Optical hydrography for seasonal and flood-induced changes in aquatic vegetation

Abstract. The increasing occurrence of extreme weather events has a far reaching impact on all types of ecosystems. This includes submerged ecosystems, where floods and droughts influence the flora and fauna. Monitoring such submerged landscapes and their aquatic vegetation has gained interest in recent decades, and advances in remote sensing technology allow detailed mapping of those ecosystems. There, changes in vegetation extent throughout such extreme weather events remain insufficiently quantified, as comprehensive data sets before and after extreme weather events are rarely obtained. Thus, this study presents an extensive case study for a vegetated pond in Lower Austria, which was subjected to major flooding during the September 2024 flood in Austria. The presented data set includes aerial imaging and bathymetric LiDAR data for six epochs over three years, four before the flood and two after. This study presents the near-extinction of the submerged vegetation after the flood event and puts the results into perspective with the seasonal changes before the flood. For the three half-year periods before the flood event, the height of the vegetation shows a median fluctuation between −0.3 m to 1 m over the pond, while the change in vegetation height after the flood event has a median value of approximately −2.9 m. Although the seasonal changes are in alignment with the expected values, based on current research regarding the type of macrophytes present in the study area, the post-flood changes present a significant decrease in the overall vegetation. This, along with changes in turbidity, indicates a long-term impact on the pond and outlines the impact of extreme weather events on aquatic ecosystems. In general, the case study presents insights into rarely documented changes due to extreme weather events and provides a remote sensing approach to quantify changes in aquatic landscapes.