The Lake Mesocosm Warming Experiment (LMWE) is a unique long-term (with 20 years so far the world’s longest running) flow-through mesocosm experiment addressing climate-change effects on lakes under contrasting nutrient levels and water clarity. Twenty-four cylindrical outdoor mesocosms are used, each with a capacity of 2.8 m3. The mesocosms receive groundwater 6 times per day, resulting in a theoretical water retention time of 2.5 months. Water in the mesocosms is continuously mixed by paddles and heated by electrical elements. Three temperature regimes are run: ambient temperature and two elevated temperatures according to IPPC climate scenarios A2 and A2 + 50%, down-scaled to local 25 25 km grid cells. The modelled temperature difference for the A2 scenario is generally higher in August to January (max. 4.4 °C in September) than during the rest of the year (min. 2.5 °C in June). Individual mesocosms of each temperature regime are run at either low or high nutrient concentrations (4 replicates per treatment). The latter are obtained by weekly dosing of N and P. In June 2023, after 19 year of the previous nutrient loadings, the nutrient treatments were reversed to study the effects of oligotrphication and eutrophication under different warming scenarios.