This study presents input-output budgets of total dissolved nitrogen (TDN), dissolved organic N (DON) and dissolved inorganic N (DIN) for a reservoir in a peatland catchment in the south Pennines (UK). This site receives high levels of atmospheric inorganic N deposition, in the range of 26 kg N ha-1 yr-1. The results show that the reservoir retains ~21 to 31% of the annual TDN input (8806±741 kg N). Approximately 39 to 55% of DON (3782±653 kg N) and 6 to 13% of DIN (5024±349 kg N) was retained/processed. A long water retention time (104 days), average annual pH of 6.5, high concentrations of DIN in the reservoir water and a deep water column suggests that denitrification is potentially a key mechanism of N retention/removal. The results also demonstrate that DON is potentially photodegraded and utilised within the reservoir, particularly during the summer season when 58 to 80% of DON input (682±241 kg N) was retained, and a net export of DIN (~ 34 kg N) was observed. The findings therefore suggest that DON may play a more crucial role in the biogeochemistry of peat-dominated acid sensitive upland freshwater systems than previously thought. Reservoirs, impoundments, and large lakes in peatland catchments may be important sites in mediating downstream N transport and speciation.