Principles of conservation, rehabilitation and restoration of estuarine and coastal habitats
Ecosystem restoration is the process of assisting the recovery of an ecosystem that has been degraded, damaged, or destroyed (SER, 2004). Habitat restoration is an essential component of ecosystem restoration.
Rehabilitation is the repair and replacement of essential ecosystem structures and functions in the context of ecoregional attainability in order to achieve specified objectives (Cooke, 2005).
- 1 Estuaries as interfaces between land, sea and the atmosphere
- 2 Preservation and the patrimonial approach
- 3 Restoration as part of an integrated approach to integrated coastal zone management
- 4 The ecosystemic approach
- 5 Global estuary restoration plan
- 6 Monitoring Methodology
- 7 Indicators
- 8 Conservation versus restoration
- 9 Costs and cost-effectiveness of coastal restoration projects
- 10 Related articles
- 11 References
- 12 Further references
Estuaries as interfaces between land, sea and the atmosphere
Amongst coastal ecosystems, estuaries need particular attention as interfaces between fresh and marine waters and the atmosphere. They are defined as zones of transition where terrestrial runoff discharges into the sea, and are closely connected to the ecosystems adjacent to them. Such interfaces are known as ecotones. Ecotones are characterized by a rapid flux of materials and organisms. They perform various functions, including mediating water flows, accumulating sediments and organic matter, processing nutrients, fertilising adjacent coastal waters and providing opportunities for recreation. Generally, the restoration of habitats and ecological functions requires a broad multi-disciplinary reflection on the definition of objectives as well as of methods. An ecosystem function is the capacity of natural processes and components to provide goods and services that satisfy human needs, either directly or indirectly. An estuarine ecosystem therefore must:
- transfer energy, and thus be organized in trophic networks;
- be composed of habitats which can shelter a diversity of species (complexity, heterogeneity);
- develop and evolve according to natural cycles, both morphodynamically and ecologically;
- interact with adjacent ecosystems.
Preservation and the patrimonial approach
Estuaries are critically threatened ecosystems because heavy human pressures have a great effect on them and on their watersheds. Their preservation and protection are likely to require unique collaboration among scientists, managers, and stakeholders. Scientists can learn a great deal from the study of these ecosystems, taking advantage of their compactness and the importance of fluxes. However a good understanding of adaptive management strategies is needed to establish a dialogue with managers and stakeholders on technical and management issues relating to the conservation of coastal ecosystems.
The goal of biodiversity conservation has been described as the conservation of diversity at three levels: 1. Ecosystem, 2. Species and 3. Genetic diversity. Developing a representative system of protected areas is often considered an effective way to achieve this goal in the marine environment. However, protecting species should not prevent restoring damaged habitats or rehabilitating lost habitats. A patrimonial approach to conservation, based on lists of species to be protected, might unfortunately block any attempt to turn back land claimed from estuaries into marine habitats. On the contrary, the development of biodiversity at fine scales (i.e. habitats) will have an increasingly important role in the identification of sites that will contribute to the various functions played by an estuary.
In large systems such as estuaries, the design of effective habitat management strategies requires attention to scale related problems. A multi-scale approach relies on sensitive socio-ecological assessment procedures, tools for evaluating ecological quality, and well-built monitoring programmes based upon pertinent indicators. An understanding of risk analysis is also important to set meaningful goals and establish logical strategies that include all of the interested parties. Managerial tools are to be used to refine strategies and make them compatible with the sustainable co-development of resources.
Restoration as part of an integrated approach to integrated coastal zone management
To restore an ecosystem consists in restoring the functions lost by this ecosystem. Rehabilitation addresses only certain functions and/or one or more selected species. Rehabilitating only some attributes or functions may lead to a “simplified” ecosystem compared to the initial ecosystem. Damaged habitats reconnection is fundamental to re-estuarisation. Longitudinally, the building of hard riparian protection structures, dykes and training walls has dislocated hydro-systems and limited access of estuarine communities. Such constructions have often led to an increase of the tidal range and a decrease of salinity gradients which force estuarine species out of the estuary. Connections with wetland habitats have been obstructed and the area and diversity of intertidal mudflats and marshes have been reduced. Restoring lateral hydrodynamic and sedimentary dynamics are a crucial element in the general project of estuarine restoration.
Too often, objectives for restoration are unclear. In some cases they target biodiversity in particular species or communities (very often birds). A pluridisciplinary approach to outline objectives and methods is needed. Restoration objectives should focus on ecosystem functioning rather than structure description. The multiplication of stakeholders complicates the management of estuaries because of divergent interests and sometimes even opposition. In order to prevent conflicts, managers and scientists need to work together towards agreed objectives. Interactions are to be understood in terms of geomorphology, sedimentology and chemistry (erosion, sedimentation, transport, adsorption, mineralisation) and human activities are to be considered as fully belonging to the ecosystem. This is why social sciences should be fully deployed in any restoration project. Wetlands and aquatic biotopes, of which intertidal zones, are the most degraded by the spiral of constructing in estuaries. A true consensus on the necessity of returning lost estuarine areas to the sea is building up in Europe, thanks to adopting an ecosystemic approach and by developing interdisciplinary synergies. Most of the time, functionalities are still found in an estuary, but more or less degraded. Loss of space and volume is obvious today on the longitudinal as well as transverse level of many estuaries. The top priority is the re-establishment of functional connections, not only hydrological but also biological.
The ecosystemic approach
Reacting against destructive developments has long been considered as normal practice by managers, proposing compensation for the loss of habitats. Promoting an ecological perspective through an ecosystemic approach would allow achieving a gradual restoration of ecological functions in estuarine ecosystems. The ecosystemic (scientific) approach relies upon the acquisition of scientific knowledge (field experiments and surveys, modelling, etc.) on hydrodynamics, sedimentology, biogeomorphology, biocoenosis] and climate change. Only such an approach can lead, on the long-term (20 – 50 years), to typical estuarine communities coming back. On the mid-term, reduced scale re-estuarisation (depolderisation experiments on demo-sites for instance) remains of interest as it might be useful for experimenting and acquiring full capability. Acquiring technical skills (ecological engineering) allows the launch of full scale pilots adapted to local conditions. After scientific selection of sites in relation to water quality, salinity and pollutants fluxes, these experiments can be used as demonstrations and tests on restoration procedures.
Global estuary restoration plan
Any intervention on an estuarine ecosystem should be incorporated into a global restoration plan, conceived for the long-term. An ecological vision on the long-term would mean analyzing the past to predict the future and promoting local activities in harmony with estuarine conditions. Based on the reconstruction of paleoenvironmental variations and history, scenarios can be proposed as a virtual image of the possible future estuary ("utopian perspective"). The sociological aspect of this should not be underestimated as the plan would serve as a potential communication tool. The following protocol can be applied in the short run to build any project of restoration:
- Analyse past experiences: which interventions were successful and which were the causes of failure?
- Identify key elements to undo the dynamics of compartmentation of the estuary.
- Ensure that development projects contribute to the restoration objectives.
- Anticipate the future with an ambitious project of restoration of aquatic habitats and wetlands at several scales (start downstream and then goes up upstream).
Resting on a rigorous scientific approach, an ecological project of restoration should consider:
- efficient procedures of socio-ecological evaluation;
- a methodology to assess the ecological quality of the systems considered;
- rigorous monitoring programs, resting on a relevant choice of indicators;
- participation of local communities,
At ecosystem level, without an holistic view of the estuary, the redevelopment of isolated aquatic systems might contribute to further patchiness in the main estuarine ecosystem with the building of new dykes and channels which increase the compartmentalisation of the area. On the larger scale, restored ecosystem can be compared to other estuaries. There is considerable opportunity for fruitful collaborations between scientists and managers.
After having laid down objectives for restoration, monitoring will help to assess the changes which take place in the ecosystem in response to measures undertaken to transform the ecosystem. In the majority of cases, in order to take account of the heterogeneity of the environment, bio-facies or biotopes will be delineated. The management and sustainable use of the natural resources may be improved as a result of accurate biotope mapping. GISs provide excellent support for computing such geographical data. Setting up a monitoring programme (from the biotope to the landscape scale) is part of the method in relation to socio-economics. It should be reoriented as often as necessary.
On each one of these biotopes, a pilot-station (or more) should be established. A priori, it would be appropriate to monitor biological diversity as a whole. For reasons of cost-efficiency, bio-indicators must be chosen. They consist classically of species or groups of species in reference to variables which can relate as well to the dynamics of the populations considered as biochemistry, cytology, physiology, ethology or ecology of the species considered. They are used for monitoring progress in the rehabilitation/restoration process. Selected species (a group of species or any other indicator) will account for overall changes taking place in the ecosystem. These variables relate to attributes that one wishes to measure according to the objectives assigned within the framework of restoration. It is advisable to establish a base line which will be used as a reference. Recently, the European programme BIOMARE proposed indicators of bio-diversity for coastal marine environments. The characteristics of a good indicator can be summarised as follows:
- Easy to include/understand
- Factual and quantitative
- Scientifically and statistically reliable
- Reacting in space and time
- Technically assigned and economically valid
- Translatable in scenarios to be proposed
- Meeting the needs for the management of the environment, in particular the user's needs
- Allowing intercomparison, being integrated at the regional, national and international levels.
The presence and the development of indicators will account for temporal and spatial changes in ecosystems. Models can be used to assess changes in the abiotic physical conditions needed by the selected indicator to develop. Numerical models can be proposed in terms, for example, of surface necessary to compensate for an impact and to restore a favourable habitat to the species in question which is then regarded as a target species. They are to be conceived like tools of communication between the scientists and the developers. They consist of information (basically of numerical data) which has a statistical significance and which is thus representative of a phenomenon.
Conservation versus restoration
The concepts of conservation and restoration diverge. It is sometimes possible to safeguard the presence of one or several species which give concern while remaining on threshold levels of presence. Restoration supposes a higher level of intervention by which one seeks to give the means of increasing the turnover of targeted populations. The quality of the habitat and its quantity are obviously crucial. Moreover, they must be connected, to exist in sufficient quantity and be of good quality. The diagnosis (then the objectives of restoration) must relate to these various aspects. These interactions also play in the sedimentary and chemical fields. Through “restoring” habitats, scientists measure the reintroduction of certain functions in the ecosystem which, in the long term, will find a new dynamic equilibrium. One will judge this new balance from the assessment of the performance of the ecosystem, for example, how the system achieves a function, for instance to recycle nutrient. In a more general way, the restoration of habitats and functions require a broad pluridisciplinary reflection on the level of definition of objectives as well as of methods. Difficulties in implementing ecological qualitative objectives often lie in the lack of rigor in the terminology and the lack of respect of ecological concepts. Preservation and restoration of habitats must rely on a robust scientific methodology.
Costs and cost-effectiveness of coastal restoration projects
Bayraktarov et al. published in 2016 an extensive review study on costs and cost-effectiveness of restoration projects. Restoration projects were selected related to the habitats Coral reefs, Mangroves, Seagrass meadows, Salt marshes and Oyster reefs. The main findings are summarized below.
- The median and average reported costs (in 2010) for restoration of one hectare of marine coastal habitat were around US$80 000 and US$1 600 000, respectively, the real total costs (median) are likely to be two to four times higher. Coral reefs and seagrass were among the most expensive ecosystems to restore. Mangrove restoration projects were typically the largest and the least expensive per hectare (in the range US$ 500 - 400,000 per ha). Reasons are likely related to the high numbers of community- or volunteer-based projects, the availability of wild mangrove seeds, seedlings, or propagules, and relatively easier access to restoration sites.
- Most marine coastal restoration projects were conducted in Australia, Europe, and USA, while total restoration costs were significantly (up to 30 times) cheaper in countries with developing economies. Marine coastal restoration can be most cost-effective in countries with developing economies if projects involve a consensus and integration of local communities and stakeholders. Investment in restoration projects in developing countries can achieve up to 30 times more unit area of habitat in developing countries compared to developed countries.
- Median survival of restored marine and coastal organisms (often assessed only within the first one to two years after restoration) was highest for salt marshes (64.8%) and coral reefs (64.5%), medium for mangroves (51.3%) and oyster reefs (56.2%) and lowest for seagrass (38.0%). Survival of restored organisms was not related to cost. Other factors are more important.
- For coral reefs, restoration projects achieving survival of ≥85% irrespective of project duration were those that used coral gardening or coral farming techniques. For oyster reefs, the establishment of no-harvest sanctuaries created by natural or artificial substrate and transplanting juvenile oysters obtained from hatchery to the reefs achieved 85% survival of restored oysters. The most cost-effective seagrass restoration project was the transplantation of seagrass cores or plugs.
- Important reported failure causes are inadequate site selection (unsuitable soil, hydrodynamic conditions, water quality), lack of community involvement, inappropriate species selection and transplantation methods, human disturbance (e.g. fishery, trampling), smothering and overgrowth by competing species.
- Restoration of marine coastal ecosystems is more expensive than for terrestrial ecosystems; this holds in particular for coral reefs, seagrass and oyster reefs. The total (median) cost estimates for the reviewed marine coastal restoration projects are 10–400 times higher than the maximum cost reported for the restoration of inland wetlands, freshwater systems, tropical and temperate forest, woodlands, and grasslands (De Groot et al. 2013).
- For most marine coastal ecosystems, only small-scale restoration projects of a few hectares or less have been reported while the scales of anthropogenic degradation are in the order of 10–1 000 000 ha. Large scale restoration projects are urgently needed to achieve socioeconomic benefits of ecosystem services delivery.
- The present literature synthesis showed that there are four main criteria for marine ecosystem restoration projects to achieve success: (1) understanding of ecosystem function (e.g., physical and biological conditions required for the ecosystem to thrive), (2) removal of the anthropogenic stressors that impede natural regeneration (e.g., altered physical conditions, pollution, eutrophication), (3) clearly defined criteria for the measurement of restoration success, and (4) long term monitoring of 15–20 years rather than <5 years.
To assess the costs-effectiveness of a coastal restoration project not only the costs and the realization of restoration targets are to be taken into consideration, but also the societal benefits that accrue from the project. It is often difficult to express these so-called ecosystem benefits in monetary terms (see Ecosystem services), but estimated monetary values are high and generally exceed the costs of wetland restoration by far. Carbon sequestration is an example of benefits that can be directly converted into monetary support for coastal restoration, see the article Blue carbon revenues of nature-based coastal protection.
- Restoration of estuarine and coastal ecosystems
- Ecological restoration of estuaries in North Western Europe
- Biogeomorphology of coastal systems
- Spatial and temporal scales in biogeomorphology
- Habitat destruction and fragmentation
- Seagrass recovery and restoration in the Wadden Sea
- Resilience and resistance
- Coastal squeeze
- Climate adaptation measures for the coastal zone
- Carrying capacity and development of the Wadden Sea
- Threats to the coastal zone
- Dynamics, threats and management of salt marshes
- Dynamics, threats and management of dunes
- Dynamics, threats and management of biogenic reefs
- Marine habitats and ecosystems
- Impact of tourism in coastal areas: Need of sustainable tourism strategy
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- US National Marine Sanctuaries
- US National Wildlife Refuge System
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- Seagrass meadows
- Estuarine ecosystems
- Marine habitats and ecosystems
- Biotopes and classification systems
- Coastal pollution and impacts
- Morphology of estuaries
- Tidal asymmetry and tidal basin morphodynamics
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