Hydromorphological restoration stimulates river ecosystem metabolism

Benjamin Kupilas,Christoph Knuth,Daniel Hering,Björn Gücker,Armin W Lorenz

doi:10.5194/bg-14-1989-2017

Abstract

Abstract. Both ecosystem structure and functioning determine ecosystem status and are important for the provision of goods and services to society. However, there is a paucity of research that couples functional measures with assessments of ecosystem structure. In mid-sized and large rivers, effects of restoration on key ecosystem processes, such as ecosystem metabolism, have rarely been addressed and remain poorly understood. We compared three reaches of the third-order, gravel-bed river Ruhr in Germany: two reaches restored with moderate (R1) and substantial effort (R2) and one upstream degraded reach (D). Hydromorphology, habitat composition, and hydrodynamics were assessed. We estimated gross primary production (GPP) and ecosystem respiration (ER) using the one-station open-channel diel dissolved oxygen change method over a 50-day period at the end of each reach. Moreover, we estimated metabolic rates of the combined restored reaches (R1 + R2) using the two-station open-channel method. Values for hydromorphological variables increased with restoration intensity (D < R1 < R2). Restored reaches had lower current velocity, higher longitudinal dispersion and larger transient storage zones. However, fractions of median travel time due to transient storage were highest in R1 and lowest in R2, with intermediate values in D. The share of macrophyte cover of total wetted area was highest in R2 and lowest in R1, with intermediate values in D. Station R2 had higher average GPP and ER than R1 and D. The combined restored reaches R1 + R2 also exhibited higher GPP and ER than the degraded upstream river (station D). Restoration increased river autotrophy, as indicated by elevated GPP : ER, and net ecosystem production (NEP) of restored reaches. Temporal patterns of ER closely mirrored those of GPP, pointing to the importance of autochthonous production for ecosystem functioning. In conclusion, high reach-scale restoration effort had considerable effects on river hydrodynamics and ecosystem functioning, which were mainly related to massive stands of macrophytes. High rates of metabolism and the occurrence of dense macrophyte stands may increase the assimilation of dissolved nutrients and the sedimentation of particulate nutrients, thereby positively affecting water quality.

Highlights

River restoration is a pivotal element of catchment management to counteract anthropogenic degradation and depletion of river health and water resources, and to increase overall biodiversity and ecosystem services provisioning (Bernhardt et al, 2005; Strayer and Dudgeon, 2010)
1120 36.6 10.5 41 673 34 218 12 381 691 19.8 0.47 10.21 27.05 3.16 2.0 × 10−4 0.8 4.4 a Width variation calculated as standard deviation; degraded: n = 42; restored 1: n = 59; restored 2: n = 54. b Data on hydrodynamic characteristics represent the final parameters obtained by one-dimensional transport modeling using OTIS-P
The share of macrophyte cover of total wetted area was highest in R2

Summary

Introduction

River restoration is a pivotal element of catchment management to counteract anthropogenic degradation and depletion of river health and water resources, and to increase overall biodiversity and ecosystem services provisioning (Bernhardt et al, 2005; Strayer and Dudgeon, 2010). In Europe, degraded river hydromorphology is considered one of the central impacts to the ecological status of rivers (EEA, 2012; Hering et al, 2015). The German national river habitat survey, which evaluates 31 hydromorphological parameters for 100 m river sections, concluded that the majority of German rivers are severely degraded (Gellert et al, 2014; UBA, 2013). As the river biota depend on suitable habitats (Beisel et al, 2000; Schröder et al, 2013), about.

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