Mapping seagrass (Zostera) by remote sensing in the Schleswig-Holstein Wadden Sea

Abstract The change of mapping methods for seagrass beds, here species of eelgrasses, from aerial surveys to automated classification of optical satellite data is described. Both methods are compared with respect to availability and suitability of their data. Differences in the detection capability of the methods are shown as well as results of the validation of the satellite image classification. In North Friesia, where the largest area of eelgrass occurs in the Wadden Sea, eelgrass beds have been mapped regularly using aerial surveys since 1994. After a significant decline in the 1930s and 1960s, monitoring results show a steady increase in the size of the area covered by eelgrass beds up to 2017. Since 2006, the aerial surveys have been complemented by ground surveys, which, however, only cover one sixth of the area of the Schleswig-Holstein Wadden Sea each year. Results show that size estimates of individual beds can vary significantly between aerial and ground surveys. In recent years, satellite-borne remote sensing technology and subsequent analysis methods have reached a level of quality, which makes them an alternative and cost-efficient method for mapping eelgrass. The technology has advantages such as the coverage of large areas at single points in time, repeatable and transferable image analysis methods, and high spatial resolution of the satellite images, as well as frequent repetition of acquisition of data. This provides standardised results, which allow direct comparisons over time and between areas.