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Ecological enhancement of coastal protection structures aims to reduce the ecosystem alteration and impoverishment that often results when applying conventional artificial hard structures. Experiments of ecological enhancement are being carried out with the incorporation of microhabitats into artificial structures. These microhabitats are designed to promote the development of marine ecosystems that thrive on hard substrate. An important further step is the integration of ecosystems that reinforce the shore protection function of artificial structures. Examples discussed in this article are ecological enhancement with reef building organisms (oysters, corals), with vegetated revetments and with vegetated forelands.




==Introduction==
Large coastal stretches around the world have been armored with hard protection structures. For example, 14% of the United States coastline (Popkin, 2015<ref>Popkin, G. 2015. Fourteen percent of U.S. coastline is covered in concrete. Science [Online]. Available: https://www.sciencemag.org/
news/2015/08/fourteen-percent-us-coastline-covered-concrete.</ref>) and 60% of the natural coastline in China have been replaced by hard seawalls (Ma et al., 2014<ref>Ma, Z., Melville, D.S., Liu, J., Chen, Y., Yang, H., Ren, W., Zhang, Z., Piersma, T. and Li, B. 2014. Rethinking China's new great wall. Science. 346:912–914</ref>). The proportion of shores protected with hard coastal structures will likely increase in future in response to sea level rise. 

The biodiversity of ecosystems that develop on artificial hard coastal protection structures is lower than the biodiversity on natural hard substrates in the same environment.
As compared to the largely horizontal and topographically complex surfaces of natural substrates, marine urban infrastructure typically has vertical, smooth surface that reduces the area for attachment and the diversity of habitat niches for organisms, and provides fewer refuges from predators, competitors and/or environmental stressors <ref name=S18>Strain, E.M.A., Olabarria, C., Mayer-Pinto,M., Cumbo, V., Morris, R.L., Bugnot, A.B., Dafforn, K.A., Heery, E., Firth, L.B., Brooks, P.R. and Bishop, M.J. 2018. Eco-engineering urban infrastructure for marine and coastal biodiversity: which interventions have the greatest ecological benefit? J. Appl. Ecol. 55: 426–441</ref>. It has been shown, for example, that surface roughness positively affects the build-up of marine biofilms, increases primary productivity and enhances further community development. Conversely, attached macro- and microalgae on concrete can reduce its exposure to variations in temperature and humidity<ref>Coombes, M. A., Viles, H. A., Naylor, L. A. and La Marca, E. C. 2017. Cool Barnacles: Do Common Biogenic Structures Enhance or Retard Rates of Deterioration of Intertidal Rocks and Concrete? Science of the Total Environment 580: 1034–1045</ref>, and calcium carbonate-forming epifauna can protect concrete structures from weathering and erosion<ref>Kawabata, Y., Kato, E. and Iwanami, M. 2012. Enhanced Long-Term Resistance of Concrete with Marine Sessile Organisms to Chloride Ion Penetration. Journal of Advanced Concrete Technology 10: 151–159</ref>, enhancing the durability of the concrete in both cases. Other possible causes of the lesser complexity and heterogeneity of ecosystems on artificial hard surfaces are disturbance by humans (periodic maintenance works) and natural factors (storms, sediment scour, …) and grazing pressure (limpets, gastropods, ...)<ref>Farrugia Drakard, V., Brooks, P.R. and Crowe, T.P. 2023. Colonisation after disturbance on artificial structures: The influence of timing and grazing. Marine Environmental Research 187, 105956</ref>. There is also evidence that first colonization by opportunistic invaders influences the subsequent successional sequence<ref>Bulleri, F. 2005. Role of recruitment in causing differences between intertidal assemblages on seawalls and rocky shores. Mar. Ecol. Prog. Ser. 287: 53–64</ref>.

==Eco-engineering artificial structures==


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[[File:EcoBlockRevetment.jpg|thumb|left|350px|Fig. 1a. Precast concrete revetment with multiple habitat structures. Photo credit: Deltares<ref name=RW>Reinders, J. and van Wesenbeeck, B. (eds.) 2013. Eco-engineering in the Netherlands. Soft interventions with a solid impact. Deltares http://publications.deltares.nl/Deltares058.pdf</ref>.]]
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[[File:EcoBlockArmor.jpg|thumb|left|350px|Fig. 1b. Precast concrete armor block with multiple habitat structures (crevices, cavities). Photo credit: Deltares<ref name=RW/>. ]]
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Minor interventions can greatly enhance the habitat function of artificial hard structures for the development of more diverse ecosystems.
A common technique for eco-engineering marine infrastructure consists of so-called 'greening the grey': increasing surface area and/or habitat complexity of the hard substrate at a range of scales (mm to metres) using either additive (i.e. attachment of protruding structures) or subtractive (i.e. drilling, removal of substrate) processes. Fine-scale (μm-cm) surface roughness greatly improves the early phase colonisation of marine organisms. The abundance and number of species that settle on these eco-engineered structures are much higher than on conventional smooth structures. Suitable adaptations of artificial hard structures can provide shading and water-retaining microhabitats for intertidal organisms to prevent thermal and desiccation stress at low tide<ref>MacArthur, M., Naylor, L.A., Hansom, J.D. and Burrows, M.T. 2020. Ecological enhancement of coastal engineering structures: Passive enhancement techniques. Science of the Total Environment 740, 139981</ref>.
The comparative analysis of Strain et al. (2018<ref name=S18/>) showed that
* in the intertidal zone, interventions that provided moisture and shade had the greatest effect on the richness of sessile and mobile organisms, while water-retaining features had the greatest effect on the richness of fish species. The taxa that responded most positively to eco-engineering in the intertidal were those whose body size most closely matched the dimensions of the resulting intervention.
* in the subtidal zone, small-scale depressions which provide refuge to new recruits from predators and other environmental stressors such as waves, had higher abundances of sessile organisms, while elevated structures had higher numbers and abundances of fish. 

[[File:ReefBallEcoshape.jpg|thumb|right|450px|Fig. 2. Left: Reef Balls colonized by marine organisms. Right: Reef Ball breakwater. Image credit Ecoshape Building with Nature https://www.ecoshape.org/en/]]

Precast concrete units have been developed that can be used for ecological enhancement of marine structures. The Reef Ball (a concrete structures in the shape of halve a ball with holes, see [[Artificial reefs]]) can be used for a variety of purposes, ranging from coral and oyster reef rehabilitation to mangrove planting. Reef Balls have also been deployed for stimulating the development of new reefs with a coastal protection function (Fig. 2). 
Other concrete units with multiple habitats have been developed for the armoring of breakwaters<ref name=F16>Firth, L.B., Knights, A.M., Bridger, D., Evans, A.J., Mieszkowska, N., Moore, P.J., O'Connor, N.E., Sheenan, E.V., Thompson, R.C. and Hawkins, S.J. 2016.  Ocean sprawl: challenges and opportunities for biodiversity management in a changing world. Oceanography and Marine Biology: An Annual Review 2016: 193-269</ref>. More complex structures, providing many different types of microhabitats, can be produced with 3D printing techniques, using ceramic binder jetting printers<ref>Berman, O., Weizman, M., Oren, A., Neri, R., Parnas, H., Shashar, N. and Tarazi, E. 2023. Design and application of a novel 3D printing method for bio-inspired artificial reefs. Ecological Engineering 188, 106892</ref>. Materials of biotic origin can be used instead of rock, concrete or ceramic. For example, oyster shell bags can serve for wave attenuation as an alternative to gabions or rock structures<ref>Allen, R.J. and Webb, B.M. 2011. Determination of Wave Transmission Coefficients for Oyster Shell Bag Breakwaters. In: Magoon, O.T., Noble, R.M., Treadwell, D.D. and Kim, Y.C. Eds. Coastal Engineering Practice. American Society of Civil Engineers, Reston, VA, USA, pp. 684–697</ref>. A different approach consists of 'seeding' concrete or rock surfaces with habitat-forming taxa such as barnacles, bivalves, canopy-forming algae, branching coralline algae or corals<ref name=S18/>. <br clear=all>

==Native and non-native species==
The introduction of artificial habitats in a natural environment promotes in general the establishment of non-native species. This is obvious when hard structures are introduced in sedimentary coastal environments where rock or other hard elements are completely absent. However, non-indigenous species also appear more prevalent on artificial structures when comparable natural habitats are nearby<ref>Mineur, F., Davies, A.J., Maggs, C.A., Verlaque, M. and Johnson, M.P. 2010. Fronts, jumps and secondary introductions suggested as different invasion patterns in marine species, with an increase in spread rates over time. Proceedings of the Royal Society B: Biological Sciences 277: 2693–2701</ref>. For example, Airoldi et al. (2015<ref>Airoldi, L., Turon, X., Perkol- Finkel, S. and Rius, M. 2015. Corridors for aliens but not for natives: effects of marine urban sprawl at a regional scale. Diversity and Distributions 21: 755–768</ref>) found that non-indigenous species were two or three times more abundant on infrastructure built along sedimentary coastlines than on natural rocky reefs or infrastructure built close to rocky coastlines in the northern Adriatic. Several possible causes have been proposed<ref name=F16>Firth, L.B., Knights, A.M., Bridger, D., Evans, A.J., Mieszkowska, N., Moore, P.J., O'Connor, N.E., Sheenan, E.V., Thompson, R.C. and Hawkins, S.J. 2016.  Ocean sprawl: challenges and opportunities for biodiversity management in a changing world. Oceanography and Marine Biology: An Annual Review 2016: 193-269</ref>. For example, the lower species diversity and density of artificial structures compared to natural habitats implies a lesser resistance to the establishment of invasive species. The high levels of physical (wave) disturbance of artificial structures may facilitate colonization by opportunistic species. However, a study comparing artificial structures with features most similar to natural habitats (natural substrates, fixed structures, upward-facing surfaces) with structures least similar to natural habitats (artificial materials, floating structures, downward-facing or vertical surfaces) found that there were no major differences in colonization by native and non-native species. It turned out that functional groups (i.e. algae, sea squirts, barnacles, bryozoans, polychaetes) and not species origin (i.e. native or non-native) were the main drivers of community differences between different types of artificial structures<ref>Schaefer, N., Bishop, M.J., Bugnot, A.B., Foster-Thorpe, C., Herbert, B., Hoey, A.S., Mayer-Pinto, M., Nakagawa, S., Sherman, C.D.H., Vozzo, M.L. and Dafforn, K.A. 2024. Influence of habitat features on the colonization of native and non-indigenous species. Marine Environmental Research 198, 106498</ref>.

Maritime infrastructure in areas frequented by ships and other transport vectors are particularly susceptible to the establishment of non-indigenous species. 

Pre-seeding substrates with native fouling species (e.g. habitat forming algae) can strongly reduce occupation by non-indigenous species<ref>Dafforn, K.A. 2017. Eco-engineering and management strategies for marine infrastructure to reduce establishment and dispersal of non-indigenous species. Management of Biological Invasions 8: 153–161</ref><ref>Bradford, T.E., Astudillo, J.C., Lau, E.T.C., Perkins, M.J., Lo, C.C., Li, T.C.H., Lam, C.S., Ng, T.P.T., Strain, E.M.A., Steinberg, P.D. and Leung, K.M.Y. 2020.  Provision of refugia and seeding with native bivalves can enhance biodiversity on vertical seawalls. Marine Pollution Bulletin 160, 111578</ref><ref>Vozzo, M., Mayer-Pinto, M., Bishop, M., Cumbo, V., Bugnot, A., Dafforn, K., Johnston, E., Steinberg, P. and Strain, E. 2021. Making seawalls multifunctional: the positive effects of seeded bivalves and habitat structure on species diversity and filtration rates. Mar. Environ. Res. 165, 105243</ref>. Traditional control methods such as chemical treatment or mechanic removal of organisms have serious drawbacks. It has been suggested that biocontrol, i.e., control by natural predators, either native or non- indigenous, could overcome these limitations<ref>Atalah, J., Newcombe, E.M., Hopkins, G.A. and Forrest, B.M. 2014. Potential biocontrol agents for biofouling on artificial structures. Biofouling 30: 999–1010</ref>. Another technique to limit colonization by non-indigenous species consists of creating artificial habitats that attract desired indigenous species. For example, promising results have been obtained with experimental reef balls designed to favor recruitment by indigenous oysters rather than the globally invasive ''Magallana'' (formerly ''Crassostrea'') ''gigas''<ref>Perog, B.D., Bowers-Doerning, C., Lopez Ramirez, C.Y., Marks, A.N., Torres Jr., R.F., Wolfe, M.L. and Zacherl, D.C. 2023.  Shell cover, rugosity, and tidal elevation impact native and non-indigenous oyster recruitment: Implications for reef ball design. Ecological Engineering 192, 106969</ref>. The experiments also showed that engineered concrete performance is context-dependent; oyster recruitment strength varied depending upon species, engineering treatment, site, and tidal elevation. 

==Ecosystem connectivity==
Local artificial habitats can either stimulate or disrupt the connections within the larger ecosystem. It can provide stepping stones for the dispersal of threatened species, but it can also facilitate the spread of harmful species. Inversely, artificial structures can disrupt connectivity in ecosystems by changing flow patterns and modifying transport pathways<ref name=F16/>. The design of large-scale deployments of artificial habitats thus requires understanding of the dispersal routes of targeted species.     

==Use of vegetation==
Combining increased erosion resistance and ecological value can also be achieved by use of vegetation, either directly on the coastal protection structure or in front of it.
===Natural grass-covered sea dikes=== 
In Germany, sea dikes usually have a grass-covered gentle slope of about 1/7 as protection against erosion by (overtopping) waves; hard revetments are used only at the most exposed locations. Studies showed that species-rich grass cover has the highest erosion resistance, which occurs for dikes where no fertilizer is used<ref name=SS>Scheres, B. and Schüttrumpf, H. 2019. Enhancing the ecological value of sea dikes. Water 11, 1617</ref>. The erosion resistance of vegetated sea dikes can be further enhanced by putting the grass cover on top of a hard or flexible geogrid structure or by seeding the grass on turf reinforcement mats<ref name=SS/>. See also [[Overtopping resistant dikes]].
===Foreland restoration and creation===
Restoration or creation of forelands are environment-friendly solutions to circumvent the need for enlarging hard coastal protection structures under [[sea level rise]]. 

On exposed sandy coasts, raising and widening the beach through [[shore nourishment]] with offshore dredged sand can provide additional protection against erosion and wave attack. Further protection can be provided by beach vegetation, see [[Shore protection vegetation]]. 

On less energetic coasts, wave attenuation can be achieved by restoring or stimulating the development of mangroves (in tropical and subtropical zones, see [[Mangroves]]) or salt marshes (in temperate zones, see [[Restoration of estuarine and coastal ecosystems]] and [[Nature-based shore protection]]).

==Biogenic reefs==
Shore protection does not necessarily require artificial hard structures. Several marine organisms are capable to produce hard structures that can fulfil a shore protection function. These organisms are sometimes called ecosystem-engineers. Examples are:
* ''Sabellaria alveolata'', a sedentary tube-dwelling polychaete that constructs tubes from suspended sediment and shell fragments on exposed open coasts in temperate climate zones. These tubes form reefs, which are generally found at the lower level of the intertidal zone, are up to 1.5 m in height and can develop to cover acres of sand flats, see [[Nature-based shore protection]].
* Corals, which grow wave resistant rock structures in tropical and subtropical waters. Coral reefs are created by calcium carbonate secreting animals and plants, see [[Coral reefs]]. 
* Oysters, which naturally aggregate and attach themselves to older shells, rocks, or submerged surfaces, creating a rocklike reef structure, see [[Oyster reef shore protection]].
* Mussels (''Mytilus spp.''), which form rough and sediment retaining mussel beds that dissipate wave energy, see [[Nature-based shore protection]].

See also: [[Biogenic reefs of Europe and temporal variability]], [[Dynamics, threats and management of biogenic reefs]].



==Related articles==
:[[Nature-based shore protection]]
:[[Rocky shore habitat]]
:[[Artificial reefs]]
:[[Hard coastal protection structures]]
:[[Restoration of estuarine and coastal ecosystems]]
:[[Mangroves]]


==References==
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{{author
|AuthorID=120
|AuthorFullName=Job Dronkers
|AuthorName=Dronkers J}}

[[Category:Coastal protection]]
[[Category:Coastal and marine ecosystems]]
[[Category:Soft coastal interventions]]