Landscape or artificially enhance the seabed (natural habitats)
Overall effectiveness category Unknown effectiveness (limited evidence)
Number of studies: 3
Background information and definitions
Landscaping or artificial enhancement can be undertaken as a restoration method aimed to promote the recovery and recolonization processes of subtidal benthic invertebrates. This could also constitute a pre-restoration method to improve the seabed and potentially enhance further restoration. For instance, following cessation of marine aggregate extraction, to try recreating the natural substrate, a layer of gravel or shell can be added to the seabed or the sediment can be landscaped (Cooper et al. 2011; de Jong et al. 2015). Broken limestone rubble or empty shellfish shells have also been laid on the seabed to enhance the natural habitat and enhance recolonization of natural biodiversity (Fariñas-Franco et al. 2013; Jones & Nithyanandan 2013; Meyer & Townsend 2000).
Cooper K., Ware S., Vanstaen K. & Barry J. (2011) Gravel seeding - A suitable technique for restoring the seabed following marine aggregate dredging? Estuarine, Coastal and Shelf Science, 91, 121–132.
de Jong M.F., Baptist M.J., Lindeboom H.J. & Hoekstra P. (2015) Short-term impact of deep sand extraction and ecosystem-based landscaping on macrozoobenthos and sediment characteristics. Marine Pollution Bulletin, 97, 294–308.
Fariñas-Franco J.M., Allcock L., Smyth D. & Roberts D. (2013) Community convergence and recruitment of keystone species as performance indicators of artificial reefs. Journal of Sea Research, 78, 59–74.
Jones D.A. & Nithyanandan M. (2013) Recruitment of marine biota onto hard and soft artificially created subtidal habitats in Sabah Al-Ahmad Sea City, Kuwait. Marine Pollution Bulletin, 72, 351–356.
Meyer D.L. & Townsend E.C. (2000) Faunal utilization of created intertidal eastern oyster (Crassostrea virginica) reefs in the southeastern United States. Estuaries, 23, 34–45.
Supporting evidence from individual studies
A controlled, before-and-after study in 2005–2007 in a sandy seabed area in the southern North Sea, UK (Cooper et al. 2011) found that depositing gravels to recreate natural habitat after ceasing aggregate extraction changed invertebrate community composition and increased species richness, abundance and biomass, after 12 months. Community composition became less similar to that of a site without gravel and more similar to that of a natural site (similarity with site without gravel presented as graphical analyses; similarity with natural site community increased from 14% to 28%). Invertebrate species richness increased from 46/m2 before gravel deposition to 118/m2 after 12 months. There were also increases in invertebrate abundance (before: 222; after: 3,081 individuals/m2) and biomass (before: 0.6; after: 7.5 g/m2). In May 2005, July 2005, July 2006 and May 2007, invertebrates were surveyed at three sites at 22–33 m depths. Two sites were historically subjected to aggregate extraction (1996–2000), of which one was added 4,444 m3 of gravels in July 2005 and the other left without gravel. The third site was natural (never subjected to aggregate extraction). Ten samples/site/survey were collected using sediment grabs (0.1 m2). Invertebrates (>0.5 mm) were dried, weighed, and counted.Study and other actions tested
A before-and-after, site comparison study in 2004–2009 of two sites in one sandy seabed area in the Westerschelde estuary, southwestern Netherlands (van der Wal et al. 2011) found that disposing of dredge material in a shallow subtidal zone to enhance natural habitat did not affect invertebrate community composition, nor promote species richness, abundance, or biomass after up to five years. Invertebrate community composition did not change over time and remained different to that of the natural site (before: 65%; after: 31% similarity). After five years, average species richness (1.8 species/sample), abundance (data not reported) and biomass (5.4 mg/m2) remained similar to pre-disposal values (species: 1.9; biomass: 7.2) and to values found at a nearby natural site (species: 1.7–1.8; biomass: 6.2–6.8). Dredged sand (500,000 m3) was disposed at one site in November–December 2004. A second site (2 km away) was left natural. Yearly in spring and autumn between 2004 and 2009, three sediment cores (30 cm depth, 8 cm diameter) were taken (then pooled) at each of twenty locations/site. Invertebrates (> 1mm) were identified, counted, and dry-weighed.Study and other actions tested
A site comparison study in 2004–2011 in one soft seabed area in the Persian Gulf, southern Kuwait (Jones & Nithyanandan 2013), found that seabed sites artificially enhanced by adding broken coral limestone rubble developed similar invertebrate species richness and abundance compared to natural sites, within two to six years. Results were not tested for statistical significance. Within two to six years, invertebrate species richness appeared similar in artificially enhanced (38.3–129 species/site) and natural site (66 species/site), and within one to two years abundance appeared higher in artificially enhanced (206–19,404 individual/m3) than natural sites (2,263 individual/m3). Following the construction of Sabah Al-Ahmad Sea City, waterways were gradually opened to the sea between 2004–2011 and sections of the seabed were artificially enhanced to promote colonisation. Annually in 2004–2011, samples were collected at 3–12 enhanced sites (3–4 m depth) using a sediment grab (three grabs/site). Invertebrates (>0.5 mm) were identified and counted. Data for the natural sites were obtained from surveys of 13 sites sampled in 2002–2004 by the United Nations Claim Commission (156 surveys in total, methodology not described).Study and other actions tested