Barbara Herbstritt

1 Introduction Floodplain forests are important ecosystems, located at the interface between water and land (Wohl 2021). They are shaped by water and sediment dynamics and provide a unique setting for high biodiversity and productivity (Tockner and Stanford 2002; Zhang et al. 2023). However, these ecosystems are threatened by anthropogenic alterations of flow regimes, for example, due to dam construction (de Resende et al. 2019), land use change (Rajib et al.

1 INTRODUCTION Stable isotopes of water (18O and 2H) have been widely used to study water cycling in ecohydrological, biogeochemical, and atmospheric research.1-5 In particular, the analyses of soil and xylem water isotopic compositions have been used to determine plants' source water, providing useful insights into many ecohydrological processes (e.g.,6-9).

1 INTRODUCTION Lentic small water bodies (LSWB) are of high abundance worldwide (Kristensen & Globevnik, 2014; Lischeid et al., 2018; Tiner, 2003; van der Kamp & Hayashi, 2009). LSWB with an area below 0.1 ha have been estimated to make up circa 8.6% of the global water surface area of lakes and ponds (Holgerson & Raymond, 2016). Particularly in Europe, LSWB are numerous.