Conservation and restoration of coastal and estuarine habitats
This article provides a description of the role of coastal habitats and ecosystems in transitional waters and their conservation needs.
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 emerges a water court which 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 reflexion on the definition of objectives as well as of methods. A function is an activity of the ecosystem leading to a precise result. It is carried out thanks to a certain number of processes (or operations) to which living organisms take part, through their interactions. Such an approach will thus consider that an estuarine ecosystem which functions 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
- Interact with adjacent ecosystems
Preservation and the patrimonial approach
Because heavy human pressures have a great effect on them and their watersheds, estuaries are critically important zones. 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 surrogates 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 help 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, replacing superfluous functions or replacing it entirely by another ecosystem which will fulfil the functions concerned. Rehabilitation relates to 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 dykes has dislocated hydro-systems and limited access of estuarine communities. Such constructions have led to an increase of tidal effects and salinity which force estuarine species out of the estuary. Transversally, dykes have broken connections between aquatic habitats and reduced the area and diversity of marshes. In parallel, decreasing fresh water areas have been lost to fish. It will be thus this circulation and the dynamics which results from it which could be used as a thread in the general project of estuarine restoration. Too often, objectives for restoration are unclear. In some cases they target biodiversity, in others 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 (silting, transportation, erosion, adsorption, and oxidation areas – sinks and source) 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 (or more precisely of volume) is obvious today on the longitudinal as well as transverse level of many estuaries. On the transverse level, the construction of dykes has broken connections between the various aquatic environments while reducing surface and diversity of the habitats. 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, ecomorphology, biocenotics 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, working on 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 fundamental principle consists in increasing the volume of water entering the estuary to include the valley as well as the limiting hills or cliffs. The following protocol can be applied in the short run to build any project of restoration:
- Analyse past experiences: which main ideas function and which are the causes of failure?
- Identify key elements to break the dynamics of compartmentation of the estuary,
- Improve projects with environmental vocation,
- Anticipate the future with an ambitious project of restoration of aquatic habitats and wetlands at several scales (one starts 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) will be established. A priori, it would be appropriate to monitor biological diversity as a whole. For reasons of cost-efficiency, bio-indicators will 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 which 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 indeed 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 (transport, delivery points, of erosion, adsorption, salting out, oxidation). 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 very broad pluridisciplinary reflexion on the level of definition of objectives as well as of methods .Difficulties in implementing ecological qualitative objectives often lie in the lack of rigour in the terminology and the lack of respect of ecological concepts. However estuaries are well delimited ecosystems, at least upstream, and human impacts understood often well. Preservation and restoration of habitats rely on a robust scientific methodology.
Bio-geo-morphology / Ecomorphology
The significance of biodiversity in marine sediments to ecosystem processes is poorly understood, but individual species and functional groups are known to carry out activities in relation to the substratum in which they develop. Macrofaunal activity impacts global carbon, nitrogen and sulphur cycling, transport, burial and metabolism of pollutants, secondary production including commercial species, and transport of sediments. These processes are included in the concept of the biotope defined as the physical 'habitat' with its biological 'community'; a term which refers to the combination of physical environment (habitat) and its distinctive assemblage of conspicuous species. So, a biotope combines the concepts of habitat and community for defining geographical sub-units inside an ecosystem.
Habitat valuation is an essential tool for tracking changes in habitat quality and in adjudicating environmental mitigation. All current methods for estimating habitat values of coastal marine sites rely heavily on the opinion of experts or on data variables that can readily be manipulated to influence the outcome. As a result, unbiased, quantitative comparisons between the values of different marine habitats are generally unavailable. Any robust and objective technique for the valuation of marine habitats should make use of reliable numerical. Such data are commonly gathered in surveys of marine animal populations (birds, fish, invertebrates, etc) assessing density, fidelity, mean size, etc. For instance, changes in the structure of sedimentary benthic assemblages in relation to anthropogenic perturbations are based on the recognition of ecological groups of different sensitivity to disturbances. Reaction of these groups, when faced with organic inputs and/or chemical-contaminants, is different from each other.
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