Braving the extremes: foraminifera document changes in climate-induced and anthropogenic stress in Wadden Sea salt marshes

Tidal wetlands are highly dynamic ecosystems that are susceptible to changes in sea level and flooding from storm surges. Among them, salt marshes play a key role in coastal protection as they contribute to wave attenuation through their regulating ecosystem services, thereby promoting sediment deposition and shoreline stabilization. However, the resilience of salt marshes, particularly those that have been modified and cultivated for centuries, is questionable in the face of accelerated sea-level rise (SLR) and increasing run-up heights of storm surges. In this context, this study aims to investigate the historical foraminiferal records of two sedimentary salt marsh archives from the Wadden Sea area (Dithmarschen and North Frisia, Germany) that have been modified to varying degrees by human management activities over the last century. The foraminiferal records document how physico-chemical traits of salt marshes of the central Wadden Sea have responded to storm tide inundation over the last century, providing information about salt marsh stability and vulnerability. Abnormally grown tests of the salt marsh indicator species Entzia macrescens increased in number between 1950 CE and the late 1980s, indicating the concurrent increase of environmental stress caused by the effects of times of increased salt marsh flooding. These trends can be linked to observations of amplified North Sea storm surges, corroborating that salt marsh ecosystems respond to changing climate conditions. Differences in the number of abnormal foraminifera between the studied salt marshes suggest a particularly high vulnerability of intensively human-modified coastal wetland ecosystems to amplified storm climate conditions

Braving the extremes: foraminifera document changes in climate-induced and anthropogenic stress in Wadden Sea salt marshes

Tidal wetlands are highly dynamic ecosystems that are susceptible to changes in sea level and flooding from storm surges. Among them, salt marshes play a key role in coastal protection as they contribute to wave attenuation through their regulating ecosystem services, thereby promoting sediment deposition and shoreline stabilization. However, the resilience of salt marshes, particularly those that have been modified and cultivated for centuries, is questionable in the face of accelerated sea-level rise (SLR) and increasing run-up heights of storm surges. In this context, this study aims to investigate the historical foraminiferal records of two sedimentary salt marsh archives from the Wadden Sea area (Dithmarschen and North Frisia, Germany) that have been modified to varying degrees by human management activities over the last century. The foraminiferal records document how physico-chemical traits of salt marshes of the central Wadden Sea have responded to storm tide inundation over the last century, providing information about salt marsh stability and vulnerability. Abnormally grown tests of the salt marsh indicator species Entzia macrescens increased in number between 1950 CE and the late 1980s, indicating the concurrent increase of environmental stress caused by the effects of times of increased salt marsh flooding. These trends can be linked to observations of amplified North Sea storm surges, corroborating that salt marsh ecosystems respond to changing climate conditions. Differences in the number of abnormal foraminifera between the studied salt marshes suggest a particularly high vulnerability of intensively human-modified coastal wetland ecosystems to amplified storm climate conditions

Foraminifera census counts at salt-marsch archive GeoHH-FK (Friedrichskoog, south-eastern North Sea)

Here we have evaluated and compared two benthic foraminiferal records derived from sedimentary salt-marsh archives from the south-eastern North Sea coast, covering the past ~100 years. The particular focus was on the agglutinated benthic salt-marsh foraminifera Entzia macrescens, finding a rising number of deformed tests at times of strengthened North Sea storminess and associated more frequent salt-marsh flooding between the mid-century and late 1980s. The study is based on sediment sequences GeoHH-FK (Friedrichskoog, Dithmarschen) and TB13-1 (Bay of Tümlau, Eiderstedt Peninsula; Müller-Navarra et al., 2019; doi:10.1016/j.ecss.2018.12.022) that were retrieved from the salt marshes' erosional cliffs in November 2016 and August 2013. Sediment sequence GeoHH-FK was sampled at 0.5 cm spacing, with every second sample considered for benthic foraminiferal analysis, resulting in a total of 116 samples. All samples were wet-sieved for the 63–500 µm sediment fraction and benthic foraminiferal analysis was based on allocate splits in order to obtain approximately 100 individuals per split. Subsequent taxonomical identification was carried out on the wet sediment samples. Normal and irregular tests of E. macrescens were distinguished and counted separately. The benthic foraminiferal record of TB13-1 was provided by Müller-Navarra et al. (2019; doi:10.1016/j.ecss.2018.12.022). As benthic foraminifera are very sensitive to environmental changes, the consideration of deformed tests of the salt-marsh indicator species E. macrescens allowed for the evaluation of the salt marshes' vulnerability to changing climate conditions, in dependence on the degree of their modification by human interventions

Organic carbon, XRF scans (Br, Cl, Rb, Zr), and relative grain size distribution, obtained from sediment sequences TB13-1 and GeoHH-FK (south-eastern North Sea)

The sedimentological and geochemical proxy data shown in here were used to evaluate the response of coastal wetlands to 20th-century storm-climate variability at the south-eastern North Sea coastal area. The identification and evaluation of storm-surge layers within well-stratified salt-marsh archives and associated marine influences on the salt marshes was based on mean grain sizes, organic carbon (Corg) and XRF scanning datasets (Br, Cl, Rb and Zr). Local changes in the sedimentary organic matter supply were illustrated by the ln(Br/Cl) ratio, while the Br/Corg ratio represented the marine versus terrestrial organic matter supply, and the ln(Zr/Rb) ratio was used as a proxy for the relative grain-size distribution. It was found that abrupt drops in the ln(Br/Cl) ratio mainly coincided with siliciclastic sand layers, whereas the Br/Corg and ln(Zr/Rb) records revealed an increasing long term trend starting from the mid-century towards today, resembling the observed strengthening in North Sea storminess. Here, we focused on sedimentary salt-marsh sequences from two different salt-marsh sites at the German North Sea coast, both of which were influenced by natural processes but exposed to human activities to different degrees. These sites comprise: I) a more natural developed and un-grazed salt marsh (Bay of Tümlau, Eiderstedt Peninsula), and II) an intensely human-modified and grazed salt marsh (Friedrichskoog, Dithmarschen). Field work was carried out in August 2013 in the Bay of Tümlau (Müller-Navarra et al., 2019; DOI: 10.1016/j.ecss.2018.12.022) and in November 2016 in Friedrichskoog. The erosional cliffs that fringe both salt marshes at their seaward edges were chosen for sediment recovery. For this purpose, 10–15 plastic U-channels open at one long side were pressed vertically into the previously cleaned erosional cliff face. The recovered sediment sequences are TB13-1 (Bay of Tümlau; Müller-Navarra et al., 2019; DOI: 10.1016/j.ecss.2018.12.022) and GeoHH-FK (Friedrichskoog). As coastal wetlands are increasingly exposed to rising sea level and associated inundation by storm surges, this study helped to better understand how and to what extent changes in storm-surge climate are translated into the sediment archives and how human-modified salt marshes have so far been able to copy to changing climate conditions over the last century