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As lateral erosion can threaten valuable saltmarsh habitats and their capacity to protect the hinterland from waves and floods, there is a need to understand the mechanisms of erosion. We monitored lateral saltmarsh erosion at high spatiotemporal resolution during a 2.5 year period. We performed terrestrial laser scans (TLS) with centimetric spatial resolution, ca monthly, as well as before and after 2 wind events to assess morphological change of a saltmarsh cliff (

Heatwaves affect tidal flat ecosystems by altering the bioturbating behavior of benthic species, with potential consequences for sediment oxygenation, particle mixing, and erodibility. Although the frequency and duration of heatwaves are expected to increase under global warming scenarios, we lack insights into how heatwaves' temporal dynamics affect bioturbating behaviors. Using the widely distributed bioturbator Cerastoderma edule as model species, we quantified how heatwaves with identical heat-sum but different temporal dynamics (i.e., 3- vs. 6-d heating and normal temperature cycles) affect bioturbating behaviors and the sediment mixing processes in tidal mesocosms. Our results show that short but frequent 3-d heatwave cycles increased the magnitude of bioturbating behaviors, thereby resulting in more bio-mixed sediment than observed under infrequent prolonged 6-d heatwave cycles. This unexpected result could be ascribed to the weakening health condition indicated by a high death rate (47.37%) under 6-d heatwave cycles than in 3-d and no-heatwave control cycles. Present findings reveal that the impact of heatwaves on sediment bioturbation will strongly depend on the temporal dynamics of future heatwaves: bioturbation will be enhanced unless the heatwave duration exceeds species resistance and increases mortality.

Estuarine intertidal sediments are important centres for organic matter remineralization and nutrients recycling. Nevertheless, there is limited understanding regarding how these processes occur along the salinity gradient and their seasonality. Here, we report on the seasonal biogeochemical cycles from three types of intertidal sedimentary habitats (freshwater, brackish and marine) located in the Western Scheldt estuary (The Netherlands and Belgium). A full year of solute fluxes, porewater nutrient and sediment pigment concentrations at a monthly resolution revealed clear differences in the biogeochemistry of the three sites, indicating that environmental conditions determined the local nutrient dynamics. Temperature controlled sediment oxygen consumption rates and nutrient fluxes, but also affected pore water nutrient concentrations up to 14 cm deep. Fresh and brackish sediments had a net influx of dissolved inorganic nitrogen (DIN) (−1.62 mmol m−2 d−1 and -2.84 mmol m−2 d−1, respectively), while only the freshwater sediments showed a net influx of phosphate (−0.07 mmol m−2 d−1). We estimated that intertidal sediments remineralized a total of 10,000 t C y−1, with 97% of mineralization occurring in the brackish and marine parts. Overall, sediments removed 11% (1500 t N y−1) and 15% (∼200 t P y−1) of the total nitrogen and phosphorus entering the estuary from riverine input. Moreover, observations revealed the historical improvement of water quality resulting from water treatment policies. This spatiotemporal study of OM remineralization and early diagenesis in estuarine systems highlights the importance of intertidal sediments for estuarine systems. Our observations can be used in models to predict estuarine biogeochemistry or assess climate change scenarios.

In degraded landscapes, recolonization by pioneer vegetation is often halted by the presence of persistent environmental stress. When natural expansion does occur, it is commonly due to the momentary alleviation of a key environmental variable previously limiting new growth. Thus, studying the circumstances in which expansion occurs can inspire new restoration techniques, wherein vegetation establishment is provoked by emulating natural events through artificial means. Using the salt‐marsh pioneer zone on tidal flats as a biogeomorphic model system, we explore how locally raised sediment bed forms, which are the result of natural (bio)geomorphic processes, enhance seedling establishment in an observational study. We then conduct a manipulative experiment designed to emulate these facilitative conditions in order to enable establishment on an uncolonized tidal flat. Here, we attempt to generate raised growth‐promoting sediment bed forms using porous artificial structures. Flume experiments demonstrate how these structures produce a sheltered hydrodynamic environment in which suspended sediment and seeds preferentially settle. The application of these structures in the field led to the formation of stable, raised sediment platforms and the spontaneous recruitment of salt‐marsh pioneers in the following growing season. These recruits were composed primarily of the annual pioneering Salicornia genus, with densities of up to 140 individuals/m2 within the structures, a 60‐fold increase over ambient densities. Lower abundances of five other perennial species were found within structures that did not appear elsewhere in the pioneer zone. Furthermore, recruits grew to be on average three times greater in mass inside of the structures than in the neighboring ambient environment. The success of this restoration design may be attributed to the combination of three factors: (1) enhanced seed retention, (2) suppressed mortality, and (3) accelerated growth rates on the elevated surfaces generated by the artificial structures. We argue that restoration approaches similar to the one shown here, wherein the conditions for natural establishment are actively mimicked to promote vegetation development, may serve as promising tools in many biogeomorphic ecosystems, ranging from coastal to arid ecosystems.

Estuarine systems filter nutrients and organic matter from riverine input and lower concentrations reaching the sea. Sediments within these ecosystems play a significant role in the mineralization and retention of nutrients and organic matter within the estuary. Such processes are influenced by abiotic (e.g. salinity, temperature, etc.) and biological (e.g. fluctuations in the benthic community) parameters which may contrast remarkably between intertidal or subtidal zones. Despite their relative importance, few studies have investigated the biogeochemistry of intertidal sediments with high spatiotemporal resolution. This study reports the results of monthly biogeochemical monitoring in intertidal muddy sediments along the salinity gradient of the Western Scheldt estuary (NL). Budgets of OM mineralization and nutrient retention were calculated for the fresh, brackish, and marine water zones. Temperature controlled sediment oxygen consumption rates and nutrient fluxes. Fresh and brackish sediments had a net influx of dissolved inorganic nitrogen (DIN) (-1.62 mmol DIN m-2 d-1 and -2.84 mmol DIN m-2 d-1, respectively), while the freshwater area had the only net influx of phosphate (-0.07 mmol m-2 d-1). Marine sediments showed net effluxes of DIN and DIP. Despite the net influx observed in freshwater sediments, geospatial analysis showed that their contribution to the total estuarine filtering capacity was minimal due to their small area. In contrast, brackish and marine regions had a more important contribution to the estuarine filter because of their larger surface area. Overall, sediments removed 11% (1,500 t N y-1) and 15% (~200 t P y-1) of the total nitrogen and phosphorus entering the estuary from riverine input. Our findings highlight the importance of using spatially-resolving remineralization budgets to improve models and nutrient cycling estimates in estuarine systems.

Storm-induced erosion events may alter the diversity of tidal flat communities by selecting species that can better tolerate such disturbances. Introduced and invasive species are highly adaptable to a wide range of abiotic characteristics, and this adaptability may make them better able to withstand erosion events. With a novel flume method, we compared the ability of two bivalve species to resist storm-induced erosion: Cerastoderma edule, a native species to the Scheldt estuary in the Netherlands, and Ruditapes philippinarum, an introduced species that is successful in the Netherlands and worldwide. We used three sediment erosion rates to simulate storms of increasing severity. At the 10.6 and 15.9 cm h−1 sediment erosion rates,all R. philippinarum were surfaced, whereas only half C. edule were surfaced. However, after being brought to the sediment surface, C. edule were more readily transported by currents and waves than R. philippinarum due to differences in their shell shape. We concluded that the two bivalve species had different strategies to avoid mortality by severe storm erosion: C. edule avoided being surfaced and R. philippinarum avoided being transported. In this case, it appears that extreme storms favor the specific adaptations of a native species over the broad adaptability of a non-indigenous one. Indeed, C. edule may be more likely to survive moderately extreme storms than R. philippinarum, though the most extreme storms would be equally devastating to both species.

Dit rapport is opgedeeld in verschillende hoofdstukken die een overzicht geven van het onderzoek en de monitoring zoals uitgevoerd in het kader van de Roggenplaatsuppletie. Voor de hoofdstukken waarin resultaten beschreven worden is voor elk subonderdeel een korte beschrijving gegeven van de methodiek indien deze afwijkt van de T0, en anders een verwijzing naar de methodiek beschreven in de T0 rapportage (Walles et al. 2021), direct gevolgd door de resultaten.

Predictive models accounting for the effect of bioturbation on sediment resuspension must be based on ecological theory as well as on empirical parametrizations. The scaling trend of individual metabolic and activity rates with body mass may be a key to the mechanistic understanding of the observed patterns. With this study we tested if general size scaling rules in bio-mediated sediment resuspension may apply to a broad range of physical contexts for the endobenthic bivalve Cerastoderma edule. The effect on sediment resuspension of populations of C. edule differing by individual size was measured across physical gradients of current velocity and sediment composition in terms of fraction of fine particles. C. edule were able to enhance the resuspension of sediment containing silt, while they had scarce effect on the resuspension of coarse sediment. The effect of bioturbation was maximal at intermediate current velocity, when the hydrodynamic forcing is not strong enough to overcome the abiotic sediment resistance but it is able to suspend the bioturbated sediment. Although differences in sediment silt content and intensities of hydrodynamic stress have a relevant influence in determining the bioturbators individual contribution to sediment resuspension, the observed mass scaling trend is consistent across all treatments and close to theoretical expectation for size scaling of individual metabolic rates. This observation supports the hypothesis that the contribution of individual bioturbators to sediment resuspension is directly related to their energy use. Therefore, the proposed approach allows the formulation of expectations of biotic contribution to sediment resuspension based on the general size scaling laws of individual energy use.

Bioturbating benthic organisms have typically been characterised by how they modify the vertical sediment erosion thresholds. By means of several annular flume experiments, we aimed to understand how benthic organisms may affect grain-size sediment properties over time, and how this depends on the sediment type and the sediment loading of the water column. We compared the effect of two bioturbating macroinvertebrate species: a local dominant species, the cockle Cerastoderma edule and a spreading non-indigeneous species, the clam Ruditapes philippinarum. Our results indicate that the effect of benthic organisms on sediment dynamics is strongly dependent on both the prevailing environmental conditions and the benthic species present. If sediment is sandy, the benthos can gradually enhance the silt content of the sediment by mixing in part of the daily tidal sediment deposition. In contrast, if sediment is muddy, benthos can gradually decrease the silt content of the sediment by specifically suspending the fine fraction. Moreover, we observed that the native cockles had a stronger impact than invasive clams. Therefore, bioturbating benthos can have an important effect in determining the local sediment properties, with the outcome depending both on the species in question and the environmental conditions the bioturbator lives in. Our findings show that sediment bioturbation may have strong implications for tidal flat stability undergoing major changes from natural or anthropogenic sources.

Coastal systems are increasingly threatened by multiple local anthropogenic and global climatic stressors. With the difficulties in remediating global stressors, management requires alternative approaches that focus on local scales. We used manipulative experiments to test whether reducing local stressors (sediment load and nutrient concentrations) can improve the resilience of foundation species (canopy algae along temperate rocky coastlines) to future projected global climate stressors (high wave exposure, increasing sea surface temperature), which are less amenable to management actions. We focused on Fucoids (Cystoseira barbata) along the north-western Adriatic coast in the Mediterranean Sea because of their ecological relevance, sensitivity to a variety of human impacts, and declared conservation priority. At current levels of sediment and nutrients, C. barbata showed negative responses to the simulated future scenarios of high wave exposure and increased sea surface temperature. However, reducing the sediment load increased the survival of C. barbata recruits by 90.24% at high wave exposure while reducing nutrient concentrations resulted in a 20.14% increase in the survival and enhanced the growth of recruited juveniles at high temperature. We conclude that improving water quality by reducing nutrient concentrations, and particularly the sediment load, would significantly increase the resilience of C. barbata populations to projected increases in climate stressors. Developing and applying appropriate targets for specific local anthropogenic stressors could be an effective management action to halt the severe and ongoing loss of key marine habitats.

Intertidal benthic macroalgae are a biological quality indicator in estuaries and coasts. While remote sensing has been applied to quantify the spatial distribution of such macroalgae, it is generally not used for their monitoring. We examined the day-to-day and seasonal dynamics of macroalgal cover on a sandy intertidal flat using visible and near-infrared images from a time-lapse camera mounted on a tower. Benthic algae were identified using supervised, semi-supervised and unsupervised classification techniques, validated with monthly ground-truthing over one year. A supervised classification (based on maximum likelihood, using training areas identified in the field) performed best in discriminating between sediment, benthic diatom films and macroalgae, with highest spectral separability between macroalgae and diatoms in spring/summer. An automated unsupervised classification (based on the Normalised Differential Vegetation Index NDVI) allowed detection of daily changes in macroalgal coverage without the need for calibration. This method showed a bloom of macroalgae (filamentous green algae, Ulva sp.) in summer with >. 60% cover, but with pronounced superimposed day-to-day variation in cover. Waves were a major factor in regulating macroalgal cover, but regrowth of the thalli after a summer storm was fast (2. weeks). Images and in situ data demonstrated that the protruding tubes of the polychaete Lanice conchilega facilitated both settlement (anchorage) and survival (resistance to waves) of the macroalgae. Thus, high-frequency, high resolution images revealed the mechanisms for regulating the dynamics in cover of the macroalgae and for their spatial structuring. Ramifications for the mode, timing, frequency and evaluation of monitoring macroalgae by field and remote sensing surveys are discussed.