Urban streams are often severely impaired due to channelization, high loads of nutrients and contaminants, and altered land cover in the watershed. Physical restoration of stream channels is widely used to offset the effects of urbanization on streams, with the goal of improving ecosystem structure and function. However, these efforts are rarely guided by strategic analysis of the factors that mediate the responsiveness of stream ecosystems to restoration. Given that ecological gradients from headwater streams to mainstem rivers are ubiquitous, we posited that location within a river network could mediate the benefits of channel restoration. We studied existing stream restorations in Milwaukee, Wisconsin, to determine (1) whether restorations improve ecosystem function (e.g., nutrient uptake, whole‐stream metabolism) and (2) how ecosystem responses vary by position in the urban river network. We quantified a suite of physicochemical and biological metrics in six pairs of contiguous restored and concrete channel reaches, spanning gradients in baseflow discharge (19–196 L/s) and river network position (i.e., headwater to mainstem). Hydrology differed dramatically between the restored and concrete reaches; water velocity was reduced 2‐ to 13‐fold while water residence time was 50–5,000% greater in adjacent restored reaches. Restored reaches had shorter nutrient uptake lengths for ammonium, nitrate, and phosphate, as well as higher whole‐stream metabolism. Furthermore, the majority of reaches were autotrophic (i.e., gross primary production > ecosystem respiration), which is not common in stream ecosystems. The difference in ecosystem functioning between restored and unrestored reaches was generally largest in headwaters and declined to equivalence in mainstem restorations. Our results suggest that headwater sites offer higher return on investment compared to larger downstream channels, where ecosystem responsiveness is low. If this pattern proves to be general, the scaling of ecosystem responses with river size could be integrated into planning guidelines for urban stream restorations to enhance the societal and ecological benefits of these expensive interventions.

中文翻译：

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生态系统对河道恢复的响应随着城市河网的水流大小而下降。

由于河道化，大量养分和污染物以及流域土地覆盖的变化，城市河流经常受到严重损害。河道的物理修复被广泛用来抵消城市化对河道的影响，目的是改善生态系统的结构和功能。然而，这些努力很少以对介导河流生态系统对恢复的响应能力的因素进行战略分析为指导。考虑到从上游水源到主干河的生态梯度无处不在，我们认为河网中的位置可以介导河道恢复的好处。我们研究了威斯康星州密尔沃基市现有的溪流修复物，以确定（1）修复物是否改善了生态系统功能（例如养分吸收，全流代谢）和（2）生态系统的响应如何随城市河网中的位置而变化。我们量化了六对连续恢复的和混凝土河道到达范围内的一系列理化和生物学指标，这些变化跨越了基流流量（19-196 L / s）和河网位置（即源头到主干）的梯度。修复后的河段与混凝土河段之间的水文差异很大。水速降低了2到13倍，而相邻恢复河段的水停留时间增加了50-5,000％。恢复的河段对铵，硝酸盐和磷酸盐的养分吸收长度较短，并且全流代谢较高。此外，大部分河段是自养的（即初级生产总值>生态系统呼吸作用），这在河流生态系统中并不常见。恢复的河段和未恢复的河段之间的生态系统功能差异通常在上游水源中最大，而在主干河水恢复中则降至同等水平。我们的研究结果表明，与较大的下游渠道相比，上游源头的生态系统响应速度较慢，而上游渠道的投资回报率更高。如果这种模式被证明是普遍的，则可以将生态系统响应随河流规模的变化纳入城市河流恢复规划指南中，以增强这些昂贵干预措施的社会和生态效益。生态系统响应度低的地方。如果这种模式被证明是普遍的，则可以将生态系统响应随河流规模的变化纳入城市河流恢复规划指南中，以增强这些昂贵干预措施的社会和生态效益。生态系统响应度低的地方。如果这种模式被证明是普遍的，则可以将生态系统响应随河流规模的变化纳入城市河流恢复规划指南中，以增强这些昂贵干预措施的社会和生态效益。