This dataset contains data on a mesocosm experiment in which we exposed 100 intact fen peat cores (80 cm, 20 cm Ø) to five different water levels (0, 20, 40, 60 cm and variable - surface), two nutrient application levels to mimic continued agricultural use, and two water origins (surface water and groundwater). Over an eight-month period, we harvested above-ground biomass five times and sampled pore water at two depths each month. Samples were analysed on nutrients. The results of these analyses can be found in this dataset.