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 the North Basin of Windermere in Cumbria, England. Monitoring began in 1945 for some variables. The data comprise surface temperature, surface oxygen saturation, Secchi depth, alkalinity, pH, ammonium, nitrate, soluble reactive phosphate, total phosphorus, dissolved reactive silicon and phytoplankton chlorophyll a. Water samples are based on a sample integrated from 0 to 7 m. Measurements are made from a boat at a marked location at the deepest part of the lake. All data are from January 2014 until the end of 2018 and were collected by the UK Centre for Ecology & Hydrology (UKCEH). This work was supported by the Natural Environment Research Council award number NE/R016429/1 as part of the UK-SCAPE programme delivering National Capability. Full details about this dataset can be found at https://doi.org/10.5285/bee7003f-90d8-412b-90b0-0c06dec9c1b6