Lakes interact with groundwater in complex spatial and temporal patterns. Hence changes in regional groundwater flow can have siginificant impacts on lake ecosystems. To assess and control such impacts a thorough quantification of exchange fluxes is of utmost importance. In this project new methods to quantify those fluxes will be developed and tested. Study site is an acidic mine lake in Eastern Germany (Lausatia) where previous studies have found strong influences of groundwater inflow on biogeochemical processes at the lake-groundwater interface. Results of the project (temporal dynamics and spatial patterns of lake-groundwater exchange) will be utilized in two joint projects, which investgate groundwater-influenced biogeochemical and microbiological processes in the lake sediments. In addition to classical field measurements such as seepage meters and mini-piezometers, highly resolved temperature measurments will be used to qualitatively map spatial exchange patterns as well as to quantify flux rates by inverse modeling of vertical temperature profiles. Results from the field measurements will be used to refine and calibrate an existing steady-state groundwater model in transient mode. Aim of the model is to simulate the seasonal dynamics and spatial patterns of lake-groundater exchange for current conditions as well as for scenarios of regional groundwater development. In a field experiment the lake water level will be maniuplated through a weir to measure exchange fluxes for a low and a high lake water level. The calibrated model can be used to assess the long term biogeochemical development of the lake.