Daphne van der Wal

Spatial synchrony in intertidal benthic algal biomass in temperate coastal and estuarine ecosystems.

Microphytobenthos plays a vital role in estuarine and coastal carbon cycling and food webs. Yet, the role of exogenous factors, and thus the effects of climate change, in regulating microphytobenthic biomass is poorly understood. We aimed to unravel the mechanisms structuring microphytobenthic biomass both within and across ecosystems. The spatiotemporal distribution of the biomass of intertidal benthic algae (dominated by diatoms) was estimated with an unprecedented spatial extent from time-series of Normalized Differential Vegetation Index (NDVI) derived from a 6-year period of daily Aqua MODIS 250-m images of seven temperate, mostly turbid, estuarine and coastal ecosystems. These NDVI time-series were related to meteorological and environmental conditions. Intertidal benthic algal biomass varied seasonally in all ecosystems, in parallel with meteorology and water quality. Seasonal variation was more pronounced in mud than in sand. Interannual variation in biomass was small, but synchronized year-to-year biomass fluctuations occurred in a number of disjointed ecosystems. Air temperature explained interannual fluctuations in biomass in a number of sites, but the synchrony was mainly driven by the wind/wave climate: high wind velocities reduced microphytobenthic biomass, either through increased resuspension or reduced emersion duration. Spatial variation in biomass was largely explained by emersion duration and mud content, both within and across ecosystems. The results imply that effects on microphytobenthic standing stock can be anticipated when the position in the tidal frame is altered, for example due to sea level rise. Increased storminess will also result in a large-scale decrease of biomass.

Windows of opportunity for salt marsh vegetation establishment on bare tidal flats: The importance of temporal and spatial variability in hydrodynamic forcing

Understanding the mechanisms limiting and facilitating salt marsh vegetation initial establishment is of widespread importance due to the many valuable services salt marsh ecosystems offer. Salt marsh dynamics have been investigated by many previous studies, but the mechanisms that enable or disable salt marsh initial establishment are still understudied. Recently, the “windows of opportunity” (WoO) concept has been proposed as a framework providing an explanation for the initial establishment of biogeomorphic ecosystems and the role of physical disturbance herein. A WoO is a sufficiently long disturbance-free period following seedling dispersal, which enables successful establishment. By quantifying the occurrence of WoO, vegetation establishment pattern can be predicted. For simplicity sake and as prove of concept, the original WoO framework considers tidal inundation as the only physical disturbance to salt marsh establishment, whereas the known disturbance from tidal currents and wind waves is ignored. In this study, we incorporate hydrodynamic forcing in the WoO framework. Its spatial and temporal variability is considered explicitly in a salt marsh establishment model. We used this model to explain the observed episodic salt marsh recruitment in the Westerschelde Estuary, Netherlands. Our results reveal that this model can significantly increase the spatial prediction accuracy of salt marsh establishment compared to a model that excludes the hydrodynamic disturbance. Using the better performing model, we further illustrate how tidal flat morphology determines salt marsh establishing elevation and width via hydrodynamic force distribution. Our model thus offers a valuable tool to understand and predict bottlenecks of salt marsh restoration and consequences of changing environmental conditions due to climate change.

Patterns and drivers of daily bed-level dynamics on two tidal flats with contrasting wave exposure

Short-term bed-level dynamics has been identified as one of the main factors affecting biota establishment or retreat on tidal flats. However, due to a lack of proper instruments and intensive labour involved, the pattern and drivers of daily bed-level dynamics are largely unexplored in a spatiotemporal context. In this study, 12 newly-developed automatic bed-level sensors were deployed for nearly 15 months on two tidal flats with contrasting wave exposure, proving an unique dataset of daily bed-level changes and hydrodynamic forcing. By analysing the data, we show that (1) a general steepening trend exists on both tidal flats, even with contrasting wave exposure and different bed sediment grain size; (2) daily morphodynamics level increases towards the sea; (3) tidal forcing sets the general morphological evolution pattern at both sites; (4) wave forcing induces short-term bed-level fluctuations at the wave-exposed site, but similar effect is not seen at the sheltered site with smaller waves; (5) storms provoke aggravated erosion, but the impact is conditioned by tidal levels. This study provides insights in the pattern and drivers of daily intertidal bed-level dynamics, thereby setting a template for future high-resolution field monitoring programmes and inviting in-depth morphodynamic modelling for improved understanding and predictive capability.

Zooming in and out: Scale dependence of extrinsic and intrinsic factors affecting salt marsh erosion

Salt marshes are valuable ecosystems that provide important ecosystem services. Given the global scale of marsh loss due to climate change and coastal squeeze, there is a pressing need to identify the critical extrinsic (wind exposure and foreshore morphology) and intrinsic factors (soil and vegetation properties) affecting the erosion of salt marsh edges. In this study, we quantified rates of cliff lateral retreat (i.e., the eroding edge of a salt marsh plateau) using a time series of aerial photographs taken over four salt marsh sites in the Westerschelde estuary, the Netherlands. In addition, we experimentally quantified the erodibility of sediment cores collected from the marsh edge of these four marshes using wave tanks. Our results revealed the following: (i) at the large scale, wind exposure and the presence of pioneer vegetation in front of the cliff were the key factors governing cliff retreat rates; (ii) at the intermediate scale, foreshore morphology was partially related to cliff retreat; (iii) at the local scale, the erodibility of the sediment itself at the marsh edge played a large role in determining the cliff retreat rate; and (iv) at the mesocosm scale, cliff erodibility was determined by soil properties and belowground root biomass. Thus, both extrinsic and intrinsic factors determined the fate of the salt marsh but at different scales. Our study highlights the importance of understanding the scale dependence of the factors driving the evolution of salt marsh landscapes.

Publisher Correction: Patterns and drivers of daily bed-level dynamics on two tidal flats with contrasting wave exposure

A correction to this article has been published and is linked from the HTML version of this paper. The error has been fixed in the paper.

Conditional effects of tides and waves on short-term marsh sedimentation dynamics

Salt marshes are in danger of degradation due to human impact and climate change. A thorough understanding of mechanisms controlling sedimentation and erosion in salt marshes is essential for their conservation and restoration. To understand short‐term dynamics of sediment availability and deposition around marsh edges, two contrasting marshes, Rattekaai and Sint Annaland, were studied in the Oosterschelde (southwest Netherlands). Suspended sediment concentration (SSC) was measured by siphon samplers along four transects perpendicular to the marsh edge in each marsh, during nine flood tides between March and December 2013. Each transect was comprised of four sampling sites (−10 m and −1 m on the mudflat and +1 m and +10 m on the marsh plateau, relative to the marsh edge). Sediment deposition was measured along the transects on the marsh, at +1 m and +10 m from the marsh edge, over seven c. 14‐day intervals during the same 10‐month period. Two types of sediment traps were used, one measuring gross sediment deposition (TTD – tube trap deposition) and one measuring net sediment deposition (FTD – filter trap deposition). Wave loggers were deployed 10 m away from the marsh edge on the mudflat at each marsh. The results showed that both SSC and sediment deposition varied greatly through space, both between the two marshes and within each marsh along the marsh edge. The SSC and gross sediment deposition were much higher at Rattekaai than at Sint Annaland. SSC was significantly correlated with wind speed during sampling. Sediment deposition rates (TTD and FTD) and retention ratio (FTD/TTD) were significantly correlated with cumulative wave energy during the measurement period. A conceptual model of local sediment dynamics is proposed to explain the sediment dynamics around the marsh edge. This study highlights the importance of incorporating local sediment dynamics when evaluating marsh vulnerability and stability.

Opportunities for Protecting and Restoring Tropical Coastal Ecosystems by Utilizing a Physical Connectivity Approach

Effectively managing human pressures on tropical seascapes (mangrove forests, seagrass beds and coral reefs) requires innovative approaches that go beyond the ecosystem as the focal unit. Recent advances in scientific understanding of long-distance connectivity via extended ecosystem engineering effects and on-going rapid developments in monitoring and data-sharing technologies provide viable tools for novel management approaches that make use of positive across-ecosystem interactions (for example, hydrodynamics). Scientists and managers can now use this collective knowledge to develop monitoring and restoration protocols that are specialised for cross ecosystem fluxes (waves, sediments, nutrients) on a site-specific basis for connected tropical seascape (mangrove forests, seagrass beds and coral reefs).