The spatio-temporal variability of benthic communities was studied in a modified system whose main physical characteristics are controlled by humans: the Rance basin, with its tidal power station. Soft-bottom communities were sampled in 1976 and 1995 by a quantitative survey of 105 and 113 stations, respectively, to assess spatial and temporal changes, to determine factors involved in benthic assemblage changes and to lay a base for the management of this coastal area. A total of 205 and 240 species were recorded in 1976 and 1995, respectively. Ascendant hierarchical classification and factorial correspondence analysis identified 7 and 6 assemblages in 1976 and 1995, respectively. The main groups are: 1) the Amphioxus lanceolatus-Glycymeris glycymeris; 2) the Abra alba-Corbula gibba and 3) the oligospecific Macoma balthica communities. A large consortium of common species linked the main assemblages described in 1976 as in 1995. From comparison of their distribution and structure, local spatio-temporal changes were identified in response to modifications of the habitats due to 1) silting in the brackish sector; 2) the decrease in the mean low water level and 3) the proliferation of the mollusc Crepidula fornicata. As a consequence, species richness 1) decreased in the brackish part as a result of the ‘homogenisation’ of habitats induced by silting and 2) increased in parts of the basin through the proliferation of Crepidula fornicata and the apparent diversification of the associated habitat. However, despite reduction in species abundances, the structure of the identified assemblages between 1976 and 1995 remained stable at the scale of the Rance basin. This long-term relative stability seems to reflect stable environmental conditions. In the basin, tide is moderate with reduced range variations and benthic communities are preserved from the effects of swells. However, the new balance is entirely dependant upon the mode of the installation: by modifying water levels with appropriate time intervals, managers can chose to favour intertidal or subtidal communities according to their respective ecological interests for avifauna and flatfishes, local socio-economical preoccupations or recreational activities carried out in the basin.
Changes in the structure of coastal benthic communities depend on natural factors, which cause seasonal or stochastic variations of abundances and artificial perturbations induced by anthropogenic actions. These may either be chronic (i.e., pollutants from domestic, industrial or agricultural sources), accidental (i.e., hydrocarbon spills) or as a result of habitat management caused by the construction of dams, harbours, hydroelectric stations. According to the different situations, humans may either be spectators of environmental changes, by studying natural changes or those induced by human activities, or actors by controlling physical parameters and the associated biological changes.
In the case of the management of a tidal power station, the main characteristics of the physical environment are controlled. Consequently, a continuous pressure is exerted on faunal and floristic communities. Numerous attempts have been made to predict the effects of such human pressure in various parts of the world (Martin, 1970; Gordon and Longhurst, 1979; Shaw, 1980; Daborn and Dadswell, 1988; Marfenin et al., 1997; Semenov, 1997). The Rance basin (Figure 1), on the northern coast of Brittany (France) offers considerable potential to make a full-scale assessment of the ecological impact of a tidal power station after 30 years of operation. To determine modifications induced by management of the Rance estuary, benthic communities were used as bioindicators of environmental change (Dauvin, 1993). The original bionomic studies, which were carried out in the basin during 30 years, were integrated over a large spatial scale and extended to all communities. Although this approach provides only limited information on the kinetics of change, or on their causes (Pearson et al., 1985; Reise and Schubert, 1987), pertinent scales to assess change in a disturbed ecosystem are at population, or better, community size.
Environmental effects of construction and functioning stages are different (Retière, 1979). The construction stage can be compared to an accidental event. For example, the closing of the estuary for 3 years (1963–1966) resulted in the disappearance of all benthic marine fauna and flora except the most tolerant species. However, based on benthic sampling conducted in 1971, Retière (1979, 1994) concluded that 5 years were sufficient for the recolonisation of the tidal basin. Recolonisation may have occurred through the sluices and/or turbines by planktonic larval stages, but also by juveniles and adults passively carried by strong currents from adjacent populations (Retière, 1979; Clavier et al., 1983).
The sampling was repeated in 1976 to examine changes in the soft-bottom benthic communities, however, only data for the annelid assemblages were analysed (Lechapt, 1983). To assess changes over several decades, a new programme of sampling was conducted in 1995 and quantitative data recorded in 1976 were reconsidered for statistical analysis.
The objectives of this paper are to summarize the spatial distribution and the structure of soft bottom benthic communities in 1976 and 1995; to assess temporal change; and to determine the incidence of physical stability/instability in the observed distributions and changes. Results are discussed in terms of implications of, and for, benthic biodiversity management in coastal areas.
Material and methods
The basin (48°50′N, 1°94′W) is the ancient estuary of the Rance: a medium size river of approximately 100 km in length, with a low discharge (average 7 m3 sec− 1). Shaped like a comma of approximately 20 km, the estuary forms a succession of narrow channels and wide basins.
The tidal power station was built at the mouth of the river, between the towns of Dinard and St. Malo (Figure 1). Construction began in 1963 and was completed in 1966, creating a reservoir with a capacity of 184 million m3 occupying a surface of 22 km2 at the +13.50 m level. Maximum water depth is 17 m at low tide, but the main part of the basin is 5 to 6 m deep. The structure is 750 m in length and anchored to granite bedrock 13 m below zero datum level. The structure comprises a lock, the generating station proper, a rock dike and a 115 m wide removable dam made up of 6 sluice gates.
Prior to the scheme, the Rance was a ria, with slight saline stratification and a maritime downstream sector, a high salinity middle section giving way to a low salinity sector (Fisher, 1929). Actually, only two areas can be identified (Figure 1): the marine reservoir, in which the deepwater salinity remains higher than 30 psu and the upstream estuary, of brackish water (Retière, 1989). The junction between brackish and marine waters has moved from St. Suliac to Port-St-Hubert since the scheme was built.
The operating constraints of the installation impose highly specific ‘tidal’ conditions on the waters in the basin: 1) periods of slack water lasting particularly long, are out of phase with high water, 2) mean water level is raised by approximately 3 m and 3) tidal range varies between 4.0 and 7.0 m, depending on whether the turbines are operating in one or both directions. Reduction in tidal range is correlated with a reduction in the surface area of the intertidal zone: the exposed zone accounts today for 50% of the total area of the basin. At a larger temporal scale, except for a period of relative instability following the initiation of the power station (1967–1975), water level fluctuations were regular (Figure 2). The decrease in water level since 1993 is linked to an initiative to protect banks and habitation.
The violent sluice and turbine currents have eroded parts of the riverbed. Sandbanks closest to the dam have shifted and the bed is more or less covered with gravel or pebbles (Retière, 1979). At the same time, long periods of slack water have promoted the deposition of fine particles in coves and bays (Retière, 1989; Bonnot-Courtois and Lafond, 1991). A 1994 map of the superficial sediment distribution shows that two granulometric gradients prevail transversely and longitudinally (Bonnot-Courtois, 1997). From downstream to upstream of the estuary, pebbles and coarse sands are replaced by medium and fine sands, muddy sands and finally muds upstream beyond Port-St-Hubert. A similar sequence is observable from the central channel to the banks. Natural silting of the basin is presumed to have increased since the tidal power station started. In the upstream part of the basin, sedimentation rate increased from 0.5 cm y−1 before the scheme to 2.7 cm y−1 after (C. Bonnot-Courtois, Ecole Pratique des Hautes Etudes, Dinard, France, pers. comm.).
The sampling grid of 80 stations used in 1971 (see Retière, 1979) was repeated in the 1976 and 1995 sampling programs. To account for changes in substrate composition (erosion or silting), 28 and 33 stations were added to the basic grid in 1976 and 1995, making up a total of 108 and 113 stations respectively sampled in 1976 and 1995 (Figure 1). Two replicate samples were collected at each station using a 0.1 m2 Smith MacIntyre grab according to Clavier (1981). In 1995, 7 intertidal stations were sampled using a hand corer (5 replicates; area 1/55 m2) to a depth of 20 cm (Clavier, 1981). Although densities of organisms were extrapolated to a standard surface area, some bias was unavoidably introduced due to the different characteristics of sampling gear.
Macrofaunal distribution, as retained on a 1 mm mesh sieve, was determined from surveys conducted in May 1976 and 1995 prior to the spring recruitment. All samples were gently washed in situ through a 1 mm sieve and preserved in 4.5% formalin before being sorted, identified and counted in the laboratory.
Each station sampled in 1995 was associated with a particular sedimentary type (coarse-medium sand, muddy sand, sandy mud, mud) based on the sedimentary map established in 1994 (Bonnot-Courtois, 1997). A new category, not previously considered by sedimentologists, was defined: the muddy heterogeneous sediments, characterised by the co-occurrence of two textural modes: coarse elements (e.g., gravel, shell fragments) and muddy particles. Unfortunately, no data from 1976 were available on the sediment granulometrics.
A hierarchical ascendant classification was performed on all data. This distinguished groups of stations by using the coefficient M, with a group average sorting method (Clifford and Stephenson, 1975). Affinity assemblages between both species and stations were established from the stations-species matrix using factorial correspondence analysis (FCA; Benzecri, 1973; Legendre and Legendre, 1979). Owing to programming limitations, species recorded at fewer than 5 stations were omitted in data analysis procedures. Thus, faunal data sets of 108 stations × 103 species and of 113 stations ×115 species were respectively adopted for 1976 and 1995 surveys. To determine the important species of each station group, constancy [Cij = (nij/nj) × 100] and the fidelity index [Fij = (Cij∑j = 1kCij) × 100] were calculated, where nij is the number of occurrences of species i in the station group j, and nj is the number of stations in station group j (with j = 1 to k). Characteristic species were qualified as constant (C ≥50%), common (50% > C ≥ 25%), elective (F ≥ 90%), or preferent (90% > F ≥ 66.7%; Retière, 1979). Finally, comparisons of assemblage structure were performed with Shannon-Weaver diversity index and rank-frequency diagrams (Frontier, 1976; 1985). From this logarithmic representation of individual numbers as a function of species rank, the quantitative shift in species occurrence can be followed from pioneer to mature communities. Stage 1 (concave diagram) indicates the development of a pioneer community with a predominance of one or a few species; stage 2 (convex diagram) characterises the stage of maximum diversity with a more complex network of interactions as new species appear. A more linear diagram results in stage 3 with a slight decrease of diversity and perhaps the beginning of the ageing of the ecosystem, while stage 1′ is intermediate between stage 1 and 2.
General description of the benthos
Samples from 1971, 1976 and 1995 showed that species richness of soft bottom fauna increased each year with 114, 205 and 240 species recorded, respectively. Of the 205 and 240 species of macrobenthic invertebrates respectively collected in 1976 and 1995, polychaetes were the most species-rich group, comprising ≈ 40–45% of the species (Table 1). Other groups, ranked in order of decreasing species richness, were crustaceans, molluscs, and diverse (anthozoans, phoronidians, echinoderms and sipunculids). Oligochaetes were neither sorted nor counted. Species richness per station ranged from 2 to 71 with a mean of 26.1 species in 1976 and from 2 to 85 with a mean of 26.0 species in 1995.
In terms of abundance, polychaetes accounted for ≈ 80% in 1976 and 1995 (Table 1). Other groups, that is, crustaceans, molluscs, anthozoans, phoronidians, echinoderms and sipunculids were minor. Density ranged from 3.5 to 1415.5 ind 0.1 m− 2 with a mean of 306.0 ind 0.1 m− 2 in 1976 and from 1 to 1687.2 ind 0.1 m− 2 with a mean of 317.6 ind 0.1 m− 2 in 1995. Maximal densities (up to 1000 ind 0.1 m− 2) were constantly observed in the marine part of the basin. Despite a twofold reduction of its abundance, the polychaete Melinna palmata largely dominated the fauna in 1976 (mean abundance: 161.1 ind 0.1 m− 2) as well as in 1995 (mean abundance: 81.6 ind 0.1 m− 2). Among the 5 dominant species in 1976, 3 remained dominant in 1995, although with reduced abundances (Table 2). The analysis of 57 dominant taxa (characterised by abundances > 10 ind. 0.1 m− 2 in at least one station) between 1976 and 1995 revealed that reduction in abundance was a general trend for 50% of the taxa (30 species out of 57). In terms of occurrence, results are similar in 1976 and 1995, with 4 common species among the 5 most frequent taxa. The polychaete Nephtys hombergii remained the most frequently recorded species (Table 2).
Characterisation of assemblages and linkages with open sea communities
From hierarchical classifications and FCA, seven and six different macrobenthic assemblages were identified in 1976 and 1995, respectively. Their distribution in 1976 (represented by index 1) and 1995 (represented by index 2) are respectively given in Figures 3 and 4. Constancy and fidelity indices were calculated for each species in the different station groups, allowing the characterisation of assemblages. Despite the relative variability in faunal composition in the Rance basin, common traits characterized the homologous units over the 19 year interval.
Sandy substrate assemblages
The macrofauna of coarse to medium sands disappeared during the construction of the power station (unpub. data), but the faunal assemblages occurring on these substrates since 1969 were recolonised from open sea communities. Although characteristic species such Amphioxus lanceolatus, are rarely observed in shallow coastal waters as in the Rance basin, assemblages A1 and A2 include many species typical of the A. lanceolatus-Glycymeris glycymeris community described by Retière (1979) on similar substrates in the Normano-Breton Gulf. Among the 4 dominant species in 1976 and 1995, 2 were common: Staurocephalus kefersteini and Elhersia cornuta, with higher abundances in 1995 (Table 3). In addition to local faunal impoverishment on hydraulic sand banks (resulting from substrate instability), a particularity of this assemblage in the Rance basin is a mixture of species living on clean coarse sands with those characteristic of muddy fine sediments. Although low, the turbidity associated with long periods of slack waters may be responsible for the existence of small mud pellets and the coexistence, at a micro-spatial scale, of species with varied substrate affinities.
Due to natural deposits of mud, sandy substrates are progressively colonised by species, such as Cirriforma tentaculata, Eupolymnia nebulosa, Sthenelais boa, characteristic of muddy, heterogeneous sediments. This is exacerbated by the proliferation of Crepidula fornicata (observed throughout the Normano-Breton Gulf) whose colonies enrich the substrate in heterogeneous elements, and modify flow regime at the sediment/water interface. However, four top ranked species always remained characteristic of coarse sands in 1995 (Table 3). As a result, the assemblage of medium to coarse sands more or less heterogeneous and/or more or less muddy (B1 in 1976; B2 in 1995) progressively replaced the previous coarse to medium sand assemblage.
Muddy fine sediment assemblages
Due to their large areas, both the intertidal and/or brackish muddy fine sand and marine muddy fine sand assemblages are the main units within the basin. Most euryoecious species that survived during the construction stage of the scheme were pro parte responsible for the recolonisation of brackish bottoms, while the marine muddy fine sands were colonized mainly from open sea communities (Retière, 1979). Macrofauna of assemblages D1 and D2 show affinities with the Abra alba-Corbula gibba community, whose distribution overlaps both mud and muddy sands. The four dominant species (Melinna palmata, Chaetozone setosa, Euclymene oerstedi and Ampelisca tenuicornis) occurred in practically the same rank order (Table 3) in 1976 and 1995, although their abundances were reduced by half.
The gravely sand sediment is enriched in coarse elements by erosion from the adjacent cliffs (pers. obs.) or by the proliferation of Crepidula fornicata. The marine muddy fine sand assemblage was progressively replaced by muddy heterogeneous fine sand assemblage (E1 in 1976; E2 in 1995), which, due to their small surface area, should be considered as an ecotonal group (Dewarumez et al., 1992). In 1995, other species were dominant but with low abundances (Table 3).
The surface covered by the intertidal mud and/or brackish muddy fine sand assemblages (F1/F2) increased considerably in both the brackish area and marine areas between 1976 and 1995 (Figures 3, 4). In 1976, marine (Chaetozone setosa, Melinna palmata) and brackish species (Ampharete acutifrons, Pygospio elegans) dominated this assemblage, whereas in 1995 most of the dominant species collected were typical of brackish areas with high silt content (Ampharete acutifrons, Hydrobia ulvae and Abra tenuis; Table 3). This assemblage represents two forms depending on location: 1) In the marine area, this assemblage is located on intertidal mudflats, with relatively high species richness in 1995 (93 species). The decrease in mean low water level, initiated in 1993, allowed the expansion of this assemblage. Considering its faunal composition and its species richness, this assemblage may represent a pioneer stage of the muddy fine sand assemblage. The ‘perturbation’ due to emersion time may induce a regression of the muddy fine sand assemblage, a decrease in diversity, the disappearance of recently established species and the persistence of robust species (Frontier and Pichod-Viale, 1991). 2) In the brackish zone, silting was responsible for the change of assemblages established on fine to medium sands, on medium to coarse sands and on heterogeneous sediments, towards an intertidal and/or brackish muddy fine sand assemblage. The main components of the Macoma balthica oligospecific community of Petersen (1913; 1918) were observed. Due possibly to the unique tidal conditions in the Rance basin, the assemblage described is characterised by high species richness (68 and 70 species in 1976 and 1995) and its low densities of Macoma balthica (only 2 specimens collected in 1995).
As a response to the increase salinity variation, the brackish subtidal mud assemblage (G1 and G2), belonging to the Macoma balthica community progressively replaced the previously-observed assemblage in the most upstream zone of the basin. Among species collected, only Hediste diversicolor was dominant in both 1976 and 1995 (Table 3), with increasing abundances.
The lightly silted fine to medium sand with Nephtys cirrosa assemblage (C1) described in 1976 in the brackish zone disappeared. Its absence in 1995 was due to the silting of the clean fine sand hills in the brackish zone. Despite its oligospecificity (17 species in 1976), it was similar to the medium to fine clean sand with Donax variegates-Armandia polyophtalma community identified by Retière (1979) in the eastern part of the Normano-Breton Gulf.
Spatial variability of assemblages
Medium to coarse clean sand or heterogeneous silty sand assemblages (A1, B1, A2, B2) covered respectively 1550 and 1625 m2 in 1976 and 1995 (i.e., 7.5 to 8.5% of surface area; Table 4). The area covered by the medium to coarse clean sand assemblage decreased between 1976 and 1995 from 1210 m2 to 525 m2. Jointly, the surface area covered by the assemblages of medium to coarse sands more or less heterogeneous and/or more or less muddy (B1, B2) increased by 3.
Fine sediment assemblages (C1 to G1 and D2 to G2) covered about 15000 m2 (about 75% of the basin). Whereas the main assemblage in 1976 was established on muddy fine sand (D1≈ 9400 m2), the pattern was different in 1995 (Table 4). Indeed, this unit partially evolved towards the muddy heterogeneous fine sand assemblage and the brackish subtidal mud assemblage, which covered 7900 m2 in 1995 versus 4500 m2 in 1976. The lightly silted up to fine/medium sand assemblage (C1) described in 1976 progressively disappeared due to the silting up of the brackish area.
Structural variability of assemblages
Medium to coarse clean sand assemblage (A1/A2)
Rank Frequency Diagrams (RFD) were convex and stable in time (Figure 5). Shapes can be compared to stage 1′ or 2 as characterised by Frontier (1985), indicating the existence of a complex network of interactions within the community. The pronounced convexity of the RFD in 1995 revealed a more equilibrated assemblage, dominated by about ten species. Diversity H' increased from 1976 (3.93) to 1995 (4.79).
Assemblage of medium to coarse sands more or less heterogeneous and/or more or less muddy (B1/B2)
RFD are very similar to those mentioned above and reflect faunal units in stage 1′ or 2 (Figure 5). Rectilinear shape of the diagram, more pronounced in 1995, is caused by the dominance of the polychaete Pomatoceros lamarckii, which colonised coarse sediment surfaces. The assemblages were homogeneous and well structured as indicated by the high Shannon index value (4.54 in 1976/4.34 in 1995).
Muddy fine sand assemblage (D1/D2)
The quasi-identical shape of RFD established from 1976 and 1995 data illustrates the remarkable long-term stability of this assemblage (stage 1′ or 2; Figure 5). Values of diversity index were high and stable: 3.77 in 1976 and 3.84 in 1995. As 70% of species constituted common faunal elements, assemblages described in 1976 and 1995 logically refer to the same unit.
Muddy heterogeneous fine sand assemblage (E1/E2)
Abundances of the dominant taxa were reduced twofold between 1976 and 1995 (Table 3) and only Chaetozone setosa remained in the four top ranked species. RFD are convex and stepped as shown in Figure 5. The overlapping of coarse sand and muddy fine sand assemblages constituted this non-homogeneous assemblage, whose structure remained stable. Diversity indices were high (4.42 in 1976 and 4.80 in 1995).
Intertidal and/or brackish muddy fine sand assemblage (F1/F2)
Presence of steps in RFD established from 1976 data indicated the co-existence of two faunal groups characteristic of marine and brackish environments (Figure 5). As a consequence of the increase in dominance of euryoecious species, the assemblage was more structured in 1995 (stage 1′ or 2), as indicated by RFD and H′ (3.65 in 1976 and 4.39 in 1995) despite the general decrease of abundances (Table 3).
Brackish subtidal mud assemblage (G1/G2)
Rectilinear curves reflect a ‘juvenile’ assemblage (stage 1 described by Frontier (1985); Figure 5). Oligospecificity of this assemblage, as illustrated by relatively low diversity values (2.23 in 1976 and 1.74 in 1995), resulted from environmental factors such as silting, duration of tidal emersion and salinity decrease associated with continuous flow of fresh water.
Species richness in soft-bottom assemblages increased between 1971, 1976 and 1995. Substrates depopulated during the construction scheme were progressively colonised mainly from populations closest to the estuary (Retière, 1994). Introductions may be unsuccessful or initiate a colonisation succession, thereby inducing variability of faunistic composition at large spatial scales. As a consequence, 55 species disappeared and 90 new ones were recorded in the basin between 1976 and 1995.
Based on the 1976 and 1995 faunal data, FCA revealed seven and six major groups of stations in the Rance basin, referring to two sedimentary categories: 1) coarse to medium sands and 2) fine sediments. As shown by the results, assemblages differ more in abundances of species groups than in species compositions, because taxa are common from one assemblage to another. All assemblages correspond to well-defined communities established on similar biotopes in the southern part of the Normano-Breton Gulf.
Biological diversity and habitat heterogeneity
Changes in species richness in the marine and brackish areas may result from changes in habitat spatial heterogeneity (Levin, 1981; Barry and Dayton, 1991; Marquet et al., 1993; Levin, 1994), despite the fact that correlation between high biological diversity and high spatial heterogeneity is not totally established (Levin, 1994). In the Rance basin, changes in the habitat heterogeneity in the brackish and marine zones are distinct.
In the brackish part, the decrease in species number observed between 1976 (110) and 1995 (70) could have resulted from silting of the area and the ‘homogenisation’ of the associated habitats. Although such an environment may appear homogeneous at a large spatial scale, the existence of spatial structures (channels, burrows) may induce small scale spatial heterogeneity.
In contrast, the apparent diversification of habitat in the marine area corresponded to an increase in species richness (95 species in 1976 versus 171 in 1995). Due to the proliferation of Crepidula fornicata, substrates became progressively more heterogeneous. Slipper limpet colonies modify hydrodynamics in the boundary layer and produce faeces and pseudofaeces, favouring muddy particle deposits. As a consequence, the faunal assemblages where limpets proliferate were enriched with species characteristic of heterogeneous sediments, as reported by De Montaudoin et al. (1999). These observations corroborate results of De Montaudoin and Sauriau (1999), which showed that in muddy sediments, the presence of Crepidula fornicata apparently stimulates zoobenthic community diversity and abundance, whereas in coarser sediments, macrofauna communities are different (more suspension feeders) from the community associated with Crepidula.
Trends of benthic communities
The number of species recorded in 1971, after the initiation of the tidal power station, was low compared to the species richness observed in 1995. Although rudimentary, the RFD method remains well adapted to study changes in community structure. Such observations allow conclusions on the structural stability and relative maturity of the different communities between 1976 and 1995, especially those established on marine muddy fine sands where hierarchy of dominant species remained constant (Frontier, 1985; Frontier and Pichod-Viale, 1991).
According to the theory of ecosystem dynamics (Frontier and Pichod-Viale, 1991), communities evolve from a pioneer stage towards an ageing stage in the absence of major perturbation, whereas a stress induces a ‘rejuvenation’ of communities. Frontier and Pichod-Viale (1991) indicate that following the complexification of an ecosystem, the readjustment of species proportion is accompanied by a reduction in diversity. In the Rance basin, species richness may either have dropped after having reached a maximal value between 1976 and 1995 or still be increasing. If the reductions in abundances observed in 1995 are associated with a decrease in species richness, this could be a sign of ecosystem senescence, and the communities could be at an intermediate stage between mature and ageing. On the contrary, if species richness continues to increase, the trends would be reversed. A monthly sampling of the muddy fine sand community performed from February 1995 to February 1997 (Desroy, 1998) suggested that the apparent reduction in abundances could be a consequence of low water temperature (≈ 5°C) recorded during the winter 1994–1995 whereas temperature observed during the winter 1975–1976 was ≈ 8°C. Low temperatures, by inducing mortality of species, could be a perturbation responsible for a ‘rejuvenation’ of assemblages. Beukema (1979, 1982, 1989) showed that cold winters corresponded to reductions in abundances and species richness of the macrofauna living on intertidal mudflats of the Wadden Sea.
Despite this inter-annual variability of abundances, probably induced by a cold winter, community structure remained remarkably stable and species diversity was high. However, the stability of communities at this large temporal scale does not illustrate some drastic impacts of the power station's functioning, such as the irregular fluctuations in water levels observed in the 1980's (Le Mao, 1986). A slow evolution of biological systems can occur in relatively stable environments as the Rance basin where fluctuations of environmental conditions are regular and constant (i.e., moderate tide with reduced range variations). The basin's benthic communities are protected from swell effects and catastrophic events like storms. According to the ‘intermediate disturbance hypothesis’ of Connell (1979), high biological diversity should be maintained by recurrent moderate perturbations such as progressive drop of the ebb tide level or prolonging the duration of intermediate water levels. However, in a relatively protected physical environment such as the Rance basin, biological interactions may be more important to population and community regulation than in the open sea.
From biological studies to management perspectives
Responses of benthos to environmental variation have been shown to be a well-adapted tool for taking decisions on the management of the coastal environment. To understand environmental change in a managed area such the Rance basin, it is highly recommended that studies within the whole estuary (minimal spatial scale of investigation) be coupled with studies of adjacent open sea communities. Monitoring of benthic invertebrate populations, aiming to understand and predict temporal and spatial patterns of abundances, is critical for the assessment of the resilience of the communities to major disturbance events (Powers et al., 2002). Considering the short response time of the benthos to variations in the power station's functioning, sampling should be performed on a 5 year basis.
The marine environment of the Rance basin constitutes a significant economic resource supporting a number of human-based activities, such as tourism and recreation. In addition, the site supports important marine biological features that provide both economic and social benefits. Soft bottom substrates are important spawning and nursery grounds for commercially important fishes (sole, plaice, bass, mullet, herring, sardine; Le Mao (1986)) and of national importance for birds (such as Branta bernicla, Calidris alpina,Caradrius hiaticula, Pluvialis squatarola, Sterna hirundo, Tadorna tadorn). Our study emphasises that the new community balance is entirely dependent upon the mode of operation of the power installation. The surface occupied by intertidal or subtidal communities is a function of the mean low water level reached. Managers can favour intertidal or subtidal communities according to their ecological interests or local socio-economical wishes, by modifying water levels with appropriate rhythms. An increase in area occupied by subtidal mudflats will profit fish populations whereas extension of the intertidal mudflats will benefit migratory shorebird populations. Whatever the choice, variations in environmental factors (associated with production requirements) should be progressive, to prevent severe ecological effects at various levels in both space and time (Retière, 1994). Past experiences have showed that maintenance of excessive prolonged low water levels could be injurious to benthic invertebrates especially when they occur during periods of recruitment or during sunny days (Le Mao, 1986). Consequences on fishes may be direct (mortality by desiccation or predation by birds) or/and indirect (mortality of benthic prey species). Thus, responsible management and decision-making would balance the long-term sustainable exploitation of these marine resources with power generation. One of the crucial questions is whether specific actions can be taken to manage habitats and species diversity. For example, should clearing operations or sedimentary deposits be removed in the brackish area to enhance biodiversity? Should Crepidula fornicata population(s) be regulated as their presence at moderate densities increases biological diversity?
The challenge will be to improve interaction between ecological, management and conservation decisions, since they are associated with different economic values. Integrated coastal management requires a compromise to be reached between the necessary human uses and the protection of the environment. A zero level of anthropogenic perturbations is utopian and management plans must actually be defined in term of ‘acceptable threshold’ of perturbation.
This study forms part of the GDR ‘Manche’ contribution to the study of the variability of coastal systems. The authors thank ‘Electricité de France’ for their support, the crew of NO ‘Louis Fage’ for their valuable assistance in field work and K. Gherstos, M. Taimour-Jolly and M. Walkey for help in correcting the English text.