This is part of an ongoing long-term monitoring dataset of surface temperature, surface oxygen, water clarity, water chemistry and phytoplankton chlorophyll a from fortnightly sampling at Blelham Tarn in Cumbria, England, that began in 1945 for some variables. The data have been collected by the UK Centre for Ecology & Hydrology (UKCEH). The data available to download comprise surface temperature (TEMP) in degree Celsius, surface oxygen saturation (OXYG) in % air-saturation, Secchi depth (SECC) in metres, alkalinity (ALKA) in µg per litre as CaCO3 and pH. Total ammoniacal nitrogen (NH4N), total oxidised nitrogen (TON), soluble reactive phosphate (PO4P), total phosphorus (TOTP), dissolved reactive silicon expressed as SiO2 (SIO2) and phytoplankton chlorophyll a (TOCA) are all given in µg per litre. Water samples are based on a sample integrated from 0 to 5 m. Water chemistry values below the detection limit, are indicated by a < sign in the column titled ‘sign(if<LOD)’. Measurements are made from a boat at a marked location (buoy) at the deepest part of the lake. When it was not possible to visit the buoy, samples were taken from the shore along with surface temperature and oxygen measurements. Water samples were not integrated on these occasions, marked as Flag 2.All data are from January 2024 until the end of 2024. Full details about this dataset can be found at https://doi.org/10.5285/b8efacd7-c849-40e1-a2d8-5c8355e95179