| Bfn Properties habitat | |
|---|---|
| Context | Bfn Natural values of solid dikes |
| Type | Outcome |
| Decompositie type | IOR |
Optimization of hard substrate surface structure
Modern revetments are mostly uniform and monotone constructions, focused on coastal security, that display lower habitat complexity and heterogeneity compared to natural rocky shores (Chapman & Bulleri, 2003) (Moschella et al., 2005). Complexity expresses the abundance of similar structures in a habitat, per unit of area or par unit of volume, while heterogeneity encompasses the relative abundance of different structures in a habitat. Combined they provide variation in structured habitats (McCoy & Bell, 1991). Habitat complexity and heterogeneity determine local abiotic factors, which in turn influence species richness and ecological function of hard structures. Furthermore, traditional revetments are highly susceptible to various disturbances when compared to similar natural habitats, both natural (e.g. storms and sediment scouring) and anthropogenic (e.g. harvesting, trampling, maintenance work). Quality of habitat biodiversity is therefore often significantly lower on artificial hard substrates than on natural formations, dominated by opportunistic and invasive species (Airoldi et al., 2005) (Bulleri & Airoldi, 2005) (Bulleri et al., 2006).
In order to promote increase in species richness on coastal defense structures local abiotic conditions can be altered by increasing surface roughness and creating higher level of variation in its relief, such as introducing a larger number of pools, pits and crevices, whose size and shape can also be varied. Small scale differentiation from the surrounding area in abiotic conditions, will create a specific microhabitat. Optimization of physical properties of revetments through changing habitat structure, can create a varied set of these microhabitats, that can be suitable for diverse species communities, in order to strengthen the ecological functions provided by the revetments and increase community resilience.
Species living on hard substrate in Eastern Scheldt
A significant amount of species living in the intertidal zone of Eastern Scheldt inhabit hard substrates for at least a part of their life cycle. Sessile species attach themselves to hard substrate and remain on it for the rest of their lives, for example, barnacles, cnidarians and oysters. Species with higher motility may use hard structures in the substrate as a refuge, habitat or nursery, such as crustaceans and some fish. For instance, fish of Myoxocephalus scorpius and Pholis gunnellus species lay eggs in crevices or prepared nests on hard substrate.
In case of small invertebrates, suitability of hard substrate for their habitat depends on the substrate’s ability to offer shelter from predation, desiccation and wave action. Holes and crevices in the hard substrate should be large enough to allow the invertebrate species to shelter inside them, but small enough to prevent predators from catching the organisms. The size of these refuge spaces must also provide protection against wave action and prevent excessive evaporation during low tide. Therefore, one of the criteria for determining a hard substrate’s suitability for a specific invertebrate species is the available size of the refuge places (Menge et al., 1983). Furthermore, rougher surfaces are a more preferable substrate for encouraging settlement of algae as they provide bigger attachment possibilities when compared smooth surfaces. Algae communities can provide additional ecological benefits, as they provide refuge and food for other organisms. Therefore, heterogeneity of available of refuge spaces and surface structure are important factors for determining possible biodiversity on hard substrate habitats.
Optimal maximum dimensions for refuge spaces in the artificial hard substrate can be determined by considering the maximum size of invertebrate and fish species, that could inhabit these artificial structures, and the functions that this substrate may provide for these organisms. Discerning minimum sizes is not as important, as settlement of most species inhabiting the hard substrate occurs early in their life cycle during the larval (planktonic) stage. Species found in the Eastern Scheldt show a large variability in sizes, from black gobies (Gobius niger) up to 150 mm long, who nurse and guard their 1-2 mm large egg clutches, to barnacles of the Balanidae family up to 15 mm in width, and lobsters such as Hommarus gammarus with carapace length of 380 mm for adults. In order to achieve maximal possible biodiversity, the hard substrate should provide a wide range of refuge space sizes to accommodate individuals of various sizes from a large assortment of species. Albeit further research is necessary to determine the shapes of hiding places required or preferred by specific species, it can be assumed that increase in biodiversity can be achieved with increasing heterogeneity (and decreasing uniformity) in both size and shape of possible refuge spaces in the hard substrate.
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