Frédéric Azémar

Tom Maris

Benoît Mialet

Hendrik Segers

Stefan Van Damme

Patrick Meire

Michèle Tackx

Frederic Azémar

Micky Tackx

Rotifers in the Schelde estuary (Belgium): a test of taxonomic relevance

Keywords: Rotifera, Schelde estuary, taxonomic sufficiency, spatio-temporal distribution Rotifera, Schelde estuary, taxonomic sufficiency, spatio-temporal distribution 14

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Introduction 1

The worldwide concern to maintain the ecological quality and biodiversity of ecosystems 2 calls for understanding of how various taxa respond to environmental conditions. On the one hand, 3 this requires a solid knowledge of the environmental factors that influence the various taxa within a 4 community, and the interplay between them. On the other hand, it requires the ability to distinguish 5 the various taxa at an ecologically-relevant level. The former is essentially obtained through 6 multivariate correlation analysis, intended to identify as precisely as possible the environmental 7 variables with which the various taxa present in a community are associated. The appropriate level 8 of taxonomic resolution and the associated strength in representing the ecological quality and 9 response of interest is currently subject to considerable debate. The topic is generally considered 10 from a cost/benefit angle, where cost consists of the time, skills and resources needed for species 11 level identification [START_REF] Quijón | The use of coarser taxonomic resolution in studies of 6 predation on marine sedimentary fauna[END_REF], and benefit is the ability of the dataset obtained 12 to answer the questions posed. These questions often concern the community response to 13 anthropogenic stress, or evaluating the diversity of ecosystems in a conservation context (Fleishman 14 et al., 2005). Using 'coarser' taxonomic levels, such as genera or families (Somerfield and Clarke, 15 1995), is one of the strategies proposed to optimize ecological research and survey strategies. In the 16 aquatic environment, benthic organisms are usually used for quality-assessment studies. The 17 feasibility of using coarser taxonomic levels for benthic communities has been investigated in [START_REF] Maes | The predation impact of juvenile herring Clupea 19 harengus and sprat Sprattus sprattus on estuarine zooplankton[END_REF] freshwater habitats [START_REF] Bowman | Does taxonomic resolution affect the multivariate 17 description of the structure of freshwater benthic macroinvertebrate communities? Can[END_REF][START_REF] Marshall | Taxonomic resolution and quantification of 22 freshwater macroinvertebrate samples from an Australian dryland river: The benefits and 23 costs of using species abundance data[END_REF], rocky shores (Pagola- Carte 19 and Saiz-Salinas, 2001), gravel and sandy beaches [START_REF] Schoch | The intertidal biota of Pudget Sound gravel beaches. 16[END_REF]Defeo and Lercari, 20 2004), lagoons [START_REF] Mistri | Taxonomic sufficiency in lagoonal ecosystems[END_REF], coastal zones (e.g. [START_REF] Gray | Analysis of community attributes of the benthic of 19 Frierfjord/Langesundfjord and in a mesocosm experiment[END_REF][START_REF] James | Sieve mesh-size and taxonomic 3 resolution needed to describe natural spatial variation of marine macrofauna[END_REF]21 Somerfield and Clarke, 1995) and deep-sea sedimentary communities (Narayanaswasny et al., 22 2003; [START_REF] Quijón | The use of coarser taxonomic resolution in studies of 6 predation on marine sedimentary fauna[END_REF]. However, little consideration of this type has so far been 23 applied to pelagic organisms. In view of the ecological importance of rotifers in estuaries, and the 24 difficulty of routinely determining them, it seemed to be worth considering this aspect of the rotifer 25 population of the Schelde estuary. 26 27 Within this context, our paper focuses on the planktonic rotifers of the Schelde estuary 28 (Belgium/The Netherlands). This macrotidal estuary is one of the few European estuaries that still 29 has an extensive freshwater tidal zone (< 0.5) in its upper reaches [START_REF] Meire | The 27 Scheldt estuary: A description of a changing ecosystem[END_REF]. The Schelde 30 estuary, and especially its freshwater stretch, was known to be one of the most polluted estuaries in 31 Europe during the 1970s and 1980s [START_REF] Soetaert | Carbon flows in the Westerschelde estuary (The 27 Netherlands) evaluated by means of an ecosystem model (MOSES)[END_REF]. Since the 1990s, management 32 efforts, including controlling pollutant levels and wastewater treatment, have resulted in an 33 improvement of the water quality. Over the past ten years, several environmental factors have 1 changed substantially, and these changes have been most marked in the freshwater stretch. Indeed, 2 in the upstream part of the Schelde estuary, a considerable increase in the oxygen concentration 3 and chlorophyll a, and decreases in the NH 4 + concentration and in BOD5 values have been 4

observed [START_REF] Cox | From heterotrophy to autotrophy: a freshwater estuarine ecosystem 25 recovering from hypereutrophication[END_REF]. Associated with the rise in oxygen concentration, there has been an 5 increase in NO [START_REF] Fontaneto | Molecular evidence 4 for broad-scale distributions in bdelloid rotifers: everything is not everywhere but most 5 things are very widespread[END_REF] -concentration as a result of more intensive nitrification. Furthermore, in the 6 summer silica concentrations (SiO 2 or DSi (Dissolved Silica)) now drop below limiting values more 7 often than ten years ago (Cox et al., in prep). Runoff, which is an important factor in structuring 8 estuarine spatio-temporal conditions, has varied irregularly over the past decade. While runoff is 9 basically regulated by precipitation and climatic conditions in general, it is also controlled to some 10 extent by human action (e.g., sluice management), and hence is highly relevant to water 11 management. A more extensive overview of the ecological changes that have occurred in the 12 Schelde estuary and current management perspectives is reported by [START_REF] Meire | The 27 Scheldt estuary: A description of a changing ecosystem[END_REF], 13 Meire et al., 2005and Soetaert et al., 2006. 14 At present, the Schelde can be considered to be an estuary on the way to restoration. To 15 provide managers of this estuary with appropriate advice, the Government of Flanders sponsored a 16 multi-disciplinary monitoring program, 'OMES'. The general aim of the OMES study, which began 17 in 1996, is to describe the changes in the estuarine community during its restoration, and to 18 understand which environmental conditions (or changes in these conditions) have led to this pattern. [START_REF] Carlton | Pattern, process, and prediction in marine invasion ecology[END_REF] This information is intended for incorporation in models used to predict impact of management 20 measures on various compartments of the system [START_REF] Meire | The 27 Scheldt estuary: A description of a changing ecosystem[END_REF]. 21 Because of their key role in the trophic functioning of the estuary [START_REF] Appeltans | 2 Zooplankton in the Schelde estuary (Belgium/The Netherlands). The distribution of 3 Eurytemora affinis: effect of oxygen?[END_REF]22 Maes et al., 2005), as well as their potential role as bioindicators [START_REF] Appeltans | 2 Zooplankton in the Schelde estuary (Belgium/The Netherlands). The distribution of 3 Eurytemora affinis: effect of oxygen?[END_REF], 23 zooplankton are an important compartment in the OMES research. This present paper reports a 24 study of the rotifer species composition in the brackish -freshwater reach of the Schelde estuary, as 25 well as its spatio-temporal distribution and diversity during 2002. Rotifers are omnipresent in 26 aquatic systems [START_REF] Pourriot | Food and feeding habits of Rotifera[END_REF][START_REF] De Ridder | Monogonont Rotifera recorded in the World literature (except 32 Africa) from 1960 to 1992[END_REF][START_REF] Park | The trophic contributions of rotifers in tidal freshwater and 1 estuarine habitats[END_REF]Wallace et 27 al., 2006;[START_REF] Segers | Global diversity of rotifers (Rotifera) in freshwater[END_REF]. They are found mainly in freshwater: a total of 1800 to 2000 species have 28 been reported world-wide, with less than 100 strictly marine species. Estuaries present a particular 29 interesting setting to look at affinities of species and environmental variables. In a highly 30 heterotrophic system such as the Schelde estuary [START_REF] Soetaert | Carbon flows in the Westerschelde estuary (The 27 Netherlands) evaluated by means of an ecosystem model (MOSES)[END_REF], rotifers are likely to 31 form an important link between the microbial web and higher trophic levels [START_REF] Havens | The importance of rotiferan and crustacean zooplankton as grazers of algal 24 productivity in a freshwater estuary[END_REF]32 Gasparini and Castel, 1997;[START_REF] Griffin | Zooplankton grazing dynamics: top-down control of 22 phytoplankton and its relationships to an estuarine habitat[END_REF][START_REF] Froneman | Trophic cascading in an oligotrophic temperate estuary, South Africa[END_REF]. Unfortunately, there 33 are few species-level datasets on rotifer communities in the literature. This is due to a combination 1 of three factors. Their small size usually requires a compound microscope for identification; 2 illoricate species are more easily identified from living material; few competent taxonomists are 3 currently trained in rotifer taxonomy. Moreover, in estuaries, high concentrations of suspended 4 particulate matter hamper microscopic observation. To the best of our knowledge, the only such 5 report available for a European Atlantic estuary is that for the Elbe estuary [START_REF] Holst | Temporal and spatial dynamics of 36 planktonic rotifers in the Elbe Estuary during spring[END_REF]. 6 Our study presents the taxonomic composition (at species level in as far as possible) and 7 the spatio-temporal distribution of the rotifer community in the Schelde estuary. Its relationship to 8 environmental variables is studied, paying special attention to those variables which have changed 9 in the past, and are expected to go on changing as a consequence of restoration. 10

To test the feasibility of monitoring the rotifer community at the genus rather than species 11 level, the analysis was carried out at both species and genus level. The results were compared to 12 evaluate whether we obtain the same information about the association between taxa and 13 environmental factors when considering the data at the genus level or at the species level. Diversity 14 indices calculated from species-and genus-level assessments were also examined in relation to the 15 environmental factors. 16

Method 17

Study site 18

The Schelde River arises in France and runs through Belgium and The Netherlands, where 19 it enters the North Sea at Vlissingen (Fig. 1). The Schelde estuary, which extends inland as far as 20 the city of Gent, comprises the final 160 km of the river, which extends inland as far as Gent. Three 21 successive salinity zones can be recognized in this stretch: a marine zone (> 15 PSU Practical 22 Salinity Units), a brackish zone 5 (0.5 to 15 PSU) and a freshwater zone (< 0.5 PSU).The tidal 23 amplitude varies between 5.2 m near Antwerp (km 78.5) and 2 m near Gent (km 160). [START_REF] Maurer | The dark side of taxonomic sufficiency (TS)[END_REF] Depending on the tidal cycle and the amount of freshwater input, the river has low salinity 25 upstream from roughly Rupelmonde (km 85) to Vlassenbroek (km 118). Downstream, the brackish 26 zone extends to around the Dutch/Belgian border (km 57.5). 27

Sampling and physico-chemical analyses 28

From February 2002 to December 2002, 16 stations situated along the brackish and 29 freshwater reaches of the estuary were sampled monthly in the middle of the stream (Fig. 1). Water 30 sampling was done just below the surface using a 15-liter Niskin bottle at each sampling station, 31 ), orthophosphate (PO 4 -P) and silica (SiO 2 ) within 5 24h after sampling. They were stored at 4°C, and analyzed colorimetrically using a SKALAR SA 6 5100 segmented flow analyzer, except for silica (SiO 2 ), which was analyzed by ICP-OES (Iris). 7

Samples for the determination of dissolved organic carbon (DOC) were filtered on Gelman 8 fiberglass filters of 0.45-µm porosity. For DOC determination, a preliminary treatment was 9 performed consisting of H 2 SO 4 acidification, and flushing with nitrogen gas to remove background 10 CO 2 . The DOC was then released by further oxidation to CO 2, and analyzed using a SKALAR 11 (phenolphthalein, detection at 550 nm). The 5-day biochemical oxygen demand (BOD5) was 12 estimated using a WTW OXI 96 oxymeter. Suspended particulate matter (SPM) was determined 13 gravimetrically after filtering on pre-combusted Whatman GF/F 0.7 µm filters which were dried at 14 60°C. Water samples were filtered on 0.45 µm porosity filters and frozen for chlorophyll a (Chl a) 15 determination. The Chl a was extracted by adding N,N-dimethylformamide, and then quantified 16 colorimetrically at 647 and 664 nm [START_REF] Meire | The 27 Scheldt estuary: A description of a changing ecosystem[END_REF]. Discharge data were obtained from 17 the Flemish administration for waterways and maritime affairs (AWZ). For zooplankton sampling, 18 50 liters of the surface water were collected in a bucket, and filtered through a 50 µm mesh. [START_REF] Carlton | Pattern, process, and prediction in marine invasion ecology[END_REF] Carbonated water was added to the sample to narcotize the rotifers before fixing them with formalin 20 at a final concentration of 4% [START_REF] Siegfried | Planktonic rotifer community 24 structure in Adirondack, New York, U.S.A. lakes in relation to acidity, trophic status and 25 related water quality characteristics[END_REF][START_REF] Joaquim-Justo | Feeding of planktonic rotifers 6 on ciliates: a method using natural ciliate assemblages labelled with fluorescent 7 microparticles[END_REF]. 21

Analyses of zooplankton samples 22

In the laboratory, samples were stained with 3-5 drops of erythrosine, prepared at 0.8 mg 23 per 100 ml of water, to make it easier to detect the organisms in the detritus rich samples. After at 24 least 12 hours, the samples were mixed thoroughly. A subsample was taken using a wide-bore 25 syringe, and screened in a counting wheel using a Leica MZ 9.5 stereomicroscope (9x-90x). 26

Specimens were counted and identified to species whenever possible. When necessary, they were 27 mounted on a slide in glycerin and further observed with a microscope Nikon Optiphot-2 (50x-28 600x) using Differential Interference Contrast (DIC). 29

Data analysis 30

Spatio-temporal trends in the rotifer community, and their relationships to some 31 environmental variables (5- prior to the analysis to obtain a normal distribution. The modality of the taxa distribution was first 11 analyzed by a Detrended Correspondence Analysis (DCA), using detrending by segments. As the 12 total inertia observed was less than 2.6, a predominance of linear species response curves could be 13 expected, and so we used redundancy analysis (RDA), a technique in which the ordination axes are 14 constrained to be linear combinations of provided environmental variables to investigate the 15 relationships between environmental factors and taxa composition. Data were centered and 16 standardized by species prior to analysis. Forward selection of variables was used to select those 17 most closely associated with the spatio-temporal structure of the rotifer community, and to quantify 18 their relative importance. The statistical significance was tested with Monte Carlo permutation tests 19 (499 unrestricted permutations) (p< 0.05) and a Bonferroni correction for multiple test was applied. 20

The minimum model so obtained explains the distribution without co-linear extra fitting. RDA 21 biplots are shown using all environmental variables in order to show relations between the most 22 important variables (minimum model) and the others. Moreover, the variation partitioning of the 23 ordination was estimated as described in [START_REF] Borcard | Partialling out the Spatial Component of 15 Ecological Variation[END_REF]Borcard andLegendre, 1994, 24 using only the variables selected in the minimum model. Due to the curve shape of the estuary and 25 for a better consideration of the connectivity between the stations in the specific case of an estuary, 26 the distance to the mouth (km) was used as the spatial variable, considering a second degree 27 polynomial, rather than the geographical coordinates. [START_REF] Mistri | Taxonomic sufficiency in lagoonal ecosystems[END_REF] To test the relevance of the taxonomic level (species versus genus) in detecting the 29 association between taxa and environmental factors, a data reduction to the genus level was made. 30

This new dataset was then analyzed as described above, and the results were compared to those 31 obtained at species level. 32 52 rotifer taxa, belonging to 26 genera, were identified. Most of the taxa belonged to the 3 Monogononta (Table 1). About 42% (22 taxa) were new reports for the Schelde estuary. With the 4 exception of Keratella cruciformis (Thompson, 1892), all taxa were detected in the freshwater 5 reach, whereas only 28 taxa were found in the brackish water. 6

Spatio-temporal distribution 7

As shown in Fig. 2, rotifer abundance was low during winter, and then increased up to its 8 peak in May, reaching 2500 rotifers per liter. The summer abundances were lower, with a maximum 9 of about 500 rotifers per liter between July and October. Considering the year as a whole, the most 10 abundant species were Brachionus calyciflorus Pallas, 1766, Keratella cochlearis (Gosse, 1851) 11 and Brachionus angularis Gosse, 1851, all three of which were found every month. K. cochlearis 12 occurred mainly in July and August, when the two Brachionus species were less numerous. 13 From April to October, the abundances were much higher in the fresh water (stations at km 14 85 to 155) than in the brackish water (stations at km 57.5 to 78), whereas they were similar in these 15 two segments in March and November (Fig. 2). In December, the same trend was observed but only 16 5 stations were sampled in freshwater. Unfortunately, data were lacking for some stations through 17 the winter months. 18

Relationship with environmental variables: analysis at the species level 19

For the rotifer community analysis at the species level, 52 taxa were considered in 154 20 samples. All environmental factors considered in the analysis contributed significantly (p<0.05) to 21 explaining variability in the distribution of rotifers. Considering the Bonferroni correction for 22 multiple test, 8 environmental factors remained significant: chlorinity, discharge, chlorophyll a, 23 temperature, SiO 2 , cyclopoid abundance, NH 4+ and NO 3-.The marginal and conditional effects for 24 each variable are shown in Table 2. When considering the importance of the variables themselves, 25 without co-variability of other ones, as estimated by their conditional effects, chlorinity, discharge 26 and cyclopoid abundance are the main factors influencing rotifer distribution (Table 2). The sum of 27 all eigenvalues is 0.494 for analysis with all environmental variables or 0.435 with the minimum 28 model. The first and second RDA axis had eigenvalues of 0.182 and 0.174 respectively, using all 29 environmental variables; 0.179 and 0.169 respectively, in the minimum model. The sum of all axes 30 accounted for 85.8% of the species variation using all environmental variables, and 93.6% when 1 using only the variables selected for the minimum model. 2

This order corresponds to both the salinity and seasonal variations. The spatial partitioning 3 of the salinity (chlorinity) in our study zone becomes obvious when we plot the samples grouped by 4 station which show the brackish stations (km 58, 63, 71 and 78) spread out on the right hand side 5 and the remaining freshwater ones (km 85 to 151) grouped on the left hand side of the plot (Fig. 6 3a). The seasonal trend is clearly illustrated by the distribution of the samples in terms of the date 7 collected (Fig. 3b). In this graph we can clearly see the change along the temperature axis from the 8 winter months in the bottom right corner to the summer months in the top left corner (Fig. 4a). 9

The species -environmental factors biplot is shown in The biplot of axes 1,3 completes the picture of the association of rotifer species with 3 environmental factors (Fig. 4b). It essentially confirms the relationship shown in the axis 1,2 biplot 4 and accentuates the importance of the cyclopoid abundance which is, in this graph, the second most 5 important vector. In this biplot discharge and oxygen concentration, as well as SiO 2 and NO 3 -are 6 located at the opposite side to NH 4 + . 7

The variation partitioning showed that the environmental variables used in the minimum 8 model explain 43.8% of the variation in the species matrix, with 33.3% and 10.5% due to non 9 spatial environmental and spatial environmental factors respectively (Fig. 5). Of the spatial species 10 variation, 1.7% is not shared by the environmental variables. 54.5% of the variation is unexplained. 11 12

Comparison with analysis at the genus level 13

The RDA biplot of genus vs. environmental factors is shown in Fig. 6. All environmental 14 factors tested were found to be significant at p<0.05 but only 6 remained significant considering the 15 Bonferroni correction for multiple test. The sum of all eigenvalues is 0.474 for analysis with all 16 environmental variables or 0.403 with the minimum model. This result is comparable to those 17 obtained in the species level analysis (0.494 and 0.435 using all environmental variables or the 18 minimum model). The sum of all the axes accounts for 86.8% of the genus variation using all 19 environmental variables, and 97.1% when using only the variables selected for the minimum model. 20

The main factors organizing distribution of rotifers considered at genus level are chlorinity and the 21 discharge (Table 2), as was seen in the species data analysis. Other contributing factors are BOD5, 22 temperature, silica and cyclopoid abundance. These results are comparable to those found in the 23 species level RDA, especially when considering the conditional effects of variables (Table 2). [START_REF] Maurer | The dark side of taxonomic sufficiency (TS)[END_REF] Some examples illustrated in Fig. 4a,b temperature, and its negative correlation with discharge levels and oxygen are confirmed. Both 1 species reach similar maximum abundances, but A. priodonta was found more frequently, occurring 2 practically throughout the entire period that A. brightwellii was absent (Fig. 2). When this analysis 3 was performed using data reduced to the level of genus, Asplanchna occurred at a position 4 comparable to that of A. priodonta in the 1,2 biplot, and at one comparable to that of A. brightwellii 5 in the 1,3 biplot (Fig. 6a,b non spatial environmental and spatial environmental factors respectively (Fig. 5). Spatial genus 1 variation that is not shared by the environmental variables is negligible with only 0.2%. As 2 observed with the species level analysis, a large part of the variation (59.4%) is unexplained. 3 4

Diversity at the species level 5

The rotifer taxonomic richness, R, was generally much lower in the brackish zone (0 -8 6 taxa) than in freshwater zone (8 -16 taxa), except during the winter, when the value of R was 7 similar (8 -10 taxa) throughout the study area (Fig. 7a). Rotifer diversity, according to the Shannon 8 diversity index, H', (Fig. 7b) and its evenness, E H' , (Fig. 7c), was low in the brackish zone from 9 spring to autumn. The highest values of H' and E H' both occurred in winter at all the stations and 10 from early summer till winter just upstream from the brackish zone (km 78 to 120). 11

12

Comparison with analysis at the genus level 13

It can be seen that taxonomic richness follows a very similar course over time and space, 14 whether calculated at species or at genus level (Fig. 8). Similar trends are observed for H' and E H 15

indices (not shown). Pearson's correlation coefficients between R, H' and E H indices calculated 16

from species and genus level data were 0.88, 0.91, and 0.92 respectively (p<0.01). 17

Correlation between the diversity indices and environmental factors 19

The correlation between diversity indices obtained from both datasets is given in Table 3. 20 28 significant correlations were observed at the species level, versus 31 at the genus level. Most, but 21 not all, of the correlations revealed (70%) concern the same indices and factors. The species level, 22 for example, shows a correlation between R and E H and SPM, which is not detected at genus level. 23 On the other hand, more significant correlations are detected with temperature, DOC and discharge 24 when using the genus-level instead of the species-level dataset. 25 26 Discussion 27

Taxonomic composition 28

The first aim of this study was to inventory the rotifer taxonomic composition of the 29 Schelde estuary. The taxonomic list for the Schelde estuary has increased since previous studies (De 30 The rotifer abundance observed in the Schelde is about the same as that observed in the 9

Elbe estuary [START_REF] Holst | Temporal and spatial dynamics of 36 planktonic rotifers in the Elbe Estuary during spring[END_REF], the nearest estuary for which rotifer data are available. Moreover, 10 most species are found in both these estuaries, exhibiting similar temporal patterns. The most 11 abundant species occurring in the Schelde estuary (B. calyciflorus, B. angularis and K. cochlearis) 12 are cosmopolitan planktonic species [START_REF] Pontin | A key to British Freshwater Planktonic Rotifera[END_REF]; De Ridder and Segers, 1997). K. cochlearis and 13 K. quadrata are considered to be generalist rotifers, feeding on bacteria, detritus and flagellates 14 [START_REF] Pourriot | Food and feeding habits of Rotifera[END_REF]Starkweather and Bogdan, 1980;[START_REF] Arndt | Rotifers as predators on components of the microbial web (bacteria, heterotrophic 5 flagellates, ciliates) -a review[END_REF], and as a consequence are well 15 adapted to high concentrations of suspended particulate matter (SPM). The genera Cephalodella, 16

Colurella, Lecane, Lepadella, are generally benthic or periphytic [START_REF] Pontin | A key to British Freshwater Planktonic Rotifera[END_REF]Nogrady et al., 17 1995;[START_REF] Segers | Rotifera. Volume 2: The Lecanidae (Monogononta). Guides to the Identification 18 of The Microinvertebrates of the Continental Waters of the World[END_REF][START_REF] Duggan | The ecology of periphytic rotifers[END_REF]. As expected, they were rare in our surface samples. Keratella 18 tropica, Brachionus variabilis and Lecane decipens (Murray, 1913) are the only exotic rotifers 19 observed in the Schelde. Their occurrence and ecology has been detailed in [START_REF] Azémar | New occurrence of Lecane decipiens 7 (Murray, 1913) and some other alien rotifers in the Schelde estuary (Belgium)[END_REF]. [START_REF] Segers | Annotated checklist of the rotifers (Phylum Rotifera), with notes on 21 nomenclature, taxonomy and distribution[END_REF] 21

Relationship between rotifer species distribution and environmental variables 22

Earlier analyses of the distribution of the Schelde zooplankton community have shown 23 salinity and temperature to be the main structuring factors for the total zooplankton community 24 [START_REF] Soetaert | Spatial and temporal patterns of the zooplankton in the 33 Westerschelde estuary[END_REF]Tackx et al., 2004). Both these reports considered rotifers as a 25 group. Our study shows that, also for the rotifer community, salinity (chlorinity) is the main 26 structuring (spatial) factor (Fig. 4, Fig. 5, Fig. 6). Temperature is less important in explaining rotifer 27 community structure and is preceded by other typically seasonal factors such as discharge and Chl 28 a. Moreover, considering the Bonferroni correction, temperature is not significant while cyclopoids, 29 mainly present during warm season, are the third environmental parameter structuring the rotifer 30 distribution. This might be explained by the fact that, while the mean temperature over the entire 31 Besides the seasonal aspect, the effect of the discharge level can also be explained through 3 the positioning of the salinity gradient. In winter, from November till March, when the freshwater 4 flow is greatest, the rotifer population seems to be displaced downstream (Fig. 2). No difference in 5 rotifer abundance was observed between the so called freshwater and brackish water reaches during 6 this period, in contrast to the differences reported in other temperate estuaries or in tropical estuaries 7 The importance of Chl a and SiO 2 as structuring variables of the rotifer community can be 13 explained by their higher concentrations in the upstream part of the transect than the downstream 14 part, as well as their seasonal variations. (Fig. 2; Fig. 4). Species such as B. quadridentatus and 15 K. cochlearis, which are associated with high Chl a concentrations (Fig. 4), are known to be 16 herbivorous, and hence are likely to benefit from high phytoplankton concentrations (Reynolds, 17 1984; [START_REF] Hlawa | Experimental Studies into the feeding biology of rotifers in 30 brackish-water[END_REF][START_REF] Heerkloss | Feeding biology of two brachionid rotifers: Brachionus 26 quadridentatus and Brachionus plicatilis[END_REF]. Effect of temperature on rotifer 18 species composition and abundance, whether directly through its physiological effect or indirectly 19 through its association with the phytoplankton growth season, is clear from Fig. 2. 20

The fact that cladoceran and cyclopoid copepod populations also peak during summer 21 (Azémar, unpublished data) explains their association with temperature-related rotifer taxa, 22 although populations of some rotifers are probably directly correlated to the crustacean abundance. 23 For instance, Brachionus rubens is known to be an epizoic rotifer associated with cladocerans 24 [START_REF] Galliford | Notes on the distribution and ecology of the Rotifera and Cladocera of North 14 Wales[END_REF][START_REF] May | Epizoic and parasitic rotifers[END_REF][START_REF] Iyer | Effect of the epizoic rotifer Brachionus rubens on the population-1 growth of three cladoceran species[END_REF]. The introduced B. variabilis which is 25 occasionally found in Belgium [START_REF] Dumont | Biogeography of rotifers[END_REF][START_REF] Azémar | New occurrence of Lecane decipiens 7 (Murray, 1913) and some other alien rotifers in the Schelde estuary (Belgium)[END_REF] also has been reported to be 26 epizoic but less frequently [START_REF] May | Epizoic and parasitic rotifers[END_REF]. Both Brachionus species are associated with the cladoceran 27 abundance vector in our analysis. Some specimens of B. rubens were observed fixed on Daphnia 28 spp. bodies during the sample analysis. The small number of such observations is probably due to 29 the mechanical effect of the filtration process, and the addition of the fixative. All of the environmental factors considered contribute significantly to explaining the 1 spatio-temporal distribution of rotifers in the Schelde, at both the species and the genus levels but 2 the Bonferroni correction limits this significance to respectively 8 and 6 main factors. This is due, in 3 part, to the colinearity between the factors, most of which change gradually across the estuarine 4 chlorinity distribution. This colinearity is well shown by differences between marginal and 5 conditional effects (Table 2): after having taken into account the variance explained by the 2 first 6 variables (Cl -and discharge), the variance explained by following variables is considerably reduced 7 in the conditional effect. 8

The variables considered here are those that have been chosen for the routine monitoring of 9 the restoring Schelde estuary, because of they are known to represent water quality (Van Damme et 10 al., 2005). Moreover, these environmental factors have all changed as a consequence of the ongoing 11 restoration process, and are likely to continue changing in the future. Thus, for the practical purpose 12 of advising those managing the estuary, our findings do make it possible to find out which taxa are, 13 for example, favored by high Chl a concentrations, or those that are not hindered by high NH 4 + 14 concentrations. These factors are, to some extent , manageable. 15 16

Relationships with environmental variables: comparison of species and genus level 17 18

The analyses performed with the dataset reduced to the genus level (Fig. 6) and using all 19 the environmental variables generally showed the same trends as those of the most abundant 20 species. The sum of all eigenvalues (0.474 and 0.494 at the genus and the species level respectively) 21 and the variation explained in the dataset (86.8% and 85.8% at the genus and the species level 22 respectively) were also comparable in both cases. In our data, genera were often represented by one 23 dominant species accompanied by a few other relatively rare ones. As shown by the examples of 24

Brachionus and Keratella, the genus position in the ordination plot in these cases corresponds 25 closely to that of the most abundant species within the genus considered. However, the position 26 found for the genus does not give an adequate picture of the association between the less abundant 27 species, or those that are only present for a short period, and environmental factors. The example of 28 the genus Asplanchna, illustrates this observation. In this case, the two species had similar 29 abundance, but their mean value did not adequate represent either. 30

In our study, however, the use of the genus level as surrogate for the species, while 31 inevitably resulting in a loss of ecological information, particularly pertaining to the less abundant 32 partitioning of the data considered at the species and the genus level do not show noticeable 3 differences. The spatially linked environmental variation explained is relatively small (10.5% at the 4 species level). This is probably due to the absence of real gradients in the distribution of the 5 environmental factors measured all along the transect. In fact, most variables vary differently inside 6 the small brackish area (3 stations) and the extensive freshwater area (13 stations) (Van Damme et 7

al., 2005). 8

Considering the taxonomic richness, R, (Fig. 7a), the area investigated can also be divided 9 into two zones, corresponding to the brackish-water and the freshwater zones respectively. Fairly 10 homogeneous values of richness are observed within both zones. Except during winter, the rotifer 11 community in the brackish zone of the Schelde is less diverse than that in the freshwater zone. Few 12 species are able to cope with the wide variations of environmental factors in the brackish zone 13 [START_REF] Remane | Die Biologie des Brackwassers[END_REF]). In the Schelde, the most abundant rotifers occurring in the brackish 14 water zone consist of a few tolerant freshwater species (B. calyciflorus, B. quadridentatus, K. 15 cochlearis, K. quadrata). As most rotifers currently live in freshwater and simply follow the 16 current, they die as salinity values increase. Within the freshwater zone, in contrast to the 17 homogeneity of the taxonomic richness, both Shannon's diversity index (H', Fig. 7b) and evenness 18 (E H' , Fig. 7c) display some noticeable differences in the relative abundance of taxa between the 19 upper and lower freshwater reaches. The upper reach of the freshwater tidal zone (km 78-105) is 20 dominated by few taxa (H' generally below 0.6, and E H' below 0.4). In comparison, the downstream 21 reach of the freshwater zone has about the same richness R but shows higher values of H' and E H' 22 (except in spring). Moreover, rotifers are numerically less abundant in the lower reach of this 23 freshwater zone than further upstream (Fig. 2). This reach often corresponds to a maximum 24 turbidity zone (MTZ) [START_REF] Baeyens | General description of 10 the Scheldt estuary[END_REF] As shown in Fig. 8, richness measured at the genus or species level followed a very similar 29 trend over space and time. This was also the case for H' and Eh. In our dataset, 9 out of 27 genera 30 were plurispecific, and 18 monospecific. However, the plurispecific genera contained 64% of the 31 species observed. The monospecific genera include 10 cases in which the species could not be 32 identified, so we cannot be certain that they were indeed monospecific. The co-variation of diversity 33 measured at species and genus level in our dataset is partly due to the fact that calculating diversity 1 at the species and genus level produced the same result for 36% of the species. It is difficult to say 2 whether this type of rotifer population composition is common in estuaries. The taxonomic 3 resolution used by Holst [START_REF] Holst | Temporal and spatial dynamics of 36 planktonic rotifers in the Elbe Estuary during spring[END_REF] in the study on the Elbe rotifers is different from ours 4 for some genera. These authors used non fixed samples, which enabled them to identify more non-5 loricate species. 6

As diversity at both taxonomic levels varies over both time and space, it is not surprising to 7 find that the indices obtained from both taxonomic levels also generally correlate significantly with 8 the same environmental factors (Table 3). As we have already mentioned, most of the 9 environmental factors considered in Table 3 can be expected to continue to change with the ongoing 10 restoration of the Schelde estuary. Diversity is typically of interest in water management, and so 11 compiling an inventory rotifer diversity offers relevant information related to the evolution of 12 environmental variables. The fact that this can be done at the genus level facilitates including the 13 rotifer community in routine monitoring programs. 14

In general, the feasibility of higher taxa as surrogates for species-level patterns appears to 15 depend mainly on the taxonomic composition of the taxonomic group considered. In the case of the 16 marine benthic invertebrates, [START_REF] Maurer | The dark side of taxonomic sufficiency (TS)[END_REF] judges that loss of ecological information resulting 17 from the use of Taxonomic Sufficiency (TS) make this method unacceptable even though it does 18 save cost. The exclusion of rare species has serious effects on general ecological observations and 19 theory, and runs counter to current biodiversity assessment and bio indicator research (Maurer, 20 2000). From this point of view, the use of the coarser identification level is not appropriate, or 21 should at least be limited to well explored areas, where the biodiversity has already been well 22 documented [START_REF] Quijón | The use of coarser taxonomic resolution in studies of 6 predation on marine sedimentary fauna[END_REF]. The time saved by identifying organisms to a coarser 23 level depends on the number of species within each group, on whether the numerically dominant 24 species belong to several taxonomically complicated groups or not, and on the level of taxonomic 25 expertise available [START_REF] Dauvin | Taxonomic sufficiency: an overview of its 27 use in the monitoring of sublittoral benthic communities after oil spills[END_REF]. In estuarine environments, the high concentration of 26 suspended particulate matter poses a serious problem for studying plankton samples, and rotifers in 27 particular. Finding small transparent animals in the samples is very difficult, even after staining. In 28 the freshwater part of the Schelde estuary, the SPM consists mainly of organic matter, so that it is 29 not possible to use ludox separation of the organisms from the heavier fraction as applied to 30 zooplankton samples by [START_REF] Soetaert | Spatial and temporal patterns of the zooplankton in the 33 Westerschelde estuary[END_REF] in the brackish zone of the estuary. On 31 the other hand, once they are found within the suspended matter, an experienced person using a 32 good stereomicroscope can identify most rotifers present up to species level without major 33 problems. Nevertheless, the rotifer classification is largely perfectible: all areas are not equally 1 intensively studied so literature is not consistent everywhere. Identifications are usually based on 2 morphology which is insufficient for numerous group or species as attested by molecular studies 3 [START_REF] Fontaneto | On the global distribution of 7 microscopic animals: New worldwide data on bdelloid rotifers[END_REF][START_REF] Fontaneto | Molecular evidence 4 for broad-scale distributions in bdelloid rotifers: everything is not everywhere but most 5 things are very widespread[END_REF][START_REF] Fontaneto | Extreme levels of hidden 9 diversity in microscopic animals (Rotifera) revealed by DNA taxonomy[END_REF][START_REF] Kaya | Inconsistent estimates of 12 diversity between traditional and DNA taxonomy in bdelloid rotifers[END_REF]. Thus, the 4 Brachionus and Keratella from the Schelde can be identified at the species level at a glance, but one 5 should bare in mind the existence of species complexes, such as, for example, Brachionus plicatilis, 6

Keratella cochlearis. Most of the rotifers in the estuary are loricated. The surface ornamentation of 7 the lorica is of taxonomic importance (Wallace et al., 2006), because they include traits that are 8 visible even on fixed samples. Soft bodied rotifers, such as Synchaeta, Rotaria and other Bdelloids, 9 once fixed with formalin often have their corona and foot retracted, making it difficult to identify 10 them even to genus level. Live samples or scanning electron microscopy (SEM) would be required 11 to identify them, which would considerably increase the methodological constraints for monitoring 12 purposes [START_REF] Hollowday | Familly Synchaetidae[END_REF]. 13 From this study of the rotifer population in the context of the restoration of the Schelde 14 estuary, we conclude that, in such studies, it is very informative but not essential to start with a 15 species-level inventory. A lack of taxonomic competence for species-level identification should not 16 prevent scientists from monitoring estuarine rotifer populations. At least in the case of the Schelde 17 estuary, most of the ecological information (community structure as a function of spatio-temporal 18 variations in environmental conditions, the relationship between diversity and changes in 19 environmental factors) are also obtained when using the genus level analysis. [START_REF] Segers | Annotated checklist of the rotifers (Phylum Rotifera), with notes on 21 nomenclature, taxonomy and distribution[END_REF] 21 See Table 1 for nomenclature of species, and Table 3 for the abbreviations of the environmental 8 factors. 9 10 Fig. 5. Variation partitioning of the rotifer data matrix at the species and genus level considering 11 environmental variables selected in the minimum model, using the distance to the mouth as typical 12 spatial factor. 13 14

F

  environmental variables were measured: pH and temperature using a CONSORT 1 C832 electrode and dissolved oxygen concentration (O 2 ) (WTW OXI 325, equipped with Clark 2 electrode). 3 Samples were taken for the determination of the concentrations of chlorine (Cl -

33

 33

Fig. 4 .10

 4 Chlorinity, the most 10 important factor influencing the distribution of rotifer community, is located close to the first axis in 11 the upper right quadrant. Calanoid copepods, SPM and pH are situated in the right-hand, chlorine-12 associated part of the plot. Discharge levels and oxygen concentration are found in the lower right 13 quadrant, opposite to the cyclopoid abundance and the temperature. Cyclopoid abundance, 14 chlorophyll a and temperature are all associated with the upper left of the ordination, together with 15 the cladoceran abundance and PO 4 -P concentration. To a lesser extent (non significant after the 16 Bonferroni correction), NO 2 -concentration and DBO5 (lower left of the ordination) are all 17 negatively associated with chlorinity. The SiO 2 , NH 4 + and NO 3 -concentration vectors are found 18 almost parallel to the second axis (lower half of the biplot). 19 When considering the species distribution in this biplot, the most abundant species are 20 mainly situated away from the center of the biplot (Fig. 4a), whereas the rare species are all 21 clustered near the center. Two haline rotifer species, Synchaeta bicornis Smith, 1904 and Keratella 22 cruciformis, are positively correlated with chlorinity. All the other taxa show greater affinity for 23 freshwater conditions. Brachionus quadridentatus Hermann 1783, B. rubens Ehrenberg. 1838, 24 Pompholyx sulcata Hudson, 1885, Keratella tropica (Apstein, 1907) and Asplanchna brightwellii 25 Gosse, 1850 are all positively correlated with high temperatures and chlorophyll a concentrations, 26 and negatively correlated to discharge and oxygen concentration, corresponding to summer 27 conditions in the top left corner of the biplot. In this quadrant we also find cyclopoid copepods and 28 cladocerans. The abundant species B. calyciflorus, B. angularis and K. cochlearis, and also Filinia 29 longiseta (Ehrenberg, 1834) and K. quadrata (Müller, 1786) are situated close to the NO 2 -and 30 BOD5 vectors. B. leydigii Cohn, 1862, A. priodonta Gosse 1850, Lecane sp. and Synchaeta sp. are 31 correlated to high SiO 2, NO 3 -and NH 4 + concentrations, whereas Euchlanis dilatata Ehrenberg, 1832, 32 Notholca acuminata (Ehrenberg. 1832) and Epiphanes sp. are positively correlated with the 1 discharge levels. 2

  and Fig.6a,b allow us to compare results obtained in the 25 species and genus level analyses. Genus Asplanchna is represented by two species. A. brightwellii, a 26 summer species, is located in the top left corner of the axis 1,2 biplot (Fig.4a, ringed). This species 27 is positively associated with high temperatures and Chl a concentrations, and negatively with 28 discharge and oxygen concentration. A. priodonta is situated at the opposite position to 29A. brightwellii (Fig.4a, ringed), and is associated with SiO 2 , NO 3 -, DOC and NH 4 + . In the 1,3 biplot 30 A. priodonta , which is located rather close to the origin on the first axis, shows little association 31 with environmental factors. On the other hand, the positive correlation between A. brightwellii and 32

  B. diversicornis (Daday, 1883) are situated among the rare species quite close to the center, and do 12 not seem to be greatly influenced by the factors being considered here. When we performed our 13 analysis at the level of genus for Brachionus (Fig.6, open squares) we found that the genus plot was 14 situated at the extreme left of the second axis, positively correlated with BOD5 and NO 2 -, and 15 negatively correlated with chlorinity. This position corresponds to the character of the most 16 abundant Brachionus species we found (i.e. B. calyciflorus and B. angularis), but the associations 17 between the less abundant species and other parameters are not clear. 18 Six species of Keratella occur in the estuary (Fig. 4, open triangles). K. cruciformis, a haline 19 species, is located at the top right hand corner of the plot, and is positively associated with 20 chlorinity. K. valga (Erhenberg, 1834), situated quite close to the origin, next to the second axis 21 (upper half), does not show any trend relative to the environmental factors considered. K. testudo 22 (Erhenberg, 1832) is situated in the bottom left quadrant on the NH 4 + vector, K. quadrata and 23 K. cochlearis are associated with high BOD5 values and NO 2 -concentration, and K. tropica with 24 cyclopoid abundance and temperature. When we performed our analysis at the level of genus, 25 Keratella (Fig. 6, open triangles) was found on the left side of the ordination plot, strongly 26 associated with NO 2-concentration and BOD5. This position corresponds fairly closely to the 27 'mean' for the most abundant species (K. cochearis, K. quadrata and K. tropica), but does not 28 reflect that of the haline species K. cruciformis. Also K. testudo, a species which is not associated 29 with the typical left side conditions of high NO 2 -, and BOD5, Chl a and temperature values, is not 30 well represented by this position of the genus. 31Concerning the variance partitioning, the environmental variables considered in the 32 minimum model explain 40.4% of the variation in the species matrix, with 31.1% and 9.3% due to 33

18

 18

6 rare

 6 [START_REF] De Pauw | Bijdrage tot de kennis van milieu en plankton in het Westerschelde estuarium[END_REF] Tackx et al., 2004), with 22 new taxa being reported. The main reason for this increase 1 was probably the taxonomic effort made in this study, although introduced species are commonly 2 observed in areas with heavy shipping activity such as the Schelde estuary[START_REF] Carlton | Pattern, process, and prediction in marine invasion ecology[END_REF];Johnson 3 and Padilla, 1996;[START_REF] Ruiz | Global spread of microorganisms by ships[END_REF] Wasson et al., 2001). 4In the Schelde estuary as in other estuaries[START_REF] Holst | Temporal and spatial dynamics of 36 planktonic rotifers in the Elbe Estuary during spring[END_REF][START_REF] Park | The trophic contributions of rotifers in tidal freshwater and 1 estuarine habitats[END_REF] 5 Rougier et al., 2005;[START_REF] Lam-Hoai | Seasonal change of community structure and size 16 spectra of zooplankton in the Kaw River estuary (French Guiana)[END_REF], rotifers originating from freshwater are dominant. The Synchaeta bicornis and Keratella cruciformis are the only typically brackish rotifers(Koste, 7 1978;[START_REF] Holst | Temporal and spatial dynamics of 36 planktonic rotifers in the Elbe Estuary during spring[END_REF][START_REF] Segers | Annotated checklist of the rotifers (Phylum Rotifera), with notes on 21 nomenclature, taxonomy and distribution[END_REF] found in our samples. 8

  4.2 end 23.6°C in time, it varies maximally 2.6°C between stations within 1 each sampling campaign. 2

  during the rainy season[START_REF] Holst | Temporal and spatial dynamics of 36 planktonic rotifers in the Elbe Estuary during spring[END_REF][START_REF] Park | The trophic contributions of rotifers in tidal freshwater and 1 estuarine habitats[END_REF][START_REF] Rougier | Ecological patterns of the rotifer 12 communities in the Kaw River estuary (French Guiana)[END_REF] Lam- 8 Hoai et al., 2006). However, during high discharge periods, the entire transect sampled in this study 9 consisted of freshwater (< 0.5 PSU). Unfortunately, rotifers were not studied further downstream, in 10 the Dutch part of the estuary. More complete data are required before we can attempt to 11 characterize the rotifer community and its geographical distribution during winter. 12

  rare species, does not substantially change the general pattern of the spatio-temporal distribution 1 of the rotifer community as a function of environmental factors. Comparison between the variation 2

  ; Herman and Heip, 1999; Chen et al., 2005; Meire et 25 al., 2005; Van Damme et al., 2005) where the concentration of particulate matter and pollutants 26 generate restricting ecological conditions (Soetaert and Van Rijswijk, 1993; Van Damme et al., 27 2005). 28

Fig. 1 . 24 Fig. 2 .Fig. 3 . 4 Fig. 4 .

 1242344 Fig. 1. Map of the Schelde estuary indicating the marine, brackish and freshwater stretches, and the 21 positions of the sampling stations (white circles). Stations are denoted according to their distance 22 from the mouth at Vlissingen in km. 23

Fig. 6 . 20 Fig. 7 . 24 Fig. 8 .

 6207248 Fig. 6. RDA biplot a) axis 1,2 and b) axis 1,3 showing the distribution of the rotifers at genus level 15 in the Schelde estuary as a function of the environmental parameters. Most significant 16 environmental variables (minimum model) are shown as full arrows. Other variables are shown as 17 doted arrows. See Table1for nomenclature of genera, and Table3for the abbreviations of 18 environmental factors.[START_REF] Carlton | Pattern, process, and prediction in marine invasion ecology[END_REF]

  RDA biplot showing the distribution of the samples grouped by station location and by date 286x147mm (600 x 600 DPI) RDA biplot showing the distribution of the rotifers at species level in the Schelde estuary as a function of the environmental parameters 286x119mm (600 x 600 DPI) Variation partitioning of the rotifer data matrix at the species and genus level 202x91mm (600 x 600 DPI) the distribution of the rotifers at genus level in the Schelde estuary as a function of the environmental parameters. 286x147mm (600 x 600 DPI) Species richness, R, calculated from species-(black diamonds) and from genus-(open squares) level data along the transect sampled for each month of the year. 230x196mm (600 x 600 DPI)

  This research was conducted within the framework of OMES (Onderzoek Milieu-Effecten 2 Sigmaplan), financed by the Flemish Administration for Waterways and Maritime Affairs 3 'Zeeschelde division'). We are indebted to Antoine Lecerf for help with statistics, and to the crews 4 of the vessels Veremans and Scaldis I for assistance during sampling. R. L. Starkweather, P.L. and Bogdan, K.G. (1980) Detrital feeding in natural zooplankton communities: 1 discrimination between live and dead algal foods. Hydrobiologia, 73, 83-85. 2 Tackx, M., De Pauw, N., Van Mieghem, R., Azémar, F., Hannouti, A., Van Damme, S., Fiers, F., 3 Daro, N. and Meire, P. (2004) Zooplankton in the Schelde estuary, Belgium and The 4 Netherlands. Spatial and temporal patterns. J. Plankton Res., 26, 133-141. 5

	1
	5 6 6 7	Wallace and an anonymous reviewer provided valuable critics to a previous version of the manuscript. M. Ghosh Tackx, M.L.M., Herman, P.M.J., Gasparini, S., Irigoien, X., Billiones, R. and Daro, M.H. (2003) Selective feeding of Eurytemora affinis (Copepoda, Calanoida) in temperate estuaries:
	7 8	corrected the English. model and field observations. Est. Coast. Shelf Sci., 56, 305-311.
	9	Ter Braack, C.J.F. (1987) Ordination. In: Data analysis in community and landscape ecology.
	10	R.H.G. Jongman et al. (Eds). Pudoc, Wageningen, Netherlands, pp. 91-173.
	11	Ter Braack, C.J.F. (1994) Canonical community ordination. Part I: basic theory and linear methods.
	12 13 14 15	o r F o r F o r F o r F o r F o r Ecoscience, 1, 127-140. Van Damme, S., Struyf, E., Maris, T., Ysebaert, T., Dehairs, F., Tackx, M., Heip, C. and Meire, P. (2005) Spatial and temporal patterns of water quality along the estuarine salinity gradient of the Scheldt estuary (Belgium and The Netherlands): results of an integrated monitoring F o r
	16 17 18 19 20 21	P e e r P e e r P e e r P e e r P e e r P e e r approach. Hydrobiologia, 540, 29-45. Wallace, R.L., Snell, T.W. and Ricci, C. (2006) Rotifera. Volume 1: Biology, ecology and P systematics (2nd edition). Guides to the Identification of The Microinvertebrates of the Continental Waters of the World, 23, H.J. Dumont (Ed) Backhuys Publishers, Leiden. 299 e pp. Wasson, K., Zabin, C.J., Bedinger, L., Diaz, M.C. and Pearse, J.S. (2001) Biological invasions of e r
	22 23	R R R R R R estuaries without international shipping: the important of intraregional transport. Biol. Conserv., 1002, 142-153. R
	24	e e e e e e e
		v i e v i e v i e v i e v i e v i e v i e
		w w w w w w w

Table 1 . 6 Table 2 .

 162 Taxonomic composition of the rotifer fauna of the Schelde estuary. The designation codes 3 used for the multivariate analyses are shown in bold. Symbols: * -new taxa for the Schelde; b -taxa 4present in the brackish water zone; f -taxa present in the freshwater zone. 5 Variance explained by the environmental variables tested with the rotifer dataset at the 7 species and the genus level. Variables were considered using the complete set or only the most 8 significant ones, after a Bonferroni correction (minimum model). Marginal and conditional effects 9

	1	Table and Figure legends
	2
	10	are shown.
	11

Table 3 .

 3 Significance of Spearman rank correlations obtained between R, H' and E H' from both 12 datasets (i.e. species and genus level) with environmental factors (indicated with the abbreviations 13 used for following analyses). *: significant at p<0.05. ***: significant at p<0.01. Dark gray: 14 significance detected with both datasets; dark shading: significance detected with the species dataset 15 only. Light gray: significance detected with the genus level dataset only.[START_REF] Bowman | Does taxonomic resolution affect the multivariate 17 description of the structure of freshwater benthic macroinvertebrate communities? Can[END_REF]

Table 1 1 2 3

 12 Monogononta

	Anuraeopsis sp.	Axx * f	Keratella valga (Ehrenberg, 1834)	Kva	f
	Asplanchna brightwellii Gosse, 1850	Abr b f	Lecane bulla (Gosse, 1851)	Lbu * b f
	Asplanchna priodonta Gosse 1850	Apr b f	Lecane closterocerca (Schmarda, 1859)	Lcl * f
	Brachionus angularis Gosse, 1851	Ban b f	Lecane decipiens (Murray, 1913)	Lde * f
	Brachionus calyciflorus Pallas, 1766	Bca b f	Lecane flexilis (Gosse, 1886)	Lfl	f
	Brachionus diversicornis (Daday, 1883)	Bdi * f	Lecane hamata (Stokes, 1896)	Lha * f
	Brachionus leydigii Cohn, 1862	Bley * b f	Lecane luna (Müller, 1776)	Llu * f
	Brachionus quadridentatus Hermann 1783 Bqu b f	Lecane sp.	Lxx b f
	Brachionus rubens Ehrenberg, 1838	Bru	f	Lepadella ovalis (Müller, 1786)	Lov	f
	Brachionus urceolaris Müller, 1773	Bur b f	Notholca acuminata (Ehrenberg, 1832)	Nac b f
	Brachionus variabilis Hempel, 1896	Bva	f	Platyias quadricornis (Ehrenberg, 1832)	Pqu * b f
	Cephalodella sp.	Cex	f	Ploesoma sp.	Plx * f
	Colurella sp.	Cox	f	Ploesoma hudsoni (Imhof, 1891)	Phu * f
	Epiphanes sp.	Epx b f	Polyarthra sp.	Pox b f
	Euchlanis dilatata Ehrenberg, 1832	Edi b f	Pompholyx sulcata Hudson, 1885	Psu * b f
	Filinia brachiata (Rousselet, 1901)	Fbr	f	Rhinoglena frontalis Ehrenberg, 1853	Rfr * b f
	Filinia longiseta (Ehrenberg, 1834)	Flo b f	Synchaeta sp.	Sxx b f
	Gastropus hyptopus (Ehrenberg, 1838)	Ghy * b f	Synchaeta bicornis Smith, 1904	Sbi * b f
	Hexarthra sp.	Hxx	f	Testudinella sp.	Tex	f
	Kellicottia longispina (Kellicott, 1879)	Klo * b f	Testudinella patina (Hermann, 1783)	Tpa * f
	Keratella cochlearis (Gosse, 1851)	Kco b f	Trichocerca pusilla (Jennings, 1903)	Tpu	f
	Keratella cruciformis (Thompson, 1892)	Kcr * b		Trichocerca similis (Wiersejski, 1886)	Tsi	f
	Keratella quadrata (Müller, 1786)	Kqu b f	Trichocerca sp.	Trx	f
	Keratella testudo (Ehrenberg, 1832)	Kte * b f	Trichotria tetractis (Ehrenberg, 1830)	Tte * f
	Keratella tropica (Apstein, 1907)	Ktr b f
	Bdelloidea
	Dissotrocha sp.	Dxx * f	Rotaria neptunia (Ehrenberg, 1832)	Rne	f
	Rotaria sp.	Rox b f	Other Bdelloids	Bdel b f

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