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

几个世纪以来，英国和欧洲的河流已经清除了大量的木质碎屑，以减少这些障碍和堵塞造成的潜在洪水风险。然而，大量木质碎屑的清除已被证明严重影响河流提供的生态系统服务，包括栖息地功能以及作为物理和生物化学屏障而减少营养和沉积物负荷的潜力。最近欧盟和英国的环境政策(特别是欧洲水框架指令)正在推动管理实践的重大转变，大力强调重新引入溪流中的木材，通过增加其减少过量硝酸盐污染的能力来提高低地河流生态系统对环境变化的适应能力。作为河流中的障碍物，木质碎屑预计会增加地表水向河床的下沉，并增加其在非常活跃的河床区的接触时间，导致化学物质周转增加和硝酸盐衰减。然而，大型木质碎屑对河床生物地球化学过程的主要驱动因素、地下水与地表水之间的交换通量以及水在河床中的停留时间的影响却知之甚少。为了改进大型木质碎屑结构的设计，以有效地去除河床中的硝酸盐，流域管理和恢复方案需要科学证据证明不同的大型木质碎屑设计对改善水质的有效性。因此，该项目将提供所需的科学证据，以评估不同的永久性和移动式大型木质碎屑设计提高低地河流对氮的吸收的效率。我们的研究特别关注低地溪流，因为这些溪流代表了英国大多数受到严重高硝酸盐浓度威胁的河流。该项目的结果将直接为河流恢复实践提供信息，并决定是否在英国低地溪流中部署大型木质碎屑，以减少极端高的硝酸盐负荷，程度如何，采用什么设计。我们将结合新的监测技术和环境示踪技术与创新的数值模拟方法，以确定大型木质碎屑促进生物地球化学周转的河床热点的发生。我们将量化在这个反应热点地区可以将过剩的硝酸盐浓度降低到什么程度，以及哪些不同设计的大型木质碎片结构最有利于这种生态系统服务。我们将改进复杂的数值模型，以量化永久的和可移动的大型木质垃圾如何有效地减少来自低地河流的过量硝酸盐。通过开发和模拟一系列情景，假设更广泛的环境条件，然后通过实验观察，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质碎屑结构如何有效地减少硝酸盐污染。了解溪流中的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对溪流中的生物群和人类使用的水质都有好处的技术。该项目与我们将制定的拟议政策咨询文件相结合，将为形成以科学为基础的修复活动提供一个理想的机会，这些活动能够以很少的或不额外的成本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.