Vulnerable Hotspots: Understanding the hydrology of small German lakes and ponds

Small lakes and ponds are hotspots of biodiversity, biogeochemical reactions, and hydrological interactions in the landscape. While providing the same functions as larger lakes, they often do so at higher rates per unit area. Despite their ecological importance and vulnerability to climate and land use changes, they are excluded from monitoring programs at the European and German national scales. This limits our ability to assess their responses to a changing climate and the cascading effects on surrounding ecosystems. The ecosystem functions of small lakes are closely tied to the permanent or seasonally consistent presence of water, making the understanding of their water budgets a crucial research priority.

In this study, we utilized Sentinel-2 satellite imagery to reconstruct water area time series for approximately 700 German small lakes and ponds (0.005–0.5 km²) from 2017 to 2024. These time series were used to calibrate simple water balance models and investigate the susceptibility of these lakes to temporary or permanent lack of water under current and projected climate conditions. Sensitivity analyses and climate projections, combined with lake characteristics from the German Small Lake and Pond Inventory (GSLPI), allowed us to identify key attributes—such as morphology, geographic location, and connectivity to river networks—that best explain the risk of falling dry.

Our findings indicate that many small lakes and ponds are at risk of transitioning from permanent to seasonal water presence in the coming decades. Importantly, individual lake characteristics, rather than regional hydro-climate conditions, are the strongest predictors of these changes. This work underscores the urgent need to include small lakes and ponds in monitoring frameworks to better understand their ecological functions and vulnerabilities in a rapidly changing climate.