The impact of surface water exchange on the nutrient and particle dynamics in side-arms along the River Danube, Austria

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Abstract

Results of two monitoring programs obtained in the free-flowing section of the Danube downstream of Vienna were used to evaluate the effects of river restoration designed to increase surface water inputs into side-arms. Functional descriptors like hydrochemical parameters and plankton react immediately to restored hydrological conditions and offer the opportunity to elucidate the hydrological control on organic processing as an important ecosystem function in fluvial landscapes. Two hydraulic parameters were estimated and linked to basic ecological properties. The level of hydrological connectivity was defined as the average annual duration (days per year) of upstream surface connection and can be used as a ‘simple to estimate’ parameter within the planning phase. Water age, an adapted measure of residence time based on more detailed information, allow description of the temporal development in side-arms. Greater hydrological connectivity leads to lower conductivity levels and increased nutrient concentrations due to the shift of the dominating source to river water. The contribution of river flow is indicated by higher suspended solid concentrations in side-arms than disconnected water bodies. The phytoplankton biomass shows the highest mean values at a duration of integration of 1 month a−1 and decrease with increasing connectivity. The relationships point to a more ‘main channel like’ hydrochemical situation in the side-arms, with a medium level of phytoplankton biomass and increased autochthonous carbon export. No evidence of eutrophication was found due to the shift of the side-arm from an organic matter sink to a source. On a more detailed level, water age demonstrates the temporal patterns of riverine input, the development of plankton production and the shift between hydrological and biological control of phytoplankton vs. riverine flow in a side-arm. The hydrologic parameters were useful predictors for evaluating the effects of restoration measures in river floodplain systems.

Introduction

Running waters are important links in the global biogeochemical cycles. They transport organic matter from terrestrial sources, produce organic material within aquatic environments and degrade organic matter on its way downstream (Hedges et al., 2000). The net effect of processes such as storage, breakdown and transformation of matter is a basic ecological property of lotic ecosystems (Fisher et al., 1998). In large rivers, these processes are influenced by the spatial and temporal availability of subsystems with higher hydrological retention, slack water areas, such as inshore retention zones, side-arms, riparian zones, floodplains, backwaters and wetlands (Reckendorfer et al., 1999, Thorp and Delong, 2002). The availability of certain slack water areas or dead zones is controlled by the riverine discharge, morphology and the exchange condition, e.g. during lower water tables inshore retention can foster the processing of organic matter (Schiemer et al., 2001). In contrast, during high water, floodplains and riverine wetlands play a key role for matter retention (Tockner et al., 1999). In river reaches with a variety of subsystems integrated in the riverine flow or with a frequent water exchange with the main channel, the local aquatic production and the particulate organic matter (POM) accumulation are emphasized. Therefore, higher ecosystem stability can be expected (Fisher et al., 1998).

Human impacts (e.g. on river systems like regulation) canalization and flood protection significantly reduce the retention capacities of the riverine landscape and limit the exchange of matter to short periods of high flow (Tockner et al., 1999). In particular, embankments and lateral dams lead to a significant decrease of hydrologic exchange of surface waters. The riverine landscape with secondary channels and all the various water bodies within the floodplain were disconnected from the river flow for long periods. Lotic conditions were reduced to the regulated main channel and lentic conditions prevail in all other water bodies as shown for the Austrian Danube (Tockner et al., 1998). In large rivers, therefore, restoration needs to focus on the availability of slack water areas with an active hydrologic exchange at a wide range of discharge conditions to improve the ecological integrity of these riverine systems (Bradshaw, 1996). As an example, the ‘Danube Restoration Project’ (DRP) was developed along the Austrian Danube to enhance the hydrological connectivity between the main channel and former side-arms by increased duration of lotic conditions and hydrologic exchange (Tockner et al., 1998, Schiemer et al., 1999).

The Danube transports high amounts of nutrients and suspended particles (Prazan, 1994, Hein et al., 1996), which could lead to a further eutrophication in the floodplain or lead to higher nutrient transformation within certain river stretches. Carbon, fixed in algal biomass in connected side arms, could either serve as a basis for the riverine community corresponding to export or lead to increased sedimentation depending on the hydraulic regime in the side-arm (Amoros et al., 1987). The main aim of our study was to understand how the hydrologic exchange conditions impact the role of side-arms as sink or source of matter and mediate the transfer of nutrients and organic matter. Within the restoration projects the applicability of hydrologic parameters and their predictive strength were tested. The exchange conditions were quantified by a ‘simple-to-estimate’ parameter: the level of hydrological connectivity expressed as the average days of integration in the flow regime and its predictive value for the evaluation of restoration programmes are discussed. The hydrologic parameter ‘water age’, an adapted metric of residence time measuring water exchange in multi input systems, was used to demonstrate the control of biotic processes on a more detailed level in one side-arm (Baranyi et al., 2002). Therefore, the main objectives of the present study were to determine: (i) nutrient and organic matter dynamics of side-arms in relation to the duration of integration in the riverine flow regime; and (ii) to evaluate the effect of water age on abiotic and biotic processes in a restored side-arm.

In consequence for future restoration activities, these issues were used to predict what effects of restoration measures on nutrient and organic matter cycling can be expected.

Section snippets

Study area

The Danube is one of the main drainage systems in Europe (817 000 km2). The Danube in Austria drains 104 000 km2 and is of the ninth river order. The flow is characterized by an alpine regime with highly variable and stochastic patterns. The mean discharge is 1900 m3 s−1 (Q95: 950 m3 s−1, Q1: 5040 m3 s−1). Like all large rivers in the industrialized world (Petts et al., 1989, Ward, 1998), the ecology of the Danube has been considerably affected by changed land-use, by pollution and most

Basic hydrologic properties of side-arms

The estimated duration of integration covered a wide range of lotic situations for side-arms in regulated river systems. Periods of less than 1 month were characterized by flowing conditions only during high water, whereas periods longer than 160 d a−1 refer to upstream connection close to mean water. The relevance of the two side-arms can be demonstrated by the portion of discharge flowing through them. In both side-arm systems, RB and OR, approximately 1% and up to 12% of the total discharge

The effect of increased hydrological connectivity

Generally, the hydrochemical conditions of the floodplain segments approached the riverine situation with increasing days of integration (Table 2). The geochemical situation of the floodplains was significantly different to the mean main channel situation until 46 d a−1. The low frequency of upstream surface water input and the higher portion of seepage supply was responsible for the higher values of geochemical parameters (Steininger, 2002). SRP and NH4–N concentrations showed for all

Conclusion

Restoration concepts for large river–floodplain systems require a profound insight into ecological functioning. The evaluation of restoration measures can be improved by the development of eco-hydraulic predictors, which are easy to estimate during the planning phase and have a high potential to predict the changes on important ecosystem function like matter processing and organic matter production.

The mean duration of integration, as one of these eco-hydraulic predictors, was significantly

Acknowledgements

This work was funded by the Austrian Science Fund (grant # P11720 bio), by the Austrian River Authority (in Regelsbrunn the project ‘Gewaesservernetzung Regelsbrunn’) and by the National Park Authority (Life98NAT/A/005422 in Orth). A. Aschauer, G. Heiler, C. Holarek, H. Kraill, P. Riedler and M. Schagerl contributed to the data collection and analysis. The authors would like to thank T.J. Battin and two anonymous reviewers for their comments on an earlier version of the manuscript.

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