Indicators of Expansion and Retreat of Phragmites Based on Optical and Radar Satellite Remote Sensing: a Case Study on the Danube Delta

Reed is an important wetland species. In some places it provides valuable ecosystem services, while in other places it poses a threat as an invasive species. Thus, monitoring and predicting reed dynamics is crucial. We not only detected changes in reed area using remote sensing, but also developed indicators for the stability of reed wetlands based on remote sensing that would allow to predict its future development. We used satellite imagery to study reed development in the Danube Delta in Romania over a period of 22-years and identified expanding, stable and retreating reedlands. We then compared optical vegetation indices and radar backscatter among those three different reed development categories. We found clear spatial differences in long-term reed dynamics. We also revealed a clear difference in radar backscatter, but no difference in the optical signal of expanding, stable and decreasing reed areas. The radar data showed the largest seasonal variation in locations where reed was expanding and smallest seasonal variation where reed was decreasing. Overall, our study shows that the stability of reed ecosystems, and their services, can be monitored by quantifying seasonal changes in backscatter of reed-lands using radar satellites. This principle looks promising for monitoring other ecosystems as well

Synchronized high-resolution bed-level change and biophysical data from 10 marsh–mudflat sites in northwestern Europe

Tidal flats provide valuable ecosystem services such as flood protection and carbon sequestration. Erosion and accretion processes govern the ecogeomorphic evolution of intertidal ecosystems (marshes and bare flats) and, hence, substantially affect their valuable ecosystem services. To understand the intertidal ecosystem development, high-frequency bed-level change data are thus needed. However, such datasets are scarce due to the lack of suitable methods that do not involve excessive labour and/or costly instruments. By applying newly developed surface elevation dynamics (SED) sensors, we obtained unique high-resolution daily bed-level change datasets in the period 2013–2017 from 10 marsh–mudflat sites situated in the Netherlands, Belgium, and the United Kingdom in contrasting physical and biological settings. At each site, multiple sensors were deployed for 9–20 months to ensure sufficient spatial and temporal coverage of highly variable bed-level change processes. The bed-level change data are provided with synchronized hydrodynamic data, i.e. water level, wave height, tidal current velocity, medium sediment grain size (D50), and chlorophyll a level at four sites. This dataset has revealed diverse spatial morphodynamics patterns over daily to seasonal scales, which are valuable to theoretical and model development. On the daily scale, this dataset is particularly instructive, as it includes a number of storm events, the response to which can be detected in the bed-level change observations. Such data are rare but useful to study tidal flat response to highly energetic conditions