Reduce the slope of intertidal artificial structures
Overall effectiveness category Awaiting assessment
Number of studies: 2
Background information and definitions
Definition: ‘Reducing the slope’ includes actions taken to reduce the inclination of structures without increasing the footprint, with the aim of enhancing their biodiversity.
The slope of substrate surfaces can influence the species that colonize in intertidal rocky habitats (Harmelin-Vivien et al. 1995; Vaselli et al. 2008; but see Cacabelos et al. 2016; Firth et al. 2016). Artificial structures tend to have steeper slopes than natural reefs, with narrower bands of intertidal habitat. This means that space for organisms is scarce and competitive interactions and other environmental processes differ (Chapman & Underwood 2011). Steep surfaces can also be associated with the presence of non-native species (Dafforn 2017).
Although fundamental aspects of structure designs, such as their slope, are likely to be driven by engineering and cost requirements, there may be opportunities to reduce the slope of intertidal artificial structure surfaces with the aim of enhancing their biodiversity. This may, however, lead to an increase in the physical footprint of structures and associated impacts on the receiving environment (Perkins et al. 2015). For this reason, studies that test the effects of creating additional artificial habitat in front of existing structures to create horizontal or gently sloping surfaces are not included in this synopsis, although such actions can deliver biodiversity benefits and are informative (e.g. Liversage & Chapman 2018; Toft et al. 2013).
See also: Create small protrusions (1–50 mm) on intertidal artificial structures; Create large protrusions (>50 mm) on intertidal artificial structures; Create small ridges or ledges (1–50 mm) on intertidal artificial structures; Create large ridges or ledges (>50 mm) on intertidal artificial structures.
Cacabelos E., Martins G.M., Thompson R., Prestes A.C.L., Azevedo J.M.N. & Neto A.I. (2016) Factors limiting the establishment of canopy-forming algae on artificial structures. Estuarine, Coastal and Shelf Science, 181, 277–283.
Chapman M.G. & Underwood A.J. (2011) Evaluation of ecological engineering of “armoured” shorelines to improve their value as habitat. Journal of Experimental Marine Biology and Ecology, 400, 302–313.
Dafforn K.A. (2017) Eco-engineering and management strategies for marine infrastructures to reduce establishment and dispersal of non-indigenous species. Management of Biological Invasions, 8, 153–161.
Firth L.B., White F.J., Schofield M., Hanley M.E., Burrows M.T., Thompson R.C., Skov M.W., Evans A.J., Moore P.J. & Hawkins S.J. (2016) Facing the future: the importance of substratum features for ecological engineering of artificial habitats in the rocky intertidal. Marine and Freshwater Research, 67, 131–143.
Harmelin-Vivien M.L., Harmelin J.G. & Leboulleux V. (1995) Microhabitat requirements for settlement of juvenile sparid fishes on Mediterranean rocky shores. Hydrobiologia, 300, 309–320.
Liversage K. & Chapman M.G. (2018) Coastal ecological engineering and habitat restoration: incorporating biologically diverse boulder habitat. Marine Ecology Progress Series, 593, 173–185.
Perkins M.J., Ng T.P.T., Dudgeon D., Bonebrake T.C. & Leung K.M.Y. (2015) Conserving intertidal habitats: what is the potential of ecological engineering to mitigate impacts of coastal structures? Estuarine, Coastal and Shelf Science, 167, 504–515.
Toft J.D., Ogston A.S., Heerhartz S.M., Cordell J.R. & Flemer E.E. (2013) Ecological response and physical stability of habitat enhancements along an urban armored shoreline. Ecological Engineering, 57, 97–108.
Vaselli S., Bertocci I., Maggi E. & Benedetti-Cecchi L. (2008) Assessing the consequences of sea level rise: effects of changes in the slope of the substratum on sessile assemblages of rocky seashores. Marine Ecology Progress Series, 368, 9–22.
Supporting evidence from individual studies
A replicated, controlled study (year not reported) on an intertidal seawall in Sydney Harbour estuary, Australia (Chapman & Underwood 2011) found that reducing the slope of the seawall did not increase the abundance of macroalgae, oysters Saccostrea glomerata or mobile invertebrates on seawall surfaces. Over 24 months, the abundances of macroalgae, oysters and mobile invertebrates were similar on surfaces of a new sloping seawall and on remnants of the original vertical wall that it replaced (data not reported). The slope of a seawall was reduced by replacing a vertical concrete wall with a sloping wall of boulders. This increased the extent of the intertidal area from high to low shore by 2–3 m (timing and other details of the intervention not reported). Macroalgae and invertebrates were counted on 10 surfaces (dimensions not reported) in each of four sites on the new sloping wall, and 10 on a remnant of the original vertical wall, during low tide over 24 months.Study and other actions tested
A before-and-after study in 2012–2013 on an intertidal seawall in Sydney Harbour estuary, Australia (Heath & Moody 2013) reported that reducing the slope of the seawall, along with creating rock pools on the wall, increased the macroalgae, invertebrate and fish species richness on the wall. A total of 25 macroalgae, invertebrate and fish species were recorded on the seawall and in pools after the slope was reduced and pools were created, compared with 10 species on the seawall before (data not statistically tested). It is not clear whether these effects were the direct result of reducing the slope of the seawall or creating rock pools. The slope of a sandstone boulder seawall was reduced during reconstruction in July 2012 (details not reported). Three large rock pools (area: 2 m2; depth: 300 mm; volume: 600 l) were also created on the wall. Macroalgae, invertebrates and fishes were counted during low tide on the wall before reconstruction and on the wall and in pools after reconstruction in 2013 (sampling details and month not reported).Study and other actions tested