# Benthic community impacts from sediment dredging and disposal: A comparison of sampling gear

Xavier de Montaudouin, Hugues Blanchet, Benoit Gouillieux, Suzie Humbert,

Lise Latry, Lucas Crovetto, Nicolas Lavesque

► To cite this version:

Xavier de Montaudouin, Hugues Blanchet, Benoit Gouillieux, Suzie Humbert, Lise Latry, et al. Ben-
thic community impacts from sediment dredging and disposal: A comparison of sampling gear. Marine
Pollution Bulletin, 2023, 194, pp.115278. 10.1016/j.marpolbul.2023.115278. hal-04224614

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Distributed under a Creative Commons Attribution 4.0 International License
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Benthic community impacts from sediment dredging and disposal: a

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comparison of sampling gear

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Xavier de Montaudouin<sup>a,*</sup>, Hugues Blanchet<sup>a</sup>, Benoît Gouillieux<sup>a</sup>, Suzie Humbert<sup>a,b</sup>, Lise Latry<sup>a</sup>, Lucas

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Crovetto<sup>a,†</sup>, Nicolas Lavesque<sup>a</sup>

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<sup>a</sup> Univ. Bordeaux, CNRS, Bordeaux INP, EPOC, UMR 5805, Station Marine d'Arcachon, F-33600 Pessac,

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France

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<sup>b</sup> PATRINAT, OFB-MNHN-CNRS-IRD, F-75005 Paris, France

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*Corresponding Author: xavier.de-montaudouin@u-bordeaux.fr

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Keywords: Dredging; Disposal; Benthic recovery; Sampling gear; Sediments; Arcachon Bay

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<footnote n="1">¹ Present address : Centre Scientifique de Monaco, Département de Biologie Marine, 98000 Monaco</footnote>
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18 Abstract

19 The effect of sediment dredging and disposal work on benthic communities was compared according
to two sampling strategies, grab (1-mm mesh) and dredge (1-cm mesh). Nine subtidal, sandy sites
20 were selected in Arcachon Bay (French Atlantic coast), where these operations were performed
21 during winter. Fauna was sampled several months before and one year after the dredging or disposal
22 work. The sediments were fine to medium sands and their characteristics were not modified. In
23 dredging sites, abundance, diversity and community structure of grab sampled fauna were minimally
24 affected by the activities, while abundance and community structure displayed significant changes
25 for dredge sampled fauna. In disposal sites, there was no work effect on fauna, although
26 environmental conditions changed, especially when initial sediments were covered by mussels or
27 seagrass. This study suggests that dredge sampling can be an alternative to grab sampling for
28 monitoring changes related to dredging in sandy shallow channels.
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32 1. Introduction

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34 Dredging, trawling and disposal activities (also named “work” or “operations”, hereafter) are among
35 the most visible sources of stress sustained by coastal ecosystems because of the large-scale means
36 that are implemented to perform these works (Mazor et al., 2021). By contrast, the effects of these
37 activities on the environment are often difficult to detect, especially in high-energy sites (van Dalfsen
et al., 2000). In general, the capacity of an ecosystem to re-establish after work depends on the
benthic community types, the season in which the activity is carried out, its intensity, the life-cycles
of species, and local hydrological and sedimentological conditions (Sánchez-Moyano et al., 2004).
The target sites are often characterized by sandy bottoms (Bolam and Rees, 2003) which usually
harbour relatively poor benthic communities, displaying a high potential of (re)colonization (Guerra-
García and García-Gómez, 2006). Analyses of benthic communities are the most utilized tools to
compare the structure of these communities Before and After the work, and/or in Control and
Impacted sites (BACI monitoring (Clarke and Tully, 2014)). In soft-sediment, the most investigated
living group is the macrofauna defined as the part of the fauna retained by a sieve with a mesh size
of 500 or 1000 µm following authors (Bachelet et al., 2000; Buchanan, 1984). When analysing
parameters which characterize communities sampled during or right away after the work (< 1 mo.),
there is a general consensus to highlight a decrease of at least species richness and abundance (Cruz-
Motta and Collins, 2004; Powilleit et al., 2006; Vivan et al., 2009). On the other hand, many authors
also agree that the recolonization process by macrofauna is often complete after 2 years, in both
dredging (Guerra-García et al., 2003; Kenny and Rees, 1996; Sheridan, 2004; Vivan et al., 2009) and
disposal (van der Val et al., 2011; Wilber et al., 2008) conditions, although in some areas full recovery
was attained after more than 5 years (Boyd et al., 2005; Wan Hussin et al., 2012). These authors
often qualify this recovery as fast. However, contrasted results are reported among the multitude of
impact studies for the mid-temporal scale which would be between one month and two years after
the work. In this range, recovery can be total or null (see review by Bolam and Rees, 2003). Sediment
characteristics (e.g. grain-size median, silt and clay content, organic matter content, pollutants, etc.)
are major drivers of macrofauna community composition and structure (Blanchet et al., 2005; Newell
et al., 1998; Snelgrove and Butman, 1994; Thrush, 1991). Thus, changes in sediment composition
should have implications for resident and recolonizing fauna, resulting in the establishment of a
faunal community that differs from the assemblage present before the works (Cooper et al., 2011;
Somerfield et al., 1995; Zimmerman et al., 2003). However, a return to the initial sediment
composition may not always be a prerequisite for faunal recovery (Lamarque, 2022; Seiderer and Newell, 1999). Finally, the conclusion that the degree of impact of dredging/disposal activities on macrofaunal communities is largely site (i.e., environment) specific (Bolam and Rees, 2003; Gimenez et al., 2014; Ware et al., 2010) highlights the relevance of examining the particular case of the poorly studied faunal communities associated with subtidal sediments in high energetic systems. In these systems, species are used to physical stress and can either move or recolonize rapidly from adjacent unspoiled areas (Kenny and Rees, 1996; Robinson et al., 2005).

Species which will take the longer time to recover after operations should be long-lived species and/or rarer species which do not benefit from surrounding dense parental stock. For macrofauna, the difficulty to recover the initial community structure after the work was often due to the disappearance of the rare species (Sánchez-Moyano et al., 2004), which can also be linked to interannual variability in regional benthic recruitment rather than stressful conditions induced by the dredging or disposal operations. Also, we can expect that the fauna's fate will depend on the species' biological traits, existence and duration of planktonic larval stage, feeding mode, motility, position in/on the sediment (Guerra-García and García-Gómez, 2006; Hinchey et al., 2006; López-Jamar and Mejuto, 1988; Sánchez-Moyano et al., 2004). In the present study, we will compare the ratio of infauna/epifauna before and after the work, hypothesizing that unfixed epifaunal species in soft sediments have more opportunity to escape perturbation and after to recolonize from adjacent unspoiled areas (Robinson et al. 2005, Targusi et al. 2019). Finally, when closely looking at the different lists of macrofauna available in the articles describing the effects of marine operations on benthic macrofauna, we can also detect species that were caught by the 1-mm mesh of the sieve but which could have also been sampled with a 1-cm mesh (Hill et al., 1999; Sánchez-Moyano et al., 2004; Smith and Rule, 2001). Therefore, it seems more relevant to compare the structure of benthic communities according to the type of sampling gear (and mesh) rather than according to the size of individuals. Thus, our first objective was to clearly separate the structure of benthic community according to the type of sampling strategy: grab samples sieved with a 1-mm mesh vs. dredge
samples caught with a 1-cm mesh (Mazor et al., 2021). The second objective was to analyse, using
data from both sampling strategies, the effects of dredging activity vs. the effects of disposal activity
on benthic communities. Finally, concerning disposal operations and their impact on the benthos, we
compared sites with naked sediments with sites with biogenic structures (seagrass meadow and
mussel bed). The predicted immediate impact is the destruction of the initial habitat by suffocation
(Duarte et al., 1997; Reed and Hovel, 2006). But the question remains whether all benthic
communities will be evenly impacted ca. one year after the work.

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## 2. Material and methods

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### 2.1. Study sites

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Arcachon Bay is a 180-km² lagoon on the French northeastern Atlantic coast (Figure 1). At low tide, the largest part (120 km²) is exposed, with *Zostera noltei* seagrass beds and oyster parks occupying 46 and 7 km², respectively (Plus et al., 2010; Scourzic et al., 2011). The remaining intertidal area is composed of muddy flats, sand beaches and banks. The lagoon is connected to the Atlantic Ocean by narrow channels at its southwestern entrance through which important volumes of water are exchanged at each tide, ranging from 370-400 x 10⁶ m³ for spring tide to 130-200 x 10⁶ m³ for neap tide (Bouchet, 1968). Inside the bay, the main channel splits into two channels that subdivide repeatedly towards the North-East with decreasing depth. The tidal channels extend over 60 km², of which 1 km² is covered by *Zostera marina* beds (Plus et al., 2010), while most of the bottoms are unvegetated and consist in gravel to sandy mud. The tidal amplitude ranges from 0.9 to 4.9 m, according to coefficient (spring vs. neap tide) and location in the bay (Gassiat, 1989). The maximum
water depth is 20 m (except very limited “holes” approaching 30 m) but most channels are 5-10 m
deep (SIBA, 2020).

Mean sea temperature annually fluctuates between 6-8°C and 21-22°C (Robert et al., 1987).
Freshwater inputs, mainly from the Eyre River in the South-East side of the lagoon, but also from
ground waters, represent 2.5-3.7 x 10⁶ m³ d⁻¹ (Auby, 1991). The salinity ranges from 32-35, close to
the lagoon entrance, to 15-20 in the northern and eastern parts of the Arcachon bay (Robert et al.,
1987).

Between 2003 and 2005, many subtidal areas underwent dredging work, mainly to calibrate
navigation channels. Sediments were not polluted and were deposited in deeper areas of the lagoon.
Grab and dredge sampling were always performed in February, usually one or two years before the
work and ca. one year after the work. Initially, there was no overall sampling strategy because
stakeholders and dredging workers often modified their plans. However, we always followed the
same sampling protocol and thus achieved a rather homogeneous dataset, although we sometimes
needed to adapt and change the number of replicates as well as the sampled surface.

2.2 *Sampling*

Nine different subtidal sites representing different situations in terms of work (dredging, disposal) and substrates (presence of biogenic structure or not) were selected (Figure 1). In each site, different stations were systematically sampled, many of them being replicated (Tables 1 and 2). The sampling took place in average 22 months before the work but with a high variability (4 to 40 months). Sampling was reiterated in average 11 months after the work, with a narrower temporal range among campaigns (8 to 15 months). These discrepancies were due to heavy operational constraints.
Firstly, fauna and sediment samples were collected with an Eckman grab (0.0225 m²). However, when sediments were apparently very poor in fauna, two grabs in the same station were pooled as a unique replicate (0.0450 m²), before and after the work (Table 1). Each sample was gently washed with a 1-mm mesh sieve, fixed in 4% formaldehyde and stained with Rose bengal. Individuals were sorted and identified at the lowest taxonomic level possible (Suppl. 1). A supplementary sample was taken in order to characterise sediments. Median sediment grain-size was determined after sieving pre-weighted dried sediment through a wet column of sieves with decreasing aperture (1000 µm, 500 µm, 250 µm, 125 µm and 63 µm). Percentage of silt and clay (particles < 63 µm) was calculated as the difference of dry sediment weight before and after sieving. Percentage of organic matter in the sediment was assessed after ignition (450°C, 4 h) of a dried aliquot of sediment (Buchanan, 1984).

Secondly, fauna was sampled again in the same locations than those for the grab sampling but using a dredge with a 1 m width opening, a 20 cm height and a 1 cm mesh net. Collected individuals were identified and counted (Suppl. 1). Each sampling length was controlled with a GPS and averaged around 133 m (Table 2).

Median faunal abundance was calculated per site, before and after the work. For statistical comparison and due to sampling size discrepancy (i.e., large range of sampled surfaces between sites), the diversity of fauna sampled with the grab was calculated as the E(Sn), which is the expected number of species in a sample with > n individuals. According to the number of individuals in our samples, E(S10), E(S25) or E(S50) was calculated. When n<10 individuals in a station, the station was discarded in the statistical analysis. E(S10) per site were correlated to total species richness per site (Spearman test, r=0.68, p=0.002). Thus, in order to visualize diversity among sites, total species richness values per site were also provided (Figure 2A). Concerning the diversity that was sampled with the dredge, the species richness was retained due to a very small number of collected
individuals, especially after the work, precluding E(Sn) methodology. However, this species richness
was weighted according to the sampled surface (species richness/sampled area in m²).

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164 2.3. *Data analysis*

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166 In this study the word “period” was dedicated to the two following situations: before and after the
work.
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168 For each type of work (dredging or disposal), a Wilcoxon pairwise test was used in order to compare
169 values before and after the work, in terms of abundance, diversity E(Sn) (or species richness for
170 dredge samples), infauna ratio (=infauna abundance/total fauna), grain-size median, silt and clay and
171 organic matter content in the sediment. Within a site, variability among stations was not tested in
172 order to compare the two periods with a maximum of replicates and because of a lack of replicates
173 per station at few occasions (Tables 1 and 2). Bonferroni correction was applied due to multiple
174 pairwise comparisons (N=18). Thus the p-value from each test was compared at a significance level of
175 0.0028.
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Dredge sampled fauna and grab sampled fauna were analysed separately. Original data consisted in
"stations x taxa" matrix. Abundance data were square-root transformed before computing the
dissimilarity coefficient. Following Clarke and Warwick (2001), a zero-adjusted Bray-Curtis
dissimilarity coefficient between samples was computed in order to cope with the scarcity of fauna in
many samples. This procedure consisted in adding a « dummy species » with abundance equal to 1
before computing the Bray-Curtis coefficient. As a result, two samples devoid of fauna would be
considered as identical (dissimilarity=0) while these two samples would be considered as completely
different (dissimilarity=1) by classical Bray-Curtis coefficient because they share no species in
common. Dissimilarity matrices were computed considering either grab sampled fauna or dredge
sampled fauna. The ability of each sampling method (dredge or grab) to detect changes in benthic
communities following dredging or disposal operations was then tested using two-ways PERMANOVA
(Anderson et al., 2008) using BEFORE-AFTER as fixed factor and SITE as random factor.

## 3. Results

Five shallow sites were considered, the surface of which was affected by the work ranging between 20 ha (CLV) and 100 ha (BLV) (Figure 1). Before and after the work, all stations were naturally covered with medium sands, with no significant difference of the median grain-size, silt and clay content in the sediment, nor organic matter content in the sediment between the two periods (Table 3).

Grab sampled communities before the work displayed rather poor species richness (Figure 2A) and the diversity ($E(S_n)$) remained similar before and after the work (Table 3). The abundance varied between 100 and 578 ind.m⁻² (Figure 2B), Table 3) with no difference between the two periods (Table 3). The fauna sampled before the work was dominated by infauna (77% of total fauna), which didn't significantly change after the work (87%) (Figure 2C, Table 3). Moreover, the PERMANOVA showed that the structure of the benthic communities differed between sites (Table 4), yet benthic fauna community structure remained similar before and after the work (Table 4). Three taxa were common to all sites and periods, two peracarid crustaceans (*Bathyporeia* spp., *Urothoe* spp.) and *Nemertea*, representing 15% of all collected individuals (Suppl. 1). Per site, 22 to 36% (mean = 28%) of species
richness was shared before and after the work, corresponding to 24 to 68% (mean = 51%) of
collected individuals.

Dredge sampled communities before the work also displayed poor species richness (Figure 3A),
which remained similar after the work (Table 3). However, abundance decreased significantly
between before (0.11 ind.m⁻²) and after (0.02 ind.m⁻²) the operations (Figure 3B, Table 3). The
percentage of infauna (88%) was not significantly different between the two periods (88% before vs.
75% after) (Table 3). The PERMANOVA showed that the structure of the dredge sampled benthic
communities differed between sites (Table 4). Only the bivalve *Mactra glauca* was found common to
all sites, although it disappeared in the site GRA after the work (Suppl. 1). The community structure
was also significantly different between the periods (Table 4). Per site, 19 to 38% (mean = 29%) of
species were the same before and after the work, corresponding to 23 to 63% (mean = 44%) of
collected individuals (Suppl. 1).

For grab and dredge sampled communities, a significant interaction between the period (before vs.
after) and the site factors was observed (Table 4).

## 3.2. Sediment disposal stations

In this section, mussels from the mussel bed (station MPY) were discarded for the calculation of
abundances and for the community structure analysis (see Supplementary data for abundance
values). Before and after the work, all stations were naturally covered with fine to medium sands,
with no significant changes in median grain-size, silt and clay content, and organic matter content in
the sediment (Table 3).

Grab sampled displayed rather poor species richness (Figure 2A), the diversity ($E(S_n)$) remained
similar (Table 3) and the abundance did not vary between before and after the work (Figure 2B, Table
3). The infauna/epifauna ratio was balanced (55%/45%). After the work, the increase of infauna (90%
of the total fauna) was not significant (Figure 2C, Table 3). The PERMANOVA showed that the structure of the benthic communities differed between sites (Table 4), but not between periods (Table 4). One taxon was common to all sites and periods, Nemertea, but represented only 1% of all collected individuals (Suppl. 1).

Dredge sampled community descriptors also displayed low values with, before the work (Figures 3A,B). Species richness (per m² and per site) and abundance were similar before and after the work (Table 3). The work didn't affect the faunal assemblage sampled with the dredge, which was dominated by epifauna (99%/94% of total abundance (Figure 3C, Table 3). The PERMANOVA showed that the structure of the benthic communities differed between sites (Table 4). Conversely, benthic fauna community structure was similar before and after the work (Table 4).

As for naked sand dredged stations, a significant interaction between the period (before vs. after) and the site factors was observed for grab sampled communities, while there was no interaction for dredge sampled communities (Table 4).

A focus was then performed to analyse benthic communities in sites with and in sites without biogenic structure on the sediment, with no statistical comparison due to low replication.

### 3.2.1. Sites with biogenic structure (seagrass and mussel bed)

Before the work, MAT was characterized by the presence of a *Zostera marina* seagrass bed. However, the seagrass bed was completely destroyed. The abundance of the macrofauna decreased from 6022 ind.m⁻² before the work to 1500 ind.m⁻² after the work, and the species richness from 41.5 to 15 species (Figure 2). Out of a total of 80 species, 33 were sampled in both periods, 34 only before and 13 only after the operations (Suppl. 1). Out of 35 species, seven were sampled in both periods, 22 only before and six only after the work (Suppl. 1).

Before the work, MPY was characterized by the presence of a mussel (*Mytilus edulis*) bed. After the work, the mussel bed had disappeared (Suppl. 1). For grab sampled fauna, out of 49 species, six were
sampled in both periods, 19 only before work and 24 only after the work (Suppl. 1). In this mussel bed, for dredged sampled fauna, six out of 13 species were sampled in both periods, four only before and three only after the work (Suppl. 1).

### 3.2.2. Sites without biogenic structure ("naked sands")

Two shallow sites were considered, with a surface affected by disposal work ranging between 6 ha (PAC) and 11 ha (PYL).

The results concerning grab sampled communities were contrasted between the sites PAC and PYL. In PAC (fine sands), the initial community was abundant and diverse (Figure 2). After the work, the diversity was similar, but the abundance was divided by 2.5 (Figure 2). Three polychaete species accounting for 62 % of the total abundance before the work, became rare or absent (5 %) after the work (Suppl. 1). In PYL (medium sands), the grab sampled community was characterized by low abundance and species richness, both before and after the disposal work (Figure 2).

For dredged sampled fauna in PAC, only two species (*Cerastoderma edule* and *Tritia reticulata*) out of 14 were still collected after the work with no new species (Suppl. 1). In PYL, the number of collected individuals remained similar and two (*Echinocardium cordatum* and *Mactra glauca*) out of the four species collected after the disposal work had also been collected before (Suppl. 1).

Finally, the variability in the recovery processes between sites was also observed in our results through the significant interaction terms of the PERMANOVAs (except for dredge sampled communities in deposit stations). These interactions highlighted the different trajectories of benthic assemblages according to the site and revealed by different impacts on abundance, diversity, infauna proportion (Figures 2 and 3) and community structure (Table 4, Suppl. 1).

## 4. Discussion
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This multi-site study revealed contrasted results concerning the fate of benthic communities based on grab sampled fauna sieved with a 1-mm mesh vs. dredged sampled fauna using a 1-cm mesh net, about one year after the work, with a noticeable difference according to the type of work (dredging vs. disposal). Concerning grab samples, after dredging work, sediment characteristics were similar and fauna abundance and diversity were recovered. Also, fauna community structure (PERMANOVA) did not highlight discrimination between "before" and "after" dredging work, infauna remaining dominant. These results are consistent with those of many other studies and reviews, performed in such sandy and unstable environments (Newell et al., 1998). Indeed, recolonization generally allows a global recovery in less than one year (Kenny and Rees, 1996; Newell et al., 1998; Sánchez-Moyano et al., 2004), except when bottoms are repeatedly and intensively perturbed (Boyd et al., 2005; Dernie et al., 2003; Paradis et al., 2021). The macrofauna recovery is most of the time related to opportunistic and dominant species which can quickly respond to stressful conditions by their high reproduction rate and/or high dispersal ability allowing them to recolonize from adjacent undisturbed zones (Robinson et al., 2005), considered as "refuge areas" (van Dalfsen et al., 2000).

Besides, the sensitivity of macrofaunal community decreases when the level of natural physical disturbance increases (Cooper et al., 2011), strong hydrodynamic conditions being the characteristic of the presently investigated areas (Plus et al., 2009). Of course, the use of the term "recovery" implies a return to preplacement ecological conditions that are neither a reality nor a practical expectation for areas that are subject to disturbances (Szymelfenig et al., 2006; Wilber et al., 2008).

In particular, biomass or total somatic production, which were not taken into account in this study, are often more affected than the other univariate parameters (species richness and abundance) (Cooper et al., 2011; Kenny and Rees, 1996). To summarize, in these conditions of sediment (fine/medium sands) and activity (punctual dredging), and at this temporal scale (ca. one year), macrofauna communities sampled through a 1-mm mesh can fail in detecting physical stressful events.
By contrast, dredge-sampled communities were significantly affected by these dredging activities, with a lower abundance after one year, and a significant effect on community structure (PERMANOVA). This fauna is generally composed of large long-lived species, which will take more time to recolonize. In the five dredged sites of Arcachon Bay, molluscans were the dominant species before the work (Suppl. 1). In particular, large bivalve species like *Mactra glauca*, *Cerastoderma edule* and *Donax vittatus* were strongly and durably affected by the work. This resulted in a lower proportion of infaunal species, although not significant (p=0.0166). Thus, benthic dredge sampled fauna analysis allowed to clearly discriminate “before” dredging situations from “after” dredging situations.

Concerning disposal activity in high energy sites, the duration of recovery by benthic macrofauna (sampled with a grab and washed on a 1-mm sieve) is highly fluctuating following studies. Results range from “no significant change” due to the fact that there was a fast dispersal of sediments by marine currents (Smith and Rule, 2001; van der Val et al., 2011; Vivan et al., 2009), to moderate impact vanishing after a few months (Cruz-Motta and Collins, 2004; Wilber et al., 2008), or to long-term disturbance (Harvey et al., 1998; Menn et al., 2003). This variability was also observed in our four disposal sites, with no significant effect after one year. However, our disposal sites highlighted differences in terms of habitat and associated communities. The variability of response is related to several factors. Firstly, disposal of sediment implies accumulation of sediments which could (or not) display different characteristics than the underlying ones, and we know that granulometry can directly affect any species which has sediment type requirements (Snelgrove and Butman, 1994; Somerfield et al., 1995). Secondly, disposed sediment may move with a more or less return to initial conditions (van der Val et al., 2011). Lastly, there is variability in the capacity of organisms to resist to burial (Hinchey et al., 2006). In the Baltic Sea, three groups of species were distinguished: the ones which survive to the burial, the ones which escape or immigrate to unaffected areas and the sensitive taxa which collapse (Powilleit et al., 2006). Unless the resident organisms are mobile enough to burrow through the sand, they will be suffocated and will die after the disposal (Bolam,
2011; Targusi et al., 2019). By contrast, in our study, fauna was not significantly impacted one year after the sediment disposal, whatever the sampling gear. A slight but non-significant (p=0.0281) increase of infauna after disposal was observed, related to a decrease of epifauna which could be less resistant to burial than infauna (Bolam, 2011; Harvey et al., 1998; Hinchey et al. 2006).

Sediment disposal can have even more detrimental effects when target bottoms are initially covered by biogenic structures like, in our case, a mussel bed or a seagrass meadow. These habitats usually harbour a high diversity (Blanchet et al., 2004; Bouma et al., 2009; Duffy, 2006). The immediate effect of sediment disposal is the loss of this substrate heterogeneity, although, for seagrass, the critical threshold for turbidity and sedimentation varies greatly among species (Erftemeijer and Lewis III, 2006). The expected effect is a modification of the structure of macrofauna benthic communities, characterized by an impoverishment of the diversity and a decrease of the abundance and biomass (Duarte et al., 1997; McLaverty et al., 2020; Reed and Hovel, 2006). Although, this couldn't be statistically verified due to low replication, this was also observed in MAT (initially a seagrass site, loss of 78% of the abundance and of 64% of the species richness) but not in MPY (initially a mussel bed site). In situ experimental studies showed that many infaunal species in subtidal position can be more influenced by organic matter in the sediment than by the overlying substrate heterogeneity (Cottet et al., 2007; Edgar, 1999), which could be the case in MPY.

## CONCLUSION

All studies dealing with the impact of dredging and/or disposal on benthic communities insist on the importance and need for long-term data sets in order to better understand the difference between natural and human-induced variability in benthic assemblages (Barrio Froján et al., 2008). It is difficult to draw any general conclusions concerning the impact of these operations on the benthic communities or recovery rates (Bolam and Rees, 2003), due to a great variability of situations and
results (Bolam et al., 2006a). In the framework of short-term assessment (ca. one year) in sandy, shallow subtidal areas, dredge sampling seems a good (or at least complementary) alternative to grab sampling in order to detect environmental effects of dredging/disposal activities. Another advantage of dredge sampling is that the methodology is simpler in terms of sorting and taxa identification, at least in the type of habitat that was studied here. However, this technique remains invasive and sometimes imprecise in terms of quantification (i.e. surface assessment). Concerning large epifauna, an underwater camera could be a good conservative technique (Roegner et al., 2021; Yu et al., 2020), but this requires clear waters which is rarely the case in the areas subjected to dredging (estuaries, harbours, ...).

Acknowledgements

Authors are grateful to all colleagues and students working in the "biodiversity platform" of UMR EPOC, at the marine station in Arcachon. Sampling was performed with the help of the staff and ships Planula III and IV fitted out by University Bordeaux 1 and CNRS/INSU (Flotte Océanographique Française), respectively. Many thanks to the three reviewers who helped us in improving this paper. Thank you to Leslie Stout for reading and correcting the last version.

Funding

These studies were partly financed by Syndicat Intercommunal du Bassin d'Arcachon (SIBA), F-33120 Arcachon.

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554
Table 1

List of sites where benthic fauna was sampled with a grab, mentioning the presence or absence of biogenic structures (Biog., seagrass or mussel bed), the kind of work (DR: dredging; DI: disposal) with the involved area between brackets (ha), the mean depth (± 1 SD), the sampling period compared to the activity date (Before/After), the number of months before/after the activity, the date of the activity, the number of stations per site/replicates per station, and the grab surface (m²)

<table><thead><tr><th>Sites</th><th>Biog.</th><th>Work (Area in ha)</th><th>Depth in m (SD)</th><th>Period</th><th>N mo.</th><th>Date W</th><th>Stations/Repl.</th><th>Grab surf.</th></tr></thead><tbody><tr><td>GRA</td><td>No</td><td>DR (44)</td><td>3.7 (0.8)</td><td>Be</td><td>16</td><td>Feb-03</td><td>2/2</td><td>0.0450</td></tr><tr><td>GRA</td><td>No</td><td>DR (44)</td><td>4.0 (0.8)</td><td>Af</td><td>8</td><td>Feb-03</td><td>2/2</td><td>0.0450</td></tr><tr><td>JAB</td><td>No</td><td>DR (44)</td><td>5.0 (1.7)</td><td>Be</td><td>40</td><td>Feb-05</td><td>4/2</td><td>0.0450</td></tr><tr><td>JAB</td><td>No</td><td>DR (44)</td><td>6.3 (0.3)</td><td>Af</td><td>9</td><td>Feb-05</td><td>4/4</td><td>0.0225</td></tr><tr><td>BLV</td><td>No</td><td>DR (100)</td><td>4.8 (3.3)</td><td>Be</td><td>28</td><td>Feb-04</td><td>5/2</td><td>0.0450</td></tr><tr><td>BLV</td><td>No</td><td>DR (100)</td><td>4.2 (2.3)</td><td>Af</td><td>8</td><td>Feb-04</td><td>5/2</td><td>0.0450</td></tr><tr><td>CLV</td><td>No</td><td>DR (20)</td><td>0.1 (0.2)</td><td>Be</td><td>16</td><td>Feb-03</td><td>3/2</td><td>0.0450</td></tr><tr><td>CLV</td><td>No</td><td>DR (20)</td><td>1.1 (0.8)</td><td>Af</td><td>9</td><td>Feb-03</td><td>3/2</td><td>0.0450</td></tr><tr><td>BEE</td><td>No</td><td>DR (82)</td><td>5.7 (0.8)</td><td>Be</td><td>22</td><td>Feb-03</td><td>5/2</td><td>0.0225</td></tr><tr><td>BEE</td><td>No</td><td>DR (82)</td><td>5.2 (3.1)</td><td>Af</td><td>15</td><td>Feb-03</td><td>5/2</td><td>0.0450</td></tr><tr><td>PAC</td><td>No</td><td>DI (6)</td><td>8.5 (1.2)</td><td>Be</td><td>16</td><td>Feb-03</td><td>2/2</td><td>0.0450</td></tr><tr><td>PAC</td><td>No</td><td>DI (6)</td><td>7.1 (0.2)</td><td>Af</td><td>8</td><td>Feb-03</td><td>2/2</td><td>0.0450</td></tr><tr><td>MAT</td><td>Seagrass</td><td>DI (0.8)</td><td>2.0 (0.0)</td><td>Be</td><td>4</td><td>Feb-05</td><td>2/2</td><td>0.0450</td></tr><tr><td>MAT</td><td>No</td><td>DI (0.8)</td><td>0.1 (0.1)</td><td>Af</td><td>9</td><td>Feb-05</td><td>4/2</td><td>0.0450</td></tr><tr><td>MPY</td><td>Mussel</td><td>DI (5.4)</td><td>7.2 (2.9)</td><td>Be</td><td>27</td><td>Feb-03</td><td>3/1</td><td>0.0450</td></tr><tr><td>MPY</td><td>No</td><td>DI (5.4)</td><td>9.1 (3.2)</td><td>Af</td><td>15</td><td>Feb-03</td><td>3/1</td><td>0.0450</td></tr><tr><td>PYL</td><td>No</td><td>DI (11)</td><td>9.8 (0.6)</td><td>Be</td><td>27</td><td>Feb-03</td><td>3/1</td><td>0.0450</td></tr><tr><td>PYL</td><td>No</td><td>DI (11)</td><td>5.5 (0.6)</td><td>Af</td><td>15</td><td>Feb-03</td><td>6/2</td><td>0.0450</td></tr></tbody></table>

561

562
<table><caption>Table 2</caption><caption>List of sites where benthic fauna was sampled with a dredge, mentioning the presence or absence of biogenic structures (Biog., seagrass or mussel bed), the kind of work (DR: dredging; DI: disposal), the sampling period compared to the activity date (Before (Be)/After (Af)), the number of stations per site/replicates per station, the surface of each replicate (m²) and the surface of the sum of all replicates (m²). Depth and dates were the same than those reported in Table 1</caption><thead><tr><th>Sites</th><th>Biog.</th><th>Work</th><th>Period</th><th>Stations/Repl.</th><th>Area (mean)</th><th>Area (cumulative)</th></tr></thead><tbody><tr><td>GRA</td><td>No</td><td>DR</td><td>Be</td><td>2/2</td><td>116</td><td>465</td></tr><tr><td>GRA</td><td>No</td><td>DR</td><td>Af</td><td>2/2</td><td>108</td><td>433</td></tr><tr><td>JAB</td><td>No</td><td>DR</td><td>Be</td><td>4/2</td><td>84</td><td>670</td></tr><tr><td>JAB</td><td>No</td><td>DR</td><td>Af</td><td>4/2</td><td>108</td><td>861</td></tr><tr><td>BLV</td><td>No</td><td>DR</td><td>Be</td><td>5/2</td><td>105</td><td>1051</td></tr><tr><td>BLV</td><td>No</td><td>DR</td><td>Af</td><td>5/2</td><td>188</td><td>1880</td></tr><tr><td>CLV</td><td>No</td><td>DR</td><td>Be</td><td>3/2</td><td>103</td><td>616</td></tr><tr><td>CLV</td><td>No</td><td>DR</td><td>Af</td><td>3/2</td><td>116</td><td>693</td></tr><tr><td>BEE</td><td>No</td><td>DR</td><td>Be</td><td>6/1</td><td>123</td><td>736</td></tr><tr><td>BEE</td><td>No</td><td>DR</td><td>Af</td><td>4/1</td><td>205</td><td>819</td></tr><tr><td>PAC</td><td>No</td><td>DI</td><td>Be</td><td>2/2</td><td>138</td><td>550</td></tr><tr><td>PAC</td><td>No</td><td>DI</td><td>Af</td><td>2/2</td><td>138</td><td>552</td></tr><tr><td>MAT</td><td>Seagrass</td><td>DI</td><td>Be</td><td>2/1</td><td>219</td><td>438</td></tr><tr><td>MAT</td><td>No</td><td>DI</td><td>Af</td><td>2/1</td><td>217</td><td>434</td></tr><tr><td>MPY</td><td>Mussel</td><td>DI</td><td>Be</td><td>6/1</td><td>143</td><td>859</td></tr><tr><td>MPY</td><td>No</td><td>DI</td><td>Af</td><td>6/1</td><td>112</td><td>669</td></tr><tr><td>PYL</td><td>No</td><td>DI</td><td>Be</td><td>3/1</td><td>71</td><td>213</td></tr><tr><td>PYL</td><td>No</td><td>DI</td><td>Af</td><td>3/1</td><td>104</td><td>312</td></tr></tbody></table>

<table><thead><tr><th>Sites</th><th>Biog.</th><th>Work</th><th>Period</th><th>Stations/Repl.</th><th>Area (mean)</th><th>Area (cumulative)</th></tr></thead><tbody><tr><td>GRA</td><td>No</td><td>DR</td><td>Be</td><td>2/2</td><td>116</td><td>465</td></tr><tr><td>GRA</td><td>No</td><td>DR</td><td>Af</td><td>2/2</td><td>108</td><td>433</td></tr><tr><td>JAB</td><td>No</td><td>DR</td><td>Be</td><td>4/2</td><td>84</td><td>670</td></tr><tr><td>JAB</td><td>No</td><td>DR</td><td>Af</td><td>4/2</td><td>108</td><td>861</td></tr><tr><td>BLV</td><td>No</td><td>DR</td><td>Be</td><td>5/2</td><td>105</td><td>1051</td></tr><tr><td>BLV</td><td>No</td><td>DR</td><td>Af</td><td>5/2</td><td>188</td><td>1880</td></tr><tr><td>CLV</td><td>No</td><td>DR</td><td>Be</td><td>3/2</td><td>103</td><td>616</td></tr><tr><td>CLV</td><td>No</td><td>DR</td><td>Af</td><td>3/2</td><td>116</td><td>693</td></tr><tr><td>BEE</td><td>No</td><td>DR</td><td>Be</td><td>6/1</td><td>123</td><td>736</td></tr><tr><td>BEE</td><td>No</td><td>DR</td><td>Af</td><td>4/1</td><td>205</td><td>819</td></tr><tr><td>PAC</td><td>No</td><td>DI</td><td>Be</td><td>2/2</td><td>138</td><td>550</td></tr><tr><td>PAC</td><td>No</td><td>DI</td><td>Af</td><td>2/2</td><td>138</td><td>552</td></tr><tr><td>MAT</td><td>Seagrass</td><td>DI</td><td>Be</td><td>2/1</td><td>219</td><td>438</td></tr><tr><td>MAT</td><td>No</td><td>DI</td><td>Af</td><td>2/1</td><td>217</td><td>434</td></tr><tr><td>MPY</td><td>Mussel</td><td>DI</td><td>Be</td><td>6/1</td><td>143</td><td>859</td></tr><tr><td>MPY</td><td>No</td><td>DI</td><td>Af</td><td>6/1</td><td>112</td><td>669</td></tr><tr><td>PYL</td><td>No</td><td>DI</td><td>Be</td><td>3/1</td><td>71</td><td>213</td></tr><tr><td>PYL</td><td>No</td><td>DI</td><td>Af</td><td>3/1</td><td>104</td><td>312</td></tr></tbody></table>
573 Comparison of sediment grain size (median), silt and clay content (% of sediment dry weight), organic matter content (% of sediment dry weight), total abundance (median abundance in ind.m⁻²), infauna proportion (infauna abundance/total fauna abundance), E(Sn) (for grab samples) or species richness (dredge samples: number of species/sampled surface (m²)) according to the type of work and the type of sampling gear. For all variables, median (min-max) is mentioned, before and after the work. 574 575 576 577 E(Sn) is the expected number of species in a sample of >n individuals (see Material and methods). N is 578 579 580 (Bonferroni correction). Significant results (p-value<0.0028) are in bold.

<table><thead><tr><th>Work</th><th>Gear</th><th>Variable</th><th>Before / After</th><th>N</th><th>Z</th><th>p-value</th></tr></thead><tbody><tr><td>DR</td><td>grab</td><td>Median grain size (µm)</td><td>350 (238-353) / 344 (306-348)</td><td>19</td><td>0.28</td><td>0.7764</td></tr><tr><td>DR</td><td>grab</td><td>Silt and clay (%)</td><td>2.26 (1.55-2.60) / 2.65 (1.83-3.63)</td><td>19</td><td>0.93</td><td>0.3547</td></tr><tr><td>DR</td><td>grab</td><td>Organic matter (%)</td><td>0.44 (0.39-0.71) / (0.42 (0.01-0.60))</td><td>14</td><td>0.41</td><td>0.6832</td></tr><tr><td>DR</td><td>grab</td><td>Abundance (ind.m⁻²)</td><td>178 (100-578) / 178 (133-489)</td><td>19</td><td>0.28</td><td>0.5461</td></tr><tr><td>DR</td><td>grab</td><td>Infauna proportion</td><td>0.75 (0.60-0.84) / 0.88 (0.53-0.94)</td><td>18</td><td>0.64</td><td>0.5180</td></tr><tr><td>DR</td><td>grab</td><td>E(Sn)</td><td>nd</td><td>14</td><td>0.72</td><td>0.4703</td></tr><tr><td>DR</td><td>dredge</td><td>Abundance (ind.m⁻²)</td><td>0.085 (0.071-0.132) / 0.015 (0.005-0.021)</td><td>18</td><td>3.68</td><td>0.0002</td></tr><tr><td>DR</td><td>dredge</td><td>Infauna proportion</td><td>0.75 (0.60-0.84) / 0.44 (0.11-0.63)</td><td>18</td><td>2.40</td><td>0.0166</td></tr><tr><td>DR</td><td>dredge</td><td>Species richness per m²</td><td>0.026 (0.014-0.033) / 0.010 (0.006-0.025)</td><td>18</td><td>2.72</td><td>0.0065</td></tr><tr><td>DI</td><td>grab</td><td>Median grain size (µm)</td><td>225 (183-330) / 296 (213-333)</td><td>7</td><td>0.31</td><td>0.7532</td></tr><tr><td>DI</td><td>grab</td><td>Silt and clay (%)</td><td>4.20 (3.37-5.02) / 4.45 (3.31-8.33)</td><td>4</td><td>1.10</td><td>0.2733</td></tr><tr><td>DI</td><td>grab</td><td>Organic matter (%)</td><td>1.14 (0.82-1.45) / 0.35 (0.02-1.34)</td><td>4</td><td>0.53</td><td>0.5930</td></tr><tr><td>DI</td><td>grab</td><td>Abundance (ind.m⁻²)*</td><td>1100 (178-6022) / 439 (178-1400)</td><td>10</td><td>2.19</td><td>0.0284</td></tr><tr><td>DI</td><td>grab</td><td>Infauna proportion*</td><td>0.55 (0.22-0.93) / 0.90 (0.84-0.96)</td><td>10</td><td>2.29</td><td>0.0281</td></tr><tr><td>DI</td><td>grab</td><td>E(Sn)</td><td>nd</td><td>7</td><td>0.33</td><td>0.7353</td></tr><tr><td>DI</td><td>dredge</td><td>Abundance (ind.m⁻²)**</td><td>0.150 (0.059-0.862) / 0.021 (0.009-0.343)</td><td>13</td><td>2.13</td><td>0.0330</td></tr><tr><td>DI</td><td>dredge</td><td>Infauna proportion**</td><td>0.16 (0.01-0.32) / 0.13 (0.00-0.50)</td><td>12</td><td>1.36</td><td>0.1730</td></tr><tr><td>DI</td><td>dredge</td><td>Species richness per m²</td><td>0.030 (0.029-0.075) / 0.025 (0.005-0.041)</td><td>13</td><td>1.15</td><td>0.2489</td></tr></tbody></table>

581 * Without mussels in the mussel bed station, MPY (mussel density in MPY: median = 1956 ind.m⁻² Before, 0 after)

582 ** Without mussels in the mussel bed station, MPY (mussel density in MPY : median = 982 ind.m⁻² Before, 0.008 ind.m⁻² after)

583 584 nd: not determined because the Wilcoxon paired test was performed with E(Sn) based on n=10 or 25 or 50, following the number of individuals per station.

587

588
589

590

<caption>Table 4</caption>

591

592

593

594

595

Results of the PERMANOVA comparing the structure of benthic communities according to the period (before or after the work) and to the different sites, for the two types of work (dredging (DR) or disposal (DI)) and the two types of sampling gear (grab with 1-mm mesh sieve or dredge with 1-cm mesh net). df is the degree of freedom. Significant results (p-value<0.05) are in bold. Mussels were discarded from MPY site.

596

<table><thead><tr><th>Work</th><th>Gear</th><th>Source</th><th>df</th><th>Pseudo-F</th><th>p-value</th></tr></thead><tbody><tr><td rowspan="4">DR</td><td rowspan="4">grab</td><td>Before/After (a)</td><td>1</td><td>1.61</td><td>0.188</td></tr><tr><td>Site (b)</td><td>4</td><td>3.02</td><td>0.001</td></tr><tr><td>(a) x (b)</td><td>4</td><td>2.14</td><td>0.001</td></tr><tr><td>Residual</td><td>28</td><td></td><td></td></tr><tr><td rowspan="4">DR</td><td rowspan="4">dredge</td><td>Before/After (a)</td><td>1</td><td>5.47</td><td>0.015</td></tr><tr><td>Site (b)</td><td>4</td><td>1.82</td><td>0.007</td></tr><tr><td>(a) x (b)</td><td>4</td><td>1.54</td><td>0.019</td></tr><tr><td>Residual</td><td>28</td><td></td><td></td></tr><tr><td rowspan="4">DI</td><td rowspan="4">grab</td><td>Before/After (a)</td><td>1</td><td>1.48</td><td>0.209</td></tr><tr><td>Site (b)</td><td>3</td><td>3.08</td><td>0.001</td></tr><tr><td>(a) x (b)</td><td>3</td><td>1.72</td><td>0.001</td></tr><tr><td>Residual</td><td>12</td><td></td><td></td></tr><tr><td rowspan="4">DI</td><td rowspan="4">dredge</td><td>Before/After (a)</td><td>1</td><td>1.33</td><td>0.299</td></tr><tr><td>Site (b)</td><td>3</td><td>2.08</td><td>0.016</td></tr><tr><td>(a) x (b)</td><td>3</td><td>1.30</td><td>0.209</td></tr><tr><td>Residual</td><td>16</td><td></td><td></td></tr></tbody></table>

597

598
599

600

Figure captions

601

Fig. 1. Location of dredging (stars: GRA, JAB, BLV, CLV, BEE) and disposal (circles: PAC, MAT, MPY,

602

PYL) sites in Arcachon Bay.

603

604

Fig. 2. Median grab sampled fauna species richness (A), abundance (B) and infauna proportion (C) per

605

site (+/- minimum and maximum values) before (Be) and after (Af) the work. DR: sediment dredging,

606

DI: sediment disposal.

607

608

Fig. 3. Median dredge sampled fauna species richness (A), abundance (B) and infauna proportion (C)

609

per site (+/- minimum and maximum values) before (Be) and after (Af) the work. DR: sediment

610

dredging, DI: sediment disposal.

611

612
<figure><img src="image_2.png" /></figure>

613

614 Fig. 1

615
616

<figure><img src="image_3.png" /></figure>

617

<figure><img src="image_4.png" /></figure>

618

<figure><img src="image_5.png" /><figcaption>Fig. 2</figcaption></figure>
A. Species richness

<figure><img src="image_6.png" /></figure>

619

B. Abundance

<figure><img src="image_7.png" /></figure>

620

C. Infauna ratio

<figure><img src="image_8.png" /></figure>

621

<caption>Fig. 3</caption>

622