Change in groundwater chemistry as a consequence of suppression of floods: the case of the Rhine floodplain

Spatio-temporal variations of nitrogen, phosphorus and base cation concentrations in groundwater were related to the drastic change in hydrological conditions of the Rhine alluvial floodplain (Eastern France), which has been disconnected from the river by canalisation. The Groundwater chemistry was studied in two alluvial forests with contrasting hydrological conditions: one in a sector unflooded for 30 years, the second one in a sector still subject to flooding. Nutrient concentrations were measured at two levels, in the root zone (1.5 m depth) and in the gravel below the root layer (4.5 m depth). In the unflooded sector, the average nitrate concentration was significantly lower in the shallow groundwater (2.06 mg l−1 NO3−) than in the deeper layer (5.84 mg l−1NO3−). In contrast, in the flooded sector the nitrate concentrations in the shallow groundwater (5.02 mg l−1 NO3−) were not significantly different from those in the deep groundwater (3.98 mg l−1 NO3−). The concentration of phosphate was similar in shallow and deep groundwater in the unflooded sector (46 and 35 μg l−1, respectively) but significantly lower in the deep groundwater of the flooded sector (47 μg l−1), than in the shallow groundwater (58 μg l−1). The major elements (cations: Ca2+, Mg2+, Na+ and associated anions: HCO3−, SO4−2, Cl−) concentrations were significantly higher in the groundwaters than in the surface water. The results are discussed in terms of changes that accompanied suppression of floods, and processes that take place during the transfer of nutrients through the groundwater–soil–plant compartments. The reduction of groundwater fluctuations in the unflooded sector modified the transfer of nitrate by reducing the resolubilisation of locally produced nitrate, and/or denitrification. The transfer of phosphate was affected to a lesser extent, because of precipitation and adsorption. Base cation concentrations reflect exchange between groundwater and the calcareous gravel

Nitrate elimination by denitrification in hardwood forest soils of the Upper Rhine floodplain – correlation with redox potential and organic matter

Denitrification in floodplains is a major issue for river- and groundwater quality. In the Upper Rhine valley, floodplain forests are about to be restored to serve as flood retention areas (polders). Besides flood attenuation in downstream areas, improvement of water quality became recently a major goal for polder construction. Redox potential monitoring was suggested as a means to support assessment of nitrogen elimination in future floodplains by denitrification during controlled flooding. To elucidate the relationship between redox potential and denitrification, experiments with floodplain soils and in situ measurements were done. Floodplain soil of two depth profiles from a hardwood forest of the Upper Rhine valley was incubated anaerobically with continuous nitrate supply. Reduction of nitrate was followed and compared with redox potential and organic matter content. The redox potential under denitrifying conditions ranged from 10 to 300 mV. Redox potential values decreased with increasing nitrate reduction rates and increasing organic matter content. Furthermore, a narrow correlation between organicmatter and nitrate reduction was observed. Experiments were intended to help interpreting redox potentials generated under in situ conditions as exemplified by in situ observations for the year 1999. Results obtained by experiments and in situ observations showed that monitoring of redox potential could support management of the flooding regime to optimize nitrogen retention by denitrification in future flood retention areas

Changes in foliar nutrient content and resorption in Fraxinus excelsior L., Ulmus minor Mill. and Clematis vitalba L. after prevention of floods

This paper focuses on the impact of flood on tree mineral nutrition through measurement of resorption (i.e. transfer of nutrients from leaves to perennial organs). Nutrient (N, P, K, Mg, Ca) concentrations in leaves of three representative species, Fraxinus excelsior L., Ulmus minor Mill. and Clematis vitalba L. were measured before and after abscission on flooded and unflooded hardwood forests of the upper Rhine plain. The nutrient concentrations in the soils, which were measured in the top layer of the study sites, were higher in the flooded sites for P but slightly lower for N and K, and identical at both types of site for Ca and Mg. The summer foliage concentrations were higher for N and P in the flooded areas, and probably related to the flooding process, which contributes to regular nutrient inputs in the flooded forest, causes high fluctuations of water level and increases bioavailability of certain nutrients. Resorption occurred for all nutrients in the three species, and was higher for N, P and K (40-70 %) than for Ca and Mg (0-45 %), but not significantly different at the two sites. This paper stresses the variability of the test species response (nutrient content and resorption) to the soil and flood water nutrient sources, and tries to specify parameters which control resorption, i.e. soil fertility, tree species or flood stress. © 1999 Inra/Éditions scientifiques et médicales Elsevier SAS

Impact of river management history on the community structure, species composition and nutrient status in the Rhine alluvial hardwood forest

The present-day Rhine alluvial hardwood forest (Querco-Ulmetum minoris, Issler 24) in the upper Rhine valley (France/Germany) is comprised of three vegetation units, one still flooded by calm waters (F) and the two others unflooded, one for 30 years (UF30) (after the river canalisation) and the other for about 130 years (UF130) (after river straightening and embankment work in the middle nineteenth century). In the three stands, species composition, structure and diversity of vegetation and nutrient content of mature leaf, leaf litter and soil have been studied. Fungi (Macromycetae) were only studied in two stands (F and UF130). The intensity of nutrient recycling was exemplified by comparing the chemical composition of rainwater, flood, throughfall, mature leaf, leaf litter, soil and groundwater in two of these stands (F and UF30). The elimination of floods has caused a change in floristic composition, tree density and plant diversity. Tree density was higher in the two unflooded stands and was related to a large increase in sapling (< 6 cm dbh) density more than to a change of stem (> 6 cm dbh) density. Sapling density increased 2 times and three times in the UF30 and the UF130 respectively, whereas the stem density increased only 12% in the first stand and decreased 29% in the second one. The saprophytic macromycete communities have been supplemented with mycorrhizal species

Impact of river management history on the community structure, species composition and nutrient status in the Rhine alluvial hardwood forest

The present-day Rhine alluvial hardwood forest (Querco-Ulmetum minoris, Issler 24) in the upper Rhine valley (France/Germany) is comprised of three vegetation units, one still flooded by calm waters (F) and the two others unflooded, one for 30 years (UF30) (after the river canalisation) and the other for about 130 years (UF130) (after river straightening and embankment work in the middle nineteenth century). In the three stands, species composition, structure and diversity of vegetation and nutrient content of mature leaf, leaf litter and soil have been studied. Fungi (Macromycetae) were only studied in two stands (F and UF130). The intensity of nutrient recycling was exemplified by comparing the chemical composition of rainwater, flood, throughfall, mature leaf, leaf litter, soil and groundwater in two of these stands (F and UF30). The elimination of floods has caused a change in floristic composition, tree density and plant diversity. Tree density was higher in the two unflooded stands and was related to a large increase in sapling (< 6 cm dbh) density more than to a change of stem (> 6 cm dbh) density. Sapling density increased 2 times and three times in the UF30 and the UF130 respectively, whereas the stem density increased only 12% in the first stand and decreased 29% in the second one. The saprophytic macromycete communities have been supplemented with mycorrhizal species

Quantification of nitrate removal by a flooded alluvial zone in the Ill floodplain (Eastern France)

The nitrate reducing capacity of a flooded system in the Ill floodplain (Eastern France) was investigated for a period of 2 years. The methodology used consisted of a spatio-temporal monitoring of stream flow and nitrate concentrations in the groundwater and surface water, calculation of input and output fluxes and modelling of groundwater fluxes and nitrate transfer through the alluvial area. A comparison of chloride flux (used as hydrological tracer) and nitrate flux was done to determine a floodplain effect on the retention of nitrate. We show that up to 95% of the nitrate load in the groundwater is retained by the system, whereas the retention in the stream network is very low. Ammonium fluxes increased from inputs to outputs in the stream and in the groundwater. The chloride input in the groundwater is higher than the output, whereas in the surface water the output is higher than the input, the amount evacuated in streams corresponding to the losses from groundwater. The nitrate removal rate calculated for the whole modelized surface area (40 km2) represented 559 t N yr-1 or 1397.7 kg N ha-1 yr-1. The ammonium fluxes exported by the system represented 102 kg N ha-1 yr-1 A part of nitrate is reduced and exported by the groundwater and stream network in the form of ammonium. These results can be explained by the duration of floods which controls the equilibrium between the various forms of nitrogen. Thus, long watering periods favour nitrogen removal (denitrification and plant uptake) and limit nitrate production which compensates elimination during the dry period

Mining Complex Hydrobiological Data with Galois Lattices

International audienceWe used Galois lattices for mining hydrobiological data about macrophytes, i.e. macroscopic plants living in water bodies. These plants are characterized by several biological traits, that are divided into several modalities. Our aim was to cluster the plants according to their common traits and modalities and to find out the relations between the traits. Galois lattices are efficient methods for such an aim, but apply to binary data. In this article, we detail a few of the approaches we used to turn complex hydrobiological data into binary data and compare the first results obtained thanks to Galois lattices

Quel système de référence pour la restauration des systèmes alluviaux rhénans ?

Floodplain ecosystems consist of a mosaic of shifting interactive habitats (forests, river channels and alluvial aquifers). Flood pulse, hydrological connectivity and geomorphological dynamics are key factors for high species diversity and productivity, complexity in successional trajectories and importance of ecotones between terrestrial and aquatic ecosystems. Most rivers in Europe have been drastically altered by dams and reservoirs, canalisation, reduction of the welland area, eutrophication and various other land-use developments. After a very long period of destruction of ftoodplain integrity, the need for protecting and restoring alluvial ecosystems slowly emerged over the last three decades. Restoration needs to define reference conditions, present conditions and restoration objective states, according to the principle of naturalness. This principle includes hydrological functionality (taking into account the flood pulse), the spatio-temporal diversity of habitat types and species, such as alluvial forest succession and connection of lateral arms to the active channel. We take examples in the upper Rhine valley to support the naturalness concept and to propose sorne principles on this basis for a restoration programme. The preservation of the hydrological functioning (i.e. flooding) ensures an efficient purifying capacity of the soil root system, with regard to phosphate and nitrate. As a consequence, groundwater nitrate and phosphate concentrations are very low under alluvial forests. Moreover the ftooding regime maintains a certain heterogeneity of aquatic and terrestrial habitats. The hydrological network, i.e. the former lateral arms, is supplied by different types of water (ground and/or surface water) and water quality differs according to their degree of conneclion to the active channel. The connected arms, is eutrophie whereas the disconnected arms evolve towards an oligotrophic state. The aquatic macrophytes communities reftect the change in water quality. The spatio- temporal variability of habitats is higher in the ftooded sectors than in the unftooded ones. As a consequence the ftooded sectors include more aquatic macrophytes communities (3 to 5 according to the hydrological year vs 2) and each community has greater species richness. A similar relationship exists for terrestrial habitats and forest communities. Hard wood forests tend to dominate the unftooded sectors whereas the flooded areas present a very rich spatio-tempoal mosaïc of forests units including all steps of the alluvial succession. The comparison of forest architecture between ftooded and unftooded sites shows that disconnection results in an increase of the ligneous density, a decrease of the tree height, the absence of treefall and a decrease of lianas (for example ivy).The Rhinau island, a still flooded sector in the Rhine floodplain can be used as a basis for the naturalness concept, due to the preservation of a certain functionnality, even if flooding duration is lower than before the Rhine canalisation (2 days versus 15 days). The two other sites, with differing durations of isolation (30 and 130 years), show different levels of change in floristic composition and structure of forests, and aquatic macrophyte communities . Accordingly, the restoration programme needs to engage more direct interventions. A key factor to emphasize should be the restoration of natural hydrological functioning. On the basis of the concept of naturalness, we therefore propose a four- step restoration programme : 1) restoration of free flow of water in the former lateral arms by connecting them to the river, 2) restoration of natural floods based on Rhine discharges, 3) development of a specific programme for the by-passed sections, 4) finally, development of the inter-connection of the restored areas along the riverLes zones alluviales, traversées par des flux d'eau et d'énergie fournis par un cours d'eau, sont organisées en ensembles fonctionnels interactifs associant des systèmes terrestres forestiers et/ou prairiaux et des systèmes aquatiques. Le fonctionnement par pulsations (« flood pulse »), la connectivité hydrologique et la dynamique géomorphologique sont les facteurs-clés de la diversité et de la productivité biologiques. Les aménagements hydrauliques des grands fleuves ont fortement réduit la surface du lit majeur et perturbé, voire supprimé, les fonctions d'écrêtement des crues, de recharge de la nappe, d'épuration des eaux et de diversité du complexe alluvial. Il est proposé de restaurer des zones inondables et de recréer ainsi des conditions favorables à la restitution de leurs fonctions. La restauration et la gestion de ces zones passent par la détermination de différents états, état de référence (celui du fleuve « sauvage » ), état actuel et état objectif sur la base du principe de naturalité. Ce principe inclut la fonctionnalité hydrologique (maintien d'un fonctionnement par pulsations), la diversité spatio-temporelle des habitats (conservation de la succession alluviale forestière, connexion des bras latéraux au fleuve) et des espèces. Au travers des exemples pris parmi les sites rhénans, présentant des degrés variables de naturalité, nous définissons les grands principes de mise en place d'un programme de restauration du Rhin supérieu

Un système d'information pour le suivi et l'évaluation de la qualité des cours d'eau

National audienceLa directive cadre européenne sur l'eau (2000) impose la mise au point de nouveaux outils pour l'évaluation et le suivi de la qualité des masses d'eau. Dans ce but, nous avons réalisé depuis 2005 divers prélèvements sur un ensemble de stations (en rivières) de la plaine d'Alsace. Une base de données a été conçue pour organiser et partager les informations collectées : informations relatives aux espèces présentes dans les cours d'eau alsaciens et résultats des prélèvements (physiques, chimiques et biologiques) effectués sur les stations. Pour simplifier et enrichir l'analyse de ces informations nous avons développé un ensemble d'outils comprenant : l'interrogation de la base, la visualisation sur carte des stations respectant des critères donnés, un classifieur traitant les caractéristiques des stations. Notre objectif à terme est de constituer un système d'information utilisable à l'échelle du bassin Rhin-Meuse

Mining a database on Alsatian rivers

International audienceWe aim at comparing the answers of the bio-indication tools of the water bodies towards the various pressures which they undergo. Therefore we have designed of a database gathering the physical, physico-chemical, floristic (diatoms and macrophytes), and faunistic data (invertebrates, oligochaetes, fishes) of the rivers streams and water areas of the Alsace plain. Besides, we are implementing data mining methods to explore the database. These methods are used to find out regularities and similarities that can be interpreted by a domain expert. Indeed, we consider data mining as a part of a more general process that is knowledge discovery in databases. We give examples of such methods and the results that can be obtained