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Ecological restoration of estuaries in North Western Europe

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Estuaries are open systems and are essential interfaces between rivers and the coast. They constitute main transition zones or ecotones between land, the ocean and the atmosphere. In the North Sea and the English Channel (North-West Europe), the role of the tide is paramount, with a tidal range over 10 m in some areas. So, large macrotidal estuaries have developed there (the Elbe, the Weser, the Scheldt, the Humber, the Thames, the Seine,...). Geomorphology is essential to understand when comparing such estuaries. It is rapidly evolving because of natural processes (mainly hydrological, due to sea level rise and increasing tides). Estuaries are also greatly influenced by changes in the watersheds. The main factors affecting the hyporheic zone are the width and the depth of the river bed, the river flow and constructions (dams, embankments, polders...) by humans. Human activities, mainly through reclamation, have accelerated natural morphological processes and worsened the degradation of estuarine resources [1]. In the estuaries presented here, the erection of dykes and the reclamation of land have dislocated the hydro-systems and limited access to estuarine animal and vegetal communities. Longitudinally, there has been an increase of tidal effects and salinity which have forced some estuarine species out of estuaries. The turbidity maximum also has a general tendency to move outwards. Transversally, dykes have broken connections between aquatic habitats and reduced the area and diversity of wet land, including intertidal flats and salt marshes. In all ecosystems, there has been a parallel decrease of fresh water tidal habitats for fish, birds, and the benthos on which they feed.

Estuaries within Europe are managed to protect features designated under EU directives, notably their habitats and species in order to build upon their conservation objectives. This approach is no different from that adopted in other countries, for example the US Clean Water Act. Ecological restoration requires to build these into an iterative environmental management system which treats the environment as an entity to be managed as a whole [2]. This is why, on the long-term, the main objective of estuarine habitat restoration in Europe is to enable the gradual re-establishment of ecological functions, leading to the (re)installation of typical estuarine communities. This can be accomplished through increasing fluxes of water circulating in the estuary and re-establishing connections between the various aquatic components of ecosystems. Adopting such objectives means considering each estuary as a whole including peri-estuarine areas such as the flood plain, associated marshes and land claimed by humans essentially over the last 150 years.

Ecological stability of estuaries

Despite radical changes in their morphology over a centenary and a half, North-West European estuaries are still productive marine ecosystems. In all of them, biodiversity reflects their ecological value, not necessarily in terms of species richness, but in terms of diversity of habitats and biotopes [3]. Biotopes constitute sub-units of ecosystems and are displayed as a mosaic in each estuary [4] . They seem to be similar in all estuaries but what makes each estuary special is the physical, chemical and biological (biogeochemical) links between the biotopes. These interrelations depend upon hydrology, sediment transport, nutrient transfer and biological cycles. Naturally, other variations between estuaries exist. The main structuring environmental factors appear to be salinity, water movements and turbidity. They affect the heterogeneity (or structure) of each ecosystem, as well as their complexity (in terms of relations between structural attributes).

Despite radical changes inflicted on them, the estuaries presented in this paper stand out as specific and valuable socio-ecosystems. They consist in highly dynamical systems and the global change is increasing the speed of change. Even with the pressure exerted by human societies on their ecology, estuaries stand as assets to humans. Similar ecological functions are found in all of them, which provide valuable goods and services to human societies [5]. Amongst those, biogeochemical cycling, in particular nutrients, water purification and mitigation of floods are much looked for. Tett et al. (2007) [6] suggested that an ecosystem impacted by anthropogenic factors may, because of its resistance to disturbance, initially show little response to increasing pressure (figure 1). Pushed beyond a certain point, however, change becomes rapid, and may culminate in a radically altered state from which recovery would be slow. An example would be the occurrence of extensive deep water anoxia, resulting in the widespread elimination of benthos and fish in the Scheldt and the Seine in the 1990s. A key operational need, when considering restoring any estuary, is to detect a trend towards such a widespread undesirable disturbance before the ecosystem has reached the limit of its resistance [7]. Case studies show however that ecosystems do not return to the same state after removal of a pressure, but to a different one. Re-estuarisation often implies re-creating damaged habitats from scratch [8].

Ecological response to increasing pressure.jpg
Figure 1: an ecosystem impacted by anthropogenic factors may, because of its resistance to disturbance, initially show little response to increasing pressure (document IECS)

The Humber

In the Humber, connectivity between the various aquatic components of the ecosystem has been shown to be the key to all restoration actions [9]. Pollution and eutrophication needed to be tackled [10][11] while setting back the shore line was required in response to sea level rise. In order to test techniques for reconnection, a large scale experimental site was chosen in the 1990s for depolderisation and habitat (re)creation. In 2010, the Humber estuary was the site of three existing managed realignment sites (former agricultural land) with the primary role of direct compensation for habitat loss. A fourth site was being created as part of a flood defence scheme. Creation of a further five sites, with the primary aim of mudflat creation, is planned over the next 20 years [12].

The Elbe

In the Elbe, concern arose in the 1990s because of accumulation of sediments which limited access to Hamburg harbour [13]. A restoration plan was launched based on a control of local sediments dynamics in order to improve navigability through the tidal areas (volumes, distribution, quality,...). This sediment management concept intends to relocate fresh, non-contaminated sediments in areas where there is less possibility for them to return to the place where they were dredged [14]. The opportunity was taken to improve environmental quality in association with engineering work realised to reverse negative geomorphological effects. Measures for restoring natural conditions along the river were taken, including conservation and development of shallow water areas, creation of alluvial forests, and salt marsh development in front of dikes. In the meanwhile, protected areas became attractions for tourists [15]. The re-creation of inter-tidal areas including salt marshes was initiated in order to increase floodable areas. It was aided by the installation of flood polders as buffers in case of storm surge. These restoration actions also provided flood risk protection in reducing storm surge water levels. Both recreational and commercial fishing will further benefit from the creation of shallow water zones. These are important spawning and hatching zones for fish and prey. An enhanced connection of tributaries also had a positive effect on the functioning of the estuarine ecosystem as migrating fish species could now reach their breeding areas with less effort. According to the Hamburg Port Authority, on the long run, the diversification of the system and the ecological improvement of the water and sediment quality very likely will increase the number of species [16]. Application of scientific results led to interesting operational aspects when dealing with dredging of huge volumes of sediments. For instance, the use of models made their management more effective.

The Scheldt

Past management and unexpected changes have had a negative impact on the delivery of ecosystem services by estuaries and hence on the resilience of ecosystems [17][18]. This resulted in growing socio-economic problems (e.g. inundations, eutrophication, siltation, ...) in the various instances considered. The carrying capacity and the assimilative capacity of ecosystems might be overrun and signs such as pollution show that their ecological functioning is affected [19].

Precisely, the consideration of the carrying capacity of the estuarine ecosystem and ecological thresholds [20] has been instrumental in putting together the restoration plan of the Scheldt estuary, in particular measuring the importance of re-creating tidal systems. In the 1990s, it was felt that tidal wetland restoration would be necessary in order to compensate loss of habitat [21].

In combination with a master plan to protect the population from storm surges, an opportunity arose to restore areas under tidal influence. One specific option of combining safety and ecology was the creation of flood control areas under the influence of a controlled reduced tide (CRT) [22]. These specific areas differ in many ways from fully tidal areas but can fulfil important ecological functions with effects on aeration, sedimentation [23][24] nitrification, denitrification, and primary production in the estuary [25]. Opportunities for ecological development within a CRT have been investigated for a specific case. The ecology within a CRT was shown to be very case specific, depending e.g. on the morphology of the area, the sluice design and the local water quality [26]. Depending on the sluice design, water quality can be improved and sedimentation can be influenced. A scientific approach to the management of these sensitive areas made it possible to design CRTs with a rich habitat variation [27]. Some ideas for the future restoration of estuaries world-wide should emanate from that approach. Any future estuarine management plan should take into consideration the type and the proportion of each habitats which needs to be (re)-created in order to provide the ecosystem with expected functionalities [28]. In parallel, expected goods and services to be provided should be adapted [29][30]. The main requirement to sustain such measures is that they should ensure resilience and adaptability [31]. So, planning should include actions to mitigate or to reverse the local effects of climate change (e.g. sea level rise) and slow down the global change (e.g. through CO2 sequestration).

The Seine

All of the sites considered earlier in this article have benefited from management schemes in order to re-establish some of the lost ecological functions: in the Elbe, the sediment dynamics, in the Scheldt, the control of floods, in the Humber, the restoration of specified ecological processes. In the Seine the restoration process was started quite differently. Compensation measures were implemented in the late 1990s in response to the extension of Le Havre port facilities. The construction of “Port 2000” was the occasion for managers and politicians to stress the importance of research for reaching “a balance between the development of economic objectives and the protection of natural aquatic environments towards an integrated management of the estuary”. The new harbour installations required the reclamation of existing wetlands [32]. Such an operation presented threats for safeguarding the sedimentary balance in the estuary and the future of mudflats. Decision makers decided that accompanying measures should be taken to minimize the hydro-sedimentary impact and to rehabilitate threatened intertidal mudflats durably. Various options were selected after mathematical modelling of circulation patterns of sediments in the estuary. The ones which were adopted consisted mainly in restoring one damaged mudflat, building a resting place for birds on a sand dune and constructing a small island, also to be a refuge for birds. Some habitats were re-created on part of the land claimed from the estuary with a view to facilitating the growth of charismatic plants [33][34][35]. The relevance of future experimental restoration measures will depend on the adequacy of scientific research in helping to build a long term vision of the estuary.

Integrated management plans

A model plan for the restauration of an estuary.jpg
Figure 2: A model plan for the restoration of an estuary

The piecemeal application of present European environmental legislation has not been sufficient to change the negative trends in the considered estuaries. Integrative management plans are still required at the scale of each estuary [36]. Such a model plan is shown on figure 2. What is interesting is that despite the different managerial approaches applied in the various countries, all actions included some degree of ecological restoration of habitats. Such actions involved more or less large-scale engineering work. From comparing these various managerial approaches in the different estuaries, it appears that only conservation objectives can translate the aim to reduce negative developments. It would seem that the best way to formulate such objectives is in computing and calculating surfaces of the different habitats necessary to sustain the resilience of the ecosystem [37][18], including geomorphological, hydrodynamic, ecological and quality aspects. The approach shown on figure 3 gives the possibility to sustain each estuary in a healthy state to reduce management costs and increase benefits from goods and services obtained from them. In order to do so, a holistic approach is needed where the system characteristics are considered in such a way that negative developments are stopped or at least slowed down. This requires a major investment in research to better understand the systems functioning and the interactions between the different compartments, including socio-economics, identify services and calculate surface of habitat needed for delivering the required services and providing the expected goods to humans.

Model plan for sustaining an estuary in a healthy state.jpg
Figure 3: A Model plan for sustaining an estuary in a healthy state

In order to define strategies compatible with conservation and sustainable development at the local, regional and European levels, environmental aspects must be integrated in the management of estuaries, which must rely on thorough collaboration between and mutual understanding of all actors and stakeholders. Resting on a rigorous scientific approach, restoring ecological functionalities in an estuary is dependent on efficient procedures of socio-ecological evaluation including a methodology to assess the ecological quality of systems considered [38][39][40]. For making interdisciplinarity work, socio-economics need to be considered in the early stages of the elaboration of any restoration programme.

Putting the project in a scientific perspective implies the application of fundamentals of ecology. Because of the popularity of certain concepts, including biodiversity, productivity, etc., definition and use of important terms may have been misinterpreted [32]. For example, most often, the general public thinks that biodiversity is at the basis of robust and productive ecosystems. Recently, Elliott & Quintino (2007) [41] have put into light the quality paradox of estuaries, where poor species richness supports high production and stability [42][31].

The concept of habitat is therefore essential as a species might disappear but a habitat remains available for shifting species. Unfortunately, European and national legislation aimed at protecting habitats are species based, locked by conservation management. With the arrival of “new” species, whether they will move in response to the climatic change or they were introduced artificially, conditions should be made to avoid “fossilisation” of protected habitats. It might be necessary to accommodate shifts in spatial distribution and alien species. However, one may ask whether the legislative framework is fit for purpose when habitats will need to be adapted to changing biophysical conditions [43]. Breakdown in geographical barriers or deliberate and inadvertent transport of species could be at the origin of new "emerging" ecosystems, of which the functional characteristics are unknown today. From all examples given, it has been shown that it is impossible to freeze an ecosystem at a particular stage of its evolution and that it is further impossible to return backward in time. Fundamental research needs to address the issue of better understanding future shifts in ecological niches. Rigorous monitoring programs, resting on a relevant choice of indicators (table 1), should be linked to research and data used more efficiently and on the long-term [44].

Scientific area Indicator Response Tools
1. Populations dynamics Biological diversity and community structure
Population numbers and turn over
Numbers equivalent to those found for resistance in normal disturbance conditions Diversity indices
Life tables
Multivariate statistics
Distribution of plant and animal propagules
Animals circulation
No physical or chemical break in connectivity and migration corridors within and between habitats Water quality compatible with organisms survival
2. Biogeography Species spatial distribution Species spatial distribution extension compatible with ecosystems dynamical balance Long-term monitoring at the eco-region scale
Alien invasive or introduced species Absent or low numbers compatible with the integrity of other species, the habitat or the ecosystem Integrated surveillance
3. Functional ecology Communities functioning
Algal or invertebrate functional groups
Sustainable on the long -term
All functional groups are represented
Trophic structure
4. Eco-morphology Spatial distribution and quantitative make-up of biotopes (bio-sedimentary facies) and/or habitats
Functional relations between biotopes
Ecosystem made of all main biotopes in sufficient quantity in normal physical and chemical conditions Sedimentary and biological parameters to be compared to reference conditions
5. Sato-Umi eco-hydrological Biogeochemical dynamics of nutriments from watershed to estuary and coastal environment approach Hydrological conditions not modified by anthropic activities
Physical and chemical quality of water masses unaltered
Mathematical modelling of primary production
Light incidence in water column
6. Patrimonial approach Protected and threatened species Incorporated in changing ecosystem dynamics Routine monitoring
Importance of protected areas Sustained or expanded according to system’s dynamics GIS
7. Natural resources exploitation Genetic diversity of cultivated and wild species of fish, molluscs and shellfish Not affected
Acknowledgement of socio-economic value
DNA sequencing
Integrated sustainable management of aquaculture and fisheries (from river to sea) Meeting socio-economic requirements and constraints (from local to global) Fish populations dynamics studies
Sustainable management of exploited species and /or des products (for example biopharmaceuticals) Potential intact Biodiversity
Population dynamics of exploited or exploitable species
Market economy
8. Anthropic impacts Existent and potential threats in and out socio-ecosystem Eliminated, reduced, attenuated or compensated
Positive evolution of biodiversity with regard to climate change
Pollution assessment in water, sediment and bios
Pressure integrated indicators
Table 1: Indicators and tools for estuarine ecological restoration

The way forward

Climate change will affect managers' views on the permanence of estuarine socio-ecosystems. In the future, the restoration of damaged habitats will be instrumental in adapting socio-economic activities [17][45] to changing environmental conditions [28]. In this context, sea level rise, presently estimated at 40 cm per century and expected to increase, according to the prognosis of IPCC, in the second part of the century, is a paramount challenge [46]. Besides the enlargement of tidal capacity, a reduction of the cross-section profile at the mouth of the estuaries ought to hold back part of the tidal energy in the event of a storm surge. What will be the implications of such events for estuarine ecological restoration? Climate change will make "habitats" of interest more fragile and less resilient. The Scheldt example has shown that a patrimonial view of ecosystems is not necessarily compatible with promoting new functions in an estuary. The Seine compensation measures showed how important habitats are as there is a need to allow species to adapt to new biophysical conditions. Nevertheless, ecosystems are dynamical and restoration needs to focus on ecosystems (and how they function), not species [41].

In order to get sustainable and successful management, harmonisation is required within and between sectors, stakeholders, regulators, mediums, estuaries, regions, countries, outcomes and implementation. This is because North-West Europe estuaries are regarded as multi-user spaces and so there are many things which need to be managed (and by whom) [2]:

  • habitats (nature conservation agencies),
  • environmental quality (Environmental Protection Agency-type organisations),
  • water space usage (port authorities),
  • navigation (port authorities),
  • infrastructure (municipalities/federal state),
  • energy extraction (private companies),
  • biological extractions (fisheries bodies),
  • estuarine water extraction (private energy companies),
  • upstream water abstraction (water supply companies),
  • land space usage (municipalities/federal state),
  • erosion and flooding control (EPA, municipalities etc),
  • industry (EPA and private companies)
  • and recreation and tourism (agencies).

Restoring functions at ecosystem level will undoubtedly help guarantee assets to human societies which depend on them. Ensuring resilience and adaptability, will allow adjusting goods and services both to new environmental conditions and to emerging human needs. But, over and above, integrated management of estuaries will be essential in adapting to local changing conditions (e.g. sea level rise) and slowing down climate change at global level. It is why, in the future, participation of local communities will be essential for the success of measures taken. Communicating with existing groups will help making visible actions taken and creating synergies with other development plans. Research and education clearly stand out as a means to achieve governance.

See also


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The main author of this article is Ducrotoy, Jean-Paul
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