Large woody debris -A river restoration panacea for streambed nitrate attenuation?

几个世纪以来，英国和欧洲的河流一直在清除大块的木屑，以减少这些障碍物和堵塞造成的潜在洪水风险。然而，大量木质碎片的清除已被证明会严重影响河流提供的生态系统服务，包括栖息地功能，以及通过充当物理和生化屏障来减少营养和沉积物负荷的潜力。这对河流内和洪泛区的生物群以及社会在提供清洁饮用水方面造成了不利影响。最近欧盟和英国的环境政策（特别是欧洲水框架指令）正在促进管理实践的重大转变，强烈强调重新引入河流内木材，通过增加低地河流生态系统减轻过量硝酸盐污染的能力来提高其对环境变化的适应能力。木屑作为河流中的障碍物，预计会增加地表水向河床的下渗，并增加其在反应性很强的河床区的接触时间，从而增加化学物质的周转和硝酸盐的衰减。然而，大型木屑对河床生物地球化学过程的主要驱动因素、地下水与地表水的交换通量以及水在河床中的停留时间的影响尚不清楚。为了改进大型木屑结构的设计以有效地去除河床中的硝酸盐，河流流域管理和恢复计划需要科学证据来证明不同大型木屑结构设计对改善水质的有效性。因此，该项目将为评估不同的永久性和移动式大型木屑设计的效率提供所需的科学证据，以提高低地河流对氮的吸收。我们的研究主要集中在低地溪流上，因为这些河流代表了英国大部分受到高硝酸盐浓度威胁的河流。该项目的研究结果将直接为河流恢复实践提供信息，并决定是否，在多大程度上以及采用何种设计，在英国低地溪流中部署大型木屑，以减少极高的硝酸盐负荷。我们将把新的监测技术和环境示踪技术与创新的数值模拟方法相结合，以确定大型木质碎屑促进生物地球化学转换增加的河床热点的发生。我们将量化在这些反应热点中过量硝酸盐浓度可以降低到何种程度，以及哪种不同的大型木屑结构设计最能促进这种生态系统服务。我们将改进复杂的数值模型，以量化永久性和可移动的大型木屑如何有效地衰减来自低地河流的过量硝酸盐。通过开发和模拟一系列场景，假设比实验观察到的更广泛的环境条件，我们将预测在不断变化的环境条件下，低地河流中不同的大型木屑结构减少硝酸盐污染的效率。了解河流内木材如何促进生物地球化学热点的发生，将有助于河流管理者采用有利于河流内生物群和水质的技术，供人类使用。该项目与我们将制定的政策建议文件相结合，将为形成基于科学的恢复活动提供一个理想的机会，这些活动能够以很少或没有额外成本的方式提供定量的水质改善。

For centuries, large woody debris has been removed from UK and European rivers to reduce potential flood risks caused by these obstacles and blockage. However, the removal of large woody debris has been shown to severely impact on ecosystem services provided by rivers, including habitat functioning and the potential to reduce nutrient and sediment loading by acting as both a physical and biochemical barriers. This caused detrimental impacts on in-stream and floodplain biota as well as society in the provision of clean drinking water.Recent EU and UK environmental policies (notably the European Water Framework Directive) are promoting a significant shift in management practices, strongly emphasizing the reintroduction of in-stream wood to improve the resilience of lowland river ecosystems to environmental change by increasing their capacity to attenuate excess nitrate pollution. As obstacle in the river, woody debris is expected to increase the down-welling of surface water into the streambed and enhance its contact time in the very reactive streambed zone, resulting in increased chemical turnover and nitrate attenuation. However, the impacts of large woody debris on the main drivers of biogeochemical processes in the streambed, the exchange fluxes between groundwater and surface water as well as the residence time of water in the streambed are poorly understood. To improve design of large woody debris structures to efficiently remove nitrate from the streambed, river basin management and restoration programmes require scientific evidence for the effectiveness of different large woody debris designs for water quality improvement.This project will therefore provide the scientific evidence required for assessing the efficiency of different permanent and mobile designs of large woody debris for enhancing the uptake of nitrogen in lowland rivers. Our research focuses specifically on lowland streams as these represent the majority of UK rivers under threat of critically high nitrate concentrations. The findings of this project will directly inform river restoration practice and decide whether, to what degree and with what design, large woody debris will be deployed in UK lowland streams to reduce critically high nitrate loads.We will combine novel monitoring techniques and environmental tracer technologies with innovative numerical modelling approaches to identify the occurrence of streambed hotspots of increased biogeochemical turnover that are facilitated by large woody debris. We will quantify to what degree excess nitrate concentrations can be reduced in this reactive hotspots and what different designs of large woody debris structures best facilitate this ecosystem service. We will improve sophisticated numerical models to quantify how effective permanent and mobile large woody debris may attenuate excess nitrate from lowland rivers. By developing and simulating a set of scenarios, assuming wider ranges of environmental conditions then observed experimentally, we will predict how efficiently different large woody debris structures in lowland rivers can reduce nitrate pollution under changing environmental conditions. Knowledge of how in-stream wood facilitates the occurrence of biogeochemical hotspots will assist river managers in adopting techniques which deliver benefits to both in-stream biota and water quality for human uses. This project, twinned with the proposed policy advice documents we will develop, will provide an ideal opportunity for the formation of science-based restoration activities which are able to deliver quantitative water quality improvements for little to no additional cost.