Ecological restoration of estuaries in North Western Europe
Introduction
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 (Elliott & Ducrotoy 2010). This is why, on the long-term, the main objective of estuarine habitats 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.
Despite radical changes in their morphology over a centenary and a half, North-West Europeestuaries 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 (Ducrotoy 2010a). Biotopes constitute sub-units of ecosystems and are displayed as a mosaic in each estuary (Olenin& Ducrotoy 2006). 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 (Costanza et al. 1993). Amongst those, biogeochemical cycling, in particular nutrients, water purification and mitigation of floods are much looked for. Tett et al. (2007) 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 fishin 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 (Diaz & Rosenberg 2008). 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 (Ducrotoy 2011). In the Humber, connectivity between the various aquatic components of the ecosystem has been shown to be the key to all restoration actions (Cave et al. 2003). Pollution and eutrophication needed to be tackled (Mazik& Elliott 2000, Mazik 2004) 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 (Environment Agency 2009). In the Elbe, concern arose in the 1990s because of accumulation of sediments which limited access to Hamburg harbour (Plüss 2004). 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 (Fickert&Strotmann 2007). 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 (Bergemann 2006). 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 (Dücker et al. 2006). 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. 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 (Folke et al. 2002, Gunderson &Folke 2005). 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 (Arrow et al. 1995). Precisely, the consideration of the carrying capacity of the estuarine ecosystem and ecological thresholds (Reynolds et al. 2006) 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 (Eertman et al. 2002). 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) (Maris et al. 2007). These specific areas differ in many ways from fully tidal areas but can fulfil important ecological functions with effects on aeration, sedimentation(Ten Brinke&Dronkers 1993, Bolle et al. 2010) nitrification, denitrification, and primary production in the estuary (Billen et al. 2005). 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 (van den Bergh et al. 2005). 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 (Maris et al. 2007).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 (Diaz et al. 2004). In parallel, expected goods and services to be provided should be adapted (Folke et al. 2002, Hughes et al. 2007). The main requirement to sustain such measures is that they should ensure resilience and adaptability (Petersen et al. 2010). 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). 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 (Ducrotoy &Dauvin, 2008). 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 (Scherrer&Galichon 2002, Dauvin et al. 2006, PortAutonome du Havre 2007). 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. 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 (Harris et al. 2006). 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 (Folke et al. 2002, Gunderson &Folke 2005), including geomorphological, hydrodynamic, ecological and quality aspects. The approach shown on figure 3gives 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. 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 (Bingham et al. 1995, Costanza et al. 1998, de Groot et al. 2002). 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 (Ducrotoy &Yanagi 2008). For example, most often, the general public thinks that biodiversity is at the basis of robust and productive ecosystems. Recently, Elliott &Quintino (2007) have put into light the quality paradox of estuaries, where poor species richness supports high production and stability (Holling 1973, Peterson et al. 2010). 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 (Harris et al., 2006). 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 (Ducrotoy 2010b). 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 (Folke et al. 2002, Hughes et al. 2007) to changing environmental conditions (Diaz et al. 2004). 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 (IPCC 2011). 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 (Elliott et al. 2007). 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) Elliott & Ducrotoy 2010):
- habitats (nature conservation agencies),
- environmentalquality (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).
References
- ↑ Berkes F, Hughes TP, Steneck RS, Wilson JA, Bellwood DR, Crona B, Folke C, Gunderson LH, Leslie HM, Norberg J, Nyström M, Olsson P, Osterblom H, Scheffer M, Worm B (2006) Ecology—Globalization, roving bandits, and marine resources.Science 311: 1557–1558.
