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Monitoring the impacts of marine aggregate extraction

Robert Lafite
Michel Desprez


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1 ADCP: the ADCP current meter vertical profiler uses the doppler effect. It measures the current characteristics (speed and direction) using the velocity of particles in suspension in water. Measurements are made at several points along the entire water column. Post-processing and calibration of the acoustic signal backscattered by the particles allows to measure SPM concentrations*.

2 Aggregate: sedimentary materials of any size (sand, gravel, pebbles) and from various sources (massive rocks, river deposits in marine and terrestrial environments), needed by industry (construction, public works).

3 Bathymetry: depth of the seabed, measured relatively to the theoretical level of the lowest water springs. Bathymetry is measured with echosounders (single or multibeam*) using the reflection of sound waves. Bathymetry monitoring allows to view topographic perturbations caused by extraction and to check that the conditions (location and deepening) conditions meet administrative requirements.

4 Benthos: free living (mobile) or attached (sessile) organisms on the seabottom.

5 Constance: frequency of encountering a species in the sampled stations.

6 CTD: a probe to measure salinity, conductivity and temperature in the environment. It is fitted on a frame, dipped into water and thus provides the information on the entire water column.

7 Demersal: a species living freely near the bottom, that is to say without actually being connected to it permanently, for example Gadidae fish. Contrarily, the term “benthic” qualifies species having a close and constant contact with the bottom (e. g. flatfish).

8 Fallow: sector temporarily abandoned after extraction to allow its physical restoration and biological recolonisation (benthos and fish).

9 Fine sand: sediment composed of particles whose size ranges between 0.063 and 0.2 mm.

10 Fisheries: describes all forms of fishing activities: business or leisure, freshwater or marine.

11 Ichthyofauna: fish groups living in a geographical area or a determined habitat.

12 LISST: laser particle sizer. It allows to measure particle size distribution—the spherical equivalent—using the laser diffraction technique. In our study, the interest of the device lies in using it in situ. It avoids laboratory analysis with samples being potentially modified (eg post-sampling flocculation). The laser technique also allows to measure particule size almost instantaneously in the water column.

13 Macrotidal: describes an environment undergoing significant tidal ranges (more than 4 to 5 meters).

14 Mini-Hamon grab sampling: a sampling device for surface sediments and the associated fauna used to describe the initial state of sediment and benthos and monitor their nature in areas subject to exploitation. The sampling system using a shovel is better suited to coarse sediments (gravel and pebbles) than skip jaws (Van Veen, Smith Mac Intyre). The sampling surface is 1/10 m².

15 Multibeam echosounder: an underwater acoustic system usually installed under the keel. It provides accurate information on the underwater topography of the bottom (bathymetry measurement) and on the nature of the seafloor (reflectivity measurement) on a broad band (up to 7 times the water depth).

16 Niskin bottles: these bottles sample the water at a given depth of the water column. Such samples are used to: (1) determine the characteristics of suspended particulate matter (concentration, nature and size) and (2) calibrate the backscattering signal recorded by ADCP (post-treatment) to estimate SPM concentration.

17 Opportunistic: describes an organism, a species able to quickly settle and develop somewhere.

18 Overflow: mixed water and fine sediment (< 0.25 mm) overflow masses discharged through the ship side-doors or through an internal well during the progressive sediment infilling of the ship hold.

19 Replicates: samples from one station to estimate the average value of the various sedimentary or biological parameters (number of species, abundance, biomass).

20 Recolonization: the process by which species and benthic communities recolonize the site once extraction has ceased; the speed of recolonization depends on the disturbance intensity and the surrounding communities. It varies from 1-2 years in poor sandy bottoms with strong hydrodynamics up to more than 10 years in more stable bottoms (gravel, cobble) with higher diversity.

21 Screening: on-board sieving process regularly practiced in the UK to remove sediment of unwanted size; this practice is now almost abandoned. It is banned in France.

22 Shipeck grab sampler: grab sampling for mechanical harvesting of surficial sediment samples. It gives direct, punctual and sedimentological information. Laboratory analyzes are used to characterize the nature and size of sediment. The results can be used to calibrate the proxy data from acoustic imaging, with a sidescan sonar for example. The Shipeck grab used in Dieppe and the Baie de Seine is cylindrical and samples a surface of 4 dm² at a maximum depth of 10 cm.

23 Side scan sonar: consists of an instrument called “fish” towed behind a boat. The sonar sends a signal to the bottom via two transducers. The signal reflected from the bottom is captured by the “fish” and sent through an electrocarrier cable to an on-board digital and graphical recorder. The side-scan sonar allows to measure the variation of the bottom backscatter coefficient which depends on the nature and morphology of the seafloor. The data are collected as 100 m-wide profile recordings to be assembled into a mosaic. They are interpreted in terms of acoustic facies and are calibrated through analyzes of the grabbed sediments. Interpreting the data allows to produce a bottom morphosedimentary map.

24 SPM: suspended particulate m atter (SPM) are inorganic or organic fine material, living or detritic, insoluble, visible to the naked eye, passively driven by waters and which contribute to turbidity.

25 Stinger: a pipe sucking sediments into the ship hold with a draghead back into the ship hold.

26 Trophic: regards the nutrition of tissues and organisms. A trophic level refers to the position occupied by an organism in a food web: primary producers (phytoplankton, higher plants…), primary consumers (herbivores: zooplankton), secondary consumers (carnivores) etc. There are four main trophic levels: primary producers, herbivores, carnivores and decomposers.

27 Turbid plume: “cloud” of suspended solids discharged with overflow water when filling the well of the dredger with sediment extracted from the sea bottom. The sand particles settle rapidly upon release of the ship while the fine particle plume may persist for more than one hour over a length of several kilometers and a width of a few hundred meters.

28 Turbidity: refers to an optical characteristic of water, namely its ability to diffuse or absorb incident light; turbidity is due to the presence of suspended particle matter (SPM) in water.

Table des illustrations

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© Presses universitaires de Rouen et du Havre, 2014

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