Modeling the Effect of Enhanced Lateral Connectivity on Nutrient Retention Capacity in Large River Floodplains: How Much Connected Floodplain Do We Need?

Stephanie Natho,Martin Tschikof,Elisabeth Bondar-Kunze,Thomas Hein

doi:10.3389/fenvs.2020.00074

Abstract

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 modelled 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 year-1) and TP (1.4- 5.7 kg ha-1 year-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.

Highlights

Rivers and their adjacent floodplain ecosystems are essential for human life and biodiversity, but are among the most threatened ecosystems globally (Tockner et al, 2008)
We explore the efficiency of nutrient retention due to side arm reconnections in a case study of the Danube River by applying two modeling techniques
Only four side arm systems in the Danube Floodplain National Park (DFNP) were partially reconnected to different degrees, resulting in no noticeable reduction of nutrients transported in the Danube River

Summary

Introduction

Rivers and their adjacent floodplain ecosystems are essential for human life and biodiversity, but are among the most threatened ecosystems globally (Tockner et al, 2008). Floodplains are seen as important nutrient sinks (Spieles and Mitsch, 1999; McClain et al, 2003; Hoffmann et al, 2011; Hopkins et al, 2018), with a higher retention potential than in the main channel (Saunders and Kalff, 2001; Venterink et al, 2003). Whereas in times of higher water levels suspended particles and inorganic nutrients are transported into side arms or inundated areas, during periods of no or low supply, internal nutrient cycling prevails (with low concentrations of inorganic forms) (Hein et al, 2004). The retention of these nutrients in floodplains depends on a variety of abiotic and biotic processes. Inorganic nitrogen (N), on the other hand, is most commonly removed through biotic processes, mainly denitrification (accounting for up to 63% TN retention according to Saunders and Kalff, 2001) as a permanent removal (Boyer et al, 2006) and together with phosphate through autotrophic and heterotrophic uptake (Fisher and Acreman, 2004; Venterink et al, 2006; Jordan et al, 2011)

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