The aim of this work was to study freshwater Nitrososphaeria and their impact on the nitrogen cycling in lakes. We performed stable isotope experiments with addition of 15N-ammonium in 2019 to analyze potential nitrification rates in the hypolimnion of Lake Constance. Water was taken with a Niskin bottle. The indicated time represents the sampling time.We incubated 4.5 L lake water of 85 m in June, July, August and November 2019 with (15NH4)2SO4-2 (10 µM final concentration) for 48 h (67 h in June) at in-situ temperature of 4°C in the dark. 15N-Nitrite and 15N-Nitrate production was then determined and converted into ammonia oxidation rates. For more details see Klotz et al. (2021).