Create grooves and small protrusions, ridges or ledges (1–50 mm) on subtidal artificial structures
Overall effectiveness category Awaiting assessment
Number of studies: 3
Background information and definitions
Definition: ‘Groove habitats’ are depressions with a length to width ratio >3:1 and depth 1–50 mm (modified from “Crevices” in Strain et al. 2018). ‘Small protrusions’ are elevations with a length to width ratio ≤3:1 that protrude 1–50 mm from the substratum (modified from “Small elevations” in Strain et al. 2018). ‘Small ridges and ledges’ are elevations with a length to width ratio >3:1 that protrude 1–50 mm from the substratum (modified from “Small elevations” in Strain et al. 2018). On vertical surfaces, vertically-orientated elevations that fit these criteria are referred to as ‘ridges’, while horizontal ones are referred to as ‘ledges’. On horizontal surfaces, these features are referred to as ‘ridges’ regardless of their orientation.
Grooves, small protrusions, ridges and ledges provide organisms refuge from predation or grazing in subtidal rocky habitats (Nelson & Vance 1979; Wahl & Hoppe 2002). Some species preferentially settle in and around them (Andrews & Anderson 2004; Bourget et al. 1994). The size and density of grooves, protrusions, ridges and ledges is likely to affect the size, abundance and variety of organisms that can use them. Small habitats can provide refuge for small-bodied organisms but may exclude larger organisms, limit their growth and get rapidly filled-up (Firth et al. 2020). Large habitats can be used by larger-bodied organisms but may not provide sufficient refuge from predators for smaller organisms. By default, horizontal protrusions/ledges (overhangs) create shaded and downward-facing surfaces, which can be associated with the presence of non-native species (Dafforn 2017).
Grooves, protrusions, ridges and ledges are sometimes present on artificial structures such as cable mattresses (Lacey & Hayes 2020) or quarried boulders (MacArthur et al. 2020), but are often absent from other types of structures. Groove habitats, small protrusions, ridges and ledges can be created on subtidal artificial structures by adding or removing material, either during construction or retrospectively. In some scenarios, creating one will automatically result in creation of the other (i.e. grooves created in between created protrusions/ridges/ledges, or vice versa). Studies containing such scenarios are considered under this joint intervention.
See also: Create textured surfaces (≤1 mm) on subtidal artificial structures; Create natural rocky reef topography on subtidal artificial structures; Create pit habitats (1–50 mm) on subtidal artificial structures; Create hole habitats (>50 mm) on subtidal artificial structures; Create groove habitats (1–50 mm) on subtidal artificial structures; Create crevice habitats (>50 mm) on subtidal artificial structures; Create small protrusions (1–50 mm) on subtidal artificial structures; Create small ridges or ledges (1–50 mm) on subtidal artificial structures.
Supporting evidence from individual studies
A replicated, randomized, paired sites, controlled, before-and-after study in 2014–2016 on a subtidal seawall in a marina in the Mediterranean Sea, Israel (Perkol-Finkel et al. 2018) found that groove habitats and small ledges created on seawall panels, along with holes and environmentally-sensitive material, supported higher macroalgae and invertebrate species diversity and richness and different community composition compared with standard-concrete seawall surfaces without added habitats. After 22 months, macroalgae and invertebrate species diversity (data reported as Shannon index) and richness was higher on panels with added habitats (9 species/quadrat) than on seawall surfaces without (5/quadrat), and compared with seawall surfaces before habitats were added (1/quadrat). Community composition differed between panels with added habitats and seawall surfaces without (data reported as statistical model results). Two non-mobile invertebrate species groups recorded on panels were absent from surfaces without. It is not clear whether these effects were the direct result of creating grooves and ledges, holes, or using environmentally-sensitive material. Groove habitats and small ledges were created on seawall panels (height: 1.5 m; width: 0.9 m; thickness: 130 mm) using a formliner. Each panel had multiple interlocking rectangular grooves and ledges (length: 50–150 mm; width/depth/height: 10–50 mm) amongst multiple holes. Panels were made from patented ECOncreteTM material. Four panels were attached to a vertical concrete seawall in November 2014. The bottom 1.2 m were subtidal. Seawall surfaces were subtidal areas of seawall cleared of organisms (height: 1.2 m; width: 0.9 m) adjacent to each panel. Macroalgae and invertebrates were counted in one 300 × 300 mm randomly-placed quadrat on each panel and seawall surface over 22 months.Study and other actions tested
A replicated, randomized, controlled study in 2016 on pontoons in a marina in the Fal estuary, UK (Hanlon et al. 2018) found that upward-facing settlement plates with groove habitats and small protrusions supported different macroalgae and invertebrate community composition, with similar species diversity but higher species richness and abundances, than upward-facing plates without grooves or protrusions, but that there were no significant differences on downward-facing plates. After six months, upward-facing plates with grooves and protrusions supported different macroalgae and invertebrate community composition (data reported as statistical model results) with similar species diversity (data not reported) but higher species richness (20 species/plate, reported from Figure 4) and macroalgae and non-mobile invertebrate live cover (29% cover), compared with plates without grooves and protrusions (15 species/plate, 13% cover). On downward-facing plates, there were no significant differences between plates with and without grooves and protrusions (both 25 species/plate; 92 vs 86% cover). Settlement plates (150 × 150 mm) were moulded with a regular grid of six groove habitats (length: 150 mm; width/depth: 10 mm) between 15 rectangular small protrusions (length: ~44 mm; width: ~22 mm; height: 10 mm) on one surface, but flat on the other. Plates were either standard-concrete or oyster-shell-concrete. Forty plates were suspended horizontally, randomly arranged, beneath floating pontoons at 2–3 m depth in April 2016. Ten of each material had grooves and protrusions facing up, while 10 of each faced down. Macroalgae and invertebrates on upward- and downward-facing surfaces were counted in the laboratory over six months.Study and other actions tested
A replicated study in 2014–2015 on a subtidal rocky reef on open coastline in the Alboran Sea, Spain (Sempere-Valverde et al. 2018) reported that settlement plates with groove habitats and small protrusions supported 33 macroalgae and non-mobile invertebrate species groups. After 11 months, plates with grooves and protrusions supported 33 species groups in total (20 macroalgae, 13 non-mobile invertebrates). On average, there were nine species/pair of plates, with 55% live cover. Settlement plates (170 × 170 mm) were cut to create a regular grid of six groove habitats (length: 170 mm; width/depth: ~7 mm) between 16 square protrusions (length/width: 30 mm; height: ~7 mm) on their surfaces. Plates were either sandstone, limestone, gabbro, slate or concrete. Two of each material were attached horizontally at 15 m depth on gneiss rocky seabed in each of three sites in June 2014. Macroalgae and non-mobile invertebrates on each pair of plates were counted from photographs over 11 months.Study and other actions tested