Analysis of multibeam echo sounding data on bed forms near the Walsoorden sandbar, a first phase in the subtidal habitat classification for the Western Scheldt

River morphodynamics and sediment transportRiver morphology and morphodynamic

Settlement of Macoma balthica larvae in response to benthic diatom films

The role of multi-species benthic diatom films (BDF) in the settlement of late pediveliger larvae of the bivalve Macoma balthica was investigated in still-water bioassays and multiple choice flume experiments. Axenic diatom cultures that were isolated from a tidal mudflat inhabited by M. balthica were selected to develop BDF sediment treatments characterized by a different community structure, biomass, and amount of extracellular polymeric substances (EPS). Control sediments had no added diatoms. Although all larvae settled and initiated burrowing within the first minute after their addition in still water, regardless of treatment, only 48-52% had completely penetrated the high diatom biomass treatments after 5 min, while on average 80 and 69% of the larvae had settled and burrowed into the control sediments and BDF with a low diatom biomass (<3.5 mu g Chl a g(-1) dry sediment), respectively. The percentage of larvae settling and burrowing into the sediment was negatively correlated with the concentration of Chl a and EPS of the BDF. This suggests higher physical resistance to bivalve penetration by the BDF with higher diatom biomass and more associated sugar and protein compounds. The larval settlement rate in annular flume experiments at flow velocities of 5 and 15 cm s(-1) was distinctly lower compared to the still-water assays. Only 4.6-5.8% of the larvae were recovered from BDF and control sediments after 3 h. Nonetheless, a clear settlement preference was observed for BDF in the flume experiments; i.e., larvae settled significantly more in BDF compared to control sediments irrespective of flow speed. Comparison with the settlement of polystyrene mimics and freeze-killed larvae led to the conclusion that active selection, active secondary dispersal and, at low flow velocities (5 cm s(-1)), passive adhesion to the sediment are important mechanisms determining the settlement of M. balthica larvae in estuarine biofilms

Natuurherstelplan Zeeschelde: drie mogelijke inrichtingsvarianten

The Schelde estuary, with its tidal flats and marshes along an uninterrupted salinity gradient from a marine to a fresh water system, is almost unique in Europe. On top of its important contributions towards national and international biodiversity it performs multiple socio-economic functions of vital importance. To combine all these functions in a sustainable way an integrated management is needed, based on the comprehension of its functioning and aimed at an optimal tuning of all processes and functions involved. With this report we hope to initiate a productive dialogue, one step ahead towards the integrated management of the Schelde estuary. It forms part of an agreement between the Administration of Waterways and Water infrastructure and some Flemish environmental organisations. The agreement was concluded as a result of protest actions against the deepening of the Westerschelde and it should result in a fruitful cooperation towards a more integrated management for the Flemish waterways. This pilot study explores possibilities to couple ecological rehabilitation and the creation of sustainable river related nature with the security measures of the SIGMA plan (a protection plan against floods) and the navigation requisites for the Zeeschelde. It also integrates the proposals from MEIRE et al. (1992), HOFFMANN (1993) and the AMIS study (general environmental impact assessment of the Sigmaplan). The decreased self-cleaning capacity of the river, the highly fragmented ecologica1 infrastructure, the accelerated sedimentation and increased tidal amplitude inhibit a proper functioning of the estuarine ecosystem. They are combined consequences of land reclamation, channelisation, land use and water management in the catchment area, discharge manipulations and the general sea level rise. They bring about the ecosystems limited carrying capacity and resilience, its increased contribution to the eutrofication of the North Sea, impoverished biotic communities, flood hazzard and reduced navigability. Possible restoration measures can be defined at three different levels: expansion of the intertidal area through dike removal and levelling of raised grounds outside the dikes, enhancement of contacts between the river and its catchment through ecological adjustments of dike structures and restoration of the contact between waters on both sides of the dike, structural and functional restoration of inland territories. Application of these measures can yield a more complete ecological infrastructure, improved migration possibilities for biota, more diverse biotic communities, limited input of energy and matter towards the river, enhanced self-cleaning capacity and retention, reduced contribution towards the eutrofication of the North Sea, increased volume and a slow down of the sedimentation rate. According to the adhered vision for the development of the estuary, the proposed restoration measures can be applied in different combinations to enhance its ecological rehabilitation/restoration. In this report three different scenario's are traced out, each of which set a different direction for the area' s development. The first two scenario's aim at a coherent development direction for the estuary, each from a different viewpoint with respect to hydrodynamics, one of the most important guiding determinants for the estuarine gradiënts along which different biotic communities follow upon each other in space and time. The scenario 'Space for the Estuary' leaves as much space as possible for the estuary to allow the tidal wave to die out laterally and longitudinally. Processes, which are steered by tidal dynamics gradually, merge into processes, which are directed by hydrodynamics of precipitation, ground and surface water. In those areas where estuarine expansion is not possible or advisable the attention is rather focused on inland nature development. The full realisation of this scenario would result in a total of 3.544 ha tidal wetland, 846 ha controlled inundation area with a reduced tidal regime and 3.472 ha of inland wetlands. The surface of functional inundation areas would vary between minimum 1.207 and maximum 3.907 ha. The scenario' Attention for the Alluvium' judges that the greater part of this alluvium in the Schelde River Basin did not come into existence under tidal influence but through seasonal floods from upstream areas. It rather aims at the restoration of the alluvial plains as nutrient-rich wetlands in a continuous transition towards higher and more dry, nutrient -poor soils. Still, in this scenario attention is also paid to estuarine rehabilitation outside the dikes. The full realisation of this scenario would result in a total of 1.663 ha tidal wetland, 774 ha controlled inundation area with a reduced tidal regime and 5.324 ha of inland wetlands. The surface of functional inundation areas would vary between minimum 1.787 and maximum 5.687 ha.The scenario ‘Funtional and Structural Basic Quality' does not really adhere to a preconceived vision for the development of the estuary. The proposed measures mainly contain the ecological adaptations to the Sigmaprojects as they were suggested in the environmental impact assessment (AMIS-45) and the ecological restoration of all raised fields and dumping grounds outside the dikes. Implementation of this scenario would suffice for the ecological rehabilitation of the estuary, it would at the very most maintain a structural and functional basic quality with the completion of all sigmaprojects, the deepening of the Westerschelde and the structural adaptations of the Zeeschelde to contemporary navigation demands. The full realisation would result in a total of 1.878 ha tidal wetland, 496 ha controlled inundation area with a reduced tidal regime and 1.075 ha of inland wetlands. The surface of functional inundation areas would vary between minimum 1.508 and maximum 1.694 ha. Not any of these scenario's constitutes a ready-made blueprint for a ‘restored’ Zeeschelde. They picture a few possible designs for the estuary which, by means of models, can be tested for their probable impacts on the ecosystem, its functioning and potential opportunities for ecotopes, species and biotic communities. As such they should be considered as a contribution towards the social discussion that, hopefully, will yield an integrated ecosystem vision in the end

Alternatieve waterkeringen: een verkenning naar nieuwe concepten voor kustverdediging in het kader van beleid Ondersteuning Programmaburea Zuidewestelijke Delta

Klimaatverandering en de daarmee samenhangende zeespiegelstijging, extremere stormen en hogere piekafvoeren van de rivieren leiden tot nieuwe opgaven voor kustverdediging in de Zuidwestelijke Delta. Deze ontwikkelingen vragen om flexibele en robuuste waterkeringen die veilig zijn maar ook mogelijkheden bieden voor aanpassingen (adaptief beheer). In dit rapport ligt de nadruk op alternatieve vormen van kustverdediging of aanpassingen/toevoegingen aan huidige keringen in getijdensystemen die meer ruimte bieden voor natuurlijke processen of deze zelfs benutten om de kust- en getijdezone veiliger (robuuster) te maken. Daarnaast wordt aandacht besteed aan de kansen die deze alternatieve waterkeringen bieden voor andere functies en medegebruik

Management options for restoring estuarine dynamics and implications for ecosystems: a quantitative approach for the Southwest Delta in the Netherlands

The Delta Works, a series of dams and barriers constructed in the 1960's–1980's changed the estuarine landscape of the Rhine-Meuse-Scheldt delta (SW Netherlands) into more stagnant and disengaged freshwater, brackish water or saltwater lakes. The remaining tidal systems were adapted by building a storm surge barrier in the Oosterschelde and dike reinforcement works along the Westerschelde. The Delta Works brought protection against flooding, but at the same time resulted in environmental and socio-economic problems, such as degradation of ecological quality and ecosystem functioning, disruption of fish migration routes, water and sediment quality problems.In this study we explore in an integrated, quantitative way the consequences of a number of management options for the Southwest Delta and their implications for the occurrence and distribution of aquatic and estuarine habitats, considering the mutual coherence between the water basins. Five scenarios were evaluated using a 1D hydraulic, water quality and primary production numerical model and GIS habitat mapping. Scenarios vary from small-scale interventions, such as changes in day-to-day management of hydraulic infrastructures or creation of small inlets in dams, feasible in the short term, to restoration of an open delta by removing dams and barriers, as a long term potential. We evaluate the outcomes in relation to the restoration of estuarine dynamics, as this is in policy plans proposed as a generic solution for the current ecological and environmental problems. Net water flow rates show more complex patterns when connectivity between water basins is increased and when sluice management is less strict. Estuarine transition zones and fish migration routes are partly restored, but only fully develop when basins are in open connection with each other. Area of intertidal habitats, tidal flats and tidal marshes, increases in each scenario, ranging between 7 and 83%, 1–56%, and 8–100% respectively, depending on scenario. Large scale infrastructural adaptations are needed to restore estuarine dynamics at large scale.The use of a 1D numerical model allowed to quantify the effect of different management measures for all water basins simultaneously, but also has its limitations. The model does not resolve more complex processes such as vertical mixing and morphodynamic changes. This requires expert judgment and more detailed 3D modelling

Settlement of <i>Macoma balthica</i> larvae in response to benthic diatom films

The role of multi-species benthic diatom films (BDF) in the settlement of late pediveliger larvae of the bivalve Macoma balthica was investigated in still-water bioassays and multiple choice flume experiments. Axenic diatom cultures that were isolated from a tidal mudflat inhabited by M. balthica were selected to develop BDF sediment treatments characterized by a different community structure, biomass, and amount of extracellular polymeric substances (EPS). Control sediments had no added diatoms. Although all larvae settled and initiated burrowing within the first minute after their addition in still water, regardless of treatment, only 48-52% had completely penetrated the high diatom biomass treatments after 5 min, while on average 80 and 69% of the larvae had settled and burrowed into the control sediments and BDF with a low diatom biomass (3.5 µg Chl a g1 dry sediment), respectively. The percentage of larvae settling and burrowing into the sediment was negatively correlated with the concentration of Chl a and EPS of the BDF. This suggests higher physical resistance to bivalve penetration by the BDF with higher diatom biomass and more associated sugar and protein compounds. The larval settlement rate in annular flume experiments at flow velocities of 5 and 15cm s1 was distinctly lower compared to the still-water assays. Only 4.6-5.8% of the larvae were recovered from BDF and control sediments after 3h. Nonetheless, a clear settlement preference was observed for BDF in the flume experiments; i.e., larvae settled significantly more in BDF compared to control sediments irrespective of flow speed. Comparison with the settlement of polystyrene mimics and freeze-killed larvae led to the conclusion that active selection, active secondary dispersal and, at low flow velocities (5cm s1), passive adhesion to the sediment are important mechanisms determining the settlement of M. balthica larvae in estuarine biofilms

Biobouwers als optimalisatie van waterveiligheid in de Zuidwestelijke Delta

Het Deltaprogramma Zuidwestelijke Delta werkt aan lange-termijn veiligheidsstrategieën voor de ZW Delta en onderzoekt de potentiële inzet van biobouwers als natuurlijke waterveiligheidsstrategie. Er is met name behoefte aan kennis over mogelijkheden voor het lokaal toepassen van biobouwers voor het optimaliseren van de waterveiligheid in de verschillende bekkens, zowel in de huidige situatie als voor mogelijke toekomst scenario’s. Deze studie onderzoekt welke biobouwers hiervoor in aanmerking komen en maakt een inschatting van de toepassingsmogelijkheden in de ZW Delta. Deze studie maakt onderdeel uit van een overkoepelende studie naar “innovatieve dijkconcepten” en is uitgevoerd binnen het Beleidsondersteunend onderzoek in het kader van EZ-programma's

Samenhang in de Delta, ontwikkelingsvarianten voor de Zuidwestelijke Delta: ecologische ontwikkeling van habitats en levensgemeenschappen (deel 2)

De Deltawerken brachten weliswaar veiligheid, maar daarmee verdween ook een groot deel van een zeer waardevol estuarien ecosysteem. Het oorspronkelijk stelsel van estuaria veranderde in een serie van elkaar gescheiden waterbekkens. Zoet en zout water zijn in de huidige situatie grotendeels gescheiden. Alleen de Westerschelde behield het karakter van een estuarium. Voorliggende rapportage geeft inzicht in de ecologische potenties van verschillende ontwikkelingsvarianten voor de Zuidwestelijke Delta (ZW Delta) als geheel en voor de verschillende waterbekkens binnen de ZW Delta. Hierbij gaat het om de ecologische potenties binnen de buitendijkse gebieden. In een eerdere studie, (IMARES, 2013), is een ecologische indeling ontwikkeld van systeem- en habitattypen die gebruikt kan worden om de verschillende ontwikkelvarianten op een gestandaardiseerde manier met elkaar te vergelijken. Voorliggende studie betreft de vervolgstap waarin een analyse is uitgevoerd naar het potentieel voorkomen van habitats en daarbij horende levensgemeenschappen in de verschillende ontwikkelvariante