## 3 Results
### Tidal Flat Dynamics Imaged by TerraSAR-X  
The tidal area close to the watershed of the Norderney basin between the eastern Riffgat channel and the mainland coast may serve as an example to demonstrate both the stability and the variability of tidal areas and their reproduction in the TerraSAR-X data. A time series of TerraSAR-X images shows the developments taking place in this area from 2009-2015 (Figure 12). The branches of the Riffgat channel, at the top of the picture, do not change their courses during this period. Likewise, the large mud field (Figure 12, Region 1) remains as such, only the shape of the southern edge, constituting the boundary to the adjacent depression, changes slightly. The gully structures within the mud field remain essentially the same, even if displacements occur in the course of the smaller branches. Since the TerraSAR-X data were recorded in April to July, the mud field is shown in the aspect of the calmer season in each of the four SAR acquisitions. Compared to 2009, the area increased slightly in 2011, 2014, and 2015. Most obvious in the SAR data, however, is the development of mussel beds in the low lying area south of the mud field (Figure 12, Region 2-4): in the field surveys of 2008/2009, this area proved to be a depression with open sediment surface, often water-covered, and in wide areas densely populated by common cockles (_C. edule_) and the polychaete worm sand mason (_Lanice conchilega_). Sand masons build tubes protruding up to a few centimeters above the sediment surface which leads to an increased roughness, particularly when they break through the surface of shallow water covering the flats (Figure 12a, Region 4, see also photography in Figure 12a). Just like the shell detritus of cockles (cf. chap. 3.2), sand masons can serve as a substrate for the settlement of blue mussels. In 2011, the first mussel settlement in this location was reproduced in the TerraSAR-X image (Figure 12b, Region 4), and in the data from 2014, the mussel bed with its internal structures is already well recognizable as such (Figure 12c, Region 4, photography in Figure 12c). The typical pattern of an established mussel bed can be seen here in 2015 (Figure 12b-d). Southwest of the mud field however, a mussel bed with open structures developed from 2009 to 2011, which in the following years recedes and confines to a few central bed structures in 2014/2015 (Figure 12, Region 3). In effect, the area of the extensive depression clearly discernible in 2008-2011, has narrowed until 2015. It has been taken up, in particular, by scattered mussel settlements but also by accumulations of muddy sediment, partly forming temporary linear structures (Figure 12c,d, Region 2) which are visible at the mud field's edge in 2014 and throughout the area of the formerly water covered depression. These may be due to the unusually turbulent summer season of that year [32; 33]. South of the mussel bed in Region 3, higher backscatter is visible especially in the 2014 acquisitions (Figure 12c). From the field surveys it is known that, in this area, fields of seagrass patches occur. Figure 11: Mussel beds in the central area of the tidal flats south of Norderney imaged by TerraSAR-X (SL of 19/07/2011, desc. ): (**a**) established old mussel bed (1) and young mussel bed (2); (**b**) yellow line represents monitoring result from aerial photography interpretation (2011). Seagrass itself was not detected in the TerraSAR-X data according to this study, as it lies flat on the sediment at low tide and is characterized mainly by its spectral features. In some cases, though, the seagrass vegetation leads to the formation of elevated surface structures, which are reflected in the SAR data. In summary, certain habitats and structures such as the mud field, mussel bed, or the water-covered depression are clearly recognizable in the TerraSAR-X data due to typical characteristics and patterns.