Increasing terrestrial input of colored dissolved organic matter (CDOM) to temperate softwater lakes has reduced transparency, distribution of pristine rosette plants and overall biodiversity in recent decades. We examined microbial and UV-induced reduction of absorption by CDOM and dissolved organic carbon pools (DOC) in humic water from a groundwater-fed softwater lake as well as groundwater received from surrounding heathland and coniferous forest. An experimental setup that mimics naturally coupled continuous UV-exposure and microbial degradation was introduced and compared with experiments applying a single initial or no UV-exposure. We found that decreases of CDOM and DOC concentrations were negligible in groundwater and very small in lake water over 30 days in the absence of UV-exposure. Initial UV-exposure increased degradation rates, but further degradation ceased after 20 days preventing determination of the natural time course of degradation. Coupled continuous UV-exposure and microbial degradation showed high and constant degradation of CDOM (340 nm) over 30 days removing 87% of the initial absorption in heathland groundwater, and 20% in forest groundwater and lake water. Declines in DOC concentrations over 30 days were 34%, 28% and 13% of the initial levels in heathland groundwater, forest groundwater and lake water, respectively. Model estimates showed that a shift in land use from a forest dominated to a heathland dominated catchment could increase lake transparency from 0.6 to 2.5 m. and expand plant-covered area from 3 to 35%. The main time delay to a new steady state of better light climate would be degradation of soil organic pools and exchange of groundwater magazines, while the delay in the lake water after a complete shift to inflow of CDOM-poorer groundwater would last only 1–2 years. Consequently, changes in CDOM levels in groundwater input should have relatively rapid and marked influence on light conditions and plant distribution in shallow softwater lakes.