Schelde Estuary Research Articles

A reference list of fish species for a heavily modified transitional water: The Zeeschelde (Belgium)

A crucial step in the development of a fish-based index for the ecological assessment of water bodies as provided by the European Water Framework Directive is to define a reference list of fish species occurring in pristine rivers. The aim of this study was to elaborate such a list. The reference corresponds to an ecological status that is referred to as Good or Maximal Ecological Potential (GEP/MEP). Based on historically-reported fish survey data of the Zeeschelde estuary (Belgian part of the Schelde estuary) and its tributaries, i.e. an affluent system, under tidal influence, presence/absence reference lists were compiled for different salinity zones and adjusted using information from recent catches. In addition, an MEP list of fishes occurring in the Westerschelde (Dutch part of the Schelde estuary), developed by JAGER & KRANENBARG (2004), is provided to present a complete overview of the Schelde estuary. Inclusion of fish species in the reference lists depended on their natural geographical distribution and ecological demands. These reference lists contain guild-specific information for the different zones within the estuary and its tidal tributaries.

Fish assemblages across a salinity gradient in the Zeeschelde estuary (Belgium)

Between 1991 and 2008 a total of 71 fish species was recorded in the brackish and fresh water zone of the Schelde estuary (Zeeschelde). The results were obtained from fish surveys from the cooling water filter screens of the power plant at Doel (between 1991 and 2008) and fyke net surveys along the length of the estuary between 1995 and 2008. Species abundance in the different salinity zones was analysed using the fyke net data only. The ten most abundant species represent 90.8% of the total number of individuals caught. In decreasing order of abundance: flounder (Platichthys flesus), roach (Rutilus rutilus), herring (Clupea harengus), eel (Anguilla anguilla), pike-perch (Sander lucioperca), sole (Solea solea), common goby (Pomatoschistus microps), seabass (Dicentrarchus labrax), three-spined stickleback (Gasterosteus aculeatus) and white bream (Blicca bjoerkna). With fyke nets 33 species were caught in the tidal freshwater zone, 43 species in the oligohaline zone and 59 species in the mesohaline zone. Each salinity zone is characterised by a typical fish assemblage, although some species are shared between all three salinity zones: e.g. three-spined stickleback (Gasterosteus aculeatus), Prussian carp (Carrasius gibelio), roach (Rutilus rutilus) and eel (Anguilla anguilla). Diadromous species occur in all zones and make up, on average up 22% of the species richness. Freshwater species comprise about 70% of the species in the tidal freshwater zone. In the oligohaline zone the contribution of the freshwater species to the species richness is less while marine migrants become more abundant. As expected, the contribution of marine migrants and estuarine species is higher in the mesohaline zone. The recent increase in species richness in the freshwater and oligohaline zone coincides with a remarkable increase in dissolved oxygen since 2007.

Do different habits affect microplastics contents in organisms? A trait-based analysis on salt marsh species

Salt marshes in urban watersheds are prone to microplastics (MP) pollution due to their hydrological characteristics and exposure to urban runoff, but little is known about MP distributions in species from these habitats. In the current study, MP occurrence was determined in six benthic invertebrate species from salt marshes along the North Adriatic lagoons (Italy) and the Schelde estuary (Netherlands). The species represented different feeding modes and sediment localisation. 96% of the analysed specimens (330) did not contain any MP, which was consistent across different regions and sites.Suspension and facultative deposit-feeding bivalves exhibited a lower MP occurrence (0.5–3%) relative to omnivores (95%) but contained a much more variable distribution of MP sizes, shapes and polymers. The study provides indications that MP physicochemical properties and species' ecological traits could all influence MP exposure, uptake and retention in benthic organisms inhabiting European salt marsh ecosystems.

Groundwater dynamics in a restored tidal marsh are limited by historical soil compaction

In places where tidal marshes were formerly embanked for agricultural land use, these marshes are nowadays increasingly restored with the aim to regain important ecosystem services. However, there is growing evidence that restored tidal marshes and their services develop slowly and differ from natural tidal marshes in many aspects. Here we focus on groundwater dynamics, because these affect several key ecosystem functions and services, such as nutrient cycling and vegetation development. We hypothesize that groundwater dynamics in restored tidal marshes are reduced as compared to natural marshes because of the difference in soil structure. In the macro-tidal Schelde estuary (Belgium), in both a natural and a restored (since 2006) freshwater tidal marsh, we measured depth profiles of soil properties (grain size distribution, LOI (loss on ignition), moisture content and bulk density) and temporal dynamics of groundwater levels along a transect with increasing distance from a tidal creek. X-ray micro CT-scanning was used to quantify soil macroporosity. The restored marsh has a two-layered soil stratigraphy with a topsoil of freshly accreted sediment (ranging in depth between 10 and 60 cm, deposited since 2006) and a subsoil of compact relict agricultural soil. We found that both the soil in the natural marsh and the topsoil of the restored marsh consist of loosely packed sediment rich in macropores and organic matter, whereas the relict agricultural soil in the restored marsh is densely packed and has few macropores. Our results show that groundwater level fluctuations in the restored marsh are restricted to the top layer of newly deposited sediment (i.e. on average 0.08 m depth). Groundwater level fluctuations in the natural marsh occur over a larger depth of the soil profile (i.e. on average 0.28 m depth). As a consequence, the reduced groundwater dynamics in restored tidal marshes are expected to alter the subsurface fluxes of water and nutrients, the source-sink function and the development of marsh vegetation.

An Update on the Invasion of Weakfish Cynoscion regalis (Bloch & Schneider, 1801) (Actinopterygii: Sciaenidae) into Europe

New information on weakfish introduction vectors, its invasive status, distribution, and use as a fishing resource arose after the publication of “The transatlantic introduction of weakfish Cynoscion regalis (Bloch & Schneider, 1801) (Sciaenidae, Pisces) into Europe” by Morais and Teodósio (2016). Currently, the first known report of weakfish in Europe dates back to September 2009, with a specimen captured in the Schelde estuary (Belgium/The Netherlands). This fact suggests that weakfish could have been introduced into Europe via multiple and independent ballast water introduction events, and not through a point-source introduction event with subsequent dispersion as previously hypothesized. It is also unlikely that Schelde weakfish migrated southwards to colonize Iberian aquatic ecosystems. Weakfish have established a population in the Gulf of Cádiz region and have already reached an invasive status in the Sado estuary (Portugal). Weakfish were also captured in several other locations along the Portuguese coast, including the Tagus and Mira estuaries at least since 2013 or 2014, and the Ria Formosa lagoon in 2017. Tagus anglers caught weakfish specimens of ~1 kg and ~40 cm in November 2016, which corresponds to fish of 3+ years of age in the native range. The presence of weakfish in the Tagus estuary is still fairly unknown to local anglers. Sado weakfish has already been sold in local fish markets in southern Portugal for 3 to 10 € kg−1. However, we consider that the weakfish sale price is underrated in comparison with other wild species (e.g., meagre, seabass, gilthead seabream). Increasing sale price will convince fishers to use weakfish as a new fishing resource; however, it is necessary to promote the species among consumers and evaluate consumers’ preference in respect to other species. A putative biological threat might turn into a new valuable fishing resource by implementing adequate management solutions.

Substrate origin and morphology differentially determine oxygen dynamics in two major European estuaries, the Elbe and the Schelde

The expansion of oxygen minimum zones (OMZ's) in estuaries can be harmful for ecology and economy, prompting the demand for expensive measures. Here we look at the oxygen dynamics in two northern temperate European estuaries, the Schelde (The Netherlands/Belgium) and the Elbe (Germany) and analysed data from the period of 2004–2009. The Schelde is characterized by two zones of increased oxygen consumption; the Elbe shows one zone of increased oxygen consumption. Despite reduction in biochemical oxygen demand in both estuaries, oxygen conditions improved in the Schelde estuary, while the oxygen minimum zone persisted in the Elbe estuary. To understand these different oxygen dynamics, we applied a one-dimensional reactive transport model to both estuaries. In the Schelde we found low oxygen concentrations to be related to organic matter and ammonium input from the major tributaries. In the Elbe, additionally to a high input of organic matter from upstream, oxygen dynamics were influenced by abrupt changes in estuarine morphology. Next, the origin of the organic matter substrate differs between the two estuaries. In the Elbe, the organic matter imported is mostly composed of algal die-off produced in the Elbe River upstream. In the Schelde the organic matter and ammonium input is mostly related to sewage input of anthropogenic origin. This implies that waste water treatment will be more effective to remediate hypoxia related problems in the Schelde than in the Elbe.

Ecosystem service delivery in restoration projects: the effect of ecological succession on the benefits of tidal marsh restoration

Long-term assessment of ecosystem restoration projects is complex because of ecological processes such as succession, particularly in highly dynamic ecosystems such as estuaries. Restoration of intertidal flats and marshes on formerly embanked land, often called managed coastal realignment (MR), became popular in estuarine management. In our study, biophysical and monetary data were collected to calculate the value of 15 (sub)ecosystem services (ES) delivered by a large tidal marsh restoration project in the Schelde estuary in Belgium and the Netherlands. We hypothesized that ES delivery changes over time due to ecological succession and hence the long-term benefits are subject to this phenomenon and need to be taken into consideration. A marsh sediment accretion model (MARSED) was used to simulate potential marsh succession scenarios. In this way, the temporal evolution of ES delivery caused by ecological succession could be evaluated. Our study shows that benefits during successional marsh stages could actually be higher than for marshes in equilibrium. This finding does not suggest that ecosystems in transition always have a higher value than systems in equilibrium, but emphasizes the need to consider long-term ecological dynamics, such as succession, in a benefit assessment for restoration projects.

Community structure and diversity of gelatinous zooplankton in the Bay of Biscay

Community structure and diversity of gelatinous zooplankton in the Bay of Biscay

Gelatinous zooplankton in the Belgian part of the North Sea and the adjacent Schelde estuary: Spatio-temporal distribution patterns and population dynamics

Many ocean ecosystems are thought to be heading towards a dominance of gelatinous organisms. However, gelatinous zooplankton has been largely understudied and the absence of quantitative long-term data for the studied area impedes drawing conclusions on potential increasing densities. This study gives a comprehensive overview of the spatio-temporal distribution patterns of gelatinous zooplankton in terms of diversity and density in the Belgian part of the North Sea and the adjacent Schelde estuary, based on monthly and seasonal samples between March 2011 and February 2012. Three Scyphozoa, three Ctenophora and 27 Hydrozoa taxa were identified, including three non-indigenous species: Mnemiopsis leidyi, Nemopsis bachei and Lovenella assimilis. In general, one gelatinous zooplankton assemblage was found across locations and seasons. Average gelatinous zooplankton densities reached up to 18ind·m−3 near the coast, gradually declining towards the open sea. In the brackish Schelde estuary, average densities remained below 3ind·m−3. Highest gelatinous zooplankton densities were recorded in summer and autumn. Overall, hydromedusae were the most important group both in terms of diversity and density. The ctenophore Pleurobrachia pileus and the hydromedusa Clytia sp. were present in every season and at every location. Gelatinous zooplankton densities never outnumbered the non-gelatinous zooplankton densities recorded from the WP3 samples. The spatial and temporal distribution patterns seemed to be mainly driven by temperature (season) and salinity (location). Other environmental parameters including (larger) non-gelatinous zooplankton densities (as an important food source) were not retained in the most parsimonious DistLM model.In terms of population dynamics, Beroe sp. seemed to follow the three reproductive cycles of its prey P. pileus and the presence of M. leidyi, which were abundant in a broad size spectrum in summer and autumn. In general, gelatinous zooplankton diversity was higher, but densities were in the same order of magnitude compared to adjacent areas in the North Sea. This study provides a baseline against which a potential increase in gelatinous zooplankton in the Belgian part of the North Sea and the Schelde estuary can be measured.

Sediment Abiotic Patterns in Current and Newly Created Intertidal Habitats from an Impacted Estuary

The controlled reduced tide system (CRT) is a new technique for restoring tidal marshes and is being tested in the Schelde estuary (Belgium). Biogeochemical processes within a CRT were hypothesized to support and improve several estuarine functions such as sediment trapping and nutrient burial. In 2006, the first pilot CRT was implemented in the freshwater zone of the estuary. Fifteen sediment physicochemical descriptors were intensively monitored over 3 years in the newly created CRT and in reference habitats from the adjacent estuary. Soil transformed rapidly in the CRT; in the most frequently flooded zones, the formation of a nutrient-rich estuarine sedimentary substrate contrasted with the estuarine sand flats where shear stress is sustained by coastal squeeze. The temporal dynamics of the sediment descriptors were investigated to identify key processes involved in the flooding of the CRT sediment. Although many processes were specific to the CRT, both reference and CRT sediment characteristics experienced similar long-term oscillations. However, despite such variations, successful CRT nutrient trapping and fine particles burial were demonstrated. This study proves that the CRT, in accordance with restoration goals, can restore ecological functions in impacted estuaries. In addition, the results highlight the complex timing of abiotic patterns in intertidal sediments.

Avian response to tidal freshwater habitat creation by controlled reduced tide system

Human activities have caused extensive loss of estuarine wetlands, and the restoration of functional habitats remains a challenging task given several physical constraints in strongly embanked estuaries. In the Schelde estuary (Belgium), a new tidal marsh restoration technique, Controlled Reduced Tide system (CRT), is being implemented in the freshwater zone. A polder area of 8.2 ha was equipped with a CRT to test the system functionality. Among different ecological compartments that are studied for assessing the CRT restoration success, avifauna was monitored over three years. The tidal regime generated a habitat gradient typical of tidal freshwater wetlands along which the distributions of bird and ecological groups were studied. 103 bird species were recorded over the three years. In addition to many generalist bird species, several specialist species typical of the North Sea coast were present. Thirty-nine species of local and/or international conservation interest were encountered, emphasising the importance of this habitat for certain species. Species communities and ecological groups were strongly habitat specific and non-randomly organized across habitats. Spatiotemporal analyses highlighted a rapid habitat colonization, and a subsequent stable habitat community structure across seasons in spite of strong seasonal species turnovers. Hence, these findings advocate CRT implementation as a means to effectively compensate for wetland habitat loss.

Bratislavia dadayi (Michaelsen, 1905) (Annelida: Clitellata: Naididae): a new non-indigenous species for Europe, and other non-native annelids in the Schelde estuary

For the first time, the freshwater oligochaete species Bratislavia dadayi (Michaelsen, 1905) is recorded in Europe. The species was found at three subtidal stations in the Schelde estuary in Belgium, where it was probably introduced from the Americas. We provide an overview of the species’ nomenclature, diagnostics, distribution, and ecology. Bratislavia dadayi is one of 11 non-indigenous annelids currently known to occur in the Schelde estuary.

Sediment macroinvertebrate community functioning in impacted and newly-created tidal freshwater habitats

Although experiencing strong anthropogenic pressures in many estuaries, the ecology of tidal freshwater areas remains largely undocumented. As part of a restoration project in the freshwater zone of the Schelde estuary (Belgium), a new tidal habitat restoration technique (Controlled Reduced Tide system, CRT) was hypothesised to successfully compensate for the impairment of contemporary habitats. The suitability of this newly-created habitat (CRT) and the estuary was investigated over a period of three years for its macroinvertebrate community development. In both the estuary and the CRT, habitats along a flooding gradient were monitored. Differences between the CRT and reference sites in community functioning were explored according to environmental characteristics and organism biological attributes using the RLQ ordination analysis together with the fourth-corner method. Frequently flooded reference sites exhibited environmental characteristics resulting from a hydrological shear stress. In the CRT, after a rapid removal of the terrestrial fauna at low and mid elevations, the low-energy hydrology led to taxonomic and functional enrichment. The RLQ analysis produced significant environmental filtering of biological attributes mainly related to the terrestrial–aquatic transition and to the environmental stressors. This provides an example of life history modification via estuarine ecosystem management.

The role of a freshwater tidal area with controlled reduced tide as feeding habitat for European eel (Anguilla anguilla, L.)

Summary Implementation of the Controlled Reduced Tide (CRT) technique could increase the total surface of tidal freshwater marshes in Europe and ease implementation of restoration projects in coastal defense and riverine ecosystems. The goal was to determine whether a regularly flooded area connected to a freshwater tidal river could act as an important foraging area for European eel, and if so, to what extent the diet of eels in this flooding area differed from that of eels foraging in the river itself. The stomach contents of eels from the River Schelde were compared with eels from the Lippenbroek, an adjacent CRT area. Prey diversity (H′) of individual eels was about four times higher in the Lippenbroek than in the River Schelde. Moreover, 12 prey categories in eel stomachs from the Lippenbroek were found whereas only three categories were retrieved from eels in the River Schelde. In the Lippenbroek, eels fed on terrestrial organisms (lumbricids, caterpillars and other insects), but also on fish and fish eggs and to a lesser extent on other aquatic prey (Lumbricullidae, chironomids and Hirudinea). In contrast, eels from the main river fed mainly on tubificids, fish, and some gammarids. Consequently, eels in the Schelde estuary are opportunistic feeders, but with a preference for large benthic prey. The number and weight of aquatic organisms ingested by eels in the Lippenbroek is not significantly different from the River Schelde. However, eels foraging in the Lippenbroek area had consumed significantly more terrestrial prey. Furthermore the total caloric value estimated for the ingested prey of eels from the Lippenbroek (derived from the literature) was about twice as high as that for eels from the River Schelde. While the condition index remained inconclusive, an Ancova revealed that eels captured in the Lippenbroek were significantly heavier for a given length than eels captured in the Schelde. The study showed that with a controlled reduced tide to restore lateral connectivity of large tidal rivers with their adjacent floodplains, high quality habitats for the European eel are created. These measures could significantly contribute to the production of eels in better condition, which have better chances to reproduce successfully. Hence, wetland restoration could enhance the recovery of the European eel stocks.

Evolution of sediment metal concentrations in a tidal marsh restoration project

The combination of flood prevention and tidal marsh restoration will be implemented on a large scale in the Schelde estuary (Belgium). Densely populated and industrialized, this estuary was found to be severely contaminated with trace metals. In this study we evaluated the effect of tidal restoration on sediment trace metal concentrations. To asses historical contamination of embanked-, a restored- and natural tidal areas, deep sediment cores were sampled while the evolution of metal concentrations was determined by means of superficial samples taken during 10 sampling campaigns spread over the first 3years of the restoration project. Metal concentrations in the natural tidal marsh reflected the estuaries' contamination history. Fertilization by irrigation caused high metal concentrations in superficial soil layers of some embanked areas. However, reintroduction of the tide resulted in deposition of a new sediment layer with lower metal concentrations, comparable to the natural tidal marsh. Despite diagenetic mobility of manganese no diagenetic movements of the trace metals were observed during these first three years. Removal of metals from the estuary and burial of contaminated sediments in the restored site emphasize the potential of these restoration projects to decrease metal contamination risks. However, more research under field conditions on the effects of changes in land use and inundation related changes in metal bioavailability is needed to draw clear conclusions on the environmental consequences.

Spatiotemporal bioturbation patterns in a tidal freshwater marsh

Bioturbation has been hypothesized to exhibit different forms of sediment mixing in aquatic systems, but few in situ tests have been conducted in estuaries, and anyone along a flooding gradient which is the main feature characterizing intertidal areas. The relationships between bioturbation and macroinvertebrate communities were studied as part of a restoration project in the tidal freshwater zone of the Schelde estuary, and highlighted specific sediment mixing patterns along a tidal gradient. Three permanent sites, evenly distributed along the flooding gradient, were monitored over a period of one year. Tidal influence engendered a clear gradient opposing newly-established aquatic communities (low elevation and strong disturbance) to remnant terrestrial communities (high elevation and low disturbance). Different bioturbative modes were identified along this gradient. Biodiffusion (random spreading of sediment particles) was the dominant mode at high and mid elevations. Low elevation was characterized by bioadvection (vertical movement of sediment particles) and higher bioturbative intensities. Maximum bioturbative intensities were observed in summer. This is the first bioturbation study, conducted along a flooding gradient, and which characterizes the bioturbative modes and intensities among tidal habitats and confirms the key role of disturbance. These findings underline the significance of the multiplicity of bioturbation modes in estuarine habitats, and the potential implications in estuarine biogeochemistry in general.

A new technique for tidal habitat restoration: Evaluation of its hydrological potentials

The inability to create an adequate tidal regime in embanked areas is a major problem for restoring estuarine habitats. The controlled reduced tide system (CRT) was previously hypothesized to overcome this constraint. As part of an estuarine management plan which combines flood protection and tidal habitat restoration, the first CRT system was implemented in the freshwater zone of the Schelde estuary (Belgium). Based on four years of high-frequency monitoring on the first CRT and the adjacent estuary, this study demonstrates the hydrological functionality of CRT. The tidal characteristics generated by this technique were suitable in the short and the long term, with a reproduction of the spring-neap tidal cycle. In both the CRT and the estuary, the spatial and temporal variability of several hydrological descriptors were comparatively analysed. In spite of some hydrological deviations from the estuarine pattern, nothing precluded a suitable ecosystem feature in CRT. The potential influences of CRT-specific hydrology on ecology and estuarine restoration are discussed. The restoration potential of the CRT system is shown to be particularly relevant for tidal marshes in early succession stage, habitats which are often lacking in embanked estuaries. Additionally, it offers a more robust and adaptable alternative to other systems. Conclusively, the CRT system should be more advocated in estuarine restoration.

Modeling photosynthesis‐irradiance curves: Effects of temperature, dissolved silica depletion, and changing community assemblage on community photosynthesis

Sets of photosynthesis‐irradiance (P‐I) curves yield more information about community photosynthesis when analyzed with proper models in mind. Based on ecosystem‐specific considerations regarding the factors that explain spatial and temporal patterns of photosynthesis, the Webb model of photosynthesis can be extended and fitted to P‐I data. We propose a method based on a series of nested models of increasing complexity to test whether supposed effects of environmental factors are reflected in the P‐I data, whether more complex models fit the data significantly better than more simple models, and whether parameters describing the presumed dependencies can be estimated from the data set. We compare a direct approach, fitting the extended model to all P‐I data at once, with a two‐step approach in which photosynthetic efficiencies and maximum photosynthetic rates of individual P‐I curves are determined first, and then related to environmental variables. A nested model approach prevents overfitting of multiparameter models. Monte Carlo analysis sheds light on the error structure of the model, by separating parameter and model uncertainty, and provides an assessment of the performance of the formulations used in ecosystem models. We demonstrate that the two‐step approach underperforms when used to compute photosynthetic rates. We apply the proposed method to an extensive P‐I data set from the Schelde estuary, where spatiotemporal patterns of photosynthesis arise from a combination of seasonality, silica depletion, phytoplankton community composition, and salinity effects.

Rotifers in the Schelde estuary (Belgium): a test of taxonomic relevance

To investigate the reaction of the zooplankton community to improving water quality in the Schelde estuary, we studied the relationship between rotifer species distribution and environmental factors, and the feasibility of using a coarser level of taxonomic resolution. Fifty-two taxa, belonging to 26 genera, were identified, including 22 taxa new for the Schelde. Brachionus calyciflorus, Keratella cochlearis and B. angularis were the most abundant species. The highest diversity and abundances were observed in the freshwater reach. Redundancy analyses (RDA) showed that the main environmental factors explaining rotifer distribution were chlorinity and seasonal factors (discharge levels, cyclopoid abundance). Analysis carried out with data at the species and the genus level gave similar ordination plots, but the positioning of a genus relative to environmental factors did not always adequately represent the associations between the various species within the genus and environmental factors. Similar patterns in space and time were observed using taxonomic richness and diversity indices for analyses at species or genus level. Thus, in the context of the restoration of the Schelde estuary, the identification of rotifer species is very informative, but not essential for detecting important ecological associations.

Restoration of tidal freshwater vegetation using controlled reduced tide (CRT) along the Schelde Estuary (Belgium)

Throughout the world, tidal marshes are being restored to obtain natural protection against recurring storm surges and sea level rise and to preserve the resources these habitats provide. Managed realignment, or breaching of the seaward dikes, is a restoration technique increasingly used; yet unsuitable site elevation has been a major constraint to habitat development. Controlled reduced tide (CRT), a new technique to implement tidal regime on low elevated sites, could offer solutions for several problems associated with reconstruction or mitigation of tidal marshes. We followed vegetation development at 3-month intervals during two years after implementation of the first pilot CRT treatment, which aimed to restore tidal freshwater marsh habitat in an agricultural site where elevation relative to the estuarine marshes has lowered. The development of the plant community and its driving forces (flooding, remnant vegetation) were studied. Arrival and establishment of typical tidal freshwater marsh vegetation was rapid, and a clear elevation gradient developed by eradication of terrestrial species and colonisation by estuarine species. Remnant vegetation substantially reduced the number of new established species. General development trajectory as well as the changing effect of driving forces and their interaction point to a swift ecosystem shift during the first two surveys, and a gradual species saturation during the seven following surveys. Vegetation communities seem to deviate from the nearby tidal freshwater marshes, but show similarities with tidal freshwater marshes described for the beginning of the 20th century. The CRT-technique provides strong potential for durable, adaptive restoration of tidal marshes on sites with low elevation. Wider implementation of the CRT technique could increase the total surface of tidal freshwater marshes with fully developed vegetation gradients.