In search of suitable habitats for water beetles (Insecta: Coleoptera) within a heavily transformed river system

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Many such beetles also have considerable capacities to disperse and migrate, which are characteristics that affect the structures of beetle communities in the floodplains of rivers, both when these are swollen and when they are carrying optimal volumes of water. At the same time, the stenotopicity and demanding habitat requirements of some species imply that they are good bioindicators of the state of surface waters (Gioria et al. 2010a(Gioria et al. , 2010bPakulnicka et al. 2015aPakulnicka et al. , 2015bTurić et al. 2020). The growing significance of biomonitoring, in combination with the need for the permanent monitoring of the water's physical and chemical parameters, has taken on a new dimension in light of the recent environmental disaster on the Odra. This is also confirmed by the new challenges currently facing ecologists (Ziglio et al. 2006).
Knowledge of the beetle fauna of groyne fields and modified oxbows provides a foundation on which specific proposals for the planning and implementation of large-river restoration measures can be based. This is not only important in light of the EU recommendations for protecting biodiversity and improving the state of the waters, contained in the Natura 2000 programme and the Water Framework Directive, but also for sound economic reasons -making use of existing habitats is more expedient than, for example, undertaking a costly and often unworkable reconstruction of river meanders.
The aims of our study were: (i) to define the species, ecological and trophic structures of the beetle assemblages inhabiting the groyne fields and oxbows; (ii) to highlight factors exerting a significant influence on the structure of these assemblages and the trophic groups of beetles in both types of habitats; (iii) to try and find an answer to the question -Can groyne fields in a large, radically transformed river offer an alternative lentic habitat for water beetles, approximating that of near-natural oxbows?

Study area
The study covered the middle and lower course of the River Odra over a distance of around 420 km, from Uraz (51°14′N, 16°51′E) to Ognica (53°04′N, 14°22′E). A series of groynes built from stone blocks have been constructed along the 350 km-long stretch of the river from Brzeg Dolny to Czelin. The spaces between them -the groyne fields -where the current is slower, are filled with sandy or muddy sediments and frequently support marshland vegetation with Doretto et al. 2020;Turič et al. 2021;Knehtl et al. 2021). The deterioration in water quality, the probable consequence of these activities, usually leads to a decline in the biodiversity of these environments (Vinson & Hawking 1998;Bates et al. 2007;Kennedy & Turner 2011).
In the last 300 years, the River Odra (Czech: Odra, German: Oder), the subject of this study, has had barrages constructed across it and, in some places, dykes built alongside. Moreover, short-cuts dug across the bases of the rivers's meanders have reduced its original length by 25% (Rast et al. 2000). This straightening out of the river's course has deprived it of the natural pools of still water, typical of large, natural rivers, which has impoverished the aquatic fauna and flora. In the 18th century, groynes began to be constructed along the river -these are extant along the 350-km stretch from Brzeg Dolny all the way to Czelin. Another type of water body that enriches the hydrographical network of the Odra valley are oxbows, remnants of the original course of the river. Even so, entry to them is very often merely screened by a groyne, so that in fact they are not completely cut off from the mainstream, thereby facilitating the free circulation of water and the migration of organisms. Both the groyne fields and the oxbows may be of great significance for the Odra's assemblage of invertebrates and the local biodiversity. These areas may also play an important part in the river's self-cleaning processes. In the context of the ecological disaster that befell the Odra in the summer of 2022 (Absalon et al. 2023;Free et al. 2023), they may act as refuges of fauna from which the mainstream can be recolonised, once the wave of contamination has passed (Allan 1998;Ward et al. 2002).
To date, these potential refuges have been little studied, however. Only a small number of detailed papers have been published on their zooplankton (Ławniczak et al. 2008), molluscs (Piechocki & Szlauer-Łukaszewska 2013), ostracods (Szlauer-Łukaszewska 2015), dragonflies , leeches (Jabłońska- Barna et al. 2017) and caddisflies (Buczyńska et al. 2018). Some relevant information is also given in the report by Schöll et al. (2003). The results of these studies are absolutely crucial and may offer points of reference as the river's renaturalisation is tracked.
An important and very numerous taxonomic group of organisms inhabiting river valleys are water beetles (Biesiadka & Pakulnicka 2004;Buczyński et al. 2011;Pakulnicka et al. 2012;Costea et al. 2013;Pakulnicka & Nowakowski, 2016;Turić et al. 2020). They are quite diverse in terms of species, trophic relationships and ecology (Gioria et al. 2010a(Gioria et al. , 2010bCostea et al. 2013;Pakulnicka et al. 2015aPakulnicka et al. , 2016. dominant Phalaris arundinacea L.; elodeids have also been recorded in this zone (Fig. 1). The central, deepest parts have sandy bottoms, upon which rest large boulders and huge amounts of shell debris. The sediments of the areas in front of the groynes, which are attacked by the oncoming, swirling waters of the river, are usually of sand and gravel. Apart from the pools that have formed between the groynes, the other lentic habitats found along this stretch of the Odra are oxbows, most of which have a permanent connection with the river. The oxbows we explored were from 520 to 5000 m long and from 20 to 630 m wide (Fig. 1). Their bottoms are covered by sandy and silty sediments with a considerable admixture of organic matter, sometimes forming a sapropel. The littoral zone is dominated by sedges Carex spp. with admixtures of other helophytes; elodeids and nymphaeids also occur. When the water levels are high, the meadow vegetation behind the banks is inundated (Piechocki & Szlauer-Łukaszewska 2013). Along the stretch of the Odra that we explored we  species, N -number of individuals, D -dominance (% contribution of the taxa).
Because the samples of aquatic beetles were collected on several different dates (to take into account the phenological aspect) from the habitats distinguished in the river and oxbows, we used a GLM (Generalized Linear Model) for repeated measures ANOVA (Hocking 1996) to determine the significance of differences in the number of species and the abundance of beetles. First, we checked the assumptions of normality (Shapiro-Wilk test) and the homogeneity of variances (Levene's test). The GLM repeated measure models were calculated on the basis of Type III sums of squares so as to take the unbalanced design into account. Significant results were tested for pairwise comparisons using Tukey's HSD post-hoc tests. The dependent variables (abundance and number of species) were transformed where necessary to satisfy the requirements of parametric tests (Saint-Germain et al. 2007;Cremona et al. 2008). Correspondence analysis (CA) (Clausen 1998) was used to determine dependences between the abundance of water beetles within the ecological groups, the functional feeding groups (FFG) and the habitats. The analysis involved two dimensions: one explained the largest part of the general chi-squared statistics (% of inertia), the other dimension increased the percentage of the explained inertia. Relationships between the FFG of beetles (N -abundance, S -number of species) and the environmental parameters at the sampling sites were determined using principal components analysis (PCA). All the calculations were performed in Statistica, ver. 13.5 (StatSoft, Tulsa, USA).
To distinguish the key environmental drivers responsible for the variation among the beetle species we used Canonical Correspondence Analysis (CCA) for two sets of variables separately: the 12 physical and chemical parameters of the water, and the 7 features describing the structure of the sites. Matrices with no data transformation were used according to O'Hara & Kotze (2010). Stepwise selection with 999 test permutations was used to filter the significant variables (p < 0.05) (Šmilauer & Lepš 2014). The computations were performed and plotted in CANOCO 5.0 statistical software (ter Braak & Šmilauer 2012).
set up a total of 27 research stations located in both environments: 15 in the groyne fields and 14 in the oxbows (Fig. 2). On the stretch below Czelin (3 stations where a total of 54 samples were collected) there are neither groynes nor groyne fields -the banks are reinforced with boulders, reedbeds emerge above the water surface -and no beetles were caught there. Therefore, those samples were not taken into consideration in the subsequent statistical analyses.

Sampling procedures
The water beetles were caught in spring, summer and autumn in both 2009 and 2010; some additional material was acquired in October 2012. A total of 539 faunistic samples were obtained. The insects were collected with a 25 cm wide handnet fitted with a 0.25 mm mesh netbag. In places where the bottom was even, the net was dragged for a specified distance to scoop up the surface layer of bottom sediment. Where the bottom was overgrown or uneven, stony or very hard, the sample was taken from a specified area by sweeping the net back and forth over it. The results were recalculated and expressed as the number of individuals per 1 m 2 . The research stations were selected so as to ensure the maximum possible coverage of all the habitats accessible during the various study periods. The adults collected (965) were identified to species level and the larvae (315) to generic level.
Twelve physical and chemical parameters of the water and seven structural parameters of the river or oxbows were measured at each station. The specific electrolytic conductivity (EC), total dissolved solids (TDS), salinity (SALIN), pH, dissolved oxygen content (O2) and temperature (TEMP) were measured using a multifunctional probe (Hach-Lange HQ40d). Visibility (VISIB) was assessed with a Secchi disc. Ammonium nitrogen (NH 4 ), nitrites (NO 2 ), nitrates (NO 3 ), phosphates (PO 4 ) and water hardness (HARD) were measured in situ with a Slandi LF 300 portable photometer for measuring contaminants in waters and effluents. At each station we measured the depth of the bottom (DEPTH), the breadth of the littoral (LITTOR), and the degree of plant cover (PLANTS) based on phytosociological relevés (Braun-Blanquet 1964). The substrate composition at each sampling site was estimated as the proportion of each of the following substrate particle size classes: mud (< 0.06 mm diameter -MUD), sand (0.06-2 -SAND), gravel (> 2-64 -GRAVEL) and rock (> 64 -STONES) (Gordon et al. 1992).

Biocoenotic indices and statistical analyses
The following indices were used for the species diversity analyses and comparisons: S -number of (6.6%), a species particularly abundant in the oxbows, was of somewhat lesser importance in the entire material. The GLM Repeated Measure ANOVA indicated significant differences in both the numbers of species (F (1;27) = 9.49906, p = 0.005) and the abundance of beetles (F (1;27) = 9.88372, p = 0.004) in the two habitats (Table 2).
This analysis also showed up significant differences in the dependence on the phenological period, i.e. the month when sampling took place, of both the numbers of species (F (6;27) = 4.31, p = 0.00047) and

Results
General comments on the beetle fauna A total of 1280 beetles belonging to 66 taxa were collected, 280 from the river (40 taxa) and 1000 from the oxbows (57 taxa) ( Table 1).
The most numerous species in the dominance structure were Laccophilus hyalinus (26.0%) and Hygrotus versicolor (9.4%), eudominants in both the groyne fields and the oxbows. Haliplus fluviatilis tors (more than 70% of the total). In the river, detritivores, mainly shredders, were quite important; against that, there were considerable numbers of largely phytophagous beetles in the oxbows. Among the latter, grazer-scrapers and polyphages were present in abundance, along with a few miners. Filter feeders were few in number in both habitats. The qualitative trophic structure was similar: the principal element here consisted of predators, whereas the species diversities in the other FFGs resembled one another.
The trophic structures of the beetle assemblages in both habitats were phenologically diverse (Fig. 6). In spring, the largest numbers of beetles in both habitats were predators. In summer, the significance of this group decreased distinctly, making way for a clear increase in the proportion of detritus feeders (like shredders and active filter feeders), and phytophages (grazer-scrapers, miners and polyphages). Polyphages were particularly numerous in the oxbows.
Relationships between the ecological and functional feeding groups of Coleoptera in both types of habitats and with respect to the habitat conditions CA analysis of the ecological structure of the beetles in both habitats revealed significant differences in the counts of beetles representing different ecological groups (χ 2 = 441.41, df = 130; p = 0.00001).
Together, the two dimensions explained 71.26 % of the total chi-squared statistic (total inertia). The most important relationship demonstrated was that rheophiles exhibited a clear preference for oxbows, the abundance of beetles (F (6;27) = 6.78, p = 0.000002). The GLM repeated measure ANOVA highlighted the significant synergistic effect between the habitat type and phenological period, with respect to both the number of species (F (6;162) = 1.431, p = 0.026) and the number of beetles (F (6;162) = 1.379, p = 0.023). The significant differences (Tukey's HSD post-hoc test) between the subclasses are illustrated in Fig. 3.
Particularly noteworthy are the statistically significant differences in the numbers of species found in both habitats in October (p = 0.008) and in the abundance of beetles in May (p = 0.027) and August (p = 0.049).
Faunistic similarities, ecological elements and functional feeding groups The species richness in the ecological structure of the entire material was the greatest among eurytopes (56.4% of all species), followed by rheophiles (14.1%) and tyrphophiles (12.8%). The ecological structure of the beetle fauna in the two habitats was much the same, albeit with more tyrphophiles and argilophiles in the oxbows and more rheobionts in the groyne fields (Fig. 4A).
However, the ecological structure differed where the abundances of the ecological groups were concerned: 61.5% of the individuals found were rheophiles, while 29.9% were eurytopes. The proportions of the other groups were not significant. Analysis of the beetle fauna in the two habitats showed that rheophiles were quantitatively more important in the oxbows, while eurybionts were likewise more important in the groyne fields (Fig. 4B).
As regards the trophic structure, the largest numbers of beetles in both habitats (Fig. 5) were preda-    Table 1.  Table 1.
whereas rheobionts displayed a pronounced affinity for the groyne fields ( Fig 7A).
Analysis of the beetles' trophic structure also highlighted significant differences in the numbers of aquatic beetle FFGs (χ 2 = 522.73, df = 140; p = 0.00031). The two dimensions together explained 71.38% (b) of the total chi-squared statistic (total inertia). The analysis upheld the positive correlations between active filter feeders (FF) and shredders (SHR) with groyne fields. In turn, grazers and scrapers (GR-SC), miners (MIN) and polyphages (POL) were shown to have a strong relationship with oxbows. Predators were common in both habitats (Fig. 7B, Table 3).
Principal component analysis (PCA) of the variables representing the parameters of habitats and trophic groups distinguished in our study (Fig. 8) showed that the first axis, representing the highest eigenvalues, corresponded most strongly with the variables "polyphages", "predators", "plants", "Cond", "TDS" and "NO 3 ", and that the second axis showed the strongest correlations with the variables "miner", "filter feeder" and "O 2 ".
Environmental factors responsible for the distribution of beetles CCA of the relationships between beetle occurrence and the physical and chemical parameters of the water in both habitats showed that both ordination axes together explained 36.3% of the overall variance. The following factors turned out to be statistically significant: "pH", explaining 12.6% of the total variability, "Visib" (12.3%), "temp" (10.8%), "hard" (10.0%) and "O2" (10.0%). The plot indicates Table 3 Correlation matrix from the PCA panel between the various functional feeding groups and the habitat parameters    Table 1.  Table 1.
CCA of the relationships between the beetle associations and structural factors indicated that the first and second ordination axes together explained 47.76% of the total variance. The statistically significant factors were "plants" (22.6% of the overall variability), "littor" (19.7%) and "depth" (14.9%) ( Table 4). The plot shows that the deeper the water, the greater the proportion of "good swimming" species, primarily Dytiscidae, and of rheobionts   Table 1.
et al. 2017). However, the total of 40 water beetle taxa found in the Odra is comparable with the species richness of these insects found in natural lowland rivers in Poland, like the Narew (52 species), Neman (44) and Bug (44) (Biesiadka & Pakulnicka 2004;Buczyński et al. 2011;Pakulnicka & Nowakowski 2012). The key to explaining this richness appears to be the presence of groynes on the Odra, which have compensated for the loss of biodiversity caused by the realignment of the bank line. This was confirmed by earlier reports of other taxa occurring along the Odra, especially Odonata  and Trichoptera (Buczyńska et al. 2018), and by the results of zoobenthos studies in other rivers (Tockner 1996;Brunke et al. 2002). Buczyński et al. (2017) drew attention to the distinctly higher species richness of dragonfly assemblages on the sections of the Odra with groynes than along the groyne-free, regulated stretches, which provides further evidence of the significance of groynes for the local biodiversity. Our data corroborate these reports. Earlier papers also demonstrated that the canalisation of a river dramatically reduces the density and species richness of water beetles (Bates et al. 2007;Kennedy & Turner 2011). This is probably due to the lack of Coleoptera from the groyne-free reaches of the Odra: beetles may well have been present there, but in such small densities that none were caught, even though these stretches were regularly surveyed. In contrast, the material acquired from the reaches with groynes was relatively rich.
In north-central Europe water beetles are a group of organisms that display a high level of eurytopicity and have considerable dispersal abilities (Lundkvist et al. 2002). In accordance with the ecological and biogeographical theory, this results from a great many causes affecting the patterns of geographic variation of biological communities (Currie 1991;Krebs 2008). Nevertheless, there are few species among the beetles with special preferences restricting them to particular habitat types; but it is these stenotopes that are the best bioindicators, facilitating the evaluation of habitat conditions (Gioria et al. 2010a(Gioria et al. , 2010bPakulnicka et al. 2015a;Turić et al. 2020). This high level of eurytopicity among the Coleoptera is distinctive in most aquatic environments, in which it is eurytopes that manifest the greatest species differentiation (Biesiadka & Pakulnicka 2004;Buczyński et al. 2011;Pakulnicka & Nowakowski 2012;Pakulnicka et al. 2015aPakulnicka et al. , 2015b. Our research has shown that the same applies to the Odra's groyne fields: this aspect is the faunistic foundation of the whole assemblage. The species richness among the rheophiles and tyrphophiles is much smaller.

Discussion
General comments on the beetle fauna of the River Odra and its oxbows Papers focusing on the ecology of invertebrates of flowing waters and their valleys have usually related to smaller streams. Large lowland rivers have seldom been explored in this respect, especially where their natural character has been artificially modified (Brunke et al. 2002;Kleinwächter et al. 2003;Lewin 2014). Allan (1998), who takes the RCC into consideration, maintains that analyses of ecological processes occurring in flowing waters should account for the heterogeneity of the environment, including factors that arise from human activities.
A good example is the radically transformed River Odra. Because there are insufficient relevant data (Piechocki & Szlauer-Łukaszewska 2013;Buczyński et al. 2017;Jabłońska-Barna et al. 2017), it is very difficult to establish the effect of these hydroengineering works on the animal and plant communities living there. The main source of data on the beetle fauna of the Odra in its entirety is the report by Schöll et al. (2003), which lists 18 taxa (species and genera) and states whether they do or do not inhabit three stretches of the river (upper, middle, lower), along with 5 others caught in its tributaries. Further fragmentary faunistic data can be found in three other papers (Roger 1856;Reitter 1870;Greń 2017). Nevertheless, it is worth attempting to evaluate the conditions offered by the Odra to the water beetles living there, and to compare them with those in other lowland rivers that have been explored in this respect to a greater or lesser extent (Biesiadka & Pakulnicka 2004;Persson Vinnersten et al. 2009;Buczyński et al. 2011;Pakulnicka & Nowakowski 2012;Turić et al. 2020). Characteristic of large lowland rivers are the various water bodies in their valleys: oxbows are of fundamental functional significance, as to a large extent they govern the biological communities of rivers, including those of beetles (Biesiadka & Pakulnicka 2004;Sanderson et al. 2005;Jurkiewicz-Karnkowska 2006;Piechocki & Szlauer-Łukaszewska 2013;Lewin 2014;Buczyński et al. 2011;da Conceição et al. 2017;Buczyńska et al. 2018).
The Odra is an example of a profoundly transformed river, the bank lines of which have been straightened. River regulation leads to a simplification of the bank line and the disappearance of important microhabitats, mostly lentic ones, and this impoverishes the species richness (Kleinwächter et al. 2003; Jurkiewicz-Karnkowska 2006; Buczyński Analysis of the trophic groups showed that most of the species in the beetle fauna of both habitats were predators, which at the same time were the most numerous. Less abundant in the groyne fields were saprophages, mainly shredders, whereas in the oxbows there were fewer phytophages and polyphages. We also noticed a phenological change in the trophic structure: the numerical preponderance of predators in spring diminished distinctly in successive months, with a concomitant rise in the abundance of the other trophic groups -saprophages, phytophages and polyphages. This is evidently associated with the new growing season, the appearance of young plants and the increasing supply of organic matter in the form of detritus (including FPOM, the presence of which is revealed by the autumn records of active filter feeders, especially in the groyne fields) and also confirmed by the results of PCA. This dependence between the characteristics of organism assemblages and the presence of plants and organic matter has been demonstrated by other authors, both from the Odra (Szlauer-Łukaszewska 2015; Buczyński et al. 2017;Buczyńska et al. 2018) and other rivers (e.g., Eggers 2006;Sanderson et al. 2005;Paula-Bueno & Fonseca-Gessner 2015). In addition, Biesiadka & Pakulnicka (2004) showed that the diminishing numbers of predators in oxbows combined with the simultaneous increase in the percentages of Hydrophilidae were a sign of their eutrophication. Hence, a greater richness of predators is indicative of the generally good ecological condition of the two habitat types we studied.
The effect of environmental parameters on the beetle assemblages Our principal component analysis (PCA) of the variables representing the parameters of the habitats and trophic groups confirmed earlier data that the presence of vegetation in aquatic environments (both fresh biomass and decomposed in the form of detritus) have a positive influence on the occurrence and abundance of species representing lower trophic levels, especially polyphages and saprophages (Eggers 2006;Sanderson et al. 2005;Paula-Bueno & Fonseca-Gessner 2015). The plentiful food resources undoubtedly favour the occurrence of predators, which are predominant in both the groyne fields and the oxbows of the Odra, as regards both species richness and abundance. Our study confirmed that the degree of vegetation cover is key to the distribution of species (CCA), and that beetles prefer habitats supporting vegetation to habitats on the mineralised bottom. They provide not only foraging areas and concealment from potential predators (Eggers 2006; The close proximity of oxbows undoubtedly affects the ecological structure of the beetles in the groyne fields; various hydroengineering modifications have ensured that most of the oxbows have a permanent water connection with the river. This means that river water flows continuously through the oxbows, which substantially retards their eutrophication (Pakulnicka & Nowakowski 2012;Pakulnicka et al. 2016). The so-called ecological integrity of the river-floodplain system, expressed by e.g. the Floodplain Index Waringer et al. 2005), is one of the most important elements in maintaing well functioning invertebrate fauna in specific reference conditions. In recent years, numerous authors have demonstrated the significant influence of catchment area factors (including the presence of other waters) on the fauna colonising a river (Pakulnicka et al. 2016;Zawal et al. 2016). Many hydrobiologists have drawn attention to the special importance of oxbows in the shaping of riverine communities (Biesiadka & Pakulnicka 2004;Jurkiewicz-Karnkowska 2006;Paula-Bueno & Fonseca-Gessner 2015;Obolewski et al. 2009;Pakulnicka & Nowakowski 2012;Lewin 2014;Pakulnicka et al. 2016). We collected 57 species of water beetles in the Odra's oxbows; this figure is only slightly smaller than that obtained for the oxbows of other lowland rivers (Biesiadka & Pakulnicka 2004;Buczyński et al. 2011;Pakulnicka & Nowakowski 2012).
Despite the qualitative dominance of eurytopes, we demonstrated a distinct quantitative predominance of rheophiles in the synecological structures of both habitat types. This has been confirmed by studies of other groups of organisms, like dragonflies and caddisflies, in the Odra valley Buczyńska et al. 2018). The most numerous among them were Laccophilus hyalinus, Hygrotus versicolor and Haliplus fluviatilis. Furthermore, these species were usually the most abundant ones found in the studies of other lowland rivers (Biesiadka & Pakulnicka 2004;Buczyński et al. 2011;Pakulnicka & Nowakowski 2012). This ecological structure, especially in the groyne fields, testifies to their excellent ecological condition, since a predominance of eurytopes in biological communities is a response to unpropitious environmental conditions (Clausnitzer 2003;Šiling & Urbanič 2016). Therefore, the highest proportion of rheophiles in both the groyne fields and the oxbows of the Odra indicates not only a high level of faunistic similarity, but also the similar habitat conditions which they imply, an aspect that has been corroborated by earlier studies of macroinvertebrates in the Odra Buczyńska et al. 2018). Pakulnicka 2004). River water regularly circulates through these oxbows, so their trophic development is limited and their ecological stability maintained. The presence of these species in groyne fields implies that, as in the oxbows, the water parameters in the former are optimal; in contrast, the presence of plants creates a stable living habitat, protected from river wave action (Tockner 1996;Brunke et al. 2002).

Conclusions
Groyne fields restore the habitat heterogeneity on a regulated river, thus compensating for losses in its biodiversity. These habitats have fairly good water parameters and are structurally appropriate, so they are eminently suitable for the occurrence of water beetles, especially rheophiles, the most distinctive element among them. Because there is considerable faunistic similarity between the beetle assemblages of groyne fields and oxbows, the latter function in much the same way as the former and can act as an important refuge for many species that inhabit large lowland rivers. This is crucial for biodiversity conservation, particularly in view of anticipated, further hydro-engineering works on those rivers and in their valleys. This implies that groyne construction is advantageous, but solely on already regulated rivers. Our data by no means offer an argument in favour of river canalisation; conversely, they indicate that strongly transformed watercourses can benefit from the creation of these alternative lentic habitats, thereby enlarging their area along the entire river valley.