Policy planning in the Oosterschelde estuary

During the execution of the Delta project the awareness of the environmental values of estuaries was growing strongly. This awareness has led to a reconsideration of the original proposal to close off the Oosterschelde, and to an alternative view of the management of the estuarine areas and of the civil engineering structures. Now, the completed storm-surge barrier and two compartmentation dams, together with the reinforced dikes, guarantee a sufficient safety against flooding.After the decision to build a storm-surge barrier it was soon recognized that a coherent plan would be needed for the management and development of the Oosterschelde. A policy plan was accordingly produced in which the broad outlines of the policy were set out regarding the various potential uses of the estuary and the possible means of effecting them. Because of the periodic improvement and adjustment of the policy plan an environmental research programme was set up. Also, environmental research played a significant role in the decision-making processes for the completion of the barrier and the compartmentation dams as well as for the management of the storm-surge barrier.The coping-stone of the Delta Project marks the beginning of a new period in the water management of the Netherlands: the period of integrated water management.     

The influence of vegetation on erosion and accretion in salt marshes of the Oosterschelde,The Netherlands

The total root strength of two plant species (Spartina anglica and Limonium vulgare) is related to salt marsh cliff erosion in the Krabbenkreek (Oosterschelde). A ranking order in cliff stability is predicted on the basis of these root strength calculations. It turns out that the S. anglica root system is more effective in reducing lateral cliff erosion than the root system of L. vulgare. Also the establishment of S. anglica by germination of seeds is studied in relation to the erosion/deposition rates at an accretion site in the Krabbenkreek. The percentage of seeds washed away depends on the mobility of the superficial sediment which increases with decreasing height above N.A.P. (Dutch Ordnance Level). Above 0.90 m + N.A.P. a germination of 20% is measured, but seedlings survive the winter period only in the zone where a patchy vegetation already exists. It is concluded that generative spread of S. anglica in the Krabbenkreek is not very likely under the present hydrodynamic conditions.Nomenclature follows Heukels & van Ooststroom (1977).Acknowledgement: The research presented herein was carried out at the Delta Department, Environmental Division of the Ministry of Transport and Public Works, Middelburg. This work forms part of the research on salt-marsh ecosystems of the Oosterschelde. Assistance with sowing and sampling was given by the Field Survey section of the Environmental Division. Analyses of root weight and root diameter were carried out by the Institute of Soil Fertility in Haren by order of the Environmental Division. I gratefully acknowledge Ir A. de Jager and J. Floris of this institute for putting a special sampling auger at my disposal. G. den Hartog, P. van Vessem and Drs P. M. Schoot of the State University of Utrecht are thanked for their assistance during the study. Special thanks are due to Drs J. M. Roels for this critical editorial review of this paper.     

The salt marshes of the Westerschelde and their role in the estuarine ecosystem

The salt marshes along the Westerschelde estuary have been influenced by various human activities of which reclamation has been a major cause for the loss of salt marsh area. The salinity gradient in the aquatic system is also mirrored in the vegetation of the salt marshes.The role of the salt marshes for the estuary as a whole is manifold but a major importance is their function as a sink for anthropogenic substances.The possible role as a carbon and mineral source for the estuary is discussed in this paper. It is estimated that the total area of salt marsh adds about 8% to the organic matter input in the estuary while the nutrient input may be as high as 25%.Communication nr. 403 of the Delta Institute, Yerseke.     

Recognition of co-existence pattern of salt marshes and mangroves for littoral forest restoration

Climate-change driven sea level rise causes a increase in salinity in coastal wetlands accelerating the alteration of the species composition. It triggers the gradual extinction of species, particularly the mangrove population which is intolerant of excessive salinity. Thus despite being crucial to a wide range of ecosystem services, mangroves have been identified as a vulnerable coastal biome. Hence restoration strategy of mangroves is undergoing rigorous research and experiments in literature at an interdisciplinary level. From a data-driven perspective, analysis of mangrove occurrence data could be the key to comprehend and predict mangrove behavior along different environmental parameters, and it could be important in formulating management strategy for mangrove rehabilitation and restoration. As salt marshes are the natural salt-accumulating halophytes, mitigating excessive salinity could be achieved by incorporating salt-marshes in mangrove restoration activities. This study intends to find a novel restoration strategy by assessing the frequent co-existence status of salt marshes, with the mangroves, and mangrove associates in different zones of degraded mangrove patches for species-rich plantation. To achieve this, we primarily design a novel methodological framework for the practice of knowledge discovery concerning the coexistence pattern of salt marshes, mangroves, and mangrove associates along with environmental parameters using a data mining paradigm of association rule mining. The proposed approach has the capability to uncover underlying facts and forecast likely facts that could automate the study in the field of ecological research to comprehend the occurrence of inter-species relationships. Our findings are based on published data gathered on the Sundarban Mangrove Forest, one of the world’s most important littoral forests. The existing literature reinforces the findings that include all the sets of frequently co-occurring mangroves, their associates, and salt marshes along the salinity gradient of coastal Sundarbans. A detailed understanding of the occurrence patterns of all these, along with the environmental variables, would be able to promote decision-making strategy. This framework is effective for both academia and stakeholders, especially the foresters/ conservation planners, to regulate the spread of salt marshes and the restoration of mangroves as well.     

The harbour seal,Phoca vitulina,in the Oosterschelde: decline and possibilities for recovery

Within a timespan of a few decadesm the harbour seal almost completely disappeared from the estuaries in the south-west of the Netherlands. In 1960 a population of around 350 animals still lived in the Oosterschelde and Westerschelde area. About a quarter of this population lived in the Oosterschelde. At present less than 17 animals can be regularly observed in the whole area. Human influences are responsible for the rapid decline of the population. Initially a high hunting pressure and later environmental pollution are the main causes. Loss of habitat and disturbance at the resting places are additional important factors. The Oosterschelde still is a suitable habitat for seals. A short term natural development of a viable population in the area is not to be expected. Only with human help through active management, i.e. reintroduction of rehabilitated seals (preferably originating from that area) and strict conservation of the extant Oosterschelde seal population, accompanied by environmental sanitation of the neighbouring waters, can the current southern Dutch harbour seal population increase.     

New multi-metric Salt Marsh Sediment Microbial Index (SSMI) application to salt marsh sediments ecological status assessment

Salt marshes are very important areas for biogeochemical cycling, sediment accretion, pollution filtration and retention and erosion and stabilization of the river margins. The high organic matter content in the salt marsh plant sediments along with the radial oxygen diffusion provided by these halophyte root systems gather the ideal conditions for the development of a microbial rhizosphere community. Due to the quick feedback of the microbial communities to an environmental change, these organisms become important monitors for environmental impact assessment. A Salt marsh Sediment Microbial Index (SSMI) that reflected physical–chemical and microbial parameters was applied to plant rhizosphere sediments of five salt marshes from three important water bodies from Portugal. The SSMI revealed to be plant-independent evaluating efficiently the different marshes according to their maturity degree and disturbance influence. Mature salt marshes SSMI classification grouped all the systems at this development stage, while the younger salt marshes are classified in different groups according to their evolution degree. Also the impact degree is reflected at this level discriminating also the more adversely impacted salt marshes. Being a multi-metric index, the SSMI sub-metrics are also susceptible of ecological interpretation, giving important backstage information about the underlying biogeochemical cycling processes.     

The Oosterschelde estuary: an evaluation of changes at the ecosystem level induced by civil-engineering works

The interest in storm-flood protection has recently gained momentum, owing to the wide international discussion on the impact of sea-level rise on society. The Oosterschelde project is technically and scientifically unique. The storm-surge barrier represents an important breakthrough in marine civil engineering. The project also offered ample opportunities to perform integrated physical, chemical, geological and biological research. Integration of the knowledge gained, raised the entire project to the level of a case study of a changing estuarine ecosystem, and demonstrated the effects of human interference in a non-polluted estuary. Notwithstanding considerable changes in the environment, the Oosterschelde has retained most of its favourable abiotic factors, labeling the estuary as a high quality marine system. The water quality in the post-barrier period more closely resembles that of the North Sea than in the period before. Significant changes in erosion and sedimentation and the consequent redistribution of fine sediments, are continuing. The ecosystem has shown responses to various factors. Effects of severe winters and impact of mussel- and cockle fisheries could be distinguished from other factors. The physical response of the ecosystem to the civil-engineering project could be quantified in terms of changes in habitat availability, maintenance of biological productivity, and restricted maintenance of the carrying capacity as an internationally recognized wetland and fisheries area.     

Measurements of import and export in the Krabbenkreek,a tidal area in the Oosterschelde (S.W. Netherlands)

Summary The Krabbenkreek is a tidal area of about 11.5 km², surrounded by dikes for 94%. Mudflats cover nearly 65% of the area and salt marshes cover 15%. The area is connected to the Oosterschelde estuary by a main channel, which allows the tides to reach the inner parts; a small channel draines the mudflats near the mouth directly to the estuary.In a series of 10 measurements the fluxes of material were observed in the two channels by measuring the flow velocities each half hour during two tidal cycles. The water was sampled concurrently. A rough calculation of the data indicated that the resulting masses of soluble and particulate material during a tidal cycle between Krabbenkreek and Oosterschelde are of the same magnitude as the error due to incorrect flow velocity measurements. A more refined calculation is under develoment, which together with checks on the flow measurements will reduce the error.A statistical analysis of the data on the soluble and particulate material gives an indication of the direction of the net flow. In this analysis the chloride is considered a conservative parameter and data of ebb and flood are treated separately. This analysis reveals that ammonia is imported over the year while nitrate is exported. Chlorophyll-a is exported throughout the year as is particulate organic carbon. Silicate is imported throughout the year. There is an import of phosphorus in spring and summer and an export during autumn and winter.     

Ecological engineering, design, and construction considerations for marsh restorations in Delaware Bay, USA

Public Service Electric & Gas of New Jersey is restoring approximately 4050 ha of salt marsh along Delaware Bay, USA, to offset possible effects on fish populations in the Bay from their existing once-through cooling system. Planning for this effort started with addressing three questions: Do marshes contribute significantly to fish production? How much marsh produces how much fish? Which marshes should be restored? There is ample evidence that salt marshes produce fish. The area of marsh necessary to offset potential losses was calculated from a simple aggregated food chain model and multiplied by four to provide a comfort level to the regulatory agencies. Marshes chosen for restoration were former salt marshes at appropriate tidal elevations. Planning involved experts in marsh ecology, hydrology, and engineering working with the company and regulatory agencies to establish clearly defined goals for the project. Design followed the advice of the experts and construction was overseen to follow the design. Long-term follow up is through adaptive management that is scheduled to continue for about a decade, depending upon progress of the restoration toward its goals.     

Nature management in coastal areas of Western Europe

The general environmental characteristics of West European coastal dunes and those of salt marshes are outlined. It is pointed out that the dynamical character of the dune system with its interaction between plants and sand in building and degeneration phases has been known for a much longer period of time than generally believed.As to the salt marshes the special significance is discussed of ungrazed marshes; the majority of the marshes having been under a grazing regime for a long time is elucidated.The distinction between internal management is mentioned, i.e. the measures taken to maintain or improve the qualities of an area, versus external management, i.e. the measures to prevent or counteract negative influences from outside the area.Nomenclature follows Heukels & Van Ooststroom (1977).     

Influence of abiotic factors on spider and ground beetle communities in different salt-marsh systems

Salt marshes are interesting and endangered ecosystems in West-Europe. Nevertheless, their arthropod fauna remains largely unknown and the factors determining assemblages at micro-habitat scale are poorly understood. Few data are also available about the effects of management measures in salt marshes and how to monitor them. The aim of the present study is to determine the major factors structuring two dominant communities of arthropods, spider and ground beetles, in natural, managed (cutting and sheep grazing) and invaded (by the grass Elymus athericus) salt marshes. The two taxa were studied during 2002 and 2003 in different salt marshes of the Mont Saint-Michel Bay (NW France) by pitfall traps and hand-collecting. A total of 12 350 spiders (57 species) and 16 355 ground beetles (34 species) were caught during the study and analysed with respect to effects of the salinity gradient and of habitat structure characteristics. Spiders and ground beetles reacted differently to environmental factors in salt marshes. Spiders could more easily cope with salinity and their presence/absence was less related to the salinity than that of ground beetles. For ground beetles, there were few other community-structuring environmental factors and these were only related to the edaphic environment: species restricted to open habitats, significant effects of moisture content and salinity revealed by CCA. Because they are likely to bring complementary information on abiotic factors, we finally suggest using both spiders and ground beetles for monitoring the effects of management practices in salt marshes.     

The environmental consequences of climatic change on British salt marsh vegetation

The present relationship between sea level and the zonation of salt marsh vegetation is discussed in terms of the salt marshes of the Essex and Kent coasts. These marshes are already decreasing in area as a result of a number of different environmental pressures, including the sinking of the land relative to the sea, at a rate of about 3 mm per year, the result of isostatic adjustment following the last glaciation. Because most British salt marshes are backed by a sea wall the marshes can not respond to rising sea levels by migrating landwards, thus increasing the impact of sea level change. In view of this and of the importance of salt marshes as protection for the sea walls themselves, a conceptual model has been developed, of the likely impact of climate change and the resulting sea level rise, on British salt marsh vegetation. The basis of this approach is the assumption that a rise in sea level will cause the drowning of certain existing vegetation zones and their subsequent replacement by new vegetation types appropriate to the changed sea level. Estimates have been made of the likely impact of rises in sea level of 0.5, 1.0 and 1.5 metres on the five major vegetation zones identified in East Anglia. The validity of this approach is discussed, together with the likely additive effect of present degenerative changes observed in the Essex salt marshes. It is estimated that over the next 60 years a sea level rise of only 0.5 m, when existing degeneration is taken to account, would cause a loss of over 40% of the present area of salt marsh in Essex and probably also in Kent. These losses would mainly effect the higher salt marsh vegetation zones which would be replaced by pioneer communities. These predictions would be greatly magnified by larger rises in sea level. The wider ecological implication of these changes and some possible remedial measures are considered. These predictions are discussed in relation to the situation in the rest of East Anglia and for Britain as a whole.     

Integrated Marsh Management (IMM): a new perspective on mosquito control and best management practices for salt marsh restoration

Salt marsh management often embraces diverse goals, ranging from the restoration of degraded marshes through re-introduction of tidal flow to the control of salt marsh mosquito production by altering marsh surface topography through Open Water Marsh Management (OMWM). However, rarely have these goals been incorporated in one project. Here we present the concept of Integrated Marsh Management (IMM), which combines the best management practices of salt marsh restoration and OMWM. Although IMM offers a comprehensive approach to ecological restoration and mosquito control, research evaluating this concept??s practical implementations has been inadequate. A long-term IMM project at Wertheim National Wildlife Refuge located in a highly urbanized watershed on Long Island, New York, USA was designed to fill this knowledge gap. A combination of restoration and OMWM techniques was employed at two treatment marshes, the results monitored before and after alterations, and compared to two adjacent control marshes. The treatment marshes experienced decreased mosquito production, reduced cover of the invasive common reed (Phragmites australis), expansion of native marsh vegetation, increased killifish and estuarine nekton species abundance, as well as increased avian species diversity and waterbird abundance. This demonstration project validated the IMM conceptual approach and may serve as a case study for similar IMM projects in the future.     

Physical stress, not biotic interactions, preclude an invasive grass from establishing in forb-dominated salt marshes

Background

Biological invasions have become the focus of considerable concern and ecological research, yet the relative importance of abiotic and biotic factors in controlling the invasibility of habitats to exotic species is not well understood. Spartina species are highly invasive plants in coastal wetlands; however, studies on the factors that control the success or failure of Spartina invasions across multiple habitat types are rare and inconclusive.

Methodology and Principal Findings

We examined the roles of physical stress and plant interactions in mediating the establishment of the smooth cordgrass, Spartina alterniflora, in a variety of coastal habitats in northern China. Field transplant experiments showed that cordgrass can invade mudflats and low estuarine marshes with low salinity and frequent flooding, but cannot survive in salt marshes and high estuarine marshes with hypersaline soils and infrequent flooding. The dominant native plant Suaeda salsa had neither competitive nor facilitative effects on cordgrass. A common garden experiment revealed that cordgrass performed significantly better when flooded every other day than when flooded weekly. These results suggest that physical stress rather than plant interactions limits cordgrass invasions in northern China.

Conclusions and Significance

We conclude that Spartina invasions are likely to be constrained to tidal flats and low estuarine marshes in the Yellow River Delta. Due to harsh physical conditions, salt marshes and high estuarine marshes are unlikely to be invaded. These findings have implications for understanding Spartina invasions in northern China and on other coasts with similar biotic and abiotic environments.     

Restoring Salt Marshes Using Small Cordgrass, Spartina maritima

The use of exotic cordgrasses in salt marsh restoration projects has caused important negative environmental impacts and little is known about the possibilities of applying the endangered cordgrass Spartina maritima as a biotool at many European estuaries where it is the only native cordgrass. This paper discusses the planning and the development of an innovative restoration project based on S. maritima plantations in Odiel marshes (S.W. Iberian Peninsula). Our ecological restoration project had four specific goals: (1) to recover native vegetation, restoring the degraded landscape; (2) to phytostabilize oil-polluted sediments; (3) to prevent erosion and stabilize banks; and (4) to promote the conservation of S. maritima . Spartina maritima was planted at two physiographical locations: slightly sloping channel banks and flat interior marshes. Nonsuccessional stands of S. maritima develop at the channel banks where the marsh surface was stabilized. In contrast, successional stands of S. maritima grown in flat interior marshes are being replaced naturally by Sarcocornia perennis .     

Hydrodynamic characteristics of the Oosterschelde in recent decades

Hydrodynamic conditions in the Oosterschelde have changed as a result of the Delta Project. Engineering works were carried out between 1960 and 1970 in the northern branch of the Oosterschelde. The Grevelingen dam was built in 1964 and the Volkerak dam was completed in the spring of 1969. With the completion of these dams, the northern branch of the Oosterschelde was cut off from other estuaries, and river flow was then regulated by a sluice complex in the Volkerak dam. Between 1980 and 1987 a storm-surge barrier was constructed across the mouth of the Oosterschelde, and two compartmentalisation dams were built in the eastern part of the basin. The storm surge barrier decreased the effective cross-sectional area at the mouth from 80 000 m² to 17 900 m². The compartmentalisation dams reduce the surface area of the Oosterschelde basin from 452 km² to 351 km². The hydraulic impact of the works is demonstrated with data from field measurements and data from model calculations.     

Salt marshes along the coast of The Netherlands

The area of salt marshes does no longer increase. The recent erosion coincides with a rise in MHT-level in the last 25 years. Despite the decrease in area, sedimentation continues, especially in the lower salt marsh, which acts as a sink of nitrogen. Assimilation and mineralization of nitrogen are in balance in most plant communities along the gradient from lower to higher salt marshes. Mineralization of nitrogen increases towards the higher salt marsh, whereas the above-ground production and the mean nitrogen content of plants decrease. There is a positive correlation between quality of food plants in salt marshes and breeding success of Brent geese in the arctic tundra. Sedimentation on mainland salt marshes can compensate for the expected sea level rise. This is not the case for island salt marshes, if the relative sea level rise is more than 0.5–1.0 cm yr⁻¹. The natural succession on salt marshes results in an accumulation of organic material, which is related to the dominance of single plant species. It is not clear to which extent this process is enhanced by eutrophication from acid deposition and seawater. Human exploitation of unprotected salt marshes is old and heavy in the system of mound settlements. Reclamation rates by dikes in the last centuries were higher than the rate of area increase. Grazing by cattle as a management practice results in both a higher plant species-richness and community diversity than abandoning; hay-making is intermediate, but shows less structural diversity than grazing with low stocking density. The invertebrate fauna is favoured by a short period of abandoning, but eventually characteristic salt marsh invertebrates are replaced by inland species. Many bird species prefer grazed salt marshes. The final section gives some perspectives. Provided that no further embankments take place the optimal nature management option for plants and animals is a vegetation pattern, which includes areas with a low canopy (grazed) and areas with a tall canopy.     

Role of biotic interactions and physical factors in determining the distribution of marsh species along an estuarine salinity gradient

Understanding the mechanisms that shape plant distribution patterns is a major goal in ecology. We investigated the role of biotic interactions (competition and facilitation) and abiotic factors in creating horizontal plant zonation along salinity gradients in the Elbe estuary. We conducted reciprocal transplant experiments with four dominant species from salt and tidal freshwater marshes at two tidal elevations. Ten individuals of each species were transplanted as sods to the opposing marsh type and within their native marsh (two sites each). Transplants were placed at the centre of 9‐m² plots along a line parallel to the river bank. In order to disentangle abiotic and biotic influences, we set up plots with and without neighbouring vegetation, resulting in five replicates per site. Freshwater species (Bolboschoenus maritimus and Phragmites australis) transplanted to salt marshes performed poorly regardless of whether neighbouring vegetation was present or not, although 50–70% of the transplants did survive. Growth of Phragmites transplants was impaired also by competition in freshwater marshes. Salt marsh species (Spartina anglica and Puccinellia maritima) had extremely low biomass when transplanted to freshwater marshes and 80–100% died in the presence of neighbours. Without neighbours, biomass of salt marsh species in freshwater marshes was similar to or higher than that in salt marshes. Our results indicate that salt marsh species are precluded from freshwater marshes by competition, whereas freshwater species are excluded from salt marshes by physical stress. Thus, our study provides the first experimental evidence from a European estuary for the general theory that species boundaries along environmental gradients are determined by physical factors towards the harsh end and by competitive ability towards the benign end of the gradient. We generally found no significant impact of competition in salt marshes, indicating a shift in the importance of competition along the estuarine gradient.     

From a civil engineering project to an ecological engineering project: An historical perspective from the Mont Saint Michel bay (France)

Because of an excess of sedimentation due to natural and anthropogenic causes, the Mont Saint Michel (France) is becoming less frequently surrounded by seawater during high tide events. This prestigious monument and its bay have both been recognized by multiple national and international institutions for cultural and ecological richness. Diverse human activities such as tourism, fisheries, and farming also occupy the Mont Saint Michel Bay. The French government has been faced with a challenge of preserving the integrity of the bay, while restoring the ‘insularity’ of what used to be an island, the Mont Saint Michel. The objective of this paper is primarily to demonstrate how environmental scientists and ecologists influenced a typical civil engineering project and modified it into a civil/ecological engineering project, more respectful of both environment and human interests. To achieve this objective, we first retrace the formation of the Mont Saint Michel Bay, under both geological and historical perspectives, to provide the necessary background for the understanding of the actual restoration project. Some of the first projects that were proposed were designed to destroy the area of salt marshes that surrounded the Mont Saint Michel, with no consideration for the ecological integrity of the site. We conclude by presenting the current project in which France will invest over 130 million USD in the next few years. If this final project cannot be recognized truly as an ecological engineering project, it has the merit of being a compromise between various partnerships which have been debating for more than a century.     

Composition and abundance of resident marsh-surface nekton: comparison between tidal freshwater and salt marshes in Virginia,USA

Previous research on intertidal nekton communities has identifiedimportant determinants of community structure and distribution; however, fewstudies have compared nekton utilization of disparate marsh habitats. Inthis study, abundance and distribution patterns of resident nekton werecompared between tidal freshwater marsh and salt marsh surfaces varying inflooding depth and duration. Nekton were collected in pit traps installedalong elevational transects at four marshes in coastal Virginia (twofreshwater, two saline) from April through November 1992–1993. Thedominant fish collected at all sites was the mummichog Fundulusheteroclitus. The daggerblade grass shrimp Palaemonetes pugio was thedominant nekton species collected at salt marsh sites, and was seasonallyabundant on tidal freshwater marshes. A positive correlation betweenflooding depth and nekton abundance was observed on salt marshes; anopposite pattern was observed on tidal freshwater marshes. Tidal floodingregime influences the abundance of resident nekton, however, the effect maybe confounded by other environmental variables, including variation insurface topography and seasonal presence or absence of submerged aquaticvegetation (SAV) in adjacent subtidal areas. In mid-Atlantic tidalfreshwater wetlands, SAV provides a predation refuge and forage site forearly life stages of marsh-dependent nekton, and several species utilizethis environment extensively. Salt marshes in this region generally lackdense SAV in adjacent subtidal creeks. Consequently, between-sitedifferences in species and size-specific marsh surface utilization byresident nekton were observed. Larvae and juveniles represented 79%and 59% of total fish collected at tidal freshwater and salt marshsites, respectively. The resident nekton communities of tidal freshwater andsalt marsh surfaces are characterized by a few ubiquitous species with broadenvironmental tolerances. This revised version was published online in July 2006 with corrections to the Cover Date.