Aquatic ecosystem functions of an isolated floodplain and their implications for flood retention and management

Authors


Corresponding author. E-mail: walter.reckendorfer@wcl.ac.at

Summary

  1. We used an isolated floodplain of the river Danube as a model system to gain an understanding on the functioning of retention areas to predict future developments and to sustain their ecological services.
  2. We applied correlation analysis and spline regression models to assess the effects of geomorphology, hydraulics, and seasonality on sediment characteristics, suspended solids, hydrochemistry and primary producers.
  3. The spatio-temporal connection to the river is the primary factor influencing the hydrochemical characteristics and sediments. Allochthonous processes such as nutrient and sediment input during high waters dominate in connected parts of the floodplain, whereas autochthonous processes, for example, the release of phosphorus from the sediments and internally driven eutrophication, dominate in isolated parts. These conditions also affect the dominating primary producers, biodiversity, the degree of floodplain aggradation and thus the potential life span of aquatic habitats.
  4. Measures to improve the functional basis for ecological services may use both allochthonous and autochthonous processes as a starting point, that is, minimizing sediment storage and nutrient input and improving the water balance to prolong the life span of isolated waters, and thus maximizing water body diversity and associated biodiversity.
  5. Based on the results of our analysis and literature, eight alternative management measures have been evaluated. As a result, we propose a stepwise adaptive approach beginning with a controlled water supply with low sediments and nutrient loads. If these measures prove insufficient to sustain ecological functions and conservation value, more radical steps must be considered.
  6. Synthesis and applications The increasing problems with catastrophic flooding have forced decision makers to seek basin-wide solutions with focus on ‘more room for the river’ and the reintegration of former floodplains as retention basins. Such reintegrations also represent opportunities to improve the ecological conditions for nature development in addition to their principal function, that is, the storage of water during floods. The results of our study can serve as an effective tool to predict the effects of alternative management options and to establish and define the design criteria of water retention areas with regard to their ecological functions, life spans and biodiversity.

Introduction

The increasing threats with the catastrophic flooding of large rivers have forced river authorities to seek basin-wide solutions with focus on ‘more room for the river’ and the reintegration of nowadays disconnected floodplains as retention basins (Fiselier & Oosterberg 2004). Besides flood-mitigation, such reintegrations also represent opportunities to improve the ecosystem functioning with regard to nutrient, carbon and sediment dynamics and the resulting landscape development. The main ecological services that are affected thereby are nature conservation, flood retention and recreation. Ecologically orientated planning and management of retention areas thus require an understanding of the relationships between their function and designs (Verhoeven et al. 2006). Within floodplains, aquatic habitat elements are most endangered and especially susceptible towards management measures which change hydrological connectivity. A high diversity of back water bodies (BWs) along with their temporal development is the basis for a unique assemblage of biota.

The management of retention areas with respect to the preservation and promotion of BWs comprises two significant aspects of ecosystem functioning, namely trophic conditions and sediment dynamics. For the control of the trophic state, the major factors are external and internal nutrient loadings and the development of littoral, macrophytic vegetations. These are in turn determined by the water source (seepage, river water and ground water; Heiler et al. 1995), the source of sediments (river borne, autumnal leaf fall and autochthonous production) and the balance between erosion and sedimentation. All these factors strongly depend on the location of a water body within the backwater chain, and their hydrological connectivity with the river.

At low hydrological connectivity, floodplain backwaters develop characteristics of shallow eutrophic lakes, exhibiting a clear water state, dominated by macrophytes or a turbid- state dominated by phytoplankton caused by nutrient input and associated stimulation of phytoplankton growth (‘alternative stable state hypothesis’; Scheffer et al. 1993; Bayley & Prather 2003).

However, with increasing connectivity, the various ‘states’ are primarily under hydrological control (Preiner et al. 2008). Disturbances such as high flow velocities, water level fluctuations or increased turbidity may result in reduction in macrophyte abundance, whereas phytoplankton is stimulated by periodical nutrient pulses (Riis & Biggs 2003).

Furthermore, the accumulation of fine sediments is significant for eutrophication by internal loading via exchange processes between the sediment and water column (Mainstone & Parr 2002; Noe & Hupp 2005).

Sediments are also important for floodplain aggradation because the balance between sedimentation and erosion determines the life spans of aquatic habitats. The primary autochthonous sources for the sediments are the deposition of submerged and emergent macrophytes, whereas significant amounts of allochthonous material are provided by the river and by autumnal leaf fall of floodplain forests (Kirschner, Riegl & Velimirov 2001). The balance between these processes controls the texture, the chemical composition and the organic characteristics of the sediments (Schwarz, Malanson & Weirich 1996).

This study is based on the hypothesis that the hydrological connectivity to the river is the primary factor for the ecology of floodplains. A significant question in this context is the extent to which autochthonous and allochthonous processes are regulatory and interacting. To answer this question, the following aspects must be analysed: (i) sediment, nutrient and organic matter dynamics in relation to the riverine flow regime and geomorphic characteristics of the BWs and (ii) the effect of water sources on abiotic and biotic processes such as nutrient dynamics and organic matter cycling.

We have used the backwater area of the river Danube at Vienna, Austria – known as Lower Lobau (LL) – as a model system to analyse the effects of geomorphology, hydraulics and seasonality on the sediment characteristics, suspended solids, hydrochemistry, and primary producers and thereby emphasize their interdependencies. The shape and topology of LL, which is a >120-year-old polder system with downstream connections to the Danube, enabled analysis of the effect of different hydromorphic gradients such as surface connectivity, seepage supply or permanence of BWs on the sediment, hydrochemistry, and phytoplankton and macrophyte development. The age of LL permitted the analysis of long-term processes (e.g. sediment accumulation and terrestrialization). We then use the results of our study to assess the outcome of eight different management options in LL. In a broader context, our study may help to define design criteria for water retention areas with regard to their ecological functions, life spans and biodiversity.

Materials and methods

Study Area

Lower Lobau is a former floodplain area c. 10-km in length (1918–1908 river kilometres (r-km); that is, the distance to the Black Sea) on the left bank of the river Danube. The total retention capacity in LL is 47·6 mio.m³ at 14000 m³s−1 discharge (maximum theoretically achievable discharge). During floods, the discharge into LL is estimated to be 45 m³s−1, which corresponds to 1% of the Danube River discharge during the mean yearly flood (Reckendorfer & Hein 2006). The water level of the Danube is thereby lowered by c. 5 cm as a result of the retention area. Besides its significance for flood control, LL is also important for Vienna as a source of drinking water supply and recreation (Schiemer, Baumgartner & Tockner 1999). The size of the LL area is 1474 ha, and it comprises 61% floodplain forest, 13% xeric gravel biotopes, 12% BWs, temporary waters and reeds, 7% meadows and 7% fields.

LL has been completely isolated from scouring floods since the major regulation scheme in the 19th century. The BWs presently comprise a river channel network whose main side channel (MSC) has a surface connection with the river at its lowermost end through a levee opening (henceforth referred to as ‘levee opening,’ ‘inflow’ or ‘backflow connection’); numerous partly disconnected backwaters and isolated BWs such as ponds are also present (Fig. 1; for details, see Schiemer, Baumgartner & Tockner 1999). The LL is subdivided into 6 basins by check dams and natural fords.

Figure 1.

Location of the study area along the Austrian Danube (inset) and a detailed map of Lower Lobau (LL) with basins (1–6), check dams and fords (KT, MF, GT, SF, ST); gauges (P1, P44, P43); hydrochemical sampling sites (black circles: BWs; white circles: MSC); the numbers in the boxes refer to the number of sediment samples in the BWs and MSC, respectively.

Management options for LL discussed within the last years range from the conservation of the present status as an isolated floodplain lake system to the restoration of the floodplain towards pristine conditions (Funk et al. 2002, Funk et al. 2012).

Sampling Design

In this study, the results of several investigations addressing hydrochemical and hydromorphic parameters and processes in the study area have been integrated. The data have been gathered between 1996 and 2007 to support conservation and restoration strategies in LL. Sampling points where chosen to cover all potential hydromorphic gradients and were thus located in every basin of the main channel as well as in the BWs (Fig. 1).

In the case of all the hydrochemical parameters and suspended solid concentrations, samples were taken in monthly intervals in the growing season from April to October in every year of the investigation at 6 sites, and in shorter intervals during special sampling programmes (e.g. during several high water events; see the hydrograph of the Danube in Data S1).

Sediments were sampled with a modified Gilson Corer (d = 5·9 cm) equipped with a closure mechanism to prevent sediment losses when pulling out the corer. All the samples for the laboratory analyses were placed in the dark and brought to the laboratory within 3 h.

Hydrology, Hydraulics and Geomorphic Parameters

The water levels of the river Danube (gauge at 1894·7 r-km) and for the basins in LL were provided by the Austrian River Authority (viadonau Österreichische Wasserstraßen GmbH) and the municipal water authority of Vienna. The Danube levels are monitored on a daily basis, while the levels in LL are checked, on an average, twice a week.

The following hydromorphic parameters were assessed or calculated.

Surrogates for connectivity – distance to inflow (DI, m): the distance of a sampling point to the inflow (Fig. 1); isolation (I, m.a.sl): height when a sampling point becomes disconnected from the river Danube; connectedness (C, 1/0): connected to the river Danube on the sampling day; water level (gauge, m.a.sl): water level at the gauge Wildungsmauer (1894·7 r-km);

Surrogate for the influence of seepage water – distance to dike (DD, m): the distance of a sampling point to the flood protection dike (inner levee in Fig. 1)

Surrogate for autumnal leaf input – direct solar irradiation (DSI; hours per day)

Surrogate for permanence of BWs – depth (dmax, m): maximum depth of a water body at the mean water level of the Danube.

The distances of the sampling points from the inflow and from the dike were delineated using the ‘Near’ tool in ArcGis 9·3 (ESRI).

Direct solar irradiation was calculated using the RadCalc extension for ArcView (ESRI) for day 150 (i.e. May 31). As inputs, the RadCalc extension requires a digital terrain model (DTM) and the geographic location of the sampling point. To include the shadowing effects of the alluvial forest, a grid of the vegetation height was calculated and added to the DTM. Tree height measurements were provided by the Vienna municipality. Water bodies were assumed to have a height of zero, while reed was assumed to have a height of 2 m. For each water body, the mean of all the grid cells within the water body was calculated.

To characterize the different BWs with respect to their maximum depth, the DTM, aerial photographs and a groundwater model were used. In the aerial photographs, BWs were delineated based on transversal check dams and natural fords. For each water body, the maximum depth at the mean water level in the Danube was assessed.

Isolation and connectedness were estimated with the DTM and the water level at the inflow. For this analysis, it was assumed that the water level in LL is equivalent to the water level at the inflow. If a continuous interlinkage between the Danube River and a sampling point is given, the sampling point can be assumed to be connected.

Sediment Characteristics

By probing from a boat or from the ground, fine sediment depths were measured in transverse sections in the MSC and in the BWs at 1401 sampling points. For probing, a U-profiled bar with a diameter of 1 cm was used. Fine sediment depth was defined as the depth that the bar can reach before touching the gravel subsoil.

For the determination of fine sediment particulate organic matter (POM) content, sediment cores were taken from 122 of the 1401 sampling points in MSC and BWs. A weighed subsample was dried at 95 °C to constant weight (dry weight) and burned at 450 °C for 6 h to obtain its ash-free dry weight (= organic matter content). The total phosphorus concentrations (Ptot) were estimated for 21 sampling points. Weighed sediment subsamples were combusted at 490 °C for 2·5 h, and the total phosphorus content was first extracted using 1 M HCl for 16 h in accordance with Ruban et al. (2001). Ptot was then measured spectrophotometrically in accordance with Strickland & Parsons (1972).

To identify different carbon sources and their potential biological availability, particulate organic carbon (POC), particulate organic nitrogen (PON), the POC/PON ratio, δ 13C and δ 15N values were estimated according to Aspetsberger et al. (2002) for 41 sampling points.

Hydrochemical Parameters, Suspended Solids and Macrophytes

The temperature, pH, conductivity and oxygen concentrations were measured in situ with portable meters at each sampling point. Water samples were taken 20 cm below the surface using 10-L polyethylene bottles and transported in dark cool boxes to the laboratory. Samples were immediately filtered through precombusted Whatman GF/F filters for the determination of suspended particles. Chemical analyses were completed within 24 h. Inorganic nutrients [soluble reactive phosphorus (SRP)], total and dissolved fractions of phosphorus and nitrogen, magnesium, calcium, alkalinity, chlorophyll a (filtered with Whatman GF/C filters) and suspended solid concentrations (dry weight and ash-free dry weight for fractions of organic matter) were analysed in accordance with standard methods (Strickland & Parsons 1972; Golterman, Clymo & Ohnstad 1978).

Between 1996 and 2006 macrophyte quantities were assessed according to Kohler (1978) at 261 stretches once or twice, giving a total of 447 samples.

Statistical Analysis

For data analysis, the statistical software SPSS 12.0G (2003) and the package Earth (Milborrow 2011) of R (R Development Core Team 2010) were used. Spline regressions (main effects and first-order interactions) were estimated to describe the relationships between the parameters of interest with the hydrological, morphological and seasonal variables. Correlation analysis was performed by the nonparametric Spearman's rank correlation.

The relationship between the parameters of interest and the distance to the inflow was chosen for a graphical representation. anova results for the graphs are given in Table S3_5 in Data S3 (Supporting information).

The physicochemical data set consisted of more than 500 cases, but for only 172, the entire data set was available. We thus ran a principal component analysis (PCA) on the reduced data set (172 cases) to eliminate correlated hydrochemical variables. The following parameters were entered in the analysis: alkalinity, conductivity, O2 saturation and concentrations of Ptot (total phosphorous), P-PO4, P-s (soluble phosphorus), N-totkj (total Kjeldahl nitrogen), N-skj (soluble Kjeldahl nitrogen), N-NO3, SiO4, magnesium, calcium, O2 and chlorophyll a. The variables that showed highest correlations with the first three principal axes were considered to be surrogates for the entire data and used for further analysis (see Yuan 2004; Linke, Norris & Pressey 2008). This procedure allowed us to analyse a considerably larger data set (>500 cases as opposed to 172). In addition to the variables with the highest loading on the three axes, chlorophyll a and macrophyte biomass were also analysed.

Results

Sediment Characteristics

The sediment characteristics exhibited high variability within the floodplain and within single BWs. The mean depth of the fine sediment layer was 43·3 ± 1·2 cm (mean ± SE). From spline regression, two major gradients influencing the sediment depth could be identified: distance to the inflow and degree of isolation. Other significant hydromorphic variables were width and depth of BWs. Fine sediment depth rapidly decreased with distance from the downstream exit and reached its maximum of 270 cm close to the first check dam. On an average, the depth was 120·3 ± 9·1 cm (mean ± SE) in the first basin. Upward of the first check dam, the mean values in the basins remained comparatively low (average values ranging from 28·4 ± 2·1 cm to 56·8 ± 3·3 cm). Isolated BWs had higher fine sediment layers than the connected ones (Fig. 2, S3_1, S3_2 in Data S3, Supporting information).

Figure 2.

Upper graph: Fine sediment depth vs. distance from the downstream exit; Lower graph: Organic content of the sediment. The median (square) and quartiles (error bar) for each basin (numbers on top of the figure) are shown; black: connected at water levels <150 m.a.sl; white: connected at water levels >150 m.a.sl.

The POM content of fine sediments ranged from 0·3 to 48·9% of the dry weight (mean ± SE, 8·1 ± 0·7). Spline regression identified the same two major gradients that influenced the POM content – the distance to the inflow and the degree of isolation. The only other hydromorphic variable with significant impact was depth. The POM concentration significantly increased with distance from the downstream exit with values of around 3% and 8% in the in the first and upper basins, respectively. The highest values were observed in shallow isolated BWs far away from the inflow (Fig. 2, S3_1, S3_2 in Data S3, Supporting information).

The total phosphorus concentrations in the sediment were significantly correlated with the POM content (r = 0·88; < 0·001). The pattern was similar, and shallow, isolated BWs had higher concentrations. The POC/PON ratio ranged from 6·8 to 21·9 (mean 9·5 ± 2·5) with significantly higher values in the shallow parts of MSC. The values also tended to be higher near the inflow, but no significant relationships with the distance from the inflow could be detected. The δ 13C values ranged from −27 to −32 and decreased significantly with the distance from the inflow and the distance from the dike. The shaded BWs had significantly higher δ 13C values. For δ 15N, a significant positive relationship with depth could be observed (S3_2 in Data S3, Supporting information).

Suspended Solids

The total suspended solid (TSS) concentrations ranged from 0·02 to 1319 mg L−1. Spline regression identified the distance to the inflow, water level of the Danube and degree of isolation as the major gradients influencing the suspended solids concentration. The TSS significantly decreased with distance from the inflow. The concentrations were highly variable in the first basin and ranged over 3 orders of magnitude: from less than 1 mg L−1 to more than 1,300 mg L−1 with a mean of 75·7 ± 14·8 mg L−1 (mean ± SE); this was followed by a distinct decrease after the first check dam (8 ± 7 mg L−1). In the other basins, the mean values remained comparatively low (average values ranging from 3·0 ± 0·2 to 11·4 ± 1·9 mg L−1) (Fig. 3, S3_3 in Data S3, Supporting information).

Figure 3.

Total suspended solid (TSS) concentration and organic content of TSS (quartiles) for each basin (numbers on top of the figure) with regression lines; Low water: <144 m.a.sl; High water: >144 m.a.sl.

The relative contribution of POM to TSS significantly increased with the distance from the inflow. The water level in the river Danube and the degree of isolation also had significant effects on the TSS and the relative contribution of POM (Fig. 3, S3_3 in Data S3, Supporting information).

Hydrochemistry

Three principle components (axes) could be extracted from the 14 variables that were entered into the PCA. The variance in hydrochemical and pigment parameters explained by the three axes totalled 70%. The ‘nutrient’ axis (PCA1) corresponded to 33% of the variance, while the ‘geochemistry’ axis (PCA2) corresponded to 20%. The third axis (PCA3) was designated as the ‘primary production’ axis, and it corresponded to 17% of the variance explained (Table 1).

Table 1. Principal component analysis (PCA) results (N = 172); bold numbers indicate r > 0.5
 PCA1‘nutrient’ axisPCA2‘geochemistry’ axisPCA3‘primary production’ axis
Ptot 0·95 −0·120·02
N-totkj 0·88 0·040·26
P-s 0·84 −0·10−0·33
P-PO4 0·79 −0·12−0·39
SiO4 0·78 0·09−0·24
N-skj 0·77 0·110·05
Alkalinity0·18 0·89 −0·20
Magnesium−0·30 0·75 0·05
Calcium0·01 0·70 −0·08
Conductivity (25 °C)−0·07 0·65 −0·09
N-NO3−0·110·620·00
O2 concentration−0·20−0·23 0·87
O2 saturation (%)−0·23−0·19 0·87
Chlorophyll a (Talling)0·480·11 0·65

The BWs were clearly influenced by the downstream connection with the Danube. This can be best observed in the geochemical parameters that increased with the distance from the inflow. The influence of the connection to the Danube River was also reflected in the variations in the alkalinity values with the water levels; high and low levels led to different alkalinity values. The average nutrient concentrations were high near the inflow, probably due to the input of nutrients from the river, while they were elevated in isolated BWs distant from the inflow. Along with the nutrient concentrations, the O2 saturation decreased with the distance from the inflow and was higher in deeper BWs (Fig. 4, S3_4 in Data S3, Supporting information).

Figure 4.

Alkalinity, Ptot, and O2 saturation (quartiles) with regression lines; Low water: <144 m.a.sl; High water: >144 m.a.sl; Connected: connected at water levels <150 m.a.sl; Isolated: connected at water levels >150 m.a.sl; Shallow: <1 m; Deep: >1 m.

Primary Producers

The chlorophyll a concentrations were most related to the depth and the degree of connectivity, with generally higher values in connected BWs, although deep isolated BWs showed the highest concentrations. Chlorophyll a concentrations were also elevated during low water levels (Fig. 5, S3_4 in Data S3, Supporting information). A significantly negative relationship with the macrophyte biomass (assessed as the sum of Kohler indices) indicates competition between the two primary producers. The importance of hydrological connectivity in balancing the two primary producers is indicated by a significant reduction in chlorophyll a during the growing season in isolated BWs, whereas chlorophyll a values remain high throughout the year in connected BWs (Fig. 6).

Figure 5.

Chlorophyll a concentration (quartiles) with regression lines; Low water: <144 m.a.sl; High water: >144 m.a.sl; Connected: connected at water levels <150 m.a.sl; Isolated: connected at water levels >150 m.a.sl; Shallow: <1 m; Deep: >1 m.

Figure 6.

Chlorophyll a concentrations (mean ± SE) in connected and isolated water bodies for the growing season and for the winter half year.

Macrophyte biomass was generally higher in deep and connected BWs and significantly positively related to macrophyte and odonate species number (Fig. 7, S2_1 in Data S2, Supporting information).

Figure 7.

ΣKohler index (quartiles) with regression lines; Legends as in Fig. 4.

Discussion

The results of our study lead to certain general conclusions on the driving forces of eco-hydrological processes in isolated floodplains; further, some recommendations for their management can be formulated.

Constriction of latitudinal connection to the river transforms a floodplain into a sediment sink and initiates aggradation processes (Schiemer, Baumgartner & Tockner 1999; Hohensinner et al. 2008). In LL, the accumulated fine sediment layer in the BWs is c. 40 cm on average with a maximum value of 2·7 m. This value corresponds to a sedimentation rate of 0·3 cm year−1 and a maximum rate of c. 2 cm year−1 (estimated on the basis that there was no sedimentation before the construction of flood protection dams in the late 19th century), which may lead to an extinction of many BWs within the next decennials (Data S4, Supporting information). The highest sedimentation occurred at areas near the backflow connection to the river Danube with an average of c. 0·9 cm year−1. The gradients in the sedimentation are also reflected in the concentrations of suspended solids: large amounts of TSS with low organic contents were found especially at high waters near the inflow.

The sedimentation rates calculated for LL have the same order of magnitude as those found for different locations in the floodplains of the Elbe, the Rhine and the river Ouse; they range from 0·007 to 1·54 cm year−1 and highest sedimentation occurred in the backwaters near the river and decreased with the distance from it (Kiffer et al. 1998; Walling, Owens & Leeks 1998; Middelkoop 2002; Krüger et al. 2006). In consequence, the shrinking and the ‘terrestrialization process’ of the BWs are generally faster near the levee opening. The significance of the backflow from the river for sediment characteristics is also clearly indicated by the POM content of TSS; the POM decreases with connectivity (distance to the inflow) and reflects the low amounts of POM in the Danube. Near the levee opening, the quality of the POM analysed by stable carbon isotopes (δ 13C) indicates the importance of river-borne POM as the dominant carbon source, whereas plankton- or macrophyte-derived organic carbon are dominant in isolated parts. This is also supported by low POC/PON ratios (mean 9·5 ± 2·5) with higher values at the inflow: fresh organic matter from protein-rich and cellulose-poor algae has C/N values ranging between 4 and 10, while cellulose-rich organic matter usually has values of 20 and more (Meyers 2003).

The positive correlation between the POM content and total phosphorus concentration in the sediment suggests that organic-bound phosphorus is the dominant phosphorus fraction. This supports the findings of Bondar et al. (2007) and Zehetner et al. (2008), who assessed the relative contributions of organic P to total P in adjacent floodplain areas. Numerous studies have clearly demonstrated the importance of organic phosphorous in the aquatic phosphorus cycle (see Mitchell & Baldwin 2005), although organic P is often described as persistent to bacterial mineralization. High organic matter and phosphorous content within the sediments – typical for isolated BWs in LL and similar systems (Ramm & Scheps 1997; Stephen, Moss & Phillips 1997; Bondar et al. 2007) – in combination with periodic anoxic conditions at the sediment–water interface might increase the release of phosphorus and lead to internally driven eutrophication. Evidence for this is provided by the higher amounts of total phosphorous concentrations in the water column in isolated BWs with high POM values in the sediment.

The spatio-temporal connection to the river is the primary factor influencing the hydrochemical characteristics of the individual BWs. Two spatial gradients are evident, and they are both related to the degree of surface connection, that is, the distance to the inflow and the degree of isolation. On the temporal scale, high water events lead to the import of inorganic nutrients in sites with high surface water connections to the river (Fig. 4). Near the inflow (basin 1), phytoplanktons are the dominant primary producers with high chlorophyll a concentrations especially under low-water conditions (Fig. 6). In general, this can be considered analogous to the phytoplankton-dominated state of the alternative stable state hypothesis (Scheffer et al. 1993) that is characterized by high turbidity and strong algal blooms and an absence of submerged vegetation. In several faunistic groups such as aquatic insects and molluscs, such backwaters are less rich and diverse in species (Data S2, Supporting information). Most other BWs in LL reside in a kind of macrophyte-dominated state, and they are characterized by clear water, submerged vegetation and depleted nutrients. This state is well known to occur world-wide in floodplain lakes of lower connectivity (Fisher 1979; Heiler et al. 1995). Moreover, beyond its conservation value, it is also appealing from aesthetic and recreational viewpoints.

To sum up, hydrological connectivity is clearly decisive for the ecology and development of BWs; connected backwaters are affected by sediments and nutrient inputs from the river, whereas in shallow isolated backwaters, autumnal leaf fall and autochthonous processes lead to aggradation and internally driven eutrophication. Measures to improve the functional basis for ecological services could thus employ both allochthonous and autochthonous processes as a starting point, that is minimizing sediment storage and nutrient inputs and improving the water balance to prolong life spans, especially of isolated backwaters, thereby maximizing habitat diversity.

Remediation Measures and Their Consequences for Ecological Functions

Based on our results and evolving knowledge on floodplain ecology, management options for flood retention basins can be proposed and their ecological consequences can be evaluated.

In general, three basic types of measures and their combinations are available; they are identified and illustrated in S5 and Fig. 8 for LL.

Figure 8.

Different management scenarios for isolated floodplains on the example of Lower Lobau.

Reintegration of the Floodplain

Depending on the extent of the reintegration, the measures may lead to very different outcomes with respect to flood protection, ecological functions and conservation state. A full reintegration (Scenario ‘A’, levee removal) with the removal of the inner levee will foster hydromorphic dynamics and shift the system from sediment storage to transport (Hohensinner et al. 2005). From an ecological viewpoint, a large-scale increase in pioneer habitats and habitats for rheophilic species can be expected (Leyer 2006; Baart et al. 2012). Within the side arms, the growth potential for macrophytes is reduced (Baart et al. 2010). From a flood control viewpoint, the decreased polder effect leads to reduced peak diminutions downstream, but flood protection may be still guaranteed by the strengthening of the remaining levee systems. Lowering the levee at specific sites (Scenario ‘B’) has similar but less pronounced hydromorphic and ecological effects, constrained to upstream parts of concerned BWs (Schiemer & Reckendorfer 2004; Hohensinner et al. 2005), where pioneer habitats and habitats for rheophilic species may develop. In downstream parts, environmental factors (velocity, nutrients, and suspended solids) may still allow growth of macrophytes (Schiemer & Reckendorfer 2004; Baart et al. 2010). However, the dimensions of the upstream openings have to be sufficiently large to allow transport of sediments through the system. Otherwise, the life spans of aquatic habitats will decrease significantly due to sedimentation near the inflow areas (compare Fig. 3). Concerning flood protection it resembles Scenario ’A’.

Managed weirs (Scenario ‘C’), which are opened during floods, bear the same problems concerning sedimentation: if flooding intensities are minor, aggradation processes may prevail over erosion and the life spans of aquatic habitats may decrease significantly. Flood pulses increase nutrient input (Fig. 4) with subsequent effects on primary producers (Figs 5 and 6). A large part of the system oscillates between lotic and lentic conditions, providing habitat only for eurytopic species (Schiemer & Reckendorfer 2004; Reckendorfer et al. 2006).

Improve the Water Balance and Thereby the Extents of Aquatic Area and the Permanence of BWs

A controlled water supply (Scenarios ‘D’) and seepage windows (Scenarios ‘E’), that is, dam sections with a high permeability, improve the water balance and thereby the extents of aquatic area and the permanence of BWs. The overall macrophyte-dominated state will be preserved. This may be a solution in the short-to-medium term because the life spans of aquatic habitats increase. The disadvantage of the second case is the occurrence of a continuous nutrient input, which may lead to shifts in some BWs to algae-dominated states (compare Fig. 4 to 6).

Decrease the Sediment Load

Creating a sedimentation area (Scenarios ‘F’) and manage the inflow (Scenarios ‘G’) to reduce sediment input to the system lead to an increase in the life spans of aquatic habitats (compare Fig. 2). A managed inflow additionally reduces the nutrient input (Fig. 4), thus some BWs may shift to a macrophyte-dominated state. The disadvantage of scenario ‘F’ is clearly the loss of ‘natural’ areas. Although there are no immediate negative effects, many conservationists have reservations regarding Scenario ‘G’ probably because they feel that it is more unnatural to completely eliminate a sidearm from the main river than to only partially do it.

conclusion

Our comparison of the different management options and their effects on functions and processes clearly demonstrate that the conservationist's plea for both maintaining a high macrophyte diversity and at the same time fostering the hydromorphic dynamics can hardly be achieved. All the scenarios that improve rejuvenation have serious impacts on macrophyte-dominated aquatic habitats either due to the direct effects of hydromorphic dynamics or due to nutrient enrichment.

Our results clearly demonstrate that the long-term effects of sediments and matter accumulation have to be the major focus in management of flood protection areas. They determine the life span of the backwaters and have significant effects on the ecosystem function and services. Our results also underpin the importance of long-term studies on the development of retention areas with regard to fine sediment, nutrient and carbon balance. Planning should incorporate nature conservation into flood retention designs and ensure that the functional values of the existing or impacted wetlands can be identified and included or reconstructed and mitigated, respectively.

Acknowledgements

This work was funded by the Austrian Ministry of Science (ProVision 133-113), the Federal Ministry of Agriculture, Forestry, Environment and Water Management, the Federal Ministry of Transport, Innovation and Technology, Municipal Authorities of Vienna (MA45, MA49), the provincial government of Lower Austria and the Nationalpark Donau-Auen GmbH.

Ancillary