Floodplains have been degraded in Central Europe for centuries, resulting in less dynamic and less diverse ecosystems than in the past. They provide essential ecosystem services like nutrient retention to improve overall water quality and thus fulfill naturally what EU legislation demands, but this service is impaired by reduced connectivity patterns. Along the second-longest river in Europe, the Danube, restoration measures have been carried out and are planned for the near future in the Austrian Danube Floodplain National Park in accordance with navigation purposes. We investigated nutrient retention capacity in seven currently differently connected side arms and the effects of proposed restoration measures using two complementary modeling approaches. We modeled nutrient retention capacity in two scenarios considering different hydrological conditions, as well as the consequences of planned restoration measures for side arm connectivity. With existing monitoring data on hydrology, nitrate, and total phosphorus concentrations for three side arms, we applied a statistical model and compared these results to a semi-empirical retention model. The latter was originally developed for larger scales, based on transferable causalities of retention processes and set up for this floodplain with publicly available data. Both model outcomes are in a comparable range for NO3-N (77–198 kg ha−1 yr−1) and TP (1.4–5.7 kg ha−1 yr−1) retention and agree in calculating higher retention in floodplains, where reconnection allows more frequent inundation events. However, the differences in the model results are significant for specific aspects especially during high flows, where the semi-empirical model complements the statistical model. On the other hand, the statistical model complements the semi-empirical model when taking into account nutrient retention at times of no connection between the remaining water bodies left in the floodplain. Overall, both models show clearly that nutrient retention in the Danube floodplains can be enhanced by restoring lateral hydrological reconnection and, for all planned measures, a positive effect on the overall water quality of the Danube River is expected. Still, a frequently hydrologically connected stretch of national park is insufficient to improve the water quality of the whole Upper Danube, and more functional floodplains are required.

中文翻译：

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模拟增强的横向连通性对大型河流泛滥平原养分保留能力的影响：我们需要多少连通泛滥平原？

几个世纪以来，中欧的洪泛区一直在退化，导致与过去相比，生态系统缺乏活力和多样性。它们提供必要的生态系统服务，如养分保留，以改善整体水质，从而自然地满足欧盟立法的要求，但这种服务因连通性模式的减少而受到损害。沿着欧洲第二长的河流多瑙河，奥地利多瑙河泛滥平原国家公园已经采取并计划在不久的将来根据航行目的采取恢复措施。我们研究了七个当前不同连接的侧臂的养分保留能力，以及使用两种互补建模方法提出的恢复措施的影响。我们在考虑不同水文条件的两种情况下模拟养分保留能力，以及计划恢复措施对侧臂连通性的影响。利用现有的三个侧臂的水文、硝酸盐和总磷浓度监测数据，我们应用了一个统计模型，并将这些结果与半经验保留模型进行了比较。后者最初是为更大规模开发的，基于保留过程的可转移因果关系，并使用公开数据为该洪泛区设置。对于 NO3-N（77-198 kg ha-1 yr-1）和 TP（1.4-5.7 kg ha-1 yr-1）保留，两种模型结果都在可比范围内，并且在计算洪泛区的更高保留时一致，其中重新连接允许更频繁的淹没事件。然而，模型结果的差异对于特定方面是显着的，特别是在高流量期间，半经验模型补充了统计模型。另一方面，当考虑到洪泛区剩余水体之间没有联系时的养分保留时，统计模型补充了半经验模型。总体而言，两个模型都清楚地表明，通过恢复横向水文重新连接可以增强多瑙河漫滩的养分保留，并且对于所有计划的措施，预计会对多瑙河的整体水质产生积极影响。尽管如此，经常与水文相连的国家公园区域不足以改善整个多瑙河上游的水质，需要更多功能性的洪泛区。