Restoration of estuarine and coastal ecosystems
This article deals with technical aspects of restoration projects. Cultural, social and economic aspects also play a crucial role. For these aspects the reader is referred to Principles of conservation, rehabilitation and restoration of estuarine and coastal habitats , Introduction of public participation and other articles in the categories Participation and governance in coastal management and Evaluation and assessment in coastal management.
Current efforts to restore estuarine salt-marsh systems in Europe and elsewhere illustrate the political and administrative value placed on the goods and services provided by this ecosystem.
The principle of managed realignment and managed retreat comes down to allowing salt-marsh areas, that were historically converted to alternative use for anthropogenic purposes (e.g. agricultural land or tourist development), to return to their natural state and area cover (Garbutt, et al., 2006). This can be done in a number of ways, but typically involves making a breach in the historically erected barrier (seawall, dike) rather than removing the whole structure. This approach reduces the costs involved, as well as the wave action depressing the development of the vegetation. Cost benefit analyses typically show a net advantage of managed realignment over other constructed defence options (Turner et al., 2007). An overview of managed realignment projects realized until 2014 has been compiled by Esteves (2014). The effectiveness of marsh restoration for coastal protection is discussed in the article Nature-based shore protection.
Full restoration of natural ecosystem function has met some complications. The substrates and biodiversity of pristine salt marshes is often markedly different from an artificial or restored system, even 100 years after natural processes have been allowed to operate (Hazelden and Boorman, 2001). Experience of estuarine restoration by managed realignment projects during the past decades has shown that the estuarine character of newly created mudflats and marshes is easily lost (Mazik et al., 2010). Marshes reactivated by managed realignment do not provide habitats and species in comparable proportions to natural marshes and do not have equivalent biological characteristics (Mossman et al., 2012). It is even questionable whether restored marshes satisfy the requirements of the EU Habitats Directive.
Two aspects, both related to sediment management, deserve particular attention when designing a marsh restoration scheme. The first aspect concerns the design of the tidal inlet structure. An adjustable inlet makes it possible to regulate the sedimentation rate in the restoration area and in this way influence the development of the creek system (Oosterlee et al., 2020). The second aspect concerns the origin of the sediment that is deposited in the restoration area. In estuaries with a small sediment supply (e.g. estuaries with strong ebb dominance), this deposition can alter the sediment balance of the estuary and cause or enhance erosion of flood protection barriers and intertidal areas elsewhere (Morris, 2012).
Because embanked land was often used for livestock farming, the soil has become compacted and poorly permeable. The freshly sedimented top layer of the restoration area has high hydraulic conductivity, organic matter content, macroporosity and a low bulk density, while the underlying relict polder soil has a very low hydraulic conductivity, a low micro- and macroporosity, a low organic matter content and a high bulk density (Van Putte et al., 2020). This hinders drainage, which supposedly affects both vegetation development and nutrient cycling. Measures to be considered are (Brooks et al., 2015; Lawrence et al., 2018): (1) deep plowing, (2) amending the soil with organic wastes to induce the development of macropore networks, (3) the creation of small creeks and hillocks. When formerly embanked land has changed too much, restored salt marshes may be considered novel ecosystem types, existing somewhere between unrestricted and tidally restricted systems (Gerwing et al. 2020).
Mudflats are the unvegetated or sparsely vegetated intertidal transition zones between the tidal channel and the higher vegetated salt marsh. They are home to a large number of organisms that provide a rich food source for fish and birds and fulfill an important nursery function for the estuarine ecosystem (Fig. 2). In addition, strong microbial activity in the sediment top layer fulfills an important water purification function. Estuarine mudflats are typically low diversity/high biomass/high abundance systems, hosting organisms adapted to alternating wet-dry and fresh-salt conditions. According to reports under the European Commission's Habitats Directive, the mudflats (habitat 1140) along the European Atlantic and Mediterranean coasts are in a poor state, especially due to the many land reclamation projects that have taken place in the past (EEA, 2015).
The removal of embankments creates new space for the expansion of mudflats. However, the high turbidity that occurs in many estuaries causes rapid sedimentation. In sheltered restoration areas, mudflats quickly evolve into salt marshes. Even in less turbid estuaries, this happens over time. Preservation of mudflats therefore requires active management involving, e.g. bed leveling, dredging and flushing (Pontee, 2014).
Sand-capping of organic-enriched estuarine sediments
Many estuaries have a long eutrophication history due to agricultural effluents enriched in nitrogen and phosphorus. Eutrophication has strongly raised the organic matter fraction in estuarine sediments, especially in the muddy sediments which are trapped in the estuary (see Estuarine turbidity maximum). These muddy deposits are easily resuspended in the water column, where the organic fraction is remineralized. The muddy deposits therefore perpetuate the eutrophication status of estuaries, even when the nutrient supply from agriculture is reduced. The hypoxia associated with high mineralization rates has a negative impact on the diversity of benthic fauna which is strongly dependent on appropriate sediment texture and redox conditions. Phytoplankton blooms in eutrophicated estuaries diminish the penetration of light trough the water column. These blooms are a major cause of the sharp decline of light dependent macrophytes, such as eelgrass (Zostera marina L., see Seagrass meadows). A strong reduction of the input of nutrients is required for the restoration of estuarine benthic ecosystems. However, this is a slow process due to the legacy of enriched sediments, even if nutrient loading is completely stopped.
To speed up the restoration of benthic ecosystems, mud deposits can be removed by dredging, but this entails high costs. An alternative is to cap the organic-enriched mud deposits with clean sand with a grain size that prevents resuspension by currents or waves. This solution is therefore only applicable for mud deposits in areas that are not subject to strong currents or strong waves. A sand cap experiment was recently conducted in Denmark's microtidal Odense Fjord, a shallow low-energy basin with organic-enriched mud deposits that are frequently stirred up if no sand cap is applied. This eutrophicated system had extensive eelgrass meadows which have disappeared during the last decades of the previous century and have not recovered since in spite of a substantial reduction of the nutrient input. An evaluation after 12 months showed that the 10 cm thick sand cap was effective to reduce the turbidity and increase light penetration to the bottom. The benthic biodiversity increased and the sand cap improved the anchoring capacity of eelgrass in comparison with the fine grained mud. However, mud from uncapped areas settled on the capped area; better results can be expected if the scale of the experiment had been enlarged to include also other mud deposits.
The sand-capping method is more commonly used to prevent the dispersion of pollutants from contaminated mud deposits. Most projects are concerned with sand capping of contaminated sites in harbors, lakes and rivers, mainly in the USA, but also in Europe. In Hongkong, offshore sand borrow pits (15 m deep) are used for disposal of contaminated sediment, which is eventually capped with a meter of sand and sealed with a few meters of clean mud.
- Principles of conservation, rehabilitation and restoration of estuarine and coastal habitats
- Dynamics, threats and management of salt marshes
- Spatial and temporal variability of salt marshes
- Nature-based shore protection
- Climate adaptation measures for the coastal zone
- Integrated Coastal Zone Management (ICZM)
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