Facilitating Salt Marsh Restoration: The Importance of Event-Based Bed Level Dynamics and Seasonal Trends in Bed Level Change

Abstract

Intertidal salt marshes provide a range of valuable ecosystem services which typically increase with marsh width. Understanding the drivers for salt marsh expansion versus retreat is thus key to managers. Previous research highlights the influence of short-term (daily/event) bed level dynamics on germination and establishment and subsequent vegetation presence. However, more recent literature suggests the importance of medium-term seasonal bed level dynamics on viable seed availability and subsequent vegetation presence. This study aims to assess event-based and seasonal bed level dynamics for vegetation presence in natural and semi-natural salt marshes and to provide generic thresholds for vegetation presence. To gain insight into bed level dynamics, data was used from autonomous Optical and Acoustic Surface Elevation Dynamics sensors (O-SED and A-SED) around the edge of natural and semi-natural salt marshes. Sensors were installed at vegetated and unvegetated measurement station Field observations from 22 O-SEDs deployed at 4 well-established natural salt marshes in the Western Scheldt estuary and 4 O-SEDs at a well-established semi-natural salt marsh in the Wadden Sea were reanalyzed. Six novel A-SEDs were deployed at a pioneer semi-natural salt marsh in the Ems-Dollard Estuary. The measurement duration at all salt marshes was at least 1 year. The A-SED sensor was successfully validated against manual measurements. Furthermore, vegetation data and water level data were obtained. No significant difference was observed between natural and semi-natural salt marshes. However, a significant difference between vegetated and unvegetated measurement stations for short-term bed level dynamics was observed. Vegetation was found to be present at locations restricted by short-term bed level variability smaller than or equal to 12 mm, emphasizing the presence of a short-term threshold. Although trends in the non-growing season were significantly different between vegetated and unvegetated stations, seasonal thresholds for vegetation presence were not found. The findings imply that knowledge of bed level-dynamics in well-established natural marshes can be used to predict vegetation presence in constructed semi-natural marshes. The importance of local short-term dynamics for vegetation presence instead of longer-term dynamics highlights possibilities for developing favorable conditions for vegetation presence in marsh restoration projects and the construction of new salt marsh ecosystems.

A comparison and coupling of two novel sensors for intertidal bed‐level dynamics observation

“…One drawback of this sensor, however, is that the phototransistors need sufficient light intensity, which means the sensor is only operative during low tides and daytime. In light of this, recently developed laser‐based surface elevation dynamics (LSED) and acoustic surface elevation dynamics (ASED) sensors have adopted laser‐ranging and echo‐ranging techniques (Hu et al 2020; Willemsen et al 2022). The active measuring methodology has increased the measuring window of the sensors.…”

“…Design diagrams of ( a ) LSED (based on Hu et al 2020) and ( b ) ASED sensor (based on Willemsen et al 2022). ( c ) Setup of the coupled LSED and ASED sensors in the field.…”

“…It is a novel autonomous acoustic‐based distance detection device and is capable of capturing bed‐level changes with high‐resolution during inundation period. It has been successfully deployed at a seminatural salt marsh in the Ems‐Dollard Estuary, the Netherlands (Willemsen et al 2022). The ASED sensor is comprised of downward‐looking measurement unit, circuit board, battery pack, and vacuum waterproof housing (Fig.…”

“…The pressure sensor measures pressure with increments of 0.1 mbar, both when the sensor is emergent and inundated. The temperature is monitored in degrees Celsius with a two‐decimal accuracy (Willemsen et al 2022). The battery enables the sensor to measure for a full year, with a burst measurement interval of 30 min and bursts of eight measurements with a frequency of 5 Hz.…”

Adventures of salt marsh recruits

“…The present species-specific data regarding seedling resistance can be used in biophysical models (e.g., Hu et al, 2015aHu et al, , 2021Schwarz et al, 2018) to predict long-term habitat suitability for restoration. Present findings also emphasize that it is important to measure short-term sediment dynamics (e.g., see Hu et al, 2015b, 2020and Willemsen et al, 2022 for state-of-the-art approaches) during the critical period of seedling establishment at the target site to optimize restoration designs. A less-exposed sedimentary field is ideal for seedling establishment, considering its performance in enhancing seedling resilience (Jørgensen et al, 2019;Marion et al, 2021).…”