbeds must be taken into account to assess and manage the impacts of human activities on river ecosystems.

河流深受人类活动干扰，多数同时受到多重胁迫因子的冲击。用于水力发电的取水工程会破坏绕流河段的生态系统功能，这类河段会交替处于自然径流期与流量缩减期，后者会扩大干涸河床的覆盖面积。与此同时，城市污染会形成包含营养物质、有机质、重金属、农药与药物的复杂混合污染物，进而成为河流的重要胁迫因子。然而，目前关于这两类胁迫因子对生态系统功能，尤其是对有机质周转——这一与食物网能量输入密切相关的关键过程——的交互影响的研究仍较为匮乏。为评估取水工程与城市污染对有机质周转的影响，我们选取了4条沿污染梯度分布且取水工程方案相似的河流，对比了分水堰上下游的河段。我们分别在有水河道与干涸河床中测量了叶片凋落物分解速率与二氧化碳（CO₂）通量。结果显示，在有水河道中，取水工程与污染未对CO₂通量产生显著影响，但会降低微生物分解速率；而在干涸河床中，二者的交互作用会降低总分解速率、微生物分解速率与CO₂通量。由此可见，相较于有水河道，两类胁迫因子对干涸河床中有机质周转的影响更为显著。本研究结果表明，在评估与管控人类活动对河流生态系统的影响时，必须将干涸河床纳入考量范畴。