What's more important for managing phosphorus: loads, concentrations or both?-猫眼课题宝

DOI：

10.1021/es405148c

发表时间：

2014

期刊：

Environmental science & technology

影响因子：

11.4

通讯作者：

中科院分区：

环境科学与生态学1区

文献类型：

作者：

C. Stamm;H. Jarvie;Thadd Scott

2013年9月，美国路易斯安那州和宾夕法尼亚州通过了一项具有里程碑意义的联邦判决，要求美国环境保护署（EPA）为磷和氮制定国家水质标准，优先考虑密西西比河流域和流入切萨皮克湾的河流。现在，人们的焦点集中在我们如何管理和调节营养物质，以及如何最好地应用这些标准和标准来改善受损水体的质量和生态。在这一观点中，我们认为磷(P)的水质标准应该考虑目标是当地的溪流，还是下游的静水体。对于溪流，标准必须解决浓度问题，而负荷是静水的相关指标。关注错误的指标可能会导致令人失望的结果。在确定了磷在富营养化中的关键作用几十年后，由于过量的磷输入，欧洲或北美的许多水体仍然未能达到良好的生态状态或被认为是“受损”。缓解措施，如引进农业最佳管理做法、在污水处理厂去除磷和禁止在洗涤剂中添加磷，已经实施了几十年，并在控制富营养化方面取得了不同程度的成功。大多数流域都有一系列磷源，其磷的输送组成和时间各不相同，从非点源的高度偶发事件驱动的磷输送，到点源和地下水的近连续磷输入。此外，生物地球化学磷循环影响生物和非生物内部池中养分保留和释放的时空格局，从而改变水体系统中的养分运输。在许多流域，磷的输出主要是由事件驱动的农业用地运输过程（如地表径流、侵蚀、优先流）造成的偶发性磷损失（见图1）。因此，在了解造成这些损失的过程和减轻这些损失方面已经付出了很多努力。诸如解决弥漫性磷损失的关键源区域3之类的概念是基于这样一种认识，即大部分磷是在短期偶发性径流事件中损失的。然而，与慢性源相比，在偶发性事件中传递的相同P负荷的生态相关性并不一定等同。水生生态系统吸收营养物质的能力取决于营养物质的停留时间和其他物理因素，如光有效性和基质稳定性。偶发性事件输出营养物质的速度比它们可能被生物利用的速度快，而连续源则使生物利用的能力最大化。因此，温带水体富营养化风险和生物养分需求具有强烈的季节性特征，春、夏季富营养化需求和生态敏感性最高；而富营养化的风险和养分需求在冬季是最小的。养分负荷只有在影响这些时期的浓度时才具有生态相关性，而磷负荷对不同的水体类型具有非常不同的相关性。一个主要的考虑因素是静水（湖泊和水库）与流动水（河流和小溪）停留时间的差异。湖泊中较长的磷停留时间（较高的水和沉积物停留时间）意味着，在富营养化风险最大的时期（春季和夏季），偶发性水文驱动的磷事件可以被保留，并有助于内部磷负荷，而与事件的确切时间无关。因此，在大多数静水中，生态相关的P度量是年负荷。相反，河流中停留时间越短，河流的生态响应与磷的关系越密切

R landmark federal judgements in the U.S. (Louisiana and Pennsylvania, September 2013), have taken a big step closer to requiring the Environmental Protection Agency (EPA) to set national water quality standards for phosphorus and nitrogen, with priorities being the Mississippi River Basin and the rivers draining into Chesapeake Bay. This now puts the spotlight firmly on how we manage and regulate nutrients and how such standards and criteria can be best applied to achieve improvements in the quality and ecology of impaired water bodies. In this Viewpoint we argue that water quality standards for phosphorus (P) should take account of whether the target is the local stream, versus a downstream standing water body. For streams the standards must address concentrations while loads are the relevant metrics for standing waters. Focusing on the wrong metrics may cause disappointing results. Decades after the pivotal role of P for eutrophication has been established, many water bodies across Europe or North America still fail to reach a Good Ecological Status or are considered “Impaired”, due to excessive P inputs. Mitigation measures like the introduction of agricultural best management practices, P stripping in wastewater treatment plants and the ban of P in detergents have been implemented for decades and have had variable successes in achieving control of eutrophication. Most watersheds have a range of P sources, with varying composition and timing of P delivery, from highly episodic event-driven P delivery from nonpoint sources, to nearcontinuous P inputs from point sources and groundwater. In addition, biogeochemical P cycling influences the spatial and temporal patterns of nutrient retention and release from internal biotic and abiotic pools, which modifies nutrient transport in aquatic systems. In many watersheds P export is dominated by episodic P losses caused by event-driven transport processes (e.g., surface runoff, erosion, preferential flow) from agricultural land (see Figure 1). Accordingly, much effort has gone into understanding the processes causing these losses and into mitigating them. Concepts like Critical Source Areas for tackling diffuse P losses 3 are based on an understanding that most of the P is lost during short episodic runoff events. However, the ecological relevance of the same P load, delivered during an episodic event, compared with a chronic source, is not necessarily equivalent. The capacity of aquatic ecosystems to assimilate nutrients depends upon the nutrient residence time and other physical factors such as light availability and substrate stability. Episodic events export nutrients faster than they may be utilized biologically while continuous sources maximize the capacity for biological utilization. Thus, there is a strong seasonal pattern in eutrophication risk and biological nutrient demand for water bodies in temperate zoneswith the highest demand and ecological sensitivity to eutrophication in the spring and summer; whereas eutrophication risk and nutrient demand are minimal during the winter. Nutrient loads are only relevant ecologically if they affect the concentrations during these periods, with P loads having very different relevance for different water body types. A major consideration is the difference in residence times within standing waters (lakes and reservoirs) compared with flowing waters (rivers and streams). The longer P residence times (higher water and sediment residence times) in lakes means that episodic hydrologically driven P events can be retained and contribute to internal P loadings during the times of greatest eutrophication risk (in spring and summer) irrespective of the exact timing of the event. Therefore, in most standing waters the ecologically relevant P metric is annual load. In contrast, the shorter residence times in rivers mean that the ecological response in rivers is more closely linked to P

What's more important for managing phosphorus: loads, concentrations or both?

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