Restoration of floodplain wetlands: Opening polders along a coastal river in Mediterranean France, Vistre marshes

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1 Ecological Engineering 18 (2002) Restoration of floodplain wetlands: Opening polders along a coastal river in Mediterranean France, Vistre marshes André Mauchamp *, Philippe Chauvelon, Patrick Grillas Station Biologique de la Tour du Valat, Le Sambuc, Arles, France Received 19 January 1999; received in revised form 16 August 1999; accepted 29 October 1999 Abstract Restoration of wetlands has become an increasing field of application of ecological research due to mitigation regulations, changes in agricultural practices and an increasing consideration of the role of wetlands in the water cycle. In areas where the history of human use of natural areas is old and intense, restoration projects must not only consider ecological objectives but also social aspects. The Vistre project was developed to answer a social demand of restoring the flood storage function of a riverine wetland, formerly drained and polderized for agriculture. The river is located in the Petite Camargue, southern France, and flows into the sea a few km downstream of the study site. Openings in the dykes, calculated after a preliminary study, partly restored the connection between the polder and the river basin. A monitoring program of flora and fauna was launched to test the hypothesis that the change in hydrological functioning would be sufficient to obtain the desired vegetation and fauna. During the first years of the project, high rainfall and uncontrolled openings of sluices due to difficulties with the local users caused abnormally high water levels. The vegetation changed to hydrophyte-dominated communities and was controlled mainly by the fluctuations in water level. The habitat objective for fish-eating birds was met and a large tree-nesting heron colony established. Solving the social problem and maintenance of the sills should allow most objectives to be reached, although more slowly than expected Elsevier Science B.V. All rights reserved. Keywords: Water regime; Wetland vegetation; Restoration; Phragmites; Wildlife; Riverine wetlands 1. Introduction In Europe and in France, concentration and local reductions of the areas devoted to agriculture and a better recognition of the functions and values of wetlands provide opportunities for their * Corresponding author. Tel.: ; fax: address: mauchamp@tourduvalat.org (A. Mauchamp). rehabilitation or restoration. The Mediterranean littoral zone of France is repeatedly exposed to severe floods caused by heavy rains in autumn exacerbated by increasing compartmentalization due to embankments, road building etc. which reduce the surface area for potential flood expansion. Out of about ha exposed to flood risk within the Languedoc Roussillon and the Camargue, the surface area of recent fallow land was estimated at nearly 8400 ha (Mathevet unpublished data). Beside those, and depending on /02/$ - see front matter 2002 Elsevier Science B.V. All rights reserved. PII: S (02)

2 620 A. Mauchamp et al. / Ecological Engineering 18 (2002) the agricultural policies, large areas of subsidized agricultural land established after drainage of wetlands may be abandoned. In the Mediterranean region, restoration of wetlands in a restricted sense appears unrealistic because disturbance is very old, measured in centuries or millenniums, and hardly reversible. The main hydrological disturbances will generally not be removed (e.g. navigation canals, major rivers, embankments), and very little ecological information exists on the status before disturbance. Moreover, even small patches of natural areas are seen very much in an anthropocentric utilitarian way for economical, social and ecological reasons. High social and economic pressures will allow the return towards less intensively managed ecosystems only when well-identified and highly demanded (desired) functions or services can be expected. Within this context, the aim of the present project was to create the conditions for the development of an ecosystem that would maintain itself with little continuous management effort. Placed on a new trajectory of natural succession (Aronson et al., 1993), the ecosystem would perform the different functions that are expected by the local users, and that may be expected from such a wetland. Our reference ecosystem was chosen as a riverine reedbed that should be dry by the end of summer in order to be able to receive the autumn and winter floods. Further objectives were to provide suitable feeding habitats for fish eating herons and waterfowl during the breeding season, limited waterfowl hunting, and, after complete expansion of the reedbed, to allow extensive grazing by domestic herbivores during the summer drawdown. Thus our objectives were not only determined by the need of restoring natural processes but also by social demand. Indeed the area does not have a sufficient size to solve the flood problem at a regional scale, but it is intended to be a pilot project encouraging and facilitating other projects. This flood control function was supposed to be obtained with a minimum permanent cost, and hence a minimum management effort on the long term. The internal ditches and levees were not removed and were thus supposed to contribute to the diversity of the site providing a wider range of water depth and sources plants and fish for further colonization. The flora and fauna were expected to colonize the area without specific intervention, and the economic use by man (grazing and thatching) was depending on this colonization. Development of wetland vegetation was not expected to be limited by distance for colonization as many species were already present as weeds in the agricultural land and along rivers and canals in or connected to, the study site. This project was an opportunity to test our ability to rebuild a wetland ecosystem and make predictions on its dynamics. Few restoration projects have been conducted on wetlands in the Mediterranean region at a significant scale (Montes et al., 1995). Some experimental work tested restoration techniques (Mesléard et al., 1995), and most of the projects concerned the restoration of habitats for birds and grazing management (Duncan and D Herbès, 1981). There is little local information on the methodology for the restoration and its evaluation within highly variable and unpredictable environmental conditions which would take into account both natural and social aspects. The design of the project was made after a preliminary study and the predictions of vegetation and ecosystem dynamics were based on the following hypotheses. The dynamics of vegetation is controlled by the availability of species on the site (established vegetation, seed bank and input of propagules) selected by the abiotic conditions (Gough et al., 1994; Grace and Pugesek, 1997), primarily flooding conditions. Competition output is predicted by maximum size of plants or ramets (Mitchley and Grubb, 1986; Gaudet and Keddy, 1988; Keddy et al., 1994). The water regime and the structure of vegetation are the most important parameters of the habitat for wildlife (Kusler and Kentula, 1990; Palmer et al., 1997). A 5-year monitoring program was launched in order to test these hypotheses, to determine which processes were most important within such a project, and to measure ecological change in the site. Results presented here were obtained after the first 3 years.

3 A. Mauchamp et al. / Ecological Engineering 18 (2002) Methods 2.1. Site description The site is located in southern France, in the Petite Camargue, within a complex and mantransformed hydrological network (Fig. 1). Rivers from the northern watershed are all connected and cut by the navigation canal from the Rhône to Sète (formerly Canal de Beaucaire) dug in the early eighteenth century, which leads to the sea south of Aigues Mortes. Large areas along the rivers are flooded by upstream floods when heavy rainfall occurs or by downstream flows when sea surges promote elevation of the water level in the canal. In that case, brackish water may enter the system. Major floods occur when both phenomena take place simultaneously. The project concerns 130 ha located at the connection of the Vistre River and the navigation canal. The area was embanked in the 1950s, drained, and was cultivated with rice, wheat, sunflower, and hay. Two polders were built, separated by a narrow strip of reedbed that remained non-modified. The polders were split into more than 50 plots separated by irrigation and drainage channels ranging from 0.5 to 1.5 m deep. In the last years before the start of the project, land use was rice field in the western polder, and dry annual crops and pasture in the eastern polder (Fig. 1). A riverine forest dominated by Fraxinus angustifolia and Salix alba covered some of the large dikes surrounding the polders. The elevations of the plots ranged from 0.45 to m ASL except the dikes and circa 2 ha where dredged sediment from the navigation canal were Fig. 1. Location of the Musette project in the hydraulic network relating the coastal rivers, the lagoons and the Mediterranean sea, and map of the area under study showing the former land uses, the location of the limnigraphs (stars), the valved outlets (circles), and the sills in the dykes (full bars along the dykes).

4 622 A. Mauchamp et al. / Ecological Engineering 18 (2002) deposited. Average elevation is 0.17 and 0.22 m in former east (ranging from 0.02 to 0.28 m) and west (ranging from 0.08 to 0.32 m) polders, respectively. The vegetation of the plots was described in 1994 during the preliminary study Hydrology Completely removing the dikes would have resulted in permanent flooding and the objectives would not have been met. A summer drawdown was chosen to enhance the flood control function, the vegetation dynamics, and the nutrient mineralization, and to allow summer grazing. The level at which dikes were lowered was calculated in a preliminary hydraulic study using an 8-year series of daily water levels in the navigation canal and meteorological information (Grillas, unpublished report). The level of the sills was calculated as a compromise between the flooding frequency in spring and the storage of water inside the polders. A higher level would have decreased flooding frequency but increased storage of water. On the basis of the preliminary hydraulic study, two openings (150 m wide, elevation +0.4 m ASL) were made in 1995 in the surrounding dikes of each of the two polders. Water level was monitored continuously (limnigraphs) at three locations: (1) at the connection of the river and the canal, (2) in former east- and (3) west-polders (Fig. 1). The preliminary study of the site included soil analysis, vegetation map and history of land use Monitoring program A 5-year ecological monitoring program of the site started in 1996, 1 year after opening the dikes. A detailed vegetation map was developed in In each plot, abundance of species was recorded according to a semi-quantitative scale (0, 0% cover; 1, cover below 20%; 2, cover below 40%; 3, cover below 60%; 4, cover below 80%; 5, cover below 100%; 6, cover 100%) in 1 m 2 quadrats regularly spaced along three parallel transects. The total number of quadrats in each plot ranged between 40 and 115 according to their surface area. A second visit in late summer was made in the plots that had dried up during summer. Different censuses of target species were made: trees for their importance for birds (structure), an exotic species (Ludwigia peploides (Kunth) P.H. Raven, invasive species that constitutes an important threat for the Mediterranean wetlands), and a protected species (Leucojum aesti um L.). In ten plots, eight permanent transects 1 m wide were installed for monitoring the encroachment/regression of the fringes of reed that surrounded each former plot. Along each transect the location of the fringe was calculated as the mean distance from a reference point to the five furthest living shoots. In two plots large numbers of seedlings of Phragmites australis (Cav.) Steudel were found. In these plots the density of seedlings was mapped in Viable seedbank was measured using the germination method in a greenhouse. On three former plots, (ricefield, annual dry crops and pasture) 90 regularly distributed samples of sediment (4 cm diameter, 4 cm depth) were collected in spring 1996 and brought to the laboratory. The sediment of each sample was dispersed as a 1 cm layer on sand in a 15 cm diameter tray. Samples were sprayed with tap water every hour from April to November After being dried during winter in order to break possible dormancy of seeds, half of the samples per plot were submitted to the same spraying conditions described previously between March and July During the same period the other half of the samples were flooded in tanks for hydrophytes which may not have germinated during the first trial. Seedlings were counted weekly and removed when identified. Unidentified plantlets were transplanted and cultivated until they could be identified. In the flooded treatment only the hydrophytes were taken into account, other species could usually not grow through the water column and be identified. Water birds using the site were counted fortnightly during the migrating and wintering period (late summer to spring). During the reproductive season the number of breeding pairs of colonial water birds were counted each year. The density of feeding herons was counted in 1997 in 17 visits between 30 April and 8 July along a fixed

5 A. Mauchamp et al. / Ecological Engineering 18 (2002) itinerary (line transect) representing a constant sample of both sectors. To establish a preliminary list of species, and determine the relative proportions per size class as an indication of food availability for fish eating herons, fish populations were evaluated during one field campaign in September 1997 combining electric fishing and passive fishinggears. At this date most of the plots had dried and water and fish concentrated in the ditches and canals. In large canals, nets with various mesh size ranging from 10 to 60 mm, and traps (1.1 m long, 0.4 m diameter, mesh size 12 mm) were used during one night. Three nets and three traps were used in a large canal inside the site and in a branch of the neighboring navigation canal. Electric fishing was performed in five sites with an Albatros apparatus (Dream Electronic) for brackish water. Electric fishing was applied during min for exhaustive capture on surface areas ranging between 65 and 150 m 2 (intensity 4 10 A, Power KW according to electric conductivity of water 5 13 ms/cm). All large fishes and a sub-sample of smaller were counted and their length measured, total biomass was evaluated per species. 3. Results 3.1. Hydrology The opening of the dikes and sill creation were followed by two winters with very high rainfall and floods (Fig. 2). Floods expanded in the site, which served as a flood retention area. The water level in the sites was identical to that in the river when higher than the basal elevation of the openings (e.g. January February 1996, Fig. 2). When water level in the river dropped below this elevation then the wetland was disconnected from the river. In the site the water level decreased progressively by evapotranspiration and outflow through valved drainage outlet. Decrease in water level in the former polders was almost linear as a function of the time elapsed since the last filling event and the fre- Fig. 2. Water levels in the two east and west units (a and c, dotted and solid lines, respectively), in the Vistre River (line on b and d), and rainfall (vertical bars on b and d) during 1996 (a, b) and 1997 (c, d). The limnigraphs are indicated by stars on Fig. 1. The theoretical elevation of the sills at +40 cm ASL, and the mean elevation of the plots at 20 cm ASL are shown. quency of those events is hence a key parameter for the hydrological functioning. The objective of summer drought could not be completely met during the first 3 years because of two major hydraulic and social problems. Erosion of the sills enhanced by coypu (Myocastor coypus) activity resulted in a decrease of their elevation thus increasing the quantity and frequency of water input from the river (e.g. March April 1997 where water level in the site follows that of the river as low as 0.2 m ASL). The second problem was repeated artificial flooding in former polder East during spring (May 1997) and late summer (September 1996 and 1997) due to unauthorized manipulation of sluices that remained on the site from former agricultural activity. The reasons for these manipulation remained unclear but were probably related to management for waterfowl (for hunting) or for fish populations. Both problems caused water levels higher than expected during spring and summer.

6 624 A. Mauchamp et al. / Ecological Engineering 18 (2002) Fig. 3. Relationship between species richness and mean elevation of the plots for the vegetation sampling performed in Vegetation A total of 160 plant species were found in the plots and surrounding ditches and levees in 1996, the second year after opening the dikes. Six species were found in more than 40 plots out of a total of 51: Potamogeton pectinatus L., P. australis, Ceratophyllum demersum L., Paspalum paspalodes (Michaux) Scribn., Scirpus maritimus L. and Zannichellia pedunculata Reichenb. Except for the exotic species P. paspalodes, the most frequent species are submerged macrophytes and tall helophytes which can cope with the high water levels that followed the opening of the dikes. P. paspalodes is a C4 late growing species, requiring high temperature (Pearcy et al., 1981; Mesléard et al., 1993), and therefore, it grows late in the season when water levels are low. The species richness per plot was significantly positively correlated (P 0.05) with its elevation (Fig. 3). ANCOVA with plot elevation used as covariate shows that the species richness differed significantly among land uses and elevations (P=0.02 and 0.007, respectively). Monofactorial linear correlations per land use between elevation and number of species were significant for pastures (R 2 =0.76, n=6) and dry crops (R 2 =0.69, n= 6), but not for former ricefields (R 2 =0.01, n= 10). The flooding stress was also illustrated by the decrease of the percentage of hydrophytes in vegetation from about 75% at 0.30 m ASL to circa 25% at 0 m ASL (R 2 =0.64). For the number of helophyte species, ANCOVA with elevation as covariate gave land use effect non-significant (P=0.86) but elevation significant (P=0.013). Detailed measurements of cover of submerged macrophyte communities in April 1997 showed significant differences between the abundance of dominant species according to former agricultural use (Table 1) and none for total cover of hydrophytes. There was no effect of elevation on the abundance of individual species of submerged macrophytes (ANCOVA with elevation as a covariate). A total number of seeds of 38 species germinated during the first phase and 6800 seeds Table 1 Mean cover ( S.E.) of hydrophyte species in the plots according to the former land uses Dry farming (n=13) Pastures (n=10) Ricefields (n=22) P values Land use Elevation Z. pedunculata P. pectinatus Myriophyllum spicatum Ranunculus baudoti Chara spp Total hydrophytes P values result from an ANCOVA with average plot elevation as covariable.

7 A. Mauchamp et al. / Ecological Engineering 18 (2002) of 16 species during the second phase. Only five new species were found during the second phase, three terrestrial species and two hydrophytes. Among these species only Lotus corniculatus L. produced a significant number of seedlings (160). The spatial distribution of species was very heterogeneous and 70 samples of sediment were necessary to get 90% of the species per plot. However, all dominant species were found within the first 30 samples. The seedbank differed significantly between the three studied plots (Table 2). In the former grassland the seedbank was characterized by the largest mean number of species per sample including more terrestrial species than in other plots. However, the total number of seedlings per sample was much lower than in the other plots. Conversely, the lowest mean number of species and number of terrestrial seedlings per sample (Table 2) characterized the former ricefield. The most abundant species that germinated from the seed bank in former grassland were Chara sp., Cyperus fuscus L., Aster squamatus (Sprengel) Hieron., L. corniculatus, Typha angustifolia L. and S. cfalba L.. In the dry crops, C. fuscus, A. squamatus and Polypogon monspeliensis L. (Desf.) dominated the seed bank while in the former ricefield C. fuscus contributed to more than 90% to the total number of germinations. For all species except Atriplex prostrata DC. (P= 0.32) and S. alba (P=0.58), the differences between plots were significant (ANOVA, P 0.01). The most striking feature of the dynamics during the studied years was the gradual shift from terrestrial/amphibious vegetation to amphibious/ aquatic communities (Table 3). The grassland was dominated in 1994 by terrestrial grasses such as Festuca arundinacea Schreber, Poa pratensis L., and leguminosae (Trifolium, Vicia, and Lotus). Two years after the project started, these species had disappeared and submerged macrophytes dominated (Chara sp., Z. pedunculata). Agriculture had been abandoned 1 year earlier in former dry crops than in other plots and in 1995 wetland vegetation was already abundant with A. squamatus, A. prostrata, J. gerardii Loisel., P. australis, P. paspalodes, P. monspeliensis, S. maritimus, Spergularia rubra (L.) J. and C. Presl. The shortest and least flood tolerant of these species (e.g. Table 2 Main characteristics of the seed bank in three plots corresponding to the three former land uses in La Musette (mean values S.E.) Grassland plot Dry crops Ricefield N species a b c N helophyte species aa bb ab N terrestrial species a b c N seedlings a b b N helophyte seedlings a b c N terrestrial seedlings a b a P. australis a b a T. angustifolia a b b C. fuscus a b c E. crus-galli a a b P. monspeliensis a b a A. squamatus a b a Chara sp a b b Numbers of species are per sample, and numbers of seedlings are per 100 cm 2. Different letters indicate significant difference among plots (P 0.05, ANOVA F-test). Spergularia, Aster, Atriplex, andpolypogon) disappeared while the tallest remained abundant (Phragmites, S. maritimus, T. angustifolia) and submerged macrophytes appeared (Z. pedunculata, Chara spp.). In former ricefields many wetland plants such as (S. maritimus, P. australis, T. angustifolia, and Echinochloa crus-galli (L.) P. Beauv.) were present as weeds. Some of them maintained significant populations (P. australis, P. paspalodes, S. maritimus) although submerged macrophytes dominated in 1996 (P. pectinatus, C. demersum). In 1997 the plots were all permanently flooded and no terrestrial plant was found. Although the specific composition of the seedbank was different from the actual vegetation in 1996,

8 626 A. Mauchamp et al. / Ecological Engineering 18 (2002) many terrestrial and amphibious species were still present as seeds. An additional soil sampling realized in 1998 will show whether those seeds are still viable after 3 years in flooded soils. Vegetative growth from the edges was expected to be the dominant process that would contribute to rapid colonization of P. australis. Aerial photographs and field surveys showed that reedbeds did not expand during the first years of the project. From the ten plots where the location of the fringe of the reedbed was measured, eight showed a regression of the reedbeds ranging between 0.7 and 3.4 m. In only one plot, did the reed bed colonize 0.5 m and on another there was no change. The depth and duration of flooding are probably responsible for this regression. There was no significant correlation between changes and elevation in the first year but a significant trend to colonization on highest plots was observed over the 3 year period (R 2 = 0.57). Other factors such as grazing by coypu and sediment characteristics may have interfered with hydrological factors. In contrast P. australis rapidly colonized the deposits of sediment where elevation is about 1 m higher than in the other plots and was very rarely flooded. Although seeds were found in the seed bank of the three sampled plots, abundant seedlings of P. australis were found in only two plots of similar rather high elevation ( 2 and 3 cm ASL). In these two plots, density was high (up to 5 per m 2 ) and most resulted from germination in 1995 and Those seedlings were deeply flooded in 1996 and 1997 and their growth was reduced. In better conditions, such colonization may lead to the very rapid development of a reedbed Bird use of the site Birds used the site during summer as a feeding Table 3 Major changes in the vegetation of the plots during the monitoring period Pastures Dry crops Rice fields SB SB SB Terrestrial A. squamatus 4.1 II III A. prostrata II III Festuca sp. III S. rubra II III Trifolium sp. III Vicia sati a III Helophytes E. crus-galli 2.5 III 6.8 Glyceria maritima II III P. australis 0.16 III III P. paspalodes II III 31.4 III 5.7 P. monspeliensis 0.42 II III 23.0 II 0.9 S. maritimus II III 15.4 III 6.5 T. angustifolia 3.12 II III II C. fuscus Hydrophytes C. demersum Chara sp P. pectinatus Z. pedunculata Only the species that were dominant in either 1994 (classes II III), 1996 ( 10%) or in the seed bank SB are shown.

9 A. Mauchamp et al. / Ecological Engineering 18 (2002) Table 4 Fish and crayfish captures in the internal channels of the Musette (data from Rosecchi, 1998) Species Origin Biomass (%) Carassius auratus gibelio Bloch. Goldfish E Cyprinus carpio L. Common carp E Gambusia affinis Baird and Girard Mosquito fish E 6.97 M. salmoides Lacépède Black bass E 5.43 M. cephalus L. a Striped mullet 4.24 L. ramada Risso a Thinlip grey mullet 2.83 Lepomis gibbosus L. Pumkinseed E 2.64 A. anguilla L. a Eel 1.47 Pseudorasbora par a Schlegel Topmouth gudgeon E 0.30 Ictalurus melas Rafinesque Catfish E 0.21 Tinca tinca L. Tench 0.13 Scardinius erythrophtalmus L. Rudd 0.09 Rutilus rutilus L. Roach 0.06 P. clarkii Girard Red swamp crayfish E xx O. limosus Rafinesque Spiny-cheek crayfish E x Origin E for exotic species. a Indicate species that reproduce at sea. and/or breeding site according on species. A colony of tree nesting herons established right after the start of the project and is currently increasing. In 1996, 17 couples of night herons Nycticorax nycticorax nested and the following year, the colony had 454 nests, amongst which were 222 Bubulcus ibis, 190 N. nycticorax, 25 Egretta garzetta and 17 Ardeola ralloides. Nesting success ranged from 2.3 (S.E. 0.45, n=18) for E. garzetta and 3.4 (S.E. 0.5, n=12) for A. ralloides. Fish eating birds were mainly herons plus ten couples of grebes (eight Tachybaptus ruficollis and two Podiceps cristatus). Bird counts on the fixed itinerary in 1997 showed that Ardea purpurea started using the area in May and had a stable number of individuals (15 to 20) till mid July. They used mainly the western half with about 2/3 of the individuals foraging in the plots and 1/3 foraging in the surrounding channels. A. cinerea was the most abundant with a peak of 45 birds at the end of June and the E. garzetta numbers were variable ranging from ten to 30 individuals. Complete countings performed in September 1997 showed that total numbers were much higher, with 750 E. garzetta and more than 160 A. cinerea on the same day. At that date, A. purpurea had already left the site Fish populations A total number of 16 species of fish were captured in the internal channels (Table 4) and three additional species were found only in the navigation canal connected to the marsh during floods (Silurus glanis, Stizostedion lucioperca, Micropterus salmoides). The fish community was typical of littoral ponds and lakes or rivers with a slow current. The connection to the sea is highlighted by species, which breed at sea (Anguilla anguilla, Mugil cephalus and Liza ramada). A remarkable feature is the abundance of exotic species contributing to more than 90% to total biomass and 60% of species. Furthermore, two exotic crayfish species were also captured (Procambarus clarkii and Orconectes limosus). All the exotic species are considered naturalized, that is are able to maintain populations without permanent input of new individuals. The proportion of exotic species is much higher than usually met in other sites in the same region (Rosecchi et al., 1997). Due to late floods between the reproductive season and fish sampling it cannot be demonstrated that some of these species reproduced on the site. However, it is very likely for some of them according to the characteristics of the site

10 628 A. Mauchamp et al. / Ecological Engineering 18 (2002) and the species requirements (Gambusia, Cyprinus.). The size distribution of fish corresponds to the objectives of the project in providing suitable food for herons. Large numbers of fish are small enough to be caught by E. garzetta and the range of size covers the requirements of the most abundant bird species (Fig. 4). 4. Discussion The summer drawdown expected from the preliminary study and surveys was not obtained during the first years. The date of complete drying out calculated from our preliminary study was very sensitive to the date of the last flood, as drying out was a gradual process where evapotranspiration dominated. Hence, the land was not dry in autumn, when the probability for flooding was the largest. However, at the end of September 1996 (Fig. 2), the water levels were low and the area still had 80% of its storage capacity. In 1997, water levels did not drop below 0 cm ASL and storage capacity was only half of the maximum in late September. In 1996, there was enough time between the last spring and the first autumn filling events (about 3 months) to reach an almost dry state. The decrease in water level after flood was enhanced by the installation of the valved Fig. 4. Size distributions of the fish captured in the internal channels of the Musette in Arrows indicate the food preferences in terms of size for the three main species of fish eating herons. drainage outlet. The exceptionally high rainfall in 1996 ( 1000 mm compared with a 620 mm mean) was not the only reason for the observed results. Early-autumn floods contributed to reduce the duration of the dry period, and further, unauthorized manipulation of sluices resulted in summer flooding in both 1996 and Erosion promoted by an exotic rodent, coypu (M. coypus), lowered the sill level and caused water inflow to start when river level was 10 to 20 cm lower than the 0.4 m planned. The sluices were, since then, blocked and a drier year in 1997/98 promoted a total dry out of the area, what was expected from our initial hypothesis. The high water levels had multiple consequences on biota and especially on vegetation dynamics. We observed a slower than expected establishment of the amphibious vegetation and a rapid dominance of submerged vegetation. To a large extent the vegetation that developed in each plot was independent from the seed bank and the species previously present in the former agricultural land (see Table 3). The correlation between the composition of the seed bank and the vegetation is often weak in wetlands (Van der Valk, 1981; Wilson et al., 1993). Moreover, in the context of restoration projects, this correlation is expected to be even weaker as many species in the seed bank are agricultural weeds rather than wetland species. Most of the species that were found in the seedbank were terrestrial or short lived amphibious species unable to withstand the high level and long duration of flooding. Among the species that were found in the site before rehabilitation, only a few were still present in Some species as the hydrophytes C. demersum or P. pectinatus were abundant in 1996, but were absent from both the 1994 inventory and from the seed bank. They were, however, present in 1994 in the permanent internal canals. Those canals formed a network of wetland vegetation patches among the former polders, which facilitated their colonization. Very few species that had not been found before in the site were observed during the 1996 inventory (e.g. Najas marina L.). They probably have colonized during floods. As hypothesized initially, the abiotic conditions, mainly flooding stress, determined the dynamics of the vegetation.

11 A. Mauchamp et al. / Ecological Engineering 18 (2002) This result highlights the impact of the abiotic factors on the species composition and dynamics of vegetation, and their predictive value for restoration of wetlands and planning (Weisner, 1991; Martinez-Taberner et al., 1992; Palmer et al., 1997). Water depth and duration of flooding are often identified as dominant factors controlling the distribution of species in wetlands (e.g. Spence, 1982). Increase in water level can result in changes in the species composition of vegetation (Wallsten and Forsgren, 1989; Van der Valk et al., 1994; Crivelli et al., 1995). Terrestrial species are the most sensitive, followed by emergent species. Submerged species have a better tolerance to an increase in water level and they are mainly limited by light attenuation and wave action. In this case the impact of increased water depth was probably enhanced by the high turbidity which further reduces light availability. The dominance of hydrophytes after 2 years resulted partly from the low cover of helophytes (Grillas, 1992). Reed was affected more than what we expected. The water depth in the site was within the range P. australis can stand in continental freshwater marshes (Haslam, 1972; Squires and Van der Valk, 1992) or in Mediterranean brackish coastal wetlands (Mauchamp, unpublished data). Possible causes for the regression of Phragmites were the anoxia caused by permanent flooding and eutrophication (Weisner and Graneli, 1989; Armstrong et al., 1996). The Vistre River feeding the wetland is heavily polluted by urban wastewater and agricultural runoff. Moreover, slightly brackish water occasionally enters the marsh. This may contribute to an increased richness in sulphate and the production of deletarious sulphide in anoxic conditions (Armstrong et al., 1996). The survival of the rhizomes of P. australis during floods and their further growth are favored by the presence of unflooded stems which conduct air to the rhizome and remove metabolic gases (Weisner, 1988; Brix, 1990). Winter flooding and wave action may have damaged old stems and prevented oxygen from reaching the rhizomes (Coops et al., 1994). During the last year, lower water level resulted in new vegetative colonization of P. australis although it was reduced by higher salinity (Hellings and Gallagher 1992). P. australis should dominate in the future as hypothesized in the preliminary study, although the process will be much slower than expected. The area was used for both feeding and nesting by a large range of species: common species such as mallard (Anas platyrhynchos) or waders, as well as for rare protected species like the Ardeids A. purpurea and A. ralloides. This component is generally recognized as the easiest aim of such a rehabilitation project (Weller, 1978; Kusler and Kentula, 1990; Tourenq et al., 1998), particularly when the objective is not a given species but rather a functional group such as the fish-eating birds. The establishment of a colony of tree nesting herons was difficult to predict although the expected structure of the site was recognized as suitable, and, therefore, had not been identified as an objective of the project but rather considered as a possibility. The occurrence of breeding colonies of water birds is also strongly influenced by external factors such as the distribution of colonies in the region, the distribution of feeding habitats, the reproductive success and possible disturbances (Fasola and Alieri, 1992; Hafner and Fasola, 1992). The colony of tree nesting herons is large and it has now become a site of regional importance. This was probably promoted by the combination of several factors such as the low disturbance by visitors during spring, the suitable structure of the breeding habitat and particularly the protection from the wind, and the abundance of fish at short distance. A general increasing trend for these populations further favored the establishment of the colony (Hafner and Fasola, 1997). The colony may persist even if the surrounding marshes dry up during the breeding season since two deep canals protect the site from predators. Similarly, after colonization by reed, the establishment of a colony of purple heron is possible on the site but would depend on similar external factors. The use of the wetland by birds during the day for feeding and roosting was very irregular in time. The hunting activity limited drastically the number of roosting ducks during winter. From spring to late summer the wetland constituted a valuable feeding site for herons. The number of

12 630 A. Mauchamp et al. / Ecological Engineering 18 (2002) birds feeding in late summer reached high values as a result of good reproductive success and suitable conditions created by the low water level in late summer concentrating preys in smaller volume of water. The development of hydrophytes provided food for herbivore ducks and the permanent flooding favored waterfowl. It can be predicted that the use by birds will decrease if the aimed summer drawdown is reached. This decrease during summer drought could be balanced by favorable conditions during the rest of the year. The final stages before complete drying out are extremely favorable habitats for feeding Ardeids. During the summer drawdown, ditches and canals acted as refuges for the fish populations. As soon as the area returned to public ownership, a strong social pressure developed around the project. On top of the initial objective of flood control, objectives related to hunting, thatching, potential grazing, fishing, and nature conservation were superimposed. They correspond to different hydrological regimes, which are not necessarily all compatible. The priority objective of flood control led to the decision for the level of sill; it is not, in all cases, optimal for other uses. Both the high water levels and the resulting poor development of the amphibious vegetation prevent the planned use of the area for extensive grazing and reed harvesting. Results from the first years suggest that grazing by domestic herbivores will be difficult to organize safely on this site. Floods occur too often, sometimes with short forecast, and the lack of safe refuge expose the animals to a severe risk. Lack of grazing might result in the longer term to a significant change in the habitat structure for birds if helophytes colonize as expected. Reedbed would then dominate on most of the area, except the deepest plots and the bird community would evolve from open water species (dabling ducks, herons, and waders.) to reedbed species with more passerine and different species of herons. The site would become more favorable to two vulnerable species (A. purpurea and Botaurus stellaris). The low hunting pressure from October to January was enough to prevent the establishment of a day-roost of ducks, and considering this effect, the site can only be used as a day feeding ground. Difficulty related to the reed cutting objective is caused by the fact that this specific target function depends on the presence or absence of one particular species. Similarly, the biological and socio-economic objectives of this project would be threatened by the development of an exotic invasive species L. peploides that was present on the site at the start of the project. 5. Conclusions About 3 years after opening the dykes the objectives of the project were only partly met. A number of technical, and social difficulties related to the multi-users character of the site, resulted in higher water level than expected in the initial plans. The level of the sill in the former dyke should be maintained at 0.4 m or slightly higher in order to achieve objectives. These problems seem to be nearly solved and the hydrological situation to converge towards the expected passive functioning depending on climate and fluctuations of sea level. Although the objective of restoring the flood storage function is widely accepted among the local population, there remain some difficulties to accept the natural fluctuations in water level that result from the rehabilitation project. All biological compartments and human-use of the area have been impacted by the hydrological situation. However, the initial hypotheses linking the abiotic situation, the vegetation dynamic and fauna appeared to be verified to a large extent and our results support the hypotheses that in wetlands, predictions on vegetation structure can be established on the basis of abiotic conditions, assuming that competitive output is correlated with plant maximum height. The establishment of the expected hydrological situation should result in colonization by P. australis. Special attention must be focussed on that species as it controls several functions and services that are expected from the wetland. Low water quality might result in reed colonizing limited to shallower water than expected, thus promoting the maintenance of open water in deeper parts of the wetland.

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