Remotely sensing the German Wadden Sea—a new approach to address national and international environmental legislation

Abstract

The Wadden Sea along the North Sea coasts of Denmark, Germany, and the Netherlands is the largest unbroken system of intertidal sand and mud flats in the world. Its habitats are highly productive and harbour high standing stocks and densities of benthic species, well adapted to the demanding environmental conditions. Therefore, the Wadden Sea is one of the most important areas for migratory birds in the world and thus protected by national and international legislation, which amongst others requires extensive monitoring. Due to the inaccessibility of major areas of the Wadden Sea, a classification approach based on optical and radar remote sensing has been developed to support environmental monitoring programmes. In this study, the general classification framework as well as two specific monitoring cases, mussel beds and seagrass meadows, are presented. The classification of mussel beds profits highly from inclusion of radar data due to their rough surface and achieves agreements of up to 79 % with areal data from the regular monitoring programme. Classification of seagrass meadows reaches even higher agreements with monitoring data (up to 100 %) and furthermore captures seagrass densities as low as 10 %. The main classification results are information on area and location of individual habitats. These are needed to fulfil environmental legislation requirements. One of the major advantages of this approach is the large areal coverage with individual satellite images, allowing simultaneous assessment of both accessible and inaccessible areas and thus providing a more complete overall picture.

SAR Imaging of Archaeological Sites on Intertidal Flats in the German Wadden Sea

“…We already used SAR images to derive surface characteristics of exposed intertidal flats and to detect bivalve beds [17]. During our analyses of SAR imagery of intertidal flats north of Pellworm we discovered bright and dark linear features that could not be of natural origin, but rather be manifestations of anthropogenic structures dating back to the times before the "Grote Mandrenke".…”

Detection of Bivalve Beds on Exposed Intertidal Flats Using Polarimetric SAR Indicators

“…The test site "Amrum" on the northern part of the German Wadden Sea (Figure 1) is located between the islands of Amrum and Föhr and was already subject to previous studies [11,12,27]. This region contains sandy and muddy sediments, vegetated areas, bivalve beds (mainly Pacific oysters and cockles, but also blue mussels) and seagrass meadows; hence it represents an area of a typical mixture of bivalve beds and sediments on exposed intertidal flats.…”

“…Blue mussels and Pacific oysters may form extensive bivalve beds on elevated intertidal flats that may also contain areas of bare sediments and water puddles. Frequent monitoring of bivalve beds in the test site "Amrum" is conducted during field campaigns as well as based on optical and (single-polarization) SAR imagery [12]. Therefore, a larger number of SAR images was available for the present investigation.…”

“…Tidal creeks can be delineated because of their rather smooth surface causing lower radar backscatter and also due to the enhanced surface roughness of the sandy sediments at their rims. As seen in Figure 2, it is difficult to infer the spatial extent of Frequent monitoring of bivalve beds in the test site "Amrum" is conducted during field campaigns as well as based on optical and (single-polarization) SAR imagery [12]. Therefore, a larger number of SAR images was available for the present investigation.…”

“…It was found that bivalve beds and salt marshes have specific SAR signatures and can thus be monitored using space-borne SAR sensors [10][11][12][13]. Those bivalves are exposed above the sediments and form a rough reef or bed structure (with sharp and jagged surfaces) and increase the surface roughness locally, which causes stronger radar backscattering signals, making them visible in SAR imagery [11,14,15].…”