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Monitoring the impacts of marine aggregate extraction

 | 
Robert Lafite
, 
Michel Desprez

Level of knowledge

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Extraction pressure

1In several countries like for example Belgium, Britain and the Netherlands which comply with regulations, the activity is electronically monitored by black boxes installed on the ship. In Dieppe and in the Baie de Seine, when obtaining information on extraction monitoring was possible, the extraction intensity (pressure) has been calculated according to the classification of Boyd et al. (2004) who identified three types of pressure: low, medium and high. This intensity was quantified at the scale of the permit area (hours. hectare-1. year-1) and at the scale of the furrows and bio-sedimentary monitoring stations (hours. are-1. year-1) in order to refine the relationship between the extraction intensity and the different levels of biological and morphological impacts. Unlike the experiment conducted in the Baie de Seine, commercial extraction in Dieppe covers a larger area (6 km²), but the average intensity of extraction is much lower than 1h. ha-1. year-1 on more than 95% of the authorized perimeter (local maximum of 39 mn). This indicates extensive exploitation with large fallow areas.

Relative surface of the two study areas (Dieppe = 6 km²; Baie de Seine = 0.6 km²) according to the extraction intensity.

Physical impacts

Effects on the coastline

2To comply with the precautionary approach, most of the marine aggregate extraction areas in Europe are located offshore so as to minimize the risk of impact on the coastline stability (>-20 m depth in the Netherlands). Pre-licensing assessment which is often based on numerical modelling is recommended to ensure that coastal wave conditions, tidal currents and natural sediment transport will not be impacted and to check that there is no negative impact on the particularly vulnerable beaches or dunes. Yet there is poor knowledge on the sediment processes and fluxes that control sediment transfer between offshore areas and the coastline. Further research on this topic is therefore needed. The potential effects of marine aggregate extraction on the coastal chalk cliffs and pebble ridges located near Dieppe’s extraction site have not been studied in the context of GIS SIEGMA.

Effects on the water column

3Previous studies have provided information on the concentrations of water overflow—from a few grams to tens of grams per liter—and on the geometry and time dispersion of the turbid plumes resulting either from overflow or from benthic origin. The plume may come with two phases: a dynamic phase, with particles rapidly settling to the seabed (5-15 mn) and a passive phase reflecting slower settling with progressive dispersion—during some hours with deposition on a distance of 1 to 8 km from the point of overflow. Studies have focused on the flocculation processes that can appear in turbid plumes.

Cumulated intensity (hour. year-1) per hectare (left) and per are (right) on site A in the Baie de Seine for the period 2007-2008.

Synthesis of the dynamics of a turbid plume created against currents in the case of an overspill through side doors.

4In the Baie de Seine, research has focused on the in situ measurement of the turbid plume characteristics. At the entrance of the overspill into the marine environment, concentration drops from a few grams per liter to a few tens or hundreds of milligrams per liter under the effect of dilution. The turbid plume that has formed at the rear of the dredge 10 mn after the overflow is 100 m wide. It passively evolves under the combined effect of lateral dispersion and the settling of particles. Concentrations gradually decrease and tend towards natural environment values after 2 hours even if the plume sometimes remains visible.

5In the macrotidal context of the Eastern English Channel, it can be stated that:

  • the extraction conditions (with or against currents, pump flow, loading capacity) strongly control the initial concentration, the geometry and the relative importance of the dynamic phase of the turbid plume;

  • the type of the dredged material (grain size and nature) and the tidal currents condition the time of dispersion (passive phase) and the extension of the turbid plume that can extend up to 6.5 km beyond the extraction zone.

Strategy and instrumentation for measuring the characteristics of the water column
In the Baie de Seine, a measurement strategy based on drifting surveys—Lagrangian method—using acoustic and optical instruments has proven suitable to understand the dynamics of the overflow turbid plume. The main parameters have been measured in the water column with the following instruments:
(1) The suspended sediment concentration* and turbidity* have been directly measured from water samples collected with a Niskin bottle* or indirectly with an ADCP* (backscatter signal)
(2) The current and swell characteristics on the whole water column have been measured with an ADCP current meter.
(3) The particle grain size has been directly measured from filtered water samples collected with the Niskin bottle or indirectly measured in situ with the LISST*.
(4) The hydrological parameters of the water column such as salinity, conductivity and temperature have been measured using a CTD*.
Additionally, a numerical modelling approach should allow to test scenarios that have not been experienced during field measurement, such as operating in a period of wave agitation.

Effects on topography and seabed sediments

6The width and depth ranges are respectively of several meters and one meter in the case of furrows resulting from repeated passages of the draghead. After several years, localized extraction generates durable depressions of a few meters in depth. Indicators of impact can thus be highlighted: in Dieppe, an increase in surficial sediment heterogeneity is identified as a marker for seabed disruption through extraction while an increase greater than 10% of the fine sand content is an indicator of the deposition of overflow plumes. In the Baie de Seine, despite annual extraction with higher intensity, such deposits of the overflow have not been observed because of the short duration of operation (less than 1 year on site A and 3 years on site B).

7Marine aggregate extraction leads to changes in the topography and nature of the seabed. These changes are induced by two phenomena: the digging of furrows associated with the passage of the draghead on the seafloor and the settling of the turbid plume, whether from overflow or from benthic origin.

8After several years of extraction in Dieppe and the Baie de Seine, the extraction furrows are 5 m wide, less than 1 m deep and with slopes greater than 5°. In the Baie de Seine, after one year of extraction, a little less than 50% of the surface of site A has been worked (0.3 km²) and the mean deepening of the seabed is 20 cm. Furrows are partially filled with fine to coarse and sometimes rippled sands.

Seabed impacted by extraction in the site of Dieppe (acoustic images). Up: recent furrows (less than 1 year, bottom) and old ones after several years of activity (up). Bottom: sandy bottoms with little dunes resulting from the natural sandy pathway and from fine sands through overflow.
Remark: the white (up) and black (down) stripes correspond to artefacts along the ship route.

Strategy and instrumentation for measuring seafloor characteristics
Monitoring the impacts and seabed restoration includes characterizing topography and sediments thanks to specialised instrumentation:
(1) The seabottom topography is measured with a multibeam sounder*.
(2) The seabed nature (grainsize, nature and/or roughness) is indirectly approached thanks to imaging data provided by a multibeam sounder and a sidescan sonar*. The data are calibrated through direct observation with video and with grab* sediment samples.
Laboratory analyzes performed on the sediment samples are used to quantify the size and nature of the surficial sediments. The video is a complementary tool that offers visualizing landscapes and morphosedimentological facies at an intermediate spatial scale between the grab sampling scale (0.1 m²) and the sounder and sonar scale (1 m²).

Recent extraction furrows (less than 1 year) in the Baie de Seine. Transversal topography of the seabed before (green curve) and after extraction (red curve); vertical scale x25.

Restoration of topography and seabed sediments

9The persistence of sedimentary and topographic changes varies according to the hydrosedimentary characteristics of the environment (current intensity, sediment size).

10When sediments are sandy and mobile, the dredging furrows disappear in a few weeks. In calm sandy areas with weak sediment transport or in stable coarse sediments, they can yet be still visible more than 10 years after activity has stopped.

11In Dieppe, at the scale of a few months of interrupted extraction, sand movements are observed on one-kilometer distances. Smoothing of the extraction furrow morphology is rapid during the first years after activity has ceased (widening of furrows to 5-7 m, decrease of slopes at 5° maximum and of depth furrows at 0.4-0.9 m). This evolution is accompanied by a partial infilling of the furrows by coarse sands that are brought through natural sediment transport.

12About 10 years after extraction has stopped, furrows are completely filled in some places and are covered with dynamic sandy-gravelly sedimentary bodies (ribbons, dunes). However they remain visible at other locations between the crests of pebbles (see upper and lower sections of the acoustic image below).

13In the absence of natural sediment transport, deposition of fine sands from overflow remains confined within the site (in the dredging furrows) and in the immediate vicinity of the site, with a corresponding change in invertebrate and fish communities. In sites with a high natural sediment transport, as in Dieppe, fine sand deposits are later reworked over a several-kilometer-wide area in the direction of tidal residual currents.

Restoration of the seabed in Dieppe. Smoothed old furrows (bottom right), during the final phase of in-filling and covering by rippled sandy ribbons (up left).

Biological Impacts

Effects on benthos

14The impact of dredging on organisms living on the seabed (benthic animals) is important in the wake of the draghead. Typical reductions of 60 to 80% in the number of species are observed, and over 90% for the abundance and biomass of benthos. The significance of this impact depends on the annual intensity of extraction but also on the number of operating years (cumulative effects).

15On the extraction site of Dieppe, depletion is maximal in the most intensely exploited area (-64% for the number of species, -86% for abundance and -91% for biomass). The consequences observed in the temporarily-unexploited area (“fallow”) are much lower due to opportunistic colonization (abundance > reference value).

16A high impact is also observed in the periphery (up to 2 km) due to a regular deposit of fine sand discharged with water overflow. In the case of extensive deposits of very fine sediments and associated organic matter, peripheral benthic community enrichment has been highlighted as in some UK sites.

Evolution of the number of species, abundance and biomass of benthic communities on the extraction site of Dieppe (relative mean values for 2003 and 2007 expressed in % of reference ones).

Measurement strategy and analyses for the characterization of benthic communities
Quantitative sampling of benthos is achieved with a mini-Hamon grab sampling a surface of 0.1 m². Three replicates* are performed at each station with coordinates being recorded with a differential GPS. The macrofauna corresponds to the animals retained after the sediment has been sieved through a 1 mm mesh. Determining down to the lowest taxonomic level possible allows to calculate the basic parameters describing the benthic community: species richness (number of species), abundance (number of individuals per m²) and biomass (grams of organic matter per m² expressed as ash-free dry weight after calcination).
At each station, sediment is sampled to perform granulometric analysis with a traditional AFNOR-sieve column (square mesh 2 mm - 40 µm) and to estimate its organic matter content.

17The experiment conducted in the Baie de Seine has allowed to quantify the impact on the main community parameters after one year of extraction (fallow test):

  • The difference in extraction intensity (A = 2.5 h. ha-1. year-1, B = 4.2 h. ha-1. year-1) mainly reflects in the evolution of the number of species (respectively -22% and -42%), while the difference is lower for biomass (-74% and -81%) and for abundance (-66% and -71%). These values are significantly lower than those observed on Dieppe’s site experiencing continuous activity but where extraction intensity is yet significantly lower (up to 40 mn.ha-1.year-1 against 4 h in the Baie de Seine) and has been repeated for several years.

  • The three basic community parameters show a significant impact only within the extraction perimeter with a 42% decrease in the number of species, 71% in abundance and 77% in biomass. After one year of extraction no significant impact of oversanding is observed on these three parameters in the either near (250 m) or far (750 m) periphery.

Comparison of the biological impact after one month (site A) and one year of extraction (site B) within the two experimental sites in the Baie de Seine (relative values expressed in % of reference ones).

Spatial extent of the biological impact of extraction from the central part up to 750 m of the experimental site in the Baie de Seine (site B) (relative values expressed in % of reference ones).

Recolonisation of benthic communities

18The nature and rate of seabed recolonisation by benthic communities are highly variable. The communities of mobile sediments which adapt to the natural instability of their habitat, show a rapid restoration of the number of species and of abundance in a few months, often performed by the migration of mobile species from adjacent sandy areas or sub-areas of the site that have not been affected by extraction.

19More stable sediments (gravels) have communities including species of greater size with slow growth and low reproductive rates—irregular recruitment. These species require several years to reach their adult size. These communities may require more than 10 years to recover their original structure and function although the biggest part of colonization by juveniles is done quickly (a few weeks to a few months).

20Observation of benthic recolonisation of the former extraction site in Dieppe shows a quick recovery of the number of species (80% of the reference value after 2 years) and then of abundance (7 years) thanks to the proliferation of some opportunistic species whereas biomass is restored to only 70%. Fifteen years after extraction has stopped, the number of species is always optimal and abundance is still 2.5 times higher than the reference value while biomass has not progressed (-40%).

Evolution of the number of species, abundance and biomass of benthic communities on the former extraction site of Dieppe between 1995 and 2010 (relative values expressed in % of reference ones.

21In the Baie de Seine, the experimental monitoring of the recolonisation process shows the recovery of species, abundance and biomass in early 2011, two years after semi-intensive extraction (4 h.ha-1.year-1) has stopped on site A which was only operated during one month (October 2008).

Evolution of the relative value of the three main community parameters in the northern part of experimental site A (fallow test) between 2007-2008 (baseline study) and 2011. The arrow indicates the exploitation period in October 2008 (relative values expressed in % of reference ones).

Effects on fish

22Marine aggregate extraction impacts the ichthyofauna either directly by temporarily disturbing and displacing species (avoidance reaction or sometimes attraction) and by modifying the substrate—soil preferences of benthic fish—or indirectly through the food chain.

23With low extraction intensity (< 1h.ha-1.y-1) in Dieppe, an impact of the activity on the number of demersal fish species could not be demonstrated both in the extraction area and in the peripheral sector subject to sediment deposition by overflow. On the contrary, a sharp decrease in abundance has been observed, both in the mining sector (-35%) and in overflow (-47%). For biomass, no impact has been observed in the extraction area while it has been reduced by 29% in the deposition zone.

The protocols used for fish monitoring in Dieppe and in the Baie de Seine are based on local fishing techniques: bottom trawling with large vertical opening off the coast in Dieppe, bottom trawling for sole in the Baie de Seine.
To optimally describe the ichthyofauna and as the objective being not just the capture of market size fish, smaller mesh is used for bottom trawling than the one complying with regulation-80 mm stretch. A double pocket with a 20 mm (stretched) mesh is therefore positioned inside the trawl.
Sampling is carried out quarterly on control traits in the area impacted by extraction, as well as on several reference traits located outside the zone of theoretical influence of dredging.
The time and exact position of the haul are recorded at the beginning and at the end of trawling to compute the numerical abundance and weight of the captured species. Specific richness is calculated by station and by season. Abundance and biomass of catches are related to a one-hour standard fishing time. The digestive tracts of some selected fish species are removed and placed into a solution of sea water and formalin (4%) before later dissection.

Number of species, abundance and biomass of demersal fish communities in the different impacted areas of the extraction site in Dieppe (relative mean values for the period 2004-2006 expressed in% of reference ones).

24The following table shows the representative species of the main areas in Dieppe:

  • plaice is the species most often observed (occurence > 75%) in each sector during the two years of monitoring;

  • for abundance and biomass, each sector is dominated by a different species: sandeel in the reference area, black seabream in the extraction zone and plaice in the deposition area.

Classification in decreasing order of frequency of the dominant species frequency (in bold) and of the main accompanying species of the fish community in the reference and the impacted areas in Dieppe for the occurence, abundance and biomass parameters.

25In the Baie de Seine, the monitoring conducted between 2007 and 2011 reveals a strong impact of the extraction activity on fish attendance, both for the number of species (-50%) and for abundance (-92%). The intensity of this impact is slightly higher than the one observed on benthos (-42% for the number of species,-71% for abundance).

26The difference in extraction intensity (4h. ha-1. year-1 in the Baie de Seine against 40 mn in Dieppe) may explain the significantly higher impact observed on fish abundance in the Baie de Seine (-90%) compared to Dieppe (-50%).

27Unlike Dieppe’s scenario (maximal impact on benthos and minimal impact on fish), the impact level observed in the Baie de Seine is consistent with the one observed on benthos which is the trophic link traditionally used as an indicator of the ecosystem’s disturbance level.

28The mapping of the number of species and abundance highlights the limited spatial extent of the impact that extends over a maximum area of a few hundred meters. Indeed, the changes observed at the end of 2008 at the directly impacted stations (A north and B north) are lower on the neighbouring stations located 300 m away (A south and B south). No impact has been detected on the reference traits located 1 km away.

29While many species such as the gurnard, the common pout and to a lesser extent the red mullet and the whiting have shown a sharp decrease in abundance due to extraction, there was instead a temporary and regular attractiveness for the common sole at the beginning of the extraction periods. This species showed temporary increases of 50 times in abundance within the extraction area and up to 6 times in the immediate vicinity thereof. The temporary nature of this phenomenon is linked to the deposition of benthos rejected by the overflow water during the dredging of the surficial sediments colonized by benthic fauna; this attraction lasts between 1 and 3.5 months.

30The same attractiveness did not work systematically for the dab.

Compared evolution of the total number of species and fish abundance during the 5 years’ monitoring period (2007-2011) between the extraction 30
site (CB1) and the reference area (REF) in the Baie de Seine. The red arrows indicate the extraction periods.

Map of the mean number of species and abundance (ind. h-1) observed in 2008-2009 on the experimental site of the Baie de Seine (extraction from November 2008 in the northern area of site B; recolonisation from July and November 2008 in the southern and northern areas of site A).

Evolution of the abundance of the common sole (relative values expressed in % of the reference ones) during the first four years of experimentation in the Baie de Seine. The arrows indicate the extraction periods (direct impact in blue and indirect impact in red).

Recovery of the fish communities

31In Dieppe, the temporary absence of extraction—2 years in the northern “fallow” area—has allowed a rapid return of the number of species while their and biomass remain lower than the reference values, 120 30% and 100 20% respectively.

32Ten years 80 after extraction has stopped, the recolonisation 60 area is featured by a fish community as diverse as 40 the one characterising the reference area, yet with abundance and biomass almost twice higher, while the recolonisation process through benthos is only nearing completion. The following table shows the evolution of the fish community’s dominant species on the extraction site during the recolonisation process:

  • the plaice is decreasingly present between the extraction phase and recolonisation in the medium term, while the black seabream remains the most common species;

  • abundance and biomass are dominated by the black seabream which is the key species at each step.

Number of species, abundance and biomass of demersal fish communities during the recolonisation process of the extraction site in Dieppe (relative values expressed in % of the reference ones).

Classification in decreasing order of frequency of the dominant (in bold) and of the main accompanying species of the fish community in the extraction, fallow and recolonisation areas of the site of Dieppe for the occurence, abundance and biomass parameters.

33In the Baie de Seine, after one month of extraction with a 4 h. ha-1 intensity, the fallow experiment (zone A) shows a limited (-33%) and temporary (1.5 years) decrease in the number of species found in the area: contrarily, abundance shows a significant (-80%) and more durable drop, with a restoration period of 2.5 years after the end of extraction to reach levels comparable to the baseline ones.

34The recolonisation process is completed in several steps:

  1. initial recovery of seasonal fluctuations for community parameters

  2. recovery of the abundance levels similar to the reference ones for minimum seasonal values

  3. then for maximal seasonal values (summer-autumn)

Temporal evolution of the number of species and abundance of demersal fish species in the northern area of site A (fallow test) and in the reference one. The arrow indicates the extraction period.

Effects on the trophic relationships between benthos and fish

35There is very little information on the consequences of reduced benthic food for the trophic chain, including fish, with the exception of original research carried out on the Dieppe site under the SIEGMA programme between 2004 and 2006 and the information acquired more recently in the Baie de Seine.

Relative importance of dominant preys for cod, black seabream and gurnard in the fallow areas of the extraction site in Dieppe. Remark: Crabs are in red, shrimps in yellow, other crustaceans in orange, bivalves in pink, other mollusks and diverse groups in mauve, annelids in green and echinids in purple.

Relative importance of dominant preys for plaice, sole and ray in the deposition area of the extraction site in Dieppe. Same remark for the colour code.

Relative importance of dominant preys for the red mullet in the different impacted areas of the extraction site in Dieppe. Same remark for the colour code.

36In Dieppe, the abundance of demersal fish species could be related to the nature of the benthic preys available:

  • The shingles from the extraction and fallow areas are characterized by the presence of cod and the abundance of black seabream which mainly feed on small opportunistic crabs (squat lobster Galathea, porcelain crab Pisidia) recolonising the site when dredging has stopped; the seabream can be considered as the key species of the dredging area with abundance and biomass at their highest level. Gurnards also feed mostly on these small opportunistic crabs but also on juvenile fish (dragonet).

  • Plaice and sole preferentially feed on annelids and bivalves which colonise the unstable fine sands characterizing the deposition area; shrimps and other crustacean species of this area represent exclusive winter prey for rays.

  • The red mullet’s stomach contents make this species the best indicator for the diversity of habitats and the associated benthic communities that characterize the various impacted areas of the Dieppe site.

37In the Baie de Seine, the small number of years of extraction has failed to show significant changes in the diet of most species (flounder, whiting, cod, pout), with the exception of the sole. Indeed this species has shown a net change in its diet with a decrease in the consumption of annelids and shrimps, an increase in bivalve species and the emergence of gravel epifauna. The latter has been made accessible by the mechanical perturbation of the seabed and the deposition of these new preys discharged with water overflow.

38The type of substrate and associated fauna thus determines the presence of various demersal fish species feeding on them; the extraction activity results in a significant decrease in the abundance of most fish species as their benthic preys are becoming scarce; only species with an opportunistic feeding behaviour can show a temporary increase in abundance linked to the important contribution of organic matter in the early stages of operation (removal of the biologically “active” surface layer).

Relative importance of dominant preys for the common sole in the different areas sampled in the experimental site of the Baie de Seine. Same remark for the colour code.

Modelling of the Eastern English Channel’s food web

39At a regional level, attention was focused on the influence of abiotic conditions on the distribution of fish species.

40The association fish-environment is determined on the basis of the statistical relationships between environmental variables (salinity, temperature…) and the presence of species. Their distribution is determined on the basis of biotic interactions and on the factors due to human activities impacting the distribution of each specific habitat.

41An Ecopath model of the Eastern English Channel representing the 1995-1996 period has been built. This modelling approach seeks to provide a brief description of the ecosystem structure and to represent its fundamental properties. The model includes 51 functional groups including mammals (2), seabirds (1), fish (29), invertebrates (15), primary producers (1), discards and detritus.

42This study documents the implementation of the ecosystem model which is a static representation of biotic flows in food webs.

43This will provide a basis for spatio-temporal dynamics simulations (Ecosim and Ecospace) in the near future. This type of study can reveal much information on the ecosystem status and on how its biological components are affected by disturbances or environmental changes. This type of model has been developed to study the ecological responses of fish species facing particular stress and namely the pressure related to anthropogenic activities such as seabed disturbance following marine aggregate extraction. The model also aims at analysing the consequences of these activities on the marine ecosystem.

Schematic representation of the trophic food-web of the Eastern English Channel showing the main fluxes connecting the different functional groups.

Modelling strategy
A crucial step in ECOPATH modelling requires to identify the functional groups in the ecosystem. Each group can be represented by a particular species or group of species with similar ecological characteristics. For this study, the functional groups have been established on the basis of indications given by Yodzis and Winemiller (1999) and Christensen et al. (2005). In ecosystems with low biodiversity, each group may include only one type of species that can be particularly abundant or ecologically important, or considered important for fisheries. In an ecosystem with high biodiversity, such as marine ecosystems, species are often grouped together in order to meet data homogeneity. Each group must be composed of species with similar ecological characteristics (trophic guilds, spatial distribution, age, size, etc.) which can then be considered as following the same “trophic dynamics” (Yodzis and Winemiller, 1999).
Equation of the Ecopath Model
For this study, we used the software “Ecopath with Ecosim and Ecospace” where the functional groups are made up of species with similar biological and ecological features (demography, food preferences, spatial distribution…). The basic equation applied to each functional group is as follows:
Production = Capture + Biomass accumulation + Predation mortality + Net migration + Other mortality.
Here, predation mortality is the essential link between the groups: it allows the flow of biomass within the food web.

Effects on habitats and biodiversity

44The extraction of marine aggregates may represent a challenge for biodiversity if mining projects are carried out in underrepresented areas within the geographical area and/or if they impact sensitive species or habitats (spawning grounds, nursery areas, biogenic reefs…).

45The impact of aggregate extraction on the seabed biodiversity of the site differs according to the selected criterion:

  • the number of benthic species is reduced by 42% after only one year of extraction in the Baie de Seine but by 77% after several years on the site of Dieppe;

  • while the number of species of benthic and demersal fish is not affected by extensive extraction carried out for several years in Dieppe, it has been reduced from 35 to 50% after semi-intensive extraction performed during one to several months in the Baie de Seine;

  • the number of EUNIS habitats and associated communities has increased in Dieppe with several structural and functional habitats having been highlighted in the different surveyed areas:

  • The extraction perimeter is characterized by the presence of pebble crests created during exploitation EUNIS Habitat A5.121). These crests are quickly colonised by an epifauna of opportunistic species like the tubicolous worm Pomatoceros which represents more than 90% of the new benthic community; the main accompanying species are the crabs Pisidia and Galathea; they are the dominant prey species for black seabream, cod and gurnards which characterize the new fish community of the extraction site.

  • The outlying area, where sands outwashed with overspill are regularly deposited, has a finer sediment coverage than areas with original coarse sands; yet the local strong tidal currents make this fine sand coverage unstable; a few species characterizing mobile fine sands (annelids, bivalves) inhabit this area (EUNIS Habitat A5.231) mainly frequented by sole and plaice.

  • The bottoms of the former extraction site (recolonisation area) are more heterogeneous (pebble crests, muddy furrows, coarse sands brought by tidal currents); their benthic community is always dominated by annelids and opportunistic crabs, with shrimps and young fish species (dragonet) being the main prey species of gurnards which dominate the fish community of this area.

Number of benthic species and of demersal fish species observed on the extraction sites of Dieppe and the Baie de Seine (relative values expressed in% of the reference ones) and relative number of habitats in Dieppe.

46It is important to note the recent sampling (2010) of a biogenic reef with Sabellaria spinulosa—OSPAR list of sensitive species; habitat EUNIS A5.6; Natura 2000 habitat 1170—in the former extraction site of Dieppe which may be correlated to occasional fine sand deposits from the overflow of a contiguous site (“positive” cumulative effect); this extra sand deposition facilitates the annelid’s tube construction and provides the worm with an increased quantity of organic matter of benthic origin.

Dissemination of marine aggregate extraction issues to the public

47Information sharing needs to be developed in France contrarily to Britain where a major effort has been made to share information with the public and schools (conferences, exhibitions, applied programs…) and to inform sea users on the issues related to marine aggregate extraction and on the way to protect environmental resources.

48One topic of the SIEGMA programme is dedicated to knowledge dissemination among end-users of the sea, maritime administrations and the public. Teaching material for exhibition and a website are available to date: www.siegma.fr

Table des illustrations

Légende Relative surface of the two study areas (Dieppe = 6 km²; Baie de Seine = 0.6 km²) according to the extraction intensity.
URL http://books.openedition.org/purh/docannexe/image/115/img-1.jpg
Fichier image/jpeg, 156k
Légende Cumulated intensity (hour. year-1) per hectare (left) and per are (right) on site A in the Baie de Seine for the period 2007-2008.
URL http://books.openedition.org/purh/docannexe/image/115/img-2.jpg
Fichier image/jpeg, 192k
Légende Synthesis of the dynamics of a turbid plume created against currents in the case of an overspill through side doors.
URL http://books.openedition.org/purh/docannexe/image/115/img-3.jpg
Fichier image/jpeg, 108k
Légende Seabed impacted by extraction in the site of Dieppe (acoustic images). Up: recent furrows (less than 1 year, bottom) and old ones after several years of activity (up). Bottom: sandy bottoms with little dunes resulting from the natural sandy pathway and from fine sands through overflow.Remark: the white (up) and black (down) stripes correspond to artefacts along the ship route.
URL http://books.openedition.org/purh/docannexe/image/115/img-4.jpg
Fichier image/jpeg, 76k
Légende Recent extraction furrows (less than 1 year) in the Baie de Seine. Transversal topography of the seabed before (green curve) and after extraction (red curve); vertical scale x25.
URL http://books.openedition.org/purh/docannexe/image/115/img-5.jpg
Fichier image/jpeg, 48k
Légende Restoration of the seabed in Dieppe. Smoothed old furrows (bottom right), during the final phase of in-filling and covering by rippled sandy ribbons (up left).
URL http://books.openedition.org/purh/docannexe/image/115/img-6.jpg
Fichier image/jpeg, 40k
Légende Evolution of the number of species, abundance and biomass of benthic communities on the extraction site of Dieppe (relative mean values for 2003 and 2007 expressed in % of reference ones).
URL http://books.openedition.org/purh/docannexe/image/115/img-7.jpg
Fichier image/jpeg, 48k
Légende Comparison of the biological impact after one month (site A) and one year of extraction (site B) within the two experimental sites in the Baie de Seine (relative values expressed in % of reference ones).
URL http://books.openedition.org/purh/docannexe/image/115/img-8.jpg
Fichier image/jpeg, 60k
Légende Spatial extent of the biological impact of extraction from the central part up to 750 m of the experimental site in the Baie de Seine (site B) (relative values expressed in % of reference ones).
URL http://books.openedition.org/purh/docannexe/image/115/img-9.jpg
Fichier image/jpeg, 20k
Légende Evolution of the number of species, abundance and biomass of benthic communities on the former extraction site of Dieppe between 1995 and 2010 (relative values expressed in % of reference ones.
URL http://books.openedition.org/purh/docannexe/image/115/img-10.jpg
Fichier image/jpeg, 20k
Légende Evolution of the relative value of the three main community parameters in the northern part of experimental site A (fallow test) between 2007-2008 (baseline study) and 2011. The arrow indicates the exploitation period in October 2008 (relative values expressed in % of reference ones).
URL http://books.openedition.org/purh/docannexe/image/115/img-11.jpg
Fichier image/jpeg, 76k
Légende Number of species, abundance and biomass of demersal fish communities in the different impacted areas of the extraction site in Dieppe (relative mean values for the period 2004-2006 expressed in% of reference ones).
URL http://books.openedition.org/purh/docannexe/image/115/img-12.jpg
Fichier image/jpeg, 32k
Légende Classification in decreasing order of frequency of the dominant species frequency (in bold) and of the main accompanying species of the fish community in the reference and the impacted areas in Dieppe for the occurence, abundance and biomass parameters.
URL http://books.openedition.org/purh/docannexe/image/115/img-13.jpg
Fichier image/jpeg, 84k
Légende Compared evolution of the total number of species and fish abundance during the 5 years’ monitoring period (2007-2011) between the extraction 30site (CB1) and the reference area (REF) in the Baie de Seine. The red arrows indicate the extraction periods.
URL http://books.openedition.org/purh/docannexe/image/115/img-14.jpg
Fichier image/jpeg, 48k
Légende Map of the mean number of species and abundance (ind. h-1) observed in 2008-2009 on the experimental site of the Baie de Seine (extraction from November 2008 in the northern area of site B; recolonisation from July and November 2008 in the southern and northern areas of site A).
URL http://books.openedition.org/purh/docannexe/image/115/img-15.png
Fichier image/png, 477k
Légende Evolution of the abundance of the common sole (relative values expressed in % of the reference ones) during the first four years of experimentation in the Baie de Seine. The arrows indicate the extraction periods (direct impact in blue and indirect impact in red).
URL http://books.openedition.org/purh/docannexe/image/115/img-16.jpg
Fichier image/jpeg, 168k
Légende Number of species, abundance and biomass of demersal fish communities during the recolonisation process of the extraction site in Dieppe (relative values expressed in % of the reference ones).
URL http://books.openedition.org/purh/docannexe/image/115/img-17.jpg
Fichier image/jpeg, 124k
Légende Classification in decreasing order of frequency of the dominant (in bold) and of the main accompanying species of the fish community in the extraction, fallow and recolonisation areas of the site of Dieppe for the occurence, abundance and biomass parameters.
URL http://books.openedition.org/purh/docannexe/image/115/img-18.jpg
Fichier image/jpeg, 96k
Légende Temporal evolution of the number of species and abundance of demersal fish species in the northern area of site A (fallow test) and in the reference one. The arrow indicates the extraction period.
URL http://books.openedition.org/purh/docannexe/image/115/img-19.jpg
Fichier image/jpeg, 172k
Légende Relative importance of dominant preys for cod, black seabream and gurnard in the fallow areas of the extraction site in Dieppe. Remark: Crabs are in red, shrimps in yellow, other crustaceans in orange, bivalves in pink, other mollusks and diverse groups in mauve, annelids in green and echinids in purple.
URL http://books.openedition.org/purh/docannexe/image/115/img-20.jpg
Fichier image/jpeg, 124k
Légende Relative importance of dominant preys for plaice, sole and ray in the deposition area of the extraction site in Dieppe. Same remark for the colour code.
URL http://books.openedition.org/purh/docannexe/image/115/img-21.jpg
Fichier image/jpeg, 136k
Légende Relative importance of dominant preys for the red mullet in the different impacted areas of the extraction site in Dieppe. Same remark for the colour code.
URL http://books.openedition.org/purh/docannexe/image/115/img-22.jpg
Fichier image/jpeg, 128k
Légende Relative importance of dominant preys for the common sole in the different areas sampled in the experimental site of the Baie de Seine. Same remark for the colour code.
URL http://books.openedition.org/purh/docannexe/image/115/img-23.jpg
Fichier image/jpeg, 152k
Légende Schematic representation of the trophic food-web of the Eastern English Channel showing the main fluxes connecting the different functional groups.
URL http://books.openedition.org/purh/docannexe/image/115/img-24.png
Fichier image/png, 827k
Légende Number of benthic species and of demersal fish species observed on the extraction sites of Dieppe and the Baie de Seine (relative values expressed in% of the reference ones) and relative number of habitats in Dieppe.
URL http://books.openedition.org/purh/docannexe/image/115/img-25.jpg
Fichier image/jpeg, 24k
URL http://books.openedition.org/purh/docannexe/image/115/img-26.jpg
Fichier image/jpeg, 328k
URL http://books.openedition.org/purh/docannexe/image/115/img-27.jpg
Fichier image/jpeg, 356k
URL http://books.openedition.org/purh/docannexe/image/115/img-28.jpg
Fichier image/jpeg, 346k

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