Background information and definitions
'Aggregates’ is the collective term for sand, gravel and crushed rock. They are used as raw materials for the construction industry as well as for beach replenishment schemes. Aggregate extraction can impact subtidal benthic invertebrates through physical removal, loss and alteration of seabed and habitat, and direct physical damage from the machinery used (De Groot 1996). Aggregate extraction could be ceased (for instance following end of licence or voluntary cessation of activities) or prohibited (through legislation) in an area, and the site and its invertebrates left to naturally recover over time (Desprez 2000). Evidence for other interventions related to aggregate extraction is summarised under “Threat: Energy production and mining – Mining and quarrying”, and in “Habitat restoration and creation – Refill disused borrow pits” and “Landscape or artificially enhance the seabed”.
De Groot S.J. (1996) The physical impact of marine aggregate extraction in the North Sea. ICES Journal of Marine Science, 53, 1051–1053.
Desprez M. (2000) Physical and biological impact of marine aggregate extraction along the French coast of the Eastern English Channel: short-and long-term post-dredging restoration. ICES Journal of Marine Science, 57, 1428–1438.
Supporting evidence from individual studies
A before-and-after, site comparison study in 1994–1997 of eight sites in one area of sandy and gravelly seabed in the English Channel, off the coast of France (Desprez 2000) found that 16–28 months after ceasing aggregate extraction, invertebrate species richness, abundance, and biomass appeared to have increased, and had become more similar to that of adjacent natural sites where extraction did not occur. Data were not statistically tested. After cessation, species richness at extraction sites increased and appeared to be more similar to natural sites (before: 37% that of natural sites; after 16 months onwards: >100%). Increases were also observed for biomass (before: 17%; after 16 months: 35%; after 28 months: 75% that of natural sites) and abundance (before: 14%; after 16 months: 56%; after 28 months: 57% that of natural sites). Aggregate extraction took place between 1980 and 1994. In 1994 (prior to cessation), 1996 (after 16 months) and 1997 (after 28 months), invertebrate communities were surveyed at five extracted sites and three natural sites (1 km outside the extraction area). Samples were collected using a sediment grab (0.1 m2; three samples/site/year) and invertebrates (>1 mm) identified, counted, and dry-weighed.Study and other actions tested
A before-and-after, site comparison study in 2001–2004 of 10 sites in one area of sandy seabed in the northern Mediterranean Sea, Italy (Simonini et al. 2007) found that from 18 months after ceasing aggregate extraction, invertebrate community composition, species richness, abundance and diversity were more similar to that of pre-extraction, and adjacent sites where extraction did not occur. Similarity in community composition with that of pre-extraction increased over time following cessation of extraction, from 9% similarity after one month, to 41–48% after 18–30 months. In addition, community composition became statistically similar to that of adjacent unextracted natural sites over time (59–66% similarity). Invertebrate species richness at extraction sites after 30 months (60–78) was similar to before extraction (54–72), and to unextracted sites (72–79). This was also true for invertebrate abundance (after: 2,300–2,500; before: 1,400–2,400; unextracted: 1,800–2,900 individual/m2), and diversity (as a diversity index) (these data were not statistically tested). Three sites (40–42 m depth) were dredged for aggregate extraction in April–May 2002. These and seven adjacent unextracted sites were surveyed once before (March 2001) and once during (April 2002) dredging, and six times following cessation (after one, six, 12, 18, 24 and 30 months). Three sediment samples/site/survey were collected using a grab (24 dm3) and pooled. Invertebrates (>0.5 mm) were identified and counted.Study and other actions tested
Referenced paperSimonini R., Ansaloni I., Bonini P., Grandi V., Graziosi F., Iotti M., Massamba-N’Siala G., Mauri M., Montanari G., Preti M., De N.N. & Prevedelli D. (2007) Recolonization and recovery dynamics of the macrozoobenthos after sand extraction in relict sand bottoms of the Northern Adriatic Sea. Marine Environmental Research, 64, 574-589
A before-and-after study in 1978–2004 in one area of soft seabed of the Kwintebank, North Sea, Belgium (Vanaverbeke & Vincx 2008) found that following prohibition of aggregate extraction, nematode worm community composition changed but remained different to that of pre-extraction after 8–12 months, and worm abundance and diversity did not change. Community composition after cessation was different to that of during intense extraction, but also to that of before intense extraction began (presented as graphical analyses). Worm abundance and diversity were similar before and 8–12 months after extraction stopped and ranged between 84 and 228/10 cm2 (abundances for each time period not presented; diversity reported as 10 different indices). In February 2003, extraction was prohibited where aggregate extraction had occurred since 1976. Two to three stations were sampled in 1978 (prior extraction intensification), in 1997 and 2001 (during intense extraction), and in October 2003 and February 2004 (eight and 12 months after extraction stopped, respectively). Sediment samples were collected using a 10 cm2 core, and nematode worms (between 38 µm and 1 mm) identified and counted.Study and other actions tested
A systematic review of 22 case studies reported between 1977 and 2007 of marine aggregate extraction sites across the world (Waye-Barker et al. 2015) found that, after extraction stopped, invertebrate communities took between nine months (Bristol Channel, UK) and several decades (Thames, UK) to ‘recover’ (terminology not explained) and become similar to communities occurring at non-impacted sites or prior to-extraction. Invertebrate community recovery time varied with seabed type and current strength (data not statistically tested). The shortest average ‘recovery’ time (4.5 years) was recorded for shallow mixed sediment plains with moderate currents (1.8–4 Nm2). The longest average ‘recovery’ time (10.8 years) was recorded for shallow coarse sediment plains with weak currents (0–1.8 Nm2). Case studies were identified by using set search terms, and included peer-reviewed publications (n=18), technical reports (n=2), unpublished data (n=1) and personal communication (n=1). Aggregate extraction sites were categorised by seabed type and current strength. Invertebrate ‘recovery’ times were extracted from community composition, abundance, biomass and diversity data for each site (data not presented).Study and other actions tested
A site comparison study in 2004 of seven sites of sandy seabed in the southern North Sea, off the coast of Belgium (Barrio-Frojan et al. 2011) found that 21 months after ceasing aggregate extraction at sites, invertebrate species richness and abundance were similar to that of nearby natural sites where extraction did not occur. Extracted sites had similar number of invertebrate species (16–18/site) compared to natural sites (12–17/site) after 21 months, and in similar abundance (extracted: 700–990 individuals/m2; natural: 480–860 individuals/m2). In February 2003, aggregate extraction ceased in the Kwinte Bank licence area. In November 2004, invertebrates were surveyed at three extracted sites and at four natural sites in the nearby Middelkerke Bank. Five samples/sites were collected using a sediment grab, and invertebrates (>1 mm) were identified and counted.Study and other actions tested
A site comparison study in 2001–2004 of four sites of sandy seabed in the southern North Sea, UK (Bonne 2010) found that ceasing aggregate extraction did not lead to changes in invertebrate community composition, or increases in species richness, biomass or abundance, after five years, which all remained different to that of two natural sites where extraction did not occur. Invertebrate community composition did not change from one year to another at any of the sites. After five years, community composition at the extracted sites was only 32.5% similar to that of the natural sites. In addition, average invertebrate species richness, biomass and abundance did not change from one year to another at any of the sites, and was consistently lower at the extracted sites (richness: 12–20 species/sample in 2001, 12–15 in 2004; biomass: 0.03 g/sample in 2001, 0.12 in 2004; abundance: 20–42 individuals/samples in both 2001 and 2004) compared to the natural sites (richness: 55 in 2001, 45 in 2004; biomass: 0.58 in 2001, 0.32 in 2004; abundance: 141 in 2001, 563 in 2004). In 1999, aggregate extraction ceased in a licence area. Between 2001–2004, invertebrates were surveyed yearly at two extracted sites within the licence area and two natural sites 1–15 km outside. Ten samples/survey/extracted sites and five/survey/natural sites were collected using a sediment grab (0.1 m2). Invertebrates (>0.5 mm) were identified, counted and dry-weighed.Study and other actions tested
A site comparison study in 2001–2011 of three sites in one area of sandy seabed off the southeast coast of England, North Sea, UK (Foden et al. 2009) found that, 15 years after ceasing aggregate extraction and letting the seabed recover naturally, invertebrate community composition, species richness, abundance and biomass were similar to that of adjacent sites where extraction did not occur. Although still different after five years, invertebrate community composition at the extraction site became more similar to that of the non-extraction sites over time and was undistinguishable after 15 years (data presented as graphical analyses and statistical model results). After 15 years, extraction and non-extraction sites had similar invertebrate species richness (55 vs 62), abundance (171 vs 183 individuals/0.1 m2), and biomass (0.6 vs 0.7 g/0.1 m2). In 2011, ten samples were collected using a sediment grab (0.1 m2) at a site where intense aggregate extraction had ceased in 1997, and five at each of two adjacent non-extracted sites, all at 27–35 m depths. Invertebrates (1 mm) were identified, weighed, and counted. Data were combined with prior surveys undertaken using the same sampling design in 2001, 2002, 2003, 2004 and 2007.Study and other actions tested