Ecological enhancement of coastal protection structures

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 For example, 14% of the United States coastline (Popkin, 2015^[1]) and 60% of the natural coastline in China have been replaced by hard seawalls (Ma et al., 2014^[2]). 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 ^[3]. 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^[4], and calcium carbonate-forming epifauna can protect concrete structures from weathering and erosion^[5], 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, ...)^[6]. There is also evidence that first colonization by opportunistic invaders influences the subsequent successional sequence^[7].

Eco-engineering artificial structures

Fig. 1a. Precast concrete revetment with multiple habitat structures. Photo credit: Deltares[8].

Fig. 1b. Precast concrete armor block with multiple habitat structures (crevices, cavities). Photo credit: Deltares[8].

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^[9]. The comparative analysis of Strain et al. (2018^[3]) 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.

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^[10]. More complex structures, providing many different types of microhabitats, can be produced with 3D printing techniques, using ceramic binder jetting printers^[11]. 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^[12]. 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^[3].

Native and non-native species

The introduction of artificial habitats in a natural environment promotes 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^[13]. For example, Airoldi et al. (2015^[14]) 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^[10]. 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 facilitates colonization by opportunistic species. Marine 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^[15][16]. 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^[17]. 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^[18]. 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^[10]. 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^[19]. 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^[19]. 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