We investigated which variables, including environmental variables and food availability, could predict the spatial distribution and dynamics of benthic macrofauna on an intertidal flat. A time series of macrobenthos and sediment grain size samples was complemented by time series of microphytobenthos and saltmarsh vegetation biomass and sediment grain size from airborne hyperspectral remote sensing, and elevation from laser altimetry. Response models were constructed to predict biomass and species richness of macrobenthos as a function of the environmental variables. Total biomass and species richness was best predicted by a combination of microphytobenthos biomass and sediment characteristics as explanatory variables. Deep deposit feeders and surface deposit feeders also responded best to a combination of variables, with deep deposit feeders responding more strongly to sediment grain size and surface deposit feeders responding more strongly to microphytobenthos biomass. The environmental conditions to reach maximum biomass differed for each macrobenthos species. Application of the response models to the remote sensingderived maps of the environmental variables enabled significant predictions of the spatial distribution of macrobenthos biomass, demonstrating the differences in distribution of the macrobenthos species. The models also revealed the sensitivity of the macrobenthic community to environmental change. In situ and remote sensing data demonstrated a significant fining of the sediment and a (temporal) increase in average microphytobenthos biomass. Field observations also showed a significant increase in species richness and changes in the relative abundance of species, with a decrease in Bathyporeia pilosa, and an increase in Nereis diversicolor, Pygospio elegans and Heteromastus filiformis. Such changes in macrobenthos biomass and species richness were indeed predicted from the response models. The study demonstrates that the synoptic remote sensing techniques combined with field sampling allow efficient ecological mapping and monitoring.

As ecosystem-based adaptation to global change is gaining ground, strategies to protect coastal and estuarine areas from increasing flood hazards are starting to consist of natural tidal wetland conservation and restoration in addition to conventional coastal defense structures. In this study, the capacity of tidal wetlands to locally attenuate peak water levels during storm tides is analyzed using a two-dimensional hydrodynamic model (TELEMAC2D) for a 3000 ha intertidal marsh (SW Netherlands). Model results indicate that peak water level reduction largely varies between individual flooding events and between different locations in the marsh. Model scenarios with variable dike positions show that attenuation rates can be minimized by blockage and set up of water levels against dikes or other structures confining the marsh size. This blockage only affects peak water level attenuation across wetlands if the duration of the flood wave is long compared to the marsh size. A minimum marsh width of 6 to 10 km is required to completely avoid blockage effects for the storm tidal cases assessed in this study. If blockage does not affect flood wave propagation, variations in attenuation rates between different locations in the marsh and between tides with varying high water levels can be explained with a single relationship based on the ratio between the water volume on the marsh platform and the total water volume on the platform and in the channels. Attenuation starts to occur when this ratio exceeds 0.2-0.4 and increases from there on up to a maximum of 29 cm/km for a ratio of about 0.85. Furthermore, model scenarios with varying marsh channel depth show that marsh scale attenuation rates increase by up to 4 cm/km if the channel elevation is raised by 0.7 m on average. Conversely, marsh scale attenuation rates decrease by up to 2 cm/km for scenarios in which the channels are lowered by 0.9 m on average. The marsh platform elevation has little effect on the maximum attenuation, but it determines which tides are attenuated. In particular, only overmarsh tides that inundate the platform are attenuated, while undermarsh tides that only flood the marsh channels are not attenuated or even amplified. These findings may assist coastal communities and managers in the optimization of the coastal defense function of tidal wetlands in combination with dikes.

Abstract. Coastal lowlands and estuaries are subjected to increasing flood risks during storm surges due to global and regional changes. Tidal wetlands are increasingly valued as effective natural buffers for storm surges by dissipating wave energy and providing flood water storage. While previous studies focused on flood wave attenuation within and behind wetlands, this study focuses on the effects of estuarine wetland properties on the attenuation of a storm tide that propagates along the length of an estuary. Wetland properties including elevation, surface area, and location within the estuary were investigated using a numerical model of the Scheldt estuary (Belgium, SW Netherlands). For a spring tide lower wetland elevations result in more attenuation of high water levels along the estuary, while for a higher storm tide higher elevations provide more attenuation compared to lower wetland elevations. For spring and storm tide a larger wetland surface area results in a better attenuation along the estuary up to a threshold wetland size for which larger wetlands do not further contribute to more attenuation. Finally a wetland of the same size and elevation, but located more upstream in the estuary, can store a larger proportion of the local flood volume and therefore has a larger attenuating effect on upstream high water levels. With this paper we aim to contribute towards a better understanding and wider implementation of ecosystem-based adaptation to increasing estuarine flood risks associated with storms.

2 Highlights: Richness of subtidal estuarine macrofauna depended on currents and sediment properties  Community composition and functional traits shifted with currents and sediment properties  Salinity, water depth and distance from tidal flats had a minor effect  Hydrodynamic models can predict hotspots and coldspots in ecological richness 3 ABSTRACTVariations in abundance and diversity of estuarine benthic macrofauna are typically described along the salinity gradient. The influence of gradients in water depth, hydrodynamic energy and sediment properties are less well known. We studied how these variables influence the distribution of subtidal macrofauna in the polyhaline zone of a temperate estuary (Westerschelde). Macrofauna density, biomass and species richness, combined in a so-called ecological richness, decreased with current velocities and median grain-size and increased with organic carbon of the sediment, in total explaining 39% of the variation. The macrofauna community composition was less well explained by the three environmental variables (ca 12-15% in total, with current velocity explaining ca 8%). Salinity, water depth and distance to the intertidal zone had a very limited effect on both ecological richness and the macrofauna community.The proportion of (surface) deposit feeders (including opportunistic species), decreased relative to that of omnivores and carnivores with increasing current velocity and sediment grain-size. In parallel, the proportion of burrowing sessile benthic species decreased relative to that of mobile benthic species that are able to swim. Correspondingly, spatial variations in hydrodynamics yielded distinct hotspots and coldspots in ecological richness. The findings highlight the importance of local hydrodynamic conditions for estuarine restoration and conservation. The study provides a tool based on a hydrodynamic model to assess and predict ecological richness in estuaries.